研究者データベース

明楽 浩史(アケラ ヒロシ)
工学研究院 応用物理学部門 量子物性工学分野
教授

基本情報

所属

  • 工学研究院 応用物理学部門 量子物性工学分野

職名

  • 教授

学位

  • 理学博士(東京大学)

ホームページURL

科研費研究者番号

  • 20184129

J-Global ID

研究キーワード

  • 量子井戸   半導体   電子輸送   スピントロニクス   量子ホール効果   スピン緩和   スピンネルンスト効果   スピンホール効果   物性理論   2次元電子系   

研究分野

  • 自然科学一般 / 半導体、光物性、原子物理
  • 自然科学一般 / 磁性、超伝導、強相関系

所属学協会

  • 日本物理学会   The Physical Society of Japan   

研究活動情報

論文

  • Ryosuke Otsuto, Yuya Yatabe, Hiroshi Akera
    Physical Review B 104 3 035431-1 - 035431-8 2021年07月27日 [査読有り][通常論文]
  • Tomonori Iijima, Yoshiyuki Egami, Hiroshi Akera
    Japanese Journal of Applied Physics 59 10 100901-1 - 100901-5 2020年10月01日 [査読有り][通常論文]
  • Tomonori Iijima, Hiroshi Akera
    Physical Review Applied 13 6 064075-1 - 064075-10 2020年06月30日 [査読有り][通常論文]
  • Kenji Hayashida, Hiroshi Akera
    Phys. Rev. B 101 035306-1 - 035306-7 2020年01月 [査読有り][通常論文]
  • Toshiya Ishikawa, Hiroshi Akera
    Phys. Rev. B 100 125307-1 - 125307-10 2019年09月 [査読有り][通常論文]
  • Y. Egami, H. Akera
    Appl. Phys. Express 10 6 063007-1 - 063007-4 2017年06月01日 [査読有り][通常論文]
     
    The incorporation of a symmetric electrostatic potential into quantum wells (QWs) is proposed as a method for modifying the coefficient α of the Rashba spin-orbit interaction. In a symmetric QW for which α = asoEz, where Ez is the perpendicular electric field, the constant aso can be controlled by the symmetric potential. The sign reversal of aso with the increasing strength of the symmetric potential is demonstrated in (001)- oriented GaAs/AlGaAs QWs via a tight-binding model. The present findings can be used to realize structures with vanishing α in nonzero Ez.
  • Yoshiyuki Egami, Hiroshi Akera
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 88 212 - 217 2017年04月 [査読有り][通常論文]
     
    First-principles calculations are performed for electron transmission through a metallic zigzag carbon nanotube with substitutional BN dimers parallel to the nanotube axis. The transmission coefficient is calculated in the energy range (around the charge neutrality point) in which there exist two degenerate subbands for each spin. Wave functions in the circumferential direction of one of the degenerate subbands can be chosen so as to have nodes at the position of a carbon dimer parallel to the nanotube axis. It is shown that the transmission probability of an incident wave with such wave-function nodes depends crucially on positions of BN dimers relative to the nodes. By placing each of dimers at one of the nodes, the transmission probability is substantially enhanced and is well described by the Born approximation in spite of spatially extended scattering potential due to ionized B and N. This suggests that the arrangement in the circumferential direction of various impurities influences transport through metallic zigzag carbon nanotubes.
  • Hiroshi Akera, Hidekatsu Suzuura, Yoshiyuki Egami
    PHYSICAL REVIEW B 95 4 045301-1 - 5 2017年01月 [査読有り][通常論文]
     
    The coefficient alpha of the Rashba spin-orbit interaction is calculated in an asymmetric quantum well consisting of Ga0.47In0.53As (well), Al0.48In0.52As (left barrier), and AlxGa1-xAsySb1-y (right barrier) as a function of the external electric field perpendicular to the well E-z(ex) which is controlled by the gate voltage. This coefficient alpha, which depends on the band offset, can be tuned to be zero by adjusting the Al fraction x in the right barrier layer to the optimum value x(0) in the case where the wave function vanishes at the left heterointerface. Such a composition-adjusted asymmetric quantum well is proposed as a structure in which the magnitude of a can be switched by changing the polarity of E-z(ex). The calculation shows that, when vertical bar x - x(0)vertical bar < 0.01, the on/off vertical bar alpha vertical bar ratio > 40 for a large enough vertical bar E-z(ex)vertical bar (vertical bar E-z(ex)vertical bar > 10(7) V/m for a well width of 20 nm), which results in the on/off spin-relaxation-rate ratio exceeding 10(3) in the Dyakonov-Perel mechanism.
  • 飯島 智徳, 明楽 浩史
    日本物理学会講演概要集 71 1136 - 1136 一般社団法人 日本物理学会 2016年 

    前回我々は、Rashbaスピン軌道相互作用の大きさが電場の正負により著しく異なる非対称量子井戸が存在することを理論的に示した。今回我々は、n枚の界面を持つ半導体ヘテロ構造についてRashbaスピン軌道相互作用の電場依存性を議論する。

