研究者詳細 - 早田　智也

論文 - 早田　智也

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Non-Hermitian Hubbard model without the sign problem

Hayata T., Yamamoto A.

Physical Review B （AMER PHYSICAL SOC） 104 （ 12 ） 2021年09月

査読有り, ISSN 24699950

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Thermalization of Yang-Mills theory in a (3+1)-dimensional small lattice system

Hayata T., Hidaka Y.

Physical Review D （AMER PHYSICAL SOC） 103 （ 9 ） 2021年05月

査読有り, ISSN 24700010

We study the real-time evolution of SU(2) Yang-Mills theory in a (3+1)-dimensional small lattice system after interaction quench. We numerically solve the Schrödinger equation with the Kogut-Susskind Hamiltonian in the physical Hilbert space obtained by solving Gauss law constraints. We observe the thermalization of a Wilson loop to the canonical state; the relaxation time is insensitive to the coupling strength and estimated as τeq∼2π/T with temperatures T at steady states. We also compute the vacuum persistence probability (the Loschmidt echo) to understand the relaxation from the dynamics of the wave function.

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Second order chiral kinetic theory under gravity and antiparallel charge-energy flow

Hayata T., Hidaka Y., Mameda K.

Journal of High Energy Physics （SPRINGER） 2021 （ 5 ） 2021年05月

査読有り, ISSN 1029-8479

We derive the chiral kinetic theory under the presence of a gravitational Riemann curvature. It is well-known that in the chiral kinetic theory there inevitably appears a redundant ambiguous vector corresponding to the choice of the Lorentz frame. We reveal that on top of this conventional frame choosing vector, higher-order quantum correction to the chiral kinetic theory brings an additional degrees of freedom to specify the distribution function. Based on this framework, we derive new types of fermionic transport, that is, the charge current and energy-momentum tensor induced by the gravitational Riemann curvature. Such novel phenomena arise not only under genuine gravity but also in a (pseudo-)relativistic fluid, for which inhomogeneous vorticity or temperature are effectively represented by spacetime metric tensor. It is especially found that the charge and energy currents are antiparallelly induced by an inhomogeneous fluid vorticity (more generally, by the Ricci tensor R0i), as a consequence of the spin-curvature coupling. We also briefly discuss possible applications to Weyl/Dirac semimetals and heavy-ion collision experiments.

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Thermalization of Yang-Mills theory in a ( )-dimensional small lattice system

T Hayata, Y Hidaka

Physical Review D 103 (9), 094502 2021年

Chiral vortical effect in relativistic and nonrelativistic systems

Atsuo Shitade, Kazuya Mameda, Tomoya Hayata

Physical Review B （American Physical Society (APS)） 102 （ 20 ） 2020年11月

査読有り, ISSN 2469-9950

We formulate the chiral vortical effect (CVE) and its generalization called generalized vortical effect using the semiclassical theory of wave packet dynamics. We take the spin-vorticity coupling into account and calculate the transport charge current by subtracting the magnetization one from the Noether local one. We find that the transport charge current in the CVE always vanishes in relativistic chiral fermions. This result implies that it cannot be observed in transport experiments in condensed matter systems such as Dirac/Weyl semimetals with the pseudo-Lorentz symmetry. We also demonstrate that the anisotropic CVE can be observed in nonrelativistic systems that belong to the point groups D-n, C-n (n = 2, 3, 4, 6), and C-1, such as n-type tellurium.

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String confinement in two-form lattice gauge theory

Tomoya Hayata, Arata Yamamoto

Physical Review D （AMER PHYSICAL SOC） 100 （ 7 ） 2019年10月

筆頭著者, 責任著者, 査読有り, ISSN 2470-0010

We study the confinement between vortex strings in the lattice gauge theory of the dual Abelian Higgs model. The dual lattice gauge theory is described by a two-form gauge field coupled with a one-form gauge field. We calculate the string-antistring potential from the surface operator of the two-form gauge field. The linear confining potential appears in the string confinement phase, and it disappears in the string deconfinement phase. The phase diagram of the theory is also obtained.

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Negative refractive index in cubic noncentrosymmetric superconductors

Hayata Tomoya

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS （OXFORD UNIV PRESS INC） 2019 （ 3 ） 2019年03月

筆頭著者, 責任著者, 査読有り, ISSN 2050-3911

We study the negative refractive index in cubic noncentrosymmetric superconductors. We consider the Maxwell equations under the Meissner effect, and the magnetoelectric effect arising due to inversion-symmetry breaking. We derive the dispersion relations of electromagnetic waves and show that the refractive index becomes negative at frequencies just below the plasma gap. We find that the chiral mechanism of the negative refractive index, which is usually discussed in chiral materials with negative permittivity, can be directly applied to noncentrosymmetric superconductors with positive permittivity. A rough estimation from experimental values of the penetration depth of LiPt3B indicates that a negative refractive index may appear in UV regions.

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A new route to negative refractive index from topological metals

Hayata Tomoya

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS （OXFORD UNIV PRESS INC） 2018 （ 8 ） 2018年08月

筆頭著者, 責任著者, 査読有り, ISSN 2050-3911

We discuss the possibility of realizing a negative refractive index in Weyl/Dirac semimetals. We consider the Maxwell equations with the plasma gap and chiral magnetic effect. By computing the dispersion relations of electromagnetic waves, we show that the refractive index becomes negative at frequencies (just) below the plasma frequency. We find that the axial anomaly, or, more specifically, the negative magnetoresistance (an electric current parallel to magnetic fields) opens a new route to realizing a negative refractive index. As an experimental implication, we compute the reflection and transmission coefficients of electromagnetic waves in a slab of Weyl/Dirac semimetals.

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Chiral magnetic effect of light

Tomoya Hayata

Physical Review B （American Physical Society） 97 （ 20 ） 2018年05月

筆頭著者, 責任著者, 査読有り, ISSN 2469-9969

We study a photonic analog of the chiral magnetic (vortical) effect. We discuss that the vector component of magnetoelectric tensors plays a role of "vector potential," and its rotation is understood as "magnetic field" of a light. Using the geometrical optics approximation, we show that "magnetic fields" cause an anomalous shift of a wave packet of a light through an interplay with the Berry curvature of photons. The mechanism is the same as that of the chiral magnetic (vortical) effect of a chiral fermion, so that we term the anomalous shift "chiral magnetic effect of a light." We further study the chiral magnetic effect of a light beyond geometric optics by directly solving the transmission problem of a wave packet at a surface of a magnetoelectric material. We show that the experimental signal of the chiral magnetic effect of a light is the nonvanishing of transverse displacements for the beam normally incident to a magnetoelectric material.

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Lattice simulation with the Majorana positivity

Arata Yamamoto, Tomoya Hayata

EPJ Web of Conferences （EDP Sciences） 175 2018年03月

査読有り, ISSN 2100-014X

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Topological properties of the chiral magnetic effect in multi-Weyl semimetals

Tomoya Hayata, Yuta Kikuchi, Yuya Tanizaki

PHYSICAL REVIEW B （AMER PHYSICAL SOC） 96 （ 8 ） 2017年08月

査読有り, ISSN 2469-9950

We compute the chiral magnetic effect (CME) in multi-Weyl semimetals (multi-WSMs) based on the chiral kinetic theory. Multi-WSMs are WSMs with multiple monopole charges that have nonlinear and anisotropic dispersion relations near the Weyl points, and we need to extend the conventional computation of the CME in WSMs with linear dispersion relations. The topological properties of the CME in multi-WSMs are investigated in detail for not only static magnetic fields but also time-dependent (dynamic) ones. We propose an experimental setup to measure the multiple monopole charge via the topological nature hidden in the dynamic CME.

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Kinetic theory and anomalous transports in the presence of nonabelian phase-space Berry curvatures

Tomoya Hayata, Yoