MF研究者総覧

研究者データベース

詳細
浅野　泰寛(アサノ　ヤスヒロ)
工学研究院　応用物理学部門　量子物性工学分野
准教授
researchmap個人ページ
Last Updated :2023/11/02

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

  • 准教授

学位

  • 博士(工学）（名古屋大学）

ホームページURL

J-Global ID

研究キーワード

  • 超伝導   メゾスコピック物理   物性理論   トポロジー   スピン三重項超伝導   奇周波数超伝導   ジョセフソン接合   トポロジカル物質   

研究分野

  • 自然科学一般 / 磁性、超伝導、強相関系

担当教育組織

職歴

  • 2011年04月 - 現在 北海道大学 工学(系)研究科(研究院) 准教授

学歴

  • 1992年04月 - 1995年03月   名古屋大学   大学院工学研究科   応用物理学専攻

所属学協会

  • 日本物理学会   

研究活動情報

論文

  • Fulde-Ferrell-Larkin-Ovchinnikov state in a superconducting thin film attached to a ferromagnetic cluster
    S.-I Suzuki, T. Sato, A. A. Golubov, Y. Asano
    Physical Review B 108 064509(1) - 064509(9) 2023年07月 [査読有り]
  • Dakyeong Kim, Takumi Sato, Shingo Kobayashi, Yasuhiro Asano
    Journal of the Physical Society of Japan 92 5 054703(1) - 054703(9) 2023年05月15日 [査読有り]
  • Koki Maeno, Yuki Kawaguchi, Yasuhiro Asano, Shingo Kobayashi
    Physical Review B 107 6 064511(1) - 064511(17) 2023年02月27日 [査読有り]
  • Shu-Ichiro Suzuki, Takumi Sato, Yasuhiro Asano
    Physical Review B 106 10 104518(1) - 104518(13) 2022年09月29日 [査読有り][通常論文]
  • Dakyeong Kim, Shingo Kobayashi, Yasuhiro Asano
    Journal of the Physical Society of Japan 90 10 104708(1) - 104708(9) 2021年10月15日 [査読有り][通常論文]
  • Satoshi Ikegaya, Jaechul Lee, Andreas P. Schnyder, Yasuhiro Asano
    Physical Review B 104 2 L020502(1) - L020502(6) 2021年07月08日 [査読有り][通常論文]
  • Dakyeong Kim, Shingo Kobayashi, Yasuhiro Asano
    Physical Review B 103 18 184516(1) - 184516(9) 2021年05月27日 [査読有り][通常論文]
  • Jaechul Lee, Satoshi Ikegaya, Yasuhiro Asano
    Physical Review B 103 10 104509(1) - 104509(14) 2021年03月15日 [査読有り][通常論文]
  • Takumi Sato, Yasuhiro Asano
    Physical Review B 102 2 024516(1) - 024516(10) 2020年07月17日 [査読有り][通常論文]
  • S. Ikegaya, K. Yada, Y. Tanaka, S. Kashiwaya, Y. Asano, D. Manske
    Physical Review B 101 22 220501R(1) - 220501R(6) 2020年06月01日 [査読有り][通常論文]
  • Keimei Sakurai, Maria Teresa Mercaldo, Shingo Kobayashi, Ai Yamakage, Satoshi Ikegaya, Tetsuro Habe, Panagiotis Kotetes, Mario Cuoco, Yasuhiro Asano
    Physical Review B 101 17 174506(1) - 174506(19) 2020年05月08日 [査読有り][通常論文]
  • Akihiro Sasaki, Satoshi Ikegaya, Tetsuro Habe, Alexander A. Golubov, Yasuhiro Asano
    Physical Review B 101 18 184601(1) - 184601(8) 2020年05月04日 [査読有り][通常論文]
  • Rikizo Yano, Masao Koyanagi, Hiromi Kashiwaya, Kohei Tsumura, Hishiro T. Hirose, Takao Sasagawa, Yasuhiro Asano, Satoshi Kashiwaya
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 89 3 2020年03月 [査読有り][通常論文]
     
    The transport properties of a topological Josephson junction fabricated from a magnetically doped topological insulator (TI) were investigated. The conductance spectra of the Nb/Fe-Bi2Te2Se/Nb junction below 1 K showed an unusual trident-shaped zero-bias conductance peak with a tiny peak width of similar to 6 mu V. The central peak of the trident peak presents the dc-Josephson current, and the side peaks may reflect an induced unconventional Cooper pairing. Additionally. the critical currents followed inverse to temperature, which may also reflect the presence of an unconventional proximity effect. Furthermore, microwave irradiation drived a drastic change in the conductance spectra from the peak structure into oscillatory ones, a hallmark of the ac-Josephson supercurrent. The current-phase relation of the ac-Josephson effect under high power radiofrequency-irradiation was found to be 4 pi-periodic. The results suggest that the junction based on magnetically doped 3D TIs may realize an unconventional Cooper pairing, thus enabling access to the basic physics of Majorana bound states and unconventional superconductivity.
  • Taketoki Yamashita, Jaechul Lee, Tetsuro Habe, Yasuhiro Asano
    PHYSICAL REVIEW B 100 9 094501(1) - 094501(11) 2019年09月 [査読有り][通常論文]
     
    We study theoretically the proximity effect in a ferromagnetic semiconductor with the Rashba spin-orbit interaction. The interplay between the exchange potential and the spin-orbit interactions enriches the symmetry variety of Cooper pairs depending on degree of disorder in a ferromagnet. In the ballistic limit, spin-singlet s-wave Cooper pairs are the most dominant in the presence of strong spin-orbit interaction because the spin-momentum locking stabilizes a Cooper pair consisting of two electrons of time-reversal partner to each other. We will show that the spin-orbit interactions generate equal-spin-triplet p-wave pairs. In the dirty regime, on the other hand, equal-spin-triplet s-wave pairs are dominant because random impurity potentials release the locking. The exchange splitting in the conduction band causes the imbalance between two equal-spin pairing components. In a half-metallic ferromagnet, only an equal-spin pairing component survives and carries the spin-polarized supercurrent. We discuss the effects of the spin-orbit interaction on the Josephson current.
  • S. I.-Suzuki, A. A. Golubov, Y. Asano, Y. Tanaka
    Physical Review B 100 024511(1) - 024511(11) 2019年 [査読有り][通常論文]
  • S. Ikegaya, Y. Asano, D. Manske
    Physical Review Letters 123 207002(1) - 207002(6) 2019年 [査読有り][通常論文]
  • Yasuhiro Asano, Alexander A. Golubov
    Physical Review B 97 21 214508(1) - 214508(15) 2018年06月12日 [査読有り][通常論文]
     
    We study the effects of random nonmagnetic impurities on the superconducting transition temperature Tc in a two-band superconductor, where we assume an equal-time spin-singlet s-wave pair potential in each conduction band and the hybridization between the two bands as well as the band asymmetry. In the clean limit, the phase of hybridization determines the stability of two states, called s++ and s+-. The interband impurity scatterings decrease Tc of the two states exactly in the same manner when time-reversal symmetry is preserved in the Hamiltonian. We find that a superconductor with larger hybridization shows more moderate suppression of Tc. This effect can be explained by the presence of odd-frequency Cooper pairs, which are generated by the band hybridization in the clean limit and are broken by impurities.
  • Satoshi Ikegaya, Shingo Kobayashi, Yasuhiro Asano
    Physical Review B 97 17 174501(1) - 174501(9) 2018年05月01日 [査読有り][通常論文]
     
    We discuss the symmetry property of a nodal superconductor that hosts robust flat-band zero-energy states at its surface under potential disorder. Such robust zero-energy states are known to induce the anomalous proximity effect in a dirty normal metal attached to a superconductor. A recent study has shown that a topological index NZES describes the number of zero-energy states at the dirty surface of a p-wave superconductor. We generalize the theory to clarify the conditions required for a superconductor that enables NZES≠0. Our results show that NZES≠0 is realized in a topological material that belongs to either the BDI or CII class. We also present two realistic Hamiltonians that result in NZES≠0.
  • Yasuhiro Asano, Akihiro Sasaki, Alexander A. Golubov
    New Journal of Physics 20 4 043020(1) - 043020(9) 2018年04月01日 [査読有り][通常論文]
     
    We study theoretically the effects of random nonmagnetic impurities on the superconducting transition temperature Tc in a two-band superconductor characterized by an equal-time s-wave interband pairing order parameter. Because of the two-band degree of freedom, it is possible to define a spin-triplet s-wave pairing order parameter as well as a spin-singlet s-wave order parameter. The former belongs to odd-band-parity symmetry class, whereas the latter belongs to even-band-parity symmetry class. In a spin-singlet superconductor, Tc is insensitive to the impurity concentration when we estimate the self-energy due to the random impurity potential within the Born approximation. On the other hand in a spin-triplet superconductor, Tc decreases with the increase of the impurity concentration. We conclude that Cooper pairs belonging to odd-band-parity symmetry class are fragile under the random impurity potential even though they have s-wave pairing symmetry.
  • S. V. Bakurskiy, Ya. V. Fominov, A. F. Shevchun, Y. Asano, Y. Tanaka, M. Yu. Kupriyanov, A. A. Golubov, M. R. Trunin, H. Kashiwaya, S. Kashiwaya, Y. Maeno
    Physical Review B 98 134508(1) - 134508(16) 2018年 [査読有り][通常論文]
  • Keimei Sakurai, Satoshi Ikegaya, Yasuhiro Asano
    PHYSICAL REVIEW B 96 22 224514(1) - 224514(9) 2017年12月 [査読有り][通常論文]
     
    We theoretically study the Josephson current in a superconductor/quantum anomalous Hall insulator/superconductor junction by using the lattice Green function technique. When an in-plane external Zeeman field is applied to the quantum anomalous Hall insulator, the Josephson current J flows without a phase difference across the junction theta. The phase shift phi appearing in the current-phase relationship J alpha sin(theta - phi) is proportional to the amplitude of Zeeman fields and depends on the direction ofZeeman fields. Aphenomenological analysis of the Andreev reflection processes explains the physical origin of phi. In a quantum anomalous Hall insulator, time-reversal symmetry and mirror-reflection symmetry are broken simultaneously. However, magnetic mirror-reflection symmetry is preserved. Such characteristic symmetry properties enable us to have a tunable phi junction with a quantum Hall insulator.
  • Satoshi Ikegaya, Yasuhiro Asano
    PHYSICAL REVIEW B 95 21 214503(1) - 214503(11) 2017年06月 [査読有り][通常論文]
     
    We discuss the stability of highly degenerate zero-energy states that appear at the surface of a nodal superconductor preserving time-reversal symmetry. The existence of such surface states is a direct consequence of the nontrivial topological numbers defined in the restricted Brillouin zones in the clean limit. In experiments, however, potential disorder is inevitable near the surface of a real superconductor, which may lift the high degeneracy at zero energy. We show that an index defined in terms of the chiral eigenvalues of the zero-energy states can be used to measure the degree of degeneracy at zero energy in the presence of potential disorder. We also discuss the relationship between the index and the topological numbers.
  • Kohei Kawai, Keiji Yada, Yukio Tanaka, Yasuhiro Asano, Alexander A. Golubov, Satoshi Kashiwaya
    PHYSICAL REVIEW B 95 17 174518(1) - 174518(11) 2017年05月 [査読有り][通常論文]
     
    We study Josephson currents between s-wave/spin-triplet superconductor junctions by taking into account details of the band structures in Sr2RuO4, such as three conduction bands and spin-orbit interactions in the bulk and at the interface. We assume five superconducting order parameters in Sr2RuO4: a chiral p-wave symmetry and four helical p-wave symmetries. We calculate the current-phase relationship I (psi) in these junctions, where. is the macroscopic phase difference between the two superconductors. The results for a chiral p-wave pairing symmetry show that a cos(psi) term appears in the current-phase relation because of time-reversal symmetry (TRS) breaking. On the other hand, this cos(psi) term is absent in the helical pairing states that preserve TRS. We also study the dependence of the maximum Josephson current I-c on an external magnetic flux Phi in a corner junction. The calculated I-c (Phi) obeys I-c (Phi) not equal Ic (-Phi) in a chiral state and I-c (Phi) = I-c (-Phi) in a helical state. We calculate I-c (Phi) in a corner superconducting quantum interference device (SQUID) and a symmetric SQUID geometry. In the latter geometry, I-c (Phi) = I-c (-Phi) is satisfied for all the pairing states and it is impossible to distinguish a chiral state from a helical one. On the other hand, a corner SQUID always gives I-c (Phi) not equal I-c (-Phi) and Ic (Phi) = Ic (-Phi) for a chiral and a helical state, respectively. Experimental tests of these relations in corner junctions and SQUIDs may serve as a tool for unambiguously determining the pairing symmetry in Sr2RuO4.
  • Shu-Ichiro Suzuki, Yasuhiro Asano
    PHYSICAL REVIEW B 94 15 2016年10月 [査読有り][通常論文]
     
    We study theoretically the spontaneous edge current in a small chiral superconductor with surface roughness. We obtained self-consistent solutions of the pair potential and the vector potential by solving the quasiclassical Eilenberger equation and the Maxwell equation simultaneously. We then employed them to calculate numerically the spatial distribution of the chiral edge current in a small superconductor. The characteristic behavior of the spontaneous edge current depends strongly on the symmetries of the order parameters such as chiral p-, chiral d-, and chiral f -wave pairing. The edge current is robust under the surface roughness in the chiral p- and chiral d- wave superconductors. In the chiral d- wave case, the surface roughness tends to flip the direction of the chiral current. On the other hand, the edge current in a chiral f -wave superconductor is fragile when there is surface roughness. We also discuss the temperature dependence of a spontaneous magnetization, which is a measurable value in standard experiments.
  • S. Ikegaya, Y. Asano
    JOURNAL OF PHYSICS-CONDENSED MATTER 28 37 375702(1) - 375702(6) 2016年09月 [査読有り][通常論文]
     