  • Hiroshi Akera, Hidekatsu Suzuura, Yoshiyuki Egami
    PHYSICAL REVIEW B 92 20 205311-1 - 9 2015年11月 [査読有り][通常論文]
     
    The spin relaxation due to the spin-orbit interaction (SOI) is studied theoretically in a quantum well with electrons occupying only the ground subband. First, it is shown that the coefficient of the Rashba SOI is proportional to b(off) - 1, in which the parameter boff, determined by the band offsets and the band gaps, passes through unity, for example, by changing x in Ga0.47In0.53As(well)/AlxGa1-xAsySb1-y (barrier). Second, it is derived that the transition matrix element of each spin-flip phonon scattering has the same proportionality factor b(off) - 1, in addition to the impurity scattering previously studied by the same authors [Phys. Rev. B 89, 075314 (2014)]. These findings suggest the possibility of strongly suppressing the spin-relaxation rate by choosing appropriate materials.
  • 江上 喜幸, 明楽 浩史, 広瀬 喜久治
    日本物理学会講演概要集 70 2855 - 2855 一般社団法人日本物理学会 2015年
  • 江上 喜幸, 明楽 浩史, 広瀬 喜久治
    日本物理学会講演概要集 70 3192 - 3192 一般社団法人日本物理学会 2015年
  • Kousuke Shima, Hiroshi Akera
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 63 204 - 210 2014年09月 [査読有り][通常論文]
     
    The spatial dependence of the Hall potential induced in a two-dimensional electron system (2DES) by AC source-drain voltage is studied theoretically in the incoherent linear transport in the strong-magnetic-field regime. The local capacitance approximation is employed in which the potential at each point of the 2DES is proportional to the induced charge at the same point It is shown that the frequency dependence of the induced charge distribution is described by three time constants, tau(e) for transport through an edge channel of the electron injected from a current contact, tau(eb) for transition between the edge channel and the bulk state, and tau(b) for diffusion into the bulk, which are quite different in magnitude: tau(e) << tau(eb) << tau(b) in the quantum Hall regime of a typical sample. These three time constants also determine how the Hall potential evolves in the 2DES after the source drain voltage is turned on. Calculated two-dimensional distribution of the Hall potential as a function of the frequency reveals that the Hall potential develops by penetrating into the bulk from source and drain contacts as well as from the edge channel., (C) 2014 Elsevier By. All rights reserved.
  • H. Akera, H. Suzuura, Y. Egami
    Phys. Rev. B 89 7 075314-1 - 9 2014年02月 [査読有り][通常論文]
     
    The spin relaxation of a two-dimensional electron system (2DES) formed in a symmetric quantum well is studied theoretically when the quantum well is parallel to the (110) plane of the zinc-blende structure, the spin polarization is perpendicular to the well, and electrons occupy only the ground subband. The spin-relaxation rate is calculated as a function of the distribution of donor impurities which are placed in the well layer. Considered processes of the spin relaxation are (1) the intrasubband process by impurity-potential-induced spin-orbit interaction (SOI), which is the Elliott-Yafet mechanism in the 2DES, and (2) virtual intersubband processes consisting of a spin flip by (2a) well-potential-induced SOI or (2b) the Dresselhaus SOI, and a scattering from an impurity. It is shown that all of the above processes disappear when all impurities are located on the center plane of the well. Even if impurities are distributed over three (110) atomic layers, the spin-relaxation rate is two orders of magnitude lower than that for the uniform distribution over the well width of 7.5 nm. In GaAs/AlGaAs type-I quantum wells, the processes (1) and (2a) interfere constructively, being dominant over (2b) for the well width of similar to 10 nm, while in some type-II quantum wells, they can interfere destructively.
  • Hiroshi Akera, Hidekatsu Suzuura
    PHYSICAL REVIEW B 87 7 075301-1 - 7 2013年02月 [査読有り][通常論文]
     
    The spin accumulation due to the spin current induced by the perpendicular temperature gradient (the spin Nernst effect) is studied in a two-dimensional electron system (2DES) with spin-orbit interaction by employing the Boltzmann equation. The considered 2DES is confined within a symmetric quantum well with delta doping at the center of the well. A symmetry consideration leads to the spin-orbit interaction which is diagonal in the spin component perpendicular to the 2DES. As origins of the spin current, the skew scattering and the side jump are considered at each impurity on the center plane of the well. It is shown that, for repulsive impurity potentials, the spin-Nernst coefficient changes its sign at the impurity density where contributions from the skew scattering and the side jump cancel each other out. This is in contrast to the spin Hall effect in which the sign change of the coefficient occurs for attractive impurity potentials. DOI: 10.1103/PhysRevB.87.075301
  • H. Akera
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 43 6 1240 - 1248 2011年04月 [査読有り][通常論文]
     
