Tsuneya Yoshida

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Yoshida

First Name

Tsuneya

ORCID

0000-0002-2276-1009

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Publications 1 - 9 of 9

Band structures of generalized eigenvalue equations and conic sections
Isobe, Takuma; Yoshida, Tsuneya; Hatsugai, Yasuhiro (2025)
Physical Review A
Generalized eigenvalue equations describe the band structures of various metamaterials, where complex bands can emerge even if the involved matrices are Hermitian. In this paper, we provide a geometrical understanding of the real-complex transition of the band structures. Specifically, our analysis, based on auxiliary eigenvalues, clarifies the correspondence between the real-complex transition of the generalized eigenvalue equations and the Lifshitz transition in electron systems. Furthermore, we demonstrate that real (complex) bands of a photonic system correspond to the Fermi surfaces of type-II (type-I) Dirac cones in electron systems when the permittivity ε and the permeability μ are independent of frequency. In addition, our analysis reveals that exceptional points are induced by the frequency dependence of permittivity ε and permeability μ in our photonic system.
Multifractal statistics of non-Hermitian skin effect on the Cayley tree
Hamanaka, Shu; Iliasov, Askar A.; Neupert, Titus; et al. (2025)
Physical Review B
Multifractal analysis is a powerful tool for characterizing the localization properties of wave functions. Despite its utility, this tool has been predominantly applied to disordered Hermitian systems. Multifractal statistics associated with the non-Hermitian skin effect remain largely unexplored. Here, we demonstrate that the tree geometry induces multifractal statistics for the single-particle skin states on the Cayley tree by deriving the analytical expression of multifractal dimensions. This sharply contrasts with the absence of multifractal properties for conventional single-particle skin effects in crystalline lattices. Our work uncovers the unique feature of the skin effect on the Cayley tree and provides a novel mechanism for inducing multifractality in open quantum systems without disorder.
Non-Hermitian Topology in Hermitian Topological Matter
Hamanaka, Shu; Yoshida, Tsuneya; Kawabata, Kohei (2024)
Physical Review Letters
Non-Hermiticity gives rise to distinctive topological phenomena absent in Hermitian systems. However, connection between such intrinsic non-Hermitian topology and Hermitian topology has remained largely elusive. Here, considering the bulk and boundary as an environment and system, respectively, we demonstrate that anomalous boundary states in Hermitian topological insulators exhibit non-Hermitian topology. We study the self-energy capturing the particle exchange between the bulk and boundary, and show that it detects Hermitian topology in the bulk and induces non-Hermitian topology at the boundary. As an illustrative example, we reveal non-Hermitian topology and concomitant skin effect inherently embedded within chiral edge states of Chern insulators. We also identify the emergence of hinge states within effective non-Hermitian Hamiltonians at surfaces of three-dimensional topological insulators. Furthermore, we comprehensively classify our correspondence across all the tenfold symmetry classes of topological insulators and superconductors. Our Letter uncovers hidden connection between Hermitian and nonHermitian topology, and provides an approach to identifying non-Hermitian topology in quantum matter.
Winding topology of multifold exceptional points
Yoshida, Tsuneya; König, J. Lukas K.; Rødland, Lukas; et al. (2025)
Physical Review Research
Despite their ubiquity, a systematic classification of multifold exceptional points, n-fold spectral degeneracies (EPns), remains a significant unsolved problem. In this article, we characterize the Abelian eigenvalue topology of generic EPns and symmetry-protected EPns for arbitrary n. The former and the latter emerge in (2n-2)- and (n-1)-dimensional parameter spaces, respectively. By introducing topological invariants called resultant winding numbers, we elucidate that these EPns are stable due to topology of a map from a base space (momentum or parameter space) to a sphere defined by resultants. In a D-dimensional parameter space (D≥c), the resultant winding numbers topologically characterize (D-c)-dimensional manifolds of generic (symmetry-protected) EPns, whose codimension is c=2n-2 (c=n-1). Our framework implies fundamental doubling theorems for both generic EPns and symmetry-protected EPns in n-band models.
Exceptional points and non-Hermitian skin effects under nonlinearity of eigenvalues
Yoshida, Tsuneya; Isobe, Takuma; Hatsugai, Yasuhiro (2025)
Physical Review B
