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Takaaki Koga
Faculty of Information Science and Technology　Electronics for Informatics　Advanced Electronics
Associate Professor
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Last Updated :2024/05/09

Researcher Profile and Settings

Affiliation

  • Faculty of Information Science and Technology　Electronics for Informatics　Advanced Electronics

Job Title

  • Associate Professor

Degree

  • Ph.D.（Harvard University）
  • S.M.（Harvard University）
  • Licentiate of Engineering（Stockholm Institute of Technology in Stockholm）

URL

J-Global ID

Research Interests

  • 半導体スピン物性   Semiconductor Spintronics   

Research Areas

  • Nanotechnology/Materials / Crystal engineering
  • Nanotechnology/Materials / Applied materials
  • Natural sciences / Semiconductors, optical and atomic physics

Educational Organization

Academic & Professional Experience

  • 2001 - 2005 (独)科学技術振興機構 さきがけ研究者
  • 2001 - 2005 Researcher, Precursory Research for Embryonic Science and Technology (PRESTO),Japan Science and Technology Agency
  • 2004 - 北海道大学 大学院情報科学研究科 助教授
  • 2004 - Associate Professor,Graduate School of Information Science and Technology, Hokkaido University
  • 2000 - 2001 NTT(株)物性科学基礎研究所 リサーチアソシエイト
  • 2000 - 2001 Research Associate,NTT Basic Research Laboratories
  • 1994 - 2000 Harvard University
  • 1994 - 2000 Graduate Student,Harvard University

Education

  •        - 2000  Harvard University
  •        - 2000  Harvard University  Graduate School of Arts and Sciences  Division of Engineering and Applied Sciences, Applied Physics
  •        - 1995  Harvard University
  •        - 1995  Harvard University  Graduate School of Arts and Sciences  Division of Engineering and Applied Sciences, Applied Physics
  •        - 1994  ストックホルム王立工科大学
  •        - 1994  Royal Institute of Technology in Stockholm  School of Physics  Department of Condensed Matter Physics
  •        - 1992  The University of Tokyo  The Faculty of Engineering
  •        - 1992  The University of Tokyo  Faculty of Engineering  Department of Industrial Chemistry

Association Memberships

  • 社団法人 応用物理学会   日本物理学会   米国物理学会   The Japan Society of Applied Physics   The Physical Society of Japan   American Physical Society   

Research Activities

Published Papers

  • A. Sawada, S. Faniel, S. Mineshige, S. Kawabata, K. Saito, K. Kobayashi, Y. Sekine, H. Sugiyama, T. Koga
    Physical Review B 97 (19) 2469-9969 2018/05/08 [Refereed][Not invited]
     
    We report an approach for examining electron properties using information about the shape and size of a nanostructure as a measurement reference. This approach quantifies the spin precession angles per unit length directly by considering the time-reversal interferences on chaotic return trajectories within mesoscopic ring arrays (MRAs). Experimentally, we fabricated MRAs using nanolithography in InGaAs quantum wells which had a gate-controllable spin-orbit interaction (SOI). As a result, we observed an Onsager symmetry related to relativistic magnetic fields, which provided us with indispensable information for the semiclassical billiard ball simulation. Our simulations, developed based on the real-space formalism of the weak localization/antilocalization effect including the degree of freedom for electronic spin, reproduced the experimental magnetoconductivity (MC) curves with high fidelity. The values of five distinct electron parameters (Fermi wavelength, spin precession angles per unit length for two different SOIs, impurity scattering length, and phase coherence length) were thereby extracted from a single MC curve. The methodology developed here is applicable to wide ranges of nanomaterials and devices, providing a diagnostic tool for exotic properties of two-dimensional electron systems.
  • A. Sawada, T. Koga
    PHYSICAL REVIEW E 95 (2) 2470-0045 2017/02 [Refereed][Not invited]
     
    We have developed a method to calculate the weak localization and antilocalization corrections based on the real-space simulation, where we provide 147 885 predetermined return orbitals of quasi-two-dimensional electrons with up to 5000 scattering events that are repeatedly used. Our model subsumes that of Golub [L. E. Golub, Phys. Rev. B 71, 235310 (2005)] when the Rashba spin-orbit interaction (SOI) is assumed. Our computation is very simple, fast, and versatile, where the numerical results, obtained all at once, cover wide ranges of the magnetic field under various one-electron interactions H' exactly. Thus, it has straightforward extensibility to incorporate interactions other than the Rashba SOI, such as the linear and cubic Dresselhaus SOIs, Zeeman effect, and even interactions relevant to the valley and pseudo spin degrees of freedom, which should provide a unique tool to study new classes of materials like emerging 2D materials. Using our computation, we also demonstrate the robustness of a persistent spin helix state against the cubic Dresselhaus SOI.
  • S. Souma, A. Sawada, H. Chen, Y. Sekine, M. Eto, T. Koga
    PHYSICAL REVIEW APPLIED 4 (3) 2331-7019 2015/09 [Refereed][Not invited]
     
    We propose a lateral spin-blockade device that uses the interband Rashba effect in a symmetric double quantum well (QW), where the Rashba effect in the conventional sense vanishes because of its inversion symmetry. The interband Rashba effect manifests itself in the off-diagonal term (represented by the parameter eta) in the QW space using the bonding and antibonding basis [Esmerindo Bernardes, John Schliemann, Minchul Lee, J. Carlos Egues, and Daniel Loss, Phys. Rev. Lett. 99, 076603 (2007)]. In such a system, spin selection is possible by tuning the device length, gate electric field and in-plane magnetic field. We particularly show illustrative mechanisms using a one-dimensional model with k = (k(F), 0), where the selected spin can be blocked completely in the presence of the in-plane magnetic field. While the inclusion of the finite k(y) and/or the gate electric field deteriorates the spin polarization P, finite values remain for P (P>11%). Our proposal can also be regarded as an effective way of enhancing a variation of the Rashba-Edelstein effect, the generation of bulk spin polarization by electric current, based on semiconductor band engineering technology.
  • Rai Moriya, Kentarou Sawano, Yusuke Hoshi, Satoru Masubuchi, Yasuhiro Shiraki, Andreas Wild, Christian Neumann, Gerhard Abstreiter, Dominique Bougeard, Takaaki Koga, Tomoki Machida
    PHYSICAL REVIEW LETTERS 113 (8) 0031-9007 2014/08 [Refereed][Not invited]
     
    The spin-orbit interaction (SOI) of a two-dimensional hole gas in the inversion symmetric semiconductor Ge is studied in a strained-Ge/SiGe quantum well structure. We observe weak antilocalization (WAL) in the magnetoconductivity measurement, revealing that the WAL feature can be fully described by the k-cubic Rashba SOI theory. Furthermore, we demonstrate electric field control of the Rashba SOI. Our findings reveal that the heavy hole (HH) in strained Ge is a purely cubic Rashba system, which is consistent with the spin angular momentum m(j) = +/- 3/2 nature of the HH wave function.
  • H. Nakamura, T. Koga, T. Kimura
    PHYSICAL REVIEW LETTERS 108 (20) 206601  0031-9007 2012/05 [Refereed][Not invited]
     
    We present evidence of cubic Rashba spin splitting in a quasi-two-dimensional electron gas formed at a surface of (001) SrTiO3 single crystal from the weak localization or antilocalization (WAL) analysis of the low-temperature magnetoresistance. Our WAL data were well fitted by the model assuming m(j) = +/- 3/2 for the spin-split pair, in which 2 pi rotation of the electron wave vector k(parallel to) in the k(x)-k(y) plane accompanies 6 pi rotation of the spin quantization axis. This finding pertains to the p symmetry of the t(2g) electronic band derived from d electrons in SrTiO3, which provides insights into the surface electronic state of (001) SrTiO3.
  • Takaaki Koga, Toru Matsuura, Sebastien Faniel, Satofumi Souma, Shunsuke Mineshige, Yoshiaki Sekine, Hiroki Sugiyama
    IEICE TRANSACTIONS ON ELECTRONICS E95C (5) 770 - 776 0916-8524 2012/05 [Refereed][Invited]
     
