2003 (with abstract) |
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| ▼ Ordered by first author |
1. T. Akasaka, T. Nishida, Y. Taniyasu, M. Kasu, T. Makimoto, and N. Kobayashi
"Reduction of threading dislocations in crack-free AlGaN by using multiple thin SixAl1-xN interlayers"
Applied Physics Letters 83 (20)4140-4142 (2003).
ABSTRACT: Crack-free AlGaN thin films were directly grown on SiC substrates by metalorganic vapor phase epitaxy, and their threading dislocation density was reduced by one order of magnitude using 1-2 nm thick, heavily Si-doped AlN multiple interlayers. The interlayers form SixAl1-xN ternary alloys, where the Si molar fraction ranges typically from 0.07 to 0.17. This technique enables us to grow crack-free AlGaN films, since the film thickness of about 1 mum is much smaller than that required in conventional epitaxial lateral overgrowth techniques. Both termination and looping of threading dislocations were observed near the interlayers using cross-sectional transmission electron microscopy. Light emitting devices with the SixAl1-xN multiple interlayers showed a remarkable improvement in the intensity and spectral width of electroluminescence and the series resistance. (C) 2003 American Institute of Physics.
2. T. Akasaka, S. Ando, T. Nishida, T. Saitoh, and N. Kobayashi
"Current-confining structure of InGaN hexagonal microfacet lasers by selective incorporation of Mg during selective-area MOVPE"
Journal Of Crystal Growth 248537-541 (2003).
ABSTRACT: The combination of patterned dry etching and selective-area regrowth by MOVPE enables us to fabricate the group III-nitride-based lasers of various shapes and sizes and to integrate these lasers on substrates. The vertical cavity mirrors are formed by selective-a rea regrowth of the dry-etched inclined facets. No growth occurs on the substrate surface exposed by dry etching. In this method. current-confining structures are self-organizing because of the change in Mg incorporation during the vertical facet formation. The selectively regrown Mg-doped GaN consisted of three regions by the regrowth of top. inclined. and vertical facet Surfaces, respectively. These regions exhibited different contrast in scanning electron microscope observation and different cathodeluminescence spectra due to the difference in probability of Mg incorporation into them. This phenomenon was successfully applied to form current-confining structures of the lasers, These lasers lased at room temperature by pulsed Current injection. (C) 2002 Elsevier Science B.V. All rights reserved.
3. T. Akiyama, and H. Kageshima
"Microscopic mechanism of interfacial reaction during Si oxidation"
Applied Surface Science 216 (1-4)270-274 (2003).
ABSTRACT: The reaction of atomic oxygen at the SiO2/Si(1 0 0) interface is studied based on total-energy electronic-structure calculations. It is found that the reaction in which an 0 atom in the oxide inserts into the Si-Si bond of the substrate is largely exothermic. The calculated energy barrier of 0.87eV for this reaction is lower than the experimentally reported activation energy (2.0 eV) for the interfacial reaction in the dry oxidation. The results would help us clarify the microscopic mechanism of Si oxidation, especially in identifying the form of oxidant in the dry oxidation. (C) 2003 Elsevier Science B.V. All rights reserved.
4. L. Alff, Y. Krockenberger, B. Welter, M. Schonecke, R. Gross, D. Manske, and M. Naito
"A hidden pseudogap under the 'dome' of superconductivity in electron-doped high-temperature superconductors"
Nature 422 (6933)698-701 (2003).
ABSTRACT: The ground state of superconductors is characterized by the long-range order of condensed Cooper pairs: this is the only order present in conventional superconductors. The high-transition-temperature (high-T-c) superconductors, in contrast, exhibit more complex phase behaviour, which might indicate the presence of other competing ground states. For example, the pseudogap(1,2)-a suppression of the accessible electronic states at the Fermi level in the normal state of high-T-c superconductors has been interpreted as either a precursor to superconductivity(3,4) or as tracer of a nearby ground state that can be separated from the superconducting state by a quantum critical point(5,6). Here we report the existence of a second order parameter(7) hidden within the superconducting phase of the underdoped (electron-doped) high-T-c superconductor Pr2-xCexCuO4-y and the newly synthesized electron-doped material La2-xCexCuO4-y (ref. 8). The existence of a pseudogap when superconductivity is suppressed excludes precursor superconductivity as its origin. Our observation is consistent with the presence of a (quantum) phase transition at T = 0, which may be a key to understanding high-T-c superconductivity. This supports the picture that the physics of high-T-c superconductors is determined by the interplay between competing and coexisting ground states.
5. K. Anagawa, T. Hamatani, T. Shibauchi, T. Watanabe, and M. Suzuki
"Temperature and field dependence of the gap structure in Bi2Sr2CaCu2O8+delta studied by short-pulse interlayer tunneling spectroscopy"
Physica C-superconductivity And Its Applications 388289-290 (2003).
ABSTRACT: We measure tunneling characteristics of Bi2Sr2CaCu2O8+delta intrinsic junctions by a sub-microsecond pulse technique to investigate temperature T and the magnetic field H dependence of the bulk-intrinsic gap structure without excess Joule heating. The conductance curve shows a peak at the superconducting gap voltage (V-p) at low temperatures, and the peak becomes broader when we increase either T or H that is a pair breaking parameter. However, V-p has a non-monotonic T-dependence with a dip near T-c, while V-p increases monotonically with H. This difference indicates that V-p(T) cannot be explained in terms of a single gap model, and implies that the superconducting gap is distinct from the pseudogap, which is represented by V-p(T) above T-c. (C) 2003 Elsevier Science B.V. All rights reserved.
6. F. Ancilotto, D. G. Austing, M. Barranco, R. Mayol, K. Muraki, M. Pi, S. Sasaki, and S. Tarucha
"Vertical diatomic artificial molecule in the intermediate-coupling regime in a parallel and perpendicular magnetic field"
Physical Review B 67 (20), 205311 (2003).
ABSTRACT: We present experimental results for the ground-state electrochemical potentials of a few electron semiconductor artificial molecule made by vertically coupling two quantum dots, in the intermediate-coupling regime, in perpendicular and parallel magnetic fields up to Bsimilar to 5 T. We perform a quantitative analysis based on local-spin density functional theory. The agreement between theoretical and experimental results is good, and the phase transitions are well reproduced.
7. J. Anderberg, M. S. Colclough, D. B. Crum, Y. Tokura, D. N. Paulson, and R. L. Fagaly
"LTS SQUID microscope with micron spatial resolution"
Ieee Transactions On Applied Superconductivity 13 (2)231-234 (2003).
ABSTRACT: We describe A multichannel LTS SQUID microscope with micron spatial resolution. The system achieves micron resolution by the use of small (14 mum) detection coils and narrow gap between the coils and the object(s) being scanned. Samples ire mounted inside an exchange-gas can at the lower end of a cryogenic probe. This houses all of the cryogenic portions of the microscope and is filled with helium exchange gas. A 5 min x 5 mm scanning stage is used to scan the sample, which is at cryogenic temperatures. Stepping motors on the scanning stage allow step sizes as small as 0.16 mum. The SQUIDs are mounted on it cantilever structure with nine separate detection coils on the end of the structure. Flux noise of the SQUIDs is better than 5 muPhi(o)/rootHz. Sensitivity is better than 106 pT/rootHz with a bandwidth of dc-10 kHz. Open architecture software provides control of all critical system components,along with data acquisition and analysis. We have demonstrated spatial resolutions better than 2 mum. We discuss the impact of the external field coils for susceptibility measurements.
8. D. G. Austing, S. Sasaki, K. Muraki, K. Ono, S. Tarucha, M. Barranco, A. Emperador, M. Pi, and F. Garcias
"Density functional theory application to double quantum dots: Influence of mismatch on the addition energy spectra of vertical diatomic artificial molecules"
International Journal Of Quantum Chemistry 91 (3)498-503 (2003).
ABSTRACT: Local spin density functional theory calculations, for double quantum dots are compared with experimental data to explain the evolution of addition energy spectra with the interdot distance. Realistic features such as finite thickness and nonidentical constituent dots are included in the calculation. We can thus evaluate the heteronuclear character of the diatomic artificial quantum molecules due to a small unavoidable mismatch introduced in their fabrication. (C) 2002 Wiley Periodicals, Inc.
9. P. Baars, A. Richter, and U. Merkt
"Microwave properties of ballistic Nb/InAs(2DES)/Nb (SNS) Josephson junctions"
Superconductor Science & Technology 16 (12)1470-1474 (2003).
ABSTRACT: We present investigations of Shapiro steps in ballistic Nb/InAs(2DES)/Nb (SNS) Josephson junctions. Since the standard RCSJ model developed for tunnel junctions does not fit our data, we have extended it taking into account a voltage-dependent excess current term that has been derived from the microscopic OTBK model. This RCSJ+EX model not only recovers the overall shape of our I-V characteristics but also fits the power and temperature dependence of the Shapiro step widths quantitatively. Furthermore, it neatly explains the considerably enhanced step widths compared to the ones in tunnel junctions as a consequence of the excess currents.
10. S. Bhunia, T. Kawamura, Y. Watanabe, S. Fujikawa, and K. Tokushima
"Metalorganic vapor-phase epitaxial growth and characterization of vertical InP nanowires"
Applied Physics Letters 83 (16)3371-3373 (2003).
ABSTRACT: Highly dense and free-standing InP nanowire structures of vertical orientation were grown by the metalorganic vapor-phase epitaxial technique using colloidal Au nanoparticles as the catalyst. Scanning electron microscopy and transmission electron microscopy showed that the nanowires were single crystalline with <111> growth direction and of uniform length of about 700 nm, and most of them had diameter in the range of 20-25 nm. Photoluminescence measurements, carried out at room temperature as well as at 77 K, showed a significant blueshift in the peak position compared to bulk InP due to the quantum confinement of the carriers in the nanowires. The successful growth of these nanowires opens up the possibility of realizing various nanoscale devices on the wafer scale in the bottom-up approach. (C) 2003 American Institute of Physics.
11. S. Bhunia, T. Kawamura, Y. Watanabe, S. Fujikawa, J. Matsui, Y. Kagoshima, Y. Tsusaka, K. Uchida, N. Sugiyama, M. Furiya, S. Nozaki, and H. Morisaki
"Real-time measurement of rocking curves during MOVPE growth of GaxIn1-xP/GaAs"
Applied Surface Science 216 (1-4)382-387 (2003).
ABSTRACT: Measurement of real-time rocking curves of semiconductor heterostructures at various stages of metal organic vapor epitaxy (MOVPE) process may provide useful informations about the composition, thickness and in-built strain in the growing epilayer. In this study, we have used a previously grown lattice-matched GaInP/GaAs heterostructure as the reference substrate on which Ga(x)ln(1-x)P epilayers of different composition and thickness were successively grown by MOVPE while recording rocking curves of each layer in real-time during the growth by using synchrotron X-ray source. Strain redistribution at the interface of the GaInP/GaAs substrate due to the different linear thermal expansion coefficients of GaInP and GaAs was determined from rocking curve of the heterostructure measured at 570 degreesC. We could detect the change in rocking curve due to the growth of as thin as 16 nm of In-rich Ga0.42In0.58P epilayer at the initial stage of growth. Data from the simulation of each intermediate rocking curve during growth was systematically used to grow a lattice-matched GaInP epilayer. We believe, this is the first report of measurement of rocking curves at high temperature and during MOVPE growth of GaxIn1-xP. (C) 2003 Elsevier Science B.V. All rights reserved.
12. H. Deng, G. Weihs, D. Snoke, J. Bloch, and Y. Yamamoto
"Polariton lasing vs. photon lasing in a semiconductor microcavity"
Proceedings Of The National Academy Of Sciences Of The United States Of 100 (26)15318-15323 (2003).
ABSTRACT: Nearly one decade after the first observation of Bose-Einstein condensation in atom vapors and realization of matter-wave (atom) lasers, similar concepts have been demonstrated recently for polaritons: half-matter, half-light quasiparticles in semiconductor microcavities. The half-light nature of polaritons makes polariton lasers promising as a new source of coherent and nonclassical light with extremely low threshold energy. The half-matter nature makes polariton lasers a unique test bed for many-body theories and cavity quantum electrodynamics. In this article, we present a series of experimental studies of a polariton laser, exploring its properties as a relatively dense degenerate Bose gas and comparing it to a photon laser achieved in the same structure. The polaritons have an effective mass that is twice the cavity photon effective mass, yet seven orders of magnitude less than the hydrogen atom mass; hence, they can potentially condense at temperatures seven orders of magnitude higher than those required for atom Bose-Einstein condensations. Accompanying the phase transition, a polariton laser emits coherent light but at a threshold carrier density two orders of magnitude lower than that needed for a normal photon laser in a same structure. It also is shown that, beyond threshold, the polariton population splits to a thermal equilibrium Bose-Einstein distribution at in-plane wave number k(parallel to) > 0 and a nonequilibrium condensate at kparallel to similar to 0, with a chemical potential approaching to zero. The spatial distributions and polarization characteristics of polaritons also are discussed as unique signatures of a polariton laser.
13. J. M. Elzerman, R. Hanson, J. S. Greidanus, L. H. van Beveren, S. De Franceschi, L. M. Vandersypen, S. Tarucha, and L. P. Kouwenhoven
"Few-electron quantum dot circuit with integrated charge read out"
Physical Review B 67 (16), 161308 (2003).
ABSTRACT: We report on the realization of a few-electron double quantum dot defined in a two-dimensional electron gas by means of surface gates on top of a GaAs/AlGaAs heterostructure. Two quantum point contacts are placed in the vicinity of the double quantum dot and serve as charge detectors. These enable determination of the number of conduction electrons on each dot. This number can be reduced to zero, while still allowing transport measurements through the double dot. Microwave radiation is used to pump an electron from one dot to the other by absorption of a single photon. The experiments demonstrate that this quantum dot circuit can serve as a good starting point for a scalable spin-qubit system.
14. T. Ernst, S. Cristoloveanu, G. Ghibaudo, T. Ouisse, S. Horiguchi, Y. Ono, Y. Takahashi, and K. Murase
"Ultimately thin double-gate SOI MOSFETs"
Ieee Transactions On Electron Devices 50 (3)830-838 (2003).
ABSTRACT: The operation of 1-3 nm thick SOI MOSFETs, in double-gate (PG) mode and single-gate (SG) mode (for either front or back channel), is systematically analyzed. Strong interface coupling and threshold voltage variation, large influence of substrate depletion underneath the buried oxide, absence of drain current transients, degradation in electron mobility are typical effects in these ultra-thin MOSFETs. The comparison of SG and DG configurations demonstrates the superiority of DG-MOSFETs: ideal subthreshold swing and remarkably improved transconductance (consistently higher than twice the value in SG-MOSFETs). The experimental data and the difference between SG and DG modes is explained by combining classical models with quantum calculations. The key effect in ultimately thin DG-MOSFETs is volume inversion, which primarily leads to an Improvement in mobility, whereas the total inversion charge is only marginally modified.
15. D. Fattal, C. Santori, J. Vuckovic, G. S. Solomon, and Y. Yamamoto
"Indistinguishable single photons from a quantum dot"
Physica Status Solidi B-basic Research 238 (2)305-308 (2003).
ABSTRACT: We report a set of experiments designed to test the quantum indistinguishability of single photons emitted by a resonantly pumped InAs/GaAs quantum dot, embedded in a microcavity. We first performed high resolution interferometric measurements on the source to infer its coherence time. By comparison with the exciton lifetime, obtained by time-resolved spectroscopy, we found that some quantum dots were emitting almost transform-limited photons. The measured ratio of the coherence time to the pulse duration gives a lower bound on the dephasing time which agrees well with previous studies. We then directly measured the mean overlap between wave packets of consecutive photons emitted 2 ns apart, through a Hong-Ou-Mandel type two-photon interference experiment. For the best dot, an overlap as high as 0.81 was inferred. This result suggests that quantum dots could be a practical single photon source to perform quantum information processing experiments. We stress the importance of the microcavity for identical photon generation.
16. T. Fujisawa, D. G. Austing, Y. Tokura, Y. Hirayama, and S. Tarucha
"Spin selection rules in single-electron transport through a few-electron quantum dot"
Physics Of Semiconductors 2002, Proceedings 171245-252 (2003).
ABSTRACT: Since a quantum dot has an extremely long spin relaxation time, we must consider two spin selection rules for its transport characteristics. Namely, the total spin of the quantum dot is conserved during internal transitions, and it should change by +/-1/2 during a single-electron tunneling transition. We find that these selection rules are strictly obeyed in transport through a few-electron vertical quantum dot, and that related non-equilibrium transport can break the single-electron tunneling scheme.
17. T. Fujisawa, D. G. Austing, Y. Tokura, Y. Hirayama, and S. Tarucha
"Electrical pulse measurement, inelastic relaxation, and non-equilibrium transport in a quantum dot"
Journal Of Physics-condensed Matter 15 (33)R1395-R1428 (2003).
ABSTRACT: We review electrical pulse experiments carried out to probe inelastic energy relaxation processes and related non-equilibrium transport characteristics of quantum dots (QDs) in the Coulomb blockade (CB) regime. In contrast to the relatively short momentum relaxation time (similar to10 ns) that can be understood on the basis of acoustic phonon emission, the spin-flip relaxation time is found to be extremely long (similar to200 mus). The spin relaxation process in our QDs is actually dominated by a cotunnelling process, and thus the intrinsic spin relaxation should have a longer relaxation time. The long relaxation time is discussed in terms of potential applications to spin-based quantum information Storage. On the other hand, the extremely long spin relaxation process can induce considerable fluctuation of the spin, charge, and total energy of the QD. The absence of efficient spin relaxation processes can cause highly non-equilibrium transport, which actually 'breaks down' the single-electron tunnelling scheme. The non-equilibrium effects must be considered when electrons and spins are manipulated in the CB regime.
18. T. Fujisawa, D. G. Austing, Y. Hirayama, and S. Tarucha
"Electrical pump and probe measurements of a quantum dot in the coulomb blockade regime"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (7B)4804-4808 (2003).
ABSTRACT: We describe two kinds of electrical pump and probe measurements to investigate energy relaxation time in a quantum dot in the Coulomb blockade regime. Transient current driven by single-step pulses can be used to determine the tunneling rates for the two tunneling barriers as well as the relaxation rate. under a limited condition. A double-step pulse scheme is more useful for deducing longer relaxation times. The feasibility of these techniques is demonstrated by simple simulations based on rate equations that describe the time-dependent transport, and-by experiments performed on a few-electron quantum dot.
19. A. Fujiwara, S. Horiguchi, M. Nagase, and Y. Takahashi
"Threshold voltage of Si single-electron transistor"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (4B)2429-2433 (2003).
ABSTRACT: We experimentally evaluate threshold voltages of Si single-electron transistors (SET) in order to investigate the effect of offset charges. Threshold voltages show a clear relation to the gate capacitance of SETS, which is a device parameter reflecting the size of the Si island of SETS. This indicates that the fabricated Si SETS do not suffer much from random offset charges that cause the threshold voltages to fluctuate. Moreover, our theoretical analysis shows that the obtained negative threshold voltages strongly suggest the reduction of the band gap of Si islands due to oxidation-induced strain.
20. S. Fukatsu, T. Takahashi, K. M. Itoh, M. Uematsu, A. Fujiwara, H. Kageshima, Y. Takahashi, and K. Shiraishi
"The effect of partial pressure of oxygen on self-diffusion of Si in SiO2"
Japanese Journal Of Applied Physics Part 2-letters 42 (12B)L1492-L1494 (2003).
ABSTRACT: The self-diffusion coefficient of Si in thermal oxides (SiO2) formed on semiconductor silicon wafers has been determined with isotope heterostructures, (SiO2)-Si-nat/(SiO2)-Si-28, as a function of the partial pressure of oxygen mixed into argon annealing ambient. The (SiO2)-Si-nat layers contain 3.1% of Si-30 stable isotopes while the (SiO2)-Si-28 layers are depleted of Si-30 stable isotopes down to 0.003%, and the diffusion depth profiles of Si-30 isotopes from the at SiO2 to (SiO2)-Si-28 layers after thermal annealing have been determined by secondary ion mass spectrometry (SIMS). The Si self-diffusivity is found not to depend on the partial pressure of oxygen within our experimental error of about +/-33%.
21. S. Fukatsu, T. Takahashi, K. M. Itoh, M. Uematsu, A. Fujiwara, H. Kageshima, Y. Takahashi, K. Shiraishi, and U. Gosele
"Effect of the Si/SiO2 interface on self-diffusion of Si in semiconductor-grade SiO2"
Applied Physics Letters 83 (19)3897-3899 (2003).
ABSTRACT: Self-diffusion of ion-implanted Si-30 in SiO2 formed directly on Si substrates by thermal oxidation was studied as a function of the temperature and SiO2 thickness (200, 300, and 650 nm). The diffusion coefficient increases by about an order of magnitude with decreasing SiO2 thickness from 650 to 200 nm when silicon-nitride capping layers are placed on top of the SiO2, i.e., the distance between the Si-30 diffusers and Si/SiO2 interface has a strong influence. Because the stress on SiO2 by nitride estimated for such a change in diffusivity is unrealistically large, Si species, most likely SiO, generated at the Si/SiO2 interface and diffusing into SiO2 must be affecting the self-diffusion of Si in SiO2. (C) 2003 American Institute of Physics.
22. K. Furukawa, K. Ebata, D. Ichikawa, and N. Matsumoto
"Side chain effect on thermochromism and solvatochromism of end-grafted polysilane [Si(CH3)(2)SiR2](n) (R = C2H5, n-C4H9, n-C6H13, n-C8H17, n-C10H21)"
Macromolecules 36 (20)7681-7688 (2003).
ABSTRACT: We synthesized a series of polysilanes-[Si(CH3)(2)SiR2](n) - [R = C2H5, n-C4H9, n-C6H13, n-C8H17, n-C10H21] end-grafted on a quartz surface by utilizing the anionic polymerization of the corresponding source materials. We first confirmed that these symmetrically alkyl-substituted polysilanes exhibited reversible thermochromism in isooctane solution. When the polysilanes were end-grafted on the quartz surface, they also exhibited reversible thermochromism in a vacuum and reversible solvatochromisin as the ratio of the isooctane/ethanol cosolvent was varied. In all of these experiments, we observed the transition between two clearly distinguishable phases. No side chain effect was observed for the peak wavelengths of the absorption band. However, the side chain effect appeared at a critical temperature and a critical cosolvent ratio for the transitions. The packing force between the alkyl chains in the highly ordered structure of the polysilane plays a dominant role in these transitions. We also discuss the difference in the transition behavior in solution, in the solid state, and in the end-grafted state.
23. K. Furukawa
"End-grafted polysilane- An approach to single polymer science"
Accounts Of Chemical Research 36 (2)102-110 (2003).
ABSTRACT: This Account describes our approach to single polymer science in which we use end-grafted polysilane. Single polymer science, by our definition, involves isolating and observing a single polymer chain and extracting its physical properties at the single molecule level. For this purpose, we developed the end-graft technique, which is a method for fixing a polymer terminus on a solid surface by forming a chemical bond between them. We applied this technique to polysilane, a semiconducting polymer with a one-dimensional silicon-catenated backbone. Our recent experimental achievements are reviewed, which include direct observations of polysilane single-molecular structures by atomic force microscopy, the formation of unique supramolecular structures, and the observation of temperature-and solvent- dependent UV absorption spectra that are characteristic of isolated polysilane chains end-grafted on a solid surface.
24. K. Furukawa, K. Ebata, H. Nakashima, Y. Kashimura, and K. Torimitsu
"Polysilane bearing "sulfide tripod" terminus: Preparation and selective chemisorption on gold surface"
Macromolecules 36 (1)9-11 (2003).
ABSTRACT:
25. R. A. Ganeev, A. Ishizawa, T. Kanai, T. Ozaki, and H. Kuroda
"Polarization peculiarities of femtosecond laser induced harmonic generation from solid surface plasma"
Optics Communications 227 (1-3)175-182 (2003).
ABSTRACT: Polarization characteristics of harmonic generation from solid surface plasma were studied using a femtosecond CPA Ti:S-Nd:glass laser. The ratios between the harmonics intensities produced by p-polarized and s-polarized pump were 25 and 6 for the second and fifth harmonics, respectively, at pump intensities (I) below 10(17) W cm(-2). The same ratios decreased to 2.4 and 1.5 for I > 2 x 10(17) W cm(-2). This decrease was explained by the rippling caused by Rayleigh-Taylor-like instability at the critical density surface, and Faraday rotation effect due to the influence of the spontaneous magnetic fields generated in plasma. Theoretical analysis of s- and p-polarization-induced harmonic generation was presented using LPIC code. The blueshift of the harmonics has been also observed. It was found that the blueshift increases with the harmonic order. (C) 2003 Elsevier B.V. All rights reserved.
26. R. A. Ganeev, T. Kanai, A. Ishizawa, T. Ozaki, and H. Kuroda
"Coherent soft X-ray (lambda=18.9 nm) generation in a transient gain scheme in molybdenum plasma"
Optics And Spectroscopy 94 (2)291-294 (2003).
ABSTRACT: We investigate the threshold conditions for coherent soft x-ray generation using a compact hybrid (neodymium-doped glass-sapphire titanate) laser with an output energy of several hundred millijoules. Collisional excitation of molybdenum ions in a longitudinal-pumping scheme was used for x-ray (lambda = 18.9 nm) generation. The generated radiation exhibited a low divergence and a strong dependence on the delay between pico-second and femtosecond pumping radiations. (C) 2003 MAIK "Nauka/Interperiodica".
27. R. A. Ganeev, A. Ishizawa, T. Kanai, T. Ozaki, and H. Kuroda
"Generation of harmonics of femtosecond radiation from the surface of aluminum targets"
Optics And Spectroscopy 94 (2)295-299 (2003).
ABSTRACT: The results of a study of the generation of harmonics from a laser plasma resulting from the interaction of radiation of femtosecond duration (lambda = 1.06 mum, t = 475 fs, and I similar to 2 x 10(17) W cm(-2)) with aluminum targets are presented. The observed frequency shift of harmonics to the short-wavelength region (1.6 and 5.1 nm for the second and fifth harmonics, respectively) is determined by a collisionless absorption resulting from an anomalous skin effect. The efficiencies of conversion into the second and fifth harmonics in an s-polarized pumping field were lower than the conversion efficiencies in a p-polarized pumping field by a factor of eight and a factor of two, respectively (for intensities I < 10(17) W cm(-2)). With a further increase in the pumping intensity, these values decreased to 0.8 and 0.5, respectively. The mechanisms of such behavior of the conversion process are considered. (C) 2003 MAIK "Nauka/Interperiodica".
28. H. Gotoh, T. Tawara, Y. Kobayashi, N. Kobayashi, and T. Saitoh
"Piezoelectric effects on photoluminescence properties in 10-nm-thick InGaN quantum wells"
Applied Physics Letters 83 (23)4791-4793 (2003).
ABSTRACT: We report a large change in the photoluminescence (PL) properties induced by piezoelectric effects found in 10-nm-thick InGaN quantum wells. The time-resolved PL properties are measured with changing excitation intensity at 17 K. A blueshift in the PL peak of 200 meV and a decrease in the PL decay time from 3 mus to 17 ns are found with increasing excitation intensity. This large change is caused by a strong internal electric field up to 1 MV/cm and a spatial separation between an electron and a hole of as much as three times the Bohr radius. (C) 2003 American Institute of Physics.
29. H. Gotoh, H. Kamada, H. Ando, and J. Temmyo
"Exciton spin relaxation properties in zero dimensional semiconductor quantum dots"
Japanese Journal Of Applied Physics Part 1-regular Papers Brief 42 (6A)3340-3349 (2003).
ABSTRACT: Exciton spin relaxation dynamics in zero dimensional semiconductor quantum dots is studied. Exciton spin relaxation behavior in disk-shaped InGaAs quantum dots is measured by monitoring the time-evolution of exciton luminescence with resolving polarization. The spin relaxation time is studied in terms of the lateral extent of the dot. The spin relaxation time is found to depend strongly on the lateral extent of the dot. The spin relaxation time in quantum dots extending 30-40 nm is as long as 1 ns at 4 K, which is almost twice as long as the recombination lifetime and considerably longer than that in quantum wells. The spin relaxation time decreases with rising temperature and it became longer in quantum dots with larger exciton energy level separation. These results suggest the importance of the exciton-acoustic phonon interaction on spin relaxation in a semiconductor quantum dot at low temperature.
30. H. Gotoh, H. Kamada, T. Saitoh, H. Ando, and J. Temmyo
"Electric-field-induced anisotropy of excitonic optical properties in semiconductor quantum dots"
Journal Of Applied Physics 94 (1)342-347 (2003).
ABSTRACT: We report the anisotropic excitonic optical properties (polarization dependent photoemission and photoabsorption) induced by applying a lateral electric field in a single semiconductor quantum dot. The excitonic optical polarization characteristics are examined using theoretical calculation and optical measurement. The optical properties are numerically analyzed taking into account the quantum dot potential, electric field, and electron-hole Coulomb interaction. We evaluate the polarization properties from calculated exciton wave functions. The polarization properties depend strongly on the size of the quantum dots and the spatial symmetry of the hole part of the wave function. There is large electric-field-induced anisotropy in thin quantum dots to a larger lateral extent even where the polarization properties are completely isotropic without an electric field. We compare the theoretical results with experimental results for InGaAs quantum dots obtained using the microphotoluminescence technique. Qualitative agreement between the theoretical results and experimental ones is obtained. (C) 2003 American Institute of Physics.
31. V. Grillo, A. Genseki, N. Yamamoto, and Y. Watanabe
"Characterization of ultrathin InAs quantum wells by TEM-cathodoluminescence and TEM techniques"
Surface And Interface Analysis 35 (1)40-44 (2003).
ABSTRACT: Emission from ultrathin quantum wells of different thicknesses is studied. In particular, for structures with doped substrate the unintentional deposition of Cu droplets is used as a probe of electron diffusion inside the material. A large diffusion length (L = 8 +/- 3 mum) is found in spite of the presence of quantum dots. In particular, the contribution to the diffusion from re-emission of electrons from quantum dots is evaluated. Copyright (C) 2003 John Wiley Sons, Ltd.
32. O. Hanaizumi, Y. Sakurai, Y. Aizawa, S. Kawakami, E. Kuramochi, and S. Oku
"Fabrication of structures with III-V compound semiconductors embedded into 3D photonic crystals"
Thin Solid Films 426 (1-2)172-177 (2003).
ABSTRACT: We developed a dry-etching process to form holes with diameters of 5-50 mum in 7-mum thick a-Si/SiO2 three-dimensional (3D) photonic crystal layers fabricated on InP substrates by the autocloning method. We also demonstrated wet-etching processes to remove damaged surfaces of exposed InP substrates and selectively grew In0.62Ga0.38As/In0.45Ga0.31Al0.24As multiple quantum well by molecular beam epitaxy in a region surrounded by a-Si/SiO2 3D photonic crystals. Polarization dependence was observed in a spontaneous emission transmitted at 75 mum in a lateral direction in the photonic crystal layer. (C) 2002 Elsevier Science B.V. All rights reserved.
33. J. Haruyama, K. Takazawa, S. Miyadai, A. Takeda, N. Hori, I. Takesue, Y. Kanda, N. Sugiyama, T. Akazaki, and H. Takayanagi
"Injection of Cooper pairs into quasidiffusive multiwalled carbon nanotubes with weak localization"
Physical Review B 68 (16), 165420 (2003).
ABSTRACT: We study the influence of Cooper pair injection into niobium (Nb)/quasidiffusive multiwalled carbon nanotube (MWNT's)/aluminum (Al) junctions possessing weak localization. Evaporation of the Nb electrode on the open top end of MWNT's standing in nanopores of alumina membranes makes end-bonded structures possible, leading to highly transparent Nb/MWNT interfaces. We found a proximity-induced conductance increase (PIC), which was enhanced by the extremely large diffusion constant of the Cooper pair in the MWNT's, with onset temperatures as high as T=6-9 K. In contrast, we clarified that this PIC was very sensitive to the transparency of the MWNT/Al interface at low temperatures. High transparency led to reentrant conductance due to diffusion out of the Cooper pairs, while we successfully found superconductivity at T=0.6 K, implying enhanced critical magnetic fields, in some of the low-transparency samples.
34. N. Hatakenaka, M. Nishida, M. Kumagai, and H. Takayanagi
"Dual Aharonov-Casher effect in singlet-exciton systems"
Physica E-low-dimensional Systems & Nanostructures 18 (1-3)239-240 (2003).
ABSTRACT: Dual Aharonov-Casher (DAC) phase (or He-McKellar-Wilkens phase), which is defined as a quantum topological phase acquired by a neutral particle only with an electric dipole moment mu(E) being taken on a closed path around a magnetic monopoleire, is theoretically investigated in singlet-exciton systems. In the Sangster's interference scheme, a moving 2s exciton in magnetic fields feels an effective electric field in co-moving frame, which causes a superposition of opposite parity states, i.e., s> and p>, due to the motional Stark effect. The superposition gives rise to a nonvanishing electric dipole moment required for the DAC effect to the exciton. The accumulated phase is determined by detecting photon emissions from 2p states as a function of applied magnetic fields. (C) 2003 Elsevier Science B.V. All rights reserved.
35. M. Hayashi, M. Koashi, K. Matsumoto, F. Morikoshi, and A. Winter
"Error exponents for entanglement concentration"
Journal Of Physics A-mathematical And General 36 (2)527-553 (2003).
ABSTRACT: Consider entanglement concentration schemes that convert n identical copies of a pure state into a maximally entangled state of a desired size with success probability being close to one in the asymptotic limit. We give the distillable entanglement, the number of Bell pairs distilled per copy, as a function of an error exponent, which represents the rate of decrease in failure probability as n tends to infinity. The formula fills the gap between the least upper bound of distillable entanglement in probabilistic concentration, which is the well-known entropy of entanglement, and the maximum attained in deterministic concentration. The method of types in information theory enables the detailed analysis of the distillable entanglement in terms of the error rate. In addition to the probabilistic argument, we consider another type of entanglement concentration scheme, where the initial state is deterministically transformed into a (possibly mixed) final state whose fidelity to a maximally entangled state of a desired size converges to one in the asymptotic limit. We show that the same formula as in the probabilistic argument is valid for the argument on fidelity by replacing the success probability with the fidelity. Furthermore, we also discuss entanglement yield when optimal success probability or optimal fidelity converges to zero in the asymptotic limit (strong converse), and give the explicit formulae for those cases.
36. S. Hayashi, A. Takano, H. Takenaka, and Y. Homma
"SIMS study of depth profiles of delta-doped boron/silicon alternating layers by low-energy ion beams"
Applied Surface Science 203298-301 (2003).
ABSTRACT: Most recently, the semiconductor industry has been making use of low-energy ion implantation for shallow junction. It requires less than 1 nm depth resolution to evaluate the junction profile. We have started the research to develop the international standard respect to the depth profile of shallow junction in Japan. In this study, we originally made the delta-doped B (0.02 nm)/Si (20 nm) alternative layers on a silicon substrate by using high-vacuum magnetron sputter deposition instrument and evaluated the depth resolution to make use of low-energy oxygen ion beam from 0.25 to 2 keV. The higher depth resolution was given, the lower oxygen ion beam energy was performed. However, the depth resolution was made drastically worse when the ion beam with the accelerating energy of less than 0.5 keV was used. We use MRI model to extract the depth resolution parameters (atomic mixing and roughness). The factors, which make the depth resolution worse, are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.
37. T. Hayashi, T. Fujisawa, H. D. Cheong, Y. H. Jeong, and Y. Hirayama
"Coherent manipulation of electronic states in a double quantum dot"
Physical Review Letters 91 (22), 226804 (2003).
ABSTRACT: We investigate coherent time evolution of charge states (pseudospin qubit) in a semiconductor double quantum dot. This fully tunable qubit is manipulated with a high-speed voltage pulse that controls the energy and decoherence of the system. Coherent oscillations of the qubit are observed for several combinations of many-body ground and excited states of the quantum dots. Possible decoherence mechanisms in the present device are also discussed.
38. T. Hayashi, T. Fujisawa, and Y. Hirayama
"Investigation of spin state in a quantum dot by using strongly asymmetric tunnel barriers"
Physica Status Solidi B-basic Solid State Physics 238 (2)262-265 (2003).
ABSTRACT: We investigate the spin state of a GaAs lateral quantum dot (QD) by using strongly asymmetric tunnel barriers. The saturated current depends on the current polarity, which switches the direction of the dominant tunneling transition that increases or decreases the total spin. With this technique, the spin polarization and the Coulomb interaction are investigated. We find that high spin states appearing in some magnetic field regions can be understood by considering simple Coulomb interactions.
39. H. Hibino, Y. Homma, M. Uwaha, and T. Ogino
"Step wandering induced by homoepitaxy on Si(111) during "1x1"-7x7 phase transition"
Surface Science 527 (1-3)L222-L228 (2003).
ABSTRACT: We investigate step configurations during homoepitaxial growth on Si(111) near the "1 x 1"-7 x 7 phase transition temperature. The surface mass diffusion constant is larger on "1 x 1" than on 7 x 7. This difference in the surface mass diffusion constant, coupled with the preferential nucleation of 7 x 7 at the upper side of the step, causes asymmetry in the surface mass diffusion during the "1 x 1"-7 x 7 phase transition. This asymmetry leads to step wandering during homoepitaxial growth. (C) 2002 Elsevier Science B.V. All rights reserved.
40. Y. Hirayama, K. Muraki, K. Hashimoto, K. Takashina, and T. Saku
"Quantum Hall effects at Landau level crossings"
Physica E-low-dimensional Systems & Nanostructures 20 (1-2)133-142 (2003).
ABSTRACT: Interesting ferromagnetic characteristics are found in quantum Hall effects when Landau levels (LLs) cross and carrier interaction effects become important at the crossing points. Such LL crossings are studied in bilayer AlGaAs/GaAs systems and silicon quantum wells having a novel valley splitting. Detailed measurements of bilayer ferromagnetism at the LL crossings reveal easy-plane or axis characteristics depending on spin and pseudospin of the crossing states. In the fractional QH regime, interactions between electron and nuclear spins are observed at the crossings of composite-fermion LLs. (C) 2003 Elsevier B.V. All rights reserved.
41. M. Hiroki, and N. Kobayashi
"Flat surfaces and interfaces in AlN/GaN heterostructures and superlattices grown by flow-rate modulation epitaxy"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (4B)2305-2308 (2003).
ABSTRACT: AN and AlGaN with high Al composition were grown by flow-rate modulation epitaxy (FME); a growth method in which group-III sources and group-V sources are supplied alternately. The AIN growth efficiency in FME was three times higher than those by simultaneous-supply. Vapor-Solid relation was almost linear in AlGaN grown by FME with N-2 carrier gas. In contrast to rough surfaces of AlN/GaN heterostructures grown by simultaneous-supply, the surfaces by FME were flat. Intensities of X-ray diffraction satellite peaks indicate that AlN/GaN superlattices by FME (FME-SLs) have more abrupt and flat interfaces than those grown by simultaneous-supply. The threading dislocations were effectively reduced by about half on FME-SLs. These results indicate FME is an effective growth method for the fabrication of high quality AlN/GaN heterostructures and SLs with flat interfaces.
42. Y. Homma, Y. Kobayashi, T. Ogino, D. Takagi, R. Ito, Y. J. Jung, and P. M. Ajayan
"Role of transition metal catalysts in single-walled carbon nanotube growth in chemical vapor deposition"
Journal Of Physical Chemistry B 107 (44)12161-12164 (2003).
ABSTRACT: We characterize the iron and cobalt catalysts for carbon nanotube growth in chemical vapor deposition (CVD) by using electron microscopy. Nanoparticles of iron and cobalt exhibit a melting point drop in the methane ambient. Nanoparticles after nanotube growth are identified as Fe3C and Co3C for iron and cobalt, respectively. Those results indicate that a eutectic compound of metal and carbon is formed in the methane ambient, resulting in the phase separation into graphite (nanotubes) and metal carbide as the carbon uptake in the catalyst melt increases. This supports the vapor-liquid-solid mechanism for nanotube growth by CVD. Iron or cobalt silicide formation causes the poisoning of the catalysts. However, the coexistence of oxygen due to native oxide on the silicon surface or the metal surface causes formation of a SiO2 base, which can prevent silicidation of iron particles.
43. Y. Homma, H. Takenaka, F. Toujou, A. Takano, S. Hayashi, and R. Shimizu
"Evaluation of the sputtering rate variation in SIMS ultra-shallow depth profiling using multiple short-period delta layers"
Surface And Interface Analysis 35 (6)544-547 (2003).
ABSTRACT: We have developed multiple short-period delta layers as a reference material for SIMS ultra-shallow depth profiling. Boron nitride delta layers and silicon spacer layers were sputter-deposited alternately, with a silicon spacer thickness of 1-5 nm. These delta-doped layers were used to measure the sputtering rate change in the initial stage of oxygen ion bombardment. A significant variation of sputtering rate was observed in the initial 3 nm or less. The sputtering rate in the initial 3 nm was estimated to be about four times larger than the steady-state value for 1000 eV oxygen ions. Copyright (C) 2003 John Wiley Sons, Ltd.
44. Y. Homma, A. Takano, and Y. Higashi
"Oxygen-ion-induced ripple formation on silicon: evidence for phase separation and tentative model"
Applied Surface Science 20335-38 (2003).
ABSTRACT: We have investigated the ripple topography on silicon surfaces developed by 2-10 keV O-2(+) bombardment at an impact angle of 45degrees. The ripple amplitude increased with the depth of erosion and finally became comparable with the ripple wavelength. This caused the local impact angle of ripples facing the oxygen-ion beam to reach the critical angle Of SiO2 formation, promoting the phase separation between silicon dioxide and silicon. As observed before, the critical depth for SiO2 formation decreased with decreasing the primary ion energy. This appears to be related to the amount of erosion necessary to reach the critical angle of SiO2 formation. The decrease of wavelength at lower primary ion energies is interpreted as the result of reduced surface diffusivity which is enhanced by ion bombardment. (C) 2002 Elsevier Science B.V. All rights reserved.
45. T. Hong
"Spatial weak-light solitons in an electromagnetically induced nonlinear waveguide"
Physical Review Letters 90 (18), 183901 (2003).
ABSTRACT: We show that a weak probe light beam can form spatial solitons in an electromagnetically induced transparency (EIT) medium composed of four-level atoms and a coupling light field. We find that the coupling light beam can induce a highly controllable nonlinear waveguide and exert very strong effects on the dynamical behavior of the solitons. Hence, in the EIT medium, it is not only possible to produce spatial solitons at very low light intensities but also simultaneously control these solitons by using the coupling-light-induced nonlinear waveguide.
46. I. Iguchi, A. Sugimoto, and H. Sato
"Quantized vortices and diamagnetic precursor to the Meissner state in high-Tc superconductors"
Journal Of Low Temperature Physics 131 (3-4)451-460 (2003).
ABSTRACT: We report the magnetic imaging for underdoped and optimally-doped La2-xSrxCuO4 (LSCO) thin films on single substrates and nearly optimally-doped YBa2Cu3O7-x (YBCO) thin films on tricrystal substrates in the temperature range both below and above T-c using scanning SQUID microscopy. Below T-c, clear integer- and half-integer quantized vortices were observable. Above T-c, however, the inhomogeneous diamagnetic domains appeared. The local diamagnetic domains that led to the Meissner state were found in the broad temperature range for underdoped samples and in the narrow limited temperature range for optimally-doped samples. The results provide evidence that local diamagnetic domains are closely related to the pseudogap state. The continuous connection of the domain state above T-c with the state of a half-integer vortex at the tricritical point in the YBCO film below T-c also indicates that the diamagnetic domains are also closely related to the occurrence of d(x 2-y 2)-wave superconductivity.
47. H. Inokawa, and Y. Takahashi
"A compact analytical model for asymmetric single-electron tunneling transistors"
Ieee Transactions On Electron Devices 50 (2)455-461 (2003).
ABSTRACT: Analytical model for asymmetric single-electron tunneling transistors (SETTs), in which, resistance and capacitance parameters of source/drain junctions are not equal, has been developed. The model is based on-the steady-state master equation, takes only the two most-probable charging states into account, and is therefore very simple. Even so, it can accurately reproduce the peculiar behaviors of an asymmetric SETT, such as the skew in the drain current-gate voltage characteristics and the Coulomb staircase in the drain current-drain voltage characteristic. Analytical expressions for the charge in. the Coulomb island and the capacitance components of the SETT are also derived according to the same scheme, and it is demonstrated that the model can precisely describe the various aspects of the SETT behavior.
48. H. Inokawa, A. Fujiwara, and Y. Takahashi
"A multiple-valued logic and memory with combined single-electron and metal-oxide-semiconductor transistors"
Ieee Transactions On Electron Devices 50 (2)462-470 (2003).
ABSTRACT: Devices that combine single-electron and metaloxide-semiconductor (MOS) transistors are newly proposed as basic components of multiple-valued (MV) logic, such as a universal literal gate and a quantizer. We verified their operation using single-electron and MOS transistors fabricated on the same wafer by pattern-dependent oxidation of silicon. We also discuss their application to an analog-to-digital converter, a MV adder, and MV static random-access memory.