    We theoretically study the stability of more than one Majorana fermion appearing in a p-wave superconductor/dirty normal metal/p-wave superconductor junction in two-dimensions by using the chiral symmetry of a Hamiltonian. At the phase difference across the junction. being pi, we will show that all of the Majorana bound states in the normal metal belong to the same chirality. Due to this pure chiral feature, the Majorana bound states retain their high degree of degeneracy at zero energy even in the presence of a random potential. As a consequence, the resonant transmission of a Cooper pair via the degenerate Majorana bound states carries the Josephson current at phi = pi - 0(+), which explains the fractional current-phase relationship discussed in a number of previous papers.
  • Satoshi Ikegaya, Shu-Ichiro Suzuki, Yukio Tanaka, Yasuhiro Asano
    PHYSICAL REVIEW B 94 5 2016年08月 [査読有り][通常論文]
     
    We discuss the minimum value of the zero-bias differential conductance G(min) in a junction consisting of a normal metal and a nodal superconductor preserving time-reversal symmetry. Using the quasiclassical Green function method, we show that G(min) is quantized at (4e(2)/h)N-ZES in the limit of strong impurity scatterings in the normal metal at the zero temperature. The integer N-ZES represents the number of perfect transmission channels through the junction. An analysis of the chiral symmetry of the Hamiltonian indicates that N-ZES corresponds to the Atiyah-Singer index in mathematics.
  • Bo Lu, Pablo Burset, Yasunari Tanuma, Alexander A. Golubov, Yasuhiro Asano, Yukio Tanaka
    PHYSICAL REVIEW B 94 1 2016年07月 [査読有り][通常論文]
     
    We study the influence of nonmagnetic impurity scatterings on the tunneling conductance of a junction consisting of a normal metal and a disordered unconventional superconductor by solving the quasiclassical Eilenberger equation self-consistently. We find that the impurity scatterings in both the Born and the unitary limits affect the formation of the Andreev bound states and modify strongly the tunneling spectra around zero bias. Our results are interpreted well by the appearance of odd-frequency Cooper pairs near the interface and by the divergent behavior of the impurity self-energy. The present paper provides a useful tool to identify the pairing symmetry of unconventional superconductors in experiments.
  • Pablo Burset, Bo Lu, Hiromi Ebisu, Yasuhiro Asano, Yukio Tanaka
    PHYSICAL REVIEW B 93 20 2016年05月 [査読有り][通常論文]
     
    We propose an all-electrical experimental setup to detect and manipulate the amplitude of odd-frequency pairing in a double quantum dot. The odd-frequency pair amplitude is induced from the breakdown of orbital symmetry when Cooper pairs are injected in the double dot with electrons in different dots. When the dot levels are aligned with the Fermi energy, i.e., on resonance, nonlocal Andreev processes are directly connected to the presence of odd-frequency pairing. Therefore, their amplitude can be manipulated by tuning the level positions. The detection of nonlocal Andreev processes by conductance measurements contributes a direct proof of the existence of the odd-frequency pair amplitude and is available using current experimental techniques.
  • 鈴木 修, 浅野 泰寛
    日本物理学会講演概要集 71 1769 - 1769 一般社団法人日本物理学会 2016年
  • 池谷 聡, 浅野 泰寛
    日本物理学会講演概要集 71 1377 - 1377 一般社団法人日本物理学会 2016年
  • 鈴木 修, 浅野 泰寛
    日本物理学会講演概要集 71 1449 - 1449 一般社団法人 日本物理学会 2016年 

    本研究ではEilenberger方程式とMaxwell方程式を連立させて解く事により、微小な異方的超伝導体の基底状態を調べた。微小な超伝導体の表面には、奇周波数クーパー対が現れる。これら奇周波数クーパー対の磁気不安定性に起因して、微小な超伝導体は、低温で磁化する可能性があることが解った。講演では、磁化する過程と奇周波数クーパー対の役割について議論する。

  • S.-I. Suzuki, Y. Asano
    Physical Review B 94 15 155302(1) - 155302(11) 2016年 [査読有り][通常論文]
  • S. Ikegaya, S.-I. Suzuki, Y. Tanaka, Y. Asano
    Physical Review B 94 5 054512(1) - 054512(6) 2016年 [査読有り][通常論文]
  • B. Lu, P. Burset, Y. Tanuma, A. A. Golubov, Y. Asano, Y. Tanaka
    Physical Review B 94 1 014504(1) - 014504(9) 2016年 [査読有り][通常論文]
  • P. Burset, B. Lu, H. Ebisu, Y. Asano, Y. Tanaka
    Physical Review B 93 20 201402R(1) - 201402R(6) 2016年 [査読有り][通常論文]
  • Yasuhiro Asano, Akihiro Sasaki
    PHYSICAL REVIEW B 92 22 224508(1) - 224508(11) 2015年12月 [査読有り][通常論文]
     
    We discuss the appearance of odd-frequency Cooper pairs in two-band superconductors by solving the Gor'kov equation analytically. We introduce the equal-time s-wave pair potentials as realized in MgB2 and iron pnictides. Although the order parameter symmetry is conventional, the band degree of freedom enriches the symmetry variety of pairing correlations. The hybridization and the asymmetry between the two conduction bands induce odd-frequency pairs as a subdominant pairing correlation in the uniform ground state. To study the magnetic response of odd-frequency Cooper pairs, we analyze the Meissner kernel represented by the Gor'kov Green function. In contrast to the even-frequency pairs linked to the pair potential, the induced odd-frequency Cooper pairs indicate a paramagnetic property. We also discuss the relation between the amplitude of the odd-frequency pairing correlation and the stability of superconducting states in terms of the self-consistent equation for the pair potential.
  • Kohta Saitoh, Satoshi Kashiwaya, Hiromi Kashiwaya, Yasunori Mawatari, Yasuhiro Asano, Yukio Tanaka, Yoshiteru Maeno
    PHYSICAL REVIEW B 92 10 100504(1) - 100504(6) 2015年09月 [査読有り][通常論文]
     
    Clarifying the chiral domain structure of superconducting Sr2RuO4 has been a long-standing issue in identifying its peculiar topological superconducting state. We evaluated the critical current I-c versus the magnetic field H of Nb/Sr2RuO4 Josephson junctions, changing the junction dimension in expectation of that the number of domains in the junction is controlled. I-c(H) exhibits a recovery from inversion symmetry breaking to invariance when the dimension is reduced to several microns. This inversion invariant behavior indicates the disappearance of domain walls; thus, the size of a single domain is estimated at approximately several microns.
  • S. S. Mou, H. Irie, Y. Asano, K. Akahane, H. Nakajima, H. Kumano, M. Sasaki, A. Murayama, I. Suemune
    JOURNAL OF APPLIED PHYSICS 118 7 073102(1) - 073102(7) 2015年08月 [査読有り][通常論文]
     
    We studied InAs quantum dots (QDs) where electron Cooper pairs penetrate from an adjacent niobium (Nb) superconductor with the proximity effect. With time-resolved luminescence measurements at the wavelength around 1550 nm, we observed luminescence enhancement and reduction of luminescence decay time constants at temperature below the superconducting critical temperature (T-C) of Nb. On the basis of these measurements, we propose a method to determine the contribution of Cooper-pair recombination in InAs QDs. We show that the luminescence enhancement measured below T-C is well explained with our theory including Cooper-pair recombination. (C) 2015 AIP Publishing LLC.
  • M. Snelder, A. A. Golubov, Y. Asano, A. Brinkman
    JOURNAL OF PHYSICS-CONDENSED MATTER 27 31 315701(1) - 315701(9) 2015年08月 [査読有り][通常論文]
     
    To guide experimental work on the search for Majorana zero-energy modes, we calculate the superconducting pairing symmetry of a three-dimensional topological insulator in combination with an s-wave superconductor. We show how the pairing symmetry changes across different topological regimes. We demonstrate that a dominant p-wave pairing relation is not sufficient to realise a Majorana zero-energy mode useful for quantum computation. Our main result is the relation between odd-frequency pairing and Majorana zero energy modes by using Green functions techniques in three-dimensional topological insulators in the so-called Majorana regime. We discuss thereafter how the pairing relations in the different regimes can be observed in the tunneling conductance of an s-wave proximised three-dimensional topological insulator. We discuss the necessity to incorporate a ferromagnetic insulator to localise the zero-energy bound state to the interface as a Majorana mode.
  • S. S. Mou, H. Irie, Y. Asano, K. Akahane, H. Nakajima, H. Kumano, M. Sasaki, A. Murayama, I. Suemune
    PHYSICAL REVIEW B 92 3 035308(1) - 035308(9) 2015年07月 [査読有り][通常論文]
     
    We study luminescence spectra observed from InAs quantum dots (QDs) embedded in an n-type InGaAs-based heterostructure, where electron Cooper pairs penetrate from an adjacent niobium (Nb) superconductor with the proximity effect. Below the superconducting (SC) critical temperature of Nb, we observe substantial luminescence intensity enhancement and a sharp edge in luminescence spectra of InAs QDs. We explain the observed sharp edge in the luminescence spectra with the proximity effect, that is, with the consideration of opening of SC gap and modification of density of states (DOS) near the electron Fermi level in the n-type semiconductor heterostructure. We demonstrate that the sharp edge luminescence spectra are well reproduced by the SC DOS, with quasiparticle lifetime broadening and a Gaussian distribution of lowest QD state emission lines. We discuss the reason why it has been difficult to observe the sharp edge luminescence spectra in the previous quantum well-based SC light emitting diodes.
  • Shu-Ichiro Suzuki, Yasuhiro Asano
    PHYSICAL REVIEW B 91 21 214510(1) - 214510(10) 2015年06月 [査読有り][通常論文]
     
    We theoretically study the effects of surface roughness on the magnetic response of small unconventional superconductors by solving the Eilenberger equation for the quassiclassical Green function and the Maxwell equation for the vector potential simultaneously and self-consistently. The paramagnetic phase of spin-singlet d-wave superconducting disks is greatly suppressed by the surface roughness, whereas that of spin-triplet p-wave disks is robust even in the presence of the roughness. This difference derives from the orbital symmetry of paramagnetic odd-frequency Cooper pairs appearing at the surface of the disks. The orbital part of the paramagnetic pairing correlation has p-wave symmetry in the d-wave disks, whereas it has s-wave symmetry in the p-wave disks. Calculating the free energy, we also confirm that the paramagnetic state is more stable than the normal state, which indicates a possibility of detecting the paramagnetic effect in experiments. Indeed, our results are consistent with an experimental finding on high-T-c thin films.
  • Satoshi Ikegaya, Yasuhiro Asano, Yukio Tanaka
    PHYSICAL REVIEW B 91 17 174511(1) - 174511(6) 2015年05月 [査読有り][通常論文]
     
    We discuss the stability of zero-energy states appearing in a dirty normal metal attached to a superconducting thin film with Dresselhaus [110] spin-orbit coupling under an in-plane Zeeman field. The Dresselhaus superconductor preserves an additional chiral symmetry and traps more than one zero-energy state at its edges. All the zero-energy states at an edge belong to the same chirality in large Zeeman fields due to the effective p-wave pairing symmetry. The pure chiral nature of the wave function enables the zero-energy states to penetrate the dirty normal metal while retaining their high degree of degeneracy. We prove the perfect Andreev reflection into the dirty normal metal at zero energy.
  • Ya. V. Fominov, Y. Tanaka, Y. Asano, M. Eschrig
    PHYSICAL REVIEW B 91 14 144514(1) - 144514(14) 2015年04月 [査読有り][通常論文]
     
    We consider physical properties of a superconductor with a recently proposed type of odd-frequency pairing that exhibits diamagnetic Meissner response ("odd-dia state"). Such a state was suggested in order to address stability issues arising in an odd-frequency superconducting state with paramagnetic Meissner response ("odd-para state"). Assuming the existence of an odd-dia state (due to a proper retarded interaction), we study its coexistence with an odd-para state. The latter is known to be generated as an induced superconducting component in, e.g., singlet superconductor/ferromagnet proximity structures or triplet superconductor/normal metal systems. Calculating the superfluid density of the mixed odd-para/odd-dia state and the Josephson current between the odd-para and odd-dia states, we find that the expressions for the currents in both cases have nonvanishing imaginary contributions and are therefore unphysical. We show that a realization of the odd-dia state implies the absence of a Hamiltonian description of the system, and suggest that there exists no physically realizable perturbation that could give rise to the spontaneous symmetry breaking necessary for an actual realization of the odd-dia superconducting state.
  • 斎藤 広大, 柏谷 聡, 柏谷 裕美, 小柳 正男, 馬渡 康徳, 浅野 泰寛, 田仲 由喜夫, 前野 悦輝
    日本物理学会講演概要集 70 2020 - 2020 一般社団法人日本物理学会 2015年
  • 池谷 聡, 浅野 泰寛
    日本物理学会講演概要集 70 1361 - 1361 一般社団法人日本物理学会 2015年
  • 鈴木 修, 浅野 泰寛
    日本物理学会講演概要集 70 1684 - 1684 一般社団法人日本物理学会 2015年
  • Yasuhiro Asano, Yakov V. Fominov, Yukio Tanaka
    PHYSICAL REVIEW B 90 9 094512(1) - 094512(8) 2014年09月 [査読有り][通常論文]
     
    We analyze symmetries and magnetic properties of Cooper pairs appearing as subdominant pairing correlations in inhomogeneous superconductors, on the basis of the quasiclassical Green-function theory. The frequency symmetry, parity, and the type of magnetic response of such subdominant correlations are opposite to those of the dominant pairing correlations in the bulk state. Our conclusion is valid even when we generalize the theory of superconductivity to recently proposed diamagnetic odd-frequency superconductors. As a consequence, Cooper pairs are classified into eight classes in terms of their symmetries and magnetic properties. Anomalous magnetic properties of subdominant components can be probed by studying the Meissner effect.
  • Shuhei Uchida, Tetsuro Habe, Yasuhiro Asano
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 83 6 064711(1) - 064711(6) 2014年06月 [査読有り][通常論文]
     
    We theoretically study low energy electric transport in a junction consisting of a Weyl semimetal and a metallic superconductor. The characteristic features of the differential conductance depend on the relative directions between the current and the vector connecting the two Weyl points. When the electric current is perpendicular to the vector, the conductance spectra are sensitive to the direction and the amplitude of magnetic moment at the junction interface. This is a direct consequence of the chiral spin configuration on the fermi surface near the Weyl points.
    ダウンロード
  • Shu-Ichiro Suzuki, Yasuhiro Asano
    PHYSICAL REVIEW B 89 18 184508(1) - 184508(7) 2014年05月 [査読有り][通常論文]
     