    We recently determined the values of intrinsic spin-orbit (SO) parameters for In(0.52)Al(0.48)AS/In0.53Ga0.47As(10 nm)/In0.52Al0.48As (InGaAs/InAlAs) quantum wells (QW), lattice-matched to (001) InP, from the weak localization/antilocalization analysis of the low-temperature magneto-conductivity measurements [I]. We have then studied the sub-band energy spectra for the InGaAs/InAlAs double QW system from beatings in the Shubnikov de Haas (SdH) oscillations. The basic properties obtained here for the double QW system provides useful information for realizing nonmagnetic spin-filter devices based on the spin-orbit interaction [2].
  • S. Mineshige, S. Kawabata, S. Faniel, J. Waugh, Y. Sekine, T. Koga
    Physical Review B - Condensed Matter and Materials Physics 84 (23) 233305  1098-0121 2011/12/27 [Refereed][Not invited]
     
    We present a semiclassical interpretation of the time-reversal spin interference (SI) observed in the square loop arrays made of In 0.53Ga 0.47As quantum wells. The simulated amplitude of SI as a function of the Rashba parameter α captured characteristic features in the experimental results if γ8 eV3 is assumed for the bulk Dresselhaus spin-orbit constant γ. Our work proves the validity of the semiclassical approach to predict the effect of time-reversal quantum interference in mesoscopic systems and the values of the spin-orbit coefficients recently deduced from the weak localization/antilocalization experiment. © 2011 American Physical Society.
  • S. Faniel, T. Matsuura, S. Mineshige, Y. Sekine, T. Koga
    PHYSICAL REVIEW B 83 (11) 115309  1098-0121 2011/03 [Refereed][Not invited]
     
    We report the determination of the intrinsic spin-orbit interaction (SOI) parameters for In0.53Ga0.47As/In0.52Al0.48As quantum wells (QWs) from the analysis of the weak antilocalization effect. We show that the Dresselhaus SOI is mostly negligible in this system and that the intrinsic parameter for the Rashba effect, a(SO) = alpha/< E-z >, is given to be a(SO)m*/m(e) = (1.46-1.51 x 10(-17) N-S [m(-2)]) e angstrom(2), where alpha is the Rashba SOI coefficient, < E-z > is the expected electric field within the QW, m*/m(e) is the electron effective mass ratio, and NS is the sheet carrier density. These values for a(SO)m* were also confirmed by the observation of beatings in the Shubnikov-de Haas oscillations in our most asymmetric QW sample.
  • Takaaki Koga, Sebastien Faniel, Toru Matsuura, Shunsuke Mineshige, Yoshiaki Sekine, H. Sugiyama
    15TH INTERNATIONAL CONFERENCE ON NARROW GAP SYSTEMS (NGS15) 1416 38 - 41 0094-243X 2011 [Refereed][Not invited]
     
    We report the determination of the intrinsic spin-orbit interaction (SOI) parameters and phase coherence times for In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells (QWs) from the analysis of the weak antilocalization (WAL) measurements at dilution temperatures. We find that the Dresselhaus SOI is mostly negligible in this system and that the intrinsic parameters for the Rashba effect, a(SO) equivalent to alpha/< E-z >, is determined quantitatively to be a(SO)m(*)/m(e)=(1.46 -1.51 x 10(-17) N-S [m(-2)]) e angstrom(2), where N-S is the sheet carrier density. We also provide the values of the phase coherent time tau(phi) extracted from the WAL analysis and the transport mobility mu obtained from the Hall and Shubnikov-de Haas (SdH) measurements.
  • S. Faniel, T. Matsuura, S. Mineshige, Y. Sekine, T. Koga
    PHYSICS OF SEMICONDUCTORS: 30TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS 1399 649 - 650 0094-243X 2011 [Refereed][Not invited]
     
    We report weak antilocalization (WAL) measurements in a nearly symmetric In0.53Ga0.47As/In0.52Al0.48As quantum well. In this system, an enhancement of the spin-orbit scattering time, equivalently the weakening of the WAL effect, is expected at carrier densities N-s where equal magnitudes between the Rashba and Dresselhaus parameters ( a = +beta) are realized. Our magnetotransport measurements indeed exhibited a weakening of the WAL effect as a function of N-s, though the weakening took place at only one carrier density. Our interpretation is that the value of beta is so small that the two carrier densities corresponding to alpha = beta and alpha = -beta are very close to each other. It turned out that the deduced values for the bulk Dresselhaus parameter gamma in In0.53Ga0.47As/In0.52Al0.48As quantum wells is considerably smaller than the k . p value of 27 eV angstrom(3).
  • Toru Matsuura, Sebastien Faniel, Nozomu Monta, Takaaki Koga
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 42 (10) 2707 - 2710 1386-9477 2010/09 [Refereed][Not invited]
     
    We propose a mechanism to control the Rashba-induced subband splitting by a magnetic field using a symmetric double quantum well (QW) system, where the lowest two subbands are coupled by a position-dependent Rashba parameter alpha(z). In such a system, all subbands are spin degenerate due to the time reversal symmetry and the spatial inversion symmetry at zero magnetic field, despite the presence of the Rashba spin-orbit interaction. Applying an external magnetic field parallel to the QW plane (B parallel to(y) over cap) lifts this spin degeneracy breaking the time reversal symmetry, where the spin splitting energies are controllable in the range between zero and 2.9 meV, the latter being on the same order of magnitude as a typical Rashba splitting in a narrow asymmetric QW. We find that the first and second subband energy levels for a selected spin state with k(parallel to) = (k(F),0,0) anticross each other, and that the energy of the subband splitting Delta(0), equivalent to the Rashba splitting for the case of single QWs, can be determined from the value of the anticrossing magnetic field B(ac). These results suggest that the investigation in the symmetric double QWs would provide useful approaches for quantitative understanding of the Rashba spin-orbit interaction. (C) 2010 Elsevier B.V. All rights reserved.
  • S. Faniel, T. Matsuura, S. Mineshige, Y. Sekine, Takaaki Koga
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 42 (4) 990 - 993 1386-9477 2010/02 [Refereed][Not invited]
     
    We report the experimental detection of the anisotropic spin-orbit interaction (SOI) in InGaAs/InAlAs quantum wells, using spin interference experiments in arrays of rectangular loops with their sides aligned to the [1 1 0] and [1 (1) over bar 0] crystallographic directions. While the gate voltage is tuned, the time reversal Aharonov-Casher (TRAC) oscillations exhibit higher frequencies when the loops have their longer side along the [1 1 0] direction, clearly highlighting the anisotropy of the SOL We find that a simple spin interferometer model, including both the Rashba and the Dresselhaus SOIs, reproduces qualitatively the TRAC oscillations. (C) 2009 Elsevier B.V. All rights reserved.
  • S. Q. Jin, J. Waugh, T. Matsuura, S. Faniel, H. Z. Wu, Takaaki Koga
    PROCEEDINGS OF THE 14TH INTERNATIONAL CONFERENCE ON NARROW GAP SEMICONDUCTORS AND SYSTEMS 3 (2) 1321 - 1324 1875-3892 2010 [Refereed][Not invited]
     
    We point out that the electric field formed in the surface inversion layer in InAs nanowires leads to effective magnetic fields, due to the Rashba effect, that are mostly aligned along the wire axis, i.e., parallel to the external magnetic field B. While this situation leads to some similarities in spin splitting between the Zeeman and Rashba effects, extensive theoretical simulations revealed that large and small spin splittings should take place alternately at Fermi energies with increasing magnetic field B, as a result of the competition between the Rashba and Zeeman spin splittings. We suggest that an experimental detection of such characteristics should bring up quantitative insights into the relative strengths between the Rashba and Zeeman magnetic fields.
  • Takaaki Koga, S. Faniel, S. Mineshige, T. Mastuura, Y. Sekine
    PROCEEDINGS OF THE 14TH INTERNATIONAL CONFERENCE ON NARROW GAP SEMICONDUCTORS AND SYSTEMS 3 (2) 1325 - 1328 1875-3892 2010 [Refereed][Not invited]
     