49. K. Inoue, E. Waks, and Y. Yamamoto
"Differential-phase-shift quantum key distribution using coherent light"
Physical Review A 68 (2), 022317 (2003).
ABSTRACT: Differential-phase-shift quantum key distribution based on two nonorthogonal states is described. A weak coherent pulse train is sent from Alice to Bob, in which the phase of each pulse is randomly modulated by {0,pi}. Bob measures the differential phase by a one-bit delay circuit. The system has a simple configuration without the need for an interferometer and a bright reference pulse in Alice's site, unlike the conventional QKD system based on two nonorthogonal states, and has an advantage of improved communication efficiency. The principle of the operation is successfully demonstrated in experiments.
50. K. Inoue, C. Santori, E. Waks, and Y. Yamamoto
"Entanglement-based quantum key distribution without an entangled-photon source"
Physical Review A 67 (6), 062319 (2003).
ABSTRACT: A quantum key distribution scheme is described, in which a time (or phase) entangled-photon pair posteriorly generated by a single-photon source and a simple beam splitter. This scheme offers an efficient entanglement-based quantum key distribution system superior to the system using a standard parametric down-converter.
51. T. Itakura, and Y. Tokura
"Dephasing of a coupled qubit system during gate operations due to background charge fluctuations"
Superlattices And Microstructures 34 (3-6)497-501 (2003).
ABSTRACT: A quantum computer that can be constructed on the basis of superconducting nanocircuits has previously been proposed. We examine the effect of background charge fluctuations on a coupled Josephson charge qubit system used in such a computer. In previous work, the background charge fluctuations were found to be an important dephasing channel for a single Josephson qubit. We investigate the effect of fluctuations in the bias at the charge degeneracy point of a Josephson charge qubit system. The evaluated quantities are the gate fidelity and the diagonal elements of the qubit's density matrix. The fluctuation leads to gate error; however, the quantum gate operation becomes more reliable with increasing interaction between the qubit systems. (C) 2004 Elsevier Ltd. All rights reserved.
52. T. Itakura, and Y. Tokura
"Effect of multiple charge traps on dephasing rates of a Josephson charge qubit system"
Journal Of The Physical Society Of Japan 72 (11)2726-2729 (2003).
ABSTRACT: We examine the dephasing rate of a Josephson charge qubit system due to background charge fluctuations. We consider single qubit and two-charge traps. The transition probability was controlled to a state where two traps were occupied. The transition probability was affected by the Coulomb blockade effect that occurs between two charge traps. To obtain the dephasing rate, we computed the spectra of random frequency modulation signals. Our results show that the interaction between charge traps suppresses dephasing.
53. T. Itakura, and Y. Tokura
"Dephasing due to background charge fluctuations"
Physical Review B 67 (19), 195320 (2003).
ABSTRACT: In quantum computation, quantum coherence must be maintained during gate operation. However, in physical implementations, various couplings with the environment are unavoidable and can lead to a dephasing of a quantum bit (qubit). The background charge fluctuations are an important dephasing process, especially in a charge qubit system. We examined the dephasing rate of a qubit due to random telegraph noise. Solving stochastic differential equations, we obtained the dephasing rate of a qubit constructed of a coupled-dot system; we applied our results to the charge Josephson qubit system. We examined the dephasing rates due to two types of couplings between the coupled-dot system and the background charge, namely, fluctuation in the tunnel coupling constant and fluctuation in the asymmetric bias. For a strong-coupling condition, the dephasing rate was inversely proportional to the time constant of the telegraph noise. When there is fluctuation in the tunnel coupling constant, Gaussian decay occurs in the initial regime. We also examined the rate of dephasing due to many impurity sites. For a weak-coupling condition with fluctuation in the asymmetric bias, the obtained dephasing rate coincided with that obtained by the perturbation method using the spectral weight of a boson thermal bath, which is proportional to the inverse of the frequency.
54. T. Ito, K. Nakamura, Y. Kangawa, K. Shiraishi, A. Taguchi, and H. Kageshima
"Systematic theoretical investigations of miscibility in Si1-x-yGexCy thin films"
Applied Surface Science 216 (1-4)458-462 (2003).
ABSTRACT: Miscibility of C in Si1-x-yGexCy thin films is systematically investigated by using the empirical interatomic potentials. The empirical potential approach is applied to calculate excess energies for Si1-x-yGexCy thin films incorporating interface lattice constraint due to S(0 0 1). In order to compare with experimental results, we employ the content values such as x = 0.13, 0.22, 0.27, 0.31, 0.35, and y = 0.019. The calculated results imply that the lattice constraint at the interface and Si-C interatomic bond formation dramatically reduce excess energies of Si1-x-yGexCy thin films by 20-30% of those in bulk state. Therefore, the lattice constraint promotes C incorporation in Si1-x-yGexCy thin films. Furthermore, segregation phenomena of Ge and C atoms in Si0.78Ge0.2C0.02 on Si(0 0 1) is clarified by Monte Carlo (MC) simulation taking into account surface and interface structures. The simulated results reveal that Ge atoms segregate in the topmost layer and C atoms accumulate in the second layer. These calculated results suggest that the lattice constraint at the interface enhance the miscibility of C in Si1-x-yGexCy thin films, whereas the miscibility tends to reduce near the surface because of the segregation of Ge and C atoms. (C) 2003 Elsevier Science B.V. All rights reserved.
55. M. W. Jack
"Effect of atom loss on collapse and revivals of phase coherence in small atomic samples"
Physical Review A 67 (4), 043612 (2003).
ABSTRACT: A small atomic sample at a single site of an optical lattice potential is a realization of an anharmonic oscillator with atomic collisions giving rise to the nonlinearity. Due to the atomic collisions, an initial state with a well-defined phase evolves into a complex superposition of states of different phase before returning to the initial state. In this realization, three-body loss (the dominant loss process at high densities) acts as a damping mechanism and decoheres the intermediate superpositions thus reducing the ability of the phase to revive at later times. For high densities the decoherence process is rapid and the system enters the classical regime where revivals do not occur.
56. M. W. Jack, and M. Yamashita
"Signatures of the quantum fluctuations of cold atoms in an optical lattice in the three-body loss rate"
Physical Review A 67 (3), 033605 (2003).
ABSTRACT: We consider an optical lattice potential loaded with a Bose-Einstein condensate in the presence of three-body loss. The dynamics of the atoms in the lattice realizes the Bose-Hubbard model of condensed-matter physics, where on-site number fluctuations are reduced by the atomic repulsion energy but increase with increased tunneling between lattice sites. A range of quantum states with different number fluctuations can therefore be created by varying the amplitude of the optical potential, and will be reflected in the values of the on-site correlation functions G(n)(i)=
57. Y. Jimbo, N. Kasai, K. Torimitsu, T. Tateno, and H. P. Robinson
"A system for MEA-based multisite stimulation"
Ieee Transactions On Biomedical Engineering 50 (2)241-248 (2003).
ABSTRACT: The capability for multisite stimulation is one of the biggest potential advantages of microelectrode arrays (MEAs). There remain, however, several technical problems which have hindered the development of a practical stimulation system. An important design goal is to allow programmable multisite stimulation, which produces minimal interference with simultaneous extracellular and patch or whole cell clamp recording. Here, we describe a multisite stimulation and recording system with novel interface circuit modules, in which preamplifiers; and transistor transistor logic-driven solid-state switching devices are integrated. This integration permits PC-controlled remote switching of each substrate electrode. This allows not only flexible selection of stimulation sites, but also rapid switching. of the selected sites between stimulation and recording, within 1.2 ms. This allowed almost continuous monitoring of extracellular signals at all the substrate-embedded electrodes, including those used for. stimulation. In addition, the vibration-free solid-state switching made it possible to record whole-cell synaptic currents in one neuron, evoked from multiple sites in the network. We have used this system to visualize spatial propagation patterns. of evoked responses in cultured networks of cortical neurons. This MEA-based stimulation system is a useful tool for studying neuronal signal processing in biological neuronal networks, as well as the process of synaptic integration within single neurons.
58. Y. J. Jung, Y. Homma, T. Ogino, Y. Kobayashi, D. Takagi, B. Q. Wei, R. Vajtai, and P. M. Ajayan
"High-density, large-area single-walled carbon nanotube networks on nanoscale patterned substrates"
Journal Of Physical Chemistry B 107 (28)6859-6864 (2003).
ABSTRACT: High-density single-walled carbon nanotubes forming self-directed networks on nanoscale patterned substrates are produced by chemical vapor deposition. The roles of catalyst particles for the growth of single-walled carbon nanotubes are investigated. Transition-metal particles of 3-7 nm size, obtained by depositing thin catalyst films (0.5-1 nm), remain as active catalysts for the growth of high-density single-walled carbon nanotubes on select catalyst supports (SiO2). Highly organized single-walled carbon nanotube architectures with controlled density can be fabricated following the predefined submicrometer-sized substrate patterns, suggesting the possibility of building nanotube-based molecular-scale electronic devices.
59. Y. J. Jung, B. Q. Wei, R. Vajtai, and P. M. Ajayan
"Mechanism of selective growth of carbon nanotubes on SiO2/Si patterns"
Nano Letters 3 (4)561-564 (2003).
ABSTRACT: During the chemical vapor deposition of multiwalled carbon nanotubes using the vapor phase delivery of a metal-organic (ferrocene) catalyst precursor, a strong selectivity for growth on patterned SiO2/Si substrates has been observed. A mechanism for this selective growth is described here. Delivered metal particles (Fe) on Si and SiO2 regions were investigated using several high-resolution characterization techniques. Active iron catalyst (gamma iron) particles were formed on the silicon oxide surface resulting in the formation of highly aligned nanotubes on this substrate. However, in the Si regions, stable FeSi2 and Fe2SiO4 particles were formed due to chemical reactions between silicon surface and Fe particles at high temperature leading to an inhibition of nanotube growth in the Si regions.
60. H. Kageshima, A. Taguchi, and K. Wada
"Formation of stable N-V-O complexes in Si"
Physica B-condensed Matter 340626-629 (2003).
ABSTRACT: We have investigated complexes formed by nitrogen (N), vacancy (V), and oxygen (O) in Si by using first-principles calculations to reveal the N-doping effect on formation processes of oxygen precipitates in Si. We considered coupling of an O atom with a {N-2-V-2} complex, which is thought to be the most stable complex in Si. The calculated results show that the {N-2-V-2} Complex effectively captures an O atom, forming stable {N-2-V-2-O} complexes. Such stable complexes may become the heterogeneous nucleation sites of O precipitates. As a result, the N-doping drives the aggregation of O. This is consistent with the experimentally observed enhancement of 0 precipitate formation due to the N-doping. (C) 2003 Published by Elsevier B.V.
61. A. Kanazawa, A. Takano, Y. Higashi, M. Suzuki, and Y. Homma
"Observation of ripple formation on O-2(+)-irradiated GaN surfaces using atomic force microscopy"
Applied Surface Science 203152-155 (2003).
ABSTRACT: We observed the morphologies of GaN surfaces before and after ion sputtering by atomic force microscopy (AFM). The GaN surfaces were bombarded by an O-2(+)-beam in an SIMS apparatus with different bombarding energies, incident angles, ion current densities, and ion doses. The morphologies of the ion-bombarded surfaces were largely different at various incident angles. The roughness was largest around 45degrees at any bombarding energy. The roughness did not monotonously become larger by increasing the bombarding energy but was the smallest at 5 keV Both of the ripple height and wavelength were found to be almost unchanged even though the ion current density was varied. The ripple height became much higher by increasing the ion dose, whereas the wavelength became larger with only a small increment. (C) 2002 Elsevier Science B.V. All rights reserved.
62. S. Karimoto, H. Yamamoto, H. Sato, A. Tsukada, and M. Naito
"Tc versus lattice constants in MBE-grown M2CuO4 (M = La, Sr, Ba)"
Journal Of Low Temperature Physics 131 (3-4)619-623 (2003).
ABSTRACT: Cuprates with a K2NiF4 structure are prototypes of p-type high-T-c superconductors and may provide a good opportunity for understanding the relationship between structure and T-c. However, until now they have all been La-based compounds, which limits the structural parameter range. Recently, by means of molecular beam epitaxy (MBE) and subsequent ozone oxidation, we newly synthesized Sr2CuO4+/-delta (T(c)(onset)similar to75 K) and Ba2CuO4+/-delta (T(c)(onset)similar to90 K) in addition to La2CuO4+delta (T(c)(onset)similar to52 K). These new compounds enabled us to undertake an investigation over a wider range than before. The results show the tendency for a longer Cu-O-apex to give a higher T-c.
63. Y. Kashimura, H. Nakashima, K. Furukawa, and K. Torimitsu
"Fabrication of nano-gap electrodes using electroplating technique"
Thin Solid Films 438317-321 (2003).
ABSTRACT: We report a simple and controllable method for fabricating a pair of electrodes with a sub-10-nm gap, namely nano-gap electrodes, by using a conventional electroplating technique. A pair of initial electrodes with a 100-200 nm gap is fabricated by electron beam lithography, and then gold is deposited electrochemically to reduce this gap. A sub-10-nm gap is produced by halting the electroplating just before the electrodes make contact with each other. We also use our technique to fabricate nanogap electrodes with a gold and a platinum finger, and with three gold fingers. (C) 2003 Elsevier Science B.V. All rights reserved.
64. M. Kasu, T. Makimoto, W. Ebert, and E. Kohn
"Formation of stacking faults containing microtwins in (111) chemical-vapor-deposited diamond homoepitaxial layers"
Applied Physics Letters 83 (17)3465-3467 (2003).
ABSTRACT: Stacking faults containing microtwins in (111)-oriented diamond layers grown on a high-pressure high-temperature (HPHT)-synthesized diamond substrate by chemical vapor deposition start to form just on the substrate. The microtwins in the stacking faults form on the {(1) over bar 11} plane, not on the (111) substrate plane. To explain these results, we propose an atomic-scale model in which a foreign atom remains on the HPHT substrate surface and a C atom on the foreign atom cannot form a covalent bond with it. Therefore, twinning of the C atom occurs on the {(1) over bar 11} plane. The next C atoms bond with the twinned C atom in an untwinned (normal crystalline) relation. Consequently, the formation of stacking faults that contain microtwins occurs. (C) 2003 American Institute of Physics.
65. M. Kasu, and N. Kobayashi
"High mobility and high crystalline-quality chemical-vapor-deposition grown homoepitaxial diamond"
Diamond And Related Materials 12 (3-7)413-417 (2003).
ABSTRACT: We have grown a high-crystalline-quality homoepitaxial diamond (001) layer by using microwave plasma chemical vapor deposition. We did not detect any impurities, such as H, B, N and O, in the homoepitaxial layer by secondary ion-mass spectroscopy. The full width at half maximum of the Raman diamond-related peak was as low as 2.35 cm(-1). The Raman results showed that in the growth temperature range, T-g, from 650 to 730 degreesC, local crystalline quality deteriorated with the T-g and that for T-g>730 degreesC, it improved with the T-g. However, as the T-g was increased to above 780 degreesC, almost all the surface became covered with unepitaxial crystals, which obstructed hole conduction on the H-terminated surface. Consequently, we obtained high Hall mobility at a T-g's of approximately 660 and 766 degreesC. At room temperature, we obtained a Hall mobility of 814 cm(2)/Vs in the air. (C) 2003 Elsevier Science B.V. All rights reserved.
66. R. Kato, M. Uwaha, Y. Saito, and H. Hibino
"Step wandering due to the gap in diffusion coefficient on the upper and the lower terraces"
Surface Science 522 (1-3)64-74 (2003).
ABSTRACT: In a vicinal face of a crystal, wandering instability of steps may occur under nonequilibrium conditions when asymmetry in the surface diffusion field exists. Near the transition temperature of Si(111) surface between 1 x 1 and 7 x 7 structures, wandering instability is observed in growth. We provisionally attribute this instability to the gap in diffusion coefficient on the upper and the lower terraces whose surface structures are different. With linear stability analysis and Monte Carlo simulation, we show that the instability occurs in growth if the diffusion coefficient of the lower terrace is larger. The wandering pattern in a vicinal face is chaotic when evaporation is allowed, but is rather periodic when evaporation is absent. The latter result agrees with the experimental observation. (C) 2002 Elsevier Science B.V. All rights reserved.
67. T. Kawamura, S. Bhunia, Y. Watanabe, S. Fujikawa, J. Matsui, Y. Kagoshima, and Y. Tsusaka
"In situ observation of step-terrace structures on MOVPE grown InP(001) by using grazing X-ray scattering"
Applied Surface Science 216 (1-4)361-364 (2003).
ABSTRACT: Step-terrace structures on MOVPE grown InP(001) surface were investigated by using grazing X-ray scattering. After buffer layers were formed on the substrate, X-ray scattering profiles were measured in the same chamber without exposing the sample to air. Small peaks were observed at the tail of the specular reflections, suggesting the in-plane periodicity of surface morphology. The azimuth angle dependence of peak position suggests a one-dimensional structure on the surface, and considering the AFM images of the sample, the bunched step-terrace structure toward [100] is the reason of this structure. Analyses based on a simple grating approximation shows the period of the one-dimensional structure is about 550 nm, which is consistent with the spacing value determined from AFM images. (C) 2003 Elsevier Science B.V. All rights reserved.
68. T. Kawamura, Y. Watanabe, S. Fujikawa, S. Bhunia, K. Uchida, J. Matsui, Y. Kagoshima, and Y. Tsusaka
"Real-time observation of surface morphology at nanometer scale using x-ray specular reflection"
Surface And Interface Analysis 35 (1)72-75 (2003).
ABSTRACT: The changes of surface morphology at nanometer scale during metal-organic vapour-phase epitaxial growth of indium phosphide have been investigated by using x-ray specular reflection at grazing incidence. At the low growth temperature of 400 degreesC, large decreases of x-ray beam intensity were observed, suggesting the formation of nanometer-sized nuclei. At a higher growth temperature of 500 degreesC, oscillations estimated to by layer-by-layer growth were observed, and roughness changes obtained from these oscillations were <0.01 nm, suggesting small-island formation on the terrace or step-edge fluctuation during the growth. Additionally, reflectivity increases at a growth temperature of 550degreesC were observed. Considering the reflectivity dependence on t-butylphosphine flow at 400 degreesC, the change of reflectivity at 550 degreesC is explained by the existence of phosphorus layers before the growth. Copyright (C) 2003 John Wiley Sons, Ltd.
69. M. S. Kim, J. A. Skinta, T. R. Lemberger, A. Tsukada, and M. Naito
"Magnetic penetration depth measurements of Pr2-xCexCuO4-delta films on buffered substrates: Evidence for a nodeless gap"
Physical Review Letters 91 (8), 087001 (2003).
ABSTRACT: We report measurements of the inverse squared magnetic penetration depth, lambda(-2)(T), in Pr2-xCexCuO4-delta (0.115less than or equal toxless than or equal to0.152) superconducting films grown on SrTiO3 (001) substrates coated with a buffer layer of insulating Pr2CuO4. lambda(-2)(0), T-c, and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, 0.124less than or equal toxless than or equal to0.144, lambda(-2)(T) at low T is flat: it changes by less than 0.15% over a factor of 3 change in T, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in lambda(-2)(T) produce values of the minimum superconducting gap in the range of 0.29less than or equal toDelta(min)/k(B)T(c)less than or equal to1.01.
70. T. Kimura, H. Tamura, K. Kuroki, K. Shiraishi, H. Takayanagi, and R. Arita
"Quantum wire networks for superconducting quantum-dot superlattices"
Physica B-condensed Matter 3291395-1396 (2003).
ABSTRACT: Quantum wire networks have been proposed for fabricating quantum-dot superlattices with the square and the plaquette lattice structures. These artificial lattices are well represented by Hubbard models with parameters determined by the local density approximation. The superconducting transition temperature T-c (= 90 mK) for the plaquette lattice is more than twice T-c (= 40 mK) for the square lattice and is sufficiently high for achieving superconductivity in experiments. (C) 2003 Elsevier Science B.V. All rights reserved.
71. Y. Kobayashi, Y. Yamauchi, and N. K. Kobayashi
"Structural and optical properties of AlGaInN/GaN grown by MOVPE"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (4B)2300-2304 (2003).
ABSTRACT: Structural and optical properties of AlyGa1-x-yInxN with y from. 0.17 to 0.66 and x from 0.01 to 0.08 grown on GaN epitaxial layers by metalorganic vapor phase epitaxy are investigated by X-ray diffraction (XRD), room-temperature photoluminescence (RT-PL), and transmission electron microscopy (TEM). From XRD measurement, it is found that phase separation of AlGaInN occurs with an increase of Al and In contents. For AlyGa1-x-yInxN layers with higher Al (y > 50%) and In (x > 8%) contents, a long-range ordered structure is observed along the growth direction for the first time by TEM. RT-PL shows single peak band emissions from 300 to 335 nm for the AlyGa1-x-yInxN layers with Al (y > 50%) and x from 0.01 to 0.08. By controlling trimethylalminiurn flow rates and growth temperatures, growth of lattice-matched strain-free Al0.19Ga0.77In0.04N layer to GaN is confirmed by persistent oscillation of in-situ shallow-angle reflectance monitoring and XRD, and 364-nm band emission is observed for the layer in RT-PL.
72. T. Koga, J. Nitta, and S. Datta
"Nonmagnetic control of spin transport in InGaAs quantum wells"
Physica E-low-dimensional Systems & Nanostructures 18 (1-3)161-162 (2003).
ABSTRACT: We have studied the Rashba constant values a in the In0.52Al0.48As/In0.53Ga0.47As quantum wells (QW), as a function of the degree of the structural inversion asymmetry (SIA), using the weak antilocalization analysis. We control the SIA of the QWs both by the specific sample design and by the applied gate voltage. The deduced alpha values are in a quantitative agreement with the theoretical values obtained in the k (.) p-type calculation. We, then, propose a novel spin-filter device solely based on the Rashba effect as an example of the devices that utilize the Rashba spin-orbit coupling effect. (C) 2003 Elsevier Science B.V. All rights reserved.
73. T. Koga, J. Nitta, and S. Marcet
"Structural control of Rashba spin-orbit coupling in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells"
Journal Of Superconductivity 16 (2)331-334 (2003).
ABSTRACT: We investigated the correlation between the Rashba spin-orbit coefficient alpha and potential shape of the quantum wells (QW), where alpha values are experimentally deduced from the weak antilocalization analysis. We studied the gate I-V properties of the QW samples and have obtained results consistent with the potential shapes predicted for these QWs.
74. K. Kumakura, T. Makimoto, and N. Kobayashi
"Ohmic contact to p-GaN using a strained InGaN contact layer and its thermal stability"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (4B)2254-2256 (2003).
