1.         K. Ajito and K. Torimitsu

            "Near-infrared Raman spectroscopy of single particles"

            Trac-Trends Anal. Chem. 20 (5), 255-262 (2001).

 

            ABSTRACT: Raman spectroscopy using non-invasive near-infrared (NIR) laser light has become a powerful tool for the microscopic analysis of organic and biological materials. A Raman tweezers microscope (RTM) was developed by combining NIR Raman spectroscopy with the laser trapping technique, which enables us to expand the scope of single particle studies. Recent results obtained using the RTM for single droplets and polymer spheres in a micrometer range are reported. (C) 2001 Elsevier Science B.V. All rights reserved.

 

2.         T. Akasaka, S. Ando, T. Nishida, H. Saito, and N. Kobayashi

            "Room-temperature lasing of InGaN multiquantum-well hexagonal microfacet lasers by current injection"

            Appl. Phys. Lett. 79 (10), 1414-1416 (2001).

 

            ABSTRACT: InGaN multiquantum-well (MQW) hexagonal microfacet (HMF) lasers have been fabricated by electron cyclotron resonance dry etching of as-grown planar InGaN MQW laser structures followed by metalorganic vapor phase epitaxy regrowth of p-GaN to form mirror facets of the lasers. The mirror facets are smooth and vertical {11(2) over bar0} facets of the regrown p-GaN. The HMF lasers lase at 401 nm under pulsed operation at room temperature. There is no significant difference in threshold current density for different cavity lengths of 480 and 770 mum, indicating that the mirror facets act as total reflectors. (C) 2001 American Institute of Physics.

 

3.         T. Akasaka, S. Ando, T. Nishida, H. Saito, and N. Kobayashi

            "Selective area metalorganic vapor phase epitaxy of thick crack-free GaN films on trenched SiC substrates"

            Appl. Phys. Lett. 79 (9), 1261-1263 (2001).

 

            ABSTRACT: Crack-free GaN films up to 11-mum-thick have been grown by using trenched SiC substrates and selective area metalorganic vapor phase epitaxy. These crack-free GaN films have hexagonal shapes and are surrounded by trenches. 97% of the hexagonal GaN films with side lengths of 100 mum and thickness of 11 mum was crack-free. The GaN films do not crack because the lateral propagation of cracks stops at the trenches and strain is relaxed in the small-area hexagonal GaN. This strain relaxation is confirmed by micro-Raman scattering measurements and agrees well with theoretical predictions. (C) 2001 American Institute of Physics.

 

4.         L. Alff, S. Kleefisch, B. Welter, A. Marx, R. Gross, and M. Naito

            "Pseudogap like tunneling spectra in electron doped high-temperature superconductors"

            Physica C 357, 134-137 (2001).

 

            ABSTRACT: We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd1.85Ce0.15CuO4-y and Pr1.85Ce0.15CuO4-y as a function of temperature and applied magnetic field using grain-boundary junction tunneling spectroscopy. For these optimum doped samples, no excitation gap is observed in the tunneling spectra above the superconducting critical temperature. In contrast, in applied magnetic fields above the resistively determined upper critical field. a clear excitation gap at the Fermi level of comparable size to the superconducting energy gap is present. One possible interpretation of this observation is the existence of a normal state pseudogap also in the electron doped HTS. (C) 2001 Elsevier Science B.V. All rights reserved.

 

5.         S. Amaha, D. G. Austing, Y. Tokura, K. Muraki, K. Ono, and S. Tarucha

            "Magnetic field induced transitions in the few-electron ground states of artificial molecules"

            Solid State Commun. 119 (4-5), 183-190 (2001).

 

            ABSTRACT: Few- and several-electron states in a single quantum dot and vertically quantum mechanically coupled double quantum dot structures are investigated at high magnetic (B-) fields up to 12 T in circular-shaped vertical single electron transistors. The phase diagrams showing the evolution with B-field of the Coulomb oscillation peaks reflecting the electro-chemical potential of the N-electron ground states, mu (N), for N from 0 to about 20 are compared, and the origin of the differences is discussed. The vertical coupling between two dots gives rise to a set of lateral bonding states and a set of lateral anti-bonding states, each set of which is composed of Darwin- Fock- like states arising from a two-dimensional harmonic confining potential. For the most strongly coupled double dot structure, only the bonding-states are occupied for N < 12 at 0 T, and the stability of the spin-polarized maximum density droplet (MDD) at filling factor, v = 1, is much reduced compared to the single dot system. This is attributed to the effect of reduced Coulomb interactions and reduced lateral confinement in the larger volume occupied by the electrons in the double dot system. For the weakly coupled double dot structure, both bonding and anti-bonding states can be occupied, and no highly stable MDD is present because of frequent magnetic depopulation of anti-bonding states. (C) 2001 Elsevier Science Ltd. All rights reserved.

 

6.         H. Asoh, K. Nishio, M. Nakao, T. Tamamura, and H. Masuda

            "Conditions for fabrication of ideally ordered anodic porous alumina using pretextured Al"

            J. Electrochem. Soc. 148 (4), B152-B156 (2001).

 

            ABSTRACT: The conditions for the fabrication of ideally ordered anodic porous alumina with a high aspect ratio were examined using pretextured Al in oxalic acid solution. The obtained anodic porous alumina has a defect-free array of straight parallel channels perpendicular to the surface. Thr channel interval could be controlled by changing the interval of the pretextured pattern and the applied voltage. However, the depth at which perfect ordering could be maintained depended on the anodizing conditions, that is, the hole array with a high aspect ratio could be obtained only under thr appropriate anodizing voltage, which corresponded to that of the long-range ordering conditions in the oxalic acid solution. Under the most appropriate condition, ideally ordered channels with an aspect ratio of over 500 could be obtained. From these results, it was concluded that the long-range ordering conditions significantly influenced the growth of channels in anodic porous alumina even in/on the pretextured Al. (C) 2001 The Electrochemical Society. All rights reserved.

 

7.         D. G. Austing, H. Tamura, Y. Tokura, K. Muraki, S. Amaha, K. Ono, and S. Tarucha

            "Single dot and strongly coupled double dots at high magnetic fields"

            Physica E 10 (1-3), 112-116 (2001).

 

            ABSTRACT: Circular vertical semiconductor single quantum dot artificial atoms and strongly coupled double quantum dot artificial molecules are investigated at high magnetic (B-) fields up to 10 T. The phase diagrams showing the evolution with B-field of the conductance peaks reflecting the electro-chemical potential of the N-electron ground states for N from zero to about 20 are compared, and the origin of the differences is discussed. Tn particular, the stability of the spin-polarized maximum density droplet (MDD) at filling factor, v = 1, is much reduced in the double dot system compared to the single dot system. (C) 2001 Elsevier Science B.V. All rights reserved.

 

8.         D. G. Austing, Y. Tokura, S. Tarucha, P. Matagne, and J. P. Leburton

            "Addition energy spectrum of a quantum dot disk up to the third shell"

            Physica E 11 (2-3), 63-67 (2001).

 

            ABSTRACT: The 0 T addition energy spectrum for a high quality circular semiconductor quantum dot artificial atom is measured directly from the Coulomb diamonds up to 12 electrons. We focus on the way electrons fill the third shell. Attention is paid in particular to the entry of the first and second electrons to the third shell levels as deduced from the weak magnetic field dependence. The experimental third shell spectrum is then compared to spectra generated from a 3-dimensional density functional calculation, and a modified parabolic potential and constant interaction model. (C) 2001 Elsevier Science B.V. All rights reserved.

 

9.         Y. Avishai and Y. Tokura

            "Resonant electron transmission through a finite quantum spin chain"

            Phys. Rev. Lett. 8719 (19), art. no.-197203 (2001).

 

            ABSTRACT: Electron transport in a finite one-dimensional quantum spin chain (with ferromagnetic exchange) is studied within an s-d exchange Hamiltonian. Spin transfer coefficients strongly depend on the sign of the s-d exchange constant. For a ferromagnetic coupling, they exhibit a novel resonant pattern, reflecting the salient features of the combined electron-spin system. Spin-flip processes are inelastic and feasible at finite voltage or at finite temperature.

 

10.       G. Bastian and H. Takayanagi

            "Ballistic reflection at a side gate in a superconductor-semiconductor-superconductor structure"

            Physica C 352 (1-4), 219-222 (2001).

 

            ABSTRACT: We have fabricated a sub-micron-sized structure consisting of an InAs-based 2DEG, two narrow Nb leads and a gate, where the indirect ballistic transport between the non-oppositely superconducting contacts can be controlled by the voltage applied to the gate. This new kind of tunable junction can be used for applications and allows several fundamental questions related to the transport mechanism to be studied. First results of experiments carried out in this respect are presented. (C) 2001 Elsevier Science B.V. All rights reserved.

 

11.       D. J. Bottomley

            "Comment on "SiGe intermixing in Ge/Si(100) islands" [Appl. Phys. Lett. 78, 303 (2001)]"

            Appl. Phys. Lett. 79 (7), 1060-1060 (2001).

 

            ABSTRACT:

12.       D. J. Bottomley and T. Ogino

            "Alternative to the Shuttleworth formulation of solid surface stress"

            Phys. Rev. B 63 (16), 165412 (2001).

 

            ABSTRACT: We examine two derivations of the Shuttleworth equation (which is a relation between surface stress g, surface tension gamma, and surface strain), and identify the flaws we perceive. Rectifying the perceived flaws leads not to the Shuttleworth equation but to g-gamma equivalence. We conclude that surface stress is merely the generalization of the concept of surface tension to an elastically anisotropic system; the surface free-energy density is one-half of the trace of the surface stress tensor, to lowest order. In our opinion, our conclusions lead to a more coherent and elegant form of surface thermodynamics which should prove useful in controlling and in understanding nanometer-scale fabrication.

 

13.       D. J. Bottomley, H. Omi, and T. Ogino

            "The stress and strain in cubic films on (113), emphasizing Ge on Si(113)"

            J. Cryst. Growth 225 (1), 16-22 (2001).

 

            ABSTRACT: We derive analytical expressions for the non-zero stress and strain tenser elements of cubic epitaxial films on (1 1 3) surfaces. We have evaluated these expressions for systems with the greatest immediate future technological promise, emphasizing Ge on Si(1 1 3). In this case there are compressive stresses along [3 3 (2) over bar] and [(1) over bar 1 0] of 6.39 and 5.77 GPa, respectively. Comparing with previous experimental work, we conclude that the Ge nanowire growth morphology on Si(1 1 3) elongated along [3 3 (2) over bar] is not caused by film stress and strain effects. Taking other theoretical work into consideration leads to the conclusion that in this case the nanostructure elongation direction arises from the anisotropic surface stress properties of the surface monolayer, including the heteroepitaxial components. (C) 2001 Elsevier Science B.V. All rights reserved.

 

14.       S. De Franceschi, S. Sasaki, J. M. Elzerman, W. G. van der Wiel, S. Tarucha, and L. P. Kouwenhoven

            "Electron cotunneling in a semiconductor quantum dot"

            Phys. Rev. Lett. 86 (5), 878-881 (2001).

 

            ABSTRACT: We report transport measurements on a semiconductor quantum dot with a small number of confined electrons. In the Coulomb blockade regime, conduction is dominated by cotunneling processes. These can be either elastic or inelastic, depending on whether they leave the dot in its ground state or drive it into an excited state, respectively. We are able to discriminate between these two contributions and show that inelastic events can occur only if the applied bias exceeds the lowest excitation energy. Implications to energy-level spectroscopy are discussed.

 

15.       K. J. Friedland, T. Saku, Y. Hirayama, and K. H. Ploog

            "Electron coupling in weakly and strongly coupled quantum point contacts"

            Physica E 11 (2-3), 144-148 (2001).

 

            ABSTRACT: We present electron transport investigations in coupled double quantum point contacts defined by a split gate on the top of a novel narrow-barrier double-quantum-well structure. Using different barrier heights between the wells, we fabricated devices with strong and weak coupling. Mixing of the wavefunctions occurs in the strong coupling device with an (Al,Ga)As barrier, We are able to determine the sublevel spacing energy as well as the symmetric-antisymmetric sublevel splitting energy Delta (SAS). In-plane magnetic fields change the energy diagram of the double quantum point contact giving rise to new crossing and anticrossing states. For the weak coupling device with an AlAs barrier, we find an intriguing reduction of the height of the conductance steps due to the imperfect coupling between ID subbands and the two-dimensional reservoir. This allows us to quantitatively estimate the coupling. (C) 2001 Elsevier Science B.V. All rights reserved.

 

16.       T. Fujii, T. Watanabe, and A. Matsuda

            "Comparative study of transport properties of Bi2Sr2Ca2Cu3O10+delta and Bi2Sr2CaCu2O8+delta single crystals"

            Physica C 357, 173-176 (2001).

 

            ABSTRACT: The doping dependence of the in-plane resistivity rho (a) (T) and the out-of-plane resistivity rho (c) (T) have been systematically measured for the triple-layered system, Bi2Sr2Ca2Cu3O10+delta (Bi-2223). In comparison with the bilayered system, Bi2Sr2CaCu2O8+delta (Bi-2212), we found in Bi-2223 that the superconducting transition temperature T-c and pseudogap formation temperature T*, below which rho (c) shows a typical upturn, do not change from their optimum values in the overdoped region, even though doping actually proceeds. This result suggests that inequivalent hole doping occurs between the outer and inner planes. In the overdoped region, the carriers are mostly doped in the outer plane and the inner plane remains at an optimum doping level. (C) 2001 Elsevier Science B.V. All rights reserved.

 

17.       T. Fujii, T. Watanabe, and A. Matsuda

            "Single-crystal growth of Bi2Sr2Ca2Cu3O10+delta (Bi-2223) by TSFZ method"

            J. Cryst. Growth 223 (1-2), 175-180 (2001).

 

            ABSTRACT: The traveling solvent floating-zone (TSFZ) method has the potential advantage over the usual flux method that crystal growth call be performed at one point on the temperature-composition phase diagram, and provides an opportunity to grow large single crystals even if their crystallization field is very narrow. Here, we report the first successful preparation of the triple-layered system Bi2Sr2Ca2Cu3O10 + delta (Bi-2223) single crystals by improving the TSFZ method condition by applying slow growth velocity and higher temperature gradient. The obtained crystals are as large as 4 x 2 x 0.1 mm(3). X-ray diffraction pattern shows only sharp Bi-2223 peaks, confirming its good crystallinity. DC susceptibility of the as-grown samples exhibits a sharp superconducting transition with T-c = 105 K. After annealing the sample under O-2 flow at 600 degreesC, T-c increased to the maximum value (= 110 K). High-resolution transmission electron microscope images were taken to investigate the phase purity. It confirms that the crystals cleaved from the last part of grown boule are almost single-phase Bi-2223. (C) 2001 Elsevier Science B.V. All rights reserved.

 

18.       M. Fujiki

            "Experimental tests of parity violation at helical polysilylene level"

            Macromol. Rapid Commun. 22 (9), 669-674 (2001).

 

            ABSTRACT: With respect to the origin of bimolecular handedness, a long-standing issue is whether mirror-image molecules are energetically identical. A few theorists presumed that the very tiny, parity-violating weak neutral current at molecular level may distinguish between mirror-image molecules, and proposed several amplification and detection mechanisms. Here we report a possibility of detecting differences in chiroptical and achiral Si-29 NMR and viscometric data of an enantiomeric pair between certain helical organopolysilylenes bearing 96% ee chiral pendants.

 

19.       M. Fujiki

            "Optically active polysilylenes: State-of-the-art chiroptical polymers"

            Macromol. Rapid Commun. 22 (8), 539-563 (2001).

 

            ABSTRACT: Controlled synthesis, chiroptical characterization and manipulation of artificial helical polymers are challenging issues in modern polymer stereochemistry. Although many artificial polymers adopting a preferential screw-sense helical structure have been investigated optically active polysilylenes bearing chiral side chains may be among the most suitable to elucidate the inherent nature of the helical structure, since these polymers offer powerful spectroscopic probes as a result of their ideal chromophoric and fluorophoric main chain properties around 300-330 nm. The present paper will review comprehensively the helix-property-functionality relationship between side chain structure, global and local main chain conformation, (chir)optical properties, electronic properties, several helical cooperative phenomena, the effects of temperature and solvent polarity, and molecular imaging. This knowledge and understanding of the nature of the polysilylene helix might constitute a bridge between artificial polymers and biopolymers and will assist in designing and controlling new types of helical polymers directed to diverse screw-sense-related properties and applications in the future.

 

20.       M. Fujiki, J. R. Koe, M. Motonaga, H. Nakashima, K. Terao, and A. Teramoto

            "Computing handedness: Quantized and superposed switch and dynamic memory of helical polysilylene"

            J. Am. Chem. Soc. 123 (26), 6253-6261 (2001).

 

            ABSTRACT: Two new conjugating helical polymers comprising a rodlike silicon backbone and enantiopure chiral pendants, poly{(R)-3,7-dimethyloctyl-(S)-3-methylpentylsilyle} (PS.1) and its diastereomeric poly {(S)-3,7-dimethyloctyl-(S)-3-methylpentylsilylene} (PS-2), were prepared. Molecular mechanics calculations of PS-1 and PS-2 model oligomers indicated a double well potential energy curve corresponding to almost enantiomeric helices with dihedral angles of 150-160 degrees (P-motif, global minimum) and 200-210 degrees (M-motif), regardless of their tacticity. Experimentally, it was found that PS-I in dilute isooctane revealed switchable ambidextrous helicity on application of a thermal energy bias. Although PS-1 featured three distinct switching regions, viz. "region 1, between -80 and -10 degreesC", "region 2, between -10 and +10 degreesC", and "region 3, between +10 OC and +80 degreesC", the switching properties were interpreted as the result of superposed P- and M-helicities, undergoing dynamic pseudo-racemization or oscillation. Oscillating helicity in region 2 was roughly estimated to be about 13 cm(-1). The superposed helicity in region 2 was critical since it afforded molecular recognition ability with a dynamic memory function that was highly susceptible to solvent molecular topology and volume fraction. This could lead to potential as a molecular information processor to serve as a gauge of chemical properties. On the other hand, PS-2 could not switch its preferential screw-sense in the range of -80 to +80 degreesC. This may be related to greater differences the potential energy curve between P- and M-motifs.

 

21.       M. Fujiki, M. Motonaga, H. Z. Tang, K. Torimitsu, Z. B. Zhang, J. R. Koe, J. Watanabe, K. Terao, T. Sato, and A. Teramoto

            "A new screw-sense switchable polysilylene with quantized and superposed helicities"

            Chem. Lett. (12), 1218-1219 (2001).

 

            ABSTRACT: A new rod-like polysilylene bearing a P-branched alkyl group, poly [(S)-3,7-dimethyloctyl(2-ethylbutyl)silylene], was found to undergo a thermo-driven, helix-helix transition at -7 degreesC in isooctane associated with the discontinuous changes in the singlet sigma-sigma* excitation energy in the silicon backbone in the switching temperature region.

 

22.       T. Fujisawa, Y. Tokura, and Y. Hirayama

            "Energy relaxation process in a quantum dot studied by DC current and pulse-excited current measurements"

            Physica B-Condensed Matter 298 (1-4), 573-579 (2001).

 

            ABSTRACT: Energy relaxation from an excited to a ground state in a quantum dot is studied by DC current and pulse-excited current measurements. We find that all the excited states can be classified as short-lived or long-lived excited states, although they are too short or too long for us to determine the actual lifetime with our method. The relaxation time is short (less than a few ns) if the transition takes place by means of electron-phonon interaction, while it is very long (more than a few mus) if the transition involves a spin-flip. The crossover between long and short relaxation time is studied at a particular level crossing. (C) 2001 Elsevier Science B.V. All rights reserved.

 

23.       T. Fujisawa, Y. Tokura, and Y. Hirayama

            "Transient current spectroscopy of a quantum dot in the Coulomb blockade regime"

            Phys. Rev. B 6308 (8), 081304 (2001).

 

            ABSTRACT: Transient current spectroscopy is proposed and demonstrated in order to investigate the energy relaxation inside a quantum dot in the Coulomb blockade regime. We employ a fast pulse signal to excite an AlGaAs/GaAs quantum dot to an excited state, and analyze the nonequilibrium transient current as a function of the pulse length. The amplitude and time constant of the transient current are sensitive to the ground and excited spin states. We find that the spin-relaxation time is longer than, at least, a few mus.

 

24.       A. Fujiwara and Y. Takahashi

            "Manipulation of elementary charge in a silicon charge-coupled device"

            Nature 410 (6828), 560-562 (2001).

 

            ABSTRACT: The ultimate limit in the operation of an electronic device is the manipulation of a single charge, Such a limit has been achieved in single-electron tunnelling devices(1,2). However, these devices are based on multiple tunnel barriers and conductive islands, which are complex structures to fabricate. Here we demonstrate another type of device that can also manipulate elementary charge, but which is more suitable for large-scale integration, The device consists of two closely packed silicon wire-MOSFETs, which are commonly used building blocks of electronic circuits. We have developed a scheme to generate and store holes in the channels of either of these MOSFETs, Subsequently, holes can be transferred between the two MOSFETs at the level of an elementary charge, and their exact position can be monitored. This single-charge transfer device, which is operated at 25 K, is in effect a charge-coupled device(3), This is also the first realization of a silicon-based device that manipulates elementary charge.

 

25.       K. Hashimoto, S. Miyashita, T. Saku, and Y. Hirayama

            "Back-gated point contact"

            Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 40 (4B), 3000-3002 (2001).