    We report an unambiguous detection of the crytalline anisotropy of the spin-orbit interaction in (001) InAlAs/InGaAs/InAlAs quantum wells using nanofabricated rectangular loop arrays, where the sides of the constituent loops are aligned along either the [110] or [1 (1) over bar0] crystallographic axis. The fabrication and measurements were performed on the epi-wafer samples whose spin properties were characterized previously [Koga et al., Phys. Rev. Lett 89, 046801 (2002)]. We find that the experimentally observed spin interference patterns-the amplitude modulation of the Al'tshuler-Aronov-Spivak oscillations as a function of the gate voltage-are in good agreement with the results of the spin interferometer model extended for rectangular loops and including both the Rashba and Dresselhaus spin-orbit interactions.
  • Ni Jia-Ting, Liang Xiao-Wan, Chen Bin, T. Koga
    Chinese Physics Letters 26 127302 (3P)  2009 [Refereed][Not invited]
  • Jiating Ni, Bin Chen, T. Koga
    PHYSICS LETTERS A 372 (38) 6026 - 6031 0375-9601 2008/09 [Not refereed][Not invited]
     
    By using the Al'tshuler-Aronov-Spivak (AAS) model, we give the amplitude changing with Rashba spin-orbit interaction (SOI) and Dresselhaus SOI strength. In the first idea 1 D square loop (SL), Rashba SOI acts on two sides while Dresselhaus SOI acts on the other two sides. In the second SL, we consume Rashba SOI and Dresselhaus SOI act on four sides simultaneously. This model can be replaced by another one that Rashba SOI and Dresselhaus SOI act on every side independently, and each side is twice long. We theoretically illustrate the influence of the Dresselhaus SOI on node position and number. To explain the "half oscillation" phenomenon found in experiment, we apply Dresselhaus SOI to the ideal 1D SL. The conclusion is that the Dresselhaus SOI has a strong effect on the emergence of "half oscillation". (C) 2008 Elsevier B.V. All rights reserved.
  • Takaaki Koga, Yoshiaki Sekine, Junsaku Nitta
    PHYSICAL REVIEW B 74 (4) 041302  1098-0121 2006/07 [Refereed][Not invited]
     
    We succeeded in observing gate-controlled electron spin interference in nanolithographically defined square loop arrays that were fabricated in In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells (QWs). In this experiment, we demonstrated electron spin precession in ballistic channels within the QW that is caused by the Rashba effect. It turned out that the spin precession angle theta was gate controllable by more than 0.75 pi for a length of 1.5 mu m. Thus, the demonstration of the large controllability of theta by the applied gate voltage was carried out in a more direct way using spin interference of an electron wave function than the conventional beating analysis of the Shubnikov-de Haas oscillations.
  • Marc J. van Veenhuizen, Takaaki Koga, Junsaku Nitta
    PHYSICAL REVIEW B 73 (23) 235315  1098-0121 2006/06 [Refereed][Not invited]
     
    We investigate the spin-orbit induced spin-interference pattern of ballistic electrons traveling along any regular polygon. It is found that the spin interference depends strongly on the Rashba and Dresselhaus spin-orbit constants as well as on the sidelength and alignment of the polygon. We derive the analytical formulas for the limiting cases of either zero Dresselhaus or zero Rashba spin-orbit coupling, including the result obtained for a circle. We calculate the nonzero Dresselhaus and Rashba case numerically for the square, triangle, hexagon, and circle and discuss the observability of the spin interference which can potentially be used to measure the Rashba and Dresselhaus coefficients.
  • T. Koga, H. Okutani, Y. Sekine, J. Nitta
    PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 12 3 (12) 4220 - 4226 1862-6351 2006 [Refereed][Not invited]
     
    Recently, spin interference effect based on the Rashba effect in a square loop (SL) geometry is proposed theoretically [Koga et al., Phys. Rev. B 70, 161302(R) (2004)] and demonstrated experimentally [Koga et al., Phys. Rev. B 74 in press] using In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells (QW), where the spin precession angle theta per side of the SL can be controlled by the applied gate voltage via the Rashba spin-orbit coupling constant alpha. In the present work, we include the effect of the bulk inversion asymmetry, so-called Dresselhaus term, in our model calculation. We found that the agreement between the experimental and simulation results are greatly improved by the inclusion of the Dresselhaus term in the model calculation. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  • YP Lin, T Koga, J Nitta
    PHYSICAL REVIEW B 71 (4) 045328  2469-9950 2005/01 [Refereed][Not invited]
     
    We report the effect of the insertion of an InP/In0.53Ga47As Interface on the Rashba spin-orbit interaction in In0.52Al0.48As/In0.53Ga0.47As quantum wells. A small spin split-off energy in InP produces a very intriguing band lineup in the valence bands in this system. With or without this InP layer above the In0.53Ga47As well, the overall values of the spin-orbit coupling constant alpha turned out to be enhanced or diminished for samples with the front- or back-doping position, respectively. These experimental results, using weak antilocalization analysis, are compared with the results of the k.p theory. The actual conditions of the interfaces and materials should account for the quantitative difference in magnitude between the measurements and calculations.
  • FE Meijer, AF Morpurgo, TM Klapwijk, T Koga, J Nitta
    PHYSICAL REVIEW B 70 (20) 201307  1098-0121 2004/11 [Refereed][Not invited]
     
    We systematically investigate how the interplay between the Rashba spin-orbit interaction and Zeeman coupling affects the electron transport and the spin dynamics in InGaAs-based 2D electron gases. From the quantitative analysis of the magnetoconductance, measured in the presence of an in-plane magnetic field, we conclude that this interplay results in a spin-induced breaking of time reversal symmetry and in an enhancement of the spin relaxation time. Both effects are due to a partial alignment of the electron spin along the applied magnetic field, and are found to be in excellent agreement with recent theoretical predictions.
  • T Koga, J Nitta, M van Veenhuizen
    PHYSICAL REVIEW B 70 (16) 161302  1098-0121 2004/10 [Refereed][Not invited]
     
    We propose a ballistic spin interferometer using a square loop (SL) geometry, where an incident electron wave packet is split into a pair of partial waves by a "hypothetical" beam splitter. These electron partial waves, then, follow the SL path in the clockwise and counterclockwise directions, respectively, so that they interfere with each other at the incident point, retaining the spin degree of freedom. We find that the backscattering probability of an incident electron can be largely modulated by varying the magnitude of the Rashba spin-orbit coupling constant alpha. We propose to make the proposed spin interferometry experiment using an artificial nanostructure fabricated in, for example, In0.52Al0.48As/In0.53Ga0.47As/In0.52Al0.48As quantum wells.
  • Y Sekine, J Nitta, T Koga, A Oiwa, S Yanagi, T Slupinski, H Munekata
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 43 (4B) 2097 - 2100 0021-4922 2004/04 [Refereed][Not invited]
     
    The magnetization reversal process of submicrometer-scale Hall bars of the ferromagnetic semiconductor p-In0.97Mn0.03As has been investigated by measuring the Hall resistivity with changing the direction of the applied magnetic field. The angle dependence of the coercive force indicates that the magnetization reversal process is more likely governed by the magnetic domain wall displacement. Furthermore, observation of several Barkhausen jumps on a 0.7-mum-wide Hall bar makes it clear that p-In0.97Mn0.03As has a small-domain-sized multidomain structure near the coercive force. It is also shown that p-In0.97Mn0.03As has an ideal uniaxial magnetic easy axis perpendicular to the plane. The magnetization reversal process of p-In0.97Mn0.03As is distinct from that of a similar ferromagnetic semiconductor p-Ga1-xMnxAs.
  • FE Meijer, J Nitta, T Koga, AF Morpurgo, TM Klapwijk
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 22 (1-3) 402 - 405 1386-9477 2004/04 [Refereed][Not invited]
     
    We study the shape of the Fourier spectrum of Aharonov-Bohm conductance oscillations measured in a 2D ring in the presence of Rashba spin-orbit interaction. As expected, the shape of the Fourier spectrum of any single measurement is dominated by sample specific effects. This impedes the observation of possible effects due to the electron spin. We show in detail how the analysis of the ensemble averaged Fourier spectrum permits to reduce sample specific features and bow to quantify this reduction in a well-defined statistical way. We find that in the averaged Fourier spectrum a structure appears which is not due to sample specific effects. (C) 2003 Elsevier B.V. All rights reserved.
  • Y Lin, J Nitta, T Koga, T Akazaki
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 21 (2-4) 656 - 660 1386-9477 2004/03 [Refereed][Not invited]
     
