1.    T. Akasaka, Y. Kobayashi, and T. Makimoto

            "Nonpolar AIBN, (11(2)over-bar-0) and (1(1)over-bar-00) films grown on SiC substrates"

            Applied Physics Letters 91 (4), 041914 (2007).

 

            ABSTRACT: Nonpolar AlBN (1120) and (1100) films were grown using flow-rate modulation epitaxy. The transmission electron diffraction and lattice image reflect the wurtzite crystal structure of an AlBN (1120) film. The boron compositions in AlBN (1120) and (1100) films (B similar to 2%), estimated by x-ray diffraction assuming the wurtzite structure, agree well with the compositions measured by secondary ion mass spectroscopy, indicating that boron atoms are incorporated exactly into the wurtzite lattice sites. The (1120) face is more promising than the (1100) one for the growth of nonpolar AlBN because it has fewer dangling bonds of nitrogen on the surface.

 

2.    T. Akasaka, Y. Kobayashi, and T. Makimoto

            "Growth of nonpolar AlN (11(2)over-bar0) and (1(1)over-bar00) films on SiC substrates by flow-rate modulation epitaxy"

            Applied Physics Letters 90 (12), 121919 (2007).

 

            ABSTRACT: Nonpolar AlN (11(2)over bar0) and (1(1)over bar00) films were grown on SiC substrates by flow-rate modulation epitaxy (FME), wherein trimethylaluminum and NH3 were alternately supplied. FME provides both AlN (11(2)over bar0) and (1(1)over bar00) films with good crystallinity and smooth surfaces, whereas AlN (1(1)over bar00) films obtained by conventional metal-organic chemical vapor deposition exhibit poor crystallinity and rough surfaces with deep trenches consisting of (000(1)over bar) and (1(1)over bar01) N-face microfacets. FME effectively eliminates these trenches, because the microfacets are unstable and have faster growth rates because of the enhanced migration of Al atoms in the absence of excess N surface coverage under the Al-rich condition. (c) 2007 American Institute of Physics.

 

3.    T. Akasaka, Y. Kobayashi, and T. Makimoto

            "BGaN micro-islands as novel buffers for growth of high-quality GaN on sapphire"

            Journal Of Crystal Growth 298320-324 (2007).

 

            ABSTRACT: We discuss the growth mechanism of GaN films and report very high two-dimensional electron gas (2DEG) mobility in AlGaN/AlN/GaN heterostructures fabricated on sapphire using BGaN micro-islands as novel buffers by metalorganic vapor phase epitaxy. The three-dimensional growth of BGaN (formation of BGaN micro-islands) occurs due to the phase separation of BGaN. However, the surface of the overgrown GaN on the BGaN micro-islands becomes smooth and continuous through the epitaxial lateral overgrowth process. The threading dislocations (TDs) in GaN consist mainly of pure edge-type ones and are effectively annihilated using single and double layers of BGaN micro-islands from 2 x 10(10) to 2 x 10(9) and 2 x 10(8) cm(-2), respectively. An n-type GaN film shallowly doped with Si exhibits an electron concentration and high Hall mobility of 3.0 x 10(16) cm(-3) and 669 cm(2)/Vs at room temperature (RT). Very high Hall 2DEG mobility in an Al0.10Ga0.90N/AlN/GaN heterostructure is obtained: 1910 and 20,600 cm(2)/Vs at RT and 77 K, respectively. The sheet carrier density had almost constant values of 6.9-5.7 x 10(12) cm(-2) in the temperature range from 77 to 500K, indicating that the parallel conduction due to the residual electrons in the GaN underlying layer was negligible. (c) 2006 Elsevier B.V. All rights reserved.

 

4.    M. Berthe, S. Yoshida, Y. Ebine, K. Kanazawa, A. Okada, A. Taninaka, O. Takeuchi, N. Fukui, H. Shinohara, S. Suzuki, K. Sumitomo, Y. Kobayashi, B. Grandidier, D. Stievenard, and H. Shigekawa

            "Reversible defect engineering of single-walled carbon nanotubes using scanning tunneling microscopy"

            Nano Letters 7 (12)3623-3627 (2007).

 

            ABSTRACT: The experimental creation and annihilation of defects on single-walled carbon nanotubes (SWCNT) with the tip of a scanning tunneling microscope are reported. The technique used to manipulate the wall structure of a nanotube at the atomic scale consists of a voltage ramp applied at constant tunneling current between the tip and the nanotube adsorbed on a gold substrate. While topographic images show an interference pattern at the defect position, spatially resolved tunneling spectroscopy reveals the presence of localized states in the band gap of the nanotube. Removal of the defect by the same procedure demonstrates the reversibility of the process. Such a precise control in the local modification of the nanotube wall opens up new opportunities to tailor SWCNT electronic properties at will.

 

5.    S. Cherednichenko, V. Drakinskiy, K. Ueda, and M. Naito

            "Terahertz mixing in MgB2 microbolometers"

            Applied Physics Letters 90 (2), 023507 (2007).

 

            ABSTRACT: The authors report on a terahertz (600 GHz) mixing experiment with MgB2 microbolometers in the resistive state. The authors observed that for a 20 nm film a mixer gain bandwidth of 2.3 GHz can be achieved, corresponding to an energy relaxation time of 70 ps. The experimental results were analyzed using a two-temperature model. As a result, the phonon escape time of similar to 20 ps was deduced. At 1.6 THz the MgB2 mixer uncorrected noise temperature was 11 000 K. The obtained results show that MgB2 bolometers are good prospects for the terahertz range as both broadband mixers and fast direct detectors.

 

6.    K. Degawa, T. Aoki, T. Higuchi, H. Inokawa, and Y. Takahashi

            "Design of a two-bit-per-cell content-addressable memory using single-electron transistors"

            Journal Of Multiple-valued Logic And Soft Computing 13 (3)249-266 (2007).

 

            ABSTRACT: This paper presents a circuit design of a two-bit-per-cell Content-Addressable Memory (CAM) using Single-Electron Transistors (SETs). The key ideas of the proposed CAM architecture are (i) four-level data storage function implementing by a SET-based static memory cell and (ii) four-level data matching function employing periodic drain-current characteristics of SETS with dynamic phase-shift control. A simple multi-gate SET can be used to realize four-level data matching within a compact CAM cell circuit. As a result, the proposed two-bit-per-cell CAM architecture reduces both the number of transistors and the cell area to 1/3 compared with the conventional CAM architecture.

 

7.    F. Deppe, M. Mariantoni, E. P. Menzel, S. Saito, K. Kakuyanagi, H. Tanaka, T. Meno, K. Semba, H. Takayanagi, and R. Gross

            "Phase coherent dynamics of a superconducting flux qubit with capacitive bias readout"

            Physical Review B 76 (21), 214503 (2007).

 

            ABSTRACT: We present a systematic study of the phase coherent dynamics of a superconducting three-Josephson-junction flux qubit. The qubit state is detected with the integrated-pulse method, which is a variant of the pulsed switching-dc-superconducting quantum interference device (SQUID) method. In this scheme, the dc SQUID bias current pulse is applied via a capacitor instead of a resistor, giving rise to a narrow bandpass instead of a pure low-pass filter configuration of the electromagnetic environment. Measuring one and the same qubit with both setups allows a direct comparison. With the capacitive method about four times faster switching pulses and an increased visibility are achieved. Furthermore, the deliberate engineering of the electromagnetic environment, which minimizes the noise due to the bias circuit, is facilitated. Right at the degeneracy point, the qubit coherence is limited by energy relaxation. We find two main noise contributions. White noise limits the energy relaxation and contributes to the dephasing far from the degeneracy point. 1/f noise is the dominant source of dephasing in the direct vicinity of the optimal point. The influence of 1/f noise is also supported by nonrandom beatings in the Ramsey and spin echo decay traces. Numeric simulations of a coupled qubit-oscillator system indicate that these beatings are due to the resonant interaction of the qubit with at least one pointlike fluctuator, coupled especially strongly to the qubit.

 

8.    T. Fujisawa, G. Shinkai, and T. Hayashi

            "Zeeman splitting in single-electron transport through a few-electron quantum dot"

            Physical Review B 76 (4), 041302 (2007).

 

            ABSTRACT: Single-electron transport through a few-electron quantum dot is investigated under in-plane and perpendicular magnetic fields. Zeeman splitting always appears as two conductance peaks, whose conditions depend on whether the total spin is raised or lowered by single-electron tunneling. The total spin of the ground state can be identified by consecutively investigating the Zeeman splitting from a known spin state. Zeeman splitting for some excited states is also discussed.

 

9.    C. H. Fung, B. Qi, K. Tamaki, and H. K. Lo

            "Phase-remapping attack in practical quantum-key-distribution systems"

            Physical Review A 75 (3), 032314 (2007).

 

            ABSTRACT: Quantum key distribution (QKD) can be used to generate secret keys between two distant parties. Even though QKD has been proven unconditionally secure against eavesdroppers with unlimited computation power, practical implementations of QKD may contain loopholes that may lead to the generated secret keys being compromised. In this paper, we propose a phase-remapping attack targeting two practical bidirectional QKD systems (the "plug-and-play" system and the Sagnac system). We showed that if the users of the systems are unaware of our attack, the final key shared between them can be compromised in some situations. Specifically, we showed that, in the case of the Bennett-Brassard 1984 (BB84) protocol with ideal single-photon sources, when the quantum bit error rate (QBER) is between 14.6% and 20%, our attack renders the final key insecure, whereas the same range of QBER values has been proved secure if the two users are unaware of our attack; also, we demonstrated three situations with realistic devices where positive key rates are obtained without the consideration of Trojan horse attacks but in fact no key can be distilled. We remark that our attack is feasible with only current technology. Therefore, it is very important to be aware of our attack in order to ensure absolute security. In finding our attack, we minimize the QBER over individual measurements described by a general POVM, which has some similarity with the standard quantum state discrimination problem.

 

10.    K. Furukawa, K. Sumitomo, H. Nakashima, Y. Kashimura, and K. Torimitsu

            "Supported lipid bilayer self-spreading on a nanostructured silicon surface"

            Langmuir 23 (2)367-371 (2007).

 

            ABSTRACT: We report on the self-spreading behavior of a supported lipid bilayer (SLB) on a silicon surface with various 100 nm nanostructures. SLBs have been successfully grown from a small spot of a lipid molecule source both on a flat surface and uneven surfaces with 100 nm up-and-down nanostructures. After an hour, the self-spreading SLB forms a large circle or an ellipse depending on the nanostructure pattern. The results are explained by a model that shows that a single-layer SLB grows along the nanostructured surfaces. The model is further supported by a quantitative analyses of our data. We also discuss the stability of the SLB on nanostructured surfaces in terms of the balance between its bending and adhesion energies.

 

11.    T. Goto, H. Inokawa, M. Nagase, Y. Ono, K. Sumitomo, and K. Torimitsu

            "Effect of UV/ozone treatment on nanogap electrodes for molecular devices"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4A)1731-1733 (2007).

 

            ABSTRACT: We report the effect of UV/ozone treatment on nanogap electrodes for molecular devices. Gold nanogap electrodes with a nominal gap of 1-2 nm were fabricated by double oblique deposition and the break-junction technique. Self-assembled monolayers (SAMs) of 4,4'-p-terphenyldithiol (TPDT) were formed on the surfaces of the nanogap electrodes, and the electronic properties of these electrodes were measured. The device characteristics were also measured after repeated UV/ozone treatment and SAM re-formation. Although the resistance of the nanogap electrodes increased with the number of UV/ozone treatments, they could subsequently be used for molecular devices. We also observed Coulomb-diamond (CD) structures in the conductance contour plot with respect to the drain and gate voltages even after UV/ozone treatment. Some of the CDs observed after the treatment were aperiodic, presumably reflecting the discrete energy levels in TPDT.

 

12.    H. Gotoh, S. W. Chang, S. L. Chuang, H. Okamot, and Y. Shibata

            "Tunable slow light of 1.3 mu m region in quantum dots at room temperature"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2369-2372 (2007).

 

            ABSTRACT: We report tunable slow light (slowlight) in the 1.3 mu m region in quantum dots (QDs) observed at room temperature. The light transmission times are measured by the counter-propagating pump-and-probe method in waveguides with InAs QDs as active regions. The maximum slowdown factor is 2.5 for a 500-mu m-long sample. This corresponds to a 2.5 times longer propagation time in the waveguide. The slowdown can be well controlled by tuning the pump beam. This pump power dependence of the slowdown factor agrees well with theoretical predictions obtained by coherent population oscillation (CPO). We also report a method that realizes larger slow light effects.

 

13.    Y. Hirayama, T. Sogawa, K. Suzuki, V. Kanisawa, and H. Yamaguchi

            "Semiconductor heterostructure studies using emerging technologies"

            Physica Status Solidi B-basic Solid State Physics 244 (8)2988-3001 (2007).

 

            ABSTRACT: Low-temperature scanning tunnelling microscopy (LT-STM), surface acoustic wave (SAW), and mechanical cantilever are used to measure and manipulate quantum characteristics in semiconductor heterostructures. A nanoscale detection of local density of states is demonstrated for electrons confined in semiconductor quantum wells and dots by a combination of InAs-based heterostructures and LT-STM. The SAW is used to realize moving quantum wires and dots in semiconductor heterostructures. A synchronized optical measurement confirms two types of modulation, i.e. bandgap and potential modulations. A combination of mechanical cantilever and quantum heterostructures gives a new mechanical functionality and a novel tool to analyse quantum transport characteristics. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

 

14.    T. Honjo, H. Takesue, H. Kamada, Y. Nishida, O. Tadanaga, M. Asobe, and K. Inoue

            "Long-distance distribution of time-bin entangled photon pairs over 100 km using frequency up-conversion detectors"

            Optics Express 15 (21)13957-13964 (2007).

 

            ABSTRACT: We report an experimental demonstration of the distribution of time-bin entangled photon pairs over 100 km of optical fiber. In our experiment, 1.5-mu m non-degenerated time-bin entangled photon pairs were generated with a periodically poled lithium niobate (PPLN) waveguide by using the parametric down conversion process. Combining this approach with ultra-low-loss filters to eliminate the pump light and separate signal and idler photons, we obtained an efficient entangled photon pair source. To detect the photons, we used single-photon detectors based on frequency up-conversion. These detectors operated in a non-gated mode so that we could use a pulse stream of time correlated entangled photon pairs at a high repetition frequency (1 GHz). Using these elements, we distributed time-bin entangled photon pairs over 100 km of dispersion shifted fiber and performed a two-photon interference experiment. We obtained a coincidence fringe of 81.6% visibility without subtracting any background noise, such as accidental coincidence or dark count, which was good enough to violate Bell's inequality. Thus, we successfully distributed time-bin entangled photon pairs over 100 km. (c) 2007 Optical Society of America.

 

15.    T. Honjo, S. Yamamoto, T. Yamamoto, H. Kamada, Y. Nishida, O. Tadanaga, M. Asobe, and K. Inoue

            "Field trial of differential-phase-shift quantum key distribution using polarization independent frequency up-conversion detectors"

            Optics Express 15 (24)15920-15927 (2007).

 

            ABSTRACT: We report a field trial of differential phase shift quantum key distribution (QKD) using polarization independent frequency up-conversion detectors. A frequency up-conversion detector is a promising device for achieving a high key generation rate when combined with a high clock rate QKD system. However, its polarization dependence prevents it from being applied to practical QKD systems. In this paper, we employ a modified polarization diversity configuration to eliminate the polarization dependence. Applying this method, we performed a long-term stability test using a 17.6-km installed fiber. We successfully demonstrated stable operation for 6 hours and achieved a sifted key generation rate of 120 kbps and an average quantum bit error rate of 3.14 %. The sifted key generation rate was not the estimated value but the effective value, which means that the sifted key was continuously generated at a rate of 120 kbps for 6 hours. (c) 2007 Optical Society of America.

 

16.    T. Honjo, H. Takesue, and K. Inoue

            "Differential-phase quantum key distribution experiment using a series of quantum entangled photon pairs"

            Optics Letters 32 (9)1165-1167 (2007).

 

            ABSTRACT: We report what we believe to be the first differential-phase quantum key distribution experiment using a series of quantum entangled photon pairs. We employed two outstanding techniques. As an entangled photon source, we used a 1.5 mu m band entangled photon pair source based on spontaneous four-wave mixing in a cooled dispersion-shifted fiber. As receivers, photon pairs were actively phase modulated with LiNbO3 phase modulators followed by very stable planar light-wave circuit Mach-Zehnder interferometers, which provided two nonorthogonal measurements. As a consequence, we successfully generated sifted keys with a quantum bit error rate of 8.3% and a key generation rate of 0.3 bit/s and revealed the feasibility of this QKD scheme. (c) 2007 Optical Society of America.

 

17.    T. Honjo, H. Takesue, and K. Inoue

            "Generation of energy-time entangled photon pairs in 1.5-mu m band with periodically poled lithium niobate waveguide"

            Optics Express 15 (4)1679-1683 (2007).

 

            ABSTRACT: We report the generation of 1.5-mu m-band energy-time entangled photon pairs using a periodically poled lithium niobate ( PPLN) waveguide. We performed a two-photon interference experiment and obtained coincidence fringes with 77.3% visibilities without subtracting accidental coincidences. (c) 2007 Optical Society of America.

 

18.    S. C. Huang, M. Kato, E. Kuramochi, C. P. Lee, and M. Notomi

            "Time-domain and spectral-domain investigation of inflection-point slow-light modes in photonic crystal coupled waveguides"

            Optics Express 15 (6)3543-3549 (2007).