ABSTRACT: We have formed ohmic contacts to p-GaN using a strained p-InGaN contact layer, and achieved the lowest contact resistance of 1.1 x 10(-6) Omega-cm(2) at room temperature by optimizing the contact layer thickness and its In mole-fraction. We have also evaluated thermal stability of ohmic contacts to p-GaN using. the strained p-InGaN contact layer. The contact resistance decreased to 2 x 10(-7) Omega-cm(2) at 100degreesC, and increased with elevating temperature above 100degreesC. In the temperature range up to 400degreesC, the contact resistances of the samples with the p-InGaN contact layer were smaller than those of the samples without the contact layer. Furthennore, the ohmic characteristics of the strained p-InGaN contact layer were less degraded even after the thermal process, compared with those of the sample without a content layer. These results indicate that the strained p-InGaN contact layer is favorable for practical application.
75. K. Kumakura, T. Makimoto, and N. Kobayashi
"Mg-acceptor activation mechanism and transport characteristics in p-type InGaN grown by metalorganic vapor phase epitaxy"
Journal Of Applied Physics 93 (6)3370-3375 (2003).
ABSTRACT: The Mg-acceptor activation mechanism and transport characteristics in a Mg-doped InGaN layer grown by metalorganic vapor phase epitaxy are systematically investigated through their structural, optical, and electrical properties. The In mole fraction was from 0 to 0.13, and the Mg concentration varied from 1x10(19) to 1x10(20) cm(-3). X-ray rocking curves for Mg-doped InGaN layers indicate that the structural quality is comparable to that of undoped and Si-doped InGaN layers. Their photoluminescence spectra show emissions related to deep donors emerged at lower energy when Mg doping concentrations are above 2-3x10(19) cm(-3). The electrical properties also support the existence of these deep donors in the same Mg concentration range because the hole concentration starts to decrease at around the Mg concentration of 2-3x10(19) cm(-3). These results indicate that self-compensation occurs in Mg-doped InGaN at higher-doping levels. The temperature dependence of the hole concentration in Mg-doped InGaN indicates that the acceptor activation energy decreases with increasing In mole fraction. This is the reason the hole concentration in Mg-doped InGaN is higher than that in Mg-doped GaN at room temperature. In addition, the compensation ratio increases with doping concentration, which is consistent with the deep donor observed in PL spectra. For Mg-doped InGaN, impurity band conduction is dominant in carrier transport up to a relatively higher temperature than that for Mg-doped GaN, since the acceptor concentration for Mg-doped InGaN is higher than that of Mg-doped GaN. (C) 2003 American Institute of Physics.
76. E. Kuramochi, M. Notomi, I. Yokohama, J. Takahashi, C. Takahashi, T. Kawashima, and S. Kawakami
"Transmission characterization of drilled alternating-layer three-dimensional photonic crystals"
Journal Of Applied Physics 93 (11)8848-8851 (2003).
ABSTRACT: To investigate the photonic band structure of a drilled alternating-layer photonic crystal (DALPC), light. transmittances were measured. Electron beam lithography was used to prepare Si/SiO2/air DALPCs that had four periods in the z direction with air holes penetrating the whole structure. The transmittances of TE/TM polarized lights were measured for three directions (Gamma - X, Gamma - Y, Gamma - Z). There was a common transmittance attenuated band at a wavelength of around 1.4 mum in the transmission spectra for all measured directions and polarizations. This agreed well with the photonic band gap (PBG) predicted by calculation. The results supported-the good potential of Si-based DALPCs as full. PBG materials. (C) 2003 American Institute of Physics.
77. J. Kurian, and T. Morishita
"Epitaxial growth of NdBa2Cu3O7-delta films on (100) MgO buffered with Ba2NdTaO6, a potential dielectric buffer layer for NdBa2Cu3O7-delta films"
Superconductor Science & Technology 16 (3)422-425 (2003).
ABSTRACT: We report the in situ epitaxial growth of NdBa2Cu3O7-delta thin films on (100) MgO substrates with Ba2NdTaO6, a new dielectric buffer layer, by pulsed laser deposition for the first time. It is found that by using Ba2NdTaO6 dielectric material as a buffer layer for the growth of NdBa2Cu3O7-delta films over (100) MgO substrate, the in-plane misalignment of the NdBa2Cu3O7-delta grains can be eliminated. The NdBa2Cu3O7-delta films grown on Ba2NdTaO6 buffered MgO gave a zero resistance superconducting transition of 93.5 K, which was remarkably higher than the NdBa2Cu3O7-delta films grown on MgO. Also the NdBa2Cu3O7-delta films grown on Ba2NdTaO6 buffered MgO substrates show a narrower superconducting transition width (similar to0.5 K) and better normal state metallicity (R-300/R-100) than the NdBa2Cu3O7-delta films grown on bare MgO substrate. Our results indicate that Ba2NdTaO6 is a promising buffer layer material for the growth of epitaxial NdBa2Cu3O7-delta films on MgO substrate.
78. R. Kurita, K. Hayashi, K. Torimitsu, and O. Niwa
"Continuous measurement of glutamate and hydrogen peroxide using a microfabricated biosensor for studying the neurotoxicity of tributyltin"
Analytical Sciences 19 (12)1581-1585 (2003).
ABSTRACT: We first measured the effects of trace levels of an endocrine disruptor, tributyltin (TBT), on the secretion response from nerve cells using a microfabricated biosensor designed for the continuous measurement Of L-glutamate and hydrogen peroxide. We observed higher and long-lasting glutamate and hydrogen peroxide concentrations from the cells when cultured rat cortical neurons were exposed to TBT. Glutamate and hydrogen peroxide release was induced even when we reduced the TBT concentration to 10 nM. This concentration is about two orders of magnitude lower than the concentration that induced apoptosis-like cell death. We also report on the effects of NMDA and non-NMDA receptor antagonists, which can help us to understand the mechanism of TBT neurotoxicity.
79. J. Lefebvre, Y. Homma, and P. Finnie
"Bright band gap photoluminescence from unprocessed single-walled carbon nanotubes"
Physical Review Letters 90 (21), 217401 (2003).
ABSTRACT: Unprocessed single-walled carbon nanotubes suspended in air at room temperature emit bright, sharply peaked band gap photoluminescence. This is in contrast with measurements taken from nanotubes lying on the flat surface for which no luminescence was detected. Each individual nanotube has a luminescence peak of similar linewidth (similar to13 meV), with different species emitting at various different wavelengths spanning at least 1.0 to 1.6 mum. A strong enhancement of photoluminescence intensity is observed when the excitation wavelength is resonant with the second Van Hove singularity, unambiguously confirming the origin of the photoluminescence.
80. J. Q. Lin, H. Nakano, T. Nishikawa, T. Ozaki, and K. Oguri
"Guiding of high-intensity feratosecond laser pulses in evacuated and gas-filled capillaries"
Optics Communications 221 (4-6)411-417 (2003).
ABSTRACT: We investigated the guiding of femtosecond laser pulses with an intensity of similar to10(16) W/cm(2) in evacuated and gas-filled glass capillaries. We measured the laser transmission versus length of two capillaries with different inner cores in a vacuum, and also the laser propagation modes in these two capillaries. We succeeded in generating an extended plasma column that was ten times longer than the Rayleigh range by propagating high-intensity femtosecond laser pulses in a gas-filled capillary. We obtained a spectral blue shift in the transmitted femtosecond laser pulses with a 5 Torr nitrogen-filled capillary and used this shift to demonstrate extended plasma column formation in a gas-filled capillary. We also measured axial soft X-ray spectra from this laser-pumped, nitrogen-filled capillary and obtained an anomalously strong emission at 45.2 nm. (C) 2003 Elsevier Science B.V. All rights reserved.
81. Y. Lin, E. M. Gonzalez, E. E. Mendez, R. Magno, B. R. Bennett, and A. S. Bracker
"Magnetotunneling between two-dimensional electron gases in InAs-AlSb-GaSb heterostructures"
Physical Review B 68 (3), 035311 (2003).
ABSTRACT: We have observed that the tunneling magnetoconductance between two-dimensional (2D) electron gases, formed at nominally identical InAs-AlSb interfaces, most often exhibits two sets of Shubnikov-de Haas oscillations with almost the same frequency. This result is explained quantitatively with a model of the conductance in which the 2D gases have different densities and can tunnel between Landau levels with different quantum indices. When the epitaxial growth conditions of the interfaces are optimized, the zero-bias magnetoconductance shows a single set of oscillations, thus proving that the asymmetry between the two electron gases can be eliminated.
82. Y. X. Liu, A. Miranowicz, S. K. Ozdemir, M. Koashi, and N. Imoto
"Size-dependent decoherence of excitonic states in semiconductor microcrystallites"
Physical Review A 67 (3), 034303 (2003).
ABSTRACT: The size-dependent decoherence of the exciton states, resulting from the spontaneous emission, is investigated in a semiconductor spherical microcrystallite under the condition a(B)
83. T. Machida, T. Yamazaki, K. Ikushima, S. Komiyama, K. Muraki, and Y. Hirayama
"Pulsed-mode operation of nuclear spin polarization in integer quantum Hall systems"
Physica E-low-dimensional Systems & Nanostructures 18 (1-3)128-129 (2003).
ABSTRACT: We demonstrate pulsed-mode control of nuclear-spin polarization in integer quantum Hall systems. Nuclear spins in a limited region along. edge channels of a quantum Hall conductor are initially polarized through the hyperfine interaction with electron spins. The initialized nuclear-spin polarization is then manipulated with pulsed nuclear magnetic resonance using a micro-metal strip fabricated on top of the edge channels. The nuclear-spin polarization is probed by the Hall resistance. (C) 2003 Elsevier Science B.V. All rights reserved.
84. N. Maeda, T. Tawara, T. Saitoh, K. Tsubaki, and N. Kobayashi
"Doping design of GaN-based heterostructure field-effect transistors with high electron density for high-power applications"
Physica Status Solidi A-applied Research 200 (1)168-174 (2003).
ABSTRACT: For fully exploiting high potentiality of AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power applications, back-doping (BD) and channel doping (CD) designs have been proposed that make it possible to obtain high two-dimensional electron gas (2DEG) densities even for the devices with thin AlGaN barrier layers. Furthermore, a novel metal-insulator-semiconductor (MIS) HFET using a bilayer of Al2O3/Si3N4 thin film has been proposed to efficiently suppress the gate leakage current in the devices with thin barrier layers, by taking advantage of highly-resistive Al2O3 layers and excellent quality of Si3N4/AlGaN interface. In BD and CD designs where an asymmetric double-heterostructure and a single-heterostructure are employed, respectively, donor atoms are additionally doped in the backside region beneath the 2DEG position and electrons are supplied also from these doped regions. By using BD and CD designs, very high 2DEG densities around 3 x 10(13) cm(-2) have been achieved in the Al0.3Ga0.7N/GaN HFETs whose barrier layer (Al0.3Ga0.7N) is designed to be as thin as 120 Angstrom. A BD-HFET with the gate-length L-g of 1.5 mum has exhibited a high current density (I-d) of 1.2 A/mm and a high transconductance (g(m)) of 200 mS/mm, which is ascribed to high 2DEG densities and thin barrier layers in these devices. A novel MIS structure mentioned above has been applied to CD-HFET, and a fabricated MIS CD-HFET with L-g = 1.5 mum has exhibited a state-of-the-art current density of 1.6 A/min and g(m) = 145 mS/mm, along with reduced gate leakage current. Thus, BD- and CD-HFETs, combined with Al2O3/Si3N4 MIS structure, are promising for high-power applications and should be inevitably required in the future for further improving the device performance. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
85. N. Maeda, K. Tsubaki, T. Saitoh, T. Tawara, and N. Kobayashi
"Two-dimensional electron gas transport properties in AlGaN/GaN heterostructure field-effect transistors designed for high-power applications"
Optical Materials 23 (1-2)211-217 (2003).
ABSTRACT: Two-dimensional electron gas (2DEG) transport properties and device characteristics in AlGaN/GaN heterostructure field-effect transistors (HFETs) have been investigated keeping high-power applications in mind. The 2DEG mobility as the function of the 2DEG density at low and above room temperatures has systematically been examined for different Al compositions, revealing that the 2DEG mobility compared at the same 2DEG density is higher for higher Al compositions up to 0.3 above room temperature. This indicates that the high Al composition is favorable not only for high 2DEG densities but also for high 2DEG mobilities. For an HFET (Al = 0.3) with a gate-length (L-g) of 1.5 pm, excellent DC and RF characteristics have been obtained, i.e., a maximum transconductance (g(m)) of 180 mS/mm, a cutoff frequency (f(T)) of 16 GHz, and a maximum oscillation frequency (f(max)) of 50 GHz. The selective-area regrowth technique has been applied to reduce the source and drain resistance, and an improved g(m) of 215 mS/mm has been obtained in an HFET (L-g = 1.5 mum) as the result of the reduced ohmic resistance (0.3 Omega mm). Moreover, a novel layer structure with the back-doping (BD) design has been proposed where an asymmetric double-heterostructure is employed and the doping is performed also in the backside AlGaN barrier layer. A very high 2DEG density of 2.8 x 10(13) cm(-2) with a record value of the mobility-density product (2.4 x 10(16) (V s)(-1)) has been obtained in this structure, proving that the BD design is promising for high-power applications. (C) 2003 Elsevier Science B.V. All rights reserved.
86. T. Makimoto, K. Kumakura, and N. Kobayashi
"High breakdown electric field for Npn-type AlGaN/InGaN/GaN heterojunction bipolar transistors"
Compound Semiconductors 2002 174231-234 (2003).
ABSTRACT: Npn-type AlGaN/InGaN/GaN heterojunction bipolar transistors have been fabricated for high power application. Their common-emitter current-voltage characteristics showed a high breakdown voltage of 120 V, corresponding to the breakdown electric field of 2.3 MV/cm in the collector. The cross-section transmission electron microscopy image showed that the V-shape defects in InGaN were filled with wider bandgap AlGaN layers during the emitter growth. This is considered to prevent the leakage current from the base to the collector, resulting the high breakdown electric field.
87. T. Makimoto, K. Kumakura, and N. Kobayashi
"High current gain (> 2000) of GaN/InGaN double heterojunction bipolar transistors using base regrowth of p-InGaN"
Applied Physics Letters 83 (5)1035-1037 (2003).
ABSTRACT: GaN/InGaN double heterojunction bipolar transistors with a regrown p-InGaN extrinsic base have been fabricated on SiC substrates. The maximum common-emitter current gain exceeds 2000 at the collector current of 15 mA for a 50 mumx30 mum device. In addition, the offset voltage in the common-emitter current-voltage characteristics was reduced from 5 to 1 V. This indicates that the large offset voltage reported previously was mainly due to the degraded base ohmic characteristics. The regrowth of p-InGaN is effective for improving the characteristics of nitride heterojunction bipolar transistors. (C) 2003 American Institute of Physics.
88. C. P. Master, F. Yamaguchi, and Y. Yamamoto
"Efficiency of free-energy calculations of spin lattices by spectral quantum algorithms"
Physical Review A 67 (3), 032311 (2003).
ABSTRACT: Ensemble quantum algorithms are well suited to calculate estimates of the energy spectra for spin-lattice systems. Based on the phase estimation algorithm, these algorithms efficiently estimate discrete Fourier coefficients of the density of states. Their efficiency in calculating the free energy per spin of general spin lattices to bounded error is examined. We find that the number of Fourier components required to bound the error in the free energy due to the broadening of the density of states scales polynomially with the number of spins in the lattice. However, the precision with which the Fourier components must be calculated is found to be an exponential function of the system size.
89. A. Matsuda, T. Fujii, and T. Watanabe
"Gap inhomogeneity, phase separation and a pseudogap in Bi2Sr2CaCu2O8+delta"
Physica C-superconductivity And Its Applications 388207-208 (2003).
ABSTRACT: Gap inhomogeneity in Bi2Sr2CaCu2O8 has been studied by low-temperature STM. We found a systematic relation between the local gap energy and local background density of states. A model analysis of the dip-hump structure gave evidence of in-plane charge inhomogeneity. The temperature/doping dependences of the pattern shape of inhomogeneity as well as the gap distribution function were measured. We could identify the signature of phase separation into the superconducting and pseudogapped phases as the appearance of a double-peak structure in the distribution function. (C) 2003 Elsevier Science B.V. All rights reserved.
90. K. Matsuda, S. Hirono, and H. Takayanagi
"Electronic properties of super-hard carbon nanocrystallite film"
Physica B-condensed Matter 3291529-1530 (2003).
ABSTRACT: The carbon nanocrystallite (CNC) film is a new type of synthetic carbon material. Recently, it has been found that the film has large electric conductivity (similar to10(3)[S cm(-1)]) at room temperature, while its hardness is comparable to that of diamond. In this paper, we report the temperature dependence of electric conductivity from 1.7 to 500 K. While a hopping conductance has been observed at low temperature region (T < 200 K), conductivity increases with decreasing temperature for T > 250 K. Possible mechanism for the electric conductivity in the CNC films is the hopping conductivity in disordered media. (C) 2003 Elsevier Science B.V. All rights reserved.
91. H. Matsui, T. Sato, T. Takahashi, H. B. Yang, S. C. Wang, H. Ding, T. Fuji, T. Watanabe, A. Matsuda, T. Terashima, and K. Kadowaki
"Low energy excitation in Bi-2(2)n-1(n)2n+4(Sr)Ca(Cu)O (n=1-3) studied by high-resolution arpes"
International Journal Of Modern Physics B 17 (18-20)3554-3558 (2003).
ABSTRACT: We report systematic high-resolution angle-resolved photoemission spectroscopy (ARPES) on Bi2Sr2Can-1CunO2n+4 (n = 1-3) to study the electronic structure near the Fermi level (E-F) and the low energy excitation. Comprehensive ARPES data on the sudden break in the energy dispersion near EF (kink) as a function of momentum, temperature, and number Of CuO2 layers indicate that the coupling of electrons with Q = (pi, pi) magnetic mode is dominant in the superconducting state for multi-layered cuprates.
92. H. Matsui, T. Sato, T. Takahashi, S. C. Wang, H. B. Yang, H. Ding, T. Fujii, T. Watanabe, and A. Matsuda
"BCS-like Bogoliubov quasiparticles in high-T-c superconductors observed by angle-resolved photoemission spectroscopy"
Physical Review Letters 90 (21), 217002 (2003).
ABSTRACT: We performed high-resolution angle-resolved photoemission spectroscopy on triple-layered high-T-c cuprate Bi2Sr2Ca2Cu3O10+delta. We have observed the full energy dispersion (electron and hole branches) of Bogoliubov quasiparticles and determined the coherence factors above and below E-F as a function of momentum from the spectral intensity as well as from the energy dispersion based on BCS theory. The good quantitative agreement between the experiment and the theoretical prediction suggests the basic validity of BCS formalism in describing the superconducting state of cuprates.
93. H. Matsui, T. Sato, T. Takahashi, H. Ding, H. B. Yang, S. C. Wang, T. Fujii, T. Watanabe, A. Matsuda, T. Terashima, and K. Kadowaki
"Systematics of electronic structure and interactions in Bi2Sr2Can-1CunO2n+4 (n=1-3) by angle-resolved photoemission spectroscopy"
Physical Review B 67 (6), 060501 (2003).
ABSTRACT: We have performed systematic angle-resolved photoemission spectroscopy on the high-T-c superconducting family of Bi2Sr2Can-1CunO2n+4 (n=1-3). In addition to the generic features of the large Fermi surface and d-wave superconducting gap, we have found that there exists a scaling of the doping dependence of the energy gap. Moreover, comparison of the nodal dispersion in the superconducting state shows that the "kink" (the sudden change of band dispersion), while occurring at a similar binding energy, becomes more pronounced with increasing layer number n, indicating stronger coupling between electrons and a collective mode.
94. S. Matsui, Y. Igaku, H. Ishigaki, J. Fujita, M. Ishida, Y. Ochiai, H. Namatsu, and M. Komuro
"Room-temperature nanoimprint and nanotransfer printing using hydrogen silsequioxane"
Journal Of Vacuum Science & Technology B 21 (2)688-692 (2003).
ABSTRACT: Room-temperature nanoimprint lithography (RT-NIL) technology has been developed to overcome critical dimensions and pattern placement errors caused by thermal expansion in the conventional nanoimprint lithography (NIL) process. We propose RT-NIL using hydrogen silsequioxane (HSQ) instead of the poly(methylmethacrylate) used in conventional NIL. We demonstrate HSQ-replicated patterns with a 90 nm diameter hole and 50 nm linewidth for room-temperature replications. Furthermore, we have developed new nanotransfer printing technology utilizing the adhesion characteristics of HSQ. We also demonstrate the transfer of photoresist and Au Patterns from a mold to a substrate. (C) 2003 American Vacuum Society.
95. T. Matsuoka, M. Nakao, H. Okamoto, H. Harima, and E. Kurimoto
"Experimental consideration of optical band-gap energy of wurtzite InN"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (4B)2288-2290 (2003).
ABSTRACT: Wurtzite InN films were grown on a thick GaN layer by metalorganic vapor phase epitaxy (MOVPE). Growth of a (0001)oriented single crystalline layer was confirmed by Raman scattering with polarized geometry, X-ray diffraction, and reflection high-energy-electron diffraction. In the growth, an extremely high V/III ratio of around 700000 was used to suppress inclusion of metal-indium. We observed at room temperature strong photoluminescence (PL) at 0.75 eV as well as a clear absorption edge at 0.7-1.0 eV. In contrast, no PL was observed, even at high power excitation of 0.6 MW/cm(2) at similar to1.9 eV, which had been reported as the band-gap in absorption experiments on single crystalline films grown by microwave-excited MOVPE and poly-crystalline ones. Careful inspection strongly suggests that a wurtzite InN single crystal has a true band-gap energy of 0.7-1.0eV, and the discrepancy between this and previous data could be due to the difference in the crystallinity of the materials measured.
96. Y. Mizugaki, Y. Uematsu, S. J. Kim, J. Chen, K. Nakajima, T. Yamashita, H. Sato, and M. Naito
"Intrinsic Josephson junctions in c-axis-oriented La1.85Sr0.15CuO4 thin films"
Journal Of Applied Physics 94 (4)2534-2537 (2003).