    We report the gate-voltage dependence of the electron g factor in InGaAs/InAlAs heterostructures with double InAs-inserted wells. The 9 factor has been determined from a series of Shubnikov-de Haas oscillations at various angles with respect to the layers under different gate voltages. The strain in InAs layers and the penetration of the electron wave function into InGaAs and InAlAs layers are the important factors of the reduction in the \g\ factor. We have also observed the changes of the \g\ factor with respect to the gate voltage, though this dependence is not clear in the simple calculation. (C) 2003 Elsevier B.V. All rights reserved.
  • J Nitta, YP Lin, T Koga, T Akazaki
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 20 (3-4) 429 - 432 1386-9477 2004/01 [Refereed][Not invited]
     
    We report on electron g-factor in an InAs-inserted In0.53Ga0.47As/In0.52Al0.48As heterostructure. The gate voltage dependence of g-factor is obtained from the coincidence method. The obtained g-factor values are surprisingly smaller than the g-factor value of bulk InAs, and it is close to the bare g-factor value of In0.53Ga0.47As. The large change in g-factor is observed by applying the gate voltage. The obtained gate voltage dependence is not simply explained by the energy dependence of g-factor. (C) 2003 Elsevier B.V. All rights reserved.
  • Statistical significance of the fine structure in the frequency spectrum of Aharonov-Bohm conductance oscillations
    F. E. Meijer, A. F. Morpurgo, T. M. Klapwijk, T. Koga, J. Nitta
    Phys. Rev. B 69 035308  2004 [Refereed][Not invited]
  • J Nitta, YP Lin, T Akazaki, T Koga
    APPLIED PHYSICS LETTERS 83 (22) 4565 - 4567 0003-6951 2003/12 [Refereed][Not invited]
     
    The electron g factor in an InAs-inserted-channel In0.53Ga0.47As/In0.52Al0.48As heterostructure is studied by measuring the angle dependence of magnetotransport properties. The gate voltage dependence of the g factor is obtained from the coincidence method. The g-factor values are surprisingly smaller than the g-factor value of bulk InAs, and close to the bare g-factor value of In0.53Ga0.47As. A large change in the g factor is observed by applying the gate voltage. The gate voltage dependence is not simply explained by the energy dependence of the g factor. (C) 2003 American Institute of Physics.
  • J Nitta, T Koga
    JOURNAL OF SUPERCONDUCTIVITY 16 (4) 689 - 696 0896-1107 2003/08 [Refereed][Not invited]
     
    The Rashba spin-orbit interaction in InGaAs quantum wells (QW) is studied using the weak antilocalization analysis as a function of the structural inversion asymmetry (SIA). We have observed a clear cross-over from positive to negative magnetoresistance near zero-magnetic field by controlling the degree of the SIA in the QWs. This is a strong evidence of a zero-field spin splitting that is induced by the Rashba effect. The spin-interference effect in a gate-controlled mesoscopic Aharonov-Bohm ring structure is investigated in the presence of Rashba spin-orbit interaction. The oscillatory behavior appearing in ensemble averaged Fourier spectrum of h/2e oscillations as a function of gate voltage is possibly because of the Aharonov-Casher type interference. We propose a spin-filter device based on the Rashba effect using a nonmagnetic resonant tunneling diode structure. Detailed calculation using InAIAs/InGaAs heterostructures shows that the spin-filtering efficiency exceeds 99.9%.
  • J Nitta, T Koga, FE Meijer
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 18 (1-3) 143 - 144 1386-9477 2003/05 [Refereed][Not invited]
     
    The interference effect in a gate controlled mesoscopic Aharonov-Bohm ring structure is studied in the presence of spin-orbit interaction. After ensemble averaging, the Fourier spectrum of h/2e oscillations as a function of gate voltage showed an oscillatory behavior. The oscillatory behavior is possibly due to the Aharonov-Casher type interference. (C) 2003 Elsevier Science B.V. All rights reserved.
  • T Koga, J Nitta, S Datta
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 18 (1-3) 161 - 162 1386-9477 2003/05 [Refereed][Not invited]
     
    We have studied the Rashba constant values a in the In0.52Al0.48As/In0.53Ga0.47As quantum wells (QW), as a function of the degree of the structural inversion asymmetry (SIA), using the weak antilocalization analysis. We control the SIA of the QWs both by the specific sample design and by the applied gate voltage. The deduced alpha values are in a quantitative agreement with the theoretical values obtained in the k (.) p-type calculation. We, then, propose a novel spin-filter device solely based on the Rashba effect as an example of the devices that utilize the Rashba spin-orbit coupling effect. (C) 2003 Elsevier Science B.V. All rights reserved.
  • Spin-filter device based on a nonmagnetic resonant tunneling diode
    T. Koga, J. Nitta, S. Datta
    Physics of Semiconductors 2002, Proceedings of the 26th International Conference on the Physics of Semiconductors held in Edinburgh, UK, 29 July-2 August 2002, Edited by A. R. Long and J. H. Davies, Institute of Physics Conference Series Number 171, Insti 8 pages  2003 [Refereed][Not invited]
  • Electron g-factor in a double InAs-inserted In0.53Ga0.47As/In0.52Al0.48As heterostructure
    J. Nitta, T. Koga, T. Akazaki
    Physics of Semiconductors 2002, Proceedings of the 15th International Conferece on High Magnetic Fields in Semiconductor Physics, Oxford, 5-9 August 2002, Edited by A. R. Long and J. H. Davies, Institute of Physics Conference Series Number 171, Institute 6 pages  2003 [Refereed][Not invited]
  • J Nitta, T Koga, FE Meijer
    TOWARDS THE CONTROLLABLE QUANTUM STATES: MESOSCOPIC SUPERCONDUCTIVITY AND SPINTRONICS 55 - 60 2003 [Refereed][Not invited]
     
    Gate-controlled Abaronov-Bohm (AB) experiments are performed with AB-ring samples in the presence of spin-orbit interaction. The magnetoresistance at zero magnetic field shows minima in the whole gate voltage range. The observed minima in magnetoresistance at zero magnetic field can be explained in terms of anti-weak-localization. However, the resistance minima at zero magnetic field exhibits oscillatory behaviors as a function of the gate voltage. The possible reason for this oscillation can be attributed to the phase interference induced by the spin-orbit interaction.
  • T Koga, J Nitta, S Marcet
    TOWARDS THE CONTROLLABLE QUANTUM STATES: MESOSCOPIC SUPERCONDUCTIVITY AND SPINTRONICS 73 - 79 2003 [Refereed][Not invited]
     
    We have investigated the Rashba spin-orbit coupling constant alpha using the weak antilocalization analysis in In(0.52)Al(0.48)As/In(0.53)Ga(0.47)As/In(0.52)AI(0.48)As quantum wells (QW) as a function of the structural inversion asymmetry (SIA). We controlled the SIA of the QWs via the dopant densities in the carrier supplying layers as well as by doping at the substrate/buffer layer (S/B) interface. We find that both the intentional doping at the S/B interface and the unintentional background doping affect the alpha values significantly. This observation suggests that mere asymmetric doping in the vicinity of the QW is not the sufficient condition for obtaining a specific potential shape that produces a desired value of alpha.
  • Y Sekine, J Nitta, T Koga, A Oiwa, T Slupinski, H Munekata
    TOWARDS THE CONTROLLABLE QUANTUM STATES: MESOSCOPIC SUPERCONDUCTIVITY AND SPINTRONICS 105 - 110 2003 [Refereed][Not invited]
     
    The transport properties parallel to the pn-interfaces of two bilayer samples of p-In1-xMnxAs/n-InAs are studied. In one sample, the electron conductivity is dominant, however the temperature dependence of resistivity under various magnetic fields is different from that of ordinary n-InAs. On the other hand, the resistivity of the other sample would represent transport properties in which the contribution of p-In0.94Mn0.06As is about more than half, however the negative magnetic resistance due to p-In0.94Mn0.06As was not observed. These results of the pn-bilayer samples would mean that the transport properties of the pn-bilayer sample could not be explained by the simple summation of the transport properties of the single layer sample of p-In1-xMnxAs and n-InAs.
  • Structural Control of Rashba Spin-Orbit Coupling in In0.52Al0.48As/ In0.53Ga0.47As/In0.52Al0.48As Quantum Wells
    T. Koga, J. Nitta, S. Marcet
    Journal of Superconductivity: Incorporating Novel Magnetism 16 (2) 331 - 334 2003 [Refereed][Not invited]
  • J Nitta, T Schapers, HB Heersche, T Koga, Y Sato, H Takayanagi
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 41 (4B) 2497 - 2500 0021-4922 2002/04 [Refereed][Not invited]
     