Band structures of metamaterials described by a nonlinear eigenvalue problem are beyond the existing topological band theory. In this paper, we analyze non-Hermitian topology under the nonlinearity of eigenvalues. Specifically, we elucidate that such nonlinear systems may exhibit exceptional points and non-Hermitian skin effects which are unique non-Hermitian topological phenomena. The robustness of these non-Hermitian phenomena is clarified by introducing the topological invariants under nonlinearity which reproduce the existing ones in linear systems. Furthermore, our analysis elucidates that exceptional points may emerge even for systems without an internal degree of freedom, where the equation is a single component. These nonlinearity-induced exceptional points are observed in mechanical systems, e.g., the Kapitza pendulum.
Non-Hermitian Z4 skin effect protected by glide symmetry
Ishikawa, Sho; Yoshida, Tsuneya (2024)
Physical Review B
Although nonsymmorphic symmetry protects Z4 topology for Hermitian systems, non-Hermitian topological phenomena induced by such a unique topological structure remain elusive. In this paper, we elucidate that systems with glide symmetry exhibit non-Hermitian skin effects (NHSE) characterized by Z4 topology. Specifically, numerically analyzing a two-dimensional toy model, we demonstrate that the Z4 topology induces the NHSE when the topological invariant takes ν=1 and 2. Furthermore, our numerical analysis demonstrates that the NHSE is destroyed by perturbations preserving the relevant symmetry when the Z4 invariant takes ν=4.
Non-Hermitian Mott Skin Effect
Yoshida, Tsuneya; Zhang, Song-Bo; Neupert, Titus; et al. (2024)
Physical Review Letters
We propose a novel type of skin effects in non-Hermitian quantum many-body systems that we dub a "non-Hermitian Mott skin effect."This phenomenon is induced by the interplay between strong correlations and the non-Hermitian point-gap topology. The Mott skin effect induces extreme sensitivity to the boundary conditions only in the spin degree of freedom (i.e., the charge distribution is not sensitive to boundary conditions), which is in sharp contrast to the ordinary non-Hermitian skin effect in noninteracting systems. Concretely, we elucidate that a bosonic non-Hermitian chain exhibits the Mott skin effect in the strongly correlated regime by closely examining an effective Hamiltonian. The emergence of the Mott skin effect is also supported by numerical diagonalization of the bosonic chain. The difference between the ordinary non-Hermitian skin effect and the Mott skin effect is also reflected in the time evolution of physical quantities; under the time evolution spin accumulation is observed while the charge distribution remains spatially uniform.
Hinge non-Hermitian skin effect in the single-particle properties of a strongly correlated f-electron system
Peters, Robert; Yoshida, Tsuneya (2024)
Physical Review B
Non-Hermitian systems exhibit novel phenomena without Hermitian counterparts, such as exceptional points and the non-Hermitian skin effect. These non-Hermitian topological phenomena are observable in single-particle excitations of correlated systems in equilibrium, which are described by Green's functions. In this paper we demonstrate the appearance of the hinge non-Hermitian skin effect in the effective Hamiltonian that describes the single-particle properties of an f-electron system. Skin effects result in a strong sensitivity to boundary conditions, and a large number of eigenstates localize at one boundary when open boundary conditions are applied. Our system exhibits such sensitivity and hosts skin modes localized around hinges. This hinge skin effect is induced by a non-Hermitian topology of the surface Brillouin zone. The hinge skin modes are observed for one-dimensional subsystems located between one pair of exceptional points in the surface Brillouin zone. This paper highlights that correlated materials are an exciting platform for analyzing non-Hermitian phenomena.
Hopf exceptional points
Yoshida, Tsuneya; Bergholtz, Emil J.; Bzdušek, Tomáš (2026)
SciPost Physics
Exceptional points at which eigenvalues and eigenvectors of non-Hermitian matrices coalesce are ubiquitous in the description of a wide range of platforms from photonic or mechanical metamaterials to open quantum systems. Here, we introduce a class of Hopf exceptional points (HEPs) that are protected by the Hopf invariants (including the higher-dimensional generalizations) and which exhibit phenomenology sharply distinct from conventional exceptional points. Saliently, owing to their Z2 topological invariant related to the Witten anomaly, three-fold HEPs and symmetry-protected five-fold HEPs act as their own "antiparticles". Furthermore, based on higher homotopy groups of spheres, we predict the existence of multifold HEPs and symmetry-protected HEPs with non-Hermitian topology captured by a range of finite groups (such as Z3, Z12, or Z24) beyond the periodic table of Bernard-LeClair symmetry classes.

Publications 1 - 9 of 9

Tsuneya Yoshida

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