 

            ABSTRACT: In this study, we fabricated split gates on a back-gated undoped heterostructure to perform transport measurements on a quantum point contact (QPC). Using this back-gated QPC system, we obtained clear quantized conductance plateaus over a wide range of electron densities (1.2 x 10(15) m(-2) to 3.3 x 10(15) m(-2)) without being affected by imperfections. We also observed a clear 0.7 G(0) (G(0) = 2e(2)/h) anomaly. The shape of the anomaly varies with electron density. Furthermore, the conductance value at which this anomaly appears is not precisely 0.7 Go but changes with electron density.

 

26.       K. Hashimoto, K. Muraki, T. Saku, and Y. Hirayama

            "Longitudinal resistance anomaly around the 2/3 filling factor observed in a GaAs/AlGaAs single heterostructure"

            Physica B-Condensed Matter 298 (1-4), 191-194 (2001).

 

            ABSTRACT: A pronounced R, enhancement near v = 2/3 is observed in a back-gated GaAs/AlGaAs single heterostructure when the magnetic field for v = 2/3(B-2/3) is set less than 6 T using the back gate, but it almost vanishes for B-2/3 > 8 T. This tendency is consistent with a model in which the mixed state consisting of two different spin domains plays an important role for the anomalous R-xx enhancement. By taking advantage of the back-gated operation, we find a quick recovery of the R-xx enhancement in spite of prior electron depletion, and the long relaxation time of the R-xx enhancement. These results support the argument that the R-xx enhancement is not memorized by the electron system but by the alternative system such as the nuclear spin configuration leading to a long relaxation time. (C) 2001 Elsevier Science B.V. All rights reserved.

 

27.       H. B. Heersche, T. Schapers, J. Nitta, and H. Takayanagi

            "Enhancement of spin injection from ferromagnetic metal into a two-dimensional electron gas using a tunnel barrier"

            Phys. Rev. B 64 (16), 161307 (2001).

 

            ABSTRACT: Using free electron approximation, we calculated the spin dependent tunnel conductance of ballistic ferromagnet/tunnel barrier/two-dimensional electron gas (FM/I/2DEG) junctions and FM/I/2DEG/I/FM double junctions for different barrier strengths. We find that a tunnel barrier improves spin injection considerably. For sufficiently strong barriers, it is predicted that the tunnel conductance ratio between spin up and spin down channels is, in first approximation, equal to the ratio between their Fermi velocities in the FM. For single junctions, this results in a significant current polarization (similar to 10%). This corresponds to a relative resistance change of several percent between parallel and antiparallel magnetization of the two FM electrodes, respectively, for the double junction. In the weak barrier regime, the magnitude and sign of the current polarization are strongly dependent on the (controllable) electron density in the 2DEG.

 

28.       S. Heun, Y. Watanabe, B. Ressel, D. Bottomley, T. Schmidt, and K. C. Prince

            "Core-level photoelectron spectroscopy from individual heteroepitaxial nanocrystals on GaAs(001)"

            Phys. Rev. B 63 (12), art. no.-125335 (2001).

 

            ABSTRACT: Con-level spectra of individual heteroepitaxial nanocrystals were measured with a spectroscopic photoemission and low-energy electron microscope that allows laterally resolved photoemission spectroscopy. The nanocrystals were obtained by depositing nominally 2 monolayers (ML) of InAs on a Se-tenninated GaAs(001) surface. The Se-termination of GaAs results in the formation of a 2-3-ML-thick film of Ga2Se3 on top of bulk GaAs. During heteroepitaxy the InAs reacts with the Ga,Se: A phase separation takes place on the anion sublattice, while an alloying takes place on the cation sublattice. During the initial stages of growth, a submonolayer-thick wetting layer of In(x)Ga(1-x)A(s) is formed that is covered by (InyGa1-y)(2)Se-3. (InyGa1-y)(2)Se-3-covered InAs nanocrystals are formed on this surface.

 

29.       S. Heun, Y. Watanabe, B. Ressel, T. Schmidt, and K. C. Prince

            "Valence band alignment and work function of heteroepitaxial nanocrystals on GaAs(001)"

            J. Vac. Sci. Technol. B 19 (6), 2057-2062 (2001).

 

            ABSTRACT: The differences in valence band structure and work function between heteroepitaxial nanocrystals and the surrounding substrate were measured with a spectroscopic photoemission and low energy electron microscope which allows laterally resolved photoemission spectroscopy. The nanocrystals were obtained by depositing nominally 2 and 4 monolayers (ML) of InAs on a Se-terminated GaAs(001) surface. The samples showed differences in the valence band edge energy and work function both between nanocrystals and substrate as well as between 2 and 4 ML. We suggest that Se termination of the nanocrystals is the reason for these differences. (C) 2001 American Vacuum Society.

 

30.       H. Hibino, C. W. Hu, T. Ogino, and I. S. T. Tsong

            "Diffusion barrier caused by 1 x 1 and 7 x 7 on Si(111) during phase transition"

            Phys. Rev. B 64 (24), 245401 (2001).

 

            ABSTRACT: Low-energy electron microscopy was used to study thermal decay of islands and vacancy islands on Si(111) at the coexistence of 1 x 1 and 7 x 7 during the phase transition. Slower surface mass diffusion on 7 x 7 than on 1 x 1, coupled with preferential formation of 7 x 7 on the upper terrace of a step, causes a diffusion barrier during the phase transition, resulting in an asymmetry in the thermal decay rate between island and vacancy island. The diffusion barrier also makes the mass flow induced by the atom density difference between 1 x 1 and 7 x 7 asymmetric in the step-up and step-down directions.

 

31.       H. Hibino, C. W. Hu, T. Ogino, and I. S. T. Tsong

            "Decay kinetics of two-dimensional islands and holes on Si(111) studied by low-energy electron microscopy"

            Phys. Rev. B 6324 (24), 245402 (2001).

 

            ABSTRACT: Low-energy electron microscopy was used to study the decay kinetics of two-dimensional islands and holes with diameters of a few micrometers to similar to 100 nm on Si(111) near the "1x1'' to 7x7 phase-transition temperature. It is shown that the decay is governed by the diffusion of adatoms rather than the attachment/detachment of atoms at steps. The comparison between the island and hole decays gives an upper limit for the Ehrlich-Schwoebel barrier. We also show that the surface mass diffusion constant on 7x7 is smaller than that on "1x1".

 

32.       H. Hibino and T. Ogino

            "Growth of Si twinning superlattice"

            Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 87 (3), 214-221 (2001).

 

            ABSTRACT: We review our attempts to grow a new type of single crystal Si epitaxially on Si(111). The new single crystal Si is a superlattice of layers that have twinned and untwinned orientations with respect to the substrate, and is called a twinning superlattice. The Si twinning superlattices are grown using the fact that Si layers grown epitaxially on Si(111) root3 x root3-B have a twinned orientation with respect to the substrate. We investigate conditions for growing twinned epitaxial layers, and clarify that growth of twinned layers requires high surface B concentration and low densities of surface structural defects, such as steps and domain boundaries of the root3 x root3 reconstruction. We also investigate the thermal stability of the twinned layers, and clarify that the temperature at which the twinned layers are transformed into untwinned layers strongly depends on the thickness. Additionally, we establish a technique for measuring crystallographic orientations in surface regions in situ during growth. Based on these results, we finally succeed in growing the Si twinning superlattice by repeated growth of Si with a unit thickness and post-growth annealing. (C) 2001 Elsevier Science B.V. All rights reserved.

 

33.       R. J. A. Hill, A. Patane, P. C. Main, L. Eaves, B. Gustafson, M. Henini, S. Tarucha, and D. G. Austing

            "Magnetotunneling spectroscopy of an individual quantum dot in a gated tunnel diode"

            Appl. Phys. Lett. 79 (20), 3275-3277 (2001).

 

            ABSTRACT: We use an array of gate electrodes to control the electrostatic profile in a layer of self-assembled InAs quantum dots. In combination with magnetotunneling spectroscopy, this allows us to identify and measure the energy levels and wave functions associated with the ground and excited state of an individual quantum dot. (C) 2001 American Institute of Physics.

 

34.       Y. Hirayama, K. Muraki, A. Kawaharazuka, K. Hashimoto, and T. Saku

            "Backgated layers and nanostructures"

            Physica E 11 (2-3), 155-160 (2001).

 

            ABSTRACT: The characteristics of a novel backgated two-dimensional electron gas (2DEG) system and its application to layers and nanostructures are summarized. In the backgated system, the 2DEG is formed in an undoped heterostructure through a field effect, and we can expect high-mobility and precise controllability. We have successfully fabricated backgated electron bilayer systems, quantum point contacts and lateral periodic structures with a short period. The results clearly show that a backgated system has much potential for application to layers and nanostructures. (C) 2001 Elsevier Science B.V. All rights reserved.

 

35.       Y. Homma, P. Finnie, and M. Uwaha

            "Morphological instability of atomic steps observed on Si(111) surfaces"

            Surf. Sci. 492 (1-2), 125-136 (2001).

 

            ABSTRACT: We investigate the morphological instability of step motion due to the size asymmetry of terraces on either side of an atomic step. Atomic step morphology is observed by the in situ scanning electron microscopy of large Si(1 1 1) terraces that have step spacings comparable to the adatom diffusion length. Quantitative experimental results are compared with linear stability theory. For both growth and sublimation, step wandering occurs when the surface flux to or from the terrace preceding a step dominates over the surface flux to or from the terrace that lags behind the step. The wavelength associated with the wandering of an initially straight step is measured as a function of the impinging flux. The amplitude of wandering for a circular step is measured as a function of the terrace size. These quantities showed reasonable agreement with the theoretical predictions. (C) 2001 Elsevier Science B.V. All rights reserved.

 

36.       S. Horiguchi, M. Nagase, K. Shiraishi, H. Kageshima, Y. Takahashi, and K. Murase

            "Mechanism of potential profile formation in silicon single-electron transistors fabricated using pattern-dependent oxidation"

            Jpn. J. Appl. Phys. Part 2 - Lett. 40 (1AB), L29-L32 (2001).

 

            ABSTRACT: The origin of the potential profile in silicon single-electron transistors (SETs) fabricated using pattern-dependent oxidation (PADOX) is investigated by making use of the geometric structure measured by atomic force microscope (AFM), the bandgap reduction due to compressive stress generated during PADOX obtained using the first-principles calculation, and the effective potential method. A probable mechanism for the formation of the potential profile responsible for SET operation is proposed, The width reduction in the silicon wire region in the SET produces a tunnel barrier, while the compressive stress lowers the bottom of the conduction band through the bandgap reduction and forms a potential well corresponding to an island in the tunnel barrier.

 

37.       C. M. Hu, J. Nitta, A. Jensen, J. B. Hansen, and H. Takayanagi

            "Spin-polarized transport in a two-dimensional electron gas with interdigital-ferromagnetic contacts"

            Phys. Rev. B 63 (12), art. no.-125333 (2001).

 

            ABSTRACT: Ferromagnetic contacts on a high-mobility, two-dimensional electron gas (2DEG) in a narrow gap semiconductor with strong spin-orbit interaction are used to investigate spin-polarized electron transport. We demonstrate the use of magnetized contacts to preferentially inject and detect specific spin orientations. Spin dephasing and spin precession effects are studied by temperature and 2DEG channel length dependent measurements. Interdigital-ferromagnetic contacts suppress unwanted effects due to ferromagnetic microstrip inhomogeneities by averaging.

 

38.       C. W. Hu, H. Hibino, T. Ogino, and I. S. T. Tsong

            "Hysteresis in the (1 x 1)-(7 x 7) first-order phase transition on the Si(111) surface"

            Surf. Sci. 487 (1-3), 191-200 (2001).

 

            ABSTRACT: We used low-energy electron microscopy to study the (1 x 1)-(7 x 7) phase transition on step-free surfaces in two-dimensional (2D) bilayer vacancy islands and islands formed on the surface of Si(111). A small and yet distinct hysteresis effect is found in the phase transition on these step-free surfaces compared to the transition on surfaces with regularly spaced steps where no hysteresis is observed. The occurrence of hysteresis on step-free surfaces is explained thermodynamically by the consideration of the change in Gibbs free energy in the phase transition based on a 2D nucleation theory. (C) 2001 Elsevier Science B.V. All rights reserved.

 

39.       H. Inokawa, A. Fujiwara, and Y. Takahashi

            "Multipeak negative-differential-resistance device by combining single-electron and metal-oxide-semiconductor transistors"

            Appl. Phys. Lett. 79 (22), 3618-3620 (2001).

 

            ABSTRACT: A multipeak negative-differential-resistance device is proposed. The device comprises a single-electron transistor (SET) and a metal-oxide-semiconductor field-effect transistor (MOSFET), and can, in principle, generate an infinite number of current peaks. Operation of the proposed device is verified at 27 K with a SET fabricated by the pattern-dependent oxidation process and a MOSFET on the same silicon-on-insulator wafer. Six current peaks and a peak-to-valley current ratio of 2.1 are obtained, and multiple-valued memory operation is successfully demonstrated. (C) 2001 American Institute of Physics.

 

40.       K. Inoue

            "Suppression of level fluctuation without extinction ratio degradation based on output saturation in higher order optical parametric interaction in fiber"

            IEEE Photonics Technol. Lett. 13 (4), 338-340 (2001).

 

            ABSTRACT: Optical limiting operation based on output saturation in higher order parametric interaction ill dispersion-shifted fiber is reported. The output powers in a parametric amplifier configuration were measured as a function of the signal input power, with the result that new-wavelength light is efficiently generated in addition to the pump, signal, and idler, resulting from a higher order parametric process, and its output power initially increases quardratically and then saturates as the input power increases. By utilizing this output property of the higher order parametric light, suppression of level fluctuation with an improved extinction ratio was demonstrated.

 

41.       K. Inoue and T. Mukai

            "Signal-masked optical communication utilizing amplified spontaneous emission of an optical amplifier"

            Opt. Commun. 197 (1-3), 53-59 (2001).

 

            ABSTRACT: Signal-masked optical communication utilizing amplified spontaneous emission (ASE) from an optical amplifier is proposed and demonstrated. Data to be transmitted is embedded in spontaneous-spontaneous beat noise of the ASE light and is demodulated by differentially detecting the signal and reference channels. The feasibility of the scheme is demonstrated by experiments, where 622-Mbit/s data was successfully recovered at the receiver while the transmitted data was concealed in the signal channel alone. (C) 2001 Elsevier Science B.V. All rights reserved.

 

42.       K. Inoue and T. Mukai

            "Spectral hole in the amplified spontaneous emission spectrum of a fiber optical parametric amplifier"

            Opt. Lett. 26 (12), 869-871 (2001).

 

            ABSTRACT: The amplified spontaneous emission (ASE) spectrum of a fiber optical parametric amplifier in a gain-saturation condition is studied. We experimentally observed holes around the signal and the idler in the ASE spectrum of a deeply gain-saturated amplifier. This observation is explained by a theoretical model that includes parametric interactions among ASE components and the pump, signal, and idler beams. (C) 2001 Optical Society of America.

 

43.       K. Inoue and T. Mukai

            "Signal wavelength dependence of gain saturation in a fiber optical parametric amplifier"

            Opt. Lett. 26 (1), 10-12 (2001).

 

            ABSTRACT: Signal gain-saturation characteristics in a fiber optical parametric amplifier are described. Experiments and calculations for the high-gain-saturation region are presented in which the direction of optical power transfer, which is from the pump to the signal and the idler in the unsaturated region, becomes reversed at high-signal-input powers. It is shown that the signal gain soon saturates for signal wavelengths far from the pump. This wavelength dependence is explained by the power-dependent phase mismatch and its behavior in the high-signal-input region. (C) 2001 Optical Society of America OCIS codes: 060.0370, 190.4370, 190.4410, 190.4970.

 

44.       T. Ito, K. Shiraishi, and A. Taguchi

            "A simple approach to structural stability of semiconductors and their interfaces"

            J. Cryst. Growth 227, 366-370 (2001).

 

            ABSTRACT: The relative stability between wurtzite and zinc-blende structures in semiconductors is systematically investigated using a simple formula for calculating the energy difference between them. Using the well-known values of ionicity and lattice parameter for semiconductors, the energy formula gives good estimates of thc energy difference for group IV, III-V and II-VI semiconductors. The calculated energy differences agree well with those obtained by ab initio calculations ill thr literature. A simple criterion for wurtzite-zinc-blende polytypes in semiconductors is extracted in terms of ionicity from the formula. The criterion is also applied to the structural stability of GaN at various hetero-interfaces. (C) 2001 Elsevier Science B,V. All rights reserved.

 

45.       W. Izumida, O. Sakai, and S. Tarucha

            "Tunneling through a quantum dot in local spin singlet-triplet crossover region with Kondo effect"

            Phys. Rev. Lett. 8721 (21), art. no.-216803 (2001).

 

            ABSTRACT: Tunneling conductance through a quantum dot is calculated around the local spin singlet-triplet crossover region including the Kondo effect. The calculation is carried out using the numerical renormalization group method. When the potential on the dot deepens, two electrons filling a lower energy orbital redistribute to gain Hund's coupling energy. This redistribution induces a bump in the conductance between the Coulomb peaks. The Kondo temperature on the bump is high due to the fluctuation on the singlet-triplet crossover region. The behaviors agree well with recent experiment.

 

46.       H. Kageshima, M. Uematsu, and K. Shiraishi

            "Theory of thermal Si oxide growth rate taking into account interfacial Si emission effects"

            Microelectron. Eng. 59 (1-4), 301-309 (2001).

 

            ABSTRACT: A novel theory for the thermal silicon oxide growth rate is constructed based on a new picture of the oxidation process and its efficiency is theoretically discussed on the basis of both analytical and numerical approaches. In the picture, silicons are massively emitted from the oxide/silicon interface into the oxide during the growth process to release the large strain caused by a volume expansion from silicon to oxide at the interface. The flow of the emitted silicons controls the oxidation reaction rate at the interface as well as the flow of the oxidant. Our picture can consistently explain faults in the classical Deal-Grove picture, such as the failure to explain the initial enhanced oxidation. (C) 2001 Elsevier Science B.V. All rights reserved.

 

47.       H. Kamada, H. Gotoh, J. Temmyo, T. Takagahara, and H. Ando

            "Exciton Rabi oscillation in a single quantum dot"

            Phys. Rev. Lett. 8724 (24), art. no.-246401 (2001).

 

            ABSTRACT: A spectroscopic method, which enables characterization of a single isolated quantum dot and a quantum wave function interferometry, is applied to an exciton discrete excited state in an InGaAs quantum dot. Long coherence of zero-dimensional excitonic states made possible the observation of coherent population flopping in a 0D excitonic two-level system in a time-domain interferometric measurement. Corresponding energy splitting is also manifested in an energy-domain measurement.

 

48.       Y. Kangawa, T. Ito, A. Taguchi, K. Shiraishi, and T. Ohachi

            "A new theoretical approach to adsorption-desorption behavior of Ga on GaAs surfaces"

            Surf. Sci. 493 (1-3), 178-181 (2001).

 

            ABSTRACT: We propose a new theoretical approach for studying adsorption-desorption behavior of atoms on semiconductor surfaces. The new theoretical approach based on the ab initio calculations incorporates the free energy of gas phase; therefore we can calculate how adsorption and desorption depends on growth temperature and beam equivalent pressure (BEP). The versatility of the new theoretical approach was confirmed by the calculation of Ga adsorption-desorption transition temperatures and transition BEPs on the GaAs(001)-(4 x 2)beta2 Ga-rich surface. This new approach is feasible to predict how adsorption and desorption depend on the growth conditions. (C) 2001 Elsevier Science B.V. All rights reserved.

 

49.       K. Kanisawa, M. J. Butcher, Y. Tokura, H. Yamaguchi, and Y. Hirayama

            "Local density of states in zero-dimensional semiconductor structures"

            Phys. Rev. Lett. 8719 (19), 196804 (2001).

 

            ABSTRACT: The local density of states (LDOS) within tetrahedral InAs structures, formed at the surface of InAs/GaAs(111)A, has been characterized using low-temperature scanning tunneling microscopy. The LDOS of the lowest four zero-dimensional (0D) discrete levels have been imaged in structures with a comparable size to the electron wavelength. The LDOS inside the structures is observed to be higher than that of the surrounding area at intervals of the level separation. This feature indicates the singularity of the LDOS close to the 0D resonant levels.

 

50.       K. Kanisawa, M. J. Butcher, H. Yamaguchi, and Y. Hirayama

            "Imaging of friedel oscillation patterns of two-dimensionally accumulated electrons at epitaxially grown InAs(111)A surfaces"

            Phys. Rev. Lett. 86 (15), 3384-3387 (2001).

 

            ABSTRACT: The local density of states (LDOS) at the epitaxially grown InAs surface on a GaAs(111)A substrate were characterized using low-temperature scanning tunneling microscopy. Using dI/dV signal mapping, LDOS standing waves were clearly imaged at point defects and within nanostructures. Measurement of the wavelength as a function of bias voltage showed a nonparabolic dispersion relation for the conduction band. The observed wave features originate from the Friedel oscillations of the two-dimensional electron gas in the semiconductor surface accumulation layer.

 

51.       S. Karimoto, K. Ueda, M. Naito, and T. Imai

            "Single-crystalline superconducting thin films of electron-doped infinite-layer compounds grown by molecular-beam epitaxy"

            Appl. Phys. Lett. 79 (17), 2767-2769 (2001).

 

            ABSTRACT: Single-crystalline Sr1-xLaxCuO2 thin films of electron-doped infinite-layer compounds were grown by molecular-beam epitaxy. Crucial to our success was the use of KTaO3 substrates. The best film showed T-c(onset)=41.5 K and T-c(zero)=39.0 K, which is close to the highest T-c(onset) of 43 K for the bulk value. The resistivity of the optimum-doped film exhibited metallic temperature dependence with a low resistivity of 320 mu Omega cm at room temperature and 120 mu Omega cm just above T-c. (C) 2001 American Institute of Physics.

 

52.       S. Karimoto, H. Yamamoto, T. Greibe, and M. Naito

            "New superconducting Sr2CuO4-delta thin films prepared by molecular beam epitaxy"

            Jpn. J. Appl. Phys. Part 2 - Lett. 40 (2B), L127-L130 (2001).

 

            ABSTRACT: We report the synthesis of new superconducting Sr2CUO4-delta thin films (T-c(onset) similar to 75 K) using molecular beam epitaxy (MBE). This superconductor has a tetragonal structure with an in-plane lattice constant a(o) similar to 3.79 Angstrom and a c-axis lattice constant c(o) similar to 13.55 Angstrom. This c-axis lattice constant is significantly larger than previously reported for this system. Strong oxidation using ozone gas at a low temperature of around 350 degreesC is essential to the realization of phase conversion from the orthorhombic insulating phase (Sr2CuO3) to the tetragonal superconducting phase (Sr2CuO4-delta).

 

53.       N. Kasai, Y. Jimbo, O. Niwa, T. Matsue, and K. Torimitsu

            "Real-time multisite observation of glutamate release in rat hippocampal slices"

            Neurosci. Lett. 304 (1-2), 112-116 (2001).

 

            ABSTRACT: A multichannel glutamate sensor was fabricated that consists of enzyme modified electrodes and has a high sensitivity and selectivity to glutamate. We placed a rat hippocampal slice on the sensor and monitored the current at four electrodes resulting from the stimulation with muscimol, a gamma -aminobutyric acid(A) (GABA(A)) receptor agonist. We obtained different glutamate concentration increases at the different positions, suppressed by bicuculline, a GABA(A) receptor antagonist. This demonstrated that the sensor can monitor the glutamate released via GABA(A) receptors pathways, and the difference in the concentrations may indicate differences in the distribution of GABA(A) receptor as well as diverse receptor functions. This multichannel sensor may be useful for non-invasive, real-time monitoring of glutamate distribution, which would make it a valuable tool for pharmacological analysis. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.

 

54.       Y. Kasai, S. Tanda, N. Hatakenaka, and H. Takayanagi

            "Fluxon dynamics in isolated long Josephson junctions"

            Physica C 352 (1-4), 211-214 (2001).

 

            ABSTRACT: We investigate fluxon dynamics in isolated long Josephson junctions. In contrast to usual long Josephson junctions, where fluxons are described by a sine-Gordon equation, fluxons in the isolated junction obey a double sine-Gordon (DSG) equation. Due to the competition between the two sine terms in the DSG equation, pi-pi link oscillations appear as a unique feature of DSG system. We numerically study these new oscillations for the purpose of finding junction parameters in experimental observations. (C) 2001 Elsevier Science B.V. All rights reserved.

 

55.       M. Kasu and N. Kobayashi

            "Spontaneous ridge formation and its effect on field emission of heavily Si-doped AlN"

            Phys. Status Solidi A-Appl. Res. 188 (2), 779-782 (2001).

 

            ABSTRACT: From the linear relation between the anode-sample distance and the voltage, we have accurately obtained the net electric field between the anode and the sample surface. When Si is doped heavily into AlN. ridge structures with nanometer-order sharpness form spontaneously due to the facet growth. We found that the net electric field decreased with Si-dopant density (N-Si). The highest field emission (FE) current density was 0.22 A/cm(2). Numerical calculations show that a ridge lowers the energy barrier by about 1.5 eV, and this can explain well the ridge effect on FE.

 

56.       M. Kasu and N. Kobayashi

            "Field-emission characteristics and large current density of heavily Si-doped AlN and AlxGa1-xN (0.38 <= x < 1)"

            Appl. Phys. Lett. 79 (22), 3642-3644 (2001).

 

            ABSTRACT: From a linear relation between the applied bias and the anode-sample distance, the electric field necessary for field emission (FE) can be obtained reproducibly. For heavily Si-doped AlN and AlxGa1-xN (0.38 less than or equal tox <1), the band gap (Al-mole fraction, x), Si-dopant density (N-Si), and thickness dependences of the field emission are investigated. After optimizing the sample structure, we obtained a FE current density of 0.22 A/cm(2) with a 0.3-mm-diameter rod anode from 0.8-mum-thick Si-doped (N-Si:1x10(21) cm(-3)) AlN. (C) 2001 American Institute of Physics.

 

57.       M. Kasu and N. Kobayashi

            "Spontaneous ridge-structure formation and large field emission of heavily Si-doped AlN"

            Appl. Phys. Lett. 78 (13), 1835-1837 (2001).

 

            ABSTRACT: Sharp ridge structures with a 3 nm wide (0001) top facet and {1 (1) over bar 01} sidewall facets formed on the surface of a heavily Si-doped AlN layer on a 6H-SiC (0001) substrate during metalorganicvapor-phase-epitaxy growth. This is caused by {1 (1) over bar 01} facet growth induced by heavy Si doping. We obtained a large field emission (FE) current density of 11 mA/cm(2) at 84 V/mum. One of the reasons for the large FE is that the ridge-structure formation decreases the energy barrier necessary for FE by about 2.4 eV. (C) 2001 American Institute of Physics.

 

58.       M. Kasu, Y. Taniyasu, and N. Kobayashi

            "Formation of solid solution of Al1-xSixN (0 < x <= 12%) ternary alloy"

            Jpn. J. Appl. Phys. Part 2 - Lett. 40 (10A), L1048-L1050 (2001).

 

            ABSTRACT: When Si was doped into an AlN layer during metalorganic vapor-phase epitaxial growth, the Al density (N-Al) in the AlN layer decreased but the N density (N-N) did not change. The decrease in N-Al was almost the same as the Si density (N-si). As N-si increased in Si-doped AlN, the lattice constant decreased. These results can be explained by Si atoms replacing Al atoms in Si-doped AlN and subsequent Si-N bond formation. Thus, Si-doped AlN becomes a substitutional solid solution of Al1-xSixN ternary alloy. The highest Si density at which the x-ray diffraction peak still appears was 5.8 x 10(21) cm(-3) (x = 12%).

 

59.       A. Kawaharazuka, T. Saku, C. A. Kikuchi, Y. Horikoshi, and Y. Hirayama

            "Free GaAs surfaces studied using a back-gated undoped GaAs/AlxGa1-xAs heterostructure"

            Phys. Rev. B 6324 (24), art. no.-245309 (2001).

 

            ABSTRACT: We study the free GaAs surface by using a back-gated undoped GaAs/AlxGa1-xAs heterostructure. This structure is suitable in investigating the free GaAs surface since a two-dimensional electron gas is induced by the back-gate bias in the undoped heterostructure. We compare the channel depth dependence of the transport characteristics with two different models of the free GaAs. The "midgap pinning model" assumes a constant surface Fermi lever and an alternative approach called the "frozen surface model" assumes a constant surface charge density. The experimental results indicate that the frozen surface model appropriately describes free GaAs surfaces at low temperatures although the midgap pinning model is widely accepted. This is because charges cannot be transferred to the free GaAs surface at low temperatures.

 

60.       A. Kawaharazuka, T. Saku, Y. Tokura, Y. Horikoshi, and Y. Hirayama

            "Transport characteristics of electrons in weak short-period two-dimensional potential arrays"

            Appl. Phys. Lett. 79 (3), 427-429 (2001).

 

            ABSTRACT: We study the transport characteristics of electrons in weak short-period two-dimensional potential arrays formed on a back-gated undoped GaAs/AlGaAs heterostructure with a shallow channel. The period of the potential is as short as 50 nm. We achieve the condition where the unit cell is filled by less than one electron. In addition to the conventional magnetoresistance characteristics, we have found a feature that is determined solely by electron density. This feature reflects the strong Coulomb interaction between the electrons confined in the potential arrays. (C) 2001 American Institute of Physics.

 

61.       A. V. Khaetskii

            "Spin relaxation in semiconductor mesoscopic systems"

            Physica E 10 (1-3), 27-31 (2001).

 

            ABSTRACT: Phonon-assisted spin-flip transitions between the Zeeman sublevels are considered for two different mesoscopic systems: GaAs quantum dots (localized states) and the 2D electron system in the quantum Hall-regime for filling factor 1 (delocalized states). The phonon-assisted spin-flip rates for all spin-orbit-related mechanisms are evaluated. The role of the localization of the electron states in the effectiveness of the spin-lattice relaxation and the different role of the Coulomb interaction for these two systems are discussed. Tt is shown, for example, that spin-lattice relaxation for the electron localized in a quantum dot is much less effective than for the free electron. Besides, the spin-flip rates due to several other mechanisms not related to the spin-orbit interaction are estimated for the dot electron. (C) 2001 Elsevier Science B.V. All rights reserved.

 

62.       T. Kimura

            "Conductance renormalization and conductivity of a multisubband Tomonaga-Luttinger model"

            Phys. Rev. B 64 (23), 233306 (2001).

 

            ABSTRACT: We studied the conductance renormalization and conductivity of multisubband Tomonaga-Luttinger models with intersubband interactions. We found that, as in single-band systems, the conductance of a multisubband system with an arbitrary number of subbands is not renormalized due to interaction between electrons. We derived a formula for the conductivity in multisubband models. We applied it to a simplified case and found that intersubband interaction enhances the conductivity, which is contrary to the intrasubband repulsive interaction, and that the conductivity is further enhanced for a larger number of subbands.

 

63.       S. Kleefisch, B. Welter, A. Marx, L. Alff, R. Gross, and M. Naito

            "Possible pseudogap behavior of electron-doped high-temperature superconductors"

            Phys. Rev. B 6310 (10), art. no.-100507 (2001).

 

            ABSTRACT: We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd1.85Ce0.15CuO4-y and Pr1.85Ce0.15CuO4-y as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature T-c. In contrast, below T-c for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to the superconducting energy gap below T-c. Possible interpretations of this observation are the existence of a normal-state pseudogap in the electron doped HTS or the existence of a spatially nonuniform superconducting state.

 

64.       J. R. Koe, M. Fujiki, M. Motonaga, and H. Nakashima

            "Cooperative helical order in optically active poly(diarylsilylenes)"

            Macromolecules 34 (4), 1082-1089 (2001).

 

            ABSTRACT: Microscopic helical order in poly(diarylsilylene) copolymers containing enantiopure chiral (S)-2-methylbutylphenyl and achiral n-butylphenyl side chains results in macroscopically observable optical activity. These polymers are shown to adopt helical backbone conformations with a prevailing screw sense in solution through cooperative side chain interactions. Ultraviolet (UV) and circular dichroism (CD) spectroscopic studies over the temperature range -70 to 80 degreesC indicate temperature dependence of the dissymmetric ratio, g(abs) For copolymers of the type (Ar*Si-2)(x)(Ar2Si)((1-x)) (where Ar* = p-(S)-2-methylbutylphenyl, Ar = p-n-butylphenyl, and x = 0.2, 0.5, or 0.8), positive Cotton effects in the CD spectra indicate optical activity due to helical polymer backbones, optimally for the case where x = 0.5. This is in contrast to the copolymers with Ar* = m-(S)-2-methylbutylphenyl and x = 0.2, 0.5, or 0.8, for which the Cotton effects are of smaller magnitude and negative, except in the case of x = 0.2, for which a temperature-dependent inversion of Cotton effect sign is observed, as we communicated recently.(1) There is no optical activity, as expected, in the special case where x = 0, although from spectroscopic data and force field calculations, it is also concluded that even optically inactive poly(diarylsilylenes), including the above case where x = 0, adopt helical forms, but in an internal racemate. For the most highly chirally substituted bis-para copolymer poly[bis(p-(S)-2-methylbutylphenyl)(0.8)-co-bis(p-n-butylphenyl)(0.2)sil ylene], an unusual (hypsochromic) thermochromic transition is observed at 320 nm, the origin of which is postulated in a helical backbone conformation with a concomitant phenyl ring twist to inhibit phenyl ring/Si-Si conjugation. Viscometric studies indicate that incorporation of branched side chains on the aryl rings results in polymers with stiffer, more extended structures, and this is related to the observed increase in UV absorption intensities.

 

65.       J. R. Koe, M. Motonaga, M. Fujiki, and R. West

            "Synthesis and spectroscopic characterization of heteroatom polysilylenes: Poly(dialkoxysilylene)s and evidence for silicon sigma-oxygen n mixing interaction"

            Macromolecules 34 (4), 706-712 (2001).

 

            ABSTRACT: A new synthetic route to polysilylenes is reported which allows access to hitherto inaccessible heteroatom-substituted poly(dialkoxysilylene)s. Perchloropolysilane, (SiCl2)(n), undergoes substitution by alcohols in the presence of an amine to yield monomodal poly(dialkoxysilylene)s with molecular weights (M-w) up to 62 000, ultraviolet (UV) absorption maxima due to the lowest energy sigma-sigma* transition of the silicon backbone in the range 336-347 nm (red-shifted compared to dialkyl analogues), and fluorescence emission in the range 368-398 nm. Substitution with enantiopure chiral alcohols affords optically active polymers with matching UV and circular dichroism (CD) signals, indicating preferential screw sense helicity of the main chain, which is further investigated by force field calculations. Theoretical prediction of the reduction of the optical band gap due to interaction between silicon (sigma) and oxygen lone pair (n) orbitals is confirmed.

 

66.       L. P. Kouwenhoven, D. G. Austing, and S. Tarucha

            "Few-electron quantum dots"

            Rep. Prog. Phys. 64 (6), 701-736 (2001).

 

            ABSTRACT: We review some electron transport experiments on few-electron, vertical quantum dot devices. The measurement of current versus source-drain voltage and gate voltage is used as a spectroscopic tool to investigate the energy characteristics of interacting electrons confined to a small region in a semiconducting material. Three energy scales are distinguished: the single-particle states, which are discrete due to the confinement involved; the direct Coulomb interaction between electron charges on the dot; and the exchange interaction between electrons with parallel spins. To disentangle these energies, a magnetic field is used to reorganize the occupation of electrons over the single-particle states and to induce changes in the spin states. We discuss the interactions between small numbers of electrons (between 1 and 20) using the simplest possible models. Nevertheless, these models consistently describe a large set of experiments. Some of the observations resemble similar phenomena in atomic physics, such as shell structure and periodic table characteristics, Hund's rule, and spin singlet and triplet states. The experimental control, however, is much larger than for atoms: with one device all the artificial elements can be studied by adding electrons to the quantum dot when changing the gate voltage.

 

67.       N. Kumada, A. Sawada, Z. F. Ezawa, H. Azuhata, S. Nagahama, K. Muraki, T. Saku, and Y. Hirayama

            "Preferred number of flipped spins in Skyrmion excitation"

            Physica B 298 (1-4), 169-172 (2001).

 

            ABSTRACT: We measured the activation energy of the Skyrmion excitations in the monolayer quantum Hall (QH) state at the Landau-level filling factor v = 1 and in the bilayer QH state at v = 2. The number of flipped spins N-s changes from 7 to 1 in the monolayer sample and in the bilayer sample with small tunneling energy gap, while N-s changes from 14 to 7 and finally to 1 in the bilayer sample with large tunneling energy gap. These experimental data suggest the existence of the preferred number of flipped spins N-s = 7 for a Skyrmion-pair excitation. (C) 2001 Elsevier Science B.V. All rights reserved.

 

68.       K. Kumakura, T. Makimoto, and N. Kobayashi

            "Low resistance non-alloy ohmic contact to p-type GaN using Mg-doped InGaN contact layer"

            Phys. Status Solidi A-Appl. Res. 188 (1), 363-366 (2001).

 

            ABSTRACT: We investigated non-alloy Ohmic contact to p-type GaN using Mg-doped InGaN contact layer. The thickness and In mole fraction of the p-type InGaN were varied from 2 to 15 urn and from 0.14 to 0.23, respectively. Strained InGaN contact layers are effective in reducing the contact resistance. The lowest specific contact resistance of 1.1 x 10(-6) Omega cm(2) was obtained using a contact layer of 2 nm thick strained In0.19Ga0.81N. The mechanism for the lower contact resistance is ascribed to enhanced tunneling transport due to the large polarization-induced band bending at the surface as well as the high hole concentration in p-type InGaN.

 

69.       K. Kumakura, T. Makimoto, and N. Kobayashi

            "Low-resistance nonalloyed ohmic contact to p-type GaN using strained InGaN contact layer"

            Appl. Phys. Lett. 79 (16), 2588-2590 (2001).

 

            ABSTRACT: A strained InGaN contact layer inserted between Pd/Au and p-type GaN resulted in low ohmic contact resistance without any special treatments. The thickness and In mole fraction of the p-type InGaN varied from 2 nm to 15 nm and from 0.14 to 0.23, respectively. Strained InGaN layers are effective in reducing the contact resistance. A contact layer of 2 nm thick strained In0.19Ga0.81N showed the lowest specific contact resistance of 1.1x10(-6) Omega cm(2). The mechanism for the lower contact resistance is ascribed to enhanced tunneling transport due to large polarization-induced band bending at the surface as well as to the high hole concentration in p-type InGaN. (C) 2001 American Institute of Physics.

 

70.       R. Kurita, H. Tabei, K. Hayashi, T. Horiuchi, K. Torimitsu, and O. Niwa

            "Improvement in signal reliability when measuring L-glutamate released from cultured cells using multi-channel microfabricated sensors"

            Anal. Chim. Acta 441 (2), 165-174 (2001).

 

            ABSTRACT: We used a micromachining technique to fabricate a very reliable amperometric glutamate sensor consisting of dual channel thin layer flow cells for the highly selective measurement of glutamate released from cultured nerve cells. Our microfabricated sensor has two carbon film electrodes separated by a separator in the thin layer channel flow cells. We modified one carbon electrode with a bilayer of Os-gel-HRP/BSA film containing GluOx, and the other with a bilayer of Os-gel-HRP/BSA film without GluOx. Therefore, any difference between the currents at the dual electrodes should be due to the oxidation of glutamate by GluOx. This dual mode measurement technique can eliminate current changes caused by the change in the double layer capacitance when KCI solution is added to stimulate the cells or by baseline fluctuations resulting from pumping noise. As a result, we were able to monitor the transient peak caused by the glutamate release from cultured rat cortex cells with high selectivity by stimulating the cells with 100 mM of KCI solution. The specificity of the GluOx for glutamate was improved when the sensor was pre-treated with 6-diazo-5-oxo-L-norleucine (DON), which is known to be a glutaminase inhibitor. The signal we obtained at the DON-treated sensor when we measured the extracellular solution was lower than that at an untreated sensor. This result shows that the current caused by extracellular glutamine was eliminated by the DON treatment. Moreover, the extracellular glutamate concentration measured with the DON pre-treated sensor was consistent with that obtained by HPLC. (C) 2001 Elsevier Science B.V. All rights reserved.

 

71.       T. Machida, S. Ishizuka, S. Komiyama, K. Muraki, and Y. Hirayama

            "Nonequilibrium population in fractional edge states"

            Physica B 298 (1-4), 150-154 (2001).

 

            ABSTRACT: We study transport through two scattering barriers, which are formed by Schottky cross gates on Al0.3Ga0.7As/GaAs Hall bars. The gates sandwich the intermediate region adjusted to the v = 1 integer quantum-Hall (IQH) state, Nonequilibrium population is noted to be created when v =2/3, 3/5 and 1/3 are realized in the regions underneath the gales, suggesting the formation of v = 2/3, 3/5, and 1/3 fractional quantum-Hall (FQH) edge states in the bulk v = 1 IQH-state. The unequally occupied FQH-edge states carry the current over a 5 mum distance without equilibration. We discuss the width of incompressible strips in FQH-states from the equilibration length in FQH-edge states. (C) 2001 Elsevier Science B.V. AII rights reserved.

 

72.       T. Machida, S. Ishizuka, S. Komiyama, K. Muraki, and Y. Hirayama

            "Scaling in fractional quantum Hall transitions"

            Physica B 298 (1-4), 182-186 (2001).

 

            ABSTRACT: The transition between fractional quantum Hall (FQH) plateaus, v =2/3 <----> 3/5, is studied in Al0.3Ga0.7As/GaAs two-dimensional electron-gas systems. The transition width is independent of the sample-size at elevated temperatures T > 80mK, and decreases with decreasing T. With decreasing T below 80mK, the transition width splits into sample-specific values. The sample-size dependence indicates that the inelastic-scattering length (L-in) in FQH regime increases to become comparable to the size of conductor (of the order of 10 mum). We derive the absolute size of L-in as a function of T in FQH regime. We find surprising similarity of the scaling behavior in FQH transition to that in the earlier studied integer quantum Hall transitions. (C) 2001 Elsevier Science B.V. AII rights reserved.

 

73.       T. Machida, S. Ishizuka, S. Komiyama, K. Muraki, and Y. Hirayama

            "Resistance fluctuations in quantum Hall transitions: Network of compressible-incompressible regions"

            Phys. Rev. B 63 (4), 045318 (2001).

 

            ABSTRACT: Resistance fluctuations in integer and fractional quantum Hall transitions are studied in modulation-doped Al0.3Ga0.7As/GaAs heterostructures. We examine the role of coherence in the fluctuations by investigating the conductance through two scattering regions that are spatially separated but interact quantum-mechanically with each other. Though the conductor is in a coherent regime, the phase coherence is found to play an insignificant role in determining the observed pattern of fluctuations. In transition regions where the average filling factor of Landau levels takes a noninteger value, n - 1 < v < n, the electron system splits into incompressible subregions of v = n and those of v = n - 1, which are separated by percolating compressible strips. Irregular evolution of the network of compressible strips is suggested to be the origin of the resistance fluctuations in integer quantum Hall transitions. A similar mechanism is also suggested for fractional quantum-Hall transitions.

 

74.       N. Maeda, T. Saitoh, K. Tsubaki, T. Nishida, and N. Kobayashi

            "Two-dimensional electron gas transport properties in AlGaN/GaN single- and double-heterostructure field effect transistors"

            Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 82 (1-3), 232-237 (2001).

 

            ABSTRACT: Tao-dimensional electron gas transport properties have been investigated in both AlGaN/GaN single- and AlGaN/GaN/AlGaN double-heterostructure field effect transistors, by means of the Hall effect measurement under the gate-voltage application. In AlGaN/GaN single-heterostructures, the dependencies of the electron mobility on the electron density have systematically been examined from 30 to 400 K. Below room temperature, the mobility has been shown to assume a maximum value at a critical electron density where electrons begin to overflow into the AlGaN barrier layer. Above room temperature, even at high electron densities that exceed the channel electron capacity, the degradation in the mobility has been found to be small. This is a feature favorable for high-power and high-temperature device operation. In AlGaN/GaN/AlGaN double-heterostructures, a striking effect has been observed that the mobility is drastically enhanced compared with that in the AlGaN/GaN single-heterostructure at low temperatures. The dependencies of the mobility on the electron density measured at 4.2 K in both heterostructures have been analyzed from the viewpoint of the electron distribution in the channel. The observed mobility enhancement in the double-heterostructure has been shown to be mainly due to the enhanced polarization-induced electron confinement in the double-heterostructure, and additionally due to the improvement of the interface roughness in the structure. Transport properties specific to AlGaN/GaN single- and double-heterostructures have thus successfully been revealed through the mobility-density relations that have been determined by the Hall effect measurement under the gate-voltage application. (C) 2001 Elsevier Science B.V. All rights reserved.

 

75.       N. Maeda, T. Saitoh, K. Tusubaki, and N. Kobayashi

            "AlGaN/GaN heterostructure field-effect transistors with high Al compositions fabricated with selective-area regrowth"

            Phys. Status Solidi A-Appl. Res. 188 (1), 223-226 (2001).

 

            ABSTRACT: AlGaN/GaN heterostructure field-effect transistors (HFETs) have been fabricated using selective-area regrowth technique onto the AlGaN layer. A high transconductance (g(m)) of 215 mS/mm with a low knee-voltage of 2.5 V (at V-g = 0 V) has been obtained for a 1.5 mum gate-length device. This high g(m) value owes to (i) the superior two-dimensional electron gas (2DEG) transport properties (1260 cm(2)/Vs at 9.2 x 10(12) cm(-2), and 790 cm(2)/Vs at 2.2 x 10(13) cm(-2)) obtained with the unrelaxed structure with a high Al composition (0.25), and (ii) the reduced contact resistance (0.3 Ohm mm) obtained with the selectively grown source and drain regions. Thus, the selective-area regrowth technique applied to the unrelaxed HFETs with high. Al composition has been shown to be effective to obtain high g(m) values.

 

76.       N. Maeda, K. Tsubaki, T. Saitoh, and N. Kobayashi

            "High-temperature electron transport properties in AlGaN/GaN heterostructures"

            Appl. Phys. Lett. 79 (11), 1634-1636 (2001).

 

            ABSTRACT: Electron transport properties in the Al0.15Ga0.85N/GaN heterostructure field effect transistors (HFETs) have been examined from room temperature up to 400 degreesC. The temperature dependencies of the two-dimensional electron gas (2DEG) mobility have been systematically measured for the samples with different 2DEG densities. The 2DEG mobility has been shown to decrease with increasing the temperature, with the lower decrease ratio at higher temperatures, and moreover, shown to be less dependent on the 2DEG density at higher temperatures. These features well agree with those of the longitudinal optical phonon-limited mobility theoretically predicted, although the effect of alloy and interface scattering should further be examined and analyzed. The observed 2DEG mobilities at 400 degreesC were as high as from 100 to 120 cm(2)/V s, directly providing the evidence for suitability of the HFET of this material system for high-temperature applications. Moreover, Si-doped Al0.15Ga0.85N single layer has been shown to exhibit a relatively high bulk mobility of 50 cm(2)/V s at 400 degreesC, suggesting that AlGaN is attractive as the channel material when higher-voltage and higher-temperature device operation is required. (C) 2001 American Institute of Physics.

 

77.       T. Makimoto, K. Kumakura, and N. Kobayashi

            "High current gains obtained by InGaN/GaN double heterojunction bipolar transistors"

            Phys. Status Solidi A-Appl. Res. 188 (1), 183-186 (2001).

 

            ABSTRACT: InGaN/GaN double heterojunction bipolar Npn transistors have been fabricated using p-type InGaN and n-type GaN as base and collector layers, respectively. The structures were grown on SiC substrates by low-pressure metalorganic vapor phase epitaxy. The In mole fraction in the base layer and its thickness were 0.06 and 100 nm, respectively. The Mg doping concentration in the base layer was 1 x 10(19) cm(-3) corresponding to a hole concentration of 5 x 10(18) cm(-3) at room temperature. The common-emitter I-V characteristics showed good saturation characteristics and a maximum current gain of 20 was obtained at room temperature.

 

78.       T. Makimoto, K. Kumakura, and N. Kobayashi

            "High current gains obtained by InGaN/GaN double heterojunction bipolar transistors with p-InGaN base"

            Appl. Phys. Lett. 79 (3), 380-381 (2001).

 

            ABSTRACT: InGaN/GaN double heterojunction bipolar transistors have been fabricated using p-type InGaN as a base layer. The structures were grown on SiC substrates by metalorganic vapor phase expitaxy and defined by electron cyclotron resonance plasma etching. The In mole fraction in the base layer and its thickness were 0.06 and 100 nm, respectively. The Mg doping concentration in the base layer was 1x10(19) cm(-3) corresponding to a hole concentration of 5x10(18) cm(-3) at room temperature. From their common-emitter current-voltage characteristics, the maximum current gain of 20 was obtained at room temperature. (C) 2001 American Institute of Physics.

 

79.       H. Masuda, H. Asoh, M. Watanabe, K. Nishio, M. Nakao, and T. Tamamura

            "Square and triangular nanohole array architectures in anodic alumina"

            Adv. Mater. 13 (3), 189-192 (2001).

 

            ABSTRACT:

80.       A. Matsuda, S. Sugita, T. Fujii, and T. Watanabe

            "Study of pseudogap phenomena by STM and other probes"

            J. Phys. Chem. Solids 62 (1-2), 65-68 (2001).

 

            ABSTRACT: The temperature and doping dependence of The tunneling spectrum of Bi2.1Sr1.9CaCu2O, single crystals was obtained by using a low-temperature scanning tunneling microscope. Above T-c, for x less than or equal to 8.27, the tunneling density of states shows a clear gap-like feature with a larger gap value than the superconducting one, while for x > 8.27, it shows the feature expected from the conventional superconducting fluctuation SCF. We determined the SCF component in the static susceptibility (chi) BY subtracting the effect of the SCF from chi, we confirmed that the two pseudogap phase boundaries determined by the tunneling and chi coincide with each other. (C) 2000 Elsevier Science Ltd. Ail rights reserved.

 

81.       T. Matsuoka and E. Hagiwara

            "GaN growth on novel lattice-matching substrate: Tilted M-plane sapphire"

            Phys. Status Solidi A-Appl. Res. 188 (2), 485-489 (2001).

 

            ABSTRACT: As a novel substrate, nearly lattice-matched to GaN and coincident with its crystallographic symmetry, M-plane sapphire is shown to be promising by evaluating the characteristics of GaN epitaxial films grown on this substrate using a two-step growth process. For M-plane sapphire, the tilt of a wafer surface from the M-plane around the c-axis is found to be important in obtaining high-quality GaN. Tilt angles of 15 degrees and 20 degrees make possible the growth of single-crystal GaN. Single-crystal GaN grown on the surface tilted 15 degrees from the M-plane has a very smooth surface and shows the pure near-band-edge emission with strong intensity in photoluminescence. The dislocation density in this GaN is 50% higher than in GaN grown on C-plane. It is expected that GaN quality can be improved by optimizing the tilt angle and employing a growth condition suitable for the tilted surface.

 

82.       N. Y. Morgan, D. Abusch-Magder, M. A. Kastner, Y. Takahashi, H. Tamura, and K. Murase

            "Evidence for activated conduction in a single electron transistor"

            J. Appl. Phys. 89 (1), 410-419 (2001).

 

            ABSTRACT: Conductance measurements are reported for quantum dots fabricated by pattern-dependent oxidation in silicon; the small size of these devices allows observation of single electron behavior to temperatures as high as 200 K. At temperatures above 6 K the zero-bias conductance peaks, periodic in gate voltage, are asymmetric, with heights that increase rapidly with increasing temperature. These features suggest that the dominant conductance mechanism in this temperature range is thermal activation over one of the potential barriers that localize electrons in the quantum dot. Quantitative comparisons are made between the data and the theory of Matveev and Glazman, including fits of the asymmetric line shapes. However, from these fits we find a potential barrier height that is much smaller than the charging energy, indicating that our data cannot be fully explained by this theory. At lower temperatures, the conductance peaks become much less periodic, and the data in this regime suggest the presence of several quantum dots in series. (C) 2001 American Institute of Physics.

 

83.       F. Morikoshi and M. Koashi

            "Deterministic entanglement concentration"

            Phys. Rev. A 64 (2), art. no.-022316 (2001).

 

            ABSTRACT: Deterministic extraction of Bell pairs from a finite number of partially entangled pairs is discussed. We derive the maximum number of Bell pairs that can be obtained with probability 1 by local operations and classical communication. It is proved that the optimal deterministic concentration needs only a two-pair collective manipulation in each step, and that a collective manipulation of all entangled pairs is not necessary. Finally, this scheme reveals an entanglement measure for the deterministic concentration.

 

84.       K. Muraki, T. Saku, and Y. Hirayama

            "Charge excitations in easy-axis and easy-plane quantum Hall ferromagnets"

            Phys. Rev. Lett. 8719 (19), 196801 (2001).

 

            ABSTRACT: We study charge excitations in quantum Hall ferromagnets realized in a symmetric quantum well. Landau levels (LLs) with different subband and orbital indices crossing at the Fermi level act as up and down pseudospin levels. The activation energy measured as a function of the pseudospin Zeeman energy, Delta (z), reveals easy-plane and easy-axis ferromagnetism for LL filling of nu = 3 and 4, respectively, for which the crossing levels have parallel and antiparallel spin. For nu = 4, we observe a sharp reduction in the gap for Delta (z) --> 0, which we discuss in terms of a topological excitation in domain walls akin to Skyrmions.

 

85.       M. Naito and M. Hepp

            "Superconducting T '-La2-xCexCuO4 films grown by molecular beam epitaxy"

            Physica C 357, 333-336 (2001).

 

            ABSTRACT: We report the synthesis of superconducting T ' -La2-xCexCuO4 on SrTiO3(0 0 1) by molecular beam epitaxy. Although T ' -La2-xCexCuO4 with a very limited range of x can be prepared by bulk synthesis using rather complicated techniques. this compound is very easy to obtain by thin film synthesis. The best Tc was above 30 K at onset and similar to 29.5 K at zero resistance, which was obtained for x = 0.11. These values are the highest among electron-doped T ' superconductors. Moreover, the single-phase T ' structure is formed for a composition range of 0.09 < x < similar to0.4, which is much wider than for previously reported bulk synthesis (C) 2001 Elsevier Science B.V. All rights reserved.

 

86.       M. Naito, S. Karimoto, H. Yamamoto, H. Nakada, and K. Suzuki

            "Production of double-sided large-area high-T-c wafers by molecular beam epitaxy"

            IEEE Trans. Appl. Supercond. 11 (1), 3848-3851 (2001).

 

            ABSTRACT: We describe the growth and properties of high-T-c films by molecular beam epitaxy (MBE). MBE growth of high-T-c films requires a reliable rate control for individual elements and a strong activated oxygen source. By satisfying these two requirements, we have succeeded in growing high-quality Dy-123 films. In addition, we have achieved doubleside deposition on large-area substrates (so far up to 35 mm square). Films on MgO substrates typically show T-c(end) > 90 K, rho (300 K) < 300 mu Omega cm, J(c) similar to 2-5 x 10(6) A/cm(2) at 77K, R-s @ 22GHz similar to 2-4 m Omega at 77K, which are comparable to the best values for 123 films obtained at present by other growth methods.

 

87.       S. Nakakoshi, M. Kashino, A. Mizobuchi, Y. Fukada, and H. Katori

            "Disorder in sequential speech perception: A case study on pure word deafness"

            Brain Lang. 76 (2), 119-129 (2001).

 

            ABSTRACT: We described disorders of a patient which were uniquely restricted to speech perception of syllable sequences after brain damage. The results of series of experiments using syllable sequences showed "negative recency effect," in which the subject's repetition performance at the latter syllable position was remarkably poor. Experimental analyses suggested that the ''negative recency effect'' could be due to dual factors: the lower rate of processing of speech sounds and the memory load of holding processes of preceding syllables imposed on the succeeding phonological processing. The results also suggested that the holding processes which imposed the memory load on the succeeding auditory phonological coding processing were modality nonspecific. (C) 2001 Academic Press.

 

88.       H. Nakano, T. Nishikawa, and N. Uesugi

            "Enhanced K-shell x-ray line emissions from aluminum plasma created by a pair of femtosecond laser pulses"

            Appl. Phys. Lett. 79 (1), 24-26 (2001).

 

            ABSTRACT: The influence of an ultrashort prepulse on K-shell x-ray line emissions from Al plasma produced by 100 fs Ti:sapphire laser pulses is studied. By introducing an independent 100 fs prepulse, a more than 20-fold enhancement in K-shell emission was achieved. We could experimentally optimize K-alpha and He-alpha line emissions. In case of oblique incidence, resonance absorption made it possible to maximize K-alpha emission while keeping pulse duration short. On the other hand, He-alpha line emission was maximized when a separation between two laser pulses was large, resulting in pulse broadening (similar to 30 ps). (C) 2001 American Institute of Physics.

 

89.       H. Nakashima and M. Fujiki

            "Precise control of optical properties and global conformations by marked substituent effects in poly(alkyl(methoxyphenyl)silane) homo- and copolymers"

            Macromolecules 34 (21), 7558-7564 (2001).

 

            ABSTRACT: A series of poly(alkyl(methoxyphenyl)silane) homopolymers and poly [(alkyl(methoxyphenyl)silane)-co-((S)-2-methylbutyl(methoxyphenyl)silane)I copolymers containing 20%. chiral monomer units were synthesized, and marked substituent effects on their absorption characteristics and the global and local conformations in THF solution were observed. In the homopolymers, upon increase of the alkyl side chain length, the global backbone conformations changed from a considerably shrunken random coil to a stiff extended structure, and the UV absorptivity and absorption wavelength maximum in the Si main chain region were easily controlled by a combination of the alkyl substituents and position of the methoxy group on the phenyl ring. In the achiral-chiral copolymers, preferential screw-sense helical conformations of the Si backbone were induced by the cooperative effect of the chiral substituents. In addition, we found that the sign of the circular dichroism (CD) peak in the aryl absorption region of the copolymers depended on the methoxy group position in chiral monomer units.

 

90.       H. Nakashima, M. Fujiki, J. R. Koe, and M. Motonaga

            "Solvent and temperature effects on the chiral aggregation of poly(alkylarylsilane)s bearing remote chiral groups"

            J. Am. Chem. Soc. 123 (9), 1963-1969 (2001).

 

            ABSTRACT: Novel switchable chiroptical characteristics of poly(alkylarylsilane) microaggregates; controllable by the choice of good/poor solvent ratio (solvent polarity), solvent addition order, and Sample temperature are described. The formation of stable chiral aggregates depends critically on the polysilane structure and stereochemistry. Poly[n-hexyl-(p-(S)-2-methylbutoxyphenyl)silane] (1), optically inactive in molecularly dispersed THF solution due to the existence of dynamically equivalent amounts of right (P)- and left (M)-handed screw sense helical main chain domains, shows a marked bisignate CD signal due to the formation of chiral aggregates in good/poor cosolvent systems. The sign and magnitude of the CD signals are dependent on solvent polarity, solvent addition order, and thermal effects. The less sterically hindered poly[methyl-(p-(S)2-methylbutoxyphenyl)silane] (2) exhibits a weak, bisignate, nonswitchable CD signal in only the toluene/ acetonitrile system, and no CD signals are evident in pure toluene or THF due to masking of the helicity. In contrast, although the even less sterically hindered, less polar poly[methyl-(m-(S)-2-methylbutoxyphenyl) silane] (3) does show optical activity in pure THF or toluene (negative CD signal at 310 nm), the CD signal disappears on formation of aggregates in good/poor cosolvent systems.

 

91.       H. Nakashima, J. R. Koe, K. Torimitsu, and M. Fujiki

            "Transfer and amplification of chiral molecular information to polysilylene aggregates"

            J. Am. Chem. Soc. 123 (20), 4847-4848 (2001).

 

            ABSTRACT:

92.       H. Namatsu

            "Supercritical resist drying for isolated nanoline formation"

            J. Vac. Sci. Technol. B 19 (6), 2709-2712 (2001).

 

            ABSTRACT: Supercritical resist drying enables the formation of fine resist patterns with a nanometer-scale resolution. Fine isolated and dense lines of resist collapse when dried by conventional methods after development, thereby lowering the useful resolution of resist patterns. For nanolines, the collapse is due to the swelling of the resist that results from development and rinsing. To prevent collapse, the cause of the swelling must be removed from the resist before drying. The use of supercritical fluid is the most suitable way to do that because such fluid has a diffusion coefficient as large as that of a gas. Supercritical resist drying with carbon dioxide prevents the collapse of isolated lines and enables the formation of nanolines with a width of about 7 nm and a high aspect ratio of over 10 in the case of the negative tone resist of hydrogen silsesquioxane and 100 kV electron beams. (C) 2001 American Vacuum Society.

 

93.       K. G. Nath, Y. Haruyama, and T. Kinoshita

            "Surface superstructure formation, electronic structure modification and magnetic stability of Co films on oxygen-rich Cu(001): confirmation of oxygen-surfactant effect"

            Surf. Sci. 486 (3), 185-193 (2001).

 

            ABSTRACT: Low energy electron diffraction, core level photoemission spectroscopy and magnetic dichroism have been applied to study Co films grown on oxygen-rich Cu(0 0 1) surfaces. The surface superstructure, electronic structure, chemical phase and magnetic stability in Co films were investigated. It was found that the initial oxygen was mostly segregated on top of the deposited Co films and promoted formation of a well-ordered c(2 x 2) phase. Besides, the correlation-induced satellite structure in Co2p photoemission spectra for a 1 ML (monolayer) Co film on O/Cu was found to be stronger than that for a 1 ML Co film on Cu. It is predicted that the growth conditions in both cases alter the Co d-d interaction and adatom (Co3d)-substrate (Cu4s) hybridization, and hence the observed satellite intensities. Additionally, a 5 ML film on oxygen-rich Cu(001) showed a magnetically stable phase at room temperature with in-plane magnetization. This is proof of an oxygen surfactant effect for Co epitaxial growth on Cu(001). (C) 2001 Elsevier Science B.V. All rights reserved.

 

94.       K. G. Nath, F. Maeda, S. Suzuki, and Y. Watanabe

            "Modified epitaxy in Co/S/GaAs(001) and comparison with Co/GaAs(001)"

            J. Appl. Phys. 90 (3), 1222-1226 (2001).

 

            ABSTRACT: Due to the S passivation, a modified growth of Co on GaAs(001) has been found. Using reflection high-energy electron diffraction and transmission electron microscopy, we observed the formation of a hcp Co overlayer of approximately 5 nm thickness on S/GaAs(001). In contrast, a similar 5 nm Co film on GaAs(001) shows a bcc structure. The metal-semiconductor interfaces in both systems were found to be different, where Co/S/GaAs(001) showed a relatively more abrupt interface. This epitaxial modification is explained on the basis of the morphology of the initial substrate surface, chemical compositions, the nature of the chemical reaction between adatoms and substrate atoms, and the effect of atomic segregation. (C) 2001 American Institute of Physics.

 

95.       K. G. Nath, F. Maeda, S. Suzuki, and Y. Watanabe

            "Surfactant-mediated control of surface morphology for Co epitaxial film on S-passivated semiconducting substrate"

            J. Vac. Sci. Technol. B 19 (2), 384-387 (2001).

 

            ABSTRACT: Using atomic force microscopy and scanning electron microscopy, we demonstrate that a Co epitaxial film surface grown on S-passivated GaAs(001) is smoother than one grown on bare GaAs(001) and establish that the surfactant nature of sulfur plays a vital role in the formation of a smooth surface. Synchrotron radiation photoemission spectroscopy results confirm that S passivation greatly reduces the segregation of substrate atoms during film growth on a S-passivated surface. It was also found that methanol rinsing after chemical S passivation provides an even smoother surface. (C) 2001 American Vacuum Society.

 

96.       T. Natsume, L. B. Wu, T. Sato, K. Terao, A. Teramoto, and M. Fujiki

            "Chain-stiffness and lyotropic liquid crystallinity of polysilylene bearing (S)-2-methylbutyl and n-decyl substituents"

            Macromolecules 34 (22), 7899-7904 (2001).

 

            ABSTRACT: The intrinsic viscosity [eta], the radius of gyration <S-2> (1/2), and the isotropic-cholesteric phase boundary concentration c, (between the isotropic and biphasic regions) for poly [(n-decyl-(S)-2-methylbutyl)-silylene] dissolved in isooctane were measured as functions of the weight-average molecular weight M-w. By analyzing the results of [eta] and <S-2 >1/2 in terms of the wormlike-chain model, the persistence length q was determined to be 70 nm. Using this q, the scaled particle theory for the hard wormlike-spherocylinder model successfully predicted c(I) at high M-w but underestimated it at low M-w. The underestimate may be ascribed to the flexibility effect in the vicinity of the polymer-chain ends on c(1), which is more appreciable than the same effect on [eta].

 

97.       J. F. Nielsen, J. P. Pelz, H. Hibino, C. W. Hu, and I. S. T. Tsong

            "Enhanced terrace stability for preparation of step-free Si(001)-(2x1) surfaces"

            Phys. Rev. Lett. 8713 (13), art. no.-136103 (2001).

 

            ABSTRACT: We show that depositing Si white annealing patterned Si(001)-(2 x 1) substrates at sublimation temperatures enhances terrace stability, permitting larger step-free areas to be produced in a given time than possible by annealing alone. We confirm this enhanced terrace stability using real-time low-energy electron microscopy observations, and quantitative microscopic modeling of step dynamics. Our measurements can be used to estimate the lateral variation in adatom concentration across large terraces, and to estimate an adatom diffusion length lambda approximate to 10-30 mum at 1000 degreesC.

 

98.       J. F. Nielsen, J. P. Pelz, H. Hibino, C. W. Hu, I. S. T. Tsong, and J. Kouvetakis

            "Controlled striped phase formation on ultraflat Si(001) surfaces during diborane exposure"

            Appl. Phys. Lett. 79 (23), 3857-3859 (2001).

 

            ABSTRACT: We have used low-energy electron microscopy to study spontaneous step formation in "striped" domains on ultraflat Si(001)-(2x1) surfaces during B2H6 exposure at elevated temperatures. We show that the size and arrangement of striped domains are kinetically limited, and propose that the limiting factor is the supply of diffusing Si surface adatoms. By adding controlled amounts of extra Si to ultraflat terraces, it is possible to foster the formation of very large (>5 mum) single-domain striped regions with adjustable stripe widths. (C) 2001 American Institute of Physics.

 

99.       T. Nishida and N. Kobayashi

            "Ten-milliwatt operation of an AlGaN-based light emitting diode grown on GaN substrate"

            Phys. Status Solidi A-Appl. Res. 188 (1), 113-116 (2001).

 

            ABSTRACT: Ten-milliwatt output at 352 nm wavelength is obtained in the room-temperature cw operation of an AlGaN-based ultraviolet light emitting diode. The maximum internal quantum efficiency is estimated to be more than 80%. We also demonstrate the excitation of fluorescence of three basal colors by this wavelength.

 

100.    T. Nishida, H. Saito, and N. Kobayashi

            "Efficient and high-power AlGaN-based ultraviolet light-emitting diode grown on bulk GaN"

            Appl. Phys. Lett. 79 (6), 711-712 (2001).

 

            ABSTRACT: By introducing thick bulk GaN as a substrate, we improved the performance of an AlGaN-based ultraviolet (UV) light-emitting diode (LED). The output power exceeds 3 mW at the injection current of 100 mA under a bare-chip geometry. Internal quantum efficiency is estimated as more than 80%, and the peak wavelength is 352 nm. The maximum power exceeds 10 mW at a large current injection of 400 mA, with an operation voltage of less than 6 V. These results indicate that an efficient UV LED is intrinsically possible by the combination of appropriate device design and the nitride substrate. By introducing packaging technology to enhance extraction efficiency, we will have a compact and efficient UV light source in the wide wavelength range of 200-360 nm, similar to conventional longer-wavelength LEDs. (C) 2001 American Institute of Physics.

 

101.    T. Nishida, H. Saito, and N. Kobayashi

            "Milliwatt operation of AlGaN-based single-quantum-well light emitting diode in the ultraviolet region"

            Appl. Phys. Lett. 78 (25), 3927-3928 (2001).

 

            ABSTRACT: By introducing a single-quantum-well active layer and a high-Al-content carrier blocking layer, the output power of an AlGaN-based ultraviolet light-emitting diode has been improved by one order of magnitude. Optical output of 1 mW was achieved at the emission peak wavelength of 341-343 nm. (C) 2001 American Institute of Physics.

 

102.    T. Nishida, H. Saito, and N. Kobayashi

            "Submilliwatt operation of AlGaN-based ultraviolet light-emitting diode using short-period alloy superlattice (vol 78, pg 399, 2001)"

            Appl. Phys. Lett. 78 (12), 1795-1795 (2001).

 

            ABSTRACT:

103.    T. Nishida, H. Saito, and N. Kobayashi

            "Submilliwatt operation of AlGaN-based ultraviolet light-emitting diode using short-period alloy superlattice"

            Appl. Phys. Lett. 78 (4), 399-400 (2001).

 

            ABSTRACT: Over 0.1 mW ultraviolet output was achieved by an AlGaN-based light-emitting diode. To realize a highly conductive and ultraviolet-transparent layer, a short-period alloy superlattice was introduced. The device was fabricated on SiC substrate. Low electric resistivity due to the short-period alloy superlattice and the high thermal conductivity of the SiC substrate enabled large current injection of up to 1.7 kA/cm(2). The emission was monochromatic band-edge emission about 350 nm in wavelength without significant D-A and/or deep emissions. (C) 2001 American Institute of Physics.

 

104.    T. Nishikawa, H. Nakano, K. Oguri, N. Uesugi, M. Nakao, K. Nishio, and H. Masuda

            "Nanocylinder-array structure greatly increases the soft X-ray intensity generated from femtosecond-laser-produced plasma"

            Appl. Phys. B-Lasers Opt. 73 (2), 185-188 (2001).

 

            ABSTRACT: In order to increase the soft X-ray conversion efficiency for the femtosecond-laser-produced plasma, we adopted a nanocylinder-array structure target. Gold nanocylinder-array targets with 70-90 nm cylinder diameter and 100 nm cylinder pitch were made. A continuous smooth soft X-ray spectrum adequate for X-ray absorption spectroscopy was obtained. An around 20-fold soft X-ray (7-20 nm) fluence enhancement compared with a flat-surface gold foil target was obtained when the cylinder height was IS Lm. X-ray (> 0.06 keV) pulse duration was 17 ps, which is much shorter than that obtained by using the pre-pulse technique. The X-ray pulse peak intensity was 7-fold higher than that of a gold foil target.

 

105.    T. Nishikawa, H. Nakano, N. Uesugi, M. Nakao, K. Nishio, and H. Masuda

            "X-ray generation from femtosecond-laser-produced plasma on nanostructure-array targets"

            J. Phys. IV 11 (PR2), 425-428 (2001).

 

            ABSTRACT: In order to increase the soft x-ray conversion efficiency for femtosecond-laser-produced plasma, we adopted nanostructure-array targets. One structure is a nanohole-array structure. This structure was made by utilizing the anodic oxidation of an aluminum plate. Around a 30-fold x-ray fluence enhancement was achieved in the 5-20 nm wavelength range when the nanohole diameters were about 90 nm. The enhancement increases as the ionization level of Al becomes higher and the x-ray wavelength becomes shorter. Over a 50-fold enhancement was obtained at a soft x-ray wavelength around 6 nm, which corresponds to the emission from Al-8+,Al-9+ ions. Another structure is a nanocylinder-array structure. After filling up the alumina nanoholes (mean cell size: 100 nm, pore diameter: 70-90 nm) with Au by electrodeposition, the alumina part was removed. Soft x-ray fluence from these targets increases as the Au cylinder height increases, Around a 15-fold enhancement compared with a gold foil target was obtained in the 7-20 nm wavelength range when the cylinder height was 18 mum. Larger enhancement is expected by optimizing the cylinder size. Soft x-ray (> 0.1 keV) pulse durations on both targets were 17 ps, which is much shorter than that obtained by using a prepulse technique.

 

106.    J. Nitta, C. M. Hu, A. Jensen, J. B. Hansen, and H. Takayanagi

            "Spin injection and detection experiment in a two dimensional electron gas with inter-digital-ferromagnetic contacts"

            Physica E 10 (1-3), 467-471 (2001).

 

            ABSTRACT: Ferromagnetic contacts on a high-mobility two dimensional electron gas (2DEG) in a narrow gap semiconductor with strong spin-orbit interaction are used to investigate spin polarised electron transport. In ballistic samples, the predicted increase on the device resistance is observed when the: injector/detector magnetisation configuration is switched from parallel to anti-parallel. Spin dephasing and spin precession effects are studied as functions of temperature and 2DEG channel length. (C) 2001 Elsevier Science B.V. All rights reserved.

 

107.    J. Nitta, C. M. Hu, A. Jensen, J. B. Hansen, and H. Takayanagi

            "Spin injection experiment with multiple NiFe/ZnAs-2DEG/NiFe junctions"

            Physica C 352 (1-4), 215-218 (2001).

 

            ABSTRACT: Ferromagnetic contacts on a high-mobilty two-dimensional electron gas (2DEG) in a narrow gap semiconductor with strong spin-orbit interaction are used to investigate spin-polarized electron transport. We demonstrate the use of magnetized contacts to preferentially inject and detect specific spin orientations. Spin dephasing and spin precession effects are studied by temperature and 2DEG channel length dependent measurements. (C) 2001 Elsevier Science B.V. All rights reserved.

 

108.    S. Nojima

            "Laser oscillation due to light slowed-down by excitons in photonic crystals"

            J. Phys. Soc. Jpn. 70 (11), 3432-3445 (2001).

 

            ABSTRACT: The polariton-mediated photonic crystal laser is theoretically investigated, which is made of a periodic structure containing excitonic and gain materials in its unit cell-this kind of structure can also be called a potaritonic crystal. The basic concept of this laser lies in exploiting the anomalous dispersion of the polaritonic crystal as the photonic environment in which light created by stimulated emission propagates. This crystal provides a stage for photons, which enables a significant enhancement of optical gain in the vicinity of its band edge. The gain enhancement is found to be caused by the increased confinement of light in the gain region and the slowing-down of light in the presence of excitons. This effect permits the laser to oscillate with a very low threshold and in the single mode. The threshold-gain values obtained axe found to be much lower than those for the Fabry Perot lasers and the Distributed-Feedback lasers. Moreover, exciton damping, which is undesirable for laser action, is shown to be decreased to a considerable degree by designing the polaritonic crystal structure appropriately. All these advantages of polaritonic, crystals stem from the fact that the radiation field is modulated significantly by adding excitons to conventional photonic crystals.

 

109.    S. Nojima

            "Determination of optical modes in two-dimensional finite-size photonic crystals by photonic resonance scattering"

            Appl. Phys. Lett. 79 (13), 1959-1961 (2001).

 

            ABSTRACT: The analogy of the resonance scattering of an electron by an atom is used to investigate optical modes in two-dimensional finite-size photonic-crystal resonators. This analysis enables us to determine complex resonance energies from which we can estimate optical modes and photon lifetimes in the resonators. The threshold amplitude gain of the laser exploiting this photonic-crystal resonator is inversely proportional to the photon lifetime. This linear relation is found to remain the same irrespective of the dimensionality of the photonic crystal structure, the number of unit cells, and the size of unit cell, as long as the filling factor of the gain cell is fixed. This implies that the two-dimensional finite-size photonic-crystal resonators work well despite complicated feedback mechanisms and the absence of clear-cut cavity mirrors. (C) 2001 American Institute of Physics.

 

110.    S. Nojima

            "Optical-gain enhancement in two-dimensional active photonic crystals"

            J. Appl. Phys. 90 (2), 545-551 (2001).

 

            ABSTRACT: Optical-gain-enhancement effects are numerically investigated in detail for two-dimensional photonic crystals with active (gain) lattice points. First, this gain enhancement is shown to occur in the vicinity of every photonic band edge. A thorough examination is carried out to isolate the leading factors that determine this enhancement on the basis of a number of (more than 1000) numerically calculated data points. The gain-enhancement factor is uniquely determined by the product of the time for light to pass a gain rod and the confinement factor of the field energy in the gain rod, irrespective of the choice of dielectric constants, wave numbers, band indices, and polarization directions of light. In designing photonic crystals, therefore, more gentlly sloped bands and a greater contrast in the dielectric constant between gain and background materials are essential in order to intensify optical gain more effectively. (C) 2001 American Institute of Physics.

 

111.    H. Notomi, A. Shinya, K. Yamada, J. Takahashi, C. Takahashi, and I. Yokohama

            "Singlemode transmission within photonic bandgap of width-varied single-line-defect photonic crystal waveguides on SOI substrates"

            Electron. Lett. 37 (5), 293-295 (2001).

 

            ABSTRACT: It is demonstrated experimentally that a single missing-hole line defect photonic crystal waveguide fabricated on a silicon-on-insulator substrate can support a single waveguiding mode within the photonic bandgap if the waveguide width is properly tuned.

 

112.    M. Notomi, H. Suzuki, and T. Tamamura

            "Directional lasing oscillation of two-dimensional organic photonic crystal lasers at several photonic band gaps"

            Appl. Phys. Lett. 78 (10), 1325-1327 (2001).

 

            ABSTRACT: We fabricated a series of two-dimensional (2D) hexagonal organic photonic-crystal lasers whose lattice constant varies from 0.18 to 0.44 mum, and observed clear lasing oscillation at the four lowest band gap frequencies (M-1, K-2, M-2, and Gamma (1)). We used in-plane beam propagation analysis to clarify the 2D feedback mechanism at each gap frequency, which differs for different gaps. The observed K-1 lasing oscillation is due to coupling of three nonparallel diffracted waves, which has a purely 2D character. This shows that photonic crystal lasers can operate with various feedback essentially different from that of conventional 1D distributed feedback lasers. (C) 2001 American Institute of Physics.

 

113.    M. Notomi, K. Yamada, A. Shinya, J. Takahashi, C. Takahashi, and I. Yokohama

            "Extremely large group-velocity dispersion of line-defect waveguides in photonic crystal slabs"

            Phys. Rev. Lett. 87 (25), 253902 (2001).

 

            ABSTRACT: We reveal experimentally waveguiding characteristics and group-velocity dispersion of line defects in photonic crystal slabs as a function of defect widths. The defects have waveguiding modes with two types of cutoff within the photonic band gap. Interference measurements show that they exhibit extraordinarily large group dispersion, and we found waveguiding modes whose traveling speed is 2 orders of magnitude slower than that in air. These characteristics can be tuned by controlling the defect width, and the results agree well with theoretical calculations, indicating that we can design light paths with made-to-order dispersion.

 

114.    K. Oguri, H. Nakano, T. Nishikawa, and N. Uesugi

            "Cross-correlation measurement of ultrashort soft x-ray pulse emitted from femtosecond laser-produced plasma using optical field-induced ionization"

            Appl. Phys. Lett. 79 (27), 4506-4508 (2001).

 

            ABSTRACT: We propose a cross-correlation technique for measuring ultrashort soft x-ray pulse shapes using the rapid increase in Kr+ ion density caused by optical field-induced ionization, which operates as an ultrafast x-ray absorption switch. Using this technique, we measured the shape of a soft x-ray pulse near 15.6 nm emitted from W plasma produced by a 100 fs laser pulse, and found the duration to be about 4 ps assuming a Gaussian pulse. This result was in good agreement with the duration measured with an x-ray streak camera thus confirming the feasibility of our technique. The temporal resolution of this technique has the potential to overcome the limitation of the ionizing pulse duration. (C) 2001 American Institute of Physics.

 

115.    K. Ono, J. H. Oh, K. Horiba, M. Mizuguchi, M. Oshima, T. Kiyokura, F. Maeda, Y. Watanabe, A. Kakizaki, T. Kikuchi, A. Yagishita, and H. Kato

            "Performance of the high-resolution high-flux monochromator for bending magnet beamline BL-1C at the Photon Factory"

            Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equip. 467, 573-576 (2001).

 

            ABSTRACT: A grazing incidence varied line spacing plane grating monochromator (VLS-PGM) has been designed and installed at the Photon Factory bending magnet beamline, BL-1C. The monochromator is designed to cover 20 to 250eV with a high resolving power as well as a high photon flux and intended for a high-resolution angle-resolved photoemission study of quantum nano-structures. The resolving power of the beamline is observed to be more than 16,000 at around 65 eV and exceeds 10,000 at all the covered energy ranges of the monochromator with photon flux over 10(9) photons/s which shows that this monochromator reaches the expected design goals. (C) 2001 Elsevier Science B.V. All rights reserved.

 

116.    Y. Ono, K. Yamazaki, M. Nagase, S. Horiguchi, K. Shiraishi, and Y. Takahashi

            "Single-electron and quantum SOI devices"

            Microelectron. Eng. 59 (1-4), 435-442 (2001).

 

            ABSTRACT: This paper describes, from the viewpoint of device fabrication, single-electron and quantum devices using silicon-on-insulators (SOIs). We point out that control of the oxidation of Si is quite important and could be the key to their fabrication. We also introduce our technique for making single-electron transistors (SETs), which uses special phenomena that occur during the oxidation of SOIs, and show that the technique enables us to realize primary single-electron circuits as a result of its high controllability and high reproducibility. (C) 2001 Elsevier Science B.V. All rights reserved.

 

117.    Y. Ono, K. Yamazaki, and Y. Takahashi

            "Si single-electron transistors with high voltage gain"

            IEICE Trans. Electron. E84C (8), 1061-1065 (2001).

 

            ABSTRACT: Si single-electron transistors with a high voltage gain at a considerably high temperature have been fabricated by vertical pattern-dependent oxidation. The method enables the automatic formation of very small tunnel junctions having capacitances of less than 1 aF. In addition, the use of a thin (a few ten nanometers thick) gate oxide allows a strong coupling of the island to the gate, which results in a gate capacitance larger than the junction capacitances. It is demonstrated at 27 K that an inverting voltage gain, which is governed by the ratio of the gate capacitance to the drain tunnel capacitance, exceeds 3 under constant drain current conditions.

 

118.    S. K. Ozdemir, A. Miranowicz, M. Koashi, and N. Imoto

            "Quantum-scissors device for optical state truncation: A proposal for practical realization"

            Phys. Rev. A 6406 (6), art. no.-063818 (2001).

 

            ABSTRACT: We propose a realizable experimental scheme to prepare superposition of the vacuum and one-photon states by truncating an input coherent state. The scheme is based on the quantum scissors device proposed by Pegg, Phillips, and Barnett [Phys. Rev. Lett. 81, 1604 (1998)] and uses photon-counting detectors, a single photon source, and linear optical elements. Realistic features of the photon counting and single-photon generation are taken into account and possible error sources are discussed together with their effect on the fidelity and efficiency of the truncation process. Wigner function and phase distribution of the generated states are given and discussed for the evaluation of the proposed scheme.

 

119.    M. Pi, A. Emperador, M. Barranco, F. Garcias, K. Muraki, S. Tarucha, and D. G. Austing

            "Dissociation of vertical semiconductor diatomic artificial molecules"

            Phys. Rev. Lett. 8706 (6), art. no.-066801 (2001).

 

            ABSTRACT: We investigate the dissociation of few-electron circular vertical semiconductor double quantum dot artificial molecules at 0 T as a function of interdot distance. A slight mismatch introduced in the fabrication of the artificial molecules from nominally identical constituent quantum wells induces localization by offsetting the energy levels in the quantum dots by up to 2 meV, and this plays a crucial role in the appearance of the addition energy spectra as a function of coupling strength particularly in the weak coupling limit.

 

120.    K. Prabhakaran, Kvpm Shafi, A. Ulman, and T. Ogino

            "Nanoparticle-induced light emission from multi-functionalized silicon"

            Adv. Mater. 13 (24), 1859-+ (2001).

 

            ABSTRACT: A "plug and play" approach in which sono-chemically synthesized amorphous Fe2O3 nanoparticles are incorporated onto device quality Si wafers is demonstrated. After annealing the amorphous Fe2O3 nanoparticles (see Figure) they change their properties from superparamagnetic to soft ferromagnetic. The samples exhibit multiple light emissions with wavelength that are crucial for optical fiber communications (see also inside front cover).

 

121.    P. Roche, H. Perrin, D. C. Glattli, H. Takayanagi, and T. Akazaki

            "Enhanced shot noise in long quasi-diffusive S-N-S junctions"

            Physica C 352 (1-4), 73-76 (2001).

 

            ABSTRACT: We present shot noise measurements on superconducting-normal-superconducting junction (S-N-S) where the superconductor is Niobium and the normal conductor is a two-dimensional electron gas in a InAlAs/InGaAs heterojunction. The aim of the measurements was to check the recent predictions [Phys. Rev. Lett. 83 (1999) 2050] that in the limit of incoherent multiple Andreev reflection (I-MAR), i.e. in the elastic regime for Thouless energy much lower than the superconducting gap, the current-voltage characteristics should be quasi-linear but I-MAR could be detected in the shot noise. (C) 2001 Elsevier Science B.V. All rights reserved.

 

122.    S. Saito, H. Yoshimoto, Y. Y. Suzuki, and S. Kurihara

            "Is evolution from weak to strong coupling superconductivity always continuous?"

            J. Phys. Soc. Jpn. 70 (5), 1186-1189 (2001).

 

            ABSTRACT: We have performed a variational analysis on the evolution of superconductivity from weak to strong coupling regime. We have improved the Bardeen Cooper Schrieffer (BCS) wave function, and show that the electronic correlations are important in obtaining the correct behavior of superconducting condensation energy In contrast to a crossover without thermodynamic anomaly discussed in a dilute system. we show the existence of a quantum phase transition near half filling. which suggests that an evolution is not always continuous. The transition is driven by charge density waves instabilities. We have found that superconductivity and charge density waves coexist in the presence of a weak intersite repulsion. The ground state phase diagram is determined by varying both interactions and filling.

 

123.    Y. Sakurai, D. Yoshimura, H. Ishii, Y. Ouchi, H. Isaka, H. Teramae, N. Matsumoto, S. Hasegawa, N. Ueno, and K. Seki

            "Electronic structure of polycarbosilane studied by UV photoelectron spectroscopy"

            J. Phys. Chem. B 105 (24), 5626-5629 (2001).

 

            ABSTRACT: An ultraviolet photoelectron spectroscopic study was performed to examine the electronic structure of poly-(1,1,2,2-tetramethyldisilylenemethylene), [(SiMe2)(2)CH](n), which consists of repeating Si-Si-C units. We compared the observed spectra to reported gas-phase spectra of Si2Me6 and Si4Me10 and theoretical molecular orbital calculations. Theoretical calculations by the ab initio and PM3 methods agree well with the observed spectra, which enabled assignment of the observed spectral features. The broad peak derived from Si-Si bond orbitals in polycarbosilane suggests that Si-Si units, which are separated by carbon atoms, still interact with each other, to form sigma -conjugation through the backbone. The observed ionization threshold energy of the polycarbosilane[(SiMe2)(2)CH2](n), is 6.4 eV. The dispersion of the highest valence band of polycarbosilane is less than that of polysilane, indicating that the degree of sigma -conjugation of polycarbosilane is smaller than that of polysilane.

 

124.    H. Sato, M. Naito, A. Tsukada, S. Karimoto, and A. Matsuda

            "Influence of substrates on epitaxial thin films of high-temperature superconductors"

            Physica C 362, 186-194 (2001).

 

            ABSTRACT: For growth of high-quality epitaxial thin films of high-temperature superconductors (HTSCs), it is essential to choose appropriate substrates. For a thin film of La2-xMxCuO4 (M = Ba, Sr), the superconducting transition temperature (T-c) is modified by the strain due to the lattice mismatch between the film and the substrate-For the compressively strained (001) films of La2-xSrxCuO4 and La2-xBaxCuO4 on LaSrAlO4 substrates, T-c reached 44 and 47 K, respectively, which are higher than the values for bulk samples. Moreover, the T-c-x phase diagrams for the compressed films did not show a local minimum at x similar to 0.125, the so-called "1/8 anomaly". Based on the experimental results, we suggest that the reduction of antiferromagnetic spin fluctuation increases T-c in HTSCs. For thin films of Bi2Sr2-CaCu2O8, we found that substrates with a small lattice mismatch are very effective in attaining a smooth and almost precipitate-ftee surface. (C) 2001 Elsevier Science B.V. All rights reserved.

 

125.    T. Schapers, J. Nitta, H. B. Heersche, and H. Takayanagi

            "Interference ferromagnet/semiconductor/ferromagnet spin field-effect transistor"

            Phys. Rev. B 6412 (12), art. no.-125314 (2001).

 

            ABSTRACT: An interference ferromagnet/semiconductor/ferromagnet transistor is proposed, where the relative conductance difference between parallel and antiparallel magnetization oscillates as a function of gate voltage. The characteristics of a one-dimensional as well as a two-dimensional structure are calculated and compared. In both cases the interferences result in an enhanced spin signal. It is shown that by using the spin filtering effect of an interface barrier the signal can be further increased.

 

126.    A. Schulzgen, Y. Kawabe, E. Hanamura, A. Yamanaka, P. A. Blanche, J. Lee, H. Sato, M. Naito, N. T. Dan, S. Uchida, Y. Tanabe, and N. Peyghambarian

            "Two-photon resonant third-harmonic generation in La2CuO4"

            Phys. Rev. Lett. 86 (14), 3164-3167 (2001).

 

            ABSTRACT: Combining linear absorption and nonlinear third harmonic generation (THG) experiments, we investigate details of the electronic structure of the highly correlated electronic system in La2CuO4. We demonstrate strong THG mainly due to the charge transfer excitation from O (2p(sigma)) to Cu (3d(x2-y2)). The THG spectrum shows pronounced features due to three-photon and two-photon resonance enhancement as well as quantum interference effects. We obtain excellent agreement with a THG spectrum calculated in terms of the excitonic cluster model and can identify both odd and even symmetry excitation modes.

 

127.    C. Sekar, T. Watanabe, A. Matsuda, H. Shibata, Y. Zenitani, and J. Akimitsu

            "Crystal growth and transport properties of 4-leg spin-ladder compound La2Cu2O5: occurrence of insulator-metal transition"

            Int. J. Inorg. Mater. 3 (8), 1201-1203 (2001).

 

            ABSTRACT: A remarkable metal-insulator transition has been observed, for the first time, in hole-doped spin ladder compound La2Cu2O5 (4-leg) single crystals. Hole-doping was carried out by means of high-oxygen-pressure annealing at 600 degreesC under a pressure of 400 atm (20%O-2+80%Ar at a total pressure of 2000 atm using a hot-isostatic-pressing (HIP) furnace, In order to realize metallicity, a prolonged HIP-treatment for about 22 days or more is required. However, this annealing period is reduced to about 5-6 days, when the crystals are grown from the starting charges containing Ag in the form of Ag2O. Even in the as-grown state, the measured room-temperature resistivity is about two to three orders of magnitude lower than that of La2CU2O5 crystals grown from charges without silver. The results show that the silver addition facilitates the oxygenation of the La2CU2O5 crystals. (C) 2001 Published by Elsevier Science Ltd.

 

128.    C. Sekar, T. Watanabe, A. Matsuda, H. Shibata, Y. Zenitani, and J. Akimitsu

            "Effect of silver addition on structure and electrical properties of the spin ladder compound La2Cu2O5 single crystals"

            J. Cryst. Growth 233 (3), 466-472 (2001).

 

            ABSTRACT: We have investigated the influence of silver on crystallization and oxygenation or the spin ladder compound La2Cu2O5 (four-leg) single crystals. Large La2Cu2O5 single crystals were successfully grown from charges containing A920 by a self-flux method. Electrical resistivity of the silver grown La2Cu2O5 single crystals (parallel tob-axis) has been found to be about three orders of magnitude lower when compared with the similar crystals grown without silver. Characterizations with optical microscope, XRD, EPMA and HRTEM indicate that the silver does not exist either in free state in the crystal or in the crystal lattice. The results show that the silver facilitates oxygenation in La2Cu2O5 crystals during their growth. (C) 2001 Elsevier Science B.V. All rights reserved.

 

129.    C. Sekar, T. Watanabe, A. Matsuda, H. Shibata, Y. Zenitani, and J. Akimitsu

            "Crystal growth, structure, and transport properties of the five-leg spin ladder compound La8Cu7O19"

            J. Solid State Chem. 156 (2), 422-427 (2001).

 

            ABSTRACT: The crystallization region of La8Cu7O19 exists in a narrow temperature (1045-1050 degreesC) and composition range (1:4.5-1:5.0; La2O3:CuO molar ratio) in the La2O3-CuO system in an air ambient. In this narrow range, we have succeeded in growing large La8Cu7O19 single crystals, for the first time, by a modified slow cooling method. Single-crystal X-ray diffraction analysis reveals that La8Cu7O19 crystallizes in a monoclinic structure with space group C2/c and has the lattice parameters a = 13.82(7) Angstrom, b = 3.734(8) Angstrom, c = 34.77(10) Angstrom, and beta = 99.3(4)degrees. The electrical transport behavior of the as-grown and hole-doped La8Cu7O19 single crystals are semiconducting. A lowest resistivity of 11.0 m Omega -cm is measured at 293 K along the ladder direction (// b-axis) for the hole-doped samples. (C) 2001 Academic Press.

 

130.    Y. Sekine, N. Takeshita, N. Mori, M. Isobe, Y. Ueda, M. Kosaka, and Y. Uwatoko

            "Dielectric and magnetic properties of alpha '-NaV2O5 under multi-extreme conditions"

            J. Phys. Soc. Jpn. 70 (12), 3660-3666 (2001).

 

            ABSTRACT: We developed a method of precise measurement of the dielectric constant under multi-extreme conditions that is under high pressures high magnetic fields and low temperatures. We measured the dielectric constants and the magnetic susceptibility of single crystals of alpha ' -NaV2O5 under multi-extreme conditions. Anisotropic behavior of a relative dielectric constant was observed at the charge-ordering transition temperature under ambient pressure. With increasing hydrostatic pressure the transition temperature decreased and another transition appeared below the first transition temperature. The susceptibility exhibited it sharp drop corresponding to the transition to the spin singlet state at the first transition temperature, while it showed no anomaly at the transition temperature. These two transition temperature, were found to exhibit very weak magnetic field dependence in comparison with typical spin-peierls compounds.

 

131.    T. Sekitani, H. Nakagawa, N. Miura, and M. Naito

            "Negative magneto-resistance of the normal state in Nd2-xCexCuO4 below T-C and the effect of high magnetic fields"

            Physica B 294, 358-362 (2001).

 

            ABSTRACT: We have measured the low-temperature magneto-resistance in the normal state of Nd2-xCexCuO4 by suppressing the superconductivity with high magnetic fields below T-C. The normal-state rho -T curve shows an upturn at low temperatures, which nearly has a log T dependence, in a similar manner to previous reports on hole-doped La2-xSrxCuO4 and electron-doped Pr2-xCexCexCuO4. Notably, for all the samples of various doping levels, this log T-dependent upturn tends to be suppressed by applying magnetic field as high as 40 T, resulting in novel negative magneto-resistance. This tendency leads the optimal- and over-doped samples to show a prominent crossover from insulating to metallic conduction by magnetic field. In under-doped samples, the log T dependence persists to higher magnetic fields than in optimum and over-doped samples. We discuss these findings on the basis of the Kondo scattering originating from the magnetic moments of Cu ions. (C) 2001 Elsevier Science B.V. All rights reserved.

 

132.    K. Semba and A. Matsuda

            "Superconductor-to-insulator transition and transport properties of underdoped YBa2Cu3Oy crystals"

            Phys. Rev. Lett. 86 (3), 496-499 (2001).

 

            ABSTRACT: The carrier-concentration-driven superconductor-to-insulator (SI) transition as well as transport properties in underdoped YBa2Cu3Oy twinned crystals is studied. The SI transition takes place at y similar or equal to 6.3, carrier concentration n(H)(SI) similar or equal to 3 x 10(20) cm(-3), anisotropy rho (c)/rho (ab) similar or equal to 10(3), and the threshold resistivity rho (SI)(ab) similar to 0.8 m Omega cm which corresponds to a critical sheet resistance h/4e(2) similar or equal to 6.5 k Omega per CuO2 bilayer. The evolution of a carrier, n(H) proportional to y - 6.2, is clearly observed in the underdoped region. The resistivity and Hall coefficient abruptly acquire strong temperature dependence at y similar or equal to 6.5 indicating a radical change in the electronic state.

 

133.    H. Shibata

            "Double Josephson plasma in SmLa1-xSrxCuO4-delta"

            Physica C 362, 92-96 (2001).

 

            ABSTRACT: Josephson plasma in T* phase SmLa1-xSrxCuO4-delta is studied by the sphere resonance method for powder samples and by the reflectivity measurement of single crystals. In the sphere resonance method, two peaks appear below Tc. Both peaks shift to higher frequencies as the doping increases and are observed at 12 and 30 cm(-1) for SmLa0.85Sr0.15CuO3.985 (T-c = 31 K). The oscillator strength of the peak at the higher frequency is much smaller than that at the lower frequency. The same changes are also observed in the reflectivity measurement along the c-axis; two reflectivity edges are observed below T-c, the edges shift to higher frequencies as the doping increases, and the edge at higher frequency is smaller than that at lower frequency. These observations confirm that the system can be regarded as the. superconductor/insulatorl/superconductor/insulator2/superconductor.- type Josephson junction array. The reason for the smallness of the plasma at higher frequency, which is inconsistent with the prediction of the multilayer model, is also discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

 

134.    H. Shibata

            "Transverse Josephson plasma mode in T* phase SmLa1-xSrxCuO4-delta single crystals"

            Phys. Rev. Lett. 86 (10), 2122-2125 (2001).

 

            ABSTRACT: Far-infrared reflectivity along the c axis in T* phase SmLa1-xSrxCUO4-delta Single crystals is measured down to 8 cm(-1). Below T-c, the conductivity peak is observed at 25 cm(-1) for x = 0.15 (Tc = 30 K) along with two reflectivity edges at 13 and 27 cm(-1). The conductivity peak is attributed to the transverse Josephson plasma mode between two longitudinal Josephson plasma modes, while the oscillator strength of the peak is found to be smaller than that calculated using the Josephson-coupled multilayer model. The difference is explained by assuming that only a few junctions at the disordered (La, Sr)(2)O(2-delta)block layer take part in the plasma oscillation with omega (pI') = 27 cm(-1).

 

135.    D. J. Shin and A. Chavez-Pirson

            "Diffraction by a subwavelength-sized aperture in a metal plane"

            J. Opt. Soc. Am. A-Opt. Image Sci. Vis. 18 (7), 1477-1486 (2001).

 

            ABSTRACT: We present an experimental study on the diffraction of light by an aperture small compared with the wavelength. The aperture is illuminated by laser light guided in a metal-clad tapered optical fiber. We investigate different orientations of the aperture in the plane: normal to the cleaved plane, oblique to the cleaved plane, and off-center. We measure the far-field, two-dimensional intensity distributions of the diffracted light as functions of angle coordinates theta and phi in a full half-space for various polarization states and analyze the patterns by using low-order multipole fields. We also examine the near- and far-field effects of placing small periodic corrugations near the aperture, focusing on the role of surface-wave excitations. We measure the near-field intensity distributions near the aperture with a near-field scanning optical microscope and discuss their relation to the far-field diffracted fields. (C) 2001 Optical Society of America.

 

136.    K. Shiraishi, H. Tamura, and H. Takayanagi

            "Design of a semiconductor ferromagnet in a quantum-dot artificial crystal"

            Appl. Phys. Lett. 78 (23), 3702-3704 (2001).

 

            ABSTRACT: We present the theoretical design of quantum-dot (QD) artificial ferromagnetic crystals. The electronic structure calculations based on local spin density approximation show that our designed QD artificial crystal from a structure comprising the crossing 0.104 mum wide InAs quantum wires (an effective Kagome lattice) has flat band characteristics. Our examined QD artificial crystal has the ferromagnetic ground state when the flat band is half filled, even though it contains no magnetic elements. The ferromagnetic and paramagnetic states can be freely switched by changing the electron filling via a gate voltage. (C) 2001 American Institute of Physics.

 

137.    T. Sogawa, P. V. Santos, S. K. Zhang, S. Eshlaghi, A. D. Wieck, and K. H. Ploog

            "Transport and lifetime enhancement of photoexcited spins in GaAs by surface acoustic waves"

            Phys. Rev. Lett. 87 (27), 276601 (2001).

 

            ABSTRACT: We demonstrate spin transport and spin lifetime enhancement in GaAs quantum wells induced by the traveling piezoelectric field of a surface acoustic wave (SAW). Spin transport lengths of about 3 mum corresponding to spin relaxation times during transport over 1 ns are observed, which are considerably longer than the exciton spin diffusion lengths in the absence of a SAW. The slow spin relaxation is attributed to a reduced electron-hole exchange interaction, when the carriers are spatially separated by the lateral potential modulation induced by the SAW.

 

138.    T. Sogawa, P. V. Santos, S. K. Zhang, S. Eshlaghi, A. D. Wieck, and K. H. Ploog

            "Dynamic band-structure modulation of quantum wells by surface acoustic waves"

            Phys. Rev. B 63 (12), art. no.-121307 (2001).

 

            ABSTRACT: The dynamical modulation of the band structure of GaAs quantum wells by a surface acoustic wave (SAW) is investigated using photoluminescence (PL) spectroscopy. The strain field of the SAW modulates the excitonic transitions, leading to a splitting and to a polarization anisotropy of the excitonic PL lines. The oscillator strength of the split line gives direct information about the spatial distribution of carriers in the potential modulation induced by the SAW.

 

139.    G. S. Solomon, M. Pelton, and Y. Yamamoto

            "Single-mode spontaneous emission from a single quantum dot in a three-dimensional microcavity"

            Phys. Rev. Lett. 86 (17), 3903-3906 (2001).

 

            ABSTRACT: The spontaneous emission from an isolated semiconductor quantum dot state has been coupled with high efficiency to a single, polarization-degenerate cavity mode. The InAs quantum dot is epitaxially formed and embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single, quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 Fs, resulting in a single-mode spontaneous emission coupling efficiency of 78%.

 

140.    B. Starmark, E. Hurfeld, T. Henning, P. Delsing, A. N. Korotkov, R. S. Shaikhaidarov, T. Akazaki, E. Toyoda, and H. Takayanagi

            "Noise in the single electron transistor and controlled Josephson current in ballistic three terminal devices"

            Physica C 352 (1-4), 101-104 (2001).

 

            ABSTRACT: Two separate research topics are discussed. We discuss noise measurements of single electron transistors and the gain dependence of the noise. We will also describe the fabrication and preliminary measurements on several different superconductor/two-dimensional electron gas/superconductor structures. (C) 2001 Elsevier Science B.V. All rights reserved.

 

141.    O. M. Stoll, A. Wehner, R. P. Huebener, and M. Naito

            "Vortex-glass melting transition and scaling behavior in the cuprate superconductor Nd2-xCexCuOy at low temperatures"

            Physica C 363 (1), 31-40 (2001).

 

            ABSTRACT: We have investigated the vortex-glass transition for the cuprate superconductor Nd2-xCexCuOy (NCCO) in the region of low temperatures (T < 4.4 K) and moderate up to high magnetic fields (3 T < B < 5 T) compared to the upper critical field B-c2 (B-c2 approximate to 6 T for NCCO). We analyzed our data using the vortex-glass theory and found an excellent scaling behavior of the current-voltage characteristics in this temperature region. The critical exponents have been calculated and were compared to the few available investigations for NCCO and to the results for other materials. Additionally we determined the melting line for low temperatures and compared it to the available literature data. In this way we could draw a complete picture of the mixed-state phase diagram for this material. The knowledge about the phase diagram is crucial for the understanding of the transport properties of the high-T-c cuprates in the mixed state. It is found that for NCCO, thermal fluctuations play an important role even at temperatures below 4.2 K. This is possibly due to the relatively high anisotropy of this material. An extrapolation procedure is proposed in order to overcome our experimental limitation of the temperature range and to obtain the melting line for T > 4.4 K. A kink in the extrapolated portion of the melting line is found and explained in terms of a loss of universality of the critical exponents for temperatures T > 6 K. (C) 2001 Elsevier Science B.V. All rights reserved.

 

142.    M. Stopa

            "Periodic spin polarization of small quantum dots"

            Physica E-Low-Dimensional Systems & Nanostructures 10 (1-3), 103-106 (2001).

 

            ABSTRACT: We demonstrate, using spin density functional theory, that a quantum dot in a weak transverse magnetic field undergoes spontaneous spin polarization which fluctuates periodically with field strength B. The fluctuations are related to anti-crossing of developing Landau levels at the Fermi surface. For electron number N even, a crossing of two levels is sufficient to cause an instability to spontaneous polarization. For N odd, a simultaneous crossing of three or more levels is required. Such multiple crossings are characteristic of a circularly parabolic potential. Our calculations show that the realistic confining potential of a dot, calculated self-consistently in 3D, is sufficiently close to parabolicity for the multiple crossings to persist. (C) 2001 Elsevier Science B.V. All rights reserved.

 

143.    T. Suemitsu, T. Ishii, and Y. Ishii

            "Gate and recess engineering for ultrahigh-speed InP-based HEMTs"

            IEICE Trans. Electron. E84C (10), 1283-1288 (2001).

 

            ABSTRACT: InP-based high electron mobility transistors (HEMTs) with gate lengths reduced to 30 nm were fabricated and characterized, and the effect of the gate recess on the high-frequency characteristics was studied. The cutoff frequency, which is regarded as a function of the gate length and the average carrier velocity in a first-order approximation, depends on the size of the gate recess when the gate length becomes short. The size of the gate recess is optimized by taking the feed-back capacitance and the parasitic resistance into account. For HEMTS having the gate recess with an InP surface, an appropriate widening of the gate recess gives a record cutoff frequency of 368 GHz for the 30-nm-gate HEMTs with a lattice-matched channel.

 

144.    N. Susa

            "Threshold gain and single-mode oscillation of two-dimensional photonic bandgap defect lasers"

            IEEE J. Quantum Electron. 37 (11), 1420-1426 (2001).

 

            ABSTRACT: Defect laser behavior was simulated as a function of various parameters. When the crystal size was increased from 8 x 8 to 12 x 12 cylinders, the quality factor of the defect cavity increased from 6 X 10(3) to 5 x 10(5) while the threshold gain decreased by two orders of magnitude. It was predicted that a triangular-lattice photonic defect laser would be polarized in the Gamma -M direction. The defect laser studied usually showed multi-mode oscillation with a large defect radius, where the threshold gain was small. When the refractive indices of the cylinders and the defect, surrounded by air, were both 3.5, single-mode oscillation and a small threshold gain (4 cm(-1)) were, nevertheless, achieved at suitable device parameters that made the side-mode suppression ratio sufficiently large at 103. A single-mode oscillation and a small threshold gain (less than 20 cm(-1)) were also predicted over the entire defect radius range when the defect refractive index was decreased from 3.5 to 1.5.

 

145.    N. Susa

            "Threshold gain due to distributed-feedback in two-dimensional triangular- and square-lattice photonic crystal lasers"

            Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 40 (1), 142-145 (2001).

 

            ABSTRACT: Numerical simulation of 2D square (SQ)- and triangular (TR)-lattice infinitr-width photonic crystal distributed-feedback (DFB) lasers showed that the threshold gain (g(th)) for the 3rd photonic band was the smallest in the Gamma -X direction (SQ-lattice), and was next smallest in the Gamma -M direction (TR-lattice), but one order of magnitude larger. The g(th) values for the 3rd photonic bands were qualitatively proportional to the slopes approximated for the 3rd bands, but the g(th) values for the 1st bands were independent of the lattice structure and direction. All the g(th) values for the 1st and 3rd bands decreased to about 1/50 of the original values when the thickness was increased from 8 to 32 cylinders because of the approaching of the threshold wavelength to the band edge and the longer optical path.

 

146.    N. Susa

            "Threshold gain and gain-enhancement due to distributed-feedback in two-dimensional photonic-crystal lasers"

            J. Appl. Phys. 89 (2), 815-823 (2001).

 

            ABSTRACT: The threshold gain (g(th)) of a two-dimensional (2D) photonic crystal distributed-feedback (DFB) laser composed of 8x8 dielectric cylinders was one order of magnitude smaller than that of an 8-pair 1D DFB laser with the identical refractive indexes. In the 2D finite-width photonic crystal laser, g(th) using the 1st photonic band was smaller than that using the higher photonic band, contrary to the expectation from the flatness of the photonic band structures. This unexpected g(th) is probably due to the longer optical path caused by the reflection at the side boundary. Moreover, g(th) using the 1st photonic band was the smallest in the Gamma -X direction of the square-lattice photonic crystal. The gain-enhancement using the 1st-3rd photonic bands were 10-30 in the 2D photonic crystal and that using the 3rd band was the largest. The gain-enhancement using the 1st photonic band of the 2D photonic crystal consisting of dielectric cylinders was larger than that of air cylinders. (C) 2001 American Institute of Physics.

 

147.    K. Suzuki, S. Miyashita, and Y. Hirayama

            "Back-gate control in an InAs-based two-dimensional system"

            Physica C 352 (1-4), 125-127 (2001).

 

            ABSTRACT: Growing a heterostructure on a conductive substrate, the channel carrier density can be controlled by an applied gate voltage to the substrate. We made InAs/Al/GaSb single-quantum wells using n- and p-type InAs substrates as a back-gate. The devices perform well and the gate-leakage characteristics are better for the n-type substrates than for the p-type substrates. (C) 2001 Elsevier Science B.V. All rights reserved.

 

148.    M. Suzuki, K. Anagawa, M. Lmouchter, and T. Watanabe

            "Distinct superconducting gap and the pseudogap in the interlayer tunneling spectroscopy for Bi2Sr2CaCu2O8+delta"

            Physica C 362, 164-168 (2001).

 

            ABSTRACT: Tunneling spectroscopy by use of intrinsic Josephson junctions in the Bi2Sr2CaCu2O8+delta has provided evidence that the superconducting gap and the pseudogap are definitely different. In underdoped specimens, the tunneling spectrum exhibits two conductance peaks, indicating that the superconducting gap is distinct from the pseudogap. This result is in. conflict with some superconductivity models in which the superconducting gap remains finite at T-c and connects smoothly with the pseudogap above T-c. (C) 2001 Elsevier Science B.V. All rights reserved.

 

149.    S. Suzuki, C. Bower, T. Kiyokura, K. G. Nath, Y. Watanabe, and O. Zhou

            "Photoemission spectroscopy of single-walled carbon nanotube bundles"

            J. Electron Spectrosc. Relat. Phenom. 114, 225-228 (2001).

 

            ABSTRACT: The electronic structures and the work functions of pristine and Cs-intercalated single-walled carbon nanotube bundles were investigated by C 1s and valence band photoemission spectroscopy. The C 1s spectrum of the pristine material showed a Doniach-Sunjic type asymmetry, indicating the existence of metallic tubes. The work function of the pristine bundles was found to be 4.8 eV, which is about 0.2 eV larger than that of graphite. A drastic decrease of the work function to about 2.0 eV was observed in the Cs-intercalated sample. The Cs intercalation also caused a nearly two-order increase in the spectral intensity at the Fermi level. (C) 2001 Elsevier Science B.V. All rights reserved.

 

150.    S. Suzuki, T. Kiyokura, F. Maeda, K. G. Nath, Y. Watanabe, T. Saitoh, and A. Kakizaki

            "Observation of Ga 3d two-hole states from GaAs surfaces"

            J. Electron Spectrosc. Relat. Phenom. 114, 421-425 (2001).

 

            ABSTRACT: Ga 3d two-hole states were observed from GaAs(lll)A-(2X2), (001)-c(4x4), (001)-(2X4), and (110)-(1X1) surfaces using Ga 3p --> 4s resonant photoemission spectroscopy. The Slater integrals F-2 and F-4 Of the Ga 3d orbitals were determined to be 14.4 and 6.0 from the relative energy positions of multiplet terms (1)G, D-1, and F-3. Surface components of the two-hole states were observed in the spectra of the (111)A-(2X2), (001)-(2X4) and (110)-(1X1) surfaces involving Ga atoms at the surface. The intensities of the surface components of the two-hole states were found to be considerably smaller than those of the d(9) states. This is ascribed to the reduced absorption probability of the 3p electrons at the surface caused by surface relaxation. (C) 2001 Elsevier Science B.V. All rights reserved.

 

151.    S. Suzuki, Y. Watanabe, T. Kiyokura, K. G. Nath, T. Ogino, S. Heun, W. Zhu, C. Bower, and O. Zhou

            "Electronic structure at carbon nanotube tips studied by photoemission spectroscopy"

            Phys. Rev. B 6324 (24), 245418 (2001).

 

            ABSTRACT: The valence band and C 1s photoemission spectra were measured from the tips and the sidewalls of multiwalled carbon nantoubes grown on Si substrates. The results show an overall spectral shift to the higher-binding-energy side, a larger density of states at the Fermi level, and a lower work function at the tips. These results can be explained by assuming that the Fermi level is slightly shifted upward and located inside the conduction band at the tips. We suggest that the electronic structure at the tips is considerably affected by defects.

 

152.    A. Taguchi, K. Shiraishi, and T. Ito

            "First-principles study of Si incorporation processes on a GaAs(111)A surface"

            J. Cryst. Growth 227, 83-87 (2001).

 

            ABSTRACT: Si incorporation processes on a GaAs(1 1 1)A surface were investigated by using the first-principles pseudopotential method. We found that the most stable adsorption site for a single Si atom is a Ga lattice site of the GaAs lattice, but that the Si atom is pushed out from the site when Ga-As-Si microstructures are formed on the surface. We also found that a Si atom remains at the Ga lattice sire when it couples with three As adatoms. These findings qualitatively explain the experimentally observed properties of Si-doped GaAs layers, i.e., that the layers are p-type under conventional growth conditions and they become n-type under very high As pressure. (C) 2001 Elsevier Science B.V. All rights reserved.

 

153.    A. Taguchi, K. Shiraishi, T. Ito, and Y. Kangawa

            "Theoretical investigations of adatom adsorptions on the As-stabilized GaAs(111)A surface"

            Surf. Sci. 493 (1-3), 173-177 (2001).

 

            ABSTRACT: We investigated adsorptions of Ga and As atoms on the As-stabilized GaAs(111)A surface by using first-principles calculations. We found that a single As adatom does not occupy the As lattice site on the surface, but the As lattice site becomes active for an As adatom when the As adatom couples with Ga adatoms. This finding indicates that Ga has a self-surfactant effect on the As-stabilized surface. We also found that adatoms do not form stable microstructures on the As-stabilized surface. This is consistent with the reported experimental results that the layer-by-layer growth is difficult. We discuss epitaxial growth processes based on the calculation results. (C) 2001 Elsevier Science B.V. All rights reserved.

 

154.    H. Takayanagi, T. Akazaki, E. Toyoda, and H. Nakano

            "Quantum transport in superconductor-semiconductor junctions"

            Physica C 352 (1-4), 95-100 (2001).

 

            ABSTRACT: Experimental and theoretical results of quantum transport in superconductor-semiconductor junctions are summarized. We will first show the experimental results of reentrant behavior of the conductance as well as of giant Andreev backscattering. Then Andresv reflection in the quantum Hall regime is described. The charging effect on the proximity correction is finally discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

 

155.    H. Takenaka, K. Nagai, H. Ito, Y. Muramatsu, T. Kawamura, E. Gullikson, and R. C. C. Perera

            "Soft X-ray reflectivity and structure evaluation of CoCr/C multilayer X-ray mirrors for spectral region around 6 nm"

            Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equip. 467, 337-340 (2001).

 

            ABSTRACT: The development of highly-reflective multilayer mirrors for use in the 6-nm region is desired for X-ray photoemission spectroscopy for inner-shell excitation using a Schwarzschild objective. For this application, reflectivity is the most critical parameter determining the performance of multilayer mirrors, because the reflectivities of multilayers in the 6-nm region are generally very low, We have designed CoCr/C multilayer mirrors with a comparatively high reflectivity at around normal incidence and have fabricated them by magnetron sputtering. The measured peak reflectivity is about 11.5% at a wavelength of around 6 nm and an incident angle of 88 degrees. Thermal annealing was found to markedly improve the reflectivity, and a high value of 13% was obtained by annealing at 400 in an Ar atmosphere for 1 h. (C) 2001 Elsevier Science B.V. All rights reserved.

 

156.    H. Tamura, K. Shiraishi, and H. Takayanagi

            "Semiconductor ferromagnetism in quantum dot array"

            Phys. Status Solidi B-Basic Res. 224 (3), 723-725 (2001).

 

            ABSTRACT: Ferromagnetism is theoretically predicted in non-magnetic semiconductor-dot arrays. We propose two types of bipartite dot-arrays which show flat-band ferromagnetism according to the Lieb theorem. We verify spin-polarized ground states at half-filling by performing an exact diagonalization calculation. The ferromagnetic states can be fi eely switched on and off by changing the electron-filling via the gate voltage.

 

157.    H. Tanaka, Y. Sekine, S. Saito, and H. Takayanagi

            "Dc SQUID sensitivity for readout of entangled state quantum bits"

            Supercond. Sci. Technol. 14 (12), 1161-1165 (2001).

 

            ABSTRACT: We propose a scheme for entangled state measurement at flux qubits (quantum bits) depending on the sensitivity measurement of a dc SQUID (superconducting quantum interference device). The dc SQUID is used as a flux qubit readout device. The switching current distribution of the dc SQUID is sufficiently narrow to distinguish between two ground states of three Josephson junction qubits with a large mutual inductance between a qubit and the dc SQUID. However, discrimination between two ground states becomes difficult with the smaller mutual inductance that is preferable for qubit coherence. We employed averaging to increase the effective sensitivity. However the averaging smears out the entanglement over two qubits. We found that a Bell-pair measurement can be performed with flux qubits to prove the existence of an entangled state over two qubits. The pre-ensembled measurement scheme, in which our flux qubit readouts work, is preferable to entangled state measurement.

 

158.    K. Tanaka, K. Shimano, K. Inoue, S. Kuwano, T. Kitagawa, and K. Oguchi

            "Optical label switching using optical label based on wavelength and pilot tone frequency"

            IEICE Trans. Electron. E84C (5), 501-508 (2001).

 

            ABSTRACT: This paper describes a new optical label switching technique; wavelength and pilot, tone frequency are combined to form labels that are used to control transport network routing. This technique is very attractive for achieving simple nodes that offer extremely rapid forwarding. Experimental results on the discrimination of optical labels and all-optical label conversion are also presented.

 

159.    K. Tanaka, K. Shimano, K. Inoue, S. Kuwano, T. Kitagawa, and K. Oguchi

            "Optical label switching using optical label based on wavelength and pilot tone frequency"

            IEICE Trans. Commun. E84B (5), 1127-1134 (2001).

 

            ABSTRACT: This paper describes a new optical label switching technique; wavelength and pilot tone frequency are combined to form labels that are used to control transport network routing. This technique is very attractive for achieving simple nodes that offer extremely rapid forwarding. Experimental results on the discrimination of optical labels and all-optical label conversion are also presented.

 

160.    H. Z. Tang, M. Fujiki, M. Motonaga, and K. Torimitsu

            "Circular dichroism and circularly polarized photoluminescence of helical polyfluorenes"

            Abstr. Pap. Am. Chem. Soc. 221, U296-U296 (2001).

 

            ABSTRACT:

161.    H. Z. Tang, M. Fujiki, Z. B. Zhang, K. Torimitsu, and M. Motonaga

            "Nearly pure blue photoluminescent poly{2,7-[9-{3,5-bis[3,5-bis(benzyloxy)benzyloxy]benzyl}-9-(3,6-dioxaoct yl)]fluorene} in film"

            Chem. Commun. (23), 2426-2427 (2001).

 

            ABSTRACT: The first unsymmetrically substituted polyfluorene bearing a bulky poly(benzyl ether) dendron and less bulky 3,6-dioxaoctyl groups in the 9-position was designed and synthesized, which gives almost a pure bluish photoluminescence with negligible weak greenish excimer emission around 520 nm even in a thermally annealed thin solid film.

 

162.    Y. Taniyasu, M. Kasu, and N. Kobayashi

            "Lattice parameters of wurtzite Al1-xSixN ternary alloys"

            Appl. Phys. Lett. 79 (26), 4351-4353 (2001).

 

            ABSTRACT: Lattice constants and bond lengths of wurtzite Al1-xSixN ternary alloys (0 less than or equal tox less than or equal to0.08) were determined by reciprocal lattice maps around Al1-xSixN (0002) and (11-24) reflections. The measured lattice constants obtained directly from as-grown Al1-xSixN layers were scattered because they include the factor of residual strain. Therefore, the lattice constants in the strain-free case were calculated from the measured lattice constants taking the residual strain into account. We found that the a-axis and c-axis lattice constants of the strain-free Al1-xSixN linearly decreased with the Si content as a(0)=3.1113-0.1412x (Angstrom) and c(0)=4.9814-0.2299x (Angstrom). Further, we obtained the bond length as d(0)=1.86818-0.0862x (Angstrom). The bond length is nearly equal to the interpolation between the Al-N bond and the Si-N bond. (C) 2001 American Institute of Physics.

 

163.    S. Tarucha, D. G. Austing, S. Sasaki, T. Fujisawa, Y. Tokura, J. M. Elzerman, W. van der Wiel, S. de Franseschi, and L. P. Kouwenhoven

            "Novel Kondo anomaly in quantum dots"

            Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 84 (1-2), 10-16 (2001).

 

            ABSTRACT: Experimental observation of novel Kondo effects in quantum dots is reviewed with particular emphasis on the effect of magnetic field. The electronic configuration including spins in quantum dots can be modified by even a small magnetic field. We first use a vertical quantum dot with good tunability of orbital degeneracy in the few-electron regime but having a fixed tunnel coupling between the dot and leads. By tuning the spin configuration out of two electrons as a function of magnetic field, we observe a strong Kondo effect when the spin singlet and triplet are degenerate. This is due to the increased number of coherent co-tunneling processes. In the second we use a lateral quantum dot having good tunability of the tunnel coupling but some ambiguity in the electronic configuration. By adjusting both of the tunnel coupling and magnetic field, we observe the Kondo effect in the unitary limit. This is only observed in the presence of a small magnetic field, and can also be due to the change in the spin configuration. (C) 2001 Elsevier Science B.V. All rights reserved.

 

164.    S. Tarucha, D. G. Austing, S. Sasaki, Y. Tokura, J. M. Elzerman, W. van der Wiel, S. de Franseschi, and L. P. Kouwenhoven

            "Spin effects in semiconductor quantum dot structures"

            Physica E-Low-Dimensional Systems & Nanostructures 10 (1-3), 45-51 (2001).

 

            ABSTRACT: The spin effects on the electronic properties are studied for two different quantum dots: a quantum dot well isolated from the leads and that strongly coupled to the leads. For the first quantum dot, we use a magnetic field to adjust the single-particle state degeneracy, and observe a singlet-triplet transition at zero and non-zero magnetic fields, favored by direct Coulomb and exchange interactions. The spin configurations in an arbitrary magnetic field are well explained in terms of two-interacting electron model. For the second quantum dot we adjust the single-particle state degeneracy as a function of magnetic field and observe a novel Kondo effect associated with the singlet-triplet degeneracy. This Kondo anomaly appears when the quantum dot holds an even number of electrons, and the characteristic energy scale is much larger than in the ordinary half-spin case. (C) 2001 Elsevier Science B.V. All rights reserved.

 

165.    J. Temmyo, H. Kamada, E. Kuramochi, H. Ando, and T. Tamamura

            "Semiconductor nanostructures via self-organization for a two-state system"

            J. Korean Phys. Soc. 39, S368-S371 (2001).

 

            ABSTRACT: This paper reviews the present status of semiconductor nanostrutures via self-organization. We mainly describe self-organization in a strained InGaAs/AlGaAs system oil a GaAs (311)B surface. This self-organizing growth mode in metal-organic-vapor phase epitaxy automatically produces very small confined nanostructure of quantum disks during growth interruption under a high growth temperature. The micro-photoluminescence (PL) and PL excitation spectroscopies exhibit very narrow excitonic emission and absorption lines, indicating that a zero-dimensional nanostructure, is formed in the disk. Discrete excitonic states in the zero-dimensional disk may be applicable to a quantum two-state system.

 

166.    A. Teramoto, K. Terao, Y. Terao, N. Nakamura, T. Sato, and M. Fujiki

            "Interplay of the main chain, chiral side chains, and solvent in conformational transitions: Poly{[(R)-3,7-dimethyloctyl]-[(S)-3-methylpentyl]silylene}"

            J. Am. Chem. Soc. 123 (49), 12303-12310 (2001).

 

            ABSTRACT: Light scattering, sedimentation equilibrium, viscosity, circular dichroism (CD), and UV absorption (UV) measurements were made on dilute solutions of Poly{[(R)-3,7-dimethyloctyl]-[(S)-3-methylpentyl]silylene} (PRS) as functions of molecular weight. From light scattering and viscosity data, PRS is found to be a very stiff polymer of persistence length q as large as 103 nm at 25 degreesC, essentially a 7(3) helix found in the solid state; q increases only gradually with lowering temperature between -15 and 25 degreesC. The CD data show that PRS undergoes a conformational transition around 3 degreesC in isooctane (transition temperature T-c). The CD signal is largely positive at low temperatures, passes through zero at T-c, and becomes largely negative at higher temperatures: T-c is independent of sample's chain length N. This is a highly cooperative helix (M)-to-helix (P) transition depending remarkably on N, as PRS is substantially rodlike. The CD data are converted to the fraction f(P) of P helix as a function of N and analyzed successfully by a statistical mechanical theory based on a helix reversal model, where a polymer chain consists of M and P helices intervened by helix reversals, with the result that the free energy difference DeltaG(h) between P and M shows a temperature dependence similar to that of 2f(P)-1, whereas the helix reversal energy is substantially constant at 1.2 x 10(4) J mol(-1); the latter value means that the helix reversal occurs only once in 100 Si units or less. This DeltaG(h) change and solvent dependence of T-c are explained by a double-well potential for the rotation about Si-Si bonds, which incorporates into DeltaG(h) the solvent interactions with the helical grooves of side chains surrounding the main chain. Detailed features of UV absorption spectra at different temperature and molecular weights are also presented.

 

167.    K. Terao, Y. Terao, A. Teramoto, N. Nakamura, M. Fujiki, and T. Sato

            "Conformational transitions in poly{n-hexyl-[(S)-3-methylpentyl]silylene} in dilute solution: Temperature and molecular weight dependence detected by circular dichroism"

            Macromolecules 34 (18), 6519-6525 (2001).

 

            ABSTRACT: Ten narrow-distribution samples of poly{n-hexyl-[(S)-3-methylpentyl]silylene} (PH3MPS) ranging in weight-average molecular weight from 3.1 x 10(3) to 8.7 x 10(5) in isooctane, n-hexane, and methylcyclohexane were studied by circular dichroism and ultraviolet absorption over a wide temperature range (from -75 to 85 degreesC). To follow the conformational transition, Kuhn's dissymmetry factor g(abs) as a measure of helicity was determined as a function of molecular weight and temperature from the ratio of circular dichroism to absorbency. The molecular weight dependence of g(abs) was analyzed by a statistical mechanical theory based on a conformational picture of an alternating sequence of right-handed and left-handed helices intervened by helix reversals. The free energy of the helix reversal was much smaller than those for polyisocyanates investigated previously [poly((R)-1-deuterio-n-hexyl isocyanate) and poly((R)-2-deuterio-n-hexyl isocyanate)] at the same temperatures although its solutions showed large circular dichroism. Therefore, it was shown that the correlation lengths of the helix for this polymer were much shorter than the polyisocyanates at the same temperature. Thus, the helix reversal is the dominant molecular process in the conformational properties of PH3MPS.

 

168.    K. Terao, Y. Terao, A. Teramoto, N. Nakamura, M. Fujiki, and T. Sato

            "Temperature and solvent dependence of stiffness of poly{n-hexyl-[(S)-3-methylpentyl]silylene} in dilute solutions"

            Macromolecules 34 (13), 4519-4525 (2001).

 

            ABSTRACT: Eleven samples of poly{n-hexyl- [(S)-3-methylpentyl]silylene} (PH3MPS) ranging in weight-average molecular weight M-w from 3.1 x 10(3) to 8.7 x 10(5) in isooctane have been studied by light scattering at 25 degreesC, sedimentation equilibrium at 25 degreesC, and viscometry at five temperatures (-27,-15, 5, 25, and 45 degreesC) to determine their z-average radii of gyration (S-2)(x), second virial coefficients, and intrinsic viscosities [eta] The (S-2)(x) and [eta] data are analyzed quantitatively by the current theories on the basis of the wormlike chain with excluded volume over the entire range of M, studied. PH3MPS is shown to be a typical semiflexible polymer with the persistence length changing from 15.4 to 5.0 nm at -27 to 45 degreesC. The viscometry has also been made on a PH3MPS sample in n-hexane and methylcyclohexane at 25 degreesC. In these solvents, the global conformation of PH3MPS is essentially the same as that in isooctane at the same temperature.

 

169.    K. Terao, Y. Terao, A. Teramoto, N. Nakamura, I. Terakawa, and T. Sato

            "Stiffness of polysilylenes depending remarkably on a subtle difference in chiral side chain structure: Poly{n-hexyl-[(S)-2-methylbutyl]silylene)} and poly{n-hexyl-[(S)-3-methylpentyl]silylene}"

            Macromolecules 34 (8), 2682-2685 (2001).

 

            ABSTRACT: Light scattering and vicosity measurements were made on fractionated samples of poly-{n-hexyl-{(S)-2-methylbutyl] silylene)} (PH2MBS, 1) and poly{n-hexyl-[(S)-3-methylpentyl]silylene} (PH3MPS, 2) in isooctane to determine their stiffness, which are expressed in terms of the persistence length q of Kraty and Pored. The q of polymer 1 is as large as 85 nm and depends only slightly on temperature, indicating that its conformation is almost rodlike, whereas the a of polymer 2 is 6.2 nm, its global conformation being close to a random coil. It is concluded that the global conformation of polysilylene is determined by its side chain; polysilylene with a beta -branch is much stiffer compared to those without it.

 

170.    Y. Tokura, S. Sasaki, D. G. Austing, and S. Tarucha

            "Excitation spectra and exchange interactions in circular and elliptical quantum dots"

            Physica B 298 (1-4), 260-266 (2001).

 

            ABSTRACT: The quantum states of interacting electrons in circular and elliptical quantum dots in a magnetic field are investigated. We model the elliptical dot with two orthonormal parabolic confinement potentials of different strengths and utilize analytical forms of the single-electron states. The high-spin state of the four electron system consistent with Hund's first rule present in a circular dot near zero field becomes unstable as the dot shape becomes deformed from the circular symmetry. The excitation spectra of the three and four electron systems are compared with experiments. The stability of high-spin states realized near single-electron level crossings an finite magnetic fields is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.

 

171.    S. Toyoda and M. Fujiki

            "Origin of broad visible photoluminescence from poly(alkylarylsilylene) derivatives"

            Macromolecules 34 (8), 2630-2634 (2001).

 

            ABSTRACT: We investigated the absorption, photoluminescence (PL), and PL anisotropy spectra of a variety of poly(alkylarylsilylene)s in solution at room temperature and in thin film form in the 20-296 K range. The poly(alkylarylsilylene)s include poly(methyl-p-tert-butylphenylsilylene) (1), poly(methylphenylsilylene) (2), poly(ethylphenylsilylene) (3), poly(n-pentylphenylsilylene) (4), poly(n-hexylphenylsilylene) (5), and poly(n-pentyl-m-tolylsilylene) (6). Although there was always a visible PL band from 1, 2, and 3 in the visible region, there was almost no visible PL band with thin films of 4, 5, and 6 in the room temperature to 20 K range. We ascribe the difference in the PL characteristics to the extended global shape with a spatially homogeneous local conformational structure in solution and in solid films.

 

172.    S. Toyoda and M. Fujiki

            "Cooperative preferential helical ordering in poly(alkylarylsilylene) copolymers"

            Macromolecules 34 (3), 640-644 (2001).

 

            ABSTRACT: We prepared two series of poly[((S)-2-methylbutyl(phenyl)silylene)(x)-co-(methylphenylsilylene)(1- x)] (0 less than or equal to x less than or equal to 0.5) (1) with coiled conformations and poly [((S)-2-methylbutyl(phenyl)-silylene)(x)-co-(n-hexyl(m-tolyl)silylene)(1 -x)] (0 less than or equal to x less than or equal to 0.5) (2) with stiffer conformations; however, we could not produce the poly[((S)-2-methylbutyl(phenyl)silylene)] homopolymer due to steric overcrowding. We conclude that even optically inactive 1 with x = 0 and 2 with x = 0 adopt helical conformations with an equal quantity of P-and M-screw-senses by means of UV, circular dichroism (CD), fluorescence (FL) spectra, FL anisotropy, and molecular mechanics calculations. We found that the preferential screw-sense is subject to a noticeable cooperative induction effect in both copolymer systems as the chiral silylene moiety increases. This is based on a quantative analysis of the dissymmetry ratio (g = Delta epsilon/epsilon) as a function of chiral molar composition. However, the helical cooperative effects are markedly different between 1 and 2, which is believed to arise from their differences regarding chain stiffness, global conformation, and persistence length.

 

173.    K. Tsubaki

            "Aharonov-Bohm oscillation in rings with permalloy"

            Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 40 (3B), 1902-1905 (2001).

 

            ABSTRACT: Aharonov-Bohm (A-B) rings with and without an inserted permalloy rod were fabricated from an AlGaAs/GaAs two dimensional electron gas (2DEG) wafer. The permalloy was a soft ferromagnetic alloy metal with a relative permeability A of about 10(4). so that the effective susceptibility, including the demagnetizing effect, could be calculated from only the diameter and height of the permalloy rod. Magnetoresistance of the A-B rings with and without the permalloy rod was measured at T = 0.3 K using an external magnetic field. The magnetoresistance had oscillations of different periods in the two cases. The main oscillation periods of the rings with and without the permalloy rod were 7.5 Oe and 28 Oe. respectively. clearly indicating that the permalloy rod reduced the oscillation period. The reduction factor (26%) was consistent with the demagnetizing effect of the permalloy rod. If the permalloy rod is designed to be free of the demagnetizing effect, the reduction factor should reach the order of the reciprocal of relative permeability mu.

 

174.    K. Ueda and M. Naito

            "As-grown superconducting MgB2 thin films prepared by molecular beam epitaxy"

            Appl. Phys. Lett. 79 (13), 2046-2048 (2001).

 

            ABSTRACT: Superconducting thin films of magnesium diboride (MgB2) were prepared on various substrates [SrTiO3 (001), sapphire R, sapphire C and Si (111)] by molecular beam epitaxy. The growth temperature was examined in the ambient to 650 degreesC. Only films formed at temperatures between 150 and 320 degreesC showed superconductivity. The best T-C(onset) of 36 K with a sharp transition width of similar to1 K was observed. The T-C of the as-grown superconducting MgB2 thin films is close to the bulk value. (C) 2001 American Institute of Physics.

 

175.    M. Uematsu, H. Kageshima, and K. Shiraishi

            "Oxidation simulation of heavily phosphorus-doped silicon based on the interfacial silicon emission model"

            Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 40 (9A), 5197-5200 (2001).

 

            ABSTRACT: Silicon oxidation of heavily phosphorus-doped substrates is simulated based on the interfacial silicon emission model. We assume that double negatively charged vacancies (V2-) from the substrates reduce the interfacial silicon emission, which governs the oxidation rate at the interface. The simulation is done by reducing the rate of Si-atom emission according to the concentration of V2- estimated from the carrier concentration of the substrates. In addition, the equilibrium concentration of oxygen in the oxide is increased with increasing P concentration to fit the experimental oxide thickness.

 

176.    M. Uematsu, H. Kageshima, and K. Shiraishi

            "The effect of chlorine on silicon oxidation: Simulation based on the interfacial silicon emission model"

            Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 40 (4A), 2217-2218 (2001).

 

            ABSTRACT: Silicon oxidation in dry oxygen containing small amounts (1-3%) of chlorine gas is simulated based on the interfacial silicon emission model. We assume that the presence of chlorine (Cl) reduces the interfacial silicon emission, which governs the oxidation rate at the interface. The simulation is done by reducing the rate of Si-atom emission in the presence of Cl. In addition, the equilibrium concentration of oxygen in the oxide is increased in proportion to Cl concentration to fit the experimental oxide thickness.

 

177.    M. Uematsu, H. Kageshima, and K. Shiraishi

            "Simulation of wet oxidation of silicon based on the interfacial silicon emission model and comparison with dry oxidation"

            J. Appl. Phys. 89 (3), 1948-1953 (2001).

 

            ABSTRACT: Silicon oxidation in wet ambients is simulated based on the interfacial silicon emission model and is compared with dry oxidation in terms of the silicon-atom emission. The silicon emission model enables the simulation of wet oxidation to be done using the oxidant self-diffusivity in the oxide with a single activation energy. The amount of silicon emission from the interface during wet oxidation is smaller than that during dry oxidation. The small emission rate for wet oxidation is responsible for the insignificant initial oxidation enhancement and the linear pressure dependence of the oxidation rate observed in wet oxidation. Using a unified set of parameters, the whole range of oxide thickness is fitted for both (100) and (111) substrates in a wide range of oxidation temperatures (800 degreesC-1200 degreesC) and pressures (1-20 atm). (C) 2001 American Institute of Physics.

 

178.    Y. Ueno, K. Ajito, Y. Yamada, Y. Maruo, O. Niwa, K. Torimitsu, and T. Ichino

            "Near-infrared raman spectra of azo dye produced by a nitrogen-dioxide-gas-selective coloration reaction in a porous glass chip"

            Appl. Spectrosc. 55 (9), 1151-1154 (2001).

 

            ABSTRACT: We directly observed azo dye in a porous glass chip by near-infrared (NIR) Raman spectroscopy. We produced the dye by a modified Saltzman reaction by using sulfanilamide (SFA) and N, N-dimethyl-1-naphthylamine (DMNA) as reagents and used it for the selective sensing of atmospheric nitrogen dioxide gas (NO2). The use of NIR laser excitation is particularly advantageous for the direct measurement of azo dye because it avoids the problem of interference from fluorescence. We obtained the 1597 cm(-1) band assigned to the phenyl group stretching mode and the 1415 cm(-1) band assigned to the azo group stretching mode. The Raman intensity and visible absorption intensity of the peaks of the azo dye in a porous glass chip increased as we increased the exposure time to NO2. This allowed us to monitor the NO2-sensing reaction in a porous glass chip using NIR Raman spectroscopy. Furthermore, we determined the depth profiles of the azo dye concentration in the porous glass chip by confocal NIR Raman spectroscopy and found that they were different in the early and later NO2-exposure stages. This result indicates that the consumption of the NO.-sensing reagents that accompany azo dye production occurs preferentially at the surface to the deeper regions of the porous glass chip.

 

179.    N. Uesugi, H. Nakano, T. Nishikawa, and P. Lu

            "Efficient soft X-ray generation from femtosecond-laser-produced plasma and its application to time resolved spectroscopy"

            J. Phys. IV 11 (PR2), 397-403 (2001).

 

            ABSTRACT: The experimental results on efficient soft x-ray generation from fs-laser-produced plasma and the application of short x-ray pulse to time-resolved spectroscopy are presented. Soft x-ray generation properties were evaluated for both flat Al targets and nanohole - alumina targets. The experimental results for flat metal targets revealed the fundamental properties of soft x-ray emission such as broadband continuum spectra and short pulse duration. By adopting nano-structured targets, a more than 30-hold enhancement of x-ray generation yield is achieved compared with that for flat targets of the same materials with a slight, increase of pulse duration. The time-resolved measurement of the inner-shelf absorption change of Si during the irradiation with a high-intensity fs laser pulse is achieved by using a short soft x-ray pulse as a probe pulse in pump-probe experiments for the first time. A more than 10% increase in the absorption of Si membrane at the L-II,L-III edge (around 100eV) was observed, The recovery time of the absorption change was measured to be about 20ps. This absorption change is assumed to be the bandgap renormarization of Si.

 

180.    W. G. van der Wiel, T. Fujisawa, S. Tarucha, and L. P. Kouwenhoven

            "A double quantum dot as an artificial two-level system"

            Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 40 (3B), 2100-2104 (2001).

 

            ABSTRACT: We study an artificial two-level system formed by a double quantum dot. Microwave spectroscopy experiments are discussed, in which we show that the inter-dot coupling can be tuned from ionic-like to covalent-like. The current through the double dot system is dependent on the applied microwave power and, in case of strong inter-dot coupling, the separation between the bonding and anti-bonding state is power dependent as well. We measure the elastic and inelastic transitions between the two levels. The inelastic transition rates are well described by Bose-Einstein statistics. Acoustic phonons are the most effectively coupled bosons in the solid-state environment of the two-level system.

 

181.    J. Watanabe, H. Kamee, and M. Fujiki

            "First observation of thermotropic cholesteric liquid crystal in helical polysilane"

            Polym. J. 33 (6), 495-497 (2001).

 

            ABSTRACT:

182.    T. Watanabe, C. Sekar, H. Shibata, T. Fujii, A. Matsuda, Y. Zenitani, and J. Akimitsu

            "Novel approaches to crystallize materials with narrow liquidus lines: application to spin ladder compound La4+4nCu8+2nO14+8n (n=2,3) and high-T-c cuprate Bi-2223"

            J. Cryst. Growth 229 (1), 316-320 (2001).

 

            ABSTRACT: We propose two novel approaches, a modified slow cooling (MSC) method and an improved traveling solvent floating zone (I-TSFZ) method, to crystallize materials with narrow liquidus lines. The MSC method uses a flux-poor starting composition to attain sufficient supersaturation, and involves a rapid cooling from high temperature to a proper crystallization temperature to avoid the formation of unwanted phases. The I-TSFZ method uses a very slow growth velocity to grow crystals under small supersaturation, and a steep temperature gradient to precisely control the growth temperature. The MSC method and the I-TSFZ method have been successfully applied to grow spin ladder compounds La4+4nCu8+2nO14+8n (n = 2,3) and a high-T-c, cuprate Bi2Sr2Ca2Cu3O10+delta, respectively, (C) 2001 Elsevier Science B.V. All rights reserved.

 

183.    T. Watanabe, C. Sekar, H. Shibata, A. Matsuda, Y. Zenitani, and J. Akimitsu

            "Crystal growth and transport properties of spin ladder compounds La4+4nCu8+2nO14+8n (n=2, 3)"

            Physica C 357, 380-383 (2001).

 

            ABSTRACT: Here we summarize recent progress on the study of spin ladder compounds, La2Cu2O5 (four-leg) and La8Cu7O19 (five-leg), n = 2 and 3 members of the homologous series La4+4nCu8+2nO14+8n. Following the successful crystal growth of La2Cu2O5, we grew a pure-phase five-leg system La8Cu7O19 by a modified slow cooling method. The room-temperature resistivity of the as-grown La8Cu7O19 crystal was 1.9 Omega cm, which is much lower than that of the La2Cu2O5 crystals (approximate to1.7 x 10(3) Omega cm). A significant drop in the resistivity (11.0 m Omega cm at 300 K) was observed upon hole doping by means of high oxygen-pressure annealing, although no transition was observed from the semiconducting behavior. In addition, a new crystal growth method to enhance hole doping level of La2Cu2O5 is shown. (C) 2001 Elsevier Science B.V. All rights reserved.

 

184.    A. Wehner, O. M. Stoll, R. P. Huebener, and M. Naito

            "Flux-flow resistance in the cuprate superconductor Nd2-xCexCuOy: Spatial domains and spontaneous oscillations"

            Phys. Rev. B 6314 (14), art. no.-144511 (2001).

 

            ABSTRACT: Under quasi-voltage-bias, time-averaged measurements in epitaxial c-axis-oriented films of Nd2-xCexCuOy (NCCO) show the nucleation and growth of a high-electric-field domain. Time-resolved resistance measurements indicate spontaneous oscillations, apparently due to a breathing movement of the domain boundary. During the experiments the samples were immersed in superfluid helium at 1.92 K. The observed nonlinear phenomena suggest a strong electric-field dependence of the quasiparticle scattering rate. Since the symmetry of the pair wave function affects the electronic structure of the quasiparticle system, the correct symmetry in NCCO represents a crucial input of a model for explaining these observations.

 

185.    K. Yamada, H. Morita, A. Shinya, and M. Notomi

            "Improved line-defect structures for photonic-crystal waveguides with high group velocity"

            Opt. Commun. 198 (4-6), 395-402 (2001).

 

            ABSTRACT: Through the analysis of the guided modes in simple photonic-crystal (PC) slab waveguides, we have established a simple and essential interpretation for the dispersion structure of the guided modes: the structure of the dispersion curves is the result of interactions between the index-confined mode and the gap-confined mode. This interpretation gave us guidelines for increasing group velocities near the zone boundary, or for solving the severe light-line problem in PC slab waveguides with dielectric claddings. Applying these guidelines, we proposed novel waveguide structures that promise fine transmission characteristics, (C) 2001 Elsevier Science B.V. All rights reserved.

 

186.    H. Yamaguchi, R. Dreyfus, Y. Hirayama, and S. Miyashita

            "Excellent electric properties of free-standing InAs membranes"

            Appl. Phys. Lett. 78 (16), 2372-2374 (2001).

 

            ABSTRACT: We fabricated semiconducting free-standing-beam and Hall-bar structures with a high slenderness ratio. a minimum thickness of 50 nm. and a typical length of several tens of microns using InAs membranes processed from InAs/GaAs heterostructures. These structures showed clear electric conductivity without any intentional doping. We obtained the carrier concentration and mobility by means of standard Hall measurements, thus confirming that both parameters were much larger than those of as-grown heterostructure samples. These results indicate that this material system is promising for micro/nanoelectromechanical system applications. (C) 2001 American Institute of Physics.

 

187.    R. Yano, Y. Hirayama, S. Miyashita, H. Sasabu, N. Uesugi, and S. Uehara

            "Pump-probe spectroscopy of low-temperature grown GaAs for carrier lifetime estimation: arsenic pressure dependence of carrier lifetime during MBE crystal growth"

            Phys. Lett. A 289 (1-2), 93-98 (2001).

 

            ABSTRACT: We have performed reflection-type degenerate pump-probe spectroscopy to measure the carrier lifetime of GaAs grown by molecular beam epitaxy (MBE) at 250 degreesC under four different effective arsenic pressures. The temporal behavior of the reflected probe pulse intensity strongly depends on both the wavelength and the intensity of the exciting laser pulse. The overall feature can be interpreted by the carrier-density- and wavelength-dependent refractive index change caused by the bandgap shrinkage, band filling, and plasma effect. We also found that the carrier lifetime depends on the effective arsenic pressure during the MBE growth, which is considered to determine the density of the defects in the GaAs. A carrier lifetime of 1.9 ps was obtained when GaAs was grown under the highest effective arsenic pressure at 250 degreesC and subsequently annealed at 650 degreesC. (C) 2001 Elsevier Science B.V. All rights reserved.

 

188.    Z. B. Zhang and M. Fujiki

            "Synthesis and molecular weight dependent optical properties of mono-alkoxy substituted polythiophenes"

            Polym. J. 33 (8), 597-601 (2001).

 

            ABSTRACT: Poly (3-alkoxythiophene)s with moderately high molecular weights, which exhibit good solubility and film-forming ability, were successfully synthesized by the nickel (0) coupling reaction in good yields. The introduction of a branched side chain resulted in a molecular weight higher than that of the straight one. Broad photoluminescence (PL), ranging from 579 to 684 nm, was readily obtained by choosing the appropriate molecular weight sample of a mother polymer, This may provide a new method for evaluating organic luminescent materials.

 

189.    Y. G. Zhao, E. Li, T. Wu, S. B. Ogale, R. P. Sharma, T. Venkatesan, J. J. Li, W. L. Cao, C. H. Lee, H. Sato, and M. Naito

            "Optical Cooper pair breaking spectroscopy of cuprate superconductors"

            Phys. Rev. B 6313 (13), 132507 (2001).

 

            ABSTRACT: The photon energy dependence of the optical Cooper pair breaking rate (CPBR) is studied for compressibly strained La1.85Sr0.15CuO4 (LSCO) films and YBa2Cu2.92Zn0.08O7-delta (YBCZO) thin films, and compared to that in YBa2Cu3O7-delta (YBCO). Unlike YBCO, the CPBR for LSCO does not show an obvious photon energy dependence. In YBCZO, the CPBR shows a strong energy dependence similar to YBCO, but with a redshift in the peak position. Analysis of these results strongly favors a physical picture based on electronic phase separation in high-Tc superconductivity.

 

190.    N. M. Zimmerman, W. H. Huber, A. Fujiwara, and Y. Takahashi

            "Excellent charge offset stability in a Si-based single-electron tunneling transistor"

            Appl. Phys. Lett. 79 (19), 3188-3190 (2001).

 

            ABSTRACT: We have measured the long-term drift and the short-term 1/f noise in the charge offset Q(0)(t) in two Si-based single-electron tunneling transistors (SETTs). In contrast to metal-based SETTs, these devices show excellent charge stability, drifting by less than 0.01e over weeks. The short-term 1/f noise magnitude is similar to the metal-based devices, demonstrating that different mechanisms are responsible for the short-term noise versus the long-term drift. Finally, we show that, in addition to the excellent stability over time, it may be possible to make the devices more robust with respect to voltage-induced instability as well. (C) 2001 American Institute of Physics.