    The diamagnetism is an essential property of all superconductors. However, we will show that small topological (or unconventional) superconductors can be intrinsically paramagnetic by solving the quasiclassical Eilenberger equation and the Max well equation self-consistently on two-dimensional superconducting disks in weak magnetic fields. Because of the topologically nontrivial character of the wave function, the unconventional superconductors host the zero-energy surface Andreev bound states, which always accompany so-called odd-frequency Cooper pairs. The paramagnetic property of the odd-frequency pairs explains the paramagnetic response of the disks at low temperature.
    ダウンロード
  • Tetsuro Habe, Yasuhiro Asano
    PHYSICAL REVIEW B 89 11 115203(1) - 115203(5) 2014年03月 [査読有り][通常論文]
     
    We discuss topological electronic states described by the Dirac Hamiltonian plus an additional one in three dimensions. When the additional Hamiltonian is an element of an Abelian group, electronic states become topologically nontrivial even in the absence of the fundamental symmetries such as the time-reversal symmetry and the particle-hole one. Such symmetry-breaking topological states are characterized by the Chern number defined in the two-dimensional partial Brillouin zone. The topological insulators in Zeeman fields are an example of the symmetry-breaking topological electric state. We show the crossover from the topological insulating phase to the topological semimetal one in strong Zeeman fields.
  • Jeongmin Yoo, Tetsuro Habe, Yasuhiro Asano
    PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES 55 48 - 54 2014年01月 [査読有り][招待有り]
     
    We discuss Andreev bound states appearing at the interface between two different superconductors characterized by different nontrivial topological numbers such as one-dimensional winding numbers and Chern numbers. The one-dimensional winding number characterizes d(xy) and p(x) wave superconductors. The Chern number characterizes chiral superconductors. The number of interfacial bound states at the zero-energy is equal to the difference between the topological numbers on either sides of the Josephson junction. We also discuss relation between properties of the Andreev bound states at the zero-energy and features of Josephson current at low temperature. (C) 2013 Elsevier B.V. All rights reserved.
    ダウンロード
  • Shiro Kawabata, Yasuhiro Asano, Yukio Tanaka, Alexander A. Golubov
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 82 12 124702(1) - 124702(9) 2013年12月 [査読有り][通常論文]
     
    We have investigated the proximity effect in superconductor/ferromagnet junctions in a systematic manner to discuss the relationship between the zero-energy peak (ZEP) of the local density of states (LDOS) and spin-triplet odd-frequency pairing. By exactly solving the nonlinear Usadel equations, we have found that the ZEP is realized in a wide range of geometrical and material parameters in the case of the noncollinear magnetization. This strongly suggests the robustness of the ZEP induced by spin-triplet odd-frequency pairing in such systems. We also found that the crossover from singlet pairing to triplet pairing can be detected by measuring the F layer thickness dependence of the ZEP height. Furthermore, we show how to observe signatures of spin-triplet odd-frequency pairing and the pairing crossover by LDOS measurement. Our results provide a direct way to experimentally detect signatures of the odd-frequency pairing state.
    ダウンロード
  • Tetsuro Habe, Yasuhiro Asano
    PHYSICAL REVIEW B 88 15 155442(1) - 155442(12) 2013年10月 [査読有り][通常論文]
     
    We theoretically study subgap states appearing at the interface between two three-dimensional topological insulators which have different configurations in the spin-orbit interactions from each other. The coupling of spin sigma with momenta p is configured by a material dependent 3 x 3 matrix A as sigma(mu) A(mu)(nu)P(nu). The spectra of the interface subgap states depend on the relative choices of A's in the two topological insulators where the two A are connected by the unitary transformation including the inversion and the rotation in momentum space. The gapless states appear at the interface when the transformation includes the inversion. The two topological insulators can be distinguished by using the topological numbers defined in the two- or one-dimensional partial Brillouin zone, which explains the presence of the gapless interface states. We also discuss the robustness of such gapless states under perturbations breaking the time-reversal symmetry or the band-inversion symmetry.
    ダウンロード
  • P. M.R. Brydon, Wei Chen, Yasuhiro Asano, Dirk Manske
    Physical Review B - Condensed Matter and Materials Physics 88 5 054509(1) - 054509(13) 2013年08月20日 [査読有り][通常論文]
     
    We study the Josephson effect in a triplet superconductor-ferromagnet- singlet superconductor junction. We show that the interaction of tunneling Cooper pairs with the interface magnetization can permit a Josephson current at the lowest order of a tunneling Hamiltonian perturbation theory. Two conditions must be satisfied for this to occur: The magnetization of the ferromagnet has a component parallel to the d vector of the triplet superconductor, and the gaps of the superconductors have the same parity with respect to the interface momentum. The resulting charge current displays an unconventional dependence on the orientation of the magnetic moment and the phase difference. This is accompanied by a phase-dependent spin current in the triplet superconductor, while a phase-independent spin current is always present. The tunneling perturbation theory predictions are confirmed using a numerical Green's function method. An analytical treatment of a one-dimensional junction demonstrates that our conclusions are robust far away from the tunneling regime and reveals signatures of the unconventional Josephson effect in the critical currents. © 2013 American Physical Society.
    ダウンロード
  • P. M. R. Brydon, Wei Chen, Yasuhiro Asano, Dirk Manske
    PHYSICAL REVIEW B 88 5 2013年08月 [査読有り][通常論文]
     
    We study the Josephson effect in a triplet superconductor-ferromagnet-singlet superconductor junction. We show that the interaction of tunneling Cooper pairs with the interface magnetization can permit a Josephson current at the lowest order of a tunneling Hamiltonian perturbation theory. Two conditions must be satisfied for this to occur: The magnetization of the ferromagnet has a component parallel to the d vector of the triplet superconductor, and the gaps of the superconductors have the same parity with respect to the interface momentum. The resulting charge current displays an unconventional dependence on the orientation of the magnetic moment and the phase difference. This is accompanied by a phase-dependent spin current in the triplet superconductor, while a phase-independent spin current is always present. The tunneling perturbation theory predictions are confirmed using a numerical Green's function method. An analytical treatment of a one-dimensional junction demonstrates that our conclusions are robust far away from the tunneling regime and reveals signatures of the unconventional Josephson effect in the critical currents.
  • Tetsuro Habe, Yasuhiro Asano
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 82 6 064704(1) - 064704(5) 2013年06月 [査読有り][通常論文]
     
    . We discuss the excitation spectra around the Dirac node on a surface of a three-dimensional topological insulator. By using the diagrammatic expansion, we show that the coupling of an electron with the gauge field in the presence of impurity scatterings opens a gap around the Dirac node. The results are consistent with a recent experimental finding by Sato et al. [Nat. Phys. 7 (2011) 840]. We also discuss the consistency between the present results and the bulk-boundary correspondence of the topological insulator in the presence of time reversal symmetry. The conclusion can be applied to any two-dimensional massless fermions.
    ダウンロード
  • Yasuhiro Asano, Yukio Tanaka
    PHYSICAL REVIEW B 87 10 104513(1) - 104513(10) 2013年03月 [査読有り][通常論文]
     
    We discuss a strong relationship between Majorana fermions and odd-frequency Cooper pairs which appear at a disordered normal (N) nanowire attached to a topologically nontrivial superconducting (S) one. The transport properties in superconducting nanowire junctions show universal behaviors irrespective of the degree of disorder: the quantized zero-bias differential conductance at 2e(2)/h in NS junction and the fractional current-phase (J-phi) relationship of the Josephson effect in SNS junction J alpha sin(phi/2). Such behaviors are exactly the same as those found in the anomalous proximity effect of odd-parity spin-triplet superconductors. We show that the odd-frequency pairs exist wherever the Majorana fermions stay. DOI: 10.1103/PhysRevB.87.104513
    ダウンロード
  • Yasuhiro Asano, Masahiro Ozaki, Tetsuro Habe, Alexander A. Golubov, Yukio Tanaka
    PHYSICAL REVIEW B 86 2 024510  2012年07月 [査読有り][通常論文]
     
    We discuss the surface impedance (Z = R - iX) of a normal-metal/superconductor proximity structure taking into account the spin-dependent potential at the junction interface. Because of the spin-mixing transport at the interface, odd-frequency spin-triplet s-waveCooper pairs penetrate into the normal metal and cause the anomalous response to electromagnetic fields. At low temperature, the local impedance at a surface of the normal metal shows the nonmonotonic temperature dependence and the anomalous relation R > X. We also discuss a possibility of observing such anomalous impedance in experiments.
  • Tetsuro Habe, Yasuhiro Asano
    Physical Review B - Condensed Matter and Materials Physics 85 19 195325.1-195325.8  2012年05月29日 [査読有り][通常論文]
     
    We study electronic structures at an interface between a topological insulator and a ferromagnetic insulator by using a three-dimensional two-band model. In usual ferromagnetic insulators, the exchange potential is much larger than the bulk gap size in the topological insulators and electronic structures are asymmetric with respect to the Fermi level. In such situation, we show that unusual metallic states appear under the magnetic moment pointing the perpendicular direction to the junction plane, which cannot be described by the two-dimensional effective model around the Dirac point. When the magnetic moment is in the parallel direction to the plane, the number of Dirac cones becomes even integers. The conclusions obtained in analytical calculations are confirmed by numerical simulations on tight-binding lattice. © 2012 American Physical Society.
  • Tetsuro Habe, Yasuhiro Asano
    PHYSICAL REVIEW B 85 19 195325  2012年05月 [査読有り][通常論文]
     
    We study electronic structures at an interface between a topological insulator and a ferromagnetic insulator by using a three-dimensional two-band model. In usual ferromagnetic insulators, the exchange potential is much larger than the bulk gap size in the topological insulators and electronic structures are asymmetric with respect to the Fermi level. In such situation, we show that unusual metallic states appear under the magnetic moment pointing the perpendicular direction to the junction plane, which cannot be described by the two-dimensional effective model around the Dirac point. When the magnetic moment is in the parallel direction to the plane, the number of Dirac cones becomes even integers. The conclusions obtained in analytical calculations are confirmed by numerical simulations on tight-binding lattice.
  • Ikuo Suemune, Hirotaka Sasakura, Yasuhiro Asano, Hidekazu Kumano, Ryotaro Inoue, Kazunori Tanaka, Tatsushi Akazaki, Hideaki Takayanagi
    IEICE ELECTRONICS EXPRESS 9 14 1184 - 1200 2012年 [査読有り][通常論文]
     
    Superconductivity and optoelectronics have developed almost independently and had very rare interactions with each other in science and technologies. However recent interdisciplinary research opens up the potential for developing new optoelectronic devices. This review paper presents our recent theoretical and experimental demonstrations that superconductivity significantly modifies and accelerates photon generation processes. We have prepared superconducting light emitting diodes (LEDs) emitting at similar to 1.6-mu m optical-fiber communication band for the experimental demonstrations. This new-type LED operation is based on a unique physics related to Cooper-pair interband transition in a semiconductor, and further research leads to the solid-state simultaneously generated photon-pair sources for the potential application in quantum information and communication.
    ダウンロード
  • Daisuke Yoshizaki, Alexander A. Golubov, Yukio Tanaka, Yasuhiro Asano
    JAPANESE JOURNAL OF APPLIED PHYSICS 51 1 010108.1-010108.9  2012年01月 [査読有り][通常論文]
     
    We study the proximity effect and charge transport in ferromagnet (F)/superconductor (S) and S/F/I/F/S junctions (where I is insulator) by taking into account simultaneously exchange field in F and spin-dependent interfacial phase shifts (SDIPS) at the F/S interface. We solve the Usadel equations using extended Kupriyanov-Lukichev boundary conditions which include SDIPS, where spin-independent part of tunneling conductance G(T) and spin-dependent one G(phi) coexist. The resulting local density of states (LDOS) in a ferromagnet depends both on the exchange energy E-ex and G(phi)/G(T). We show that the magnitude of zero-temperature gap and the height of zero-energy LDOS have a non-monotonic dependence on G(phi)/G(T). We also calculate Josephson current in S/F/I/F/S junctions and show that crossover from 0-state to pi. (C) 2012 The Japan Society of Applied Physics
  • Ikuo Suemune, Hirotaka Sasakura, Yujiro Hayashi, Kazunori Tanaka, Tatsushi Akazaki, Yasuhiro Asano, Ryotaro Inoue, Hideaki Takayanagi, Eiichi Hanamura, Jae-Hoon Huh, Claus Hermannstaedter, Satoru Odashima, Hidekazu Kumano
    JAPANESE JOURNAL OF APPLIED PHYSICS 51 1 010114.1-010114.10  2012年01月 [査読有り][通常論文]
     
    The injection of Cooper pairs into a normal medium such as a semiconductor is known as the proximity effect at the superconductor/normal interface. We confirm this injection as well as the contribution of Cooper pairs to a drastic enhancement of inter-band optical transitions in semiconductor heterostructures. In this paper we investigate and clarify the relation of internal quantum efficiencies and radiative lifetimes in Cooper-pair light emitting diodes (CP-LEDs). A quantitative description of the dynamic photon generation processes is given, and the contribution of the Cooper-pair recombination relative to normal-electron recombination in CP-LEDs is discussed in detail. (C) 2012 The Japan Society of Applied Physics
  • I. Suemune, H. Sasakura, Y. Hayashi, K. Tanaka, T. Akazaki, Y. Asano, R. Inoue, H. Takayanagi, E. Hanamura, J.-H. Huh, C. Hermannstädter, S. Odashima, H. Kumano
    Jpn. J. App. Phys. 51 010114  2012年 [査読有り][通常論文]
  • H. Sasakura, S. Kuramitsu, Y. Hayashi, K. Tanaka, T. Akazaki, E. Hanamura, R. Inoue, H. Takayanagi, Y. Asano, C. Hermannstaedter, H. Kumano, I. Suemune
    PHYSICAL REVIEW LETTERS 107 15 157403  2011年10月 [査読有り][通常論文]
     
    We experimentally demonstrate Cooper pairs' drastic enhancement of the band-to-band radiative recombination rate in a semiconductor. Electron Cooper pairs injected from a superconducting electrode into an active layer by the proximity effect recombine with holes injected from a p-type electrode. The recombination of a Cooper pair with p-type carriers dramatically increases the photon generation probability of a light-emitting diode in the optical-fiber communication band. The measured radiative decay time rapidly decreases with decreasing temperature below the superconducting transition temperature of the niobium electrodes. Our results indicate the possibility to open up new interdisciplinary fields between superconductivity and optoelectronics.
  • Yasuhiro Asano, Satoshi Yamano
    PHYSICAL REVIEW B 84 6 064526  2011年08月 [査読有り][通常論文]
     
    We discuss the Josephson current between two noncentrosymmetric superconductors. The coexistence of superconducting order parameters between the spin-singlet Delta(S) and the helical p-wave spin-triplet Delta(T) enriches a variety of low-temperature behavior of the Josephson current depending on their relative amplitudes. We will show that characteristic behaviors of the Josephson current for Delta(S) > Delta(T) are clearly different from those for Delta(S) < Delta(T). The topologically protected zero-energysurface bound states are responsible for the clear difference. We conclude that the Josephson current reflects well the character of the topological surface states and the pairing symmetry of noncentrosymmetric superconductors.
  • Yasuhiro Asano, Alexander A. Golubov, Yakov V. Fominov, Yukio Tanaka
    PHYSICAL REVIEW LETTERS 107 8 087001  2011年08月 [査読有り][通常論文]
     
    We discuss the dynamic response of odd-frequency Cooper pairs to an electromagnetic field. By using the quasiclassical Green function method, we calculate the impedance (Z = R - iX) of a normal-metal thin film which covers a superconductor. In contrast with the standard relation ( i.e., R << X), the impedance in spin-triplet proximity structures shows anomalous behavior ( i.e., R > X) in the low frequency limit. This unusual relation is a result of the penetration of odd-frequency pairs into the normal metal and reflects the negative Cooper pair density.
  • P. M. R. Brydon, Yasuhiro Asano, Carsten Timm
    PHYSICAL REVIEW B 83 18 180504  2011年05月 [査読有り][通常論文]
     
    A thin ferromagnetic layer on a bulk equal-spin-pairing triplet superconductor is shown to mediate a Josephson coupling between the spin up arrow and down arrow condensates of the superconductor. By deriving analytic expressions for the bound states at the triplet superconductor-ferromagnet interface, we show that this spin Josephson effect establishes an effective anisotropy axis in the ferromagnetic layer. The associated Josephson spin current is predicted to cause a measurable precession of the magnetization about the vector order parameter of the triplet superconductor.
  • Shiro Kawabata, Yasuhiro Asano
    LOW TEMPERATURE PHYSICS 36 10-11 915 - 919 2010年10月 [査読無し][通常論文]
     
    We investigate Josephson transport through ferromagnetic insulators (FIs) taking the band structure of the FIs explicitly into account. Using a recursive Green function method, we find that a pi-junction is formed in this type of system. Moreover, an atomic-scale 0-pi oscillation is induced by increasing the thickness of an FI and its oscillation period is universal, exactly equal to a single atomic layer. Based on these results, we show that a stable pi-state can be realized in junctions based on high-T-c superconductors with a La2BaCuO5 barrier. Such FI-based Josephson junctions may become an element in the architecture of future quantum computers. (C) 2010 American Institute of Physics. [doi:10.1063/1.3515524]
  • Hiroshi Kambara, Tetsuro Matsumoto, Hiromi Kashiwaya, Satoshi Kashiwaya, Hiroshi Yaguchi, Yasuhiro Asano, Yukio Tanaka, Yoshiteru Maeno
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 79 7 074708.1-074708.5  2010年07月 [査読有り][通常論文]
     
    To search for new evidence of the chiral p-wave superconducting domain in Sr2RuO4, we investigated the unconventional local transport characteristics of a microfabricated Sr2RuO4-Ru eutectic junction. We found that the anomalous hysteresis in voltage-current characteristics [as reported in Kambara et al.: Phys. Rev. Lett. 101 (2008) 267003] is strongly affected by dc currents, but not by magnetic fields. This suggests that dc current acts as a driving force to move chiral p-wave domain walls; a domain wall trapped at a pinning potential is forced to shift to the next stable position, thereby forming a larger critical current path and resulting in the anomalous hysteresis.
  • Yasuhiro Asano, Yukio Tanaka, Naoto Nagaosa
    PHYSICAL REVIEW LETTERS 105 5 056402.1-056402.4  2010年07月 [査読有り][通常論文]
     
    We discuss electric transport through a point contact which bridges Majorana fermion modes appearing at edges of two helical superconductors. The contents focus on the effects of interference and interaction unique to the Majorana fermions and the role of spin-orbit interaction (SOI). Besides the Josephson current, the quasiparticle conductance depends sensitively on the phase difference and relative helicity between the two superconductors. The interaction among the Majorana fermions causes the power-law temperature dependences of conductance for various tunneling channels. Especially, in the presence of SOI, the conductance always increases as the temperature is lowered.
  • Shiro Kawabata, Yasuhiro Asano, Yukio Tanaka, Alexander A. Golubov, Satoshi Kashiwaya
    PHYSICAL REVIEW LETTERS 104 11 117002.1-117002.4  2010年03月 [査読有り][通常論文]
     
    We predict anomalous atomic-scale 0-pi transitions in a Josephson junction with a ferromagnetic-insulator (FI) barrier. The ground state of such junction alternates between 0 and pi states when thickness of FI is increasing by a single atomic layer. We find that the mechanism of the 0-pi transition can be attributed to thickness-dependent phase shifts between the wave numbers of electrons and holes in FI. Based on these results, we show that a stable pi state can be realized in junctions based on high-T(c) superconductors with a La(2)BaCuO(5) barrier.
  • Ikuo Suemune, Yujiro Hayashi, Shuhei Kuramitsu, Kazunori Tanaka, Tatsushi Akazaki, Hirotaka Sasakura, Ryotaro Inoue, Hideaki Takayanagi, Yasuhiro Asano, Eiichi Hanamura, Satoru Odashima, Hidekazu Kumano
    APPLIED PHYSICS EXPRESS 3 5 054001.1-054001.3  2010年 [査読有り][通常論文]
     
    A light-emitting diode (LED) in the optical-fiber communication band showed special features after replacing the n-type electrode with niobium (Nb) superconducting metal. Nb electrodes prepared on an InGaAs-based semiconductor surface formed a superconductor/semiconductor/superconductor junction, and the current-voltage characteristics exhibited both DC and AC Josephson junction properties. This was a result of the injection of electron Cooper-pairs into the n-InGaAs active layer of an LED. The drastic enhancement of the electroluminescence output observed below the Nb superconducting critical temperature, T(c), demonstrates the active role of electron Cooper-pairs in radiative recombination. Lifetime measurements of this LED and accurate evaluation of the luminescence output made it possible to estimate the radiative recombination lifetimes. A theoretical formula derived for the Cooper-pair radiative recombination accurately describes both the measured steep reduction of the radiative recombination lifetime and the observed enhancement of the internal quantum efficiency below Tc. This work will assist the development of interdisciplinary physics and new applications in superconductivity and optoelectronics. (C) 2010 The Japan Society of Applied Physics
  • Yasuhiro Asano, Ikuo Suemune, Hideaki Takayanagi, Eiichi Hanamura
    PHYSICAL REVIEW LETTERS 103 18 187001.1-187001.4  2009年10月 [査読有り][通常論文]
     
    This Letter theoretically discusses the photon emission spectra of a superconducting p-n junction. On the basis of the second order perturbation theory for electron-photon interaction, we show that the recombination of a Cooper pair with two p-type carriers causes enhancement of the luminescence intensity. The calculated results of photon emission spectra explain characteristic features of observed signal in an recent experiment. Our results indicate high functionalities of superconducting light-emitting devices.
  • H. Nobukane, K. Inagaki, K. Ichimura, K. Yamaya, S. Takayanagi, I. Kawasaki, K. Tenya, H. Amitsuka, K. Konno, Y. Asano, S. Tanda
    SOLID STATE COMMUNICATIONS 149 29-30 1212 - 1215 2009年08月 [査読有り][通常論文]
     
    We observed an unconventional parity-violating vortex in single domain Sr2RuO4 single crystals using a transport measurement. The current-voltage characteristics of submicron Sr2RuO4 show that the induced voltage has anomalous components which are even functions of the bias current. The results may suggest that the vortex itself has a helical internal structure characterized by a Hopf invariant (a topological invariant). We also discuss that the hydrodynamics of such a helical vortex causes the parity violation to retain the topological invariant. (C) 2009 Elsevier Ltd. All rights reserved.
  • A. A. Golubov, Y. Tanaka, Y. Asano, Y. Tanuma
    JOURNAL OF PHYSICS-CONDENSED MATTER 21 16 164208,1-8  2009年04月 [査読有り][通常論文]
     
    We review the theory of odd-frequency pairing in superconducting heterostructures, where an odd-frequency pairing component is induced near interfaces. A general description of the superconducting proximity effect in a normal metal or a ferromagnet attached to an unconventional superconductor (S) is given within quasiclassical kinetic theory for various types of symmetry state in S. Various possible symmetry classes in a superconductor are considered which are consistent with the Pauli principle: even-frequency spin-singlet even-parity (ESE) state, even-frequency spin-triplet odd-parity (ETO) state, odd-frequency spin-triplet even-parity (OTE) state and odd-frequency spin-singlet odd-parity (OSO) state. As an example, we consider a junction between a diffusive normal metal (DN) and a p-wave superconductor (even-frequency spin-triplet odd-parity symmetry), where the pairing amplitude in DN belongs to an odd-frequency spin-triplet even-parity symmetry class. We also discuss the manifestation of odd-frequency pairing in conventional superconductor/normal (S/N) proximity systems and its relation to the classical McMillan-Rowell oscillations.
  • Hiroshi Kambara, Satoshi Kashiwaya, Hiroshi Yaguchi, Yasuhiro Asano, Yukio Tanaka, Yoshiteru Maeno
    PHYSICAL REVIEW LETTERS 101 26 267003.1-267003.4  2008年12月 [査読有り][通常論文]
     
    Using microfabrication techniques, we extracted individual channels of 3-kelvin (3-K) phase superconductivity in Sr2RuO4-Ru eutectic systems and confirmed odd-parity superconductivity in the 3-K phase, similar to pure Sr2RuO4. Unusual hysteresis in the differential resistance-current and voltage-current characteristics observed below 2 K indicates the internal degrees of freedom of the superconducting state. A possible origin of the hysteresis is current-induced chiral-domain-wall motion due to the chiral p-wave state.
  • Yasuhiro Asano, Toshihiro Yoshida, Yukio Tanaka, Alexander A. Golubov
    Physical Review B - Condensed Matter and Materials Physics 78 1 014514  2008年07月18日 [査読有り][通常論文]
     
    In a usual ferromagnet connected with a superconductor, the exchange potential suppresses the superconducting pairing correlation. We show that this common knowledge does not hold in a ferromagnet-superconductor junction on a graphene. When the chemical potential of a graphene is close to the conical point of energy dispersion, the exchange potential rather assists the charge transport through a junction interface. The loose-bottomed electric structure causes this unusual effect. © 2008 The American Physical Society.
  • Kohkichi Konno, Toyoki Matsuyama, Yasuhiro Asano, Satoshi Tanda
    PHYSICAL REVIEW D 78 2 024037  2008年07月 [査読有り][通常論文]
     
    We investigate the spacetime of a slowly rotating black hole in the Chern-Simons modified gravity. The long range feature of frame-dragging effect under the Chern-Simon gravity well explains the flat rotation curves of galaxies which is a central evidence of dark matter. Our solution provides a different scenario of rotating space from Godel's solution.
  • Yukio Tanaka, Yasuhiro Asano, Alexander A. Golubov
    PHYSICAL REVIEW B 77 22 220504  2008年06月 [査読有り][通常論文]
     
    In s-wave superconductors the Cooper pair wave function is isotropic in momentum space. This property may also be expected for Cooper pairs entering a normal metal from a superconductor due to the proximity effect. We show, however, that such a deduction is incorrect and the pairing function in a normal metal is surprisingly anisotropic because of quasiparticle interference. We calculate angle-resolved quasiparticle density of states in NS bilayers which reflects such anisotropic shape of the pairing function. We also propose a magnetotunneling spectroscopy experiment which could confirm our predictions.
  • Jacob Linder, Takehito Yokoyama, Yukio Tanaka, Yasuhiro Asano, Asle Sudbø
    Physical Review B - Condensed Matter and Materials Physics 77 17 174505  2008年05月06日 [査読無し][通常論文]
     
    Recent experimental results indicate the possible realization of a bulk odd-frequency superconducting state in the compounds CeCu2 Si2 and CeRhIn5. Motivated by this, we present a study of the quantum transport properties of a normal metal/odd-frequency superconductor junction in a search for probes to unveil the odd-frequency symmetry. From the Eliashberg equations, we perform a quasiclassical approximation to account for the transport formalism of an odd-frequency superconductor with the Keldysh formalism. Specifically, we consider the tunneling charge conductance and the tunneling thermal conductance. We qualitatively find distinct behavior in the odd-frequency case compared to the conventional even-frequency case in both the electrical and thermal current. This serves as a useful tool to identify the possible existence of a bulk odd-frequency superconducting state. © 2008 The American Physical Society.
  • Jacob Linder, Takehito Yokoyama, Yukio Tanaka, Yasuhiro Asano, Asle Sudbo
    PHYSICAL REVIEW B 77 17 2008年05月 [査読有り][通常論文]
     
    Recent experimental results indicate the possible realization of a bulk odd-frequency superconducting state in the compounds CeCU2Si2 and CeRhIn5. Motivated by this, we present a study of the quantum transport properties of a normal metal/odd-frequency superconductor junction in a search for probes to unveil the odd-frequency symmetry. From the Eliashberg equations, we perform a quasiclassical approximation to account for the transport formalism of an odd-frequency superconductor with the Keldysh formalism. Specifically, we consider the tunneling charge conductance and the tunneling thermal conductance. We qualitatively find distinct behavior in the odd-frequency case compared to the conventional even-frequency case in both the electrical and thermal current. This serves as a useful tool to identify the possible existence of a bulk odd-frequency superconducting state.
  • J. Linder, T. Yokoyama, Y. Tanaka, Y. Asano, A. Sudbø
    Physical Review B 78 14 014516_1-5 - 14516 American Physical Society 2008年 [査読有り][通常論文]
  • Evolution of Chiral Superconducting Order in 3-K Phase of Sr2RuO4
    H. Kambara, S. Kashiwaya, H. Yaguchi, Y. Asano, Y. Tanaka, Y. Maeno
    Physical Review Letters 101 267003_1-4  2008年 [査読有り][通常論文]
  • Electron Transport of a Ferromagnet-Superconductor junction on Graphene
    Y. Asano, T. Yoshida, Y. Tanaka, A. A. Golubov
    Physical Review B 14 014516_1-5  2008年 [査読有り][通常論文]
  • Yasuhiro Asano, Yuki Sawa, Yukio Tanaka, Alexander A. Golubov
    Physical Review B - Condensed Matter and Materials Physics 76 22 224525  2007年12月27日 [査読無し][通常論文]
     
    We study Josephson current in superconductor/diffusive ferromagnet/ superconductor junctions by using the recursive Green function method. When the exchange potential in a ferromagnet is sufficiently large compared to the pair potential in a superconductor, an ensemble average of Josephson current is much smaller than its mesoscopic fluctuations. The Josephson current vanishes when the exchange potential is extremely large so that a ferromagnet is half-metallic. Spin-flip scattering at junction interfaces drastically changes the characteristic behavior of Josephson current. In addition to spin-singlet Cooper pairs, equal-spin triplet pairs penetrate into a half metal. Such equal-spin pairs have an unusual symmetry property called odd-frequency symmetry and carry the Josephson current through a half metal. The penetration of odd-frequency pairs into a half metal enhances the low energy quasiparticle density of states, which could be detected experimentally by scanning tunneling spectroscopy. We will also show that odd-frequency pairs in a half metal cause a nonmonotonic temperature dependence of the critical Josephson current. © 2007 The American Physical Society.
  • Yasuhiro Asano, Yuki Sawa, Yukio Tanaka, Alexander A. Golubov
    PHYSICAL REVIEW B 76 22 2007年12月 [査読有り][通常論文]
     
    We study Josephson current in superconductor/diffusive ferromagnet/superconductor junctions by using the recursive Green function method. When the exchange potential in a ferromagnet is sufficiently large compared to the pair potential in a superconductor, an ensemble average of Josephson current is much smaller than its mesoscopic fluctuations. The Josephson current vanishes when the exchange potential is extremely large so that a ferromagnet is half-metallic. Spin-flip scattering at junction interfaces drastically changes the characteristic behavior of Josephson current. In addition to spin-singlet Cooper pairs, equal-spin triplet pairs penetrate into a half metal. Such equal-spin pairs have an unusual symmetry property called odd-frequency symmetry and carry the Josephson current through a half metal. The penetration of odd-frequency pairs into a half metal enhances the low energy quasiparticle density of states, which could be detected experimentally by scanning tunneling spectroscopy. We will also show that odd-frequency pairs in a half metal cause a nonmonotonic temperature dependence of the critical Josephson current.
  • Yasuhiro Asano, Yukio Tanaka, Alexander A. Golubov, Satoshi Kashiwaya
    PHYSICAL REVIEW LETTERS 99 6 067005.1-067005.4  2007年08月 [査読無し][通常論文]
     
    We propose a novel experiment to identify the symmetry of superconductivity on the basis of theoretical results for differential conductance of a normal metal connected to a superconductor. The proximity effect from the superconductor modifies the conductance of the remote current depending remarkably on the pairing symmetry: spin singlet or spin triplet. The clear-cut difference in the conductance is explained by symmetry of Cooper pairs in a normal metal with respect to frequency. In the spin-triplet case, the anomalous transport is realized due to an odd-frequency symmetry of Cooper pairs.
  • Yasuhiro Asano, Yukio Tanaka, Alexander A. Golubov
    PHYSICAL REVIEW LETTERS 98 10 107002.1-107002.4  2007年03月 [査読無し][通常論文]
     
    Motivated by a recent experiment [Keizer et al., Nature (London) 439, 825 (2006)], we study the Josephson effect in superconductor/diffusive half metal/superconductor junctions using the recursive Green function method. The spin-flip scattering at the junction interfaces opens the Josephson channel of the odd-frequency spin-triplet Cooper pairs. As a consequence, the local density of states in a half metal has a large peak at the Fermi energy. Therefore the odd-frequency pairs can be detected experimentally by using the scanning tunneling spectroscopy.
  • Yukio Tanaka, Yasuhiro Asano, Masanori Ichioka, Satoshi Kashiwaya
    PHYSICAL REVIEW LETTERS 98 7 077001.1-077001.4  2007年02月 [査読無し][通常論文]
     
    We present a theory of tunneling spectroscopy for normal metal/Larkin-Ovchinnikov state junctions in which the spatial periodic modulation in the pair potential amplitude is taken into account. The tunneling spectra show the characteristic line shapes reflecting the minigap structures under the periodic pair potentials depending on the boundary condition of the pair potentials at the junction interface. These features are qualitatively different from the tunneling spectra of the Fulde-Ferrell state. We propose an experimental setup which identifies the superconducting state of CeCoIn5.
  • Anomalous Josephson Current by Odd Frequency Pairs
    Y. Asano, Y. Tanaka, A. A. Golubov
    Physica C 460-462 1327-1328  2007年 [査読有り][通常論文]
  • Yasuhiro Asano
    PHYSICAL REVIEW B 74 22 220501  2006年12月 [査読無し][通常論文]
     
    This paper theoretically discusses the spin current in spin-triplet superconductor/insulator/spin-triplet superconductor junctions. At low temperatures, a midgap Andreev resonant state anomalously enhances not only the charge current but also the spin current. The coupling between the Cooper pairs and the electromagnetic fields leads to a Fraunhofer pattern in the direct current spin flow in magnetic fields and an alternative spin current under applied bias voltages.
  • M. Takigawa, M. Ichioka, K. Kuroki, Y. Asano, Y. Tanaka
    PHYSICAL REVIEW LETTERS 97 18 187002.1-187002.4  2006年11月 [査読無し][通常論文]
     
    This Letter theoretically discusses quasiparticle states and nuclear spin relaxation rates T-1(-1) in the quasi-one-dimensional superconductor (TMTSF)(2)PF6 under a magnetic field applied parallel to the conduction chains. We study the effects of Josephson-type vortices on T-1(-1) by solving the Bogoliubov-de Gennes equation for p-, d- or f-wave pairing interactions. In the presence of line nodes in pairing functions, T-1(-1) is proportional to T in sufficiently low temperatures because quasiparticles induced by vortices at the Fermi energy relax spins. We also try to identify the pairing symmetry of (TMTSF)(2)PF6.
  • Shiro Kawabata, Satoshi Kashiwaya, Yasuhiro Asano, Yukio Tanaka, Alexander A. Golubov
    PHYSICAL REVIEW B 74 18 2006年11月 [査読有り][通常論文]
     
    We theoretically investigate the macroscopic quantum dynamics of a pi junction with a superconductor (S) and a multiferroic material or a ferromagnetic insulator (FI). By deriving the effective action from a microscopic Hamiltonian, a pi-junction qubit (a S-FI-S superconducting quantum interference device ring) is proposed. In this qubit, a quantum two-level system is spontaneously generated and the effect of the quasiparticle dissipation is found to be very weak. These features make it possible to realize a quiet qubit with high coherency. We also investigate macroscopic quantum tunneling (MQT) in current-biased S-FI-S pi junctions and show that the influence of the quasiparticle dissipation on MQT is negligibly small.
  • ASANO Yasuhiro, TANAKA Yukio, YOKOYAMA Takehito, KASHIWAYA Satoshi
    Phys Rev B Condens Matter Mater Phys 74 6 064507  2006年08月 [査読無し][通常論文]
  • Yasuhiro Asano, Yukio Tanaka, Takehito Yokoyama, Satoshi Kashiwaya
    PHYSICAL REVIEW B 74 6 2006年08月 [査読有り][通常論文]
     
    The Josephson effect in superconductor/diffusive-normal-metal/superconductor junctions is studied numerically by using the recursive Green function method. In superconductors, we consider spin-singlet s- and d-wave and spin-triplet p-wave pairing symmetries. The pairing symmetry governs two interference effects in junctions: the formation of a midgap Andreev resonant state at junction interfaces and the proximity effect in diffusive normal metals. A cooperative effect between the two interference effects causes anomalous Josephson current in a p-wave symmetry.
  • T Yokoyama, Y Tanaka, AA Golubov, Y Asano
    PHYSICAL REVIEW B 73 14 2006年04月 [査読有り][通常論文]
     
    We study the Josephson effect in D/I/DN/I/D junctions, where I, DN, and D denote an insulator, a diffusive normal metal, and a d-wave superconductor, respectively. The Josephson current is calculated based on the quasiclassical Green's function theory with a general boundary condition for unconventional superconducting junctions. In contrast to s-wave junctions, the product of the Josephson current and the normal-state resistance is enhanced by making the interface barriers stronger. The Josephson current has a nonmonotonic temperature dependence due to the competition between the proximity effect and the midgap Andreev resonant states.
  • Y Asano, Y Tanaka, S Kashiwaya
    PHYSICAL REVIEW LETTERS 96 9 097007.1-097007.4  2006年03月 [査読無し][通常論文]
     
    The Josephson effect in p-wave superconductor/diffusive normal metal/p-wave superconductor junctions is studied theoretically. Amplitudes of Josephson currents are several orders of magnitude larger than those in s-wave junctions. Current-phase (J-phi) relations in low temperatures are close to those in ballistic junctions such as J proportional to sin(phi/2) and J proportional to phi even in the presence of random impurity potentials. A cooperative effect between the midgap Andreev resonant states and the proximity effect causes such anomalous properties and is a character of the spin-triplet superconductor junctions.
  • T. Yokoyama, Y. Tanaka, A. A. Golubov, Y. Asano
    Physical Review B - Condensed Matter and Materials Physics 73 14 140504  2006年 [査読無し][通常論文]
     
    We study the Josephson effect in D/I/DN/I/D junctions, where I, DN, and D denote an insulator, a diffusive normal metal, and a d -wave superconductor, respectively. The Josephson current is calculated based on the quasiclassical Green's function theory with a general boundary condition for unconventional superconducting junctions. In contrast to s -wave junctions, the product of the Josephson current and the normal-state resistance is enhanced by making the interface barriers stronger. The Josephson current has a nonmonotonic temperature dependence due to the competition between the proximity effect and the midgap Andreev resonant states. © 2006 The American Physical Society.
  • Shiro Kawabata, Satoshi Kashiwaya, Yasuhiro Asano, Yukio Tanaka, Alexander A. Golubov
    Physical Review B - Condensed Matter and Materials Physics 74 18 180502  2006年 [査読無し][通常論文]
     
    We theoretically investigate the macroscopic quantum dynamics of a π junction with a superconductor (S) and a multiferroic material or a ferromagnetic insulator (FI). By deriving the effective action from a microscopic Hamiltonian, a π -junction qubit (a S-FI-S superconducting quantum interference device ring) is proposed. In this qubit, a quantum two-level system is spontaneously generated and the effect of the quasiparticle dissipation is found to be very weak. These features make it possible to realize a quiet qubit with high coherency. We also investigate macroscopic quantum tunneling (MQT) in current-biased S-FI-S π junctions and show that the influence of the quasiparticle dissipation on MQT is negligibly small. © 2006 The American Physical Society.
  • T Yokoyama, Y Tanaka, AA Golubov, Y Asano
    PHYSICAL REVIEW B 72 21 214513  2005年12月 [査読無し][通常論文]
     
    Thermal and charge transport in diffusive normal metal (DN)/insulator/s-, d-, and p-wave superconductor junctions are studied based on the Usadel equation with the Nazarov's generalized boundary condition. We derive a general expression of the thermal conductance in unconventional superconducting junctions. Thermal conductance, electric conductance of junctions and their Lorentz ratio are calculated as a function of resistance in DN, the Thouless energy, magnetic scattering rate in DN and transparency of the insulating barrier. We also discuss transport properties for various orientation angles between the normal to the interface and the crystal axis of superconductors. It is demonstrated that the proximity effect does not influence the thermal conductance while the midgap Andreev resonant states suppress it. Dependencies of the electrical and thermal conductance on temperature are sensitive to pairing symmetries and orientation angles. The results imply a possibility to distinguish one pairing symmetry from another based on the results of experimental observations.
  • Y Tanaka, Y Asano, AA Golubov, S Kashiwaya
    PHYSICAL REVIEW B 72 14 140503.1-140503.4  2005年10月 [査読無し][通常論文]
     
    Anomalous features of the proximity effect specific to diffusive normal metal (DN) / triplet superconductor junctions are studied within the quasiclassical Green's function formalism. The pair amplitude f(N) as a function of energy epsilon in DN, satisfies an anomalous relation, f(N)(epsilon)=-f(N)(*)(-epsilon), contrary to that in singlet superconductor junctions case where f(N)(epsilon)=f(N)(*)(-epsilon). Such an unusual epsilon dependence is responsible for a zero energy peak in the local density of states and is a source of an anomalous penetration of the applied magnetic field into DN. Our results are relevant to Sr2RuO4, which is considered to have a p(x)+ip(y)-wave symmetry.
  • Y Asano
    PHYSICAL REVIEW B 72 9 092508.1-092508.4  2005年09月 [査読無し][通常論文]
     
    A formula of spin currents in mesoscopic superconductors is derived from the mean-field theory of superconductivity. Spin flow is generated by spatial gradient of d which represents a spin state of spin-triplet superconductors. We discuss a possibility of circulating spin currents in isolated p-wave superconducting rings at the zero magnetic field. The direction of spin currents depends on topological numbers which characterize spatial configurations of d on the ring.
  • S Kawabata, S Kashiwaya, Y Asano, Y Tanaka
    PHYSICAL REVIEW B 72 5 052506.1-052506.4  2005年08月 [査読無し][通常論文]
     