ABSTRACT: We investigated current-voltage characteristics (IVCs) of intrinsic Josephson junctions (IJJs) for c-axis oriented La1.85Sr0.15CuO4 thin films. We fabricated IJJs with two kinds of structures. One was a mesa structure with a junction area of 22-194 mum(2) fabricated by using conventional photolithography and Ar-ion milling, and the other was a microbridge structure with a junction area of 1.1-3.6 mum(2) fabricated by using a focused-ion-beam technique. The mesa-type IJJs exhibited resistively-shunted-junction-like IVCs with no hysteresis. The temperature dependence of their critical current followed the Ambegaokar-Baratoff relation. IVCs of the microbridge-type IJJs, on the other hand, exhibited voltage jump and clear hysteresis. The different behaviors between the mesa-type and the microbridge-type IJJs were explained by the different numbers of grain boundaries involved in the IJJs. (C) 2003 American Institute of Physics.
97. J. Mizuguchi, Y. Tanaka, S. Tamura, and M. Notomi
"Focusing of light in a three-dimensional cubic photonic crystal"
Physical Review B 67 (7), 075109 (2003).
ABSTRACT: The dielectric properties of photonic crystals are, in general, anisotropic spatially. This means that the constant-frequency surfaces of photons in wave-vector space are deformed from spherical shapes, leading to the photon focusing effects which are significantly enhanced at frequencies close to the photonic band gaps (PBG's). We find that the associated group-velocity surfaces of photons in real space are folded near PBG's and this gives rise to the appearance of photon caustics. We explicitly show these effects by illustrating theoretical images of photons transmitted through a photonic crystal. The calculation is made with the plane-wave-expansion method and numerical examples are given for a three-dimensional cubic lattice consisting of air and silicon.
98. M. Motonaga, H. Nakashima, S. Katz, D. H. Berry, T. Imase, S. Kawauchi, J. Watanabe, M. Fujiki, and J. R. Koe
"The first optically active polygermanes: preferential screw sense helicity of enantiopure chiral-substituted aryl polygermanes and comparison with analogous polysilanes"
Journal Of Organometallic Chemistry 685 (1-2)44-50 (2003).
ABSTRACT: Circular dichroism and ultraviolet spectroscopic studies of meta and para poly[methyl {(S)- 2-methylbutylphenyl} germane]s, the first enantiopure chiral substituted polygermanes, indicate that the polymer main chains are induced to adopt preferential screw sense helical conformations and the values of the Kuhn dissymmetry ratio (g(abs), = Deltaepsilon/epsilon) at - 70degreesC were evaluated as - 0.34 x 10(-4) and - 0.38 x 10(-4), respectively for the polymers above. The analogous polysilanes have g(abs), values of - 0.76 x 10(-4) and 0.81 x 10(-4), which indicate a greater preferential screw sense selectivity. It is suggested that the origin of the lower screw sense selectivity in the polygermanes lies in the longer E-E bond in the backbone, leading to reduced steric interaction between the preferential screw sense-inducing enantiopure chiral side chain moieties. (C) 2003 Elsevier Science B.V. All rights reserved.
99. Y. Mukai, T. Shiina, and Y. Jimbo
"Continuous monitoring of developmental activity changes in cultured cortical networks"
Electrical Engineering In Japan 145 (4)28-37 (2003).
ABSTRACT: During development, cortical neurons show highly synchronized spontaneous activity. This spontaneous activity probably reflects the mechanisms underlying proper network formation regulated by activity-dependent synaptic modification. To see long-term transitions in this spontaneous activity, we constructed a microelectrode-array (MEA)-based continuous monitoring system. Rat cortical neurons were cultured on MEAs with 64 embedded electrodes and maintained in a conventional CO2 incubator. A perfusion system for medium changes and recording setup for electrical signals were directly connected to the MEA in the incubator. Culture medium was continuously perfused at a very slow rate (0.1 ml/h), which was quite effective at maintaining constant conditions without contamination. Using this system, we succeeded in recording spontaneous activity of cultured cortical networks, almost continuously from 5 days to more than 1 month in vitro. It was revealed that the spontaneous activity patterns showed transition from simple synchronized bursts to a complex mixture of multiple patterns, separated by a brief silent period at approximately 2 weeks in vitro. (C) 2003 Wiley Periodicals, Inc.
100. K. Muraki, T. Saku, and Y. Hirayama
"Charge excitation and transport in pseudospin quantum Hall ferromagnets"
Physica E-low-dimensional Systems & Nanostructures 18 (1-3)107-108 (2003).
ABSTRACT: Studies on pseudospin quantum Hall ferromagnets involving the N = 1 Landau level (LL) of the symmetric state and the N = 0 LL of the antisymmetric state in strongly-coupled double quantum wells are presented. Measurements of the charge excitation gap for v = 4 (3 and 5) reveal easy-axis (easy-plane) anisotropy. Resistance spikes observed for v = 4 are ascribed to transport along domain walls. We also discuss the effects of potential asymmetry. (C) 2003 Elsevier Science B.V. All rights reserved.
101. Y. Muramatsu, M. Fujino, T. Yamamoto, E. M. Gullikson, and R. C. Perera
"Soft X-ray emission and absorption spectroscopy in the Si L region of polysilanes"
Nuclear Instruments & Methods In Physics Research Section B-beam 199260-264 (2003).
ABSTRACT: Soft X-ray emission and absorption spectra in the Si L region of a number of substituted polysilanes, (SiR2)n, have been obtained using synchrotron radiation to investigate their electronic structures. Studied polysilanes were substituted with methyl (R = CH3), ethyl (C2H5), propyl (n-C3H7), butyl (n-C4H9), pentyl (n-C5H11) and phenyl (C6H5) groups. Although similar spectral features in both X-ray emission and absorption are observed among alkyl-substituted polysilanes, slight differences are distinguished between alkyl- and phenyl-substituted ones. These spectral features are qualitatively reproduced by summing calculated density-of-state spectra for Si3s- and Si3d-orbital.s. Thus, spectral features are explained through the hybridization of electronic orbitals in both backbone Si atoms and substituent C atoms. (C) 2002 Elsevier Science B.V. All rights reserved.
102. M. Nagase, S. Horiguchi, K. Shiraishi, A. Fujiwara, and Y. Takahashi
"Single-electron devices formed by thermal oxidation"
Journal Of Electroanalytical Chemistry 55919-23 (2003).
ABSTRACT: The embedded Si structure formed by pattern-dependent oxidation (PADOX) in single-electron transistors (SET) is observed by two microscopic methods, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The top view of the Si nanostructure embedded in SiO2 is observed by a novel microscopic method, 'see-through' SEM. The side view of the embedded Si is revealed by a cross-sectional TEM observation with the focused ion beam (FIB) sample preparation. The potential profile defined by the observed shape of the embedded Si in the SET has only a single barrier due to the quantum confinement effect. This single barrier is split into two parts by the band gap reduction due to the strain effect of the oxidized Si. (C) 2003 Elsevier B.V. All rights reserved.
103. M. Nagase, H. Takahashi, Y. Shirakawabe, and H. Namatsu
"Nano-four-point probes on microcantilever system fabricated by focused ion beam"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (7B)4856-4860 (2003).
ABSTRACT: We have developed a new cantilever system for scanning probe microscopy. The microcantilever system designed for electrical resistivity measurements of nanomaterials has 16 electrodes and dual cantilevers. One cantilever with four additional electrodes is for the resistivity measurement in the four-point-probe mode. The other lever is for surface observation. The contact forces of both the levers are controlled by a piezoresistor in the self-sensitive detection mode. The nano-four-point probes on the electrical measurement lever are fabricated by using a focused ion beam. The width of the probe electrode, is approximately 200 nm. The distance between the probes is approximately 300 nm. The feasibility of electrical measurements is confirmed by measuring the contact characteristics using a graphite: sample.
104. M. Nagase, S. Horiguchi, A. Fujiwara, and Y. Takahashi
"Microscopic observations of single-electron island in Si single-electron transistors"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (4B)2438-2443 (2003).
ABSTRACT: The three dimensional shapes and, sizes of the embedded Si nanostructures in Si single-electron transistors (SETs) are observed by using microscopic methods, scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The width of the Si-wire in the SET whose conductance oscillations are observed at 25 K is in the range of 7-15 nm measured by SEM. The height of the wire is estimated to be 5-10 nm by AFM. The size of Si wire is small enough to produce the potential barrier caused by the quantum mechanical effect. The effective length of the. single-electron island is determined from the relationship between the gate capacitance and the length of the Si wire. The effective length is 23 nm shorter than the Si wire length. The distortion in the Si wire in SET is evaluated from. the high-resolution image of TEM. The length of the distorted region is almost the same as the effective length of the single-electron island. The distortion will produce the potential well in the Si wire. Microscopic observations suggested the existence of potential barriers caused by the quantum mechanical effect and that of the potential well originated from the distortion in the Si wire of SET devices, which agrees with the theoretical model of Si SETs fabricated using pattern-dependent oxidation.
105. M. Nagase, A. Fujiwara, K. Kurihara, and H. Namatsu
"Nanometrology of Si nanostructures embedded in SiO2 using scanning electron microscopy"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (1)318-325 (2003).
ABSTRACT: A novel scanning electron microscopy for the imaging of embedded nanostructures is proposed. The Si nanostructures embedded in SiO2 are clearly observed as bright contrasts at 30 kV electrons in the secondary electron mode. It is confirmed that the surface charging effect of SiO2 on an SOI substrate is modified by the existence of an embedded Si structure. The edge of the embedded structure creates a yield peak in the secondary electron profile. The comparison of the transmission electron microscopy (TEM) image and the scanning electron microscopy (SEM) image indicates that the peak position represents the edge position of the embedded structure with nano-order precision. The metrological method using this imaging technique is successfully applicable to the width measurement of sub-10-nm embedded Si structures.
106. J. Nakamura, Z. H. Zhang, K. Sumitomo, H. Omi, T. Ogino, and A. Natori
"Structural stability of the Ge/Si(113)-2 x 2 surface"
Applied Surface Science 212724-729 (2003).
ABSTRACT: We have investigated the atomic structure and electronic states for the Ge/Si(113)-2 x 2 surface using first-principles total energy calculations. We have found that the model made up of alternating [1 (1) over bar0]-oriented rows of rebonded atoms and tilted pentamers of five atoms with an interstitial atom has the lowest surface energy of the models employed. Furthermore, the local density of states calculated for this surface provides a satisfactory description of recent scanning tunneling microscope images. (C) 2003 Elsevier Science B.V. All rights reserved.
107. H. Namatsu, Y. Watanabe, K. Yamazaki, T. Yamaguchi, M. Nagase, Y. Ono, A. Fujiwara, and S. Horiguchi
"Fabrication of Si single-electron transistors with precise dimensions by electron-beam nanolithography"
Journal Of Vacuum Science & Technology B 21 (1)1-5 (2003).
ABSTRACT: Determining the relationship between wire size and the electrical characteristics of a single-electron transistor (SET) can significantly shorten the development time required to make SETs practical devices. In this study, this relationship was examined by fabricating SETs with precise dimensions using electron-beam nanolithography. The high-resolution resist HSQ provided fine wire patterns with small linewidth fluctuations. Si nanowires were made by etching using HSQ patterns as amask, and then oxidized to produce SETs. The electrical characteristics were measured to determine the wire size required for making operational SETs. First, it was found that more oxidation widens the range of wire widths for which clear Coulomb blockade oscillations are observed. This is probably because more oxidation produces more oxidation-induced stress, which deepens the potential well essential for SET operation. In addition, it was experimentally confirmed that the gate capacitance is proportional to the nanowire length. These results demonstrate that SETs can be fabricated with good control of the size. (C) 2003 American Vacuum Society.
108. M. Nishida, S. Kurihara, and N. Hatakenaka
"Josephson pi states in superfluid He-3 B-phase/A-phase/B-phase junctions"
Physica B-condensed Matter 32984-85 (2003).
ABSTRACT: The l texture in the A phase of superfluid He-3 B-phase/A-phase/B-phase hybrid junction was a key to form the pi state with higher critical current observed in Berkeley. Here we investigate the effect of n texture in the B phase on current-phase relations for the BAB junction with a fixed l texture. We show that the n texture greatly changes the current-phase relation just like l texture does. The change is due to the h-texture-induced modification of the spin structure in the A phase of the BAB junction via the AB boundary condition. As a result, current-phase relations in BAB junctions are determined by the vector in the A phase depending on both n and l textures. (C) 2003 Elsevier Science B.V. All rights reserved.
109. T. Nishida, T. Ban, and N. Kobayashi
"340-350 nm GaN-free UV-LEDs"
Physica Status Solidi A-applied Research 200 (1)106-109 (2003).
ABSTRACT: Light extraction from AlGaN-based ultraviolet light emitting diodes (UV-LEDs) is described. The device structure is free from a GaN binary layer for the suppression of the reabsorption of the UV emission in the GaN layer. The UV extraction becomes six times higher when compared with that of the normal structure consisting of a GaN buffer layer. The highest external quantum efficiencies of 350- and 340-nm LEDs are 2.2 and 1.7%, respectively. The maximum output powers are 8.6 and 5.5 mW at injection currents of less than 150 mA. We also investigated the potential of combining the 350-nm UV-LED with a mixture of plural fluorescence materials for the application field of lighting. This combination can provide ideal lighting equipment that offers high colour rendering, steady colour, and no glare even with changes of intensity and illumination angle.
110. T. Nishida, T. Ban, and N. Kobayashi
"Highly transparent structure for nitride ultraviolet light emitting diodes"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (4B)2273-2277 (2003).
ABSTRACT: We report the merits of the transparent structure for the nitride semiconductor ultraviolet light emitting diodes. The ultraviolet light emitting diodes consisting of AlGaN-based active layers provide a wide spectral range from 200 to 360nm in the ultraviolet range. To produce a transparent device structure at the emission wavelength, we used sapphire substrates, AIN-template layers, AlGaN alloy buffer layers, and p-type contact layers consisting of a short period alloy superlattice, all of which are transparent to the emission. We confirmed the transparency of the device structure from the transmission spectra. The output power characteristic is improved by a factor of four due to the transparent structure. We also demonstrate an efficient ultraviolet light emitting diode grown on an AlGaN buffer layer of low dislocation density. The external quantum efficiency is 1.4%, which is superior to that of the ultraviolet light emitting diode grown on bulk GaN substrate.
111. T. Nishida, T. Ban, and N. Kobayashi
"High-color-rendering light sources consisting of a 350-nm ultraviolet light-emitting diode and three-basal-color phosphors"
Applied Physics Letters 82 (22)3817-3819 (2003).
ABSTRACT: The high-color-rendering (HCR) potential of the light sources consisting of 350-nm ultraviolet light-emitting diodes (UV-LED) and three-basal-color phosphors was simulated and experimentally confirmed. By using an AlGaN-based UV-LED as an excitation source at the 350-nm wavelength, we measured fluorescence spectra of three-basal-color phosphors, and simulated the optimum phosphor mixture based on the Commission Internationale de l'Eclairage standard light sources D-75, D-65, D-55, D-50, C, and A, with HCR indexes (R-a) over 92. We confirmed the potential of HCR of this light source consisting of 350-nm UV-LED and three-basal-color phosphors, by obtaining R-a=86similar to89 of the incandescent type of standard light source A experimentally. Such high R-a are due to the highly monochromatic UV-LED emission and the wide spectral separation between the UV excitation (350 nm) and visible fluorescences. This combination also provides a diffusive, nonglare light source with highly steady color independent of the emission orientation. We also examined the importance of the separation between the excitation wavelength and the fluorescence emission wavelength for practical lighting applications. (C) 2003 American Institute of Physics.
112. T. Nishida, N. Kobayashi, and T. Ban
"GaN-free transparent ultraviolet light-emitting diodes"
Applied Physics Letters 82 (1)1-3 (2003).
ABSTRACT: By introducing a GaN-free layer structure, we have transparent light-emitting diodes (LEDs) at the ultraviolet emission wavelength of 348-351 nm, which is shorter than the GaN band gap wavelength of 363 nm. The buffer layer consists of an AlGaN alloy directly grown on an AlN template layer on a sapphire substrate, and a short period alloy superlattice is adopted as p-type cladding and p-type contact layers. The transparency of the epitaxially grown layer structure is confirmed from transmission spectra. The output powers of the device are 1 mW at injection currents of 20 and 7 mW at 220 mA under room temperature continuous wave operation. The highest external quantum efficiency is 1.4%. This value is superior to that of an ultraviolet LED grown on a high-quality bulk-GaN substrate, where the performance was significantly deteriorated by light absorption into the GaN substrate. The results here indicate the importance of a transparent device structure free of GaN to improve the performance of ultraviolet LEDs in wavelength ranges shorter than 363 nm. (C) 2003 American Institute of Physics.
113. T. Nishikawa, K. Oguri, S. Suzuki, Y. Watanabe, O. Zhou, and H. Nakano
"Enhanced water-window X-ray pulse generation from femtosecond-laser-produced plasma with a carbon nanotube target"
Japanese Journal Of Applied Physics Part 2-letters 42 (8B)L990-L992 (2003).
ABSTRACT: We adopt a carbon nanotube target to increase the efficiency of water-window X-ray pulse conversion from femtosecond-laser-produced plasma. The target is an array of vertically aligned multiwalled carbon nanotubes, each 30 nm in diameter and about 12-mum long. Center-to-center nanotube distance is around 150 nm. The X-ray fluence enhancement in the water-window region is seven-fold compared with a conventional carbon plate target. Further enhancement can be expected by optimizing the size of the carbon nanotubes. X-ray pulse duration is 26 ps. The results show that carbon nanotubes are very attractive as a target for femtosecond laser-produced-plasma X-ray sources in single-shot X-ray microscopy.
114. J. Nitta, Y. P. Lin, T. Akazaki, and T. Koga
"Gate-controlled electron g factor in an InAs-inserted-channel In0.53Ga0.47As/In0.52Al0.48As heterostructure"
Applied Physics Letters 83 (22)4565-4567 (2003).
ABSTRACT: The electron g factor in an InAs-inserted-channel In0.53Ga0.47As/In0.52Al0.48As heterostructure is studied by measuring the angle dependence of magnetotransport properties. The gate voltage dependence of the g factor is obtained from the coincidence method. The g-factor values are surprisingly smaller than the g-factor value of bulk InAs, and close to the bare g-factor value of In0.53Ga0.47As. A large change in the g factor is observed by applying the gate voltage. The gate voltage dependence is not simply explained by the energy dependence of the g factor. (C) 2003 American Institute of Physics.
115. J. Nitta, and T. Koga
"Rashba spin-orbit interaction and its applications to spin-interference effect and spin-filter device"
Journal Of Superconductivity 16 (4)689-696 (2003).
ABSTRACT: The Rashba spin-orbit interaction in InGaAs quantum wells (QW) is studied using the weak antilocalization analysis as a function of the structural inversion asymmetry (SIA). We have observed a clear cross-over from positive to negative magnetoresistance near zero-magnetic field by controlling the degree of the SIA in the QWs. This is a strong evidence of a zero-field spin splitting that is induced by the Rashba effect. The spin-interference effect in a gate-controlled mesoscopic Aharonov-Bohm ring structure is investigated in the presence of Rashba spin-orbit interaction. The oscillatory behavior appearing in ensemble averaged Fourier spectrum of h/2e oscillations as a function of gate voltage is possibly because of the Aharonov-Casher type interference. We propose a spin-filter device based on the Rashba effect using a nonmagnetic resonant tunneling diode structure. Detailed calculation using InAIAs/InGaAs heterostructures shows that the spin-filtering efficiency exceeds 99.9%.
116. J. Nitta, T. Koga, and F. E. Meijer
"Effect of spin-orbit interaction in an InGaAs-based Aharonov-Bohm ring structure"
Physica E-low-dimensional Systems & Nanostructures 18 (1-3)143-144 (2003).
ABSTRACT: The interference effect in a gate controlled mesoscopic Aharonov-Bohm ring structure is studied in the presence of spin-orbit interaction. After ensemble averaging, the Fourier spectrum of h/2e oscillations as a function of gate voltage showed an oscillatory behavior. The oscillatory behavior is possibly due to the Aharonov-Casher type interference. (C) 2003 Elsevier Science B.V. All rights reserved.
117. D. Okada, H. Hasegawa, T. Hasegawa, Y. Horikoshi, and T. Saitoh
"Area selective epitaxy of anti-dot structure by solid source MBE using MEE deposition sequence"
Compound Semiconductors 2002 17433-36 (2003).
ABSTRACT: We have carried out area selective epitaxial growth of GaAs anti dot structures using solid source molecular beam epitaxy (MBE) which makes it possible to achieve "damage-free" structures. However, area selective epitaxy by MBE is very difficult unless the substrate temperature is very high. This problem has been solved by using migration-enhanced epitaxy (MEE) deposition sequence. To achieve well-defined anti-dot network structures, control of sidewall formation is very important. In addition, lateral growth beyond the SiO2 mask boundaries has to be strictly prohibited. By MEE method, anti-dots with vertical sidewalls can be fabricated without shrinking holes, even though the mask diameter is as small as 30nm.
118. W. D. Oliver, G. Feve, and Y. Yamamoto
"The Rashba effect within the coherent scattering formalism with applications to electron quantum optics"
Journal Of Superconductivity 16 (4)719-733 (2003).
ABSTRACT: The influence of spin-orbit coupling in electron quantum optics experiments is investigated within the framework of the Landauer-Buttiker coherent scattering formalism. We begin with a brief review of our electron quantum optics toolbox: an electron intensity interferometer (Hanbury Brown and Twiss-type experiment), an electron collision analyzer (Hong-Ou-Mandel-type experiment), and a proposed Bell state analyzer. These experiments are performed or proposed in two-dimensional electron gas systems and, therefore, may be influenced by the Rashba spin-orbit coupling. To quantify this effect, we define the creation/annihilation operators for the stationary states of the Rashba spin-orbit coupling Hamiltonian and use them to derive the current operator within the Landauer-Bruttiker formalism. The current is expressed as it is in the standard spin-independent case, but with the spin label replaced by a new label that we call the spin-orbit coupling label. The spin-orbit coupling effects can then be represented in a scattering matrix that relates the spin-orbit coupling stationary states in different leads. We apply this new formalism to the case of a four-port beamsplitter, and it is shown to mix states with different spin-orbit coupling labels in a manner that depends on the angle between the leads. A noise measurement after the collision of spin-polarized electrons at an electron beamsplitter provides a new experimental means to measure the Rashba parameter alpha. It is also shown that the degree of electron bunching in an entangled-electron collision experiment is reduced by the spin-orbit coupling according the beamsplitter lead angle.
119. H. Omi, D. J. Bottomley, Y. Homma, and T. Ogino
"Wafer-scale strain engineering on silicon for fabrication of ultimately controlled nanostructures"
Physical Review B 67 (11), 115302 (2003).
