2024/05/14 更新

オオクマ ノブユキ
大熊 信之
OKUMA Nobuyuki
Scopus 論文情報  
総論文数: 0  総Citation: 0  h-index: 11

Citation Countは当該年に発表した論文の被引用数

所属
大学院工学研究院 基礎科学研究系
職名
准教授
外部リンク

取得学位

  • 東京大学  -  博士(理学)   2018年03月

  • 東京大学  -  修士(理学)   2015年03月

学内職務経歴

  • 2023年04月 - 現在   九州工業大学   大学院工学研究院   基礎科学研究系     准教授

論文

  • Non-Hermitian Topological Phenomena: A Review 招待有り 査読有り 国際誌

    Okuma N., Sato M.

    Annual Review of Condensed Matter Physics   14   83 - 107   2023年03月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:記事・総説・解説・論説等(学術雑誌)

    The past decades have witnessed an explosion of interest in topological materials, and a lot of mathematical concepts have been introduced in condensed matter physics. Among them, the bulk-boundary correspondence is the central topic in topological physics, which has inspired researchers to focus on boundary physics. Recently, the concepts of topological phases have been extended to non-Hermitian Hamiltonians, whose eigenvalues can be complex. Besides the topology, non-Hermiticity can also cause a boundary phenomenon called the non-Hermitian skin effect, which is an extreme sensitivity of the spectrum to the boundary condition. In this article, we review developments in non-Hermitian topological physics by focusing mainly on the boundary problem. As well as the competition between non-Hermitian and topological boundary phenomena, we discuss the topological nature inherent in non-Hermiticity itself.

    DOI: 10.1146/annurev-conmatphys-040521-033133

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  • Topological Origin of Non-Hermitian Skin Effects 査読有り 国際誌

    Okuma N., Kawabata K., Shiozaki K., Sato M.

    Physical Review Letters   124 ( 8 )   2020年02月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    A unique feature of non-Hermitian systems is the skin effect, which is the extreme sensitivity to the boundary conditions. Here, we reveal that the skin effect originates from intrinsic non-Hermitian topology. Such a topological origin not merely explains the universal feature of the known skin effect, but also leads to new types of the skin effects - symmetry-protected skin effects. In particular, we discover the Z2 skin effect protected by time-reversal symmetry. On the basis of topological classification, we also discuss possible other skin effects in arbitrary dimensions. Our work provides a unified understanding about the bulk-boundary correspondence and the skin effects in non-Hermitian systems.

    DOI: 10.1103/PhysRevLett.124.086801

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  • Boundary-dependent dynamical instability of bosonic Green's function: Dissipative Bogoliubov-de Gennes Hamiltonian and its application to non-Hermitian skin effect 査読有り 国際誌

    Okuma N.

    Physical Review B   105 ( 22 )   2022年06月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    The energy spectrum of bosonic excitations from a condensate is given by the spectrum of a non-Hermitian Hamiltonian constructed from a bosonic Bogoliubov-de Gennes (BdG) Hamiltonian in general even though the system is essentially Hermitian. In other words, two types of non-Hermiticity can coexist: one from the bosonic BdG nature and the other from the open quantum nature. In this paper, we propose boundary-dependent dynamical instability. We first define the bosonic dissipative BdG Hamiltonian in terms of Green's function in Nambu space and discuss the correct particle-hole symmetry of the corresponding non-Hermitian Hamiltonian. We then construct a model of the boundary-dependent dynamical instability so that it satisfies the correct particle-hole symmetry. In this model, an anomalous term that breaks the particle number conservation represents the non-Hermiticity of the BdG nature, while a normal term is given by a dissipative Hatano-Nelson model. Thanks to the competition between the two types of non-Hermiticity, the imaginary part of the spectrum can be positive without the help of the amplification of the normal part and the particle-hole band touching that causes the Landau instability. This leads to the boundary-dependent dynamical instability under the non-Hermitian skin effect - strong dependence of spectra on boundary conditions for non-Hermitian Hamiltonians - of the Bogoliubov spectrum.

    DOI: 10.1103/PhysRevB.105.224301

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  • Nonnormal Hamiltonian dynamics in quantum systems and its realization on quantum computers 査読有り 国際誌

    Okuma N., Nakagawa Y.O.