    The distribution of the Hall voltage induced by low-frequency AC current is studied theoretically in the incoherent linear transport of quantum Hall systems. It is shown that the Hall-voltage distribution makes a crossover from the uniform distribution to a concentrated-near-edges distribution as the frequency is increased or the diagonal conductivity is decreased. This crossover is also reflected in the frequency dependence of AC magnetoresistance. (C) 2011 Elsevier B.V. All rights reserved.
  • Hiroshi Akera
    HORIBA INTERNATIONAL CONFERENCE: THE 19TH INTERNATIONAL CONFERENCE ON THE APPLICATION OF HIGH MAGNETIC FIELDS IN SEMICONDUCTOR PHYSICS AND NANOTECHNOLOGY 334 012019-1-012019-5  2011年 [査読有り][通常論文]
     
    The distribution of the Hall voltage induced by low-frequency AC current is studied theoretically in the incoherent linear transport of quantum Hall systems with slowly-varying confining potential. It is shown that in the low-frequency limit the Hall potential has a steeper slope where the diagonal conductivity is smaller, while at higher frequencies it exhibits a peak or a dip where the Hall conductivity has a slope.
  • Hiroshi Akera
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 78 2 023708-1-023708-4  2009年02月 [査読有り][通常論文]
     
    Dynamics during the breakdown of the quantum Hall effect is studied theoretically in the Corbino geometry at a filling factor of 2. Spatio-temporal variations of the density of electrons in the first excited Landau level are calculated as well as those of the electron temperature. It is shown that the time constant of the population in this Landau level becomes quite long when the electron temperature is low compared to the Landau-level width.
  • Yuki Nagai, Hiroshi Akera, Hidekatsu Suzuura
    PHYSICS OF SEMICONDUCTORS, PTS A AND B 893 665 - + 2007年 [査読有り][通常論文]
     
    The electrochemical potential and the electron temperature are calculated in the linear-response regime in a system with a filling-factor variation along the current, and are shown to exhibit a strong spatial modulation at the filling-factor step.
  • Hiroshi Akera, Hidekatsu Suzuura
    PHYSICS OF SEMICONDUCTORS, PTS A AND B 893 667 - + 2007年 [査読有り][通常論文]
     
    Instability of uniform steady states with negative differential resistivity in the breakdown of the quantum Hall effect is studied theoretically in the condition of a fixed voltage drop along the current. The linear stability analysis suggests a formation of periodically modulated states.
  • Sadayoshi Kanamaru, Hidekatsu Suzuura, Hiroshi Akera
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 75 6 064701-1-10  2006年06月 [査読有り][通常論文]
     
    Spatial distributions of the electron temperature perpendicular to the applied current are obtained in quantum Hall systems with compressible and incompressible strips at low lattice temperatures by solving equations of electron number conservation and energy conservation as well as Poisson's equation self-consistently. In the linear-response regime, variations of the electron temperature concentrate in the incompressible strips as the lattice temperature decreases. The electron temperature indicates an antisymmetric distribution: it becomes lower than the lattice temperature in the side of a sample with a higher electrochemical potential, and higher in the opposite side. Around the breakdown of the quantum Hall effects, the electron temperature becomes much higher than the lattice temperature as the applied current increases. Reflecting the anti-symmetric distribution in the linear-response regime, the rise of the electron temperature is suppressed in the higher potential side with increasing current. In the same side, the current density becomes large and the current-induced potential shows a steep variation. At even larger currents, accompanying disappearance of the compressible and incompressible strips, the electron temperature becomes uniform.
  • S Kanamaru, H Suzuura, H Akera
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 74 7 2069 - 2075 2005年07月 [査読有り][通常論文]
     
    Spatial distributions of the electron temperature perpendicular to the applied current in quantum Hall systems with slowly-varying confining potentials are calculated in the linear-response regime by employing hydrodynamic equations for number conservation and energy conservation. The electron temperature exhibits spatial pattern, reflecting the confining potential, along the Hall field induced by the current. The local electron temperature shows oscillations as a function of the filling factor. Such spatially dependent electron temperature causes a certain change in distributions of the current density and the current-induced potential even in the linear-response regime.
  • H Akera, H Suzuura
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 74 3 997 - 1005 2005年03月 [査読有り][通常論文]
     
    A theory of thermohydrodynamics in two-dimensional electron systems in quantizing magnetic fields is developed including a nonlinear transport regime. Spatio-temporal variations of the electron temperature and the chemical potential in the local equilibrium are described by the equations of conservation with the number and thermal-energy flux densities. A model of these flux densities due to hopping and drift processes is introduced for a random potential varying slowly compared to both the magnetic length and the phase coherence length. The flux measured in the standard transport experiment is derived and is used to define a transport component of the flux density. The equations of conservation can be written in terms of the transport component only. As an illustration, the theory is applied to the Ettingshausen effect, in which a one-dimensional spatial variation of the electron temperature is produced perpendicular to the current.
  • Akera,H., Suzuura,H.:"Thermohydrodynamics in Quantum Hall Systems", J. Phys. Soc. Jpn., 74: 997-1005 (2005)*
    2005年 [査読有り][通常論文]
  • T Maeda, S Kanamaru, H Akera, H Suzuura
    Physics of Semiconductors, Pts A and B 772 551 - 552 2005年 [査読有り][通常論文]
     
    Spatial variations of the electron temperature perpendicular to the current in a quantum Hall system with a slowly-varying parabolic confining potential are calculated in the linear-response regime. Due to the Landau quantization, the sign of the induced deviation of the electron temperature is shown to exhibit spatial oscillations along the potential gradient.
  • T Nakagawa, H Akera, H Suzuura
    Physics of Semiconductors, Pts A and B 772 553 - 554 2005年 [査読有り][通常論文]
     
    Spatial variations of the electron temperature are calculated in the linear-response regime in a quantum Hall system with a potential discontinuity in the current direction. It is shown that the sign of the induced deviation of the electron temperature from the lattice temperature exhibits quantum oscillations.
  • Ise,T., Akera,H., Suzuura,H.:"Electron Temperature Distribution and Hot Spots in Quantum Hall Systems", J. Phys. Soc. Jpn., 74: 259-262 (2005)*
    2005年 [査読有り][通常論文]
  • T Ise, H Akera, H Suzuura
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 74 1 259 - 262 2005年01月 [査読有り][通常論文]
     
    The spatial variations of the electron temperature in the vicinity of metallic current contacts in a quantum Hall system are calculated based on thermohydrodynamics with an energy gain. It is shown that, at large currents, hot spots with high electron temperatures appear at diagonally opposite corners of a sample. At small currents, however, the electron temperature at one of the corners is lower than the lattice temperature, while that at the other corner is higher than the lattice temperature. As a function of the chemical potential, the electron temperature at each corner shows quantum oscillations.
  • S Kanamaru, H Akera, H Suzuura
    INTERNATIONAL JOURNAL OF MODERN PHYSICS B 18 27-29 3589 - 3592 2004年11月 [査読有り][通常論文]
     
    Spatial variations of the electron temperature perpendicular to the applied current in quantum Hall systems with slowly-varying confining potentials are calculated in the linear-response regime employing the hydrodynamic equation of the energy conservation as well as that of the charge conservation. The effects of the confining potential on the spatial variations of the electron temperature are investigated by employing model confining potentials as well as self-consistently calculated potentials. It is shown that the deviations of the electron temperature exhibit spatial oscillations along the confining potential gradient. The gradient of the electron temperature in the center of the sample exhibits quantum oscillations as a function of the filling factor.
  • Kanamaru,S., Akera,H., Suzuura,H.:"Spatial Variations of the Electron Temperature in Quantum Hall Systems with Slowly-Varying Confining Potential", International Journal of Modern Physics B, 18:3589-3592 (2004)*
    2004年 [査読有り][通常論文]
  • Akera, H. and Shimoyama, K.:"Spatial Variations of the Electron Temperature in Quantum Hall Systems", Conference workbook of the 15th International Conference on Electronic Properties of Two-Dimensional Systems, Nara (2003)
    2003年 [査読無し][通常論文]
  • T Ise, H Akera
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 71 7 1712 - 1715 2002年07月 [査読有り][通常論文]
     
    The diagonal conductivity of two-dimensional electron systems in strong magnetic fields is derived in an analytical form by considering tunneling processes through a saddle point of the slowly-fluctuating potential in the high electron-temperature regime where the breakdown of the integer quantum Hall effect occurs. The calculation is made for a two-dimensional periodic potential modulation with a fixed amplitude equal to the Landau level separation. The electron-temperature dependence of the conductivity is mainly determined by the presence of an activation energy. It is found that an estimated value of the conductivity in the present model at even-integer filling factors is smaller by an order of magnitude than the experimental value in the integer quantum Hall effect breakdown regime.
  • Akera, H.:"Hydrodynamic Equations in Quantum Hall Systems at Large Currents", J. Phys. Soc. Jpn., 71: 228-236 (2002)*
    2002年 [査読有り][通常論文]
  • H Akera
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 71 1 228 - 236 2002年01月 [査読有り][通常論文]
     
    Hydrodynamic equations (HDEQs) are derived which describe spatio-temporal evolutions of the electron temperature and the chemical potential of two-dimensional systems in strong magnetic fields in states with large diagonal resistivity appearing at the breakdown of the quantum Hall effect. The derivation is based on microscopic electronic processes consisting of drift motions in a slowly-fluctuating potential and scattering processes due to electron-electron and electron-phonon interactions. In contrast with the usual HDEQs, one of the derived HDEQs has a term with an energy flux perpendicular to the electric field due to the drift motions in the magnetic field. As an illustration. the current distribution is calculated using the derived HDEQs.
  • H Akera
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 70 6 1468 - 1471 2001年06月 [査読有り][通常論文]
     
    The hydrodynamic equation for the spatial and temporal evolutions of the electron temperature T, in the breakdown of the quantum Hall effect at even-integer filling factors: ill a uniform current density j(x) is derived from the Boltzmann-type equation, which takes into account electron-electron and electron-phonon scattering processes. The derived equation has a. drift term, which is proportional to j(x) and the first spatial derivative of T-e. When applied to the spatial evolution of T-e in a sample with an abrupt change of the width along the current direction, the equation gives a distinct dependence on the current direction as well as a critical relaxation, in agreement with results of recent experiments.
  • H Akera
    PHYSICA B 298 1-4 38 - 42 2001年04月 [査読有り][通常論文]
     