 

            ABSTRACT: We report on spectral-domain and time-domain measurements and numerical calculations of group velocities in a photonic crystal coupled waveguide, where the unique guided mode band structure has a flat band region within the photonic band gap allowing for slow light observation. The spectral dependence of group velocity, which is measured by interference method, indicates the existence of slow light modes around the inflection point of the unique flat band, rather than at the band edge. Time-domain observation of optical pulses propagating along two-dimension slab photonic crystal coupled waveguides is also demonstrated by using a high speed oscilloscope. By adjusting the wavelength of the input pulses toward the flat band of the coupled defect modes, an increasing duration time between reference and output pulses are clearly observed. An extremely small group velocity of 0.017c is thus obtained. Calculated group velocities show good agreement with our measured results. (c) 2007 Optical Society of America.

 

19.    H. Inokawa, M. Nagase, S. Hirono, T. Goto, H. Yamaguchi, and K. Torimitsu

            "Field-effect transistor with deposited graphite thin film"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2615-2617 (2007).

 

            ABSTRACT: By using a bottom-gate Lop-contact field-effect transistor structure, the field effect of graphite-rich carbon nanocrystallite thin films deposited by electron cyclotron resonance sputtering was investigated. An appreciable ambipolar field effect was observed at the film edge where the thickness was vanishing. On-off current ratios of 2 and 7 were attained at 294 and 150 K, respectively.

 

20.    R. Itakura, P. Liu, Y. Furukawa, T. Okino, K. Yamanouchi, and H. Nakano

            "Two-body Coulomb explosion and hydrogen migration in methanol induced by intense 7 and 21 fs laser pulses"

            Journal Of Chemical Physics 127 (10), 104306 (2007).

 

            ABSTRACT: Two-body Coulomb explosion with the C-O bond breaking of methanol induced by intense laser pulses with the duration of Delta t=7 and 21 fs is investigated by the coincidence momentum imaging method. When Delta t=7 fs, the angular distribution of recoil vectors of the fragment ions for the direct C-O bond breaking pathway, CH3OH2+-> CH3++OH+, exhibits a peak deflected from the laser polarization direction by 30 degrees-45 degrees, and the corresponding angular distribution for the migration pathway, CH2OH2+-> CH2++H2O+, in which one hydrogen migrates from the carbon site to the oxygen site prior to the C-O bond breaking, exhibits almost the same profile. When the laser pulse duration is stretched to Delta t=21 fs, the angular distributions for the direct and migration pathways exhibit a broad peak along the laser polarization direction probably due to the dynamical alignment and/or the change in the double ionization mechanism; that is, from the nonsequential double ionization to the sequential double ionization. However, the extent of the anisotropy in the migration pathway is smaller than that in the direct pathway, exhibiting a substantial effect of hydrogen atom migration in the dissociative ionization of methanol interacting with the linearly polarized intense laser field.

 

21.    G. H. Jeong, A. Yamazaki, S. Suzuki, H. Yoshimura, Y. Kobayashi, and Y. Homma

            "Production of single-walled carbon nanotubes with narrow diameter distribution using iron nanoparticles derived from DNA-binding proteins from starved cells"

            Carbon 45 (5)978-983 (2007).

 

            ABSTRACT: Iron nanoparticles derived from DNA-binding proteins from starved cells (Dps) were used to grow single-walled carbon nanotubes (SWCNTs) with narrow diameter distribution. An atomic force microscopy, Raman spectroscopy, and photoluminescence were used for evaluation of diameter or chirality distribution of the SWCNTs. We found that thin SWCNTs (1.1 nm diameter) were grown from the large Dps-derived nanoparticles (2.4 nm diameter) on and above the substrates. From the size comparison with ferritins and Co-filled apoferritins, we also found that SWCNTs become thinner as the catalyst becomes smaller. The synthesis of smaller catalysts (ca. 1 nm diameter) and their use for growth becomes crucial for the control of SWCNT diameter. (c) 2007 Elsevier Ltd. All rights reserved.

 

22.    G. H. Jeong, S. Suzuki, Y. Kobayashi, A. Yamazaki, H. Yoshimura, and Y. Homma

            "Size control of catalytic nanoparticles by thermal treatment and its application to diameter control of single-walled carbon nanotubes"

            Applied Physics Letters 90 (4), 043108 (2007).

 

            ABSTRACT: The authors report size control of catalytic nanoparticles by thermal annealing for diameter-controlled growth of single-walled carbon nanotubes (SWNTs). They found that Co nanoparticle-size gradually decreased through repetitive annealing at 1000 degrees C in Ar ambient. Results of x-ray photoelectron spectroscopy and secondary ion mass spectroscopy show that thermal evaporation is responsible for the decrease. After SWNT growth using this phenomenon, the authors found that thinner SWNTs with a narrower diameter distribution grew as the nanoparticles became smaller. Their results provide a rational and straightforward technique to prepare catalysts having a desirable size and uniformity toward diameter-controlled SWNT growth. (c) 2007 American Institute of Physics.

 

23.    H. Joe, T. Akiyama, K. Nakamura, K. Kamsawa, and T. Ito

            "An empirical potential approach to the structural stability of InAs stacking-fault tetrahedron in InAs/GaAs(111)"

            Journal Of Crystal Growth 301837-840 (2007).

 

            ABSTRACT: The structural stability of InAs stacking-fault tetrahedron (SFT) in InAs/GaAs (1 1 1) is theoretically investigated. Using an empirical interatomic potential, cohesive energies are calculated for the three types of InAs/GaAs(1 1 1) system where coherent InAs and relaxed InAs with the SFT and misfit dislocations (MDs). The calculated results reveal that InAs with the SFT is more favorable beyond the film thickness of 21 monolayers (MLs) than coherent InAs. The critical film thickness of 21 ML is comparable with that of 8 ML for the MDs generation. This suggests that the SFT appears in InAs thin film layers instead of MDs resulting from lowering the strain energy accumulated in InAs thin film layers. (c) 2007 Elsevier B.V. All rights reserved.

 

24.    K. Kakuyanagi, T. Meno, S. Saito, H. Nakano, K. Semba, H. Takayanagi, F. Deppe, and A. Shnirman

            "Dephasing of a superconducting flux qubit"

            Physical Review Letters 98 (4), 047004 (2007).

 

            ABSTRACT: In order to gain a better understanding of the origin of decoherence in superconducting flux qubits, we have measured the magnetic field dependence of the characteristic energy relaxation time (T-1) and echo phase relaxation time (T-2(echo)) near the optimal operating point of a flux qubit. We have measured T-2(echo) by means of the phase cycling method. At the optimal point, we found the relation T-2(echo)approximate to 2T(1). This means that the echo decay time is limited by the energy relaxation (T-1 process). Moving away from the optimal point, we observe a linear increase of the phase relaxation rate (1/T-2(echo)) with the applied external magnetic flux. This behavior can be well explained by the influence of magnetic flux noise with a 1/f spectrum on the qubit.

 

25.    K. Kanzaki, S. Suzuki, H. Inokawa, Y. Ono, A. Vijayaraghavan, and Y. Kobayashi

            "Mechanism of metal-semiconductor transition in electric properties of single-walled carbon nanotubes induced by low-energy electron irradiation"

            Journal Of Applied Physics 101 (3), 034317 (2007).

 

            ABSTRACT: Low-energy electron irradiation causes damage in single-walled carbon nanotubes and changes the electric behavior of a nanotube field-effect transistor from metallic to semiconducting at low temperature. The irradiation damage was found to form an energy barrier of several 10 meV in the nanotube channel. We show that the transition behavior can be reasonably explained by the barrier formation and gate-induced band bending. (c) 2007 American Institute of Physics.

 

26.    S. Karimoto, H. Sato, H. Shibata, and T. Makimoto

            "Deposition of NdBa2Cu3O7-delta thin films on large area of over 5 in. in diameter"

            Physica C-superconductivity And Its Applications 463927-929 (2007).

 

            ABSTRACT: We report the deposition of NdBa2Cu3O7-theta(NBCO) thin films with a large diameter of over 5 in. by molecular-beam epitaxy (MBE). We examined the uniformity of the film composition and T-c for areas with a diameter of 140 mm. The film composition was Ba/Nd:Cu/ Nd = 1.79 +/- E 0.02:2.97 +/- 0.04, which indicates a uniform deposition over a large area. The Tc values only varied from 90.4 K to 91.6 K, which also confirms the uniform deposition. Thus our deposition method appears to be promising for producing thin film on a commercial scale. (C) 2007 Elsevier B.V. All rights reserved.

 

27.    M. Kasu, K. Ueda, Y. Yamauchi, A. Tallaire, and T. Makimoto

            "Diamond-based RF power transistors: Fundamentals and applications"

            Diamond And Related Materials 16 (4-7)1010-1015 (2007).

 

            ABSTRACT: The present status of diamond-based transistors for high-frequency and high-power applications is reviewed. We have achieved the drain current density of 550 mA/mm, cut-off frequencies for current gain (f(T)) and power gain (f(MAX)) of 45 GHz and 120 GHz, respectively, and output-power density of 2.1 W/mm at 1 GHz in class-A operation of a field-effect transistor (FET) with hydrogen (H)-surface-terminated diamond. We have found that gate capacitance can be separated into depletion-layer capacitance and insulator capacitance. Concerning a stability of H-surface termination, no apparent decrease in the current for an FET without a gate contact was observed, but gate bias stress results in a slight decrease in the drain current and simultaneously an increase in the gate leakage current. (c) 2007 Elsevier B.V. All rights reserved.

 

28.    M. Kasu, K. Ueda, Y. Yamauchi, and T. Makimoto

            "Gate capacitance-voltage characteristics of submicron-long-gate diamond field-effect transistors with hydrogen surface termination"

            Applied Physics Letters 90 (4), 043509 (2007).

 

            ABSTRACT: The radio-frequency characteristics of p-type diamond field-effect transistors with hydrogen surface termination were numerically analyzed using an equivalent-circuit model. From the gate-source capacitance (C-GS)-voltage (V-GS) results extracted from measured s parameters, the authors found a plateau in C-GS within a certain V-GS range. This means that a two-dimensional hole gas channel forms parallel to the surface and that the channel is separated by a thin energy-barrier layer with an infinite height from the gate metal. At a high negative V-GS, as negative V-GS is increased, C-GS increases steeply. This results from holes penetrating the energy barrier. (c) 2007 American Institute of Physics.

 

29.    M. A. Khalafalla, Y. Ono, K. Nishiguchi, and A. Fujiwara

            "Identification of single and coupled acceptors in silicon nano-field-effect transistors"

            Applied Physics Letters 91 (26), 263513 (2007).

 

            ABSTRACT: We performed dopant mediated hole transport measurements to identify the depth position of individual boron acceptors and investigate two-acceptor coupling in silicon-on-insulator nanoscale field-effect transistors at a temperature of 6 K. The depth position is qualitatively obtained from the analysis of the acceptor-to-gate capacitances. We also observe signatures of a two-acceptor capacitive coupling in the characteristics of the conductance versus the front and back gate voltages. (c) 2007 American Institute of Physics.

 

30.    T. Kimura, S. Tsuchiya, M. Yamashita, and S. Kurihara

            "Gutzwiller study of spin-1 bosons in an optical lattice under a magnetic field"

            Laser Physics 17 (1)54-59 (2007).

 

            ABSTRACT: We study spin-1 bosons in an optical lattice under a magnetic field with the Gutzwiller approximation for the Bose-Hubbard model. Phase boundary curves between superfluids and Mott insulators depend continuously on the magnetic field, and this provides better results than those obtained with the perturbative mean-field approximation. The phase boundary curve as a function of magnetic field has a sharp cusp structure under certain circumstances. In superfluid phases, both the spin magnetizations and fluctuations in the total number of bosons show strong magnetic field dependence, which is related to the fact that both first-and second-order transitions appear on the phase boundary curve according to the magnetic field.

 

31.    M. Kobayashi, K. Sumitomo, and K. Torimitsu

            "Real-time imaging of DNA-streptavidin complex formation in solution using a high-speed atomic force microscope"

            Ultramicroscopy 107 (2-3)184-190 (2007).

 

            ABSTRACT: The direct observation of individual molecules in action is required for a better understanding of the mechanisms of biological reactions. We used a high-speed atomic force microscope (AFM) in solution to visualize short DNA fragments in motion. The technique represents a new approach in analyzing molecular interactions, and it allowed us to observe real-time images of biotinylated DNA binding to/dissociating from streptavidin protein. Our results show that high-speed AFMs have the potential to reveal the mechanisms of molecular interactions, which cannot be determined by analyzing the average value of mass reactions. (c) 2006 Elsevier B.V. All rights reserved.

 

32.    S. Kobayashi, K. Onomitsu, K. Yanagisawa, S. Takeuchi, H. Yoshitake, and H. Yamaguchi

            "Magnetization of free standing GaMnAs"

            Journal Of Crystal Growth 308 (1)204-207 (2007).

 

            ABSTRACT: We compared the magnetization curves between as-grown and free standing GaMnAs samples, which were obtained from the same epitaxially grown wafer, to study the influence of epitaxial strain on their ferromagnetic properties. We found that the magnetization and coercive field for the free standing sample have significantly increased about twice as large as those of the as-grown GaMnAs. In addition, the uniaxial ferromagnetic anisotropies, which would not be expected on the basis of crystal symmetry, are remarkably different between the two samples. These results undoubtedly demonstrate that the magnetic properties are strongly influenced by the epitaxially induced strain. (C) 2007 Elsevier B.V. All rights reserved.

 

33.    Y. Kobayashi, H. Hibino, T. Nakamura, T. Akasaka, T. Makimoto, and N. Matsumoto

            "Boron nitride thin films grown on graphitized 6H-SiC substrates by metalorganic vapor phase epitaxy"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2554-2557 (2007).

 

            ABSTRACT: The growth of thin boron nitride (BN) films on graphitized 6H-SiC substrates was investigated in an attempt to reduce the large lattice mismatch between 6H-SiC and BN, which would improve the three-dimensional ordering in BN thin films grown by metalorganic vapor phase epitaxy (MOVPE). BN thin films were grown by low-pressure (300 Torr) MOVPE using triethylboron and ammonia on graphitized 6H-SiC substrates with surfaces displaying (1 x 1) reconstruction as determined by low energy electron diffraction (LEED). The (1 x 1) surfaces were formed by annealing at 1500 degrees C in ultrahigh vacuum with a base pressure of 10(-10) Torr. The LEED patterns showed that the. surfaces were covered with single-crystal graphite several monolayers thick. X-ray diffraction revealed that the c-axis lattice constant of the BN was 6.72 angstrom, which is close to the 6.66 angstrom of bulk hexagonal BN. In contrast, BN films grown on non-graphitized 6H-SiC substrates by MOVPE under the same conditions were mostly amorphous. Use of a graphitized 6H-SiC substrate covered with graphite several monolayers thick improves the degree of three-dimensional ordering in BN thin films grown by MOVPE.

 

34.    Y. Kobayashi, T. Nakamura, T. Akasaka, T. Makimoto, and N. Matsumoto

            "Ultraviolet luminescence from hexagonal boron nitride heteroepitaxial layers on Ni(111) grown by flow-rate modulation epitaxy"

            Physica Status Solidi B-basic Solid State Physics 244 (6)1789-1792 (2007).

 

            ABSTRACT: We report room-temperature (RT) observation of near-band-gap ultraviolet luminescence at a wavelength of 227 nm in cathodoluminescence from h-BN heteroepitaxial layers. The h-BN layers were grown on single crystal Ni(111) substrates by flow-rate modulation epitaxy (FME), which is based on the alternate supply of triethylboron and ammonia. The h-BN layers grown with longer NH3 supply time exhibit stronger intensity and narrower full width of half maximum (FWHM) of the near-band-gap luminescence. X-ray diffraction measurements reveal that the FWHM of (0002) h-BN X-ray rocking curves decreases with increasing NH3 supply time. The reduction of lattice defects in h-BN grown by FME with longer NH3 supply time could be the reason for the improved near-band-gap ultraviolet luminescence at RT. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

 

35.    Y. Kobayashi, T. Nakamura, T. Akasaka, T. Makimoto, and N. Matsumoto

            "Hexagonal boron nitride on Ni (111) substrate grown by flow-rate modulation epitaxy"

            Journal Of Crystal Growth 298325-327 (2007).

 

            ABSTRACT: Single-phase (0 0 0 1) hexagonal boron nitride (h-BN) heteroepitaxial layers were successfully grown on lattice-matched Ni (I 1 1) substrate by flow-rate modulation epitaxy (FME) using triethylboron (TEB) and ammonia (NH3) at 1020 degrees C for the first time. The structural properties of the h-BN layers were investigated by high-resolution X-ray diffraction (XRD). The first-ever X-ray rocking curve (XRC) measurement of an h-BN layers was performed and the minimum full-width of half-maximum of 0.7 degrees was obtained. For the h-BN FME growth, larger NH3 flow rates and longer NH3 exposure time are preferable for improving the crystal quality. Our approach enables us to drastically improve the crystal quality of h-BN at relatively low growth temperature. (c) 2006 Elsevier B.V. All rights reserved.

 

36.    N. Kumada, K. Muraki, and Y. Hirayama

            "NMR evidence for spin canting in a bilayer nu=2 quantum hall system"

            Physical Review Letters 99 (7), 076805 (2007).