    Physical Review B   105 ( 5 )   2022年02月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    The eigenspectrum of a non-normal matrix, which does not commute with its Hermitian conjugate, is a central issue of non-Hermitian physics that has been extensively studied in the past few years. There is, however, another characteristic of a non-normal matrix that has often been overlooked: the pseudospectrum, or the set of spectra under small perturbations. In this paper, we study the dynamics driven by the non-normal matrix (Hamiltonian) realized as a continuous quantum trajectory of the Lindblad master equation in open quantum systems and point out that the dynamics can reveal the nature of unconventional pseudospectrum of the non-normal Hamiltonian. In particular, we focus on the transient dynamics of the norm of an unnormalized quantum state evolved with the non-normal Hamiltonian, which is related to the probability for observing the trajectory with no quantum jump. We formulate the transient suppression of the decay rate of the norm due to the pseudospectral behavior and derive a non-Hermitian/non-normal analog of the time-energy uncertainty relation. We also consider two methods to experimentally realize the non-normal dynamics and observe our theoretical findings on quantum computers: one uses a technique to realize nonunitary operations on quantum circuits and the other leverages a quantum-classical hybrid algorithm called variational quantum simulation. Our demonstrations using cloud-based quantum computers provided by IBM Quantum exhibit the frozen dynamics of the norm in transient time, which can be regarded as a non-normal analog of the quantum Zeno effect.

    DOI: 10.1103/PhysRevB.105.054304

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  • Non-Hermitian Skin Effects in Hermitian Correlated or Disordered Systems: Quantities Sensitive or Insensitive to Boundary Effects and Pseudo-Quantum-Number 査読有り 国際誌

    Okuma N., Sato M.

    Physical Review Letters   126 ( 17 )   2021年04月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    There is a common belief in the condensed matter community that bulk quantities become insensitive to the boundary condition in the infinite-volume limit. Here we reconsider this statement in terms of recent arguments of non-Hermitian skin effects - strong dependence of spectra on boundary conditions for the non-Hermitian Hamiltonians - in the traditional Green's function formalism. We find the criterion for quantities to be sensitive or insensitive against the boundary condition in Hermitian correlated or disordered systems, which is characterized by the residue theorem. We also discuss the uncertainty of the quasiparticle energy under the skin effects in terms of non-normal pseudospectra, which can be tested via the sharp optical absorption from the bulk-surface coupling. Our result indicates that pseudo quantum number emerges as a consequence of large nonnormality.

    DOI: 10.1103/PhysRevLett.126.176601

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  • Quantum anomaly, non-Hermitian skin effects, and entanglement entropy in open systems 査読有り 国際誌

    Okuma N., Sato M.

    Physical Review B   103 ( 8 )   2021年02月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    We investigate the roles of non-Hermitian topology in spectral properties and entanglement structures of open systems. In terms of spectral theory, we give a unified understanding of two interpretations of non-Hermitian topology: quantum anomaly and non-Hermitian skin effects, in which the bulk spectra extremely depend on the boundary conditions. In this context, the fact that the intrinsic higher-dimensional skin effects under the full open boundary condition need the presence of the topological defects is understood in terms of the anomalous fermion production such as the Rubakov-Callan effect in the presence of the magnetic monopole. By using the unified interpretation, we classify the symmetry-protected and higher-dimensional skin effects. In terms of the entanglement structure, we investigate steady states of fermionic open systems whose Liouvillian (rapidity) spectra host non-Hermitian topology. We analyze dissipation-driven Majorana steady states in zero-dimensional open systems and relate them to the Majorana edge modes of topological superconductors by using the entanglement entropy. We also analyze a steady state of a one-dimensional open Fermi system with a non-Hermitian topological spectrum and relate it to the chiral edge states of the Chern insulator on the basis of the trace index defined from the entanglement spectrum. This correspondence indicates that the entanglement generates circular nonreciprocal currents under the periodic boundary condition and the skin-effect voltage with fermion accumulation under the open boundary condition. Finally, we discuss several related topics such as pseudospectral behaviors of Liouvillian dynamics and skin effects in interacting systems.

    DOI: 10.1103/PhysRevB.103.085428

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  • Hermitian zero modes protected by nonnormality: Application of pseudospectra 査読有り 国際誌

    Okuma N., Sato M.

    Physical Review B   102 ( 1 )   2020年07月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    Recently, it was established that there exists a direct relation between the non-Hermitian skin effects, strong dependence of spectra on boundary conditions for non-Hermitian Hamiltonians, and boundary zero modes for Hermitian topological insulators. On the other hand, in terms of the spectral theory, the skin effects can also be interpreted as instability of spectra for nonnormal (non-Hermitian) Hamiltonians. Applying the latter interpretation to the former relation, we develop a theory of zero modes with quantum anomaly for general Hermitian lattice systems. Our theory is applicable to a wide range of systems: Majorana chains, nonperiodic lattices, and long-range hopping systems. We relate exact zero modes and quasi-zero modes of a Hermitian system to spectra and pseudospectra of a non-Hermitian system, respectively. These zero and quasi-zero modes of a Hermitian system are robust against a class of perturbations even if there is no topological protection. The robustness is measured by nonnormality of the corresponding non-Hermitian system. We also present explicit construction of such zero modes by using a graphical representation of lattice systems. Our theory reveals the presence of nonnormality-protected zero modes, as well as the usefulness of the nonnormality and pseudospectra as tools for topological and/or non-Hermitian physics.