ABSTRACT: We propose a method of strain distribution control on planar Si(001) and Si(111) wafers for nanostructure self-assembly, taking a long-term view toward future Si semiconductor science and technology. Oxygen ions are implanted through patterned layers on the Si wafers. The sample is then annealed at 1325 degreesC to produce bulk oxide inclusions that yield a tensile and/or compressive strain distribution on the silicon surface. We also demonstrate that strained epitaxial growth of Ge on the Si(001) substrate surface at 550 degreesC in an ultrahigh vacuum produces three-dimensional islands whose location and size distribution are well controlled. The degree of localization control is in agreement with simulations of the elastic strain distribution. Additionally, we show that increasing the Ge growth temperature 50 degreesC significantly reduces the density of stray dots that randomly grow on the Si surface. The samples were observed by atomic force microscopy and cross-sectional transmission electron microscopy. Strains on the silicon surface, produced by the buried silicon oxides, were measured by micro-Raman spectroscopy.
120. K. Ono, D. G. Austing, Y. Tokura, and S. Tarucha
"Spin selective tunneling and blockade in two-electron double quantum dot"
Physica Status Solidi B-basic Research 238 (2)335-340 (2003).
ABSTRACT: We have studied a non-linear transport of double quantum dots in a two-electron regime, and found a pronounced blockade region in addition to the ordinary Coulomb blockade. This extra blockade region appears only on one polarity of the source drain voltage, and is ascribed to the selection rule of two-electron spins for an inter-dot tunneling as well as to an irreversible tunneling from the reservoir to the dots. In this extra blockade region the two-electron spin stays in the triplet state even in a zero magnetic field. A small leakage current indicates that the lifetime of this triplet state is about 100 ns.
121. Y. Ono, N. M. Zimmerman, K. Yamazaki, and Y. Takahashi
"Turnstile operation using a silicon dual-gate single-electron transistor"
Japanese Journal Of Applied Physics Part 2-letters 42 (10A)L1109-L1111 (2003).
ABSTRACT: A single-electron turnstile has been demonstrated using a silicon-based dual-gate single-electron transistor (SET). Each gate independently controls the closing and opening of the channel acting as the SET lead, which enables single-electron transfer synchronized with ac gate biases. By applying ac biases to the dual gates with a frequency f of similar to1 MHz and a phase shift of pi, current staircases quantized in units of ef are observed in drain current vs drain voltage characteristics at 25 K.
122. Y. Ono, and Y. Takahashi
"Electron pump by a combined single-electron/field-effect-transistor structure"
Applied Physics Letters 82 (8)1221-1223 (2003).
ABSTRACT: A silicon-based single-electron pump that operates at much higher temperatures than previous pumps is fabricated on a silicon-on-insulator substrate. The pump consists of one single-electron transistor (SET) and two ultrasmall metal-oxide-semiconductor field-effect transistors (MOSFETs). We exploit, for the operation, the extremely high off-resistance of the MOSFETs. The high resistance enables us to prevent the unwanted injection (emission) of electrons into (from) the SET nearly perfectly and to make the SET work as a pump. The operation is confirmed at 25 K at the pumping frequency of similar to1 MHz. (C) 2003 American Institute of Physics.
123. T. Ota, M. Stopa, M. Rontani, T. Hatano, K. Yamada, S. Tarucha, H. Z. Song, Y. Nakata, T. Miyazawa, T. Ohshima, and N. Yokoyama
"Molecular states observed in a single pair of strongly coupled self-assembled InAs quantum dots"
Superlattices And Microstructures 34 (3-6)159-164 (2003).
ABSTRACT: Molecular states in a single pair of strongly coupled self-assembled InAs quantum dots are investigated using a sub-micron sized single-electron transistor containing just a few pairs of coupled InAs dots embedded in a GaAs matrix. We observe a series of well-formed Coulomb diamonds with charging energy of less than 5 meV, which are much smaller than those reported previously. This is because electrons are occupied in molecular states, which are spread over both dots and occupy a large volume. In the measurement of ground and excited state single-electron transport spectra with a magnetic field, we find that the electrons are sequentially trapped in symmetric and anti-symmetric states. This result is well explained by numerical calculation using an exact diagonalization method. (C) 2004 Elsevier Ltd. All rights reserved.
124. T. Ota, T. Hatano, S. Tarucha, H. Z. Song, Y. Nakata, T. Miyazawa, T. Ohshima, and N. Yokoyama
"Transport properties of a single pair of coupled self-assembled InAs quantum dots"
Physica E-low-dimensional Systems & Nanostructures 19 (1-2)210-214 (2003).
ABSTRACT: Electron transport properties of a single pair of strongly and weakly coupled self-assembled InAs quantum dots are investigated using a single electron spectroscopy technique. We fabricated vertical single electron transistors containing just a few pairs of vertically coupled two InAs dot layers embedded in a GaAs matrix. The transport through a single pair of the coupled dots is observed near the pinch-off point of the devices. Thus, observed Coulomb oscillations and Coulomb diamonds show the characteristics just like a single dot for the strongly coupled dots, whereas those for the weakly coupled dots show the irregular features due to the localization of electrons in each dot. In case more than one pair of the coupled dots contributed to the transport, the parallel conduction is proved by observation of two families of Coulomb diamonds. (C) 2003 Elsevier B.V. All rights reserved.
125. T. Ozaki, H. Nakano, and H. Kuroda
"Characteristics of longitudinally pumped nickellike molybdenum x-ray lasers generated in waveguides"
Journal Of The Optical Society Of America B-optical Physics 20 (2)402-408 (2003).
ABSTRACT: We numerically investigate the characteristics of a longitudinally pumped 18.9-nm nickellike molybdenum x-ray laser generated by use of plasma waveguides. We take into account the temporal deformation of the longitudinal pump pulse, which is shown greatly to affect the overall x-ray laser's performance. The dependence of the spatial and temporal distribution of gain within the waveguide on parameters such as electron density, pump intensity, and wavelength is investigated. The problem of small-scale density perturbations within the relatively high-density preplasma. is also addressed. (C) 2003 Optical Society of America.
126. A. Pimenov, A. V. Pronin, A. Loidl, A. Tsukada, and M. Naito
"Far-infrared conductivity in electron-doped cuprate La2-xCexCuO4"
Europhysics Letters 64 (2)246-252 (2003).
ABSTRACT: We performed far-infrared and submillimeter-wave conductivity experiments in the electron-doped cuprate La2-xCexCuO4 with x = 0.081 (underdoped regime, T-c = 25 K). The onset of the absorption in the superconducting state is gradual in frequency and is inconsistent with the isotropic s-wave gap. Instead, a narrow quasiparticle peak is observed at zero frequency and a second peak at finite frequencies, clear fingerprints of the conductivity in a d-wave superconductor. A far-infrared conductivity peak can be attributed to 4Delta(0), or to 2Delta(0) + Delta(spin), where Delta(spin) is the resonance frequency of the spin fluctuations. The infrared conductivity as well as the suppression of the quasiparticle scattering rate below T-c are qualitatively similar to the results in the hole-doped cuprates.
127. K. Prabhakaran, Y. Watanabe, Y. Homma, and T. Ogino
"Selective activation and passivation of nanoparticle catalysts through substrate mediation"
Langmuir 19 (26)10629-10631 (2003).
ABSTRACT: Fe nanoparticles are synthesized on a silicon wafer, by the reduction of Fe2O3 nanoparticles, through a bonding partner change reaction involving silicon atoms from the substrate. On a silicon surface with germanium nanostructures grown on it, the catalyst particles are selectively activated and passivated. This has been achieved primarily by exploiting the vast differences in the reduction temperature of the oxide particles on silicon and germanium surfaces. We demonstrate this by growing carbon nanotubes selectively interconnecting the germanium nanostructures. The mechanisms of activation and passivation are discussed.
128. K. Prabhakaran, Y. Watanabe, K. G. Nath, Y. Homma, T. Ogino, K. V. Shafi, and A. Ulman
"Surface chemistry of Fe2O3 nanoparticles on ultrathin oxide layers on Si and Ge"
Surface Science 545 (3)191-198 (2003).
ABSTRACT: In this paper, we report on a comparative study of the effect of Fe2O3 nanoparticles (NP), introduced onto a thin oxide layer formed on silicon and germanium surfaces, on the thermal decomposition pathway of the individual oxide layers. On both the surfaces, NP of Fe2O3 undergo a reduction reaction through a bonding partner change reaction, where the oxygen atoms change from Fe to Si or Ge. On both the surfaces, annealing results in the conversion of the suboxide-like species to dioxide-like species (SiOx to SiO2 and GeOx to GeO2 respectively for Si and Ge surfaces), until the oxide layer decomposes following the desorption of the respective monoxide species (SiO and GeO). Both the Si and Ge corelevels show a larger chemical shift (4.1 and 3.51 eV in Si 2p and Ge 3d corelevels, respectively) for the as-prepared oxide samples with the NP, at room temperature compared to that without the NP (3.7 and 3.4 eV), indicating a catalytic enhancement of the dioxide formation. Selective formation of silicon oxides leads to encapsulation of the nanoparticles and acts like a protective layer, preventing the oxidation of Fe. (C) 2003 Elsevier B.V. All rights reserved.
129. K. Prabhakaran, F. Meneau, G. Sankar, K. Sumitomo, T. Murashita, Y. Homma, G. N. Greaves, and T. Ogino
"Luminescent nanoring structures on silicon"
Advanced Materials 15 (18)1522-+ (2003).
ABSTRACT: A unique self-organization of optically active nanoparticles of CdS on a silicon surface forming nanoring structures (see Figure and inside cover) is presented. Tunneling luminescence mapping on these structures show that there is direct correspondence between the morphology and the light emission, suggesting that this novel nanomaterial system could make a significant impact in a wide variety of fields including quantum optics.
130. A. V. Pronin, A. Pimenov, A. Loidl, A. Tsukada, and M. Naito
"Doping dependence of the gap anisotropy in La2-xCexCuO4 studied by millimeter-wave spectroscopy"
Physical Review B 68 (5), 054511 (2003).
ABSTRACT: We measure the penetration depth of optimally doped and underdoped La2-xCexCuO4 in the millimeter frequency domain (4-7 cm-1) and for temperatures 2 Kless than or equal toTless than or equal to300 K. The penetration depth as a function of temperature reveals significant changes on electron doping. It shows quadratic temperature dependence in underdoped samples, but increases almost exponentially at optimal doping. Significant changes in the gap anisotropy (or even in the gap symmetry) may account for this transition.
131. B. Ressel, K. C. Prince, S. Heun, and Y. Homma
"Wetting of Si surfaces by Au-Si liquid alloys"
Journal Of Applied Physics 93 (7)3886-3892 (2003).
ABSTRACT: The behavior of liquid Au-Si alloys on Si surfaces covered by a monolayer of gold has been investigated by ultrahigh-vacuum scanning electron microscopy. On the (111) surface, the alloy displays a constant contact angle with the surface from the eutectic temperature up to a temperature of 650 degreesC and thereafter the contact angle increases linearly with temperature. As observed in previous work, the shape of the liquid droplets changes from circular at lower temperature to hexagonal at higher temperature. In contrast, on the (100) surface, the contact angle increases linearly from the eutectic temperature to high temperature. The behavior of the shape of the droplets is, however, reversed: it is polygonal (octagonal) at lower temperature and becomes round at higher temperature. This behavior is explained in terms of the relative surface energy of the two surfaces and changing line tension of the liquid-solid-vapor phase line. In addition, the behavior of Au-Si droplets on vicinal and patterned surfaces of Si has been examined. The droplets cause step bunching and modify the local surface structure. Solidification of the droplets on all surfaces leads to phase separation. (C) 2003 American Institute of Physics.
132. H. Y. Ryu, and M. Notomi
"Enhancement of spontaneous emission from the resonant modes of a photonic crystal slab single-defect cavity"
Optics Letters 28 (23)2390-2392 (2003).
ABSTRACT: Modification of the spontaneous-emission lifetime in photonic crystal single-defect resonant modes is studied with the finite-difference time domain method. We investigate spontaneous-emission enhancement from the monopole and the dipole modes of a hexagonal lattice cavity, considering the effects of the finite emitter linewidth and spectral detuning. Large spontaneous-emission enhancement of >50 is achieved numerically from the high-quality-factor monopole mode when the emitter linewidth is comparable with the resonant-mode linewidth. However, if broad-linewidth material is used and a detuning effect is included, the dipole mode with a low quality factor and a smaller mode volume could be more advantageous for spontaneous-emission enhancement. (C) 2003 Optical Society of America.
133. H. Y. Ryu, M. Notomi, and Y. H. Lee
"High-quality-factor and small-mode-volume hexapole modes in photonic-crystal-slab nanocavities"
Applied Physics Letters 83 (21)4294-4296 (2003).
ABSTRACT: Using finite-difference time-domain calculations, we investigate the hexapole mode of photonic-crystal-slab modified triangular single-defect cavity structures as a good candidate for a high-quality factor (Q) and small-mode volume (V) resonant mode. Structural parameters are optimized to obtain very large Q of even higher than 2x10(6) with small effective V of the order of cubic wavelength in material, the record value of theoretical Q/V. It is found, by the Fourier-space investigation of resonant modes, that such a high Q from the hexapole mode is achieved due both to the cancellation mechanism related to hexagonally symmetric whispering-gallery-mode distribution and to the mode delocalization mechanism. (C) 2003 American Institute of Physics.
134. H. Y. Ryu, M. Notomi, and Y. H. Lee
"Finite-difference time-domain investigation of band-edge resonant modes in finite-size two-dimensional photonic crystal slab"
Physical Review B 68 (4), 045209 (2003).
ABSTRACT: We investigate characteristics of optical modes near band edges of finite-size two-dimensional photonic crystals and photonic crystal slab structures by using finite-difference time-domain calculations. Mode patterns, spectra, and quality factors are calculated near several photonic band edges of triangular lattice air-hole structures. As the size of a photonic crystal pattern increases, the spectral peak approaches the photonic band edge and the quality factor increases. It is interesting to find that, in spite of out-of-plane radiation loss, band-edge quality factors of the photonic crystal slab is larger than those of the two-dimensional photonic crystal with equivalent effective refractive index. This fact originates from the slower group velocity near band edges of photonic crystal slab structures compared with two-dimensional photonic crystals. High quality factors > 2000 are achieved from the second G point of the photonic crystal air-bridge slab with only 15 layers of air holes.
135. T. Saitoh, M. Kumagai, H. L. Wang, T. Tawara, T. Nishida, T. Akasaka, and N. Kobayashi
"Highly reflective distributed Bragg reflectors using a deeply etched semiconductor/air grating for InGaN/GaN laser diodes"
Applied Physics Letters 82 (25)4426-4428 (2003).
ABSTRACT: High reflectivity is achieved by deeply etched InGaN/GaN distributed Bragg reflector (DBR) mirrors with tilted sidewalls, which are appropriately designed by using the finite-difference time-domain method. The predicted optimal structure is different from the simple design consisting of a lambda/(4n) semiconductor and lambda/4 air. If the sidewall of the grating is tilted by 4degrees, the reflectivity of the DBR mirrors decreases to less than 40%. However, any degradation in the reflectivity of a perfectly vertical sidewall can be suppressed to just a few percent even with a sidewall tilt of 4degrees, if the DBR structure is properly designed. We fabricated InGaN/GaN multiple-quantum well lasers based on the optimal design. The devices operate as lasers with optical pumping at a lower threshold than devices without DBR mirrors. The DBR mirror reflectivity is characterized by the relation between the threshold pump intensity and the inverse of the cavity length, resulting in a high reflectivity of 62%. (C) 2003 American Institute of Physics.
136. H. Sato, K. Cho, J. Kurian, and M. Naito
"Third-order intermodulation measurements of 2 GHz microstrip bandpass filters based on high-temperature superconductors"
Superconductor Science & Technology 16 (12)1503-1507 (2003).
ABSTRACT: To elucidate the key parameters for improving the power-handling capability of rf filters of high-temperature superconductors with microstrip structures, we synthesize bandpass filters with different layouts using several kinds of thin film high-temperature superconductors, and perform third-order intermodulation measurements on them. The third-order intercept (IP3) at 70 K is found to increase from +53 dBm to +65 dBm as the film thickness increases from 0.62 mum to 0.80 mum for the filters of YBa2Cu3O7 (YBCO). We find that filters made from films of NdBa2Cu3O7 (NBCO) grown by molecular-beam epitaxy yield higher IP3 values than those made from commercially obtained YBCO films. These experimental results indicate that increasing the film thickness and utilizing MBE-grown NBCO films are effective in obtaining high-power-handling microstrip filters.
137. T. Sato, H. Matsui, T. Takahashi, H. Ding, H. B. Yang, S. C. Wang, T. Fujii, T. Watanabe, A. Matsuda, T. Terashima, and K. Kadowaki
"Observation of band renormalization effects in hole-doped high-T-c superconductors"
Physical Review Letters 91 (15), 157003 (2003).
ABSTRACT: We report a systematic high-resolution angle-resolved photoemission spectroscopy on high-T-c superconductors Bi2Sr2Can-1CunO2n+4 (n=1-3) to study the origin of many-body interactions responsible for superconductivity. For n=2 and 3, a sudden change in the energy dispersion, so called "kink", becomes pronounced on approaching (pi,0) in the superconducting state, while a kink appears only around the nodal direction in the normal state. For n=1, the kink shows no significant temperature dependence even across T-c. This could suggest that the coupling of electrons with Q=(pi,pi) magnetic mode is dominant in the superconducting state for multilayered cuprates, while the interactions at the normal state and that of single-layered cuprates have a different origin.
138. A. Sawada, D. Terasawa, N. Kumada, M. Morino, K. Tagashira, Z. F. Ezawa, K. Muraki, T. Saku, and Y. Hirayama
"Continuous transformation from spin- to pseudospin-type excitation"
Physica E-low-dimensional Systems & Nanostructures 18 (1-3)118-119 (2003).
ABSTRACT: We measured the activation energy of bilayer v = 1 quantum Hall states. By changing the density difference between layers, the tilting behavior of the pseudospin(P)-type activation energy at the equal density point gradually transforms into the spin(S)-type one at the monolayer density point. At the intermediate density difference, by increasing the tilting angle the activation energy starts to decrease as the P-type excitation gap and then increases as the S-type excitation gap: It is impossible to explain this behavior by the level crossing of the P- and S-type excitations. The result of the overall behavior indicates the excitation in a bilayer system is the simultaneous flip of spin and pseudospin. (C) 2003 Elsevier Science BY, All rights reserved.
139. T. Sekitani, M. Naito, and N. Miura
"Kondo effect in underdoped n-type superconductors"
Physical Review B 67 (17), 174503 (2003).
ABSTRACT: We present high-field magnetotransport properties of high-quality single-crystalline thin films of heavily underdoped nonsuperconducting (La,Ce)(2)CuO4, (Pr,Ce)(2)CuO4, and (Nd,Ce)(2)CuO4. All three materials show identical behavior. They are metallic at high temperatures and show an insulating "upturn" at low temperatures. The insulating upturn has a log T dependence, but saturates toward the lowest temperatures. Notably, the insulating upturn tends to be suppressed by applying magnetic fields. This negative magnetoresistance has a log B dependence, and its anisotropy shows a nonsimple behavior. We discuss these findings from the viewpoints of Kondo scattering and also two-dimensional weak localization, and demonstrate Kondo scattering as a more plausible explanation. The Kondo scatters are identified as Cu2+ spins in the CuO2 planes.
140. T. Sekitani, H. Sato, M. Naito, and N. Miura
"High-field magnetotransport in strained La2-xSrxCuO4 films"
Physica C-superconductivity And Its Applications 388345-346 (2003).
ABSTRACT: We present the transport properties of the low-temperature normal-state for strained La2-xSrCuO4 (LSCO) films achieved by suppressing superconductivity with high magnetic fields up to 50 T. Systematic measurements were performed on six MBE grown films with three different doping levels (underdope, optimumdope, and overdope) and also with either compressive or tensile epitaxial strain. The results showed that the low-temperature semiconducting upturn observed in under- or optimum-doped LSCO bulk single crystals is significantly suppressed by in-plane compressive strain. (C) 2003 Elsevier Science B.V. All rights reserved.
141. V. Seleznev, H. Yamaguchi, Y. Hirayama, and V. Prinz
"Single-turn GaAs/InAs nanotubes fabricated using the supercritical CO2 drying technique"
Japanese Journal Of Applied Physics Part 2-letters 42 (7A)L791-L794 (2003).
ABSTRACT: We have applied supercritical CO2 drying technique to fabricate free-standing semiconductor nanotubes, in order to solve the problem, of tube collapsing caused by the capillary forces in the final drying process. We have successfully fabricated damage-free single-turn nanotubes with ultra-thin walls (lesser than 3 nm) and very high tube-length/wall-thickness ratios (higher than 103), which so far could not been realized. These extremely small and light structures with unique geometry are promising for future nanoelectromechanical systems.
142. H. Shibata
"Josephson plasma in Ru- and Fe-cuprates"
Physica C-superconductivity And Its Applications 388459-460 (2003).
ABSTRACT: The Josephson plasma in RuSr2GdCu2O8 and FeSr2YCu2Oy is measured by the sphere resonance method. For ferromagnetic RuSr2GdCu2O8, the plasma is observed in a very low-frequency region (around 8.5 cm(-1) at T much less than T-c), which represents a large reduction in the Josephson coupling at ferromagnetic RuO2 block layers. For non-ferromagnetic FeSr2YCu2Oy, the plasma frequency increases to 31 cm(-1), which is comparable to that of lightly doped YBa2Cu3O6+delta. The temperature dependence of the plasma does not shift to zero frequencies (i.e. j(c) = 0) at low temperatures, indicating that there is no transition from the 0-phase to the pi-phase in these compounds. (C) 2003 Elsevier Science B.V. All rights reserved.
143. T. Shibauchi, J. Hashimoto, K. Fujita, T. Watanabe, A. Matsuda, and M. Suzuki
"Microwave-induced zero-current crossings in Bi2Sr2CaCu2O8+y intrinsic Josephson junctions"
Physica C-superconductivity And Its Applications 388461-462 (2003).
ABSTRACT: Current-voltage characteristics in mesa-structured Bi2Sr2CaCu2O8+y intrinsic junctions under in-plane magnetic fields reveal zero-current crossings (ZCCs) when microwave is applied. From measurements with controlled number of junctions (N = 5-14), we find that the number of ZCCs is just N in total of positive and negative bias sides; half of the 2N branches observed without microwave. Unlike Shapiro steps, the voltage step increases with microwave amplitude. Such features can be explained by considering the charging effect of superconducting layers and the pinning of triangular Josephson vortex lattice. (C) 2003 Elsevier Science B.V. All rights reserved.
144. K. Shimizu, and N. Imoto
"Communication channels analogous to one out of two oblivious transfers based on quantum uncertainty. II. Closing EPR-type loopholes"
Physical Review A 67 (3), 034301 (2003).