 

            ABSTRACT: We investigate the electron spin states in the bilayer quantum Hall system at total Landau level filling factor nu=2 exploiting current-pumped and resistively detected NMR. The measured Knight shift, K-S, of As-75 nuclei reveals continuous variation of the out-of-plane electronic spin polarization between nearly full and zero as a function of density imbalance. Nuclear spin relaxation measurements indicate a concurrent development of an in-plane spin component. These results provide direct information on the spin configuration in this system and comprise strong evidence for the spin canting suggested by previous experiments.

 

37.    K. Kumakura, A. Nishikawa, and T. Makimoto

            "High breakdown field of pnp GaN/InGaN/AlGaN DHBTs with AlGaN collector"

            Physica Status Solidi A-applications And Materials Science 204 (6)2037-2041 (2007).

 

            ABSTRACT: We have investigated the characteristics of pup GaN/InGaN/(Al)GaN double heterojunction bipolar transistors (DHBTs) grown by metalorganic vapour phase epitaxy. In GaN-collector DHBTs, the collector-base breakdown voltage under the open-emitter condition increased with increasing collector thickness. By fitting the experimental results to the theoretical model of the punch-through diode, we obtained the breakdown field of 3.1 MV/crn and the residual carrier concentration of 8.5 x 10(16) cm(-3) for the GaN collector layer. On the other hand, in a 500 nm thick AlGaN collector, the breakdown voltage was 190 V. Assuming that the bias was uniformly applied in the AlGaN collector, the corresponding breakdown field was calculated to be 3.8 MV/cm. The results indicate that the use of an AlGaN collector in HBTs is a promising way to increase the breakdown voltage without increasing the collector thickness. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

 

38.    K. Kumakura, and T. Makimoto

            "Pup AlGaN/InGaN/GaN double heterojunction bipolar transistors with low-base-resistance (< 100 Omega/square)"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2338-2340 (2007).

 

            ABSTRACT: We investigated the current-voltage characteristics of pup AlGaN/InGaN/GaN double heterojunction bipolar transistors grown by metalorganic vapor phase epitaxy. The InGaN base layer was heavily doped with Si in the range from 4 x 10(19) to 2 x 10(20) cm(-3). Base sheet resistance was less than 100 Omega/square for the base width of 30 nm and Si-doping concentration of 2 x 10(20) cm(-3). This is the lowest resistance ever reported for nitride-based heterojunction bipolar transistors. In this device, the current gain was 2.7 at the collector current of 23 mA. We observed no significant degradation in the crystal quality of InGaN in separate Hall-effect measurements of Si-doped InGaN and in the device operation, even though the Si-doping concentration was above 10(20) cm(-3). The capability to decrease the base resistance will be advantageous for high-frequency operation.

 

39.    K. Kumakura, T. Makimoto, N. Kobayashi, T. Hashizume, T. Fukui, and H. Hasegawa

            "Minority carrier diffusion lengths in MOVPE-grown n- and p-InGaN and performance of AlGaN/InGaN/GaN double heterojunction bipolar transistors"

            Journal Of Crystal Growth 298787-790 (2007).

 

            ABSTRACT: We fabricated InGaN p-n junction diode structures on SiC substrates by metalorganic vapor phase epitaxy, and investigated the minority carrier diffusion length in n- and p-InGaN layers by electron beam induced current measurements. The minority electron diffusion length in p-InGaN was little affected by the In content in InGaN. The diffusion length decreased with increasing Mg-doping concentration. The minority hole diffusion length in n-InGaN was little affected by Si-doping concentration but slightly decreased with increasing In content in InGaN. We also fabricated pup AlGaN/InGaN/GaN double heterojunction bipolar transistors and investigated their common-emitter current-voltage characteristics. The Si-doping concentration in the base was 4 x 10(19) cm(-3). The maximum current gain was 21 at a collector current of -10 mA for an emitter size of 30 mu m x 50 mu m. This good performance is ascribed to the large conduction band discontinuity between the AlGaN emitter and InGaN base. (c) 2006 Elsevier B.V. All rights reserved.

 

40.    C. W. Lai, N. Y. Kim, S. Utsunomiya, G. Roumpos, H. Deng, M. D. Fraser, T. Byrnes, P. Recher, N. Kumada, T. Fujisawa, and Y. Yamamoto

            "Coherent zero-state and p-state in an exciton-polariton condensate array"

            Nature 450 (7169)529-U8 (2007).

 

            ABSTRACT: The effect of quantum statistics in quantum gases and liquids results in observable collective properties among many-particle systems. One prime example is Bose-Einstein condensation, whose onset in a quantum liquid leads to phenomena such as superfluidity and superconductivity. A Bose-Einstein condensate is generally defined as a macroscopic occupation of a single-particle quantum state, a phenomenon technically referred to as off-diagonal long-range order due to non-vanishing off-diagonal components of the single-particle density matrix(1-3). The wave-function of the condensate is an order parameter whose phase is essential in characterizing the coherence and superfluid phenomena(4-11). The long-range spatial coherence leads to the existence of phase-locked multiple condensates in an array of superfluid helium(12), superconducting Josephson junctions(13-15) or atomic Bose-Einstein condensates(15-18). Under certain circumstances, a quantum phase difference of p is predicted to develop among weakly coupled Josephson junctions(19). Such a meta-stable pi-state was discovered in a weak link of superfluid 3 He, which is characterized by a 'p-wave' order parameter(20). The possible existence of such a pi-state in weakly coupled atomic Bose-Einstein condensates has also been proposed(21), but remains undiscovered. Here we report the observation of spontaneous build-up of in-phase ('zero-state') and antiphase ('pi-state') 'superfluid' states in a solid-state system; an array of exciton-polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These in-phase and antiphase states reflect the band structure of the one-dimensional polariton array and the dynamic characteristics of metastable exciton-polariton condensates.

 

41.    Y. C. Lin, E. Y. Chang, H. Yamaguchi, W. C. Wu, and C. Y. Chang

            "A delta-doped InGaP/InGaAs pHEMT with different doping profiles for device-linearity improvement"

            Ieee Transactions On Electron Devices 54 (7)1617-1625 (2007).

 

            ABSTRACT: In this paper, delta-doped InGaP/InGaAs pseudomorphic high-electron-mobility transistors (pHEMTs) with doping-profile modifications are investigated in order to improve the device linearity. The proposed modification was based on the third-order intermodulation distortion (IM3) and the third-order intercept point (IP3) analysis using a simple equivalent circuit of the devices. The correlations of the extrinsic transconductance (Gm) with IM3 and IP3 indicate that the flatness of Gin, as a function of gate-bias causes a lower IM3 level. On the other hand, a high Gm with a flatter Gm distribution results in higher IP3 value for the device. Therefore, doping modifications that improve the flatness of the Gm distribution will also improve the device linearity. Doping modifications in the Schottky layer (Schottky layer doped) and in the channel layer (channel doped) of the conventional delta-doped InGaP/InGaAs pHEMT were investigated. It was also found that extra doping, either in the channel region or in the Schottky layer, improved the flatness of the Gm distribution under different gate-bias conditions. This achieved a lower IM3 and a higher IP3 with a small sacrifice in the peak Gm value. The power performances of these devices were tested at different drain biases. Even though it had the lowest electron mobility among the three different types of devices studied, the channel-doped device demonstrated the best overall linearity performance, the highest IP3 value, the lowest IM3 level, and the best adjacent-channel power ratio under code-division multiple-access modulation.

 

42.    Y. C. Lin, H. Yamaguchi, E. Y. Chang, Y. C. Hsieh, M. Ueki, Y. Hirayama, and C. Y. Chang

            "Growth of very-high-mobility AlGaSb/InAs high-electron-mobility transistor structure on si substrate for high speed electronic applications"

            Applied Physics Letters 90 (2), 023509 (2007).

 

            ABSTRACT: The growth of the AlGaSb/InAs high-electron-mobility transistor (HEMT ) epitaxial structure on the Si substrate is investigated. Buffer layers consisted of UHV/chemical vapor deposited grown Ge/GeSi and molecular beam epitaxy-grown AlGaSb/AlSb/GaAs were used to accommodate the strain induced by the large lattice mismatch between the AlGaSb/InAs HEMT structure and the Si substrate. The crystalline quality of the structure grown was examined by x-ray diffraction, transmission electron microscopy, and atomic force microscopy. Finally, very high room-temperature electron mobility of 27 300 cm(2) /V s was achieved. It is demonstrated that a very-high-mobility AlGaSb/InAs HEMT structure on the Si substrate can be achieved with the properly designed buffer layers. (c) 2007 American Institute of Physics.

 

43.    Y. L. Liu, H. X. Li, Z. Y. Ji, Y. Kashimura, Q. X. Tang, K. Furukawa, K. Torimitsu, W. P. Hu, and D. B. Zhu

            "A new morphology of copper 7,7,8,8-tetracyano-p-quinodimethane"

            Micron 38 (5)536-542 (2007).

 

            ABSTRACT: Morphology control is a long-standing problem that needs to be solved for making the switching mechanism of copper 7,7,8,8-tetracyano-p-quinodimethane (CuTCNQ) understood all the time, but up till now how many morphologies CuTCNQ possesses and which morphology should be responsible for the on/off switching phenomenon are still unclear. A new morphology of CuTCNQ, namely the tubular structure, has been obtained and characterized in our experiment, whose formation mechanism has also been investigated. Through characterizing, we can conclude that the tubular structure belongs to the phase 1, which can be further confirmed by the electrical measurements. From the I-V plots, the carrier mobility of the tubular structure is estimated to be similar to 0.1 cm(2) V-1 s(-1), which suggests the potential application of CuTCNQ in devices. [GRAPHICS] (c) 2006 Elsevier Ltd. All rights reserved.

 

44.    A. Lupascu, S. Saito, T. Picot, P. C. De Groot, C. J. Harmans, and J. E. Mooij

            "Quantum non-demolition measurement of a superconducting two-level system"

            Nature Physics 3 (2)119-123 (2007).

 

            ABSTRACT: In quantum mechanics, the process of measurement is a subtle interplay between extraction of information and disturbance of the state of the quantum system. A quantum non-demolition (QND) measurement minimizes this disturbance by using a particular system-detector interaction that preserves the eigenstates of a suitable operator of the quantum system. This leads to an ideal projective measurement. We present experiments in which we carry out two consecutive measurements on a quantum two-level system, a superconducting flux qubit, by probing the hysteretic behaviour of a coupled nonlinear resonator. The large correlation between the results of the two measurements demonstrates the QND nature of the readout method. The fact that a QND measurement is possible for superconducting qubits strengthens the notion that these fabricated mesoscopic systems are to be regarded as fundamental quantum objects. Our results are also relevant for quantum-information processing for protocols such as state preparation and error correction.

 

45.    F. Maeda, S. Suzuki, Y. Kobayashi, D. Takagi, and Y. Homma

            "Reaction products of Co catalysts in ethanol-chemical-vapor-deposition ambient at low-pressure studied by in situ X-ray photoelectron spectroscopy"

            Japanese Journal Of Applied Physics Part 2-letters & Express Letters 46 (4-7)L148-L150 (2007).

 

            ABSTRACT: We succeeded in growing carbon nanotubes in a photoelectron spectroscopy analysis system using thermal chemical vapor deposition and analyzed the chemical states of the Co catalysts by in situ X-ray photoelectron spectroscopy before and after the growth. The carbon nanotubes were grown in an ethanol ambient at about 4 x 10(-2) Torr, a relatively low pressure that was used because we needed to subsequently measure photoelectrons under ultrahigh vacuum. We found that almost all of the Co particles are metallic after the growth. This shows that the metallic state is stable for Co under low-pressure ethanol ambient in our growth condition for carbon nanotubes.

 

46.    N. Maeda, M. Hiroki, N. Watanabe, Y. Oda, H. Yokoyama, T. Yagi, T. Makimoto, T. Enoki, and T. Kobayashi

            "Systematic study of insulator deposition effect (Si3N4, SiO2, AlN, and Al2O3) on electrical properties in AlGaN/GaN heterostructures"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (2)547-554 (2007).

 

            ABSTRACT: To systematically examine the effect Of insulator deposition on the electrical properties in AlGaN/GaN heterostructures, the Si- and Al-based insulators (Si3N4. SiO2, AlN, and Al2O3) have been deposited on Al0.3Ga0.7N/GaN heterostructures. A significant increase in two-dimensional electron gas (2DEG) density (N-s) was observed for all the insulators with the order of N-s(Al2O3) > N-2 (AlN) similar to N-2(SiO2) > N-s(Si3N4) > N-0 (N-0: N, without insulators). This resulted in a decrease in sheet resistance (R) with the smallest order of R(Al2O3) < R(AlN) < R(Si3N4) < R-0 similar to R(SiO2) (R-0: R Without insulators). This order is the same as that of N-s except for SiO2. where the 2DEG mobility largely degraded due to the diffusion Of Si atOITIS into nitride layers. The increase in N, was theoretically analyzed in terms of the change in the potential profile, and the following parameters were extracted: (i) file surface potential barrier (phi(B)), and (ii) the interface charge (N-lnt) between in insulator and AlGaN. phi(B) (eV) was estimated to be 1.7 (Si3N4), 2.2 (AlN), 2.7 (Al2O3), and 3.6 (SiO2), exhibiting a positive correlation between phi(B) and the bandgap of the insulator. N-lnt (10(13) cm(-2)) was estimated to be similar to 0 (Si3N4), 0.1 (SiO2), 0.3 (AlN), and 0.5 (Al2O3); thus, the interface Was found to be positively charged for AlN and Al2O3, whereas it was found to be almost neutral for Si3N4 and SiO2. Thus, the insulator deposition effect has been shown to be significant and to vary among insulators. The analysis shown here offers a guideline for Understanding and designing the electrical properties in AlGaN/GaN heterostructures, where insulators are deposited its surface passivation and/or gate insulators.

 

47.    H. Maki, S. Suzuki, T. Sato, and K. Ishibashi

            "Band gap narrowing and electron doping by potassium encapsulation into single-walled carbon nanotubes"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2486-2489 (2007).

 

            ABSTRACT: We have fabricated field-effect transistors (FETs) with pristine and potassium-encapsulated single-walled carbon nanotube (SWNT) films, and the effects of potassium encapsulation are investigated. The transformation from a unipolar characteristic to an ambipolar characteristic by potassium encapsulation is observed from the measurement of the gate voltage dependence (V-gs)of the current (I) for SWNT-film FETs. This result indicates that the potassium encapsulation into SWNTs causes band gap narrowing. In addition, the n-type region of the I-V-gs curve is expanded during annealing of the devices; electron transfer from potassium to SWNTs occurs owing to the removal of the adsorbates. The adsorbate removal is confirmed by photoemission spectroscopy measurement. The FET with an individual potassium-encapsulated SWNT shows an ambipolar characteristic.

 

48.    D. Moraru, Y. Ono, H. Inokawa, and M. Tabe

            "Quantized electron transfer through random multiple tunnel junctions in phosphorus-doped silicon nanowires"

            Physical Review B 76 (7), 075332 (2007).

 

            ABSTRACT: We have demonstrated numerically and experimentally that quantized electron transfer can be achieved in single-gated random multiple tunnel junctions. Extensive Monte Carlo simulations based on Coulomb blockade orthodox theory show that nonhomogeneous distributions of capacitances energetically favor one-by-one electron shuttling between the electrodes during each cycle of a gate voltage. This numerical prediction is supported by our experimental results on Si nanowire field-effect transistors with the channel moderately doped with phosphorus. Ionized dopants within the device channel locally modulate the potential, creating a naturally random one-dimensional multiple-tunnel-junction array. Under ac-gate operation, small current plateaus or inflections aligned at +/- nef appear in the I-d-V-d characteristics, suggesting that quantized electron transfer is achievable in such naturally disordered systems.

 

49.    S. Moreau, O. M. Fedorych, M. L. Sadowski, M. Potemski, S. Studenikin, G. Austing, A. S. Sachrajda, T. Saku, and Y. Hirayama

            "Response of a 2DEG to microwave irradiation"

            International Journal Of Modern Physics B 21 (8-9)1579-1583 (2007).

 

            ABSTRACT: In this paper, we study the behavior of a high mobility two dimensional electron gas under microwave irradiation by means of magneto-photoluminescence (PL) and absorption measurements. The high mobility sample investigated is a 15nm wide GaAs/AlGaAs quantum well with an electron concentration between 1-2 x 10(11) cm(-2), tunable by visible-light illumination. Structures in the microwave absorption at 40-60GHz are identified as geometrically confined magneto-plasmons.

 

50.    J. F. Morizur, Y. Ono, H. Kageshima, H. Inokawa, and H. Yamaguchi

            "Impact of space-energy correlation on variable range hopping in a transistor"

            Physical Review Letters 98 (16), 166601 (2007).

 

            ABSTRACT: Hopping conduction in transistors, i.e., under a transverse electric field, is addressed using percolation theory with a space-energy correlation in the density of states of the impurity band. The computation of the percolation threshold over an extended range of correlation parameters enables us to derive a formula, which, while giving the classical results in the low field limit, describes the emergence of a specific variable range hopping in the high field case. An application of this formula to experimentally extract the localization radius is also proposed.

 

51.    T. Mukai, C. Hufnagel, A. Kasper, T. Meno, A. Tsukada, K. Semba, and F. Shimizu

            "Persistent supercurrent atom chip"

            Physical Review Letters 98 (26), 260407 (2007).