    DOI: 10.1103/PhysRevB.102.014203

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  • Non-Bloch band theory of non-Hermitian Hamiltonians in the symplectic class 査読有り 国際誌

    Kawabata K., Okuma N., Sato M.

    Physical Review B   101 ( 19 )   2020年05月

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    記述言語:英語   掲載種別:研究論文(学術雑誌)

    Non-Hermitian Hamiltonians are generally sensitive to boundary conditions, and their spectra and wave functions under open boundary conditions are not necessarily predicted by the Bloch band theory for periodic boundary conditions. To elucidate such a non-Bloch feature, recent works have developed a non-Bloch band theory that works even under arbitrary boundary conditions. Here, it is demonstrated that the standard non-Bloch band theory breaks down in the symplectic class, in which non-Hermitian Hamiltonians exhibit Kramers degeneracy because of reciprocity. Instead, a modified non-Bloch band theory for the symplectic class is developed in a general manner, as well as illustrative examples. This nonstandard non-Bloch band theory underlies the Z2 non-Hermitian skin effect protected by reciprocity.

    DOI: 10.1103/PhysRevB.101.195147

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  • Topological Phase Transition Driven by Infinitesimal Instability: Majorana Fermions in Non-Hermitian Spintronics 査読有り 国際誌

    Okuma N., Sato M.

    Physical Review Letters   123 ( 9 )   2019年08月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    Quantum phase transitions are intriguing and fundamental cooperative phenomena in physics. Analyzing a superconducting nanowire with spin-dependent non-Hermitian hopping, we discover a topological quantum phase transition driven by infinitesimal cascade instability. The anomalous phase transition is complementary to the universal non-Bloch wave behavior of non-Hermitian systems. We show that an infinite small magnetic field drastically suppresses the non-Hermitian skin effect, inducing a topological phase with Majorana boundary states. Furthermore, by identifying the bulk topological invariant, we establish the non-Hermitian bulk-boundary correspondence that does not have a Hermitian counterpart. We also discuss an experimental realization of the system by using the spin-current injection to a quantum wire.

    DOI: 10.1103/PhysRevLett.123.097701

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  • Nonreciprocal superposition state in antiferromagnetic optospintronics 査読有り 国際誌

    Okuma N.

    Physical Review B   99 ( 9 )   2019年03月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    The absence of net magnetization, which forbids any stray magnetic fields, is one of the greatest advantages of antiferromagnets in device applications. In conventional antiferromagnets, however, spin current cannot be extracted without the aid of a static magnetic field. Here, we develop a theory of antiferromagnetic optospintronics to resolve this fundamental dilemma. By coupling a linearly polarized photon and nonreciprocal magnon bands, we construct a superposition state of left- and right-handed magnon states with opposite group velocities. We numerically demonstrate that by using this superposition state, an antiferromagnetic spin current can be efficiently generated without a net magnetic field including net magnetization. We also find that the breakdown of the superposition state induces the stripe superfluid phase of a two-component Bose-Einstein condensate. Our results lay the foundation for manipulating the superposition states of emergent particles in devices.

    DOI: 10.1103/PhysRevB.99.094401

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  • Topological classification under nonmagnetic and magnetic point group symmetry: Application of real-space Atiyah-Hirzebruch spectral sequence to higher-order topology 査読有り 国際誌

    Okuma N., Sato M., Shiozaki K.

    Physical Review B   99 ( 8 )   2019年02月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    We classify time-reversal breaking (class A) spinful topological crystalline insulators with crystallographic nonmagnetic (32 types) and magnetic (58 types) point groups. The classification includes all possible magnetic topological crystalline insulators protected by point group symmetry. Whereas the classification of topological insulators is known to be given by the K-theory in the momentum space, computation of the K-theory has been a difficult task in the presence of complicated crystallographic symmetry. Here we consider the K-homology in the real space for this problem, instead of the K-theory in the momentum space, both of which give the same topological classification. We apply the Atiyah-Hirzebruch spectral sequence (AHSS) for computation of the K-homology, which is a mathematical tool for generalized (co)homology. In the real-space picture, the AHSS naturally gives the classification of higher-order topological insulators at the same time. By solving the group extension problem in the AHSS on the basis of physical arguments, we completely determine possible topological phases including higher-order ones for each point group. Relationships among different higher-order topological phases are argued in terms of the AHSS in the K-homology. We find that in some nonmagnetic and magnetic point groups, a stack of two Z2 second-order topological insulators can be smoothly deformed into nontrivial fourth-order topological insulators, which implies nontrivial group extensions in the AHSS.