    The macroscopic quantum tunneling (MQT) in the current biased high-T-c superconductor Josephson junctions and the effect of the zero-energy bound states (ZES) on the MQT are theoretically investigated. We obtained the analytical formula of the MQT rate and showed that the presence of the ZES at the normal/superconductor interface leads to a strong ohmic quasiparticle dissipation. Therefore the MQT rate is noticeably inhibited compared with the c-axis junctions in which the ZES are completely absent.
  • Y Asano, Y Tanaka, M Sigrist, S Kashiwaya
    PHYSICAL REVIEW B 71 21 214501.1-214501.7  2005年06月 [査読無し][通常論文]
     
    The Josephson effect is theoretically studied in two types of SQUIDs consisting of s wave superconductor and Sr2RuO4. Results show various response of the critical Josephson current to applied magnetic fields depending on the type of SQUID and on the pairing symmetries. In the case of a p(x)+ip(y) wave symmetry, the critical current in a corner SQUID becomes an asymmetric function of magnetic fields near the critical temperatures. Our results well explain a recent experimental finding [Nelson , Science 306, 1151 (2004)]. We also discuss effects of chiral domains on the critical current.
  • T. Yokoyama, Y. Tanaka, A. A. Golubov, J. Inoue, Y. Asano
    Physical Review B - Condensed Matter and Materials Physics 71 9 094506.1-094506.9  2005年03月01日 [査読無し][通常論文]
     
    Charge transport in the diffusive normal metal (DN)/insulator/s- and d-wave superconductor junctions is studied in the presence of magnetic impurities in DN in the framework of the quasiclassical Usadel equations with the generalized boundary conditions. The cases of s- and d-wave superconducting electrodes are considered. The junction conductance is calculated as a function of a bias voltage for various parameters of the DN metal, resistivity, Thouless energy, the magnetic impurity scattering rate, and the transparency of the insulating barrier between DN and a superconductor. It is shown that the proximity effect is suppressed by magnetic impurity scattering in DN for any value of the barrier transparency. In low-transparent s-wave junctions this leads to the suppression of the normalized zero-bias conductance. In contrast to that, in high transparent junctions zero-bias conductance is enhanced by magnetic impurity scattering. The physical origin of this effect is discussed. For the d-wave junctions, the dependence on the misorientation angle a between the interface normal and the crystal axis of a superconductor is studied. The zero-bias conductance peak is suppressed by the magnetic impurity scattering only for low transparent junctions with α∼0. In other cases the conductance of the d-wave junctions does not depend on the magnetic impurity scattering due to strong suppression of the proximity effect by the midgap Andreev resonant states. ©2005 The American Physical Society.
  • S Kawabata, S Kashiwaya, Y Asano, Y Tanaka
    PHYSICAL REVIEW B 70 13 132505.1-132505.4  2004年10月 [査読無し][通常論文]
     
    We examine the macroscopic quantum tunneling (MQT) in high-T-c superconductor Josephson junctions with a d-wave order parameter. Using a microscopic Hamiltonian and the functional integral method, we analytically obtain the MQT rate (the inverse lifetime of the metastable state) for the c-axis twist Josephson junctions. In the case of the zero twist angle, the system shows the super-Ohmic dissipation due to the presence of the nodal quasiparticle tunneling. Therefore, the MQT rate is strongly suppressed in comparison with the finite twist angle cases.
  • S. Kawabata, S. Kashiwaya, Y. Asano, Y. Tanaka
    Physical Review B - Condensed Matter and Materials Physics 70 13 1 - 132505 2004年10月 [査読有り][通常論文]
     
    We examine the macroscopic quantum tunneling (MQT) in high-Tc superconductor Josephson junctions with a d-wave order parameter. Using a microscopic Hamiltonian and the functional integral method, we analytically obtain the MQT rate (the inverse lifetime of the metastable state) for the c-axis twist Josephson junctions. In the case of the zero twist angle, the system shows the super-Ohmic dissipation due to the presence of the nodal quasiparticle tunneling. Therefore, the MQT rate is strongly suppressed in comparison with the finite twist angle cases.
  • Y Asano, Y Tanaka, Y Tanuma, K Kuroki, H Tsuchiura
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 73 7 1922 - 1930 2004年07月 [査読無し][通常論文]
     
    Josephson effect in junctions of quasi one-dimensional superconductors at the quarter-filling electron density is discussed by using the tight-binding model on a two-dimensional square lattice. To consider the triangular lattice structures in real materials such as (TMTSF)(2)X (X = PF6, ClO4, etc), the asymmetric hopping integral is introduced among second nearest lattice sites. A combination of these characters in superconductors (quasi one-dimensionality, quarter-filling electron density and weak triangular lattice structures) seriously affects the formation of a zero-energy state at a junction interface. As a consequence, calculated results of the Josephson current show anomalous dependences on temperatures.
  • Y Asano, Y Tanaka, S Kashiwaya
    PHYSICAL REVIEW B 69 21 214509.1-214509.12  2004年06月 [査読無し][通常論文]
     
    Effects of impurity scatterings on the conductance in normal-metal/ d-wave superconductor junctions are discussed by using the single-site approximation. So far, the split of the zero-bias conductance peak has been believed to be evidence of the broken time reversal symmetry states at the surface of high- T-c superconductors. In this paper, however, it is shown that the impurity scattering near the interface also causes the split of the zero-bias conductance peak. Typical conductance spectra observed in experiments at finite temperatures and under external magnetic fields are explained well by the present theory.
  • Yasuhiro Asano, Yukio Tanaka, Satoshi Kashiwaya
    Physical Review B - Condensed Matter and Materials Physics 69 21 1 - 214509 2004年06月 [査読有り][通常論文]
     
    Effects of impurity scatterings on the conductance in normal-metal/d-wave superconductor junctions are discussed by using the single-site approximation. So far, the split of the zero-bias conductance peak has been believed to be evidence of the broken time reversal symmetry states at the surface of high-Tc superconductors. In this paper, however, it is shown that the impurity scattering near the interface also causes the split of the zero-bias conductance peak. Typical conductance spectra observed in experiments at finite temperatures and under external magnetic fields are explained well by the present theory.
  • ASANO Y, TANAKA Y, KASHIWAYA S
    Phys Rev B Condens Matter Mater Phys 69 13 134501.1-134501.11  2004年04月 [査読無し][通常論文]
  • Yasuhiro Asano, Yukio Tanaka, Satoshi Kashiwaya
    Physical Review B - Condensed Matter and Materials Physics 69 13 1 - 134501 2004年04月 [査読有り][通常論文]
     
    A conceptual consideration is given to a zero-energy state (ZES) at the surface of unconventional superconductors. The reflection coefficients in normal-metal/superconductor (NS) junctions are calculated based on a phenomenological description of the reflection processes of a quasiparticle. The phenomenological theory reveals the importance of the sign change in the pair potential for the formation of the ZES. The ZES is observed as the zero-bias conductance peak (ZBCP) in the differential conductance of NS junctions. The split of the ZBCP due to broken time-reversal symmetry states is naturally understood in the present theory. We also discuss effects of external magnetic fields on the ZBCP.
  • Y Asano, Y Tanaka, S Kashiwaya
    PHYSICAL REVIEW B 69 13 2004年04月 [査読有り][通常論文]
     
    A conceptual consideration is given to a zero-energy state (ZES) at the surface of unconventional superconductors. The reflection coefficients in normal-metal/superconductor (NS) junctions are calculated based on a phenomenological description of the reflection processes of a quasiparticle. The phenomenological theory reveals the importance of the sign change in the pair potential for the formation of the ZES. The ZES is observed as the zero-bias conductance peak (ZBCP) in the differential conductance of NS junctions. The split of the ZBCP due to broken time-reversal symmetry states is naturally understood in the present theory. We also discuss effects of external magnetic fields on the ZBCP.
  • Josephson effect in organic superconductors (TMTSF)2X
    Y. Asano, Y. Tanaka, K. Kuroki, Y. Tanuma, H. Tsuchiura, S. Kashiwaya
    Journal of the Physical Society of Japan 73 7 1922-1930  2004年 [査読有り][通常論文]
  • Y Asano, Y Tanaka, Y Matsuda, S Kashiwaya
    PHYSICAL REVIEW B 68 18 184506.1-184506.8  2003年11月 [査読無し][通常論文]
     
    The tunnel conductance in normal-metal/insulator/ProS(4)Sb(12) junctions is theoretically studied, where skutterudite ProS(4)Sb(12) is considered to be an unconventional superconductor. The conductance is calculated for several pair potentials which have been proposed in recent work. The results show that the conductance is sensitive to the relation between the direction of electric currents and the position of point nodes. The conductance spectra often deviate from the shape of bulk density of states. The sub gap spectra have peak structures in the case of the spin-triplet pair potentials. The results indicate that the tunnel conductance is a useful tool to obtain information of the pairing symmetry.
  • S Shirai, H Tsuchiura, Y Asano, Y Tanaka, J Inoue, Y Tanuma, S Kashiwaya
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 72 9 2299 - 2307 2003年09月 [査読無し][通常論文]
     
    Josephson current between two d-wave superconductors is calculated by using a lattice model. Here we consider two types of junctions, i.e., the parallel junction and the mirror-type junction. The maximum Josephson current (J(c)) shows a wide variety of temperature (T) dependence depending on the misorientation angles and the types of junctions. When the misorientation angles are not zero, the Josephson Current has anomalous temperature dependencies because of a zero energy state (ZES) at the interfaces. In the case of mirror-type junctions, J(c) has a non monotonic temperature dependence. These results are consistent with previous results based on the quasiclassical theory [Y. Tanaka and S. Kashiwaya: Phys. Rev. B 56 (1997) 892]. On the other hand, we find that the ZES disappears in several junctions because of the Friedel oscillations of the wave function, which is peculiar to the lattice model. In such junctions, the temperature dependence of J(c) is close to the Ambegaokar-Baratoff's relation.
  • N Kitaura, H Itoh, Y Asan, Y Tanaka, J Inoue, Y Tanuma, S Kashiwaya
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 72 7 1718 - 1723 2003年07月 [査読無し][通常論文]
     
    Effects of impurity scattering on tunneling conductance in dirty normal-metal/insulator/superconductor junctions are studied based on the Kubo formula and the recursive Green function method. The zero-bias conductance peak (ZBCP) is a consequence of the unconventional pairing symmetry in superconductors. The impurity scattering in normal metals suppresses the amplitude of the ZBCP. The degree of the suppression agrees well with results of the quasiclassical Green function theory. When superconductors have d + is-wave pairing symmetry, the time-reversal symmetry is broken in superconductors and the ZBCP splits into two peaks. The random impurity scattering reduces the height of the two splitting peaks. The position of the splitting peaks, however, almost remains unchanged even in the presence of the strong impurity scattering. Thus the two splitting peaks never merge into a single ZBCP.
  • Y Asano, Y Tanaka, M Sigrist, S Kashiwaya
    PHYSICAL REVIEW B 67 18 184505.1-184505.7  2003年05月 [査読無し][通常論文]
     
    The Josephson current between an s-wave and a spin-triplet superconductor Sr2RuO4 (SRO) is studied theoretically. In spin-singlet/spin-triplet superconductor junctions, there is no Josephson current proportional to sin phi in the absence of the spin-flip scattering near junction interfaces, where phi is a phase difference across junctions. Thus, a dominant term of the Josephson current is proportional to sin 2phi. The spin-orbit scattering at the interfaces gives rise to the Josephson current proportional to cos phi, which is a direct consequence of the chiral paring symmetry in SRO.
  • T Hirai, Y Tanaka, N Yoshida, Y Asano, J Inoue, S Kashiwaya
    PHYSICAL REVIEW B 67 17 174501.1-174501.11  2003年05月 [査読無し][通常論文]
     
    Tunneling conductance in ferromagnet/unconventional superconductor junctions is studied theoretically as a function of temperature and spin polarization in ferromagnets. In d-wave superconductor junctions, a zero-energy Andreev bound state drastically affects the temperature dependence of the zero-bias conductance (ZBC). In p-wave superconductor junctions, numerical results show various temperature dependences of the ZBC depending on the direction of the magnetic moment in ferromagnets and the pairing symmetry in superconductors such as p(x)-, p(y)-, and p(x)+ip(y)-wave symmetries. The last one is a candidate for the pairing symmetry of Sr2RuO4. From these characteristic features in the conductance, we may obtain information about the degree of spin polarization in ferromagnets and the direction of the d vector in spin-triplet superconductors.
  • N Yoshida, Y Asano, H Itoh, Y Tanaka, J Inoue, S Kashiwaya
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 72 4 895 - 899 2003年04月 [査読無し][通常論文]
     
    Effects of impurity-scattering on a zero-bias conductance peak in ferromagnet/insulator/d-wave superconductor junctions are theoretically studied. The impurities are introduced through the random potential in ferromagnets near the junction interface. As in the case of normal-metal/insulator/d-wave superconductor junctions, the magnitude of zero-bias conductance peak decreases with increasing the degree of disorder. However, when the magnitude of the exchange potential in ferromagnet is sufficiently large, the random potential can enhance the zero-bias conductance peak in ferromagnetic junctions.
  • Tunneling Spectra of Skutterudite PrOs4Sb12
    Y. Asano, Y. Tanaka, Y. Matsuda, S. Kashiwaya
    Physical Review B 68 184506_1-8  2003年 [査読有り][通常論文]
  • Journal of Physical Society of Japan
    S. Shirai, H. Tsuchiura, Y. Asano, Y. Tanaka, J. Inoue, Y. Tanuma, S. Kashiwaya
    Journal of Physical Society of Japan 72 9 2299-2307  2003年 [査読有り][通常論文]
  • Y Asano, K Katabuchi
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 71 8 1974 - 1977 2002年08月 [査読無し][通常論文]
     
    Josephson effect between two Sr2RuO4 (SRO) is theoretically studied based on a new formula of Josephson current. We analytically calculate the Josephson current in clean SRO/insulator/SRO junctions and SRO/dirty normal metal/SRO junctions. In SRO, we assume a spin-triplet p wave Superconductivity which breaks the chiral symmetry. When the current flows in a direction perpendicular to the c axis of SRO, the zero-energy states formed at the junction interfaces enhance the Josephson current in low temperature regime. We also report the Josephson current parallel to the c axis.
  • Y Asano
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 71 3 905 - 909 2002年03月 [査読無し][通常論文]
     