 

            ABSTRACT: Rubidium-87 atoms are trapped in an Ioffe-Pritchard potential generated with a persistent supercurrent that flows in a loop circuit patterned on a sapphire surface. The superconducting circuit is a closed loop made of a 100 mu m wide molecular-beam epitaxy-grown MgB2 stripe carrying a supercurrent of 2.5 A. To control the supercurrent in the stripe, an on-chip thermal switch operated by a focused argon-ion laser is developed. The switch operates as an on/off switch of the supercurrent or as a device to set the current to a specific value with the aid of an external magnetic field. The current can be set even without an external source if the change is in the decreasing direction.

 

52.    K. Muraki, N. Kumada, and Y. Hirayama

            "Interaction of electron and nuclear spins in quantum wells"

            International Journal Of Modern Physics B 21 (8-9)1266-1275 (2007).

 

            ABSTRACT: We describe our resistively detected nuclear-spin relaxation measurements on bilayer electron systems in double quantum wells. The measurements were carried out to study the compressible-incompressible transition at total filling factor nu(tot) = 1 and the canted antiferromagnetic order and its Goldstone mode predicted for v(tot) = 2. The data demonstrate how nuclear-spin relaxation can shed light on spin/pseudospin order and associated phase transitions that may not be visible through conventional transport or optical measurements.

 

53.    M. Nagase, and H. Yamaguchi

            "Local conductance imaging of semiconductor nanowires on an insulative substrate using an integrated nanogap probe"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (8B)5639-5642 (2007).

 

            ABSTRACT: We have developed a new scanning probe microscopy (SPM)-based tool that enables local in-plane conductance measurements using a split electrode (nanogap) probe. The nanogap integrated on a silicon cantilever was fabricated by focused ion beam (FIB) milling. The integrated nanotool with 45-nm-gap electrodes was used to perform two-terminal conductance measurements of semiconductor nanowires on an insulator substrate. A local conductance image of InAs nanowires grown on a GaAs substrate was successfully obtained with sub-100-nm resolution.

 

54.    T. Nakagawa, I. Sakaguchi, M. Uematsu, Y. Sato, N. Ohashi, H. Haneda, and Y. Ikuhara

            "Diffusion model of gallium in single-crystal ZnO proposed from analysis of concentration-dependent profiles based on the Fermi-level effect"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (7A)4099-4101 (2007).

 

            ABSTRACT: Diffusion of gallium in single-crystal ZnO (implanted at 150 keV at a dose of 2 x 10(16) cm(-2)) in the temperature range between 800 and 900 degrees C was investigated on the basis of experimentally measured and simulated profiles. The gallium concentration profiles have a characteristic tail with a constant-concentration region owing to the Fermi-level effect on the diffusion. The gallium concentration profiles were compared with the simulated profiles: both profiles are in good agreement. The simulation provides possible models for gallium diffusion based on the interstitialcy or vacancy mechanism.

 

55.    K. Nakamatsu, M. Nagase, T. Ichihashi, K. Kanda, Y. Haruyama, T. Kaito, and S. Matsui

            "Fabrication of diamond-like carbon nanosprings by focused-ion-beam chemical vapor deposition and evaluation of their mechanical characteristics"

            Ieice Transactions On Electronics E90C (1)41-45 (2007).

 

            ABSTRACT: Our investigation of diamond-like carbon (DLC) nanosprings with a 130nm spring-section diameter, which were fabricated by focused-ion-beam chemical vapor deposition (FIB-CVD), showed for the first time that nanosprings can be stretched. We observed large displacements of the FIB-CVD nanosprings using in situ optical microscopy; in other words, the nanosprings showed behavior similar to that of macroscale springs. In addition, we investigated the dependence of the spring constant of DLC nanosprings on spring diameter. The spring constants, measured using commercially available cantilevers, ranged from 0.47 to 0.07 N/m. The diameter dependence of spring constant can be accurately expressed by the conventional formula for a coil spring. The estimated shear modulus of the DLC nano-springs was about 70 GPa. This value is very close to the value of conventional coil springs made of steel. Furthermore, we measured the stiffness of a DLC nanospring annealed at 1000 degrees C in vacuum. The stiffness was decreased to approximately half of the stiffness of the nanospring without annealing.

 

56.    A. Nakamura, T. Ohashi, K. Yamamoto, J. Ishihara, T. Aoki, J. Temmyo, and H. Gotoh

            "Full-color electroluminescence from ZnO-based heterojunction diodes"

            Applied Physics Letters 90 (9), 093512 (2007).

 

            ABSTRACT: Red, green, and blue electroluminescence have been observed from ZnO-based heterojunction diodes consisting of n-ZnO/n-MgyZn1-yO/Zn1-xCdxO/p-SiC layers. The heterostructures were grown by remote-plasma-enhanced metal-organic chemical vapor deposition. The rectifying I-V characteristics at room temperature reveal the red, green, and blue wavelengths near 720, 520, and 480 nm, respectively, when the diodes are forward biased. It is observed that the emission color can be controlled by changing the cadmium content in the emission layer. (c) 2007 American Institute of Physics.

 

57.    H. Nakano, K. Kakuyanagi, M. Ueda, and K. Semba

            "Long range and selective coupler for superconducting flux qubits"

            Applied Physics Letters 91 (3), 032501 (2007).

 

            ABSTRACT: The authors propose a qubit-qubit coupling scheme for superconducting flux quantum bits (qubits), where quantized Josephson junction resonator and microwave irradiation are utilized. The junction is used as a tunable inductance controlled by changing the bias current flowing through the junction, and thus the circuit works as a tunable resonator. This enables them to make any qubits interact with the resonator. Entanglement between two of many qubits whose level splittings satisfy some conditions is formed by microwave irradiation causing a two-photon Rabi oscillation. Since the size of the resonator can be as large as submillimeters and qubits interact with it via mutual inductance, their scheme makes it possible to construct a quantum gate involving remote qubits. (C) 2007 American Institute of Physics.

 

58.    H. Nakashima, K. Furukawa, Y. Kashimura, and K. Torimitsu

            "Anisotropic assembly of gold nanorods assisted by selective ion recognition of surface-anchored crown ether derivatives"

            Chemical Communications (10)1080-1082 (2007).

 

            ABSTRACT: Gold nanorods (NRs) mixed with crown ether derivatives exhibited the efficient and selective recognition of Na+ and K+ ions, which were detected by localized surface plasmon absorption in response to dispersed and aggregated gold NRs. Furthermore, in the aggregates preferential end-to-end or side-to-side assembly of NRs was observed which was dependent on the additive concentration.

 

59.    H. Namatsu, M. Oda, A. Yokoo, M. Fukuda, K. Irisa, S. Tsurumi, and K. Komatsu

            "Chemical nanoimprint lithography for step-and-repeat Si patterning"

            Journal Of Vacuum Science & Technology B 25 (6)2321-2324 (2007).

 

            ABSTRACT: Chemical nanoimprint lithography is proposed as a new nanoimprint category. The authors investigate chemical nanoimprint utilizing an electrochemical reaction based on the anodic oxidation that occurs as a result of moisture that is present between a mold and a substrate. For stable nanoimprint patterning, the authors have developed a silicon carbide mold with low resistance. The mold also has excellent surface flatness and mechanical strength. In addition, the authors have developed the prototype of chemical nanoimprinter equipped with a step-and-repeat system. The imprinter also has a temperature-humidity controller and a mold-pressure adjustment system, in addition to a voltage supply source. (c) 2007 American Vacuum Society.

 

60.    N. Namekata, G. Fujii, S. Inoue, T. Honjo, and H. Takesue

            "Differential phase shift quantum key distribution using single-photon detectors based on a sinusoidally gated InGaAs/InP avalanche photodiode"

            Applied Physics Letters 91 (1), 011112 (2007).

 

            ABSTRACT: The authors report a quantum key distribution experiment, in which they implemented a differential phase shift quantum key distribution protocol, using single-photon detectors based on InGaAs/InP avalanche photodiodes operated with a sinusoidal gating. The single-photon detectors were operated at a repetition frequency of 500 MHz with low after pulsing probabilities and low dark counts. A sifted key generation rate of 1.5 Mbit/s was achieved over a communication distance of 15 km. Taking account of the security of the protocol against general individual attacks, secure keys can be generated with a rate of 0.33 Mbit/s. (c) 2007 American Institute of Physics.

 

61.    Y. Nishi, Y. Tokura, J. Gupta, G. Austing, and S. Tarucha

            "Ground-state transitions beyond the singlet-triplet transition for a two-electron quantum dot"

            Physical Review B 75 (12), 121301 (2007).

 

            ABSTRACT: We studied electrostatic confinement effects on correlated electronic states in vertical quantum dots in high magnetic fields. We prepared vertical quantum dots with different lateral confinement strengths and investigated the magnetic field evolution of the electronic states. We observed that the formation of the maximum density droplet state corresponding to filling factor nu=1 shifts to lower magnetic field as the lateral confinement energy becomes weaker. In addition, we found a ground-state transition in the nu < 1 regime for the most weakly laterally confined two-electron quantum dot that is predicted to appear beyond an easily accessible magnetic field in standard vertical quantum dots more typically investigated.

 

62.    K. Nishiguchi, Y. Ono, A. Fujiwara, H. Yamaguchi, H. Inokawa, and Y. Takahashi

            "Infrared detection with silicon nano-field-effect transistors"

            Applied Physics Letters 90 (22), 223108 (2007).

 

            ABSTRACT: The authors fabricated nanoscale silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) to detect an infrared (IR) signal at room temperature. The IR signal excites conduction-band electrons in an undoped channel of a MOSFET and some of them are injected through an energy barrier into a storage node (SN) electrically formed by the MOSFET. Small signals, originating from electrons, stored in the SN are detected by an electrometer with a single-electron resolution. Additionally, the MOSFET controls the number and energy of electrons injected into the SN. This enables electrical control of the sensitivity and cutoff wavelengths of IR signals, suggesting the possibility of highly functional IR sensors. (C) 2007 American Institute of Physics.

 

63.    K. Nishiguchi, A. Fujiwara, H. Inokawa, and Y. Takahashi

            "Long retention of gain-cell dynamic random access memory with undoped memory node"

            Ieee Electron Device Letters 28 (1)48-50 (2007).

 

            ABSTRACT: Low current leakage characteristics of a novel silicon-on-insulator (SOI) device are investigated in view of application to a gain-cell dynamic random access memory (DRAM). The device consists of a two-layered poly-Si gate. Since, in this device, the memory node is electrically formed by the gate in undoped SOI wire, no p-n junction is required. The retention is found to be dominated by the subthreshold leakage, which leads. to long data retention. The device also achieved a fast (10 ns) writing time and its fabrication process is compatible with those of SOI MOSFETs. The present results, thus, strongly suggest a way of conducting a gain-cell DRAM to be embedded into logic circuits.

 

64.    A. Nishikawa, K. Kumakura, and T. Makimoto

            "Temperature dependence of current-voltage characteristics of npn-type GaN/InGaN double heterojunction bipolar transistors"

            Applied Physics Letters 91 (13), 133514 (2007).

 

            ABSTRACT: We investigated the temperature dependence of the common-emitter current-voltage (I-V) characteristics of npn-type GaN/InGaN double heterojunction bipolar transistors. Although the current gain decreases with increasing measurement temperature, the current gain measured at 300 degrees C is still as high as 308. The reduction of the current gain with temperature is attributed not only to the hole back-injection current from the base into the emitter but also to the shorter minority carrier diffusion length due to the increase in the carrier concentration of the p-InGaN base. (c) 2007 American Institute of Physics.

 

65.    A. Nishikawa, K. Kumakura, and T. Makimoto

            "High critical electric field exceeding 8 MV/cm measured using an AlGaN p-i-n vertical conducting diode on n-SiC substrate"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2316-2319 (2007).

 

            ABSTRACT: We have succeeded in obtaining the high critical electric field exceeding 8 MV/cm measured using an AlGaN p-i-n vertical conducting diode on n-SiC substrate grown by low-pressure metal organic vapor phase epitaxy. The critical electric field of AlGaN with Al composition of 57% is as high as 8.1 MV/cm, the highest among semiconductors with a doping concentration of less than 10(17) cm(-3), at which the avalanche multiplication process takes place. The critical electric field is proportional to the bandgap energy to a power of 2.7. In the forward current-voltage characteristics, the on-state resistance of the diode increases with increasing Al composition. Since there is a tradeoff between the breakdown voltage (VB) and the on-state resistance (R-on) the figure of merit V-B(2)/R-on has its maximum when the Al composition is about 30% and is twice as high as that for GaN-based diodes. This indicates that AlGaN-based electronic devices are more promising for high-power operation than GaN-based ones.

 

66.    A. Nishikawa, K. Kumakura, T. Akasaka, and T. Makimoto

            "Low-resistance graded AlxGa1-xN buffer layers for vertical conducting devices on n-SiC substrates"

            Journal Of Crystal Growth 298819-821 (2007).

 

            ABSTRACT: We investigated the Al composition and thickness dependence of the resistance of graded AlxGa1-xN buffer layers for vertical conducting devices on n-type 6H-SiC substrates. To measure current-voltage characteristics, a Ti/Au ohmic contact was formed on the backside of the SiC substrate. The Al composition of the buffer layer was varied from 2% to 10%. We found that the optimal buffer layer thickness for the lowest resistance decreases with increasing Al composition because the nucleation seeds on SiC substrate coalesce faster. Since the optimal layer thickness depends on the Al composition, the sheet Al concentration, which is calculated from the Al composition and the thickness, should be considered as a critical parameter for low buffer resistance of vertical conducting devices on n-SiC substrates. (c) 2006 Elsevier B.V. All rights reserved.

 

67.    J. Nitta, and T. Bergsten

            "Time reversal Aharonov-Casher effect using Rashba spin-orbit interaction"

            New Journal Of Physics 9, 341 (2007).

 

            ABSTRACT: We propose a spin interferometer using Rashba spin - orbit interaction. A spin interference effect is demonstrated in small arrays of mesoscopic InGaAs rings. This spin interference is the time reversal Aharonov-Casher ( AC) effect. The AC interference oscillations are controlled over several periods. This result shows evidence for electrical manipulation of the spin precession angle in an InGaAs two- dimensional electron gas channel. We control the precession rate in a precise and predictable way with an electrostatic gate.

 

68.    J. Nitta, and T. Bergsten

            "Electrical manipulation of spin precession in an InGaAs-based 2DEG due to the Rashba spin-orbit interaction"

            Ieee Transactions On Electron Devices 54 (5)955-960 (2007).

 

            ABSTRACT: We demonstrate a spin interference effect in small arrays of mesoscopic InGaAs rings, which is the time reversal Aharonov-Casher effect. Spin-orbit interaction parameters obtained from the spin interference are consistent with those estimated from Shubnikov-de Haas analysis. This result shows evidence for electrical manipulation of the spin precession angle in an InGaAs 2-D electron gas channel. We control the precession rate in a precise and predictable way with an electrostatic gate.

 

69.    S. Nomura, M. Yamaguchi, T. Akazaki, H. Tamura, T. Maruyama, S. Miyashita, and Y. Hirayama

            "Enhancement of electron and hole effective masses in back-gated GaAs/AlxGa1-xAs quantum wells"

            Physical Review B 76 (20), 201306 (2007).

 

            ABSTRACT: Both the electron and the optically created hole effective masses are found to be density dependent in a two-dimensional electron system of a GaAs/Al0.33Ga0.67As back-gated quantum well by magnetophotoluminescence spectroscopy. We show that the density-dependent electron effective mass increases with a decrease in the electron density (n(s)) to n(s)< 1x10(11) cm(-2). It is found that the electron effective masses determined from the lowest and the second Landau levels are larger than those from the higher Landau levels. The hole effective mass is found to increase with a decrease in n(s) and the hole is found to localize at n(s)< 3x10(10) cm(-2). We observe an upward convex curve of the photoluminescence peak energy at 2

 

70.    M. Notomi, T. Tanabe, A. Shinya, E. Kuramochi, H. Taniyama, S. Mitsugi, and M. Morita

            "Nonlinear and adiabatic control of high-Q photonic crystal nanocavities"

            Optics Express 15 (26)17458-17481 (2007).

 

            ABSTRACT: This article overviews our recent studies of ultrahigh-Q and ultrasmall photonic-crystal cavities, and their applications to nonlinear optical processing and novel adiabatic control of light. First, we show our latest achievements of ultrahigh-Q photonic-crystal nanocavities, and present extreme slow-light demonstration. Next, we show all-optical bistable switching and memory operations based on enhanced optical nonlinearity in these nanocavities with extremely low power, and discuss their applicability for realizing chip-scale all-optical logic, such as flip-flop. Finally, we introduce adiabatic tuning of high-Q nanocavities, which leads to novel wavelength conversion and another type of optical memories. (C) 2007 Optical Society of America.

 

71.    T. Nyberg, A. Shimada, and K. Torimitsu

            "Ion conducting polymer microelectrodes for interfacing with neural networks"

            Journal Of Neuroscience Methods 160 (1)16-25 (2007).

 

            ABSTRACT: We have examined the stimulation and recording properties of conjugated polymer microelectrode arrays as interfaces with neural networks of dissociated cortical cells. In particular the stimulation properties were investigated as a means of supplying a neural network with information. The stimulation efficiency at low stimulation voltages was evaluated and referenced to bare indium tin oxide (ITO) electrodes. The polymer electrodes were electrochemically polymerized from a blend of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) and ethylenedioxythiophene (EDOT) onto ITO microelectrodes. Dissociated cortical cells were then plated on the electrodes and cultivated to form neural networks. Polymer electrode stimulation evoked a much greater response from the network than stimulation from ITO electrodes. Neural interfaces using polymer electrodes could be maintained for several months. (c) 2006 Elsevier B.V. All rights reserved.