    DOI: 10.1103/PhysRevB.99.085127

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  • Magnon Spin-Momentum Locking: Various Spin Vortices and Dirac magnons in Noncollinear Antiferromagnets 査読有り

    Okuma N.

    Physical Review Letters   119 ( 10 )   2017年09月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    We generalize the concept of the spin-momentum locking to magnonic systems and derive the formula to calculate the spin expectation value for one-magnon states of general two-body spin Hamiltonians. We give no-go conditions for magnon spin to be independent of momentum. As examples of the magnon spin-momentum locking, we analyze a one-dimensional antiferromagnet with the Néel order and two-dimensional kagome lattice antiferromagnets with the 120° structure. We find that the magnon spin depends on its momentum even when the Hamiltonian has the z-axis spin rotational symmetry, which can be explained in the context of a singular band point or a U(1) symmetry breaking. A spin vortex in momentum space generated in a kagome lattice antiferromagnet has the winding number Q=-2, while the typical one observed in topological insulator surface states is characterized by Q=+1. A magnonic analogue of the surface states, the Dirac magnon with Q=+1, is found in another kagome lattice antiferromagnet. We also derive the sum rule for Q by using the Poincaré-Hopf index theorem.

    DOI: 10.1103/PhysRevLett.119.107205

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  • Theory of the spin-Seebeck effect at a topological-insulator/ferromagnetic-insulator interface 査読有り 国際誌

    Okuma N., Masir M.R., MacDonald A.H.

    Physical Review B   95 ( 16 )   2017年04月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    The spin-Seebeck effect refers to voltage signals induced in metals by thermally driven spin currents in adjacent magnetic systems. We present a theory of the spin-Seebeck signal in the case where the conductor that supports the voltage signal is the topologically protected two-dimensional surface-state system at the interface between a ferromagnetic insulator (FI) and a topological insulator (TI). Our theory uses a Dirac model for the TI surface states and assumes Heisenberg exchange coupling between the TI quasiparticles and the FI magnetization. The spin-Seebeck voltage is induced by the TI surface states scattering off the nonequilibrium magnon population at the surface of the semi-infinite thermally driven FI. Our theory is readily generalized to spin-Seebeck voltages in any two-dimensional conductor that is exchange-coupled to the surface of a FI. Surface-state carrier-density-dependent signal strengths calculated using Bi2Te3 and yttrium iron garnet material parameters are consistent with recent experiments.

    DOI: 10.1103/PhysRevB.95.165418

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  • Microscopic derivation of magnon spin current in a topological insulator/ferromagnet heterostructure 査読有り 国際誌

    Okuma N., Nomura K.

    Physical Review B   95 ( 11 )   2017年03月

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    担当区分:筆頭著者   記述言語:英語   掲載種別:研究論文(学術雑誌)

    We investigate a spin-electricity conversion effect in a topological insulator/ferromagnet heterostructure. In the spin-momentum-locked surface state, an electric current generates nonequilibrium spin accumulation, which causes a spin-orbit torque that acts on the ferromagnet. When spins in the ferromagnet are completely parallel to the accumulated spin, this spin-orbit torque is zero. In the presence of spin excitations, however, a coupling between magnons and electrons enables us to obtain a nonvanishing torque. In this paper, we consider a model of the heterostructure in which a three-dimensional magnon gas is coupled with a two-dimensional massless Dirac electron system at the interface. We calculate the torque induced by an electric field, which can be interpreted as a magnon spin current, up to the lowest order of the electron-magnon interaction. We derive the expressions for high and low temperatures and estimate the order of magnitude of the induced spin current for realistic materials at room temperature.

    DOI: 10.1103/PhysRevB.95.115403

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担当授業科目(学内)

  • 2023年度   基礎量子力学

  • 2023年度   物理学・化学実験

  • 2023年度   物理学・化学実験

  • 2023年度   物理学・化学実験

  • 2023年度   物理学・化学実験

  • 2023年度   物理学・化学実験

  • 2023年度   物理学・化学実験

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