    A relation between amplitudes of Josephson current and nodes in excitation-gap of unconventional superconductors is studied in superconductor/dirty normal metal/superconductor junctions. It is found that ensemble average of the Josephson current vanishes when the junction interface is perpendicular to a plane which includes line- or point-zeros of the excitation-gap. A zero-energy state formed at the junction interface is a character of unconventional superconductor junctions and causes large sample-to-sample fluctuations in the Josephson current in low temperatures. The disappearance of the ensemble-averaged Josephson current and the large fluctuations occur at the same time when the Cooper pairs have d- or f-wave pairing symmetries.
  • Y Asano, Y Tanaka
    PHYSICAL REVIEW B 65 6 064522.1-064522.5  2002年02月 [査読無し][通常論文]
     
    We report the effects of interfacial roughness on a zero-bias conductance peak (ZBCP) in normal-metal/d-wave-superconductor junctions. The roughness spoils to specularity of the junction interface, and causes a diffuse Andreev reflection of a quasiparticle. The ZBCP decreases linearly with a decreasing rate of the specular Andreev reflection at the interface. A sensitivity of the ZBCP to the roughness strongly depends on a transparency of the junctions. In low transparent junctions, the ZBCP disappears even in the regime of weak roughness. We also find a splitting of the ZBCP owing to the inter-facial roughness in low transparent junctions.
  • Yasuhiro Asano
    Physical Review B - Condensed Matter and Materials Physics 66 17 1 - 6 2002年 [査読無し][通常論文]
     
    The Josephson effect in superconductor / semiconductor / superconductor junctions is discussed, where semiconductors are in the localization-transport regime. The temperature dependence of the critical Josephson current (formula presented) in a single sample is very different from those in other samples. Thus it is impossible to predict the Josephson current in experiments from the ensemble average of the Josephson current in theories. A variety in the temperature-dependence of (formula presented) can be understood by an analysis of the normal conductance near the Fermi energy. In some samples, we find that (formula presented) first increases with decreasing temperatures then decreases in agreement with an recent experiment. © 2002 The American Physical Society.
  • Retro Property of a Quasiparticle and Zero energy States in Normal metal/ d-wave superconductor Junctions
    Y. Asano, Y. Tanaka
    Physical Review B, 65 064522 _1-6  2002年 [査読有り][通常論文]
  • Y Asano
    PHYSICAL REVIEW B 64 22 224515.1-224515.9  2001年12月 [査読無し][通常論文]
     
    We derive a formula of the dc Josephson current between two superconductors with anisotropic pairing symmetry. One of the basic characters in the junctions of the anisotropic superconductors is the formation of the zero-energy bound states at the junction interfaces, which leads to the low-temperature anomaly of the Josephson current. The contribution of the zero-energy states to the Josephson current is taken into account in the present formalism.
  • Y Asano
    PHYSICAL REVIEW B 63 5 052512.1-052512.4  2001年02月 [査読無し][通常論文]
     
    We develop a numerical method to calculate the de Josephson current between the d(x2-y2)-wave superconductors. The Josephson junctions are described by the Bogoliubov-de Gennes equation on the two-dimensional tight-binding lattice. The Josephson current is expressed by the Matsubara Green function which is computed by the recursive Green-function method. We find in d-wave-superconductor/dirty-normal-metal/d-wave-superconductor junctions that the ensemble average of the Josephson current disappears for all temperature regimes when the angle between the crystal axis and the normal of the junction interface is pi /4.
  • Yasuhiro Asano
    Physical Review B - Condensed Matter and Materials Physics 64 1 014511.1-014511.8  2001年 [査読無し][通常論文]
     
    We discuss the Josephson current in superconductor/dirty normal conductor/superconductor junctions, where the superconductors have (formula presented) pairing symmetry. The low-temperature behavior of the Josephson current depends on the orientation angle between the crystalline axis and the normal of the junction interface. We show that the ensemble-averaged Josephson current vanishes when the orientation angle is (formula presented) and the normal conductor is in the diffusive transport regime. The (formula presented)-wave pairing symmetry is responsible for this fact. © 2001 The American Physical Society.
  • Y Asano, T Yuito
    PHYSICAL REVIEW B 62 11 7477 - 7482 2000年09月 [査読無し][通常論文]
     
    We study the effects of disorder on magnetoconductance oscillations in semiconductor-superconductor junctions. These conductance oscillations were predicted by recent several theoretical works in ballistic junctions, however, have never been observed yet in experiments. We find that the disorder near the junction suppresses the conductance oscillations. From the numerical results, we derive a condition that must be satisfied to find the conductance oscillations in experiments.
  • Y Asano, T Kato
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 69 4 1125 - 1135 2000年04月 [査読無し][通常論文]
     
    We discuss the magnetoconductance oscillations in small junctions of a ballistic semiconductor and a superconductor. The conductance is calculated both numerically and analytically based on the Bogoliubov-de Gennes equation. The conductance oscillations appear when the Andreev reflection is not perfect at the interface and the diameter of the cyclotron motion of a quasiparticle is comparable to the width of the junction. An interplay between the cyclotron motion of a quasiparticle near the interface and the phase shift by the magnetic field is the origin of the conductance oscillations.
  • Y Asano
    PHYSICAL REVIEW B 61 3 1732 - 1735 2000年01月 [査読無し][通常論文]
     
    The mechanism of the magnetoconductance oscillations in junctions of a ballistic semiconductor and a superconductor is discussed. The oscillations appear when both the normal and the Andreev refection occur at the interface. The interplay between the classical cyclotron motion of a quasiparticle and the phase shift caused by the magnetic field is the origin of the conductance oscillations.
  • Y Asano
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 67 12 4014 - 4017 1998年12月 [査読無し][通常論文]
     
    We studied the electronic structure and the electron transport in vertically coupled double quantum dots as a function of the number of electrons and strength of the magnetic field apl,lied in the direction perpendicular to the electric current. In weakly coupled double quantum dots, the electron correlation leads to the spin blockade in the Coulomb oscillations of the conductance. To date, however, it has been difficult to observe the spin blockade clearly since the temperature is not sufficiently low. We propose an experimental method to confirm the electron correlation effects even in the present temperature regime.
  • Y Asano
    PHYSICAL REVIEW B 58 3 1414 - 1423 1998年07月 [査読無し][通常論文]
     
    The effects of the electron correlation on the electronic structure and transport in a pair of vertically coupled quantum dots are studied as functions of the number of electrons and the distance between the two dots by using a numerical diagonalization method. The electron correlation drastically affects the spin structure of the ground states when the number of electrons and the distance between the dots are sufficiently large. The electronic states with the large (small) spin momentum are stable when the number of electrons is an odd (even) integer. A physical picture of this characteristic behavior can be understood by an analogy to that of the electronic states in the Hubbard model near half filling. The Coulomb oscillation can be seen in the conductance of the electric current through the double-quantum dots. When the distance between the dots is large, the amplitudes of several conductance peaks are suppressed by the electron correlation. [S0163-1829(98)00927-8].
  • Asano Y, Bauer GEW
    PHYSICAL REVIEW B 56 15 9972  1997年10月15日 [査読有り][通常論文]
  • Asano Y, Bauer GEW
    PHYSICAL REVIEW B 54 16 11602 - 11611 1996年10月15日 [査読有り][通常論文]
  • Yasuhiro Asano, Gerrit E. W. Bauer
    Physical Review B - Condensed Matter and Materials Physics 54 16 11602 - 11611 1996年 [査読無し][通常論文]
     
    The fluctuations in the electric conduction of a short disordered region are studied theoretically. The ensemble-averaged conductance and its fluctuations are calculated analytically in the single-site approximation. For a small number of impurities, the mean square of the fluctuations increases in proportion to the number of impurities, but is independent of the system size i.e., each impurity contributes independently to the fluctuations. Numerical simulations confirm the analytical results. The amplitude of the fluctuations can be larger than the universal values in diffusive conductors when the impurity scattering potential is sufficiently strong. In terms of random matrix theory, the enhancement can be understood by a reduced spectral rigidity of the transmission matrix eigenvalues. © 1996 The American Physical Society.
  • Y ASANO, A OGURI, J INOUE, S MAEKAWA
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 136 1-2 L18 - L22 1994年09月 [査読無し][通常論文]
     
    Numerical simulation in the recursive Green's function method is extended to a two-band tight-binding model where itinerant s- and localized d-electrons coexist. This method enables us to make more realistic calculations of the giant magnetoresistance in magnetic multilayers and granular alloys containing noble metals, such as Co/Cu and Co/Ag. By using the present method, dependence of the magnetoresistance on the size of the magnetic clusters in granular alloys is examined and compared with the experimental one as well as with that calculated before in the single-band model.
  • NONOYAMA S, OGURI A, ASANO Y, MAEKAWA S
    Phys Rev B Condens Matter Mater Phys 50 4 2667-2670  1994年07月15日 [査読無し][通常論文]
  • ASANO Y, OGURI A, INOUE J, MAEKAWA S
    Phys Rev B Condens Matter Mater Phys 49 18 12831-12834  1994年05月01日 [査読無し][通常論文]
  • ASANO Y, OGURI A, MAEKAWA S
    Phys Rev B Condens Matter Mater Phys 48 9 6192-6198  1993年09月01日 [査読無し][通常論文]
  • A OGURI, Y ASANO, S MAEKAWA
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 61 8 2652 - 2655 1992年08月 [査読無し][通常論文]
     
    The conductance in superlattices is studied using the Landauer formula with a transfer-matrix technique. The characteristics of the superlattice, i.e., the layer structure and roughness at interfaces, are treated in a tight-binding model on a two-dimensional long-strip lattice. In this context the magnetoresistance of a magnetic Fe/Cr superlattice, which is known to show giant magnetoresistance, is discussed.

書籍

  • Andreev Reflection in Superconducting Junctions
    Yasuhiro Asano (担当:単著)
    Springer(ISBN:978-981-16-4165-7) 2021年11月
  • Fundamentals and Frontiers of the Josephson Effect
    Yasuhiro Asano, Yukio Tanaka (担当:分担執筆範囲:Sec. 4 Charge Transport in Unconventional Superconductor Junctions)
    Springer 2019年08月 (ISBN: 9783030207267)