 

72.    T. Obata, M. Pioro-Ladriere, T. Kubo, K. Yoshida, Y. Tokura, and S. Tarucha

            "On-chip micro-coil technique for single electron spin resonance with quantum dot"

            Physica E-low-dimensional Systems & Nanostructures 40 (2)351-354 (2007).

 

            ABSTRACT: We have developed a combined setup consisting of an on-chip micro-coil and a quantum dot for implementing single electron spin resonance, which operates relevantly even at dilution refrigerator temperatures. We have examined the micro-coil performance of the high-frequency response. Capacitive coupling between the coil and the quantum dot causes photon-assisted tunneling, whose signal can overlap greatly with the electron spin resonance signal. We have developed a technique to compensate for the influence of the capacitive coupling, and checked the performance using Coulomb blockade transport. (C) 2007 Elsevier B.V. All rights reserved.

 

73.    T. Obata, M. Pioro-Ladriere, T. Kubo, K. Yoshida, Y. Tokura, and S. Tarucha

            "Microwave band on-chip coil technique for single electron spin resonance in a quantum dot"

            Review Of Scientific Instruments 78 (10), 104704 (2007).

 

            ABSTRACT: Microwave band on-chip microcoils are developed for the application to single electron spin resonance measurement with a single quantum dot. Basic properties such as characteristic impedance and electromagnetic field distribution are examined for various coil designs by means of experiment and simulation. The combined setup operates relevantly in the experiment at dilution temperature. The frequency responses of the return loss and Coulomb blockade current are examined. Capacitive coupling between a coil and a quantum dot causes photon assisted tunneling, whose signal can greatly overlap the electron spin resonance signal. To suppress the photon assisted tunneling effect, a technique for compensating for the microwave electric field is developed. Good performance of this technique is confirmed from measurement of Coulomb blockade oscillations. (C) 2007 American Institute of Physics.

 

74.    K. Oguri, Y. Okano, T. Nishikawa, and H. Nakano

            "Dynamical study of femtosecond-laser-ablated liquid-aluminum nanoparticles using spatiotemporally resolved X-ray-absorption fine-structure spectroscopy"

            Physical Review Letters 99 (16), 165003 (2007).

 

            ABSTRACT: We study the temperature evolution of aluminum nanoparticles generated by femtosecond laser ablation with spatiotemporally resolved x-ray-absorption fine-structure spectroscopy. We successfully identify the nanoparticles based on the L-edge absorption fine structure of the ablation plume in combination with the dependence of the edge structure on the irradiation intensity and the expansion velocity of the plume. In particular, we show that the lattice temperature of the nanoparticles is estimated from the L-edge slope, and that its spatial dependence reflects the cooling of the nanoparticles during plume expansion. The results reveal that the emitted nanoparticles travel in a vacuum as a condensed liquid phase with a lattice temperature of about 2500 to 4200 K in the early stage of plume expansion.

 

75.    Y. Ono, K. Nishiguchi, A. Fujiwara, H. Yamaguchi, H. Inokawa, and Y. Takahashi

            "Conductance modulation by individual acceptors in Si nanoscale field-effect transistors"

            Applied Physics Letters 90 (10), 102106 (2007).

 

            ABSTRACT: The authors measured low-temperature (6-28 K) conductance in nanoscale p-channel field-effect transistors lightly doped with boron. They observed a conductance modulation, which they ascribed to the trapping/detrapping of single holes by/from individual acceptors. The statistics of the appearance of the modulation in a few ten samples indicates that the number of acceptors is small, or even just one, suggesting that what the authors have observed is single-charge-transistor operation by a single-acceptor quantum dot. (c) 2007 American Institute of Physics.

 

76.    T. Ota, G. Yusa, N. Kumada, S. Miyashita, T. Fujisawa, and Y. Hirayama

            "Decoherence of nuclear spins due to dipole-dipole interactions probed by resistively detected nuclear magnetic resonance"

            Applied Physics Letters 91 (19), 193101 (2007).

 

            ABSTRACT: We study decoherence of nuclear spins in a GaAs quantum well structure using resistively detected nuclear magnetic resonance. The transverse decoherence time T-2 of As-75 nuclei is estimated from Rabi-type coherent oscillations as well as by using spin-echo techniques. By analyzing T-2 obtained by decoupling techniques, we extract the role of dipole-dipole interactions as sources of decoherence in GaAs. Under the condition that the device is tilted in an external magnetic field, we exhibit enhanced decoherence induced by the change in strength of the direct dipole-dipole interactions between first nearest-neighbor nuclei. The results agree well with simple numerical calculations.

 

77.    T. Ota, G. Yusa, N. Kumada, S. Miyashita, and Y. Hirayama

            "Nuclear spin population and its control toward initialization using an all-electrical submicron scale nuclear magnetic resonance device"

            Applied Physics Letters 90 (10), 102118 (2007).

 

            ABSTRACT: The authors study the nuclear spin population in a GaAs quantum well structure and demonstrate its initialization using an all-electrical nuclear magnetic resonance (NMR) device. In their device, nuclear spins are dynamically polarized in a submicron scale region defined by split gates. The relative population of each nuclear spin state is estimated from resistively detected NMR spectra combined with numerical analysis. They find that nuclear spin populations are determined by electron spin configurations. By applying radio frequency pulses to the strongly polarized nuclear spins, they demonstrate the creation of two-qubit effective pure states, which is a crucial step toward NMR quantum computation. (c) 2007 American Institute of Physics.

 

78.    H. Oyanagi, A. Tsukada, M. Naito, and N. L. Saini

            "Local structure of superconducting (La,Sr)(2)CuO4 under strain: Microscopic mechanism of strain-induced T-c variation"

            Physical Review B 75 (2), 024511 (2007).

 

            ABSTRACT: High-quality polarized x-ray absorption spectroscopy data for (La,Sr)(2)CuO4 thin-film single crystals reveal strain-dependent local disorder (in the oxygen radial distribution) that correlates with the superconducting critical temperature. The temperature-dependent in-plane oxygen displacement shows that local lattice distortion strongly depends on strain, i.e., the biaxial tensile strain develops domains with the bond-stretching-type local distortion which is weakened by the compressive strain. We suggest that the two-dimensional strain modifies electronic inhomogeneity that influences the superconducting critical temperature through superfluid density, rather than band structure effects.

 

79.    S. Perraud, K. Kanisawa, Z. Z. Wang, and T. Fujisawa

            "Imaging the percolation of localized states in a multisubband two-dimensional electronic system subject to a disorder potential"

            Physical Review B 76 (19), 195333 (2007).

 

            ABSTRACT: The electronic local density of states (LDOS) in an In0.53Ga0.47As surface quantum well (QW) grown by molecular beam epitaxy was measured by low-temperature scanning tunneling spectroscopy under ultrahigh vacuum. The LDOS in the conduction band has a clear steplike energy dependence, revealing that several subbands are formed in the QW. At a given energy, the LDOS shows strong spatial fluctuations in the QW plane due to the presence of a disorder potential. Percolation of localized states with increasing energy is observed in each subband tail. The percolation threshold is determined for each subband by using a semiclassical model. The origin of the disorder potential is discussed, and is ascribed to a random distribution of native point defects located at the QW surface.

 

80.    S. Perraud, K. Kanisawa, Z. Z. Wang, and Y. Hirayama

            "Dramatic dependence of the Fermi level pinning strength on crystal orientation at clean surfaces of n-type In0.53Ga0.47As grown by molecular beam epitaxy"

            Journal Of Crystal Growth 301148-151 (2007).

 

            ABSTRACT: The electronic density of states at clean surfaces of n-type In0.53Ga0.47As, grown by molecular beam epitaxy on lattice-matched (0 0 1)and (1 1 1)A-oriented InP substrates, was measured by low-temperature scanning tunneling spectroscopy under ultra-high vacuum. It was found that the surface Fermi level (FL) pinning strength dramatically depends on crystal orientation. The FL at the (0 0 1)-(2 x 4) surface is pinned near midgap, independently of the dopant concentration in the bulk. In contrast, the FL at the (1 1 1)A-(2 x 2) surface lies in the conduction band, close to the bulk FL, and increases with dopant concentration. (c) 2007 Elsevier B.V. All rights reserved.

 

81.    M. Pioro-Ladriere, Y. Tokura, T. Obata, T. Kubo, K. Yoshida, and S. Tarucha

            "Spin-charge qubit resonance readout in lateral quantum dots"

            Physica E-low-dimensional Systems & Nanostructures 40 (2)347-350 (2007).

 

            ABSTRACT: We report on the fabrication and simulation of a lateral quantum dot (QD) designed for coherent electrical manipulation and readout of a two-level spin-charge system. The two-level system involves a slight hybridization of a single electron spin to the QD orbitals. A permanent magnet integrated with high-frequency electrode produces a static slanting magnetic field suitable for voltage controlled single qubit rotations. Resonance frequency and rotation speed are related to device parameters. Response of the nearby quantum point contact charge sensor is modeled for continuous microwave excitation, taking into account parasitic photon-assisted tunneling effects induced by the high-frequency electric field. The model predicts a well-defined resonance in the readout signal associated to driven rotations of the qubit states. (C) 2007 Elsevier B.V. All rights reserved.

 

82.    M. Pioro-Ladriere, Y. Tokura, T. Obata, T. Kubo, and S. Tarucha

            "Micromagnets for coherent control of spin-charge qubit in lateral quantum dots"

            Applied Physics Letters 90 (2), 024105 (2007).

 

            ABSTRACT: A lateral quantum dot design for coherent electrical manipulation of a two-level spin-charge system is presented. Two micron-size permanent magnets integrated to high-frequency electrodes produce a static slanting magnetic field suitable for voltage controlled single qubit gate operations. Stray field deviation from the slanting form is taken into account in the Hamiltonian describing the two-level system, which involves hybridization of a single electron spin to the quantum dot's orbitals. Operation speed and gate fidelity are related to device parameters. Sub-100-ns pi pulse duration can be achieved with lattice fluctuation coherence time of 4 ms for GaAs. (c) 2007 American Institute of Physics.

 

83.    K. Prabhakaran, B. Schwenzer, S. P. DenBaars, and U. K. Mishra

            "Triangular pattern formation on silicon through self-organization of GaN nanoparticles"

            Applied Surface Science 253 (10)4773-4776 (2007).

 

            ABSTRACT: Nanoparticles of gallium nitride, synthesized by a low-temperature reaction between triethyl gallium and ammonia, were introduced onto silicon wafers containing a thin layer of chemically prepared silicon dioxide. At room temperature, the nanoparticles form unstructured agglomerates on the surface. However, upon annealing the samples beyond the decomposition temperature of the silicon dioxide layer, the gallium nitride particles self-organize to form triangular structures. The pattern formation is attributed to the domain separation associated with the (I x 1)-(7 x 7) surface phase transformation followed by selective incorporation of the nanoparticles. (c) 2006 Elsevier B.V. All rights reserved.

 

84.    G. M. Prinz, A. Ladenburger, M. Schirra, M. Feneberg, K. Thonke, R. Sauer, Y. Taniyasu, M. Kasu, and T. Makimoto

            "Cathodoluminescence, photoluminescence, and reflectance of an aluminum nitride layer grown on silicon carbide substrate"

            Journal Of Applied Physics 101 (2), 023511 (2007).

 

            ABSTRACT: Aluminum nitride (AlN) has an ultrawide direct band gap of approximately 6.1 eV at low temperature and is fully miscible with gallium nitride. This makes AlN a promising material for ultraviolet optoelectronic applications. Here, we apply cathodoluminescence, photoluminescence, and reflectance spectroscopies to the same AlN layer grown by metalorganic vapor phase epitaxy on silicon carbide. In cathodoluminescence and photoluminescence, we observe strong near band edge emission at approximate to 6 eV. The contribution appearing at an energetic position of 5.983 eV could be identified as A free exciton recombination, strongly redshifted due to strain effects. The spectra obtained by reflectance measurements show features at 5.985 eV and approximate to 6.2 eV which we assign to the A exciton-in accordance to our luminescence measurements-and a combination of the B and C free excitons, respectively. (c) 2007 American Institute of Physics.

 

85.    C. S. Ramanujan, K. Sumitomo, M. R. de Planque, H. Hibino, K. Torimitsu, and J. F. Ryan

            "Self-assembly of vesicle nanoarrays on Si: A potential route to high-density functional protein arrays"

            Applied Physics Letters 90 (3), 033901 (2007).

 

            ABSTRACT: The authors show that 100 nm unilamellar thiol-tagged vesicles bind discretely and specifically to Au nanodots formed on a Si surface. An array of such dots, consisting of 20 nm Au-Si three-dimensional islands, is formed by self-assembly on terraces of small-angle-miscut Si(111) after Au deposition. Consequently, both the formation of the nanopattern and the subsequent attachment of the vesicles are self-organized and occur without the need for any "top-down" lithographic processes. This approach has the potential to provide the basis of a low-cost, high-density nanoarray for use in proteomics and drug discovery. (c) 2007 American Institute of Physics.

 

86.    B. Ranguelov, V. Tonchev, H. Omi, and A. Pimpinelli

            "New universality class for step bunching in the "C+-C-" model"

            Comptes Rendus De L Academie Bulgare Des Sciences 60 (4)389-394 (2007).

 

            ABSTRACT: We formulate a new (1 + 1)D step model of unstable vicinal growth. The basic assumption is that the equilibrium adatom concentrations on both sides of the step are different. We deduce equations of step motion and numerically integrate them to obtain the step positions on a discrete time set. New dynamic phenomena are observed during the bunching process. The size-scaling of the minimal interestep distance l(min) for first time is obtained as l(min) similar to N-1/(n+1), where N is the number of steps in the bunch and n is the exponent in the step-step repulsions law U similar to 1/d(n)' for two steps placed a distance d apart.

 

87.    R. Rungsawang, Y. Ueno, and K. Ajito

            "Detecting a sodium chloride ion pair in ice using terahertz time-domain spectroscopy"

            Analytical Sciences 23 (7)917-920 (2007).

 

            ABSTRACT: The hydrogen bond resonance of a sodium chloride (NaCl) ion pair trapped in aqueous ice has been observed by transmission terahertz time-domain spectroscopy. The absorption peak of a sodium chloride ion pair in ice is 1.65 THz at 83 K. By investigating the interaction of the cation and anion with other chemical compounds, we deduce that the absorption peak originates from the hydrogen bond resonance of sodium chloride and water molecules. The charge redistribution that occurs when other ion pairs are added to aqueous salt solution changes the absorption spectrum. Furthermore, the results also indicate that simple molecules such as sodium halides have fingerprints in the terahertz region when the ions are trapped in ice. NaCl ion pairs in seawater and in Ringer's solution were examined.

 

88.    S. Russo, J. Tobiska, T. M. Klapwijk, and A. F. Morpurgo

            "Adiabatic quantum pumping at the josephson frequency"

            Physical Review Letters 99 (8), 086601 (2007).

 

            ABSTRACT: We analyze theoretically adiabatic quantum pumping through a normal conductor that couples the normal regions of two superconductor-normal-metal-superconductor Josephson junctions. By using the phases of the superconducting order parameter in the superconducting contacts as pumping parameters, we demonstrate that a nonzero pumped charge can flow through the device. The device exploits the evolution of the superconducting phases due to the ac Josephson effect, and can therefore be operated at very high frequency, resulting in a pumped current as large as a few nanoamperes. The experimental relevance of our calculations is discussed.

 

89.    T. Sakata, K. Kuwabara, T. Shimamura, N. Sato, M. Nagase, N. Shimoyama, K. Kudou, K. Machida, and H. Ishii

            "Electrodeposition of water-repellent organic dielectric film as an anti-sticking coating on microelectromechanical system devices"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (9B)6454-6457 (2007).

 

            ABSTRACT: In this paper, we propose a technique of preventing both wet-release-related and in-use sticking of actuators in microelectromechanical system (MEMS) devices. The technique involves the electrodeposition of a water-repellent organic dielectric film that renders the microstructure surface inactive towards the water used for rinsing. The source material is a core/shell emulsion, which consists of sulfonium cations with epoxy groups containing water-repellent silicone polymers. Applying this technique to the encapsulation of a microstructure confirms its effectiveness in preventing both release-related sticking and in-use sticking of a MEMS structure.

 

90.    H. Sanada, H. Gotoh, K. Tateno, and H. Nakano

            "Exciton and biexciton emissions from single GaAs quantum dots in (Al,Ga)As nanowires"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2578-2580 (2007).

 

            ABSTRACT: We report the optical properties of GaAs quantum dots embedded in (AI,Ga)As nanowires grown by the vapor-liquid-solid method. We used the micro-photoluminescence (PL) technique to observe PL peaks, which are assigned as exciton and biexciton emissions from single quantum dots. In addition, unusual features appear in the excitation power dependence of the energies and linewidths of the two PL peaks. The PL also depends on the optical polarization axis, indicating that the nanostructures have a highly asymmetrical shape. The results show that our method is a promising way of engineering the positions and optical properties of GaAs/(Al,Ga)As nanostructures.