    We have investigated the magnetization process of NiFe micro-magnets, using fringing field induced local Hall effect (LHE) and magnetic force microscopy (MFM), Although the LHE reflects information only from the edge of micro-magnets, the MFM observation supports the conclusion that a rapid jump in the Hall resistance implies a sharp switching of the magnetization. The width dependence of coercive field H-c for NiFe micro-magnets obtained from LHE measurement is well reproduced by numerical calculations. The good agreement between the LHE experiment and the model calculation shows that a LHE device provides useful information on the magnetization process in micro-magnets.
  • T Koga, J Nitta, T Akazaki, H Takayanagi
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 41 (4B) 2501 - 2504 0021-4922 2002/04 [Not refereed][Not invited]
     
    We propose the fabrication of a spin rectifying diode that utilizes the Rashba spin-orbit coupling, by the application of semiconducting triple barrier structures. This spin diode makes use of spin-dependent resonant tunneling levels that are formed in the triple barrier structures. We found that, for a certain emitter-collector bias voltage, it is possible to engineer the structure in such a way that a resonant level formed within the first quantum well matches that of the second quantum well only for a selected spin state, thus realizing an electronic spin rectifier. The calculated spin-filter efficiency of the transmitted current through the device, which is defined as [I-up arrow - I-down arrow]/(I-up arrow + I-down arrow), is found to be higher than 99.9%.
  • T Koga, J Nitta, T Akazaki, H Takayanagi
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 13 (2-4) 542 - 546 1386-9477 2002/03 [Refereed][Not invited]
     
    The values of the zero-field spin-splitting energy Delta(o) in InAlAs InGaAs InAlA, heterostructures are investigated using anti-weak-localization analysis. The obtained values for A, are compared with Value, that were theoretically predicted assuming Rashba spin-orbit coupling (denoted by Delta(R)) The good agreement between Delta(o) and Delta(R) and their dependence on quantum,kell asymmetry suggest that our approach provides a useful tool for designing future spintronics devices using Rashba spin-orbit coupling. (C) 2002 Elsevier Science B.V. All rig its reserved.
  • Proposal of a Novel Spin Filter Realized in a Triple Barrier Resonant Tunnel Diode using Rashba Spin-Orbit Interaction
    T. Koga, J. Nitta, S. Datta, H. Takayanagi
    Mat. Res. Soc. Symp. Proc. 690 123 - 128 2002 [Refereed][Not invited]
  • Anti-Weak-Localization Study of Rashba Spin-Splitting Energy as a Function of Well Asymmetry in InAlAs/InGaAs/InAlAs Quantum Wells
    T. Koga, J. Nitta, T. Akazaki, H. Takayanagi
    Mat. Res. Soc. Symp. Proc. 690 71 - 76 2002 [Refereed][Not invited]
  • Rashba spin-orbit coupling probed by the weak antilocalization analysis in InAlAs/InGaAs/InAlAs quantum wells as a function of quantum well asymmetry
    T. Koga, J. Nitta, T. Akazaki, H. Takayanagi
    Phys. Rev. Lett. 89 046801  2002 [Refereed][Not invited]
  • Spin-filter device based on the Rashba effect using a non-magnetic resonant tunneling diode
    T. Koga, J. Nitta, H. Takayanagi, S. Datta
    Phys. Rev. Lett. 88 126601  2002 [Not refereed][Not invited]
  • J Nitta, T Koga, H Takayanagi
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 12 (1-4) 753 - 757 1386-9477 2002/01 [Refereed][Not invited]
     
    Gate-controlled Aharonov-Bohm (AB) experiments are performed with two AB-ring samples with different ring diameters in the presence of spin-orbit interaction. For the larger AB-ring, the magnetoresistance at zero magnetic field [R(B = 0)] shows minima in the whole gate voltage range. In both larger and smaller AB-rings, R(B = 0) shows oscillatory behaviors as a function of the gate voltage. The possible reason for this oscillation can be attributed to the phase interference induced by the spin-orbit interaction. (C) 2002 Elsevier Science B.V. All rights reserved.
  • Carrier Pocket Engineering for the Design of Low Dimensional Thermoelectrics with High ZT
    T. Koga, S.B. Cronin, M.S. Dresselhaus
    Mat. Res. Soc. Symp. Proc. 626 Z4.3 - 12 pages 2001 [Refereed][Not invited]
  • T Koga, J Nitta, T Akazaki, H Takayanagi
    PROCEEDINGS OF THE 10TH INTERNATIONAL CONFERENCE ON NARROW GAP SEMICONDUCTORS AND RELATED SMALL ENERGY PHENOMENA, PHYSICS AND APPLICATIONS 2 227 - 229 2001 [Refereed][Not invited]
     
    The values of the zero-field spin-splitting energy Delta(0) in InAlAs/InGaAs/InAlAs heterostructures are investigated using antiweak-localization (AWL) analysis. The obtained values for Delta(0) are compared with values that were theoretically predicted assuming Rashba spin-orbit coupling (denoted by Delta(R)). A good agreement between Delta(0) and Delta(R) suggests that experimental analysis of the AWL data provides a useful tool for determining the values of Delta(0).
  • T Borca-Tasciuc, WL Liu, JL Liu, TF Zeng, DW Song, CD Moore, G Chen, KL Wang, MS Goorsky, T Radetic, R Gronsky, T Koga, MS Dresselhaus
    SUPERLATTICES AND MICROSTRUCTURES 28 (3) 199 - 206 0749-6036 2000/09 [Refereed][Not invited]
     
    This paper reports temperature-dependent thermal conductivity measurements in the cross-lane direction of symmetrically strained Si/Ge superlattices, and the effect of doping, period thickness and dislocations on the thermal conductivity reduction of Si/Ge superlattices. The Si/Ge superlattices are grown by molecular beam epitaxy on silicon and silicon-insulator substrates with a graded buffer layer. A differential 3 omega method is used to mea sure the thermal conductivity of the buffer and the superlattices between 80 and 300 K. The thermal conductivity measurement is carried out in conjunction with X-ray and TEM sample characterization. The measured thermal conductivity values of the superlattices are lower than those of their equivalent composition bulk alloys. (C) 2000 Academic Press.
  • T Koga, SB Cronin, MS Dresselhaus, JL Liu, KL Wang
    APPLIED PHYSICS LETTERS 77 (10) 1490 - 1492 0003-6951 2000/09 [Refereed][Not invited]
     
    An experimental proof-of-principle of an enhanced Z(3D)T (thermoelectric figure of merit) is demonstrated using (001) oriented Si/Ge superlattices. The highest value of the experimental Z(3D)T at 300 K for a (001) oriented Si(20 Angstrom)/Ge(20 Angstrom) superlattice is 0.1 using kappa = 5 Wm(-1) K-1, for the in-plane thermal conductivity, which is a factor of seven enhancement relative to the estimated value of Z(3D)T = 0.014 for bulk Si. The good agreement between experiment and theory validates our modeling approach (denoted as "carrier pocket engineering") to design superlattices with enhanced values of Z(3D)T. Proposals are made to enhance the experimental values of Z(3D)T for Si/Ge superlattices even further. (C) 2000 American Institute of Physics. [S0003-6951(00)05136-6].
  • T. Koga, O. Rabin, M. S. Dresselhaus
    Phys. Rev. B American Physical Society 62 (24) 16703 - 16706 0163-1829 2000 [Refereed][Not invited]
     