講演・口頭発表等

  • Josephson Effect of two-band/orbital superconductors  [招待講演]
    Y. Asano
    Topological Superconductivity in Quantum Materials 2020年10月 口頭発表（招待・特別）
  • Nonlocal conductance due to a chiral edge mode  [招待講演]
    Yasuhiro Asano
    Oxide Superspin Workshop 2019 2019年07月 口頭発表（招待・特別）
  • Flat-band Andreev bound states and odd-frequency Cooper pairs  [招待講演]
    Yasuhiro Asano
    Majorana Fermions and topological Material Science 2018年07月 口頭発表（招待・特別） Ettore Majorana Foundation and Centre for Scientific Culture,\\ Erice, Sicily, Italy
  • Dirty two-band superconductivity  [招待講演]
    Yasuhiro Asano
    Oxide Superconducting spintronics 2017 2017年11月 口頭発表（招待・特別） Yukawa Institute for Theoretical Physics, Kyoto Univ., Japan
  • Flat-band Andreev bound states and odd-frequency Cooper pairs  [招待講演]
    Yasuhiro Asano
    Novel Quantum States in Condensed Matter 2017 2017年11月 口頭発表（招待・特別） Yukawa Institute for Theoretical Physics, Kyoto Univ., Japan
  • Dirty two-band superconductivity  [招待講演]
    Yasuhiro Asano
    The 6th International Conference on Mesoscopic Structures: Fundamentals and Applications 2017年07月 口頭発表（招待・特別） Hotel “Baikal”, Listvyanka, Irkutsk region, Russia
  • Anomalous magnetic response and electric transport due to odd-frequency Cooper pair  [招待講演]
    Yasuhiro Asano
    International Conference on Superconducting hybrid nanostructures: physics and application 2016年09月 口頭発表（招待・特別） Moscow Institute of Physics and Technology, Dolgoprudny, Russia,
  • Edge current in a small chiral superconductor  [招待講演]
    Yasuhiro Asano
    SPIE. photonics and spintronics 2016 2016年09月 口頭発表（招待・特別） San Diego Convention center, San Diego, U. S. A.
  • Paramagnetic response of small topological superconductors  [招待講演]
    Yasuhiro Asano
    International Conference on Interaction of Superconductivity and Magnetism in Nanosystems 2015年09月 口頭発表（基調） Moscow State Institute of Steel and Alloys, Moscow, Russia
  • Anomalous proximity effect and more than one Majorana fermion  [招待講演]
    Yasuhiro Asano
    11th International Workshop on Nanomagnetism \& Superconductivity at the Nanoscale 2015年07月 口頭発表（招待・特別） Nuva Hotel Comarruga, Coma-Ruga, Spain
  • Anomalous proximity effect and more than one Majorana fermio  [招待講演]
    Yasuhiro Asano
    11th International Conference on Ceramic Materials Components for Energy and Environmental Applications 2015年06月 口頭発表（招待・特別） Hotel Hyatt Regency, Vancouver, Canada
  • Magnetic property of Odd-frequency Cooper Pairs  [招待講演]
    Yasuhiro Asano
    International Conference on Topological Quantum Phenomena 2014年12月 口頭発表（招待・特別） Kyoto University, Kyoto, Japan,
  • Properties of Odd-frequency Pairs  [招待講演]
    Yasuhiro Asano
    日本物理学会秋の分科会, 領域6シンポジウム 2014年09月 口頭発表（招待・特別） 中部大学, 春日井
  • Paramagnetic Meisner Effect in Small Unconventional Superconductors  [招待講演]
    Yasuhiro Asano
    Physics and Applications of Superconducting Hybrid Nano-Engineered Devices 2014年09月 口頭発表（招待・特別） Grand Hotel Santamaria, Santa Maria di Castellabate, Italy
  • Paramagnetic Meisner Effect in Small Unconventional Superconductors  [招待講演]
    Yasuhiro Asano
    10th International Workshop on Nanomagnetism \& Superconductivity at the Nanoscale 2014年07月 口頭発表（招待・特別） Hotel Vita Comarruga, Coma-Ruga, Spain
  • Three-dimensional Symmetry Breaking Topological States  [招待講演]
    Yasuhiro Asano
    MTI and ITS Fall Workshop 2013: Coherent Hybrid Structures on the Mesoscale 2013年10月 口頭発表（招待・特別） Hilton hotel Orrington/Evanston, Illinois, U. S. A.
  • Surface/interfacial states of topological materials  [招待講演]
    Yasuhiro Asano
    Electron Correlation in nanostructures 2013 2013年10月 口頭発表（招待・特別） Hotel Yalta-Intourist, Yalta, Ukraine
  • Majorana fermions and Odd-frequency Cooper Pairs  [招待講演]
    Yasuhiro Asano
    MTI Nonconventional Insulators Workshop 2012 2012年11月 口頭発表（招待・特別） Argonne National Laboratory, Illinois, U. S. A.
  • Majorana fermions and Odd-frequency Cooper Pairs  [通常講演]
    Yasuhiro Asano
    9th International Conference on New Theories, Discoveries and Applications of Superconductors and Related Materials 2012年09月 Grand Hotel Villa Tuscolana, Roma, Italy
  • Majorana fermions and Odd-frequency Cooper Pairs  [招待講演]
    Yasuhiro Asano
    Workshop on Superconducting Nanohybrids 2012 2012年09月 口頭発表（招待・特別） Donostia International Physics Center, San Sebastian, Spain
  • Odd-frequency Cooper Pairs in spin-triplet superconducting junctions  [招待講演]
    Yasuhiro Asano
    The International conference "Dubna-Nano2012 2012年07月 口頭発表（基調） Bogoliubov Laboratory of Theoretical Physics, Dubna, Russia
  • Odd-frequency Cooper Pairs in ferromagnetic junctions  [招待講演]
    Yasuhiro Asano
    8th International Workshop on Nanomagnetism & Superconductivity 2012年07月 口頭発表（招待・特別） Hotel Marvel, Coma-ruga, Spain,
  • Proximity Effect of Odd-frequency Cooper Pairs  [招待講演]
    浅野泰寛
    International Conference on Topological Phenomena, TQP2012 2012年05月 口頭発表（招待・特別） Nagoya University, Japan
  • Odd-frequency Cooper Pairs and Their Effects on Surface Impedance  [招待講演]
    浅野泰寛
    Superconductivity and Magnetism, SM-2011 2011年10月 口頭発表（招待・特別） Kishinev, Moldva
  • Anomalous Surface Impedance due to Odd-frequency Cooper Pairs  [招待講演]
    浅野泰寛
    7th International Workshop on Nanomagnetism \& Superconductivity 2011年07月 口頭発表（招待・特別） Hotel Marvel, Coma-ruga, Spain
  • Anomalous Surface Impedance due to Odd-frequency Cooper Pairs  [招待講演]
    浅野泰寛
    8th International Conference on New Theories, Discoveries and Applications of Superconductors and Related Materials 2011年06月 口頭発表（招待・特別） J. W. Marriott Hotel Chongqing, China
  • Tunneling between Two Helical superconductors via Majorana Edge Channels  [招待講演]
    浅野泰寛
    Superconductivity and Magnetism 2010 2010年09月 口頭発表（招待・特別） Savoy Hotel, Paestum, Italy
  • Unusual Cooper Pairs in Superconducting Proximity Structures  [招待講演]
    浅野泰寛
    International Conference on Superconductivity and Magnetism 2010 2010年04月 口頭発表（招待・特別） Porto Bello Hotel, Antalya, Turkey
  • Proximity effect in spin-triplet superconducting junctions  [招待講演]
    浅野泰寛
    nternational Symposium on Novel Spin Pairing 2009 2009年09月 口頭発表（招待・特別） Yukawa Memorial Hall, Kyoto University, Japan
  • Josephson current through a strong ferromagnet  [通常講演]
    浅野泰寛
    International symposium on new directions of superconducting nanostructures 2009 2009年09月 口頭発表（招待・特別） Nagoya University, Japan
  • Luminescence of a superconducting pn Junction  [通常講演]
    浅野泰寛
    Nanoscience and Engineering in Superconductivity, Hybrid and Josephson Structures 2009年06月 口頭発表（招待・特別） Anacapri, Italy
  • "Josephson Effect in Chiral p-wave Superconductors  [招待講演]
    浅野泰寛
    Spin Helicity and Chirality in Superconductor and Semiconductor Nanostructures 2008年07月 口頭発表（招待・特別） Karlsurhe, Germany
  • Conductance Spectroscopy of Spin-triplet Superconductors  [招待講演]
    浅野泰寛
    Probing superconductivity at the nanoscale 2008年06月 口頭発表（招待・特別） Alicante, Spain
  • Odd-frequency Pairing State in Superconductor / Ferromagnet Junctions  [招待講演]
    浅野泰寛
    日本物理学会63回年次大会, 領域3-6シンポジウム 2008年03月 口頭発表（招待・特別） 近畿大学, 大阪
  • Conductance Spectroscopy of Spin-triplet Superconductors  [招待講演]
    浅野泰寛
    MTI International Argonne Fall Workshop on Nanophysics VII, 2007 2007年12月 口頭発表（招待・特別） National Center for Theoretical Sciences, Hsinchu, Taiwan
  • Odd-frequency Cooper Pairs in Ferromagnet/Superconductor Junctions  [招待講演]
    浅野泰寛
    Nanoscale Superconductors and their Applications 2007年11月 口頭発表（招待・特別） Institute for Material Research in Tohoku Univ., Sendai, Japan
  • Josephson Current through a Strong Ferromagnet and Odd-frequency Pairs  [招待講演]
    浅野泰寛
    Physics of Nanoscale Superconducting Heterostructures 2007年07月 口頭発表（招待・特別） Lorentz Center in Leiden Univ., Leiden, The Netherlands,
  • Proximity Effect in Half metals and Odd-frequency Cooper Pairs  [招待講演]
    浅野泰寛
    International Conference on Nanoscale Magnetism 2007年06月 口頭発表（招待・特別） Istanbul, Turkey
  • Anomalous Josephson Effect in Unconventional Superconductor Junctions  [招待講演]
    浅野泰寛
    Quantum Phenomena in Low-dimensional Systems 2007年01月 口頭発表（招待・特別）
  • Anomalous Proximity Effect due to Odd-Frequency Cooper Pairs  [招待講演]
    浅野泰寛
    Sixth International Conference on New Theories, Discoveries and Applications of Superconductors and Related Materials 2007年01月 口頭発表（招待・特別） Sydney, Austraria,
  • Josephson Current through a Diffusive Half Metal  [招待講演]
    浅野泰寛
    19th International Symposium on Superconductivity 2006年10月 口頭発表（招待・特別） Nagoya, Japan
  • Spin Current in Triplet Superconducting Rings  [招待講演]
    浅野泰寛
    International Conference on Nanoscale Magnetism 2005年07月 口頭発表（招待・特別）
  • Impurity Effect of Unconventional Superconducting Junctions  [招待講演]
    浅野泰寛
    International Workshop on Physics of Superconducting pi-Phase Shift Devices 2005年04月 口頭発表（招待・特別）
  • 異方的超伝導体接合の不純物効果  [招待講演]
    浅野泰寛
    日本物理学会秋の分科会, 領域4シンポジウム 2004年09月 口頭発表（招待・特別）
  • 超伝導ジョセフソン接合系の理論の最近の進展  [招待講演]
    浅野泰寛
    日本物理学会秋の分科会 2002年09月 口頭発表（招待・特別）
  • Symmetry of Pair Potential and Josephson Effect in Dirty SNS Junctions}  [招待講演]
    浅野泰寛
    The International Symposium on Superconducting Device Physics 2001年06月 口頭発表（招待・特別）

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：文部省科学研究費補助金(基盤研究(B))
    研究期間 : 2020年04月 -2023年03月 
    代表者 : 浅野泰寛、田仲由喜夫、水島健
  • 強相関物質のトポロジカル相
    文部科学省：科学研究費補助金（新学術領域研究）
    研究期間 : 2015年 -2019年 
    代表者 : 川上則雄
  • 文部科学省：科学研究費補助金(基盤研究(B))
    研究期間 : 2014年 -2016年 
    代表者 : 浅野 泰寛
  • 文部科学省：科学研究費補助金(新学術領域研究(研究領域提案型))
    研究期間 : 2010年 -2015年 
    代表者 : 前野 悦輝, 柏谷 聡, 赤崎 達志, 浅野 泰寛
  • 文部科学省：科学研究費補助金(基盤研究(C))
    研究期間 : 2010年 -2012年 
    代表者 : 浅野 泰寛
     
    発光ダイオード(LED)は交通信号や懐中電灯に使われるなど日々の営みに深く浸透し、省エネルギーの達成と快適な生活の維持を両立させる優れた電子素子である。半導体pn接合で生じる発光という物理現象は、このように極めて身近なものである。最近、発光現象の新たな側面に注目した研究が精力的に行われている。超伝導体を用いたLEDには量子情報操作に不可欠な量子力学的にもつれ合った光子のペアを発生出来ると期待されており、超伝導を担う電子対(クーパー対)がどのような過程で、量子力学的にもつれ合った光子ペアを生成するかを明らかにするのが本研究の目的である。2009年に申請者が報告した理論に基づきクーパー対発光の理論を進展させた。時間に依存する摂動理論を用いて接合から発光するフォトン数の長時間平均を理論的に計算する事が出来る、これは発光強度に対応する。これまでのところ、1次の摂動論の範囲では超伝導の効果が極めて小さい事、2次の摂動論を用いて初めてクーパー対の再結合が記述でき、その発光過程から大きな発光強度が説明できそうな事がわかっている。22年度は更に高次の摂動項の大きさを、見積もった結果3次以上の摂動の高次項が無視できることがわかった。この結果は、連携して研究を進めている実験グループによって見いだされた結果を矛盾なく説明する。さらに実験的には、発行寿命が超伝導転移温度以下で急激に減少する事を見いだ...
  • 文部科学省：科学研究費補助金(特定領域研究)
    研究期間 : 2008年 -2009年 
    代表者 : 浅野 泰寛
     
    超伝導には通常金属における超伝導(スピン1重項s波超伝導)の他に,銅酸化物高温超伝導(スピン1重項d波)に代表される異方的超伝導がある.超伝導体に普通の非超伝導金属(常伝導体)を接合したとき,超伝導を担うクーパー対が超伝導体から常伝導体へ染み出す現象は近接効果と呼ばれている.我々はこれまで異方的超伝導体の近接効果を調べ,近接効果はその母体となる超伝導の対称性に極めて敏感であるという重要な結論を得ている.異方的超伝導が非局所電流に及ぼす効果を明らかにする事で本研究の当初の目的のひとつである.特にスピン3重項超伝導体から染み出したクーパー対が奇周波数対称性を獲得することは、我々が明らかにした重要な性質である。奇周波数ペアは、超伝導体と強磁性体が接合した場合にも生み出され、その性質が大変注目されている。我々は、超伝導体に張り付けた金属薄膜における表面インピーダンスが超伝導体のスピン対称性に依存して特徴的なふるまいを示すことを明らかにした。また、強磁性絶縁体を超伝導体で挟んだSFS接合では、強磁性絶縁体の長さを1原子層ずつ変化させるごとにπ接合という特殊な状態が実現することを明らかにした。このπ状態と奇周波数ペアの関係は、今後の課題として残されている。
  • 文部科学省：科学研究費補助金(基盤研究(C))
    研究期間 : 2007年 -2008年 
    代表者 : 浅野 泰寛, 田仲 由喜夫, 田仲 由喜夫
     
    超伝導の対称性は、金属で発現するスピン1重項対称性と、ルテニウム酸化物や重い電子系で有力とされるスピン3重項対称性に大別される。特にスピン3重項超伝導の物理はきわめて興味深く、注目を集めている。我々は、理論的な手法にのっとり超伝導近接効果を調べ、この2種類を明確に区別する新しい実験方法を提案した。
  • 文部科学省：科学研究費補助金(特定領域研究)
    研究期間 : 2006年 -2007年 
    代表者 : 浅野 泰寛, 市岡 優典
     
    本研究課題ではスピン3重項超伝導体に固有の輸送現象の解明を目指している.18年度はその中でもスピン・ジョセフソン効果の基本的な性質の解明と,観測に向けての提案を行った.超スピン流に関する研究はヘリウム3ですでに行われているが,本研究ではスピン3重項超伝導体Sr_2RuO_4を念頭に超スピン流を制御しながら流すための理論的提案を行った.超伝導体中のクーパー対はスピンの外に電荷の自由度をもち,電荷が電磁場とゲージ結合するためスピン流の制御が可能になるという結論を得た.スピン流の基礎的性質の研究とその応用研究は現在磁性体を用いた微小素子においてきわめて盛んに行われており,デバイス応用の可能性も高くなってきた.その意味で超伝導体を用いたスピン流の研究はとても時宜を得たものと考えられる.19年度はT字形状の超伝導体と常伝導体の接合の電気伝導測定から,スピン1重項超伝導体と3重項超伝導体の峻別が可能であることを示した.この目的のためには多くの実験があるが,ルテニウム酸化物をはじめとして多くの3重項超伝導体について,スピン3重項超伝導を否定する意見もないわけではない.こうした状況の下で,超伝導の対称性をはっきりと区別できる方法の提案は,この分野の発展に大きく寄与すると考えられる.

教育活動情報

主要な担当授業

  • 相関系物理工学特論
    開講年度 : 2021年
    課程区分 : 修士課程
    開講学部 : 工学院
    キーワード : 第2量子化の方法、第2種の相転移、平均場理論、超伝導、強磁性、トポロジガル物質
  • 相関系物理工学特論
    開講年度 : 2021年
    課程区分 : 博士後期課程
    開講学部 : 工学院
    キーワード : 第2量子化の方法、第2種の相転移、平均場理論、超伝導、強磁性、トポロジガル物質
  • 極低温物理学
    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 : 低温，第二種の相転移，ボーズ凝縮, 超流動, 超伝導
  • 統計力学Ⅱ
    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 : ネルンストの定理、グランドカノニカル分布，相平衡、量子理想気体，固体の低温比熱、第二種の相転移
  • 統計力学演習Ⅱ
    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 : ネルンストの定理、グランドカノニカル分布，相平衡、量子理想気体，固体の低温比熱、第二種の相転移

Copyright © MEDIA FUSION Co.,Ltd. All rights reserved.