    A brief description is given about our recent theory on microscopic processes giving the energy gain and loss of a two-dimensional electron system in long-range potential fluctuations at the breakdown of the quantum Hall effect in the case of even-integer filling factors. Considering the Coulomb scattering within a broadened Landau level as a mechanism of the gain, we show that the electron temperature T-e and the diagonal conductivity sigma (xx) exhibit a bistability above the lower critical electric field E-c1, and that the orders of magnitude of E-c1 as well as T-e and sigma (xx) at E-c1 are in agreement with the observed ones. (C) 2001 Elsevier Science B.V. All rights reserved.
  • W Izumida, H Akera
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS 40 3B 1963 - 1965 2001年03月 [査読有り][通常論文]
     
    Transmission coefficient through a quantum dot, which contains two orbitals, is calculated in the weak-tunneling regime, taking into account the Coulomb interaction within the dot. Transmitted waves through these orbitals acquire phase shifts which differ by pi from each other in the present model. The transmission coefficient as a function of gate voltage exhibits symmetric and asymmetric line shapes, depending on temperature T. A nearly symmetric line shape appears in the temperature region of 0.05 less than or similar to k(B)T/delta less than or similar to 0.25, where delta is the single-particle energy level separation in the dot. This behavior is understood by a combined action of the above-mentioned phase difference and the Coulomb interaction. A possibility that this mechanism explains the recent experiment is suggested.
  • Izumida, W. and Akera, H.:"Transmission through a Quantum Dot with Two Orbitals", Jpn. J. Appl. Phys., 40: 1963-1965 (2001)*
    2001年 [査読有り][通常論文]
  • H Akera
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 69 10 3174 - 3177 2000年10月 [査読有り][通常論文]
     
    Microscopic processes giving the energy gain and loss of a two-dimensional electron system in long-range potential fluctuations are studied theoretically at the breakdown of the quantum Hall effect in the case of even-integer filling factors. The Coulomb scattering within a broadened Landau level is proposed to give the gain, while the phonon scattering to give the loss. The energy balance equation shows that the electron temperature T-e and the diagonal conductivity sigma (xx) exhibit a bistability above the lower critical electric field E-c1. Calculated values of E-c1 as well as T-e and sigma (xx) at E-c1 are in agreement with the observed values in their orders of magnitude.
  • Y Nakano, T Kaneko, H Akera
    PHYSICAL REVIEW B 61 15 9948 - 9951 2000年04月 [査読有り][通常論文]
     
    Correlations of two electrons and two holes are studied in unbalanced double layers in strong magnetic fields using a variational method. The ground state of an electron-hole pair (exciton) in quantized cyclotron orbits possesses an in-plane electric dipole moment, when an electron and a hole are in different Landau orbits with different radii. The resulting attractive interactions between excitons creates the possibility of novel biexciton states.
  • H Akera
    JOURNAL OF LOW TEMPERATURE PHYSICS 118 5-6 421 - 426 2000年03月 [査読有り][通常論文]
     
    A review is given on theoretical studies of roles of the spin degree of freedom in transport through a quantum dot with the total spin S for N electrons and S for N + 1 electrons in the ground state. The conductance and the phase coherency are expressed in terms of universal functions of S, (S) over tilde, and the spin splitting at small tunneling rates in the absence of spin-orbit interactions and are shown to exhibit the correlation between tunnelings of electrons with opposite spins and the dephasing due to spin flips.
  • Nakano, Y., Kaneko, T. and Akera, H.: "Biexcitons in unbalanced double layers in quantizing magnetic fields", Phys. Rev. B, 61: 9948-9951 (2000)*
    2000年 [査読有り][通常論文]
  • Akera, H.: "Electronic Processes at the Breakdown of the Quantum Hall Effect", J. Phys. Soc. Jpn., 69: 3174-3177 (2000)*
    2000年 [査読有り][通常論文]
  • H Akera
    PHYSICAL REVIEW B 60 15 10683 - 10686 1999年10月 [査読有り][通常論文]
     
    The dependence of the Coulomb staircase on the total spin of a quantum dot is calculated at a current step corresponding to a transition between the ground states of two successive electron numbers. The ratio of the step heights for the positive and negative bias is simply the ratio of the spin degeneracies of the ground states, when tunneling rates across the two barriers are strongly asymmetric. This can be used for determining experimentally the total spin and therefore identifying the spin blockade. The in-plane magnetic-field dependence of the Coulomb staircase is also calculated, and it is shown that the relative height of spin-split steps can be used alternatively to determine the total spin. [S0163-1829(99)11639-4].
  • H Akera
    PHYSICAL REVIEW B 59 15 9802 - 9805 1999年04月 [査読有り][通常論文]
     
    Variations of the Coulomb oscillations with the in-plane magnetic field (the spin splitting) are theoretically studied to find many-body effects in tunneling through a quantum dot. The conductance and the transmission coefficient are calculated at small tunneling rates in the absence of gin-orbit interactions and expressed in terms of universal functions of the spin splitting, which have no dependence on details of wave functions except the magnitude of spin. Such universal functions are shown to be useful in observing the correlation between tunnelings of electrons with opposite spins and the dephasing due to spin hips. [S0163-1829(99)11815-0].
  • Akera, H.:`` Correlations and spin flips in tunneling through a quantum dot", Phys. Rev. B 59 : 9802-9805 (1999)*
    1999年 [査読有り][通常論文]
  • Akera, H.:`` Coulomb Staircase and Total Spin in Quantum Dots", Phys. Rev. B 60 : 10683-10686 (1999)*
    1999年 [査読有り][通常論文]
  • H Akera
    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS 38 1B 384 - 387 1999年01月 [査読有り][通常論文]
     
    The coherent transmission coefficient for an electron tunneling through a quantum dot is calculated and the effects of electron interactions at the dot are studied. The previous calculations for a dot with a single level are extended to a dot with two degenerate levels. Calculations of the transmission coefficient around a resonance peak are carried out using the noncrossing approximation at temperatures much higher than the level broadening. It is shown that the magnitude of the transmission coefficient decreases in the presence of interactions and the height and the line shape of the resonance peak in the two-level dot are very different from those in the single-level dot. The result is compared with the recent Aharonov-Bohm experiment and the phase coherency in the current through the dot is discussed.
  • H Akera
    SOLID-STATE ELECTRONICS 42 7-8 1379 - 1383 1998年07月 [査読有り][通常論文]
     
    The phase coherency in the current through a strongly correlated quantum dot is studied theoretically by calculating the amplitude and phase of the coherent transmission coefficient through a single level in the dot. The result is compared with the recent experiment in a four-terminal Aharonov-Bohm (AB) configuration. Another definition of the phase coherency, which is proportional to the transmission probability calculated from the coherent transmission coefficient, is introduced and shown to be different in magnitude from the one calculated from the AB conductance oscillations at temperatures much higher than the level broadening. (C) 1998 Elsevier Science Ltd. All rights reserved.
  • H Akera, T Kaneko
    PHYSICA E 1 1-4 180 - 184 1997年12月 [査読有り][通常論文]
     
    We study theoretically correlations of electrons and holes in unbalanced double-layer electronic systems in strong magnetic fields. Calculations are made using the exact diagonalization and the variational wave function. The ground state of an electron-hole pair in quantized cyclotron orbits possesses an in-plane electric dipole moment, when an electron and a hole are in different Landau orbits with different radii. The resulting attractive interactions between pairs creates the possibility of novel states. (C) 1997 Elsevier Science B.V. All rights reserved.
  • Akera, H. and Kaneko, T. : "Magnetoexcitons in Unbalanced Double Layers", Physica E, 1(1-4) : 180-184 (1997)*
    1997年 [査読有り][通常論文]
  • Akera, H. : "Aharonov-Bohm Effect in Quantum Dots", Jpn. J. Appl. Phys., 36(6) : 3974-3977 (1997)*
    1997年 [査読有り][通常論文]
  • Akera, H., MacDonald, A. H. and Yoshioka, D. : "Electron-Hole Chains in Unbalanced Double Layers", (Landwehr, G. and Ossau, W. : High Magnetic Fields in the Physics of Semiconductors II, 127-130, World Scientific, Singapore, 1997)*
    1997年 [査読有り][通常論文]
  • Tanaka, Y. and Akera, H. : "Self-consistent calculation of the charging energy in a quantum dot coupled with leads", J. Phys. Soc. Japan, 66(1) : 15-18 (1997)*
    1997年 [査読有り][通常論文]
  • Y Tanaka, H Akera
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 66 1 15 - 18 1997年01月 [査読有り][通常論文]
     
    A self-consistent calculation scheme in the Coulomb-blockade regime is formulated to obtain the charging energy required in adding a single electron to a quantum dot which is electrostatically coupled to leads and a gate electrode. This method is applied to the quantum-dot transistor fabricated recently from a double-barrier heterostructure, showing that the electrostatic coupling to leads significantly influences the charging energy of the dot in such devices when the leads are close to the dot and the electron density is high in the leads.
  • Y Tanaka, H Akera
    PHYSICAL REVIEW B 53 7 3901 - 3911 1996年02月 [査読有り][通常論文]
     
    Exact many-body eigenstates in a quantum dot formed in double-barrier heterostructures are calculated in the limit of strong confinement, and the nonlinear coherent transport through the states is studied for temperatures larger than their level broadenings. Energy splittings between many-body states due to exchange and correlation effects manifest themselves as small steps which decorate the Coulomb staircase in the current-voltage characteristics, which strongly depends on the number of electrons in the dot. Clear many-body effects are also found in peak heights and peak separations in the Coulomb oscillation of the linear conductance.
  • Y. Tanaka and H. Akera : "Many-body effects in transport through a quantum dot", Phys. Rev. B, 53(7) : 3901-3911 (1996)*
    1996年 [査読有り][通常論文]
  • MR NORMAN, H AKERA, AH MACDONALD
    PHYSICA C 196 1-2 43 - 47 1992年06月 [査読有り][通常論文]
     
    We have solved the microscopic mean-field equations for a BCS superconductor in a strong magnetic field. Calculations were Performed for fields both above and below the semiclassical upper critical field. In the low-field regime we find small quantum corrections to the results of semiclassical theory, whereas in the reentrant strong-field regime we find that the vortex lattice state changes qualitatively.
  • T TSUCHIYA, H AKERA, T ANDO
    PHYSICAL REVIEW B 39 9 6025 - 6033 1989年03月 [査読有り][通常論文]

その他活動・業績

共同研究・競争的資金等の研究課題

  • 日本学術振興会:科学研究費助成事業(基盤研究(C))
    研究期間 : 2017年04月 -2021年03月 
    代表者 : 明楽 浩史
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 2004年 -2007年 
    代表者 : 明楽 浩史, 鈴浦 秀勝
     
    1.量子ホール系の熱流体力学理論を局所平衡仮定のもとで非線形領域も含めて構築した。この理論では、電子温度と電気化学ポテンシャルの時空間変動を電子数保存則とエネルギー保存則の方程式で記述する。この理論を適用することにより、下記のそれぞれの系における電子温度と電気化学ポテンシャルの空間変動を計算した。2.圧縮性・非圧縮性帯領域をもつ量子ホール系における電子温度分布と電流分布電流に垂直の方向の電子温度分布と電流分布を、閉じこめポテンシャルが緩やかに空間変動し圧縮性・非圧縮性帯領域をもつ場合について、線形領域および非線形領域で計算した。熱流体力学の電子数保存およびエネルギー保存の方程式とポアッソン方程式を連立することにより閉じこめポテンシャルをセルフコンシステントに計算し、次のことを明らかにした。(1)線形応答領域では、格子温度が低い場合電子温度の空間変動が非圧縮性帯領域に集中する。電子温度分布は反対称性をもち、電子温度のピークおよびディップが現れる。(2)この線形応答領域での反対称分布を反映して、量子ホール効果のブレークダウン近傍の非線形領域において電流分布の非対称性が顕著になる。3.量子ホール系と電流電極の接合部分における電子温度分布とホットスポット電流電極の近傍における電子温度の空間変動を線形および非線形領域において計算した。高電流領域では両方の電流電極の近傍に電子温度のピーク...
  • 文部科学省:科学研究費補助金(基盤研究(C))
    研究期間 : 2001年 -2003年 
    代表者 : 明楽 浩史
     
    ブレークダウン臨界電流以上の非線形領域および線形応答領域において、電子温度の空間変動と電流分布に関する理論的研究を行った。[1]量子ホール系の流体力学方程式と電子温度の空間変動ブレークダウン臨界電流以上の非線形領域における巨視的空間変動を記述する流体力学方程式を微視的な電子過程から導出した。導出された方程式により、十分長い試料について幅方向に電子温度の非対称な空間変動が現れることが明らかになった。電流分布も非対称な空間変動を示すが、この電流分布は量子ホール効果における対称な分布とは顕著に異なる。[2]高電子温度領域における対角伝導率ランダウ準位内局在状態間のトンネル遷移による対角伝導率の理輪値は実験値より1桁ほど小さいことを示した。[3]線形応答領域における電子温度分布の量子振動試料両端の電子温度差が量子振動を示すこと、格子温度が十分低い場合には電子温度差の符号が量子振動を示すことを明らかにした。[4]量子ホール系の熱流体力学の理論量子ホール系の電子温度・電気化学ポテンシャルの時空間変動を記述する理論を局所平衡仮定のもとで非線形領域も含めて構築した。この理論は[1]で提案した方程式を改善・一般化したものである。[5]緩やかな閉じこめポテンシャルをもつ系のエッティングスハウゼン効果線形応答領域において、電子温度のずれがポテンシャルの勾配に沿って空間的に振動することを示した。[6...
  • 文部科学省:科学研究費補助金(重点領域研究, 特定領域研究(A), 重点領域研究)
    研究期間 : 1996年 -2000年 
    代表者 : 川村 清, 津田 穣, 上田 正仁, 塚田 捷, 勝本 信吾, 明楽 浩史, 樽茶 清悟, 津田 穣, 上田 正仁
     