    An enhanced thermoelectric figure of merit Z_{3D}T is predicted for Bi/(111)Pb_{1-x}Eu_{x}Te superlattices. The values of Z_{3D}T obtained for x≈1 superlattices are 2.31, 1.55, and 1.61 at 300, 150, and 77 K, respectively, showing that they are promising candidates for thermoelectric elements in the temperature range 77–300 K. Even with x as small as 0.1, where the conduction-band offset ΔE_{c} is estimated to be 0.25 eV, the predicted Z_{3D}T values are 1.75, 1.16, and 1.18 at 300, 150, and 77 K, respectively. It is proposed that other families of Bi-based superlattices, such as Bi/(111)CdTe superlattices, should also be good candidates for low-temperature thermoelectric elements.
  • MS Dresselhaus, G Dresselhaus, Sun, X, Z Zhang, SB Cronin, T Koga
    PHYSICS OF THE SOLID STATE 41 (5) 679 - 682 1063-7834 1999/05 [Refereed][Not invited]
     
    The promise of low dimensional thermoelectric materials for enhanced performance is reviewed, with particular attention given to quantum wells and quantum wires. The high potential of bismuth as a low-dimensional thermoelectric material is discussed. (C) 1999 American Institute of Physics. [S1063-7834(99)00105-7].
  • Carrier Pocket Engineering to Design Superior Thermoelctric Materials Using Superlattice Structures
    T. Koga, X. Sun, S. B. Cronin, M. S. Dresselhaus
    The 18th International Conference on Thermoelectrics: ICT Symposium Proceedings, edited by G. Chen, IEEE, Piscataway, NJ 378 - 381 1999 [Refereed][Not invited]
  • Carrier Pocket Engineering to Design Superior Thermoelctric Materials Using GaAs/AlAs Superlattices
    T. Koga, X. Sun, S. B. Cronin, M. S. Dresselhaus
    Mat. Res. Soc. Symp. Proc. 545 375 - 380 1999 [Refereed][Not invited]
  • Investigation of the Mechanism of the Enhanced Thermoelectric Properties in the PbTe based Superlattices
    T. Koga, T. C. Harman, S. B. Cronin, X. Sun, M. S. Dresselhaus
    Mat. Res. Soc. Symp. Proc. 545 479 - 484 1999 [Refereed][Not invited]
  • T. Koga, T. C. Harman, S. B. Cronin, M. S. Dresselhaus
    Phys. Rev. B American Physical Society 60 (20) 14286 - 14293 0163-1829 1999 [Refereed][Not invited]
     
    A theoretical investigation of the recently observed enhanced thermoelectric power S in (111)-oriented n-type PbTe/Pb1-xEuxTe multiple-quantum-wells (x≃0.09) has been carried out, including both longitudinal acoustic phonon deformation potential scattering and polar optical phonon scattering of the two-dimensionally confined electrons in the quantum wells. An enhancement in S is observed experimentally and predicted theoretically, despite the lifting of the conduction band valley degeneracy, and an excellent agreement between the experimental and theoretical results has been obtained over a wide temperature range (80–400 K). In the low temperature regime (∼100 K), the polar optical phonons are found to be more effective in scattering carriers in the oblique L-point valleys than in scattering carriers in the longitudinal L-point valley, making the resulting S somewhat suppressed in this temperature regime. In the high temperature regime (>~300 K), the polar optical phonon scattering generally contributes to increasing S due to the particular shape of the distribution function associated with it. It is emphasized that our theoretical model requires virtually no fitting parameters. The excellent agreement between the theoretical and experimental results suggests the validity of our model of enhanced thermoelectric figure of merit in two-dimensional structures and the reliability of the values of the parameters for the superlattice deduced from other independent measurements, such as Hall carrier concentrations and band energy gaps.
  • T. Koga, X. Sun, S. B. Cronin, M. S. Dresselhaus
    Appl. Phys. Lett. 75 (16) 2438 - 2440 0003-6951 1999 [Refereed][Not invited]
  • T Koga, Sun, X, SB Cronin, MS Dresselhaus
    APPLIED PHYSICS LETTERS 73 (20) 2950 - 2952 0003-6951 1998/11 [Refereed][Not invited]
     
    A large enhancement in the thermoelectric figure of merit for the whole superlattice, Z(3D)T, is predicted for short-period GaAs/AlAs superlattices relative to bulk GaAs. Various superlattice parameters (superlattice growth direction, superlattice period, and layer thicknesses) are explored to optimize Z(3D)T, including quantum well states formed from carrier pockets at various high symmetry points in the Brillouin zone. The highest room-temperature Z(3D)T obtained in the present calculation is 0.41 at the optimum carrier concentration for either (001)- or oriented GaAs (20 Angstrom)/AlAs (20 Angstrom) superlattices, which is about 50 times greater than the corresponding ZT for bulk GaAs. (C) 1998 American Institute of Physics. [S0003-6951(98)01546-0].
  • Modeling of the Enhanced Thermoelectric Properties in PbTe/PbEuTe MQWs
    T. Koga, T. C. Harman, S. B. Cronin, X. Sun, M. S. Dresselhaus
    Mat. Res. Soc. Symp. Proc. 263 - 268 1998 [Refereed][Not invited]
  • Modeling of the Low-dimensional Thermoelectricity
    T. Koga, X. Sun, S. B. Cronin, M. S. Dresselhaus, G. Chen, K. Wang
    Journal of Computer Aided Materials Design 4 175 - 182 1997 [Refereed][Not invited]
  • AJ Stevens, T Koga, CB Agee, MJ Aziz, CM Lieber
    JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 118 (44) 10900 - 10901 0002-7863 1996/11 [Refereed][Not invited]
  • T KOGA, BM MOON, ZAKHARCHENKO, I, KV RAO, H MEDELIUS
    IEEE TRANSACTIONS ON MAGNETICS 29 (6) 3601 - 3603 0018-9464 1993/11 [Refereed][Not invited]
     
    High quality YBa(2)Cu(3)O7-delta thin films have been fabricated by pulsed ND:YAG laser deposition using an unusual 'off-axis' target-substrate geometry. In this geometry one can eliminate the deposition of the laser 'droplets.' Our films are free of particles and have smooth surfaces. The critical temperature (T-c) of these films is in the range of 87-90 K, which is as high as that of 'on-axis' deposited films. From the x-ray rocking curve measurement and the magnetization measurement, no significant differences were found between the 'on-axis' and 'off-axis' laser deposited films.
  • The Role of Oxygen Pressure on the properties of YBaCuO Films Deposited by 'Off-axis' Pulsed Laser Deposition
    T. Koga, I. Zakharchenko, B. M. Moon, K. V. Rao
    Proceedings of the 3rd IUMRS International Conference on Advanced Materials 4 pages  1993 [Refereed][Not invited]
  • YBaCuO films by pulsed laser deposition and sputtering
    T. Koga, J. H. Xu, B. M. Moon, K. V. Rao
    NATO ASI Series E: Applied Sciences 234 369 - 374 1993 [Refereed][Not invited]
     
    NATO ASI Series E: Applied Sciences - Vol. 234, 'Multicomponent and Multilayered Thin Films for Advanced Microtechnologies; Techniques, fundamental and Devices' (Kluwer Academic Publishers, 1993)
  • D POOKE, K KISHIO, T KOGA, Y FUKUDA, N SANADA, M NAGOSHI, K KITAZAWA, K YAMAFUJI
    PHYSICA C 198 (3-4) 349 - 354 0921-4534 1992/08 [Refereed][Not invited]
     
    Intercalation of iodine into the Bi2Sr2Can-1CunOy (n = 1, 2, 3) superconductors has been studied. A variety of measurements indicate charge transfer is associated with the intercalation process, both directly to the BiO layers and also to the superconducting CuO2 sheets. Changes in T(c) on intercalation can be understood in terms of hole doping to the CuO2 planes. Hall coefficient measurements on a single crystal n=2 sample supported an increase in hole concentration, and an XPS study clearly showed the ionic character for the intercalated iodine, accompanied by changes in the Fermi level. Investigation of the c-axis resistivity, also on n = 2 single crystals, indicated a change from the semiconductive behaviour of the parent crystal to an apparently metallic character, though the c-axis resistivity remained high.
  • Structural and superconducting properties of iodine intercalated Bi2Sr2Can-1CunOy (n=1, 2, 3)
    D. Pooke, K. Kishio, N. Motohira, T. Tamura, Y. Tomioka, T. Koga, K. Kitazawa, K. Yamafuji
    Advances in Superconductivity IV: Proceedings of the 4th International Symposium on Superconductivity (Iss '91, October 14-17, 1991 Tokyo), eds. H. Hayakawa and N. Koshizuka (Springer Verlag, Tokyo, 1992) 233 - 236 1992 [Refereed][Not invited]