ABSTRACT: In a recent proposal for a quantum cryptographic scheme analogous to one out of two oblivious transfers [Phys. Rev. A 66, 052316 (2002)], a sender, Bob, can encode two bits of information (X,Y) in a quantum carrier but a receiver, Alice, can decode only either X or Y dependent on her choice. Although Bob can discover her choice whenever he desires, she can detect this with a 50% probability. This paper clarifies the amount of information that Bob can expect to obtain from Alice without being detected by her by means of an Einstein-Podolsky-Rosen (EPR) type of attack. We can show that Bob inevitably fails to discover her choice with a 50% probability even though he can always completely escape her detection.
145. J. A. Skinta, M. S. Kim, T. R. Lemberger, T. Greibe, and M. Naito
"Superconducting density of states from the magnetic penetration depth of electron-doped cuprates La2-xCexCuO4-y and Pr2-xCexCuO4-y"
Journal Of Low Temperature Physics 131 (3-4)359-368 (2003).
ABSTRACT: From measurements of the magnetic penetration depth, lambda(T), from 1.6 K to T-c in films of electron-doped cuprates La2-xCexCuO4-y and Pr2-xCexCuO4-y we obtain the normalized density of states, N-s(E) at T=0 by using a simple model. In this framework, the flat behavior of lambda(-2)(T) at low T implies N-s(E) is small, possibly gapped, at low energies. The upward curvature in lambda(-2)(T) near T-c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k(B)T(c) of the Fermi surface.
146. G. S. Solomon, M. Pelton, and Y. Yamamoto
"Tuning the single optical mode spontaneous emission coupling of a quantum dot in a micropost cavity"
Journal Of Crystal Growth 251 (1-4)737-741 (2003).
ABSTRACT: Using a micropost cavity, the emission from a quantum dot (QD) is coupled to the discrete fundamental optical mode of the cavity. By adjusting the temperature of the device between 4 and 70 K, the coupling between the QD and the cavity can be tuned. The ratio of the spontaneous emission decay rate between on and off resonance is approximately 4. (C) 2003 Published by Elsevier Science B.V.
147. A. Stopa, T. Hatano, T. Ota, K. Yamada, and S. Tarucha
"A bond bending model for negative differential resistance in transport through single molecules"
Superlattices And Microstructures 34 (3-6)439-442 (2003).
ABSTRACT: We develop a model for transport through benzene-based single molecules with an NO2 side-group, which incorporates bond bending between the NO2 and the adjacent benzene ring and successfully reproduces the experimentally observed strong negative differential resistance. Transport through the molecule is assumed to be incoherent and is treated using photon assisted tunnelling. (C) 2004 Elsevier Ltd. All rights reserved.
148. Y. Sugimoto, S. Takaoka, K. Oto, T. Saku, and Y. Hirayama
"The influence of electron-electron interactions on cyclotron resonance in subband-Landau level coupling systems"
Solid State Communications 127 (9-10)629-634 (2003).
ABSTRACT: We investigate the influence of electron-electron interactions on cyclotron resonance (CR) of two-dimensional electron systems in tilted magnetic field. It is known that the electron-electron interactions affect the CR spectra in systems that have more than two cyclotron frequencies. In this study, this multi-component system is realized by applying the tilted magnetic field; the tilted magnetic field induces the resonant subband-Landau level coupling (RSLC) and a splitting of the CR spectrum is observed. Observed anomalous CR spectra cannot be explained by the RSLC with the single-electron picture. Present theoretical treatments cannot account for the result of our experiment. (C) 2003 Elsevier Ltd. All rights reserved.
149. K. Sumitomo, H. Omi, Z. H. Zhang, and T. Ogino
"Anisotropic strain relaxation of Ge nanowires on Si(113) studied by medium-energy ion scattering"
Physical Review B 67 (3), 035319 (2003).
ABSTRACT: We have investigated the strain state in Ge nanowires on Si(113) substrate using medium-energy ion scattering. We found that nanowires have negligibly relaxed compressive strain along their length, but the strain across them is almost totally relaxed. Anisotropic strain relaxation plays a role in determining the width of the nanowires.
150. N. Susa
"Towards perfect vertical photonic band gap continement in a photonic crystal slab"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (11)7157-7162 (2003).
ABSTRACT: A distributed Bragg reflector (DBR) with air rods was studied theoretically as cladding for a photonic crystal slab in an effort to obtain perfect vertical gap confinement. This DBR has photonic band gaps in the perpendicular and parallel directions, but they do not overlap. When an air-rod-free (i.e., conventional) DBR is used for the cladding, the light line decreases profoundly in the photonic band diagram and the photonic band gap also becomes small, indicating that this type of DBR is greatly inferior to a DBR with air rods as the cladding. The light line of the DBR cladding with air rods is only slightly smaller than that for the extensively used air cladding because of the large air rod radius of 0.47a (a is a photonic crystal period). In addition, the photonic band gaps between the lowest and second lowest TE-like modes and the lowest TE- and TM-like modes are still large for this DBR cladding. Thus, this cladding can be used for a photonic crystal slab. Numerical analysis also predicts that a guided mode has a nearly zero group velocity, because the DBR cladding acts as a reflector for a light propagating in the perpendicular direction.
151. N. Susa
"The large bandwidth and large group velocity for the single guided mode in an asymmetric photonic crystal slab waveguide"
Japanese Journal Of Applied Physics Part 2-letters & Express Letters 42 (6B)L724-L727 (2003).
ABSTRACT: An asymmetric photonic crystal slab waveguide was analyzed for the first time. This waveguide exhibits an inclined dispersion relation (i.e., a large group velocity) without a mini-band gap and thus has a large bandwidth for the single guided mode (14% of the center-band frequency). This structure consists of a line defect and air holes pi-phase-shifted on one side of the line defect. The present asymmetric structure prevents Bragg reflection from occurring in the light propagation direction at the interface between the photonic crystal and the line defect. The frequency of the TE-like guided mode decreases more than that of the TM-like guided mode when the waveguide width is increased. [DOI: 10.1143/JJAP.42.L724].
152. K. Suzuki, S. Miyashita, and Y. Hirayama
"Transport properties in asymmetric InAs/AlSb/GaSb electron-hole hybridized systems"
Physical Review B 67 (19), 195319 (2003).
ABSTRACT: Transport properties in asymmetric InAs/GaSb and InAs/AlSb/GaSb heterostructures sandwiched by AlGaSb layers were studied. For the InAs/GaSb structures, partially compensated quantum Hall effects arising from the electron-hole hybridization were observed. By changing the thickness of each layer, the energy positions of the conduction band and the valence band can be controlled independently. In InAs/AlSb/GaSb structures, we tried to control the hybridization strength by varying the AlSb barrier thickness. Magnetoresistance measurements, in which the magnetic field was applied perpendicular and parallel to the surface, and cyclotron resonance (CR) measurements show that there is a clear transition from the electron-hole-hybridized system to the electron-hole-independent system. The transition point appeared at around the 2-nm-barrier thickness in all experiments, suggesting a dominant role of the overlap of electron and hole wave functions. According to the absorption peak shapes in the CR measurements, the hybridized system can be further classified as strongly hybridized or weakly hybridized. Only in the weakly hybridized system was the Shubnikov-de Haas-like oscillation due to the short-range scattering by the hole potential observed in the electron CR absorption peak.
153. M. Suzuki, K. Anagawa, Y. Yamada, and T. Watanabe
"Tunneling-spectroscopic evidence for unconventional pairing interaction in Bi2Sr2CaCu2O8+delta"
Physica C-superconductivity And Its Applications 388295-296 (2003).
ABSTRACT: The quasiparticle density of states (DOS) is measured as a function of magnetic fields up to 9 T by the short-pulse interlayer tunneling spectroscopy for slightly overdoped Bi2Sr2CaCu2O8+delta. It is found that the quasiparticle DOS near the Fermi level E-F remains almost unchanged when the magnetic field parallel to the c-axis is applied up to 9 T. This is totally at variance with the conventional behavior in which the quasiparticle DOS at E-F increases with the increasing magnetic field. The present result implies that the quasiparticles involved in pairing come from higher energies apart from E-F, providing evidence for unconventional pairing mechanism. (C) 2003 Elsevier Science B.V. All rights reserved.
154. S. Suzuki, F. Maeda, Y. Watanabe, and T. Ogino
"Electronic structure of single-walled carbon nanotubes encapsulating potassium"
Physical Review B 67 (11), 115418 (2003).
ABSTRACT: Electronic structural change of single-walled carbon nanotubes induced by potassium encapsulation was studied by photoemission spectroscopy. The potassium encapsulation caused a shift of the overall valence-band spectrum toward the higher binding-energy side by about 0.5 eV, which is basically understood by the simple rigid band shift model. However, the spectral intensity increase observed near the Fermi level was much larger than that expected by assuming the simple pi(*) band filling, indicating that, in addition to the pi(*) band, a part of the density of states initially located in the unoccupied states dips below the Fermi level by the potassium encapsulation. The result is qualitatively consistent with a recent band-structure calculation, which predicted that the nearly free-electron state hybridized with the K 4s state crosses the Fermi level. The potassium encapsulation also decreased the work function by 1.4 eV.
155. T. Takahashi, H. Matsui, T. Sato, S. C. Wang, H. B. Yang, H. Ding, T. Fujii, T. Watanabe, and A. Matsuda
"Direct evidence for superconducting quasiparticle in triple-layered high-T-c superconductor"
Physica C-superconductivity And Its Applications 388305-306 (2003).
ABSTRACT: We have performed angle-resolved photoemission spectroscopy (ARPES) on triple-layered high-T-c cuprate superconductor Bi2Sr2Ca2Cu3O10+delta. We found a direct experimental evidence for Bogoliubov quasiparticle in high-T-c cuprates by observing the full energy dispersion below and above the Fermi level. The excellent quantitative agreement in the dispersion as well as the coherence factors between the ARPES experiment and Bardeen, Cooper and Shrieffer (BCS) theory establishes the validity and universality of the basic framework of BCS theory in high-T-c cuprates. (C) 2003 Elsevier Science B.V. All rights reserved.
156. T. Takahashi, S. Fukatsu, K. M. Itoh, M. Uematsu, A. Fujiwara, H. Kageshima, Y. Takahashi, and K. Shiraishi
"Self-diffusion of Si in thermally grown SiO2 under equilibrium conditions"
Journal Of Applied Physics 93 (6)3674-3676 (2003).
ABSTRACT: Self-diffusion coefficients of Si in thermally grown SiO2 on a semiconductor-grade silicon wafer have been determined at temperatures between 1150 and 1300 degreesC under equilibrium conditions using isotope heterostructures ((SiO2)-Si-nat/(SiO2)-Si-28). Si self-diffusion was induced by appropriate heat treatments, and the diffusion depth profiles of Si-30 isotope from (SiO2)-Si-nat to (SiO2)-Si-28 layers were determined by secondary ion mass spectrometry (SIMS). The diffusion coefficients found in the present study for 1150-1300 degreesC are more than two orders of magnitude smaller than the values measured with semiconductor-grade SiO2 in the presence of excess silicon, i.e., in nonequilibrium conditions, and agree very well with previously reported values of Si self-diffusion in fused silica under equilibrium conditions. (C) 2003 American Institute of Physics.
157. Y. Takahashi, Y. Ono, A. Fujiwara, and H. Inokawa
"Development of silicon single-electron devices"
Physica E-low-dimensional Systems & Nanostructures 19 (1-2)95-101 (2003).
ABSTRACT: We have developed a novel method called pattern-dependent oxidation (PADOX) for fabricating small Si single-electron transistors (SETs) and used it to make many kinds of SEDs. One of the most primitive and important features of the method is that fabricated devices operate quite stably for long-term drift. By using the method we have demonstrated various kinds of logic devices. SETs have two unique features that conventional transistors do not have. One is multi-input gates capability, and the other is oscillatory conductance as a function of gate voltage. We have exploited these features to achieve complicated functions, such as an adder and a multiple-valued memory. (C) 2003 Elsevier B.V. All rights reserved.
158. A. Takano, Y. Homma, Y. Higashi, H. Takenaka, S. Hayashi, K. Goto, M. Inoue, and R. Shimizu
"Evaluation of SIMS depth resolution using delta-doped multilayers and mixing-roughness-information depth model"
Applied Surface Science 203294-297 (2003).
ABSTRACT: Boron delta-doped multilayers are potential reference materials for the evaluation of depth resolution in secondary ion mass spectrometry (SIMS). In this work, we studied extraction of depth resolution parameters using a theoretical model, mixing-roughness-information (MRI) depth model from the measured profiles under various O-2+ bombardment conditions. Specimens used were boron delta-doped multilayers in Si (period: 3-20 nm) which had been made by magnetron-sputtering deposition. For SIMS, information depth can be regarded to be very small, so we used only the two parameters concerning mixing and roughness. Measured B profiles were fitted well to the MRI model. The depth resolution parameters could be extracted even from a profile of multilayers with a short periodicity. The combination of short-period multilayers and MRI model analysis would be useful for evaluation of depth resolution in shallow depth profiling. 2002 Elsevier Science B.V. All rights reserved.
159. H. Takesue, and T. Sugie
"Wavelength channel data rewrite using saturated SOA modulator for WDM networks with centralized light sources"
Journal Of Lightwave Technology 21 (11)2546-2556 (2003).
ABSTRACT: This paper describes a method for realizing the efficient utilization of wavelength resources in wavelength-division multiplexing networks with centralized light sources. Using a deeply saturated semiconductor optical amplifier (SOA) modulator located in a remote node (RN), we erase the data on a downstream signal with a low extinction ratio and modulate it with new data to generate an upstream signal. Thus, we use only one wavelength for bidirectional transmission between a center node and an RN, without placing lasers at the RN. In this paper, we analyze the data suppression characteristic of the SOA using a large signal model. We also estimate the bit error rate degradation in the presence of an unsuppressed downstream bit pattern in an upstream signal. We then report experimental results that confirm the basic characteristics of the wavelength channel data rewriter, which we constructed using a linear amplifier and an SOA. Finally, we provide the results of a data transmission,experiment that we undertook using the data rewriter.
160. Y. Taniyasu, M. Kasu, K. Kumakura, T. Makimoto, and N. Kobayashi
"High electron concentrations in Si-doped AlN/AlGaN superlattices with high average Al content of 80%"
Physica Status Solidi A-applied Research 200 (1)40-43 (2003).
ABSTRACT: Uniformly Si-doped AlN/Al0.5Ga0.5N superlattices were shown to have a high electron concentration. Even with high average At content of approximately 80%, the high electron concentration reached 3.2 x 10(18) cm(-3), which is about eight times higher than that of a bulk Si-doped AlGaN layer with the same Al content. In the AlN/AlGaN system, the conduction band offset is larger than the ionization energy of the Si donor in AlN. Therefore, the Si donors in the AlN barriers are fully activated and the corresponding electrons are transferred to the AlGaN wells. In addtion, the large band bending caused by the strong strain-induced piezoelectric and spontaneous polarization increases the activation of the donors in the AlGaN wells. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
161. Y. Taniyasu, M. Kasu, T. Makimoto, and N. Kobayashi
"Triode-type basic display structure using Si-doped AlN field emitters"
Physica Status Solidi A-applied Research 200 (1)199-201 (2003).
ABSTRACT: We fabricated a basic triode-type field emission display (FED) structure by using heavily Si-doped AlN and demonstrated its action. The FED consisted of the heavily Si-doped AlN field emitter, mesh grid, and phosphor-coated anode screen. We used the grid voltage to control the emission current. This device exhibited an emission current of 2 muA for an electric field of 23 V/mum, and luminescence from the phosphor excited by the field-emitted electrons was observed. The brightness of the luminescence increased as the grid voltage was increased, and it was uniform over the entire field-emission area. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
162. T. Tawara, H. Gotoh, T. Akasaka, N. Kobayashi, and T. Saitoh
"Low-threshold lasing of InGaN vertical-cavity surface-emitting lasers with dielectric distributed Bragg reflectors"
Applied Physics Letters 83 (5)830-832 (2003).
ABSTRACT: Lasing action is achieved in InGaN vertical-cavity surface-emitting lasers (VCSELs) with dielectric distributed Bragg reflectors (DBRs). We fabricated III-nitride VCSELs by removing a SiC substrate from a III-nitride cavity with a dry etching technique and then wafer bonding the cavity and SiO2/ZrO2 DBRs. These VCSELs have a high quality factor of 460 and a spontaneous emission factor of 10(-2). We observed lasing at a wavelength of 401 nm at room temperature with optical pumping. This lasing action was demonstrated at a low threshold of 5.1 mJ/cm(2) by using a high-quality crystalline cavity and quantum-well layers without surface roughening or cracking. (C) 2003 American Institute of Physics.
163. K. Ueda, and M. Naito
"Tunnel junctions on As-grown superconducting MgB2 thin films"
Ieee Transactions On Applied Superconductivity 13 (2)3249-3252 (2003).
ABSTRACT: We demonstrate that good superconductor/insulator/normal-metal tunnel junctions can be fabricated on as-grown superconducting MgB2 thin films. The as-grown films were prepared by coevaporation at low growth temperatures of around 280degreesC. The insulating barrier was formed by an Mg overlayer, which is subsequently oxidized in air. The tunneling spectra for Au/MgOx/MgB2 junctions showed a reproducible and well-defined superconducting gap (Delta =similar to 2.5 meV). The resultant 2Delta/k(B)T(c) was significantly smaller than the predicted BCS value of 3.52.
164. K. Ueda, and M. Naito
"In situ growth of superconducting MgB2 thin films by molecular-beam epitaxy"
Journal Of Applied Physics 93 (4)2113-2120 (2003).
ABSTRACT: The in situ growth of superconducting MgB2 thin films was examined from various perspectives. The paper discusses (1) growth temperature, (2) the effect of excess Mg, (3) the effect of residual gas during growth, (4) the effect of in situ annealing, (5) thickness dependence, and (6) the effect of substrates. Our results provide a guide to the preparation of high-quality superconducting MgB2 films for potential electronics applications. (C) 2003 American Institute of Physics.
165. Y. Utsugi, Y. Watanabe, and T. Nagamura
"Spectroscopy of atom movements in photo-excited silver-chalcogenide glasses"
Journal Of Non-crystalline Solids 326226-232 (2003).
ABSTRACT: Absorption current imaging and scanning tunneling spectroscopy (STS) measurements were performed to study the electrical properties of partially photo-exposed silver-selenide (Ag2Se) surface. The absorption current images reveal photo-increased conductance, revealing a newly formed Ag layer. The time-resolved STS studies clarify the dynamical aspects of Ag+ ion movement and photo-induced change in the electronic structure of the surface; from semiconductor to metal on the exposed regions and from semiconductor to semiinsulator on the unexposed ones. These phenomena resulting from photo-induced Ag movement verify that photo-fixed Ag clusters and movable Ag+ ions exist in conflict with each other on the exposed surface. (C) 2003 Elsevier B.V. All rights reserved.
166. W. G. van der Wiel, S. De Franceschi, J. M. Elzerman, T. Fujisawa, S. Tarucha, and L. P. Kouwenhoven
"Electron transport through double quantum dots"
Reviews Of Modern Physics 75 (1)1-22 (2003).
ABSTRACT: Electron transport experiments on two lateral quantum dots coupled in series are reviewed. An introduction to the charge stability diagram is given in terms of the electrochemical potentials of both dots. Resonant tunneling experiments show that the double dot geometry allows for an accurate determination of the intrinsic lifetime of discrete energy states in quantum dots. The evolution of discrete energy levels in magnetic field is studied. The resolution allows one to resolve avoided crossings in the spectrum of a quantum dot. With microwave spectroscopy it is possible to probe the transition from ionic bonding (for weak interdot tunnel coupling) to covalent bonding (for strong interdot tunnel coupling) in a double dot artificial molecule. This review is motivated by the relevance of double quantum dot studies for realizing solid state quantum bits.
167. W. G. van der Wiel, Y. V. Nazarov, S. De Franceschi, T. Fujisawa, J. M. Elzerman, E. W. Huizeling, S. Tarucha, and L. P. Kouwenhoven
"Electromagnetic Aharonov-Bohm effect in a two-dimensional electron gas ring"
Physical Review B 67 (3), 033307 (2003).
ABSTRACT: We define a mesoscopic ring in a two-dimensional electron gas interrupted by two tunnel barriers, enabling us to apply a well-defined potential difference between the two halves of the ring. The electron interference in the ring is modified using a perpendicular magnetic field and a bias voltage. We observe clear Aharonov-Bohm oscillations up to the quantum Hall regime as a function of both parameters. The electron travel time between the barriers is found to increase with the applied magnetic field. Introducing a scattering model, we develop a method to measure the nonequilibrium electron dephasing time, which becomes very short at high voltages and magnetic fields. The relevance of electron-electron interactions is discussed.
168. A. S. Verhulst, D. Maryenko, Y. Yamamoto, and K. M. Itoh
"Double and single peaks in nuclear magnetic resonance spectra of natural and Si-29-enriched single-crystal silicon"
Physical Review B 68 (5), 054105 (2003).
ABSTRACT: We report the nuclear magnetic resonance (NMR) spectra of phosphorus-doped single-crystal silicon, enriched to 97% Si-29, which display double as well as single peaks as the sample is rotated with respect to the static magnetic field. It is shown that the shape and the width of the NMR lines are predominantly due to nuclear dipolar interactions. We provide a theoretical model to determine the strength of the weaker scalar coupling between neighboring Si-29 nuclei. For naturally abundant single-crystal silicon (4.7% Si-29), the anisotropic dipolar interaction is visible as well in the NMR spectra. We have observed small side peaks for particular orientations of the crystal axes and we show that the intensity of the side peaks contains information on the distribution of Si-29 atoms in the Si-28 matrix.
169. J. Vuckovic, D. Fattal, C. Santori, and G. S. Solomon
"Enhanced single-photon emission from a quantum dot in a micropost microcavity"
Applied Physics Letters 82 (21)3596-3598 (2003).
ABSTRACT: We demonstrate a single-photon source based on a quantum dot in a micropost microcavity that exhibits a large Purcell factor together with a small multiphoton probability. For a quantum dot on resonance with the cavity, the spontaneous emission rate is increased by a factor of 5, while the probability to emit two or more photons in the same pulse is reduced to 2% compared to a Poisson-distributed source of the same intensity. In addition to the small multiphoton probability, such a strong Purcell effect is important in a single-photon source for improving the photon outcoupling efficiency and the single-photon generation rate, and for bringing the emitted photon pulses closer to the Fourier transform limit. (C) 2003 American Institute of Physics.
