1.3k citations · 2.5k across the 22 of their papers we have counts for
22 papers
Universal platform of point-gap topological phases from topological materials
Daichi Nakamura, Kazuya Inaka, Nobuyuki Okuma +1
Whereas point-gap topological phases are responsible for exceptional phenomena intrinsic to non-Hermitian systems, their realization in quantum materials is still elusive. Here we…
Topological enhancement of non-normality in non-Hermitian skin effects
Yusuke O. Nakai, Nobuyuki Okuma, Daichi Nakamura +2
The non-Hermitian skin effects are representative phenomena intrinsic to non-Hermitian systems: the energy spectra and eigenstates under the open boundary condition (OBC) drastical…
Relationship between two-particle topology and fractional Chern insulator
Nobuyuki Okuma, Tomonari Mizoguchi
Lattice generalizations of fractional quantum Hall (FQH) systems, called fractional Chern insulators (FCIs), have been extensively investigated in strongly correlated systems. Desp…
Non-Hermitian topological phenomena: A review
Nobuyuki Okuma, Masatoshi Sato
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,…
Boundary-dependent dynamical instability of bosonic Green's function: Dissipative Bogoliubov-de Gennes Hamiltonian and its application to non-Hermitian skin effect
Nobuyuki Okuma
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) Hamiltoni…
Time-crystalline long-range order in chiral fermionic vacuum
Nobuyuki Okuma
It is widely believed that there is no macroscopic time-crystalline order in the ground states of short-range interacting systems. In this paper, we consider a time-dependent corre…
Time-crystalline long-range order in squeezed ground state
Nobuyuki Okuma
It is widely believed that ground-state time crystals are not realizable in realistic macroscopic systems. In particular, Watanabe and Oshikawa proved a theorem that implies the ab…
Non-normal Hamiltonian dynamics in quantum systems and its realization on quantum computers
Nobuyuki Okuma, Yuya O. Nakagawa
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…
Nontrivial interband effect: applications to magnetic susceptibility, nonlinear optics, and topological degeneracy pressure
Nobuyuki Okuma
The interband effect is an important concept both in traditional and modern solid-state physics. In this paper, we present a theory of the nontrivial interband effect, which cannot…
Quantum anomaly, non-Hermitian skin effects, and entanglement entropy in open systems
Nobuyuki Okuma, Masatoshi Sato
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…
Non-Hermitian Skin Effects in Hermitian Correlated/Disordered Systems: Boundary-Sensitive/Insensitive Quantities and Pseudo Quantum Number
Nobuyuki Okuma, Masatoshi Sato
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 s…
Hermitian zero modes protected by nonnormality: Application of pseudospectra
Nobuyuki Okuma, Masatoshi Sato
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 Hami…
Non-Bloch band theory of non-Hermitian Hamiltonians in the symplectic class
Kohei Kawabata, Nobuyuki Okuma, Masatoshi Sato
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…
Topological Origin of Non-Hermitian Skin Effects
Nobuyuki Okuma, Kohei Kawabata, Ken Shiozaki +1
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 intr…
Topological Phase Transition driven by Infinitesimal Instability: Majorana Fermions in Non-Hermitian Spintronics
Nobuyuki Okuma, Masatoshi Sato
Quantum phase transitions are intriguing and fundamental cooperative phenomena in physics. Analyzing a superconducting nanowire with spin-dependent non-Hermitian hopping, we discov…
Topological classification under nonmagnetic and magnetic point group symmetry: Application of real-space Atiyah-Hirzebruch spectral sequence to higher-order topology
Nobuyuki Okuma, Masatoshi Sato, Ken Shiozaki
We classify time-reversal breaking (class A) spinful topological crystalline insulators with crystallographic non-magnetic (32 types) and magnetic (58 types) point groups. The clas…
Nonreciprocal superposition state in antiferromagnetic optospintronics
Nobuyuki Okuma
The absence of net magnetization, which forbids any stray magnetic fields, is one of the greatest advantages of antiferromagnets in device applications. In conventional antiferroma…
Magnon Spin-Momentum Locking: Various Spin Vortices and Dirac Magnons in Noncollinear Antiferromagnets
Nobuyuki Okuma
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…
Theory of the Spin Seebeck Effect at a Topological-Insulator/Ferromagnetic-Insulator Interface
Nobuyuki Okuma, Massoud Ramezani Masir, Allan H. MacDonald
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 i…
Microscopic derivation of magnon spin current in a topological insulator/ferromagnet heterostructure
Nobuyuki Okuma, Kentaro Nomura
We investigate a spin-electricity conversion effect in a topological insulator/ferromagnet heterostructure. In the spin-momentum-locked surface state, an electric current generates…
Unconventional Spin Hall Effect and Axial Current Generation in a Dirac Semimetal
Nobuyuki Okuma, Masao Ogata
We investigate electrical transport in a three-dimensional massless Dirac fermion model that describes a Dirac semimetal state realized in topological materials. We derive a set of…
Long-range Coulomb interaction effects on the surface Dirac electron system of a three-dimensional topological insulator
Nobuyuki Okuma, Masao Ogata
The surface state of a three-dimensional topological insulator forms a two-dimensional massless Dirac electron system. In Dirac electron systems, Coulomb interaction is not screene…