Books etc

  • 日本熱電学会創立５周年記念 熱電変換技術ハンドブック
    KOGA Takaaki (Contributor第２章第４節１．超格子，Bi2Te3系，PbTe系，GaAs/AlAs系，SiGe系 P249-261)
    株式会社 エヌ・ティー・エス 2008
  • 環境調和型新材料シリーズ 熱電変換材料，（社）日本セラミックス協会・日本熱電学会［編］
    KOGA Takaaki (Contributor3.2 人工超格子 P193-202)
    日刊工業新聞社 2005
  • Quantum Wells and Quantum Wires for Potential Thermoelectric Applications
    M. S. Dresselhaus, Y. M. Lin, T. Koga, S. B. Cronin, O. Rabin, M.R. Black, G. Dresselhaus (Joint workVol. 71, pp.1-122)
    Semiconductors and Semimetals 2000

Conference Activities & Talks

  • Determination of spin-orbit coefficients and application to the spin-filter devices using InGaAs/InAlAs heterostructures  [Not invited]
    T. Koga, T. Matsuura, S. Faniel, S. Souma, S. Mineshige, Y. Sekine, H. Sugiyama
    2011 Asia-Pacific Workshop on Fundamentals and Applications of Advanced Semiconductor Devices, AWAD 2011, June 29 (Wed)-July 1(Fri), 2011, Legend Hotel, Daejeon, Korea.  2011/06
  • Rashba and Dresselhaus Spin-Orbit Interaction in Semiconductor Quantum Wells and Mesoscopic Spin Transport  [Not invited]
    KOGA Takaaki
    Symposium on Surface and Nano Science 2010, January 15(Fri)-18(Mon), 2010 : Shizukuishi, JAPAN  2010/01
  • Spin interference devices based on the Rashba and Dresselhaus terms  [Not invited]
    KOGA Takaaki
    The 4th International Conference on Physics and Application of Spin-Related Phenomena in Semiconductors, PASPS IV, (Sendai, 2006)  2006
  • Rashba効果を用いたスピン干渉計  [Not invited]
    古賀貴亮, 関根佳明, 新田淳作, 俵谷和典, 奥谷洋
    第53回応用物理学関係連合講演会  2005
  • 低次元、超格子系熱電材料の現状と展望  [Not invited]
    KOGA Takaaki
    熱電変換シンポジウム 2001 (TEC 2001)  2001/08
  • Carrier Pocket Engineering for the Design of Low Dimensional Thermoelectrics With High Z3DT  [Not invited]
    T. Koga, S. B. Cronin, M. S. Dresselhaus
    MRS Spring Meeting 2000 (San Francisco)  2000

Works

  • International Symposium on Nanoscale Transport and Technology (ISNTT2011)
    2011
  • 第58回応用物理学関係連合講演会(2011春)
    2011
  • Sixth International Conference on Physics and Applications of Spin-related Phenomena in Semiconductors (PASPS-VI), Hongo Campus, University of Tokyo (2010)
    2010
  • 30th International Conference on the Physics of Semiconductors, ICPS2010 (July 25-30, 2010, COEX, Seoul, Korea )
    2010
  • APS March Meeting 2010 (Portland)
    2010
  • 第15回 半導体スピン工学の基礎と応用 PASPS-15
    2010
  • 日本物理学会 2010年秋季大会
    2010
  • 第29回電子材料シンポジウム
    2010
  • 日本物理学会 第65回年次大会
    2010
  • APS March Meeting 2010 (Portland)
    2010
  • 29th Electronic Materials Symposium (EMS-29)
    2010
  • The 18th International Conference on Electronic Properties of Two-Dimensional Systems (EP2DS-18)
    2009
  • The 14th International Conference on Modulated Semiconductor Structures
    2009
  • 14th International Conference on Narrow Gap Semiconductors and Systems
    2009
  • APS March Meeting 2009
    2009
  • 日本物理学会 2009年秋季大会
    2009
  • 日本物理学会 第64回年次大会
    2009
  • The 18th International Conference on Electronic Properties of Two-Dimensional Systems (EP2DS-18)
    2009
  • The 14th International Conference on Modulated Semiconductor Structures
    2009
  • 14th International Conference on Narrow Gap Semiconductors and Systems
    2009
  • APS March Meeting 2009
    2009
  • Fifth International Conference on Physics and Applications of Spin-related Phenomena in Semiconductors (PASPS-V), Foz do Iguacu, Brazil (2008)
    2008
  • APS March Meeting 2008 (New Orleans) (2008)
    2008
  • 第69回応用物理学会学術講演会(2008秋)
    2008
  • 第55回応用物理学関係連合講演会(2008春)
    2008
  • Forth International School and Conference on Spintronics and Quantum Information Technology (SPINTECH IV), MAUI, Hawaii, USA, June 17-22 (2007)
    2007
  • APS March Meeting 2007 (Denver) (2007)
    2007
  • 日本物理学会 第62回年次大会(2007)
    2007
  • 第54回応用物理学関係連合講演会(2007春)
    2007
  • 第54回応用物理学関係連合講演会（2007春）
    2007
  • 28th International Conference on the Physics of Semiconductors, ICPS2006 (Vienna, Austria)
    2006
  • The 4th International Conference on Physics and Application of Spin-Related Phenomena in Semiconductors (Sendai, 2006.8.15-8.18)
    2006
  • APS March Meeting 2006 (Baltimore) (2006)
    2006
  • 日本物理学会 2006年秋季大会（2006）
    2006
  • 第53回応用物理学関係連合講演会(2006春)
    2006
  • 日本物理学会 第61回年次大会(2006)
    2006
  • The 12th International Conference on Narrow Gap Semiconductors, NGS12 (2005).
    2005
  • 第52回応用物理学関係連合講演会(2005春)
    2005
  • The 3rd International Conference on Physics and Applications of Spin Related Phenomena in Semiconductors (2004)
    2004
  • 第65回応用物理学会学術講演会(2004秋)
    2004
  • 日本物理学会 第58回年次大会(2003)
    2004
  • 日本物理学会 第５９回年次大会(2004春)
    2004
  • 11th Int. Conf. on Narrow Gap Semiconductors (Buffalo), Mo-B-3 (2003)
    2003
  • 日本物理学会 2003年秋季大会（2003）
    2003
  • The 23rd International Conference on Low Temperature Physics (2002)
    2002
  • 2nd International Conference on Physics and Application of Spin Related Phenomena in Semiconductors (2002)
    2002
  • 26th International Conference on the Physics of Semiconductors (2002)
    2002
  • APS March Meeting 2002 (Indianapolis) (2002)
    2002
  • 日本物理学会 2002年秋季大会(2002)
    2002
  • 日本物理学会 第57回年次大会(2002)
    2002
  • Materials Research Society, Fall Meeting(2001)
    2001
  • Materials Research Society, Fall Meeting (2001)
    2001
  • 10th International Conference on Modulated Semiconductor Structures (Linz) (2001)
    2001
  • Proceedings of the 10th International Conference on Narrow Gap Semiconductors and Related Small Energy Phenomena (2001)
    2001
  • The 2001 International Conference on SOLID STATE DEVICES AND MATERIALS (Tokyo) (2001)
    2001
  • APS March Meeting 2001 (Seattle) (2001)
    2001
  • 日本物理学会 2001年秋季大会(2001)
    2001
  • 日本物理学会 第56回年次大会(2001)
    2001
  • APS March Meeting 2000 March 20-24 Minneapolis Convention Center (Minneapolis) (2000)
    2000
  • MRS Spring Meeting 2000 (San Francisco) (2000)
    2000
  • 日本物理学会 2000年秋季大会(2000)
    2000
  • Materials Research Society, Fall Meeting (1999)
    1999
  • The 18th International Conference on Thermoelectrics: ICT Symposium Proceedings (1999)
    1999
  • American Physical Society, March Meeting (Atlanta) (1999)
    1999
  • Materials Research Society, Fall Meeting (1998)
    1998
  • Materials Research Society, Fall Meeting (1997).
    1997
  • Materials Research Society, Fall Meeting (1996)
    1996
  • THE 1993 IEEE International Magnetics Conference, Stockholm, Sweden（1993）
    1993
  • Proceedings of the 3rd IUMRS International Conference on Advanced Materials (1993)
    1993
  • NATO ASI Meeting, Badwingsheim, Germany.(1992)
    1992
  • The 4th International Symposium on Superconductivity, Tokyo(1991)
    1991
  • The 4th International Symposium on Superconductivity(1991)
    1991