 

91.    Y. Sawai, B. Takimoto, H. Nabika, K. Ajito, and K. Murakoshi

            "Observation of a small number of molecules at a metal nanogap arrayed on a solid surface using surface-enhanced Raman scattering"

            Journal Of The American Chemical Society 129 (6)1658-1662 (2007).

 

            ABSTRACT: In situ Raman spectroscopic measurements with 785 nm excitation were carried out in aqueous solutions containing bipyridine derivatives. Intense Raman signals were observed when the Ag dimer structure was optimized. The SERS activity was dependent upon on the structure of the Ag dimer with a distinct gap distance, suggesting that the intense SERS originates from the gap part of the dimer. Characteristic time-dependent spectral changes were observed. Not only a spectrum which was the superposition of two bipyridine spectra but also spectra which can be assigned to one of the bipyridine derivatives were frequently observed. Observation using solutions with different concentrations proved that the spectra originated from very small numbers of molecules at the active SERS site of the dimer.

 

92.    H. Shibata, S. Karimoto, and T. Makimoto

            "Growth of (Cu,C)Ba2CuOy thin films by molecular-beam epitaxy"

            Physica C-superconductivity And Its Applications 463939-941 (2007).

 

            ABSTRACT: We report the growth of (Cu,C)Ba2CuOy thin films by molecular-beam epitaxy (MBE). The c-axis oriented film is obtained with a = 3.959 A and c = 8.140 A. The T-c values of the film on an NdGaO3 substrate are T-c(on) similar to 35 K and Tc(zero) similar to 18 K. The T-c(on) is increased to 70 K for a 12 nm-thick film deposited on an infinite-layer CaCuO2 buffer layer. This suggests that the layer-by-layer growth method using (Cu,C)Ba2CuOy and [CaCuO2](n-1) is a promising way to obtain higher member (Cu,C)Ba2Can-1CunOy (n >= 2) thin films with a higher T,. We also find that the Ba-Cu-O phase with c similar to 4.0 A, which is known as the'infinite-layer BaCUO2' phase, can be obtained only in a slightly CO2 atmosphere. This means that the phase contains carbon in its structure. (C) 2007 Elsevier B.V. All rights reserved.

 

93.    H. Shibata, S. Karimoto, A. Tsukada, and T. Makimoto

            "Thin film growth of (Cu, C)Ba2Ca(n-1)CunOy (n=1-4) superconductor by molecular beam epitaxy"

            Journal Of Crystal Growth 301684-686 (2007).

 

            ABSTRACT: We report the growth of (Cu C)Ba2Ca(n-1)CUnOY (n = 1-4) superconducting thin films by the molecular beam epitaxy method. The film was c-axis oriented for n = 1, and was changed to a-axis oriented with 90 degrees domains for n = 2-4. All the films exhibited superconductivity and their T-c's were T-c(on) = 35 K and T-c(zero) = 18 K for n = 1, T-c(on) = 80 K and T-c(zero) = 52 K for n = 2, T-c(on) = 85 K and T-c(zero) = 42 K for n = 3, and T-c(on) = 105 K and T-c(zero) = 53 K for n = 4. (c) 2006 Elsevier B.V. All rights reserved.

 

94.    G. Shinkai, T. Hayashi, Y. Hirayama, and T. Fujisawa

            "Controlled resonant tunneling in a coupled double-quantum-dot system"

            Applied Physics Letters 90 (10), 103116 (2007).

 

            ABSTRACT: The authors investigate electrostatic coupling between two double quantum dots (DQDs) defined in an AlGaAs/GaAs heterostructure by measuring the correlation between resonant tunneling currents through the DQDs. Resonant tunneling in one DQD can be controlled by the charge state of the other DQD. This controlled resonant tunneling is consistent with the capacitance model for the geometry and can be used to investigate the statistics of single-electron charge states in the DQD. The observed electrostatic coupling is strong enough to perform two-qubit quantum gates both for charge- and electron-spin-based qubit schemes. (c) 2007 American Institute of Physics.

 

95.    J. Y. Shiu, V. Desmaris, N. Rorsman, K. Kumakura, T. Makimoto, H. Zirath, and E. Y. Chang

            "DC and microwave performance of AlGaN/GaN HEMTs passivated with sputtered SiNx"

            Semiconductor Science And Technology 22 (7)717-721 (2007).

 

            ABSTRACT: The effects of sputtered and room temperature plasma enhanced chemical vapour deposition (RT-PECVD) SiNx passivation on the dc and microwave performance of AlGaN/GaN high electron mobility transistors (HEMTs) are studied. The pulsed I - V characteristics from a class B quiescent bias point and transient measurements indicate that the sputtered SiNx passivation is more efficient in suppressing lag effects in AlGaN/GaN HEMTs. Dispersion-free sputtered SiNx passivated AlGaN/GaN HEMTs were obtained using this technique. Continuous-wave (CW) measurements without active cooling give a maximum output power density of 6.6 W mm(-1) at V-gs=- 4 V, V-ds = 50 V and a maximum power added efficiency of 51.3% at V-gs=- 4 V, V-ds = 30 V at 3 GHz on 2 x 50 mu m AlGaN/GaN HEMTs on the sapphire substrate, with a gate length of 2 mu m and without field-plated gates. To the best of our knowledge, this is the highest level power density reported on the sapphire substrate without field-plate design. The extrinsic cut-off frequency (f(t)) and maximum oscillation frequency (f(max)) are 51 GHz and 100 GHz, respectively, on 2 x 50 x 0.15 mu m HEMTs. To our knowledge, the sputtered SiNx passivation for AlGaN/GaN HEMTs is a unique technique, which has never been published before.

 

96.    J. Y. Shiu, J. C. Huang, V. Desmaris, C. T. Chang, C. Y. Lu, K. Kumakura, T. Makimoto, H. Zirath, N. Rorsman, and E. Y. Chang

            "Oxygen ion implantation isolation planar process for AlGaN/GaN HEMTs"

            Ieee Electron Device Letters 28 (6)476-478 (2007).

 

            ABSTRACT: A multienergy oxygen ion implantation process was demonstrated to be compatible With the processing of high-power microwave AlGaN/GaN high electron mobility transistors (HEMTs). HEMTs that are isolated by this process exhibited gate-lag- and drain-lag-free operation; A maximum output power density of 5.3 W/mm at V-gs = -4 V and V-ds = 50 V and a maximum power added efficiency of 51.5% at V-gs = -4 V and V-ds = 30 V at 3 GHz were demonstrated on HEMTs without field plates on sapphire substrate. This isolation process results in planar HEMTs, circumventing potential problems with enhanced gate leakage due to the gate contacting the 2-D electron gas at the mesa sidewall.

 

97.    W. Siemons, G. Koster, H. Yamamoto, T. H. Geballe, D. H. Blank, and M. R. Beasley

            "Experimental investigation of electronic properties of buried heterointerfaces of LaAlO3 on SrTiO3"

            Physical Review B 76 (15), 155111 (2007).

 

            ABSTRACT: We have made very thin films of LaAlO3 on TiO2 terminated SrTiO3 and have measured the properties of the resulting interface in various ways. Transport measurements show a maximum sheet carrier density of 10(16) cm(-2) and a mobility around 10(4) cm(2) V-1 s(-1). In situ ultraviolet photoelectron spectroscopy (UPS) indicates that for these samples a finite density of states exists at the Fermi level. From the oxygen pressure dependence measured in both transport as well as the UPS, we detail, as reported previously by us, that oxygen vacancies play an important role in the creation of the charge carriers and that these vacancies are introduced by the pulsed laser deposition process used to make the heterointerfaces. Under the conditions studied the effect of LaAlO3 on the carrier density is found to be minimal.

 

98.    W. Siemons, G. Koster, A. Vailionis, H. Yamamoto, D. H. Blank, and M. R. Beasley

            "Dependence of the electronic structure of SrRuO3 and its degree of correlation on cation off-stoichiometry"

            Physical Review B 76 (7), 075126 (2007).

 

            ABSTRACT: We have grown and studied high quality SrRuO3 films grown by molecular beam epitaxy as well as pulsed laser deposition. By changing the oxygen activity during deposition, we were able to make SrRuO3 samples that were stoichiometric (low oxygen activity) or with ruthenium vacancies (high oxygen activity). This ability to control the ruthenium stoichiometry has permitted us to make a systematic study of the dependence of the degree of electron correlation in SrRuO3 on stoichiometry using transport and photoemission experiments. We have compared the measured ultraviolet photoemission spectroscopy spectra with calculated density of states spectra and offer explanations for the large observed differences between the two.

 

99.    W. Siemons, G. Koster, H. Yamamoto, W. A. Harrison, G. Lucovsky, T. H. Geballe, D. H. Blank, and M. R. Beasley

            "Origin of charge density at LaAlO3 on SrTiO3 heterointerfaces: Possibility of intrinsic doping"

            Physical Review Letters 98 (19), 196802 (2007).

 

            ABSTRACT: As discovered by Ohtomo and Hwang, a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic, and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant.

 

100.    T. Sogawa, H. Gotoh, Y. Hirayama, P. V. Santos, and K. H. Ploog

            "Dimensional oscillation in GaAs/AlAs quantum wells by two-dimensional standing surface acoustic waves"

            Applied Physics Letters 91 (14), 141917 (2007).

 

            ABSTRACT: We have used spatially and time-resolved photoluminescence (PL) spectroscopy recorded using a synchronized excitation method in order to investigate the mechanisms for the dynamic PL modulation of GaAs/AlAs quantum wells by two-dimensional (2D) standing surface acoustic waves (SAWs). It is clearly demonstrated that a square array of dynamic quantum dots (DQDs) is formed, which oscillates with a repetition interval equal to half of the SAW period. A 2D flatband condition appears between the DQD formations. The spatial modulation of the PL intensity is attributed to exciton migration into the tensile-strained dots created by the strain-induced band-gap modulation. (C) 2007 American Institute of Physics.

 

101.    T. Sogawa, H. Sanada, H. Gotoh, H. Yamaguchi, S. Miyashita, and P. V. Santos

            "Photoluminescence dynamics of GaAs/AlAs quantum wells modulated by surface acoustic waves"

            Japanese Journal Of Applied Physics Part 2-letters & Express Letters 46 (29-32)L758-L760 (2007).

 

            ABSTRACT: We have investigated the dynamics of photoluminescence (PL) of GaAs/AlAs quantum wells under strain and piezoelectric modulation introduced by a surface acoustic wave (SAW). Measurements performed using a microscopic excitation spot show that the efficiency of PL quenching due to exciton ionization and the subsequent sweeping of free electrons and holes by piezoelectric potential varies significantly with quantum well thickness. This variation is attributed to the well-thickness dependence of carrier mobility and diffusivity. The relative timing between carrier generation pulse and SAW-induced band structure modulation also changes the delay of PL quenching and PL decay time under the SAW field.

 

102.    D. P. Sprunken, H. Omi, K. Furukawa, H. Nakashima, I. Sychugov, Y. Kobayashi, and K. Torimitsu

            "Influence of the local environment on determining aspect-ratio distributions of gold nanorods in solution using Gans theory"

            Journal Of Physical Chemistry C 111 (39)14299-14306 (2007).

 

            ABSTRACT: We apply Gans theory to fit the absorption spectra of gold nanorods with aspect ratios R <= 2.5 in solution using both the longitudinal and transversal surface plasmon resonance absorption peaks and the dielectric constant of the medium, epsilon(m) , as a fitting parameter. By fitting the broadened absorption peaks using the absorption spectra of a set of nanorods with a range of aspect ratios, we determine the size distribution of the nanorods in solution. The optimum value of epsilon(m) = 2.1 +/- 0.1 is substantially higher than the dielectric constant of the solvent (epsilon(m,water) = 1.77), which is most likely due to a change in the effective dielectric constant in the vicinity of the nanorods. The validity of our method is confirmed by comparing the calculated size distributions with transmission electron microscope images, and we obtain a good agreement between the experiments and our calculations. Furthermore, several other recent experimental results are compared with our fitting method, and we find that the discrepancy between Gans theory and those experimental results can be explained by using higher values of E.

 

103.    K. Suzuki, K. Kanisawa, S. Perraud, M. Ueki, K. Takashina, and Y. Hirayama

            "Imaging of interference between incident and reflected electron waves at an InAs/GaSb heterointerface by low-temperature scanning tunneling spectroscopy"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2618-2621 (2007).

 

            ABSTRACT: We measure the spatial variation of the local density of states normal to an InAs/GaSb heterointerface on a cleaved cross-sectional surface by low-temperature scanning tunneling spectroscopy. Clear standing waves originating from the interference between the incident and reflected electron waves at the interface are observed.

 

104.    K. Suzuki, K. Kanisawa, S. Perraud, M. Ueki, K. Takashina, and Y. Hirayama

            "Observation of subband standing waves in superlattices by low-temperature scanning tunneling spectroscopy"

            Journal Of Applied Physics 101 (8), 081705 (2007).

 

            ABSTRACT: The local density of states (LDOS) of InAs/GaSb long period superlattices was investigated on the cleaved (110) surface at atomic resolution by low-temperature scanning tunneling microscopy and low-temperature scanning tunneling spectroscopy (LT-STS). We present definitive features showing the broken-gap energy band profiles (the overlap of the conduction band of the InAs and the valence band of the GaSb in energy) in the LT-STS spectra complemented by distinct, atomically resolved topographic images. Furthermore, vivid standing waves in LDOS corresponding to the single quantum well-like subband confined in the InAs layer were observed.

 

105.    K. Suzuki, K. Kanisawa, S. Perraud, M. Ueki, K. Takashina, and Y. Hirayama

            "Imaging of subbands in InAs/GaSb double quantum wells by low-temperature scanning tunneling spectroscopy"

            Journal Of Crystal Growth 30197-100 (2007).

 

            ABSTRACT: The spatial distribution of the electron local density of states (LDOS) in InAs/GaSb double quantum wells (DQWs) was investigated by low-temperature scanning tunneling spectroscopy on cleaved surfaces. For DQW with a thick central barrier, clear standing wave patterns corresponding to subbands confined to each InAs single quantum well appeared in the spatial variation of LDOS spectra. In contrast, for the DQW with a thin central barrier, the standing wave patterns extended over both quantum wells. The deviation of the pattern arising from the asymmetry due to a slight difference of the well thickness appeared clearly. The observed spectra are well explained by the calculated LDOS taken to be the sum of LDOS contributed from all energetically accessible subbands. (c) 2007 Elsevier B.V. All rights reserved.

 

106.    K. Suzuki, K. Kanisawa, C. Janer, S. Perraud, K. Takashina, T. Fujisawa, and Y. Hirayama

            "Spatial imaging of two-dimensional electronic states in semiconductor quantum wells"

            Physical Review Letters 98 (13), 136802 (2007).

 

            ABSTRACT: We measure the spatial distribution of the local density of states (LDOS) at cleaved surfaces of InAs/GaSb isolated quantum wells and double quantum wells (DQWs) by low-temperature scanning tunneling spectroscopy. Distinct standing wave patterns of LDOS corresponding to subbands are observed. These LDOS patterns and subband energies agree remarkably well with simple calculations with tip-induced band bending. Furthermore, for the DQWs, coupling of electronic states between the quantum wells is also clearly observed.

 

107.    S. Suzuki, and Y. Kobayashi

            "Healing of low-energy irradiation-induced defects in single-walled carbon nanotubes at room temperature"

            Journal Of Physical Chemistry C 111 (12)4524-4528 (2007).

 

            ABSTRACT: Single-walled carbon nanotubes are damaged by low-energy electron and photon irradiation, depending strongly on the diameter. In this study, the formation and healing of the irradiation-induced defects were found to be in competition during irradiation even at room temperature or below. The diameter dependence of the damage can be mainly ascribed to a diameter dependence of the activation energy of the defect healing. The activation energy was estimated to be about 1 eV.

 

108.    D. Takagi, A. Yamazaki, Y. Otsuka, H. Yoshimura, Y. Kobayashi, and Y. Homma

            "Gold-filled apo-ferritin for investigation of single-walled carbon nanotube growth on substrate"

            Chemical Physics Letters 445 (4-6)213-216 (2007).

 

            ABSTRACT: We formed An nanoparticles of 5 nm or less in diameter in the cavity of apo-ferritin (Au-ferritin). The Au nanoparticle in Au-ferritin exhibited catalysis for the synthesis of single-walled carbon nanotube (SWCNT) by chemical vapor deposition on various substrates. One-to-one correspondence of the grown SWCNT and the Au catalyst was confirmed by scanning electron microscopy (SEM) utilizing the high secondary electron yield of Au. In SEM images, it was possible to distinguish the SWCNT growth modes from metal catalysts, whether the catalyst particle was at the growth front or root, by placing Au-ferritin particles in a specified area. (C) 2007 Elsevier B.V. All rights reserved.

 

109.    D. Takagi, H. Hibino, S. Suzuki, Y. Kobayashi, and Y. Homma

            "Carbon nanotube growth from semiconductor nanoparticles"

            Nano Letters 7 (8)2272-2275 (2007).

 

            ABSTRACT: Nanoscale metal catalysts have been indispensable for carbon nanotube (CNT) synthesis by chemical vapor deposition (CVD). We show that even semiconductor nanoparicles of SiC, Ge, and Si produce single-walled and double-walled CNTs in CVD with ethanol. This implies that nanosize structures might act as a template for the formation of CNT caps composed of five- and six-membered rings. Providing a template for cap formation is the essential role of the catalysts.