    第1班の研究目的は、班員のこれまでの実績を活かして、単電子デバイスを将来の単一電子エレクトロニクスの中核として発展させるために克服すべき諸問題を、物理学の切り口で解明することである。量子ドットを内包する半導体トランジスタ素子,量子ドットが超伝導体である単電子トランジスタ構造を作製し、軌道状態もスピン状態も人工原子としての諸特性をもつことを実験理論の両面から確認した.また,常伝導電子も超伝導電子も位相が確定しているこを実験的に確認し,位相緩和の理論も作った.さらに、シリコンやダイヤモンド表面上に金属量子ドットを選択的に形成する可能性を理論的に示した。また表面上の原子尺度構造がどのような性質を示し、機能を発現するかについて理論予測をおこなった。微小多孔質アルミナ膜の微小細孔中に金属、カーボンナノチューブ等を成長させ、自発的に単一微小トンネル接合が形成されることを示した。また、半導体微結晶の電子状態、緩和機構,またGaAs/AlGaAsヘテロ接合上にドット配列構造の磁気分散モードを観測した。その他の基礎研究として、微小強磁性トンネル接合におけるクーロン・ブロッケードの基礎理論を作り、スピン制御素子の動作原理として注目されている。
  • 文部科学省:科学研究費補助金(奨励研究(A))
    研究期間 : 1995年 -1995年 
    代表者 : 明楽 浩史
     
    2次元電子系に垂直に強い磁場を加えると、1電子準位はランダウ準位に量子化され縮重するため多電子状態では電子相関が重要になり、基底ランダウ準位では分数量子ホール効果が観測されている。本研究では、高いランダウ準位に着目し、その波動関数の形が反映された新しい多電子状態の存在を理論的に数値計算を併用して明らかにすることを目指した。具体的な状況としては半導体ヘテロ構造に形成される非対称な2重量子井戸に強い磁場を垂直に加えた系を考え、1つめの井戸の基底ランダウ準位と2つめの井戸のn番目のランダウ準位がフェルミ準位で縮重する場合について調べた。まず、2つのランダウ準位における電子の数を等しくした場合には、ランダウ準位の充填率を適当な値にすると半径の大きい軌道の中に半径の小さい軌道がスッポリおさまる電子対の状態が実現することを少数系の厳密対角化の方法で示した。また、この系の基底状態の相図を井戸間の距離、ランダウ準位の充填率、2つめの井戸のランダウ指数nの関数として求め、電子対の状態が現れる領域を明らかにした。さらに、半径の大きい軌道の電荷の符号を反転して、n番目のランダウ準位のホールと基底ランダウ準位の電子が同数である場合を考えた。1つのホールと1つの電子だけが存在する場合には、ホールの輪状の電荷密度の上に電子が乗っている状態が基底状態でありこれは電気双極子を持つので、このような電子ホール対...
  • 文部科学省:科学研究費補助金(重点領域研究)
    研究期間 : 1993年 -1993年 
    代表者 : 明楽 浩史
     
    最近実験できるようになった半導体量子ドットの輸送現象について、電子相関がどのような効果をもたらすかを明らかにするため数値計算を併用した理論研究を行なった。特にここではGaAsとAlGaAsよりなる二重障壁構造を微細加工することによって得られる量子ドットを取り上げた。障壁に挾まれた円形の量子ドットにおいてポテンシャルは放物線型に近似することができ1電子状態は離散的な準位に分かれる。この2次元放物線型ポテンシャルにおける準位は軌道縮重をもち、電子相関が重要になると考えられる。計算の作業は大きく二つの部分からなる。まず第一は孤立した量子ドットの電子状態をハミルトニアンの厳密対角化の方法を用いて厳密に求めることであり、第二は量子ドットを通しての電流電圧特性をマスター方程式を解いて計算することである。特にここでは放物線型ポテンシャルの準位間隔が電子間のクーロン相互作用に比べて十分大きい場合を考え、各準位ごとに多体の状態を解いた。これまでに得られた計算結果より、電子相関による微細構造が電流電圧特性に現われることが見いだされ、電子数が少ない場合の特徴が明らかになりつつある。電子数が多い場合は従来のように帯電効果を取り入れた1電子描像が正しいと考えられるので、今後の研究の方向としてこの両極限の間での振る舞いについての研究が重要であると考えられる。また、強磁場中2次元系における電子相関の効果を...

教育活動情報

主要な担当授業

  • 大学院共通授業科目(一般科目):自然科学・応用科学
    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 大学院共通科目
    キーワード : ナノ構造、微細加工、デバイス、システム、光機能性材料、光化学、分子性物質、導電性、磁性、薄膜、電子顕微鏡、走査型プローブ顕微鏡、ナノワイヤー、二次元電子ガス、物性理論
  • 統計力学Ⅰ
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 等確率の原理、ゆらぎ、エントロピー、ミクロカノニカル分布、カノニカル分布、分配関数、ヘルムホルツの自由エネルギー、ギブスの自由エネルギー、エネルギー等分配則、熱力学第3法則、格子比熱、ボルツマン分布
  • 統計力学演習Ⅰ
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 等確率の原理、ゆらぎ、エントロピー、ミクロカノニカル分布、カノニカル分布、分配関数、ヘルムホルツの自由エネルギー、ギブスの自由エネルギー、エネルギー等分配則、熱力学第3法則、格子比熱、ボルツマン分布
  • 連続体力学
    開講年度 : 2021年
    課程区分 : 学士課程
    開講学部 : 工学部
    キーワード : 弾性体、流体、ひずみ、応力、連続方程式


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