170. E. Waks, K. Inoue, W. D. Oliver, E. Diamanti, and Y. Yamamoto
"High-efficiency photon-number detection for quantum in-formation processing"
Ieee Journal Of Selected Topics In Quantum Electronics 9 (6)1502-1511 (2003).
ABSTRACT: The visible light photon counter (VLPC) features high quantum efficiency (QE) and low pulse height dispersion. These properties make it ideal for efficient photon-number state detection. The ability to perform efficient photon-number state detection is important in many quantum information processing applications, including recent proposals for performing quantum computation with linear optical elements. In this paper, we investigate the unique capabilities of the VLPC. The efficiency of the detector and cryogenic system is measured at 543 nm wavelengths to be 85 %. A picosecond pulsed laser is then used to excite the detector with pulses having average photon numbers ranging from 3-5. The output of the VLPC is used to discriminate photon numbers in a pulse. The error probability for number state discrimination is an increasing function of the number of photons, due to buildup of multiplication noise. This puts an ultimate limit on the ability of the VLPC to do number state detection. For many applications, it is sufficient to discriminate between I and more than one detected photon. The VLPC can do this with 99% accuracy.
171. T. Watanabe, T. Fujii, and A. Matsuda
"Structural study of inhomogeneous charge distribution of inequivalent CuO2 planes in Bi2.1Sr1.9Ca2Cu3O10+delta single crystals"
Journal Of The Physical Society Of Japan 72 (11)2924-2929 (2003).
ABSTRACT: Four-circle x-ray diffraction measurements were carried out on a single crystal of Bi2.1Sr1.9Ca2Cu3O10+delta with doping levels from underdoped to overdoped. The excess charge of each inequivalent CuO2 plane was estimated using the obtained inter-atomic distances in two ways by means of the bond-valence-sum method. The copper valence and the valence sum of the copper and the surrounding oxygen ions gave contradictory results for the inhomogeneous charge distribution of the inequivalent CuO2 planes.
172. B. Welter, Y. Krockenberger, M. Naito, L. Alff, and R. Gross
"Pseudogap and conservation of states in electron doped high-temperature superconductors"
Physica C-superconductivity And Its Applications 388299-300 (2003).
ABSTRACT: We report on tunneling spectroscopy measurements in underdoped Pr2-xCexCuO4 in the temperature range between 2 and 26 K and in magnetic fields up to 15 T. Our data show a pronounced depletion in the density of states near E-F in the normal state above B-c2. Furthermore, we find that the spectra in the superconducting state at 0 T do not follow the conservation of states rule. Yet the conservation of states is recovered after normalizing these spectra with that of the normal state in high fields, implying a coexistence of the superconducting gap and the pseudogap. (C) 2003 Elsevier Science B.V. All rights reserved.
173. K. Yamada, M. Stopa, T. Hatano, T. Ota, T. Yamaguchi, and S. Tarucha
"Variation of co-tunneling and Kondo effects by control of the strength of coupling between a vertical dot and a two-dimensional electron gas"
Superlattices And Microstructures 34 (3-6)185-189 (2003).
ABSTRACT: We have fabricated a vertical quantum dot with lateral coupling, modulated by a split gate voltage, to a two-dimensional electron. We thereby control not only electron configurations but also the strength of coupling between the dot and the lateral lead, by applying gate voltages. We have measured the conductance enhancement when the applied bias exceeds the single-electron excitation energy, in the Coulomb blockade regime. This conductance enhancement disappears as the split gate voltage decreases (reducing the coupling). This indicates that this enhancement is caused by inelastic co-tunneling. Furthermore, we observed a conductance enhancement at zero source-drain bias with stronger coupling. An anomaly is observed that we attribute to Kondo resonance between the dot and the leads. (C) 2004 Elsevier Ltd. All rights reserved.
174. Y. Yamada, K. Anagawa, T. Shibauchi, T. Fujii, T. Watanabe, A. Matsuda, and M. Suzuki
"Interlayer tunneling spectroscopy and doping-dependent energy-gap structure of the trilayer superconductor Bi2Sr2Ca2Cu3O10+delta"
Physical Review B 68 (5), 054533 (2003).
ABSTRACT: The superconducting gap, the pseudogap, and their doping and temperature dependences have been measured by the short-pulse interlayer tunneling spectroscopy for the CuO2 triple-layer high-T-c superconducting Bi2Sr2Ca2Cu3O10+delta system. It is found for a nearly optimally doped sample that the superconducting gap magnitude is approximate to80 meV and the pseudogap is approximate to120 meV, the values of which are slightly larger than those for CuO2 double-layer system. Both gap magnitudes show a clear tendency to decrease with increasing doping. In an underdoped sample, a clear dip-and-hump structure is observed, which declines with increasing doping and tends to diminish in overdoped samples. The relationship between unchanged T-c and decreasing superconducting gap in the overdoped region is discussed in terms of the proximity effect applied to the inequivalent doping model. We also discuss the dip-and-hump structure in comparison with other spectroscopic results. Finally, we argue an important implication of the increasing maximum Josephson current and the decreasing superconducting gap magnitude, both with increasing doping.
175. Y. Yamada, K. Anagawa, T. Fujii, T. Watanabe, A. Matsuda, T. Shibauchi, and M. Suzuki
"Superconducting gap and pseudogap in Bi2Sr2Ca2Cu3O10+delta by short-pulse interlayer tunneling spectroscopy"
Physica C-superconductivity And Its Applications 388285-286 (2003).
ABSTRACT: We have measured the superconducting gap, pseudogap and their doping dependence Of CuO2 trilayer high-T-c superconductor Bi2Sr2Ca2Cu3O10+delta by short-pulse interlayer tunneling spectroscopy. While both gaps exhibit temperature dependence similar to those for the bilayer system, their doping dependence behaves differently from that of T-c, presenting an anomalous relationship between T-c and the gap magnitude. In the overdoped region, T-c remains almost unchanged from its optimum value irrespective of doping, while the gap decreases with increasing doping. This is suggestive of inequivalent hole doping in the inner and outer planes, which is thought to occur in trilayer systems. (C) 2003 Elsevier Science B.V. All rights reserved.
176. H. Yamaguchi, S. Miyashita, and Y. Hirayama
"InAs/AlGaSb heterostructure displacement sensors"
Compound Semiconductors 2002 174247-250 (2003).
ABSTRACT: We have successfully fabricated a novel self-sensing mechanical displacement sensor with a surface InAs conductive layer of nanometer-scale thickness based on MBE-grown InAs/AlGaSb heterostructures. Sub-angstrom cantilever displacement is detectable and the sensitivity is increased with decreasing thickness. Tapping mode AFM characterization clarified the spatially resolved frequency response of this device, showing the lowest mode resonance frequency of about 300kHz.
177. H. Yamaguchi, and Y. Hirayama
"Fabrication and characterization of novel semiconductor nanomechanical structures"
Surface Science 5321171-1176 (2003).
ABSTRACT: As an application of the "bottom-up" self-organization growth technique to the fabrication of nanoscale mechanical structures, we selectively etched a GaAs sacrificial layer under InAs wires preferentially grown on bunched steps on misoriented GaAs(1 1 0) surfaces, which led to the successful formation of single crystal InAs nanoscale cantilevers. The lengths, widths, and thicknesses of the nanolevers are typically 50-300, 20-100 and 10-20 nm, respectively. The force constant, as measured by the force-modulation imaging technique using contact-mode atomic force microscopy, ranges from 0.5 to 10 N/m, showing good agreement with that estimated from the elastic constant of InAs. The resonance frequency is expected to reach 500 MHz for the smallest one, which promises possible application to high-speed nanomechanical devices. (C) 2003 Elsevier Science B.V. All rights reserved.
178. H. Yamaguchi, and Y. Hirayama
"Application of multi-step formation during molecular beam epitaxy for fabricating novel nanomechanical structures"
Journal Of Crystal Growth 251 (1-4)281-284 (2003).
ABSTRACT: We propose a novel application of "self-assembling" one-dimensional semiconductor nanostructures for nanoscale electromechanical systems. A sacrificial layer of a GaAs/AlGaAs supperlattice under InAs wires preferentially grown on bunched steps on misoriented GaAs (1 1 0) surfaces was selectively etched to form semiconductor cantilevers that have typical lengths, widths, and thicknesses of 50-300, 20-100 and 10-20 nm, respectively. The force constant, as measured by the force-modulation imaging technique using contact-mode atomic force microscopy, ranges from 0.5 to 10N/m, showing good agreement with that estimated from the elastic constant of InAs. (C) 2002 Elsevier Science B.V. All rights reserved.
179. H. Yamaguchi, S. Miyashita, and Y. Hirayama
"InAs/AlGaSb heterostructure displacement sensors for MEMS/NEMS applications"
Journal Of Crystal Growth 251 (1-4)556-559 (2003).
ABSTRACT: We have successfully fabricated a novel microelectromechanical displacement sensor with a surface InAs conductive layer of nanometer-scale thickness based on MBE-grown InAs/AlGaSb heterostructures. Sub-angstrom cantilever displacement is detectable and the sensitivity increases with decreasing thickness. Tapping-mode AFM characterization clarified the frequency response of this device, showing the fundamental mode resonance frequency of about 300 kHz. (C) 2002 Elsevier Science B.V. All rights reserved.
180. H. Yamaguchi, S. Miyashita, and Y. Hirayama
"Microelectromechanical displacement sensing using InAs/AlGaSb heterostructures"
Applied Physics Letters 82 (3)394-396 (2003).
ABSTRACT: We fabricated self-sensing microelectromechanical displacement sensors from InAs/AlGaSb heterostructures. The sensitivity increased with reduced InAs thickness showing that the piezoresistivity was strongly enhanced when the quantum level approached to the pinning position of surface Fermi level. The high-sensitivity allowed us to detect a subangstrom displacement induced by thermal vibration of cantilevers. (C) 2003 American Institute of Physics.
181. T. Yamaguchi, K. Yamazaki, M. Nagase, and H. Namatsu
"Line-edge roughness: Characterization and material origin"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (6B)3755-3762 (2003).
ABSTRACT: Line-edge roughness (LER) is a serious problem that we encounter in nanolithography as pattern sizes shrink. Two critical issues concerning the LER of resist patterns are its characterization and its origin. In this study, characterization involved estimating the LER of 300-nm line-&-space patterns in ZEP520 resist of various thicknesses by three standard metrological methods: top-down scanning electron microscope (SEM) method, top-down atomic force microscope (AFM) method, and sidewall AFM method. In this paper, we review these methods and compare their results. Regarding the origin, sidewall AFM measurements revealed polymer aggregates naturally contained in resist films to be the origin of LER in chain-scission-type resists. How the aggregates contribute to LER during the development process was also clarified.
182. T. Yamamoto, M. Koashi, S. K. Ozdemir, and N. Imoto
"Experimental extraction of an entangled photon pair from two identically decohered pairs"
Nature 421 (6921)343-346 (2003).
ABSTRACT: Entanglement is considered to be one of the most important resources in quantum information processing schemes, including teleportation(1-3), densecoding(4) and entanglement-based quantum key distribution(5). Because entanglement cannot be generated by classical communication between distant parties, distribution of entangled particles between them is necessary. During the distribution process, entanglement between the particles is degraded by the decoherence and dissipation processes that result from unavoidable coupling with the environment. Entanglement distillation and concentration schemes(6-9) are therefore needed to extract pairs with a higher degree of entanglement from these less-entangled pairs; this is accomplished using local operations and classical communication. Here we report an experimental demonstration of extraction of a polarization-entangled photon pair from two decohered photon pairs. Two polarization-entangled photon pairs are generated by spontaneous parametric down-conversion and then distributed through a channel that induces identical phase fluctuations to both pairs; this ensures that no entanglement is available as long as each pair is manipulated individually. Then, through collective local operations and classical communication we extract from the two decohered pairs a photon pair that is observed to be polarization-entangled.
183. Y. Yamamoto, and K. Inoue
"Noise in amplifiers"
Journal Of Lightwave Technology 21 (11)2895-2915 (2003).
ABSTRACT: The noise of linear amplifiers is discussed in terms of the fundamental law of quantum mechanics and is analyzed using an electrical circuit model. The operational principles of two types of amplifiers-negative conductance amplifier and nonlinear susceptance amplifier-are studied with a special emphasis on those at optical frequencies. The application of such optical amplifiers to optical telecommunication systems is reviewed.
184. M. Yamashita, and T. Mukai
"Stabilization of the number of Bose-Einstein-condensed atoms in evaporative cooling via three-body recombination loss"
Physical Review A 68 (6), 063601 (2003).
ABSTRACT: The dynamics of evaporative cooling of magnetically trapped Rb-87 atoms is studied on the basis of the quantum kinetic theory of a Bose gas. We carried out the quantitative calculations of the time evolution of conventional evaporative cooling where the frequency of the radio-frequency magnetic field is swept exponentially. This "exponential-sweep cooling" is known to become inefficient at the final stage of the cooling process due to a serious three-body recombination loss. We precisely examine how the growth of a Bose-Einstein condensate depends on the experimental parameters of evaporative cooling, such as the initial number of trapped atoms, the initial temperature, and the bias field of a magnetic trap. It is shown that three-body recombination drastically depletes the trapped Rb-87 atoms as the system approaches the quantum degenerate region and the number of condensed atoms finally becomes insensitive to these experimental parameters. This result indicates that the final number of condensed atoms is well stabilized by a large nonlinear three-body loss against the fluctuations of experimental conditions in evaporative cooling.
185. M. Yamashita, M. Koashi, T. Mukai, M. Mitsunaga, N. Imoto, and T. Mukai
"Optimization of evaporative cooling towards a large number of Bose-Einstein-condensed atoms"
Physical Review A 67 (2), 023601 (2003).
ABSTRACT: We study the optimization of evaporative cooling in trapped bosonic atoms on the basis of quantum kinetic theory of a Bose gas. The optimized cooling trajectory for Rb-87 atoms indicates that the acceleration of evaporative cooling around the transition point of Bose-Einstein condensation is very effective against loss of trapped atoms caused by three-body recombination. The number of condensed atoms is largely enhanced by the optimization, more than two orders of magnitude in our present calculation using relevant experimental parameters, as compared with the typical value given by the conventional evaporative cooling where the frequency of radio-frequency magnetic field is swept exponentially. In addition to this optimized cooling, it is also shown that highly efficient evaporative cooling can be achieved by an initial exponential and then a rapid linear sweep of frequency.
186. K. Yamazaki, T. Yamaguchi, and H. Namatsu
"Edge-enhancement writing for electron beam nanolithography"
Japanese Journal Of Applied Physics Part 1-regular Papers Short Notes & 42 (6B)3833-3837 (2003).
ABSTRACT: A novel method of electron-beam (EB) writing to reduce line-edge roughness (LER) called edge-enhancement writing (EEW) is described. EEW involves writing a fine line along a pattern edge at a high dose. This provides a steep profile of the energy deposited at the edge. The high exposure dose breaks up aggregates of resist polymer at the edge, which cause LER. Moreover, in EEW the background energy in unexposed regions due to proximity effects is also reduced by control of the dose. This increases the energy contrast at the edge, thereby preventing the appearance of aggregates on the sidewall due to undesired dissolution of the edge region. The resulting energy profile is quite different from that obtained with conventional proximity effect corrections. Experimental results and simulations of the deposited energy profile have demonstrated the effectiveness of EEW.
187. K. Yamazaki, and H. Namatsu
"Electron-beam diameter measurement using a knife edge with a visor for scattering electrons"
Japanese Journal Of Applied Physics Part 2-letters 42 (5A)L491-L493 (2003).
ABSTRACT: A novel knife edge (KE) for measurement of the electron beam (EB) diameter was made and evaluated. To screen off electrons scattered at the edge, the Si KE has a 'visor' made of Ta, which is formed in a body and aligned with the KE. Monte Carlo simulation revealed that the scattering electrons are not negligible to accurately measure the beam diameter using a KE, and that the visor effectively screens them out, especially at smaller diameters of a few nanometers. An EB diameter of less than 5 nm in a lithography system was accurately measured using the KE, which is consistent with the lithography results.
188. R. Yano, Y. Hirayama, S. Miyashita, N. Uesugi, and S. Uehara
"Arsenic pressure dependence of carrier lifetime and annealing dynamics for low-temperature grown GaAs studied by pump-probe spectroscopy"
Journal Of Applied Physics 94 (6)3966-3971 (2003).
ABSTRACT: Reflection-type degenerate pump-probe spectroscopy was performed for low-temperature grown (LT-) GaAs to study the effects of arsenic pressure during crystal growth and annealing on carrier lifetime and to ascertain the annealing dynamics. It was found that a sample grown under a high arsenic pressure has a shorter carrier lifetime for both as-grown and anneal conditions. It was also found that the carrier decay times of samples changed drastically when the annealing temperature was above 550 degreesC. We determined the annealing dynamics of LT-GaAs based on a model in which As-Ga antisite defects trap photoexcited carriers. An Arrhenius plot of the carrier decay rate vs. annealing temperature in the high temperature regime gave an energy E-PA that was different from the true activation energy. The annealing time dependences of E-PA obtained by the two diffusion models (self diffusion and V-Ga vacancy assisted diffusion of defects) were compared with E(PA)s of our data and other works, which proved that the annealing dynamics of As-Ga antisite defects was dominated by V-Ga vacancy assisted diffusion. (C) 2003 American Institute of Physics.
189. A. Yokoo
"Nanoelectrode lithography and multiple patterning"
Journal Of Vacuum Science & Technology B 21 (6)2966-2969 (2003).
ABSTRACT: Multiple patterning of a Si substrate is performed by nanoelectrode lithography, which transfers the nanoelectrode pattern by an electrochemical reaction. By repeating the process with a line-and-space-pattern nanoelectrode, a checked pattern is successfully fabricated. The fabricated pattern is used as an etching mask to wet etch a Si substrate, The etching process has two possible results. With one, the transferred pattern additionally acts as an etching mask. With the other, only the overlapping area of the transferred pattern can act as a mask. These results derive from the flexibility of nanoelectrode lithography. (C) 2003 American Vacuum Society.
190. A. Yokoo
"Nanoelectrode lithography"
Japanese Journal Of Applied Physics Part 2-letters 42 (2A)L92-L94 (2003).
ABSTRACT: A new lithographic method is proposed and demonstrated that has the potential to make nano-fabrication a more widely available technique. Nanoelectrode lithography works by transferring the pattern on a nanoelectrode to a target via an electrochemical reaction. This technique may provide a better throughput and flexibility than conventional lithographic techniques. This describes and proves the concept of nanoelectrode lithography.
191. M. Yuda, T. Sasaki, J. Temmyo, M. Sugo, and C. Amano
"High-power highly reliable 1.02-1.06-mu m InGaAs strained-quantum-well laser diodes"
Ieee Journal Of Quantum Electronics 39 (12)1515-1520 (2003).
ABSTRACT: By growing the InGaAs active layer at temperatures lower than in conventional growth, we extended the lasing wavelength and presented the high reliability in InGaAs strained-quantum-well laser diodes. Equivalent I-L characteristics were obtained for 1.02-, 1.05-, and 1.06-mum laser diodes with A cavity length of 1200 pin. Maximum output power as high as 800 mW and fundamental transverse mode operation at up to 400 mW were obtained at 1.06 mum and an 1800-mum cavity. Stable operation was observed for over 14000 h under auto-power-control of 225 mW at 50 degreesC for the 1.02-, 1.05-, and 1.06-mum lasers with a 900-mum cavity.
192. M. Yuda, J. Temmyo, T. Sasaki, M. Sugo, and C. Amano
"High-power highly reliable 1.06 mu m InGaAs strained-quantum-well laser diodes by low-temperature growth of InGaAs well layers"
Electronics Letters 39 (8)661-662 (2003).
ABSTRACT: High output power of about 800 mW in a chip and stable operation for over 14 000 It under 225 mW at 50degreesC have been achieved in 1.06 mum InGaAs strained-quantum-well laser diodes, which were realised by low-temperature growth of the InGaAs well layers.
193. B. Y. Zhang, G. Solomon, G. Weihs, and Y. Yamamoto
"Experimental extract and empirical formulas of refractive indices of GaAs and AlAs at high temperature by HRXRD and optical reflectivity measurement"
Journal Of Crystal Growth 251 (1-4)777-781 (2003).
ABSTRACT: Refractive indices of GaAs and AlAs at high temperature have been extracted by combining the high resolution X-ray diffraction and optical reflectivity measurement techniques. The comparison between the empirical formulas and experimental data on the indices of GaAs and AlAs has been made. A good agreement between them allows us to use empirical formulas to calculate the indices of GaAs and AlAs at high temperature over a wide wavelength range. The extracted data have been successfully employed in GaAs/AlAs DBR NIBE growth. (C) 2002 Elsevier Science B.V. All rights reserved.
194. M. Zitzlsperger, R. Onderka, M. Suhrke, U. Rossler, D. Weiss, W. Wegscheider, M. Bichler, R. Winkler, Y. Hiryama, and K. Muraki
"Chaos and open orbits in hole-antidot arrays with non-isotropic Fermi surface"
Europhysics Letters 61 (3)382-388 (2003).
ABSTRACT: Periodic antidot arrays imposed upon two-dimensional hole systems (2DHSs) display a striking dependence of the positive low-field magnetoresistance on the in-plane crystallographic orientation of the superlattice. An enhanced positive magnetoresistance can be ascribed to the 2DHSs non-circular Fermi surface which stabilizes chaotic trajectories for specific crystallographic directions. The erect provides a first example of the role of non-spherical Fermi surfaces on the classical chaotic dynamics of charge carriers. Transport calculations taking into account both the classical chaotic motion of the holes in antidot lattices and the warped Fermi contour confirm this picture.
195. Y. Zuev, T. R. Lemberger, J. A. Skinta, T. Greibe, and M. Naito
"Vortex pinning in electron-doped cuprate superconductor La2-xCexCuO4"
Physica Status Solidi B-basic Research 236 (2)412-415 (2003).
ABSTRACT: We have measured the vortex pinning strength in thin films of the electron-doped cuprate La2-xCexCuO4 for various doping x from x = 0.075 (T-c = 25 K) to x = 0.15 (T-c = 12.5 K). The optimal doping for this compound is x approximate to 0.11, at which T-c = 29 K. Films were prepared by a molecular-beam epitaxy method. A structural XRD investigation revealed no doping-induced lattice strain. We use a two-coil technique at frequencies from 10 to 100 kHz. The superfluid density n(s)(0) grows rapidly with x on the underdoped side and decreases slowly on the overdoped side. The Labusch parameter kappa, i.e. the spring constant for a pinned vortex, shows large variations as the doping changes. It increases with x rapidly to a maximum at slight underdoping and then it decreases on the overdoped side more rapidly than n(s)(0). Even at optimal doping kappa(0) is an order of magnitude lower than the highest values observed in YBCO.