 

110.    K. Takashina, M. Brun, T. Ota, D. K. Maude, A. Fujiwara, Y. Ono, Y. Takahashi, and Y. Hirayama

            "Anomalous resistance ridges along filling factor nu=4i"

            Physical Review Letters 99 (3), 036803 (2007).

 

            ABSTRACT: We report anomalous structure in the magnetoresistance of SiO2/Si(100)/SiO2 quantum wells. When Landau levels of opposite valleys are driven through coincidence at the Fermi level, the longitudinal resistance displays elevations at filling factors that are integer multiples of 4 (nu=4i) accompanied by suppression on either side of nu=4i. This persists when either the magnetic field or the valley splitting is swept, leading to resistance ridges running along nu=4i. The range of field over which they are observed points to the role of spin degeneracy, which is directly confirmed by their disappearance with the addition of an in-plane magnetic field. The data suggest a new type of many-body effect arising from the combined degeneracy due to the valley and the spin degrees of freedom.

 

111.    K. Takashina, B. Gaillard, Y. Ono, and Y. Hirayama

            "Low-temperature characteristics of ambipolar SiO2/Si/SiO2 Hall-bar devices"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (4B)2596-2598 (2007).

 

            ABSTRACT: We present the basic low-temperature characteristics of SiO2/Si/SiO2) Hall-bar devices, where each Hall-bar arm is split into p- and n-type Ohmic contacts made by selective area ion-implantation doping of boron and phosphorus, respectively. Magnetotransport measurements of a 22-nm-thick Si(001) quantum well show Shubnikov-de Haas oscillations of two-dimensional holes in addition to the electrons generated at both sides of the quantum well. We discuss applications for extracting the physical characteristics of this technologically vital material system.

 

112.    H. Takesue, Y. Tokura, H. Fukuda, T. Tsuchizawa, T. Watanabe, K. Yamada, and S. Itabashi

            "Entanglement generation using silicon wire waveguide"

            Applied Physics Letters 91 (20), 201108 (2007).

 

            ABSTRACT: We report an entanglement generation experiment that utilizes a silicon waveguide. Using spontaneous four-wave mixing in a 1.09-cm-long silicon wire waveguide, we generated 1.5 mu m, high-purity time-bin entangled photons without temperature control and observed a two-photon interference fringe with > 73% visibility. (c) 2007 American Institute of Physics.

 

113.    H. Takesue, S. W. Nam, Q. Zhang, R. H. Hadfield, T. Honjo, K. Tamaki, and Y. Yamamoto

            "Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors"

            Nature Photonics 1 (6)343-348 (2007).

 

            ABSTRACT: We report the first quantum key distribution (QKD) experiment to enable the creation of secure keys over 42 dB channel loss and 200 km of optical fibre. We used the differential phase shift QKD (DPS-QKD) protocol, implemented with a 10-GHz clock frequency and superconducting single-photon detectors (SSPD) based on NbN nanowires. The SSPD offers a very low dark count rate (a few Hz) and small timing jitter (60 ps, full width at half maximum, FWHM). These characteristics allowed us to achieve a 12.1 bit s(-1) secure key rate over 200 km of fibre, which is the longest terrestrial QKD over a fibre link yet demonstrated. Moreover, this is the first 10-GHz clock QKD system to enable secure key generation. The keys generated in our experiment are secure against both general collective attacks on individual photons and a specific collective attack on multiphotons, known as a sequential unambiguous state discrimination (USD) attack.

 

114.    H. Takesue

            "1.5 mu m band Hong-Ou-Mandel experiment using photon pairs generated in two independent dispersion shifted fibers"

            Applied Physics Letters 90 (20), 204101 (2007).

 

            ABSTRACT: This letter reports a Hong-Ou-Mandel experiment using two photon pairs in the 1.5 mu m band that are generated in two independent dispersion shifted fibers. A quantum interference with 53% visibility was observed. Thanks to the narrow bandwidth of the photon pairs, the required accuracy for photon arrival time adjustment was relaxed to a few millimeters, which is a great advantage for an application to a quantum relay. (C) 2007 American Institute of Physics.

 

115.    T. Tanabe, M. Notomi, E. Kuramochi, and H. Taniyama

            "Large pulse delay and small group velocity achieved using ultrahigh-Q photonic crystal nanocavities"

            Optics Express 15 (12)7826-7839 (2007).

 

            ABSTRACT: We systematically studied the spectral and temporal characteristics of wavelength-sized ultrahigh-Q photonic crystal nanocavities based on width-modulated line defects. By employing accurate measurements, we confirmed that the cavity exhibits an ultra-sharp resonance width ( 1.23 pm), an ultrahigh-Q ( 1.28 x 106), and an ultra-long photon lifetime ( 1.12 ns). We discussed the correlation between the spectral and temporal measurements for various cavities, and obtained extremely good agreement. In addition, we demonstrated photon trapping for the side-coupling configuration by employing ring-down measurement, which sheds light on another interesting aspect of this phenomenon. Finally, we performed pulse propagation experiments for samples with different waveguide-cavity coupling configurations, and achieved a smallest group velocity of about 4.6 km/ s for a novel configuration. These results show that we can effectively trap and delay light by using ultra-small cavities, which can potentially increase the packing density of optical buffers and bit-shifters if applied to coupled-cavity waveguides. (c) 2007 Optical Society of America.

 

116.    T. Tanabe, M. Notomi, and E. Kuramochi

            "Measurement of ultra-high-Q photonic crystal nanocavity using single-sideband frequency modulator"

            Electronics Letters 43 (3)187-188 (2007).

 

            ABSTRACT: The spectral measurement of an ultra-high quality-factor (Q) photonic crystal nanocavity using a single-sideband electro-optic modulator made of lithium niobate is described. This technique provides a simple and powerful way to measure the spectral response of ultra-high-Q cavities that have a Q of 1 million. Such cavities are usually difficult to measure with a tunable laser diode or an acousto-optic frequency shifter.

 

117.    T. Tanabe, A. Shinya, E. Kuramochi, S. Kondo, H. Taniyama, and M. Notomi

            "Single point defect photonic crystal nanocavity with ultrahigh quality factor achieved by using hexapole mode"

            Applied Physics Letters 91 (2), 021110 (2007).

 

            ABSTRACT: Photonic crystal (PhC) nanocavities with an extremely high quality factor (Q) based on a point defect are fabricated and their properties are studied. Their confinement of light is based on rotational symmetry, which forms a hexapole mode with a Q of 3.2x10(5). It demonstrates that this nanocavity is an alternative candidate for achieving an ultrahigh Q. In addition, we observed bistable behavior based on the thermo-optic effect at a threshold power of a few 100 mu W. We also investigated the dynamic properties of this cavity, where we observed that the cavity exhibits a smaller Q at a higher input power. (C) 2007 American Institute of Physics.

 

118.    T. Tanabe, M. Notomi, E. Kuramochi, A. Shinya, and H. Taniyama

            "Trapping and delaying photons for one nanosecond in an ultrasmall high-Q photonic-crystal nanocavity"

            Nature Photonics 1 (1)49-52 (2007).

 

            ABSTRACT: Light is intrinsically very difficult to store in a small space. The ability to trap photons for a long time (photon lifetime, tau(ph)) and to slow the propagation of light plays a significant role in quantum information(1-3) and optical processing(4-6). Photonic-crystal cavities with an ultrahigh quality factor (Q) are attracting attention(7,8) because of their extremely small volume; however, high-Q demonstrations have been accomplished only with spectral measurements(9-11). Here we describe time-domain measurements on photonic-crystal cavities with the highest Q among wavelength-scale cavities, and show directly that photons are trapped for one nanosecond. These techniques constitute clear and accurate ways of investigating ultrasmall and long tau(ph) systems. We also show that optical pulses are delayed for similar to 1.45 ns, corresponding to light propagation at similar to 2 x 10(-5) c the speed of light in a vacuum, which is the slowest for any dielectric slow-light medium. Furthermore, we succeeded in dynamically changing the Q within the T h, which is key to realizing the dynamic control of light(12,13) and photon-trapping memory(14).

 

119.    T. Tanabe, K. Nishiguchi, A. Shinya, E. Kuramochi, H. Inokawa, M. Notomi, K. Yamada, T. Tsuchizawa, T. Watanabe, H. Fukuda, H. Shinojima, and S. Itabashi

            "Fast all-optical switching using ion-implanted silicon photonic crystal nanocavities"

            Applied Physics Letters 90 (3), 031115 (2007).

 

            ABSTRACT: On-chip all-optical switching based on the carrier plasma dispersion in an argon ion (Ar+) implanted photonic crystal (PhC) nanocavity that is connected to input/output waveguides is described. A high dose of Ar+ is introduced, and annealing is used to recrystallize the silicon and thus create dislocation loops at the center of the PhC slab. Dislocation loops enable the fast recombination of the carriers, which allows a fast switching recovery time for PhC switches. The switching window (similar to 70 ps) is three times smaller than that without ion implantation, while the required operating energy remains almost the same (< 100 fJ). (c) 2007 American Institute of Physics.

 

120.    Y. Taniyasu, M. Kasu, and T. Makimoto

            "Radiation and polarization properties of free-exciton emission from AlN (0001) surface"

            Applied Physics Letters 90 (26), 261911 (2007).

 

            ABSTRACT: Free-exciton emission from AlN (0001) surface was characterized by angle-dependent photoluminescence (PL) measurement. As the radiation direction was inclined from the surface normal (c-axis direction), the emission intensity increased. This is because the optical transition between the conduction band and the top valence band is mainly allowed for light with the electric field parallel to the c-axis direction of AlN (E parallel to c) and consequently the free-exciton emission is strongly polarized for E parallel to c. By analyzing the angle-dependent PL intensities, the polarization ratio was estimated to be 0.995. This high polarization ratio results from the large negative crystal-field splitting energy. (c) 2007 American Institute of Physics.

 

121.    Y. Taniyasu, M. Kasu, and T. Makimoto

            "Threading dislocations in heteroepitaxial AlN layer grown by MOVPE on SiC (0001) substrate"

            Journal Of Crystal Growth 298310-315 (2007).

 

            ABSTRACT: To clarify the mechanisms governing the formation and reduction of threading dislocations (TDs) in aluminum nitride (AlN) layers grown on SiC (0 0 0 1) substrates by metalorganic vapor phase epitaxy (MOVPE), we characterized the mosaicity and the growth mode. High-density (similar to 10(11) cm(-2)) three-dimensional (3D) AlN islands nucleate on the substrate. Because the islands are slightly misoriented with respect to each other, dislocations are generated with a high density of 10(10)-10(11) cm(-2) as the islands coalesce. However, most of the dislocations are annihilated because their propagation direction changes horizontally during the island growth. Thus, at the initial growth stage, the dislocation density is drastically decreased to 10(8)-10(9) cm(-2). Consequently, as the layer thickness increases, the defect-free region becomes larger and the misorientation becomes smaller. On the other hand, we found that the TDs induce a large tensile strain and that the residual strain decreases with decreasing dislocation density. From the relationship between a- and c-lattice strains, the Poisson ratio of AlN was determined to be 0.19. (c) 2006 Elsevier B.V. All rights reserved.

 

122.    K. Tateno, G. Zhang, T. Sogawa, and H. Nakano

            "Bending at thinned GaAs nodes in GaP-based free-standing nanowires"

            Japanese Journal Of Applied Physics Part 2-letters & Express Letters 46 (33-35)L780-L782 (2007).

 

            ABSTRACT: The ability to intentionally bend nanowires at certain points would extend their applications further. Here, we present GaP-based nanowires with three GaAs bending nodes. The GaAs pans are selectively reduced by annealing. When 20-nm Au particles are used as catalysts, apparent bending is seen at the top GaAs node. At the bottom one, however, a thicker GaAs node is seen, which is due to excess growth at the GaAs side wall. How to control the bending at each node during the growth process remains an issue, but it should be possible to control it by applying an electric field.

 

123.    Y. Tokura, H. Nakano, and T. Kubo

            "Interference through quantum dots"

            New Journal Of Physics 9, 113 (2007).

 

            ABSTRACT: We discuss the effect of quantum interference on transport through a quantum dot (QD) system. We introduce an indirect coherent coupling parameter a, which provides constructive/destructive interference in the transport current depending on its phase and the magnetic flux. We estimate the current through the QD system using the non-equilibrium Green's function method as well as the Bloch equation method under a large bias voltage condition. The visibility of the Aharonov-Bohm oscillation is evaluated. For a large inter-dot Coulomb interaction, the current is strongly suppressed by the quantum interference effect, while the current is restored by applying an oscillating resonance field with the frequency of twice the inter-dot tunnelling energy.

 

124.    I. Tomita, H. Suzuki, R. Rungsawang, Y. Ueno, and K. Ajito

            "Analysis of power enhancement of terahertz waves in periodically inverted GaP pumped at 1.55 mu m"

            Physica Status Solidi A-applications And Materials Science 204 (4)1221-1226 (2007).

 

            ABSTRACT: We have generated terahertz (THz) waves by difference frequency generation (DFG) with a 1.55 mu m pump laser. 1.55 mu m pump beams were effectively converted to 1 THz waves through a periodically chi((2))- inverted GaP stack fabricated by direct wafer bonding. The output efficiency was an order of magnitude larger than that of typical chi ((2)) materials such as lithium niobate employing a similar DFG method. To investigate the increase in output power due to quasi-phase matching including optical losses, we measured its dependence on stack length. From the measured data, we analyze the conditions for effective power enhancement and show that it takes place if the pump-beam loss, alpha, is much smaller than that of the THz waves, alpha (3) (or the pump-beam attenuation length, 1/alpha, is much longer than that of the THz waves, 1/alpha(3),), which is achieved by improving the bonding quality of the stack. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

 

125.    I. Tomita, and A. Suzuki

            "On the propagation of electron screening in disordered conductors"

            Journal Of Physics And Chemistry Of Solids 68 (1)87-90 (2007).

 

            ABSTRACT: In condensed matter theories within the nonrelativistic framework, the screening of electrons in a free space takes place instantaneously, i.e., at infinite speed. In this paper, considering the screening in disordered (or random) systems, we show that it propagates at a finite speed of (l/lambda)l/tau depending on a ratio l/lambda of the electron mean free path l and screening length lambda, where l/tau is the propagation speed of electrons between impurity potentials, which is identical to the Fermi velocity, and T the electron elastic scattering time, thereby indicating that if the condition l >> lambda holds like a situation in ordinary dirty conductors, the influence of retardation on the screening can be ignored safely in the same theoretical framework as above. (c) 2006 Elsevier Ltd. All rights reserved.

 

126.    A. Tsukada, M. Naito, and H. Yamamoto

            "Valence of ce in T- and T '-La2-xCexCuO4"

            Physica C-superconductivity And Its Applications 46364-67 (2007).

 

            ABSTRACT: We have recently successfully doped the La site in T-La2CuO4 with Cc by a molecular beam epitaxy. The resultant T-La2-xCexCuO4 was insulating up to x = 0.06. This is in sharp contrast to T'-La2-xCexCuO4, which is superconducting with a T-c of about 30 K. In order to clarify the difference between T-La2-xCexCuO4 and T'-La2-xCexCuO4, we estimate the valences of Ce in T-La2-xCexCuO4 and T'-La2-xCexCuO4 by using Vegard's law. The estimated valence of Ce is >=+3.84 in T-La2-xCexCuO4 and similar to+3.78 in T'-La2-xCexCuO4. The result indicates that electron doping is actually achieved in T-La2-xCexCuO4, but that this compound remains an insulator. This contradicts the general belief that the doping with either electrons or holes conducts or superconducts parent compounds. (C) 2007 Elsevier B.V. All rights reserved.

 

127.    K. Ueda, S. Saito, K. Semba, T. Makimoto, and M. Naito

            "Interface microstructure of MgB2/Al-AlOxMgB2 Josephson junctions studied by cross-sectional transmission electron microscopy"

            Japanese Journal Of Applied Physics Part 2-letters & Express Letters 46 (12-16)L271-L273 (2007).

 

            ABSTRACT: We investigated the interface microstructure of sandwich-type MgB2/Al-A1O(x)/MgB2 Josephson tunnel junctions by cross-sectional transmission electron microscopy (TEM) in order to clarify the non-idealities in the junction characteristics. The results indicate that there are poor-crystal line MgB2 layers and/or amorphous Mg-B composite layers of a few nanometers between the AlOx barrier and upper MgB2 layer. The poor-crystalline upper Mg-B layers seem to behave as normal metal or deteriorated superconducting layers, which may be the principal reason for all non-idealities of our MgB2/Al-AIO(x)/MgB2 junctions.

 

128.    K. Ueda, M. Kasu, and T. Makimoto

            "High-pressure and high-temperature annealing as an activation method for ion-implanted dopants in diamond"

            Applied Physics Letters 90 (12), 122102 (2007).

 

            ABSTRACT: The authors show that high-pressure and high-temperature (HPHT) annealing is very effective for the activation of ion-implanted dopants in diamond. The HPHT annealing condition is located in the thermodynamically stable region for diamond in the phase diagram and is, therefore, much more efficient for the recovery of implantation-induced damage and for the activation of ion-implanted dopants than thermal annealing in vacuum. The B-implanted film after HPHT annealing showed a high mobility of 632 cm(2)/V s with a sheet hole concentration of 4.8x10(10) cm(-2) at 300 K and the doping efficiency of similar to 7%. The mobility is the highest so far for ion-implanted diamond. In the entire annealing temperature range, the HPHT annealing is more efficient than the thermal annealing in vacuum. (c) 2007 American Institute of Physics.

 

129.    M. Uematsu, M. Gunji, M. Tsuchiya, and K. M. Itoh

            "Enhanced oxygen exchange near the oxide/silicon interface during silicon thermal oxidation"

            Thin Solid Films 515 (16)6596-6600 (2007).

 

            ABSTRACT: The effect of the SiO2/Si interface on oxygen exchange diffusion during oxidation was investigated using oxygen isotopes. A 40-mu thick Si O-18(2) layer was first grown in O-18(2) and then the sample was reoxidized in O-16(2) at 1100 degrees C. The O-18 diffusion in Si-16 O1 during the O-16(2) oxidation was investigated by secondary ion mass spectrometry measurements. A significant broadening of the O-18 profile toward the newly grown (SiO2)-O-16 was observed. The average oxygen diffusivity was initially about one order of magnitude larger than the reported thermal diffusivity in the SiO2 network. In addition, the O-18 diffusion became slower with oxidation time and hence with increasing distance between O-18 diffusion region and the interface. This distance-dependent O-18 self-diffusion was simulated taking into account the effect Of Si O-16 molecules generated at the interface upon oxidation and diffusing into SiO2 to enhance the oxygen exchange. The simulation fits the SIMS profiles and shows that the SiO diffusion is greatly retarded by the oxidation with O-2 from the oxygen-containing atmosphere. Therefore, the (SiO)-O-16 concentration becomes smaller as the interface leaves the O-18 region and the oxygen exchange becomes slower with time. (C) 2006 Elsevier B.V. All rights reserved.

 

130.    Y. Ueno, and K. Ajito

            "Terahertz time-domain spectra of aromatic carboxylic acids incorporated in nano-sized pores of mesoporous silicate"

            Analytical Sciences 23 (7)803-807 (2007).

 

            ABSTRACT: Terahertz time-domain spectroscopy (THz-TDS) is used to study the intra- and intermolecular vibrational modes of aromatic carboxylic acids, for example, o-phthalic acid, benzoic acid, and salicylic acid, which form either intra- or intermolecular hydrogen bond(s) in different ways. Incorporating the target molecules in nano-sized spaces in mesoporous silicate (SBA-16) is found to be effective for the separate detection of intramolecular hydrogen bonding modes and intermolecular modes. The results are supported by an analysis of the differences in the peak shifts, which depend on temperature, caused by the different nature of the THz absorption. Raman spectra revealed that incorporating the molecules in the nano-sized pores of SBA-16 slightly changes the molecular structures. In the future, THz-TDS using nanoporous materials will be used to analyze the intra- and intermolecular vibrational modes of molecules with larger hydrogen bonding networks such as proteins or DNA.

 

131.    Y. D. Wang, F. Xue, Z. Song, and C. P. Sun

            "Detection mechanism for quantum phase transition in superconducting qubit array"

            Physical Review B 76 (17), 174519 (2007).

 

            ABSTRACT: We describe a mechanism to detect quantum phase transition (QPT) in a system by a coherent probe weakly coupled to it. We illustrate this mechanism by a circuit QED architecture where a superconducting Josephson junction qubit array interacts with a one-dimensional superconducting transmission line resonator (TLR). The superconducting qubit array is modeled as an Ising chain in transverse field. Our investigation shows that the QPT phenomenon in the superconducting qubit array can be evidently revealed by the correlation spectrum of TLR output: At the critical point, the drastic broadening of spectrum indicates the occurrence of QPT. We also show the generalization of this mechanism to other QPT systems.

 

132.    S. Watanabe, J. Harada, S. Sasaki, and Y. Hirayama

            "Dynamical suppression of nuclear-spin decoherence time in Si and GaAs using inversion pulses"

            Journal Of Magnetism And Magnetic Materials 310 (2)2167-2169 (2007).

 

            ABSTRACT: We found that nuclear-spin decoherence is suppressed by applying inversion pulses such as alternating phase Carr-Purcell (APCP) and Carr-Purcell-Meiboom-Gill (CPMG) sequences in silicon and GaAs. The decoherence time reaches similar to 1.3 s by applying inversion pulses, which is similar to 200 times as long as the characteristic decay time obtained from the Hahn echo sequence (similar to 6 ms) in silicon. (c) 2006 Elsevier B.V. All rights reserved.

 

133.    F. Xue, Y. D. Wang, Y. X. Liu, and F. Nori

            "Cooling a micromechanical beam by coupling it to a transmission line"

            Physical Review B 76 (20), 205302 (2007).

 

            ABSTRACT: We study a method to cool down the vibration mode of a micromechanical beam using a capacitively coupled superconducting transmission line. The Coulomb force between the transmission line and the beam is determined by the driving microwave on the transmission line and the displacement of the beam. When the frequency of the driving microwave is smaller than that of the transmission line resonator, the Coulomb force can oppose the velocity of the beam. Thus, the beam can be cooled. This mechanism, which may enable us to prepare the beam in its quantum ground state of vibration, is feasible under current experimental conditions.

 

134.    F. Xue, Y. D. Wang, C. P. Sun, H. Okamoto, H. Yamaguchi, and K. Semba

            "Controllable coupling between flux qubit and nanomechanical resonator by magnetic field"

            New Journal Of Physics 9, 35 (2007).

 

            ABSTRACT: We propose an active mechanism for coupling the quantized mode of a nanomechanical resonator to the persistent current in the loop of a superconducting Josephson junction (or phase slip) flux qubit. This coupling is independently controlled by an external coupling magnetic field. The whole system forms a novel solid-state cavity quantum electrodynamics (QED) architecture in the strong coupling limit. This architecture can be used to demonstrate quantum optics phenomena and coherently manipulate the qubit for quantum information processing. The coupling mechanism is applicable for more generalized situations where the superconducting Josephson junction system is a multi-level system. We also address the practical issues concerning experimental realization.

 

135.    H. Yamaguchi, H. Okamoto, Y. Maruta, S. Ishihara, S. Miyashita, and Y. Hirayama

            "Giant magneto-piezoresistance and internal friction in a two-dimensional electron system"

            Japanese Journal Of Applied Physics Part 2-letters & Express Letters 46 (25-28)L658-L660 (2007).

 

            ABSTRACT: We use a micromechanical cantilever with an integrated two-dimensional electron system to show that an extremely small strain of the order of 10(-4) induces a localized-delocalized electronic state transition. This strong strain effect improves the piezoresistive gauge factor by more than two orders of magnitude compared to the conventional Si cantilever. Furthermore, we found that the cantilever mechanical motion is affected considerably by friction exerted by the electron systems.

 

136.    T. Yamaguchi, and H. Yamaguchi

            "Pattern transfer of laterally aligned lamellar domains of diblock copolymers"

            Journal Of Photopolymer Science And Technology 20 (5)767-769 (2007).

 

            ABSTRACT:

 

137.    A. Yamamoto, T. Yamashita, M. Yamashita, and N. Kawakami

            "Ground-state properties of 1D Hubbard model in optical superlattice"

            Journal Of Magnetism And Magnetic Materials 310 (2)910-912 (2007).

 

            ABSTRACT: We study the ground-state properties of a cold Fermi gas trapped in a one-dimensional optical superlattice with periodically modulated lattice potentials. By applying the density matrix renormalization group method to the Hubbard model with harmonic confinement, we compute the density pro. le and the spin correlation function for several different choices of superlattice potential. The ground state is characterized by the coexistence phase with the metallic and several types of Mott-insulating domains. It is shown that the spin correlation can be enhanced between the two spatially separated Mott-insulating domains. (c) 2006 Elsevier B.V. All rights reserved.

 

138.    M. Yamashita, and M. W. Jack

            "Spin structures of spin-1 bosonic atoms trapped in an optical lattice with harmonic confinement"

            Physical Review A 76 (2), 023606 (2007).

 

            ABSTRACT: We study the ground-state properties of spin-1 bosonic atoms trapped in a one-dimensional optical lattice with harmonic confinement. We have developed a highly efficient numerical method based on the Gutzwiller approximation to clarify the inhomogeneous magnetic phases induced by the harmonic confining potential. A quantum phase transition from a superfluid to a Mott insulator is examined for the respective cases with antiferromagnetic and ferromagnetic interactions. It is shown that the profile of local spins over the lattice sites changes as a function of the interaction strength forming the characteristic structures, which closely correlates with the average number of atoms at the sites. For antiferromagnetic cases, we find that there is a crossover from a superfluid polar condensate to Mott-insulator domains consisting of either a spin-singlet pair or a single spin at each site.

 

139.    K. Yamazaki, T. Yamaguchi, and H. Yamaguchi

            "Modulation of young's modulus of poly(methyl methacrylate) nanobeam due to electron-beam exposure"

            Japanese Journal Of Applied Physics Part 2-letters & Express Letters 46 (45-49)L1225-L1227 (2007).

 

            ABSTRACT: We fabricated suspended poly(methyl methacrylate) (PMMA) micro/nanobeams using a novel two-step development process. Young's moduli measured for the fabricated beams were found to decrease with additional electron-beam (EB) exposure. This means that we can locally control Young's modulus in PMMA structure using EB exposure and thus effectively design nanoelectromechanical systems using this organic material. The effect was more significant for smaller beams, suggesting that the modulus of the near-surface region is largely affected.

 

140.    K. Yamazaki, S. Etaki, H. S. van der Zant, and H. Yamaguchi

            "Piezoresistance of suspended InAs/AlGaSb heterostructure nanobeam"

            Journal Of Crystal Growth 301897-901 (2007).

 

            ABSTRACT: Double-clamped InAs/AlGaSb nanobeams 50 nm thick and 50-200 nm wide were fabricated using electron beam lithography, and their piezoresistance was measured and analyzed. Deep etching of a sacrificial layer produced an arched beam due to the shear stress arising from lattice mismatch, while shallow etching yielded a straight beam. The measured gauge factor and piezoresistance coefficient were positive or negative depending on whether the beam was straight or arched, respectively, and were much larger in magnitude than those for bulk InAs. These surprising phenomena are roughly explainable in terms of the piezoelectric effect in the thin films of the heterostructure. An analysis of the calculated piezoelectricity (PE) and piezoresistance yielded guidelines for obtaining a higher piezoresistance, and thus a higher sensitivity, in nanomechanical sensors made using such a heterostructure. (c) 2006 Elsevier B.V. All rights reserved.

 

141.    A. Yokoo, K. Wada, and L. C. Kimerling

            "Pattern size reduction of nanoprint-fabricated structures oil heat-shrinkable film"

            Japanese Journal Of Applied Physics Part 1-regular Papers Brief 46 (9B)6395-6397 (2007).

 

            ABSTRACT: By combination of nanoimprint technology and shrinking of the flexible film, we can fabricate a pattern which has a smaller pitch than initial mold pattern. Direct nanoprinting technique is used for pattern fabrication onto a heat shrinkable film. After pattern transferring, the film is shrunk by heating. Simultaneously, the pattern on the film can be shrunk. We find that a surface protection layer on the film keeps a pit pattern during heat-shrinking process. We successfully shrink a pattern with a pitch of 100 nm to a pattern with a pitch of about 60 nm.

 

142.    G. Zhang, M. Adachi, S. Ganjil, A. Nakamura, J. Temmyo, and Y. Matsui

            "Vertically aligned single-crystal ZnO nanotubes grown on gamma-LiAlO2(100) substrate by metalorganic chemical vapor deposition"

            Japanese Journal Of Applied Physics Part 2-letters & Express Letters 46 (29-32)L730-L732 (2007).

 

            ABSTRACT: Vertically aligned single-crystal ZnO nanotubes were grown on a gamma-LiAlO2(100) substrate by metalorganic chemical vapor deposition (MOCVD) under proper growth conditions. The diameter of these nanotubes varies from 15 to 100 nm and their wall thickness is in the range of 5-30 nm. Photoluminescence measurements indicate that these nanotubes have enhanced green emission due to a much improved surface area, which is predominant over the near-band edge emission. The synthesis of ZnO nanotubes provides a new route and a new material system for forming nanostructures such as a quantum ring.

 

143.    G. Q. Zhang, Y. Hashimoto, A. Nakamura, A. Tanaka, J. Temmyo, and Y. Matsui

            "Very thin single-walled carbon nanotubes self-assembled on 6H-SiC(0001) substrate by surface decomposition method"

            Japanese Journal Of Applied Physics Part 2-letters & Express Letters 46 (1-3)L53-L56 (2007).

 

            ABSTRACT: Vertically aligned carbon nanotubes (CNTs) have been synthesized by a surface decomposition method on 6H-SiC(0001) substrate with in-situ introduction of molecular hydrogen into a vacuum chamber. It has been found that very thin singlewalled carbon nanotubes (SWNTs) with a diameter around I nm are formed, which was proved to be caused by addition of molecular hydrogen by comparing the experimental results with and without introduction of molecular hydrogen. The result indicate the modified surface decomposition method by addition of molecular hydrogen may serve as an efficient way by which very thin SWNTs can be synthesized.

 

144.    Q. Zhang, X. P. Xie, H. Takesue, S. W. Nam, C. Langrock, M. M. Fejer, and Y. Yamamoto

            "Correlated photon-pair generation in reverse-proton-exchange PPLN waveguides with integrated mode demultiplexer at 10 GHz clock"

            Optics Express 15 (16)10288-10293 (2007).

 

            ABSTRACT: We report 10-ps correlated photon pair generation in periodically-poled reverse-proton- exchange lithium niobate waveguides with integrated mode demultiplexer at a wavelength of 1.5-mu m and a clock of 10 GHz. Using superconducting single photon detectors, we observed a coincidence to accidental count ratio (CAR) as high as 4000. The developed photon-pair source may find broad application in quantum information systems as well as quantum entanglement experiments. (c) 2007 Optical Society of America.

 

145.    W. C. Zhang, K. Nishiguchi, Y. Ono, A. Fujiwara, H. Yamaguchi, H. Inokawa, Y. Takahashi, and N. J. Wu

            "Transfer and detection of single electrons using metal-oxide-semiconductor field-effect transistors"

            Ieice Transactions On Electronics E90C (5)943-948 (2007).

 

            ABSTRACT: A single-electron turnstile and electrometer circuit was fabricated on a silicon-on-insulator substrate. The turnstile, which is operated by opening and closing two metal-oxide-semiconductor field-effect transistors (MOSFETs) alternately, allows current quantization at 20 K due to single-electron transfer. Another MOSFET is placed at the drain side of the turnstile to form an electron storage island. Therefore, one-by-one electron entrance into the storage island from the turnstile can be detected as an abrupt change in the current of the electrometer, which is placed near the storage island and electrically coupled to it. The correspondence between the quantized current and the single-electron counting was confirmed.

 

146.    M. Zhao, K. Nakajima, M. Suzuki, K. Kimura, M. Uematsu, K. Torii, S. Kamiyama, Y. Nara, H. Watanabe, K. Shiraishi, T. Chikyow, and K. Yamada

            "Isotopic labeling study of the oxygen diffusion in HfO2/SiO2/Si"

            Applied Physics Letters 90 (13), 133510 (2007).

 

            ABSTRACT: The characteristic oxygen diffusion in HfO2/SiO2/Si structure during the annealing in oxygen has been investigated by high-resolution Rutherford backscattering spectroscopy in combination with oxygen isotope substitution at 900 degrees C in 0.1 Torr O-18(2). The observed O-18 profile suggests that oxygen molecules are decomposed into atomic oxygen in the HfO2 layer and diffuse through the oxide layer via exchange mechanism. This is also supported by the observed activation energy of similar to 0.6 eV for the growth of the interfacial SiO2 layer. (c) 2007 American Institute of Physics.

 

147.    Y. L. Zhong, H. Nakano, T. Akazaki, K. Kanzaki, Y. Kobayashi, and H. Takayanagi

            "Superconducting proximity magnetoconductance fluctuations correction to conductance and in random network carbon nanotubes"

            Physica E-low-dimensional Systems & Nanostructures 40 (1)169-174 (2007).

 

            ABSTRACT: We investigated the superconducting proximity effect in carbon nanotubes (CNTs). Reentrant behavior caused by the proximity effect was observed in a sandwich structure of network-like CNTs coupled to NbN electrodes. The proximity effect gives rise to enhanced magnetoconductance fluctuations that are similar to universal conductance fluctuation. The proximity effect correction to the conductance and fluctuation amplitude disappears at low temperature and reaches a maximum value at about 8 K, which corresponds to Thouless energy. This reentrant behavior was also observed in the magnetic field dependance of fluctuation amplitude. We found that a correlation magnetic field of about 3 T induces the dephasing of the interference. This high magnetic field is due to the CNTs with small diameter. We discuss this proximity effect in these quasi-one-dimensional CNTs with random network structure in experiment and theory. (c) 2007 Published by Elsevier B.V.

 

148.    N. M. Zimmerman, B. J. Simonds, A. Fujiwara, Y. Ono, Y. Takahashi, and H. Inokawa

            "Charge offset stability in tunable-barrier Si single-electron tunneling devices"

            Applied Physics Letters 90 (3), 033507 (2007).

 

            ABSTRACT: The problem of charge offset drift in single-electron tunneling (SET) devices can preclude their useful application in metrology and integrated devices. We demonstrate that in tunable-barrier Si-based SET transistors there is excellent stability, with a drift that is in general less than 0.01e; these devices exhibit some unwanted sensitivity to external perturbations including temperature excursions. Finally, we show that these devices can be "trained" to minimize their sensitivity to abrupt voltage changes. (c) 2007 American Institute of Physics.