MISC

Industrial Property Rights

  • 特願2013-015190:スピン検出器およびスピン検出方法  2014年/01/30
    関根佳明, 赤崎達志, 古賀貴亮, 末岡和久
  • 特願2012-096287:スピン素子  2012年
    古賀貴亮, 関根佳明
  • 特願2010-189336:スピン偏極装置  2010年
    古賀貴亮, 関根佳明
  • 特願2010-161379:スピン偏極装置  2010年
    古賀貴亮, 関根佳明
  • 特開2004-165426:スピンフィルター装置  2004
    古賀貴亮, 新田淳作
  • 特開2004-193200:超格子熱電材料  2004
    古賀貴亮, 渡辺正裕
  • 特開2004-165438:スピンフィルター  2004
    古賀貴亮, 新田淳作
  • 特許U.S. Patent No. 6,627,809:Superlattice Structures Having Selected Carrier Pockets and Related Methods    2003/09/30
    T. Koga, M. S. Dresselhaus, X. Sun, S. B. Cronin
  • 特開2002-343958:半導体スピンフィルター  2002
    古賀貴亮, 新田淳作, 赤崎達志, 高柳英明
  • 特許U.S. Patent No. 6,060,656:Si/SiGe Superlattice Structures for Use in Thermoelectric Devices    2000/05/09
    M. S. Dresselhaus, T. C. Harman, S. B. Cronin, T. Koga, X. Sun, K. L. Wang

Awards & Honors

  • 2004/03 NTT物性科学基礎研究所 所長表彰業績賞
     ゲートを用いた半導体中のスピン制御 
    受賞者: KOGA Takaaki
  • 1999/08 The 11th International Conference on Thermoelectrics Best Paper Award
     Carrier Pocket Engineering to Design Superior Thermoelctric Materials Using Superlattice Structures 
    受賞者: KOGA Takaaki

Research Grants & Projects

  • 二重量子井戸を用いた非磁性半導体スピンフィルタの研究
    日本電信電話株式会社NTT物性科学基礎研究所：共同研究費
    Date (from‐to) : 2012/05 -2013/02 
    Author : SUEOKA Kazuhisa
  • 半導体スピントロニクスにおけるスピン軌道相互作用を利用したスピンフィルタの研究
    日本電信電話株式会社NTT物性科学基礎研究所：共同研究費
    Date (from‐to) : 2011/09 -2012/02 
    Author : KOGA Takaaki
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2011 -2012 
    Author : KOGA Takaaki, KAWABATA Shiro, SOUMA Satofumi, SAITO Keiji, MATSUURA Toru
     
    For InP-lattice-matched InGaAs quantum wells, we have quantitatively revealed the zero-field spin splitting term. Not only in showing the presence of the Rashba term for the first time which has been the target of controversy for long time, but also we have succeeded in elucidating its size quantitatively. More specifically, we have shown for the first time the linear relation between the Rashba coefficient alpha and the electric field inside the well from the theoretical analysis of the weak antilocalization effect that is seen in the magneto-conductivity data at dilution temperatures. In addition, we have found that the Dresselhaus term in this system is very small. Finally, we have also measured the time-reversal quantum interference effect in a mesoscopic loop array structure that is fabricated in a wafer whose can be varied across zero by gate and demonstrated the effectiveness of the semi-classical approach in theory.
  • 半導体スピントロニクスにおける界面電場と結晶場の電子スピンに与える影響評価の研究
    日本電信電話株式会社NTT物性科学基礎研究所：共同研究費
    Date (from‐to) : 2010/06 -2011/03 
    Author : KOGA Takaaki
  • 半導体へテロ界面場による電子スピン制御技術の確立とスピントロニクス応用
    国立大学法人 北海道大学：創成科学機構 研究部 流動研究部門
    Date (from‐to) : 2006/10 -2011/03 
    Author : KOGA Takaaki
  • 半導体スピントロニクスにおける電子スピン制御の研究
    日本電信電話株式会社NTT物性科学基礎研究所：共同研究費
    Date (from‐to) : 2009/10 -2010/02 
    Author : KOGA Takaaki
  • Ministry of Education, Culture, Sports, Science and Technology：Grants-in-Aid for Scientific Research(若手研究(A))
    Date (from‐to) : 2007 -2010 
    Author : Takaaki KOGA
     
    Spin-orbit interaction in semiconductor can be considered as a fundamental principle on which next generation electronics, such as quantum information devices, are based. In this research, we chose the In_<0.53>Ga_<0.47>As/In_<0.52>Al_<0.48>As quantum well system as a standard material for spintronics and revealed the spin-orbit parameters quantitatively by electron transport measurements at low temperatures including spin interference effect.
  • スピン軌道相互作用を用いた半導体量子情報素子の開発
    (財)テレコム先端技術研究支援センター：研究費助成
    Date (from‐to) : 2006/06 -2009/05 
    Author : KOGA Takaaki
  • 狭ギャップ半導体量子細線の作製とスピン軌道相互作用に基づく異常磁化発現
    （財）池谷科学技術振興財団：
    Date (from‐to) : 2007/04 -2008/03 
    Author : KOGA Takaaki
  • スピン制御半導体の研究
    日本電信電話株式会社NTT物性科学基礎研究所：受託研究費
    Date (from‐to) : 2005/04 -2008/03 
    Author : KOGA Takaaki
  • 半導体スピンエンジニアリング
    (独)科学技術振興機構：CREST (分担)
    Date (from‐to) : 2005/04 -2007/03 
    Author : NITTA Junsaku
  • 半導体ナノ構造中でのスピン軌道相互作用を用いた､スピン輸送､スピン干渉､スピン操作の研究
    (財)村田学術振興財団：研究助成
    Date (from‐to) : 2005/08 -2006/07 
    Author : KOGA Takaaki
  • 量子コンピュータ実現に向けた半導体スピン干渉デバイスの研究
    （財）稲森財団：研究助成金
    Date (from‐to) : 2005/04 -2006/03 
    Author : KOGA Takaaki
  • GaAs/AlGaAs界面による電子スピン制御の検討
    (独)科学技術振興機構：戦略的創造研究推進事業 特別課題調査
    Date (from‐to) : 2006 -2006 
    Author : KOGA Takaaki
  • 半導体ナノ構造中に現れる新スピン物性の制御と応用
    （独）科学技術振興機構：さきがけ研究 ナノと物性領域
    Date (from‐to) : 2001/12 -2005/03 
    Author : KOGA Takaaki

Educational Activities

Teaching Experience

  • Physics and Mathematics for Electrical Engineering
    開講年度 : 2021
    課程区分 : 修士課程
    開講学部 : 情報科学研究科
    キーワード : 基礎数学、古典力学、量子力学、統計力学、特殊相対性理論
  • Physics and Mathematics for Electrical Engineering
    開講年度 : 2021
    課程区分 : 修士課程
    開講学部 : 情報科学院
    キーワード : 基礎数学、古典力学、量子力学、統計力学、特殊相対性理論
  • Physics and Mathematics for Electrical Engineering
    開講年度 : 2021
    課程区分 : 博士後期課程
    開講学部 : 情報科学研究科
    キーワード : 基礎数学、古典力学、量子力学、統計力学、特殊相対性理論
  • Physics and Mathematics for Electrical Engineering
    開講年度 : 2021
    課程区分 : 博士後期課程
    開講学部 : 情報科学院
    キーワード : 基礎数学、古典力学、量子力学、統計力学、特殊相対性理論
  • Applied Quantum Mechanics
    開講年度 : 2021
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 前期量子論、波動関数の対称性、波動関数の時間発展、軌道角運動量、スピン角運動量、全角運動量、量子電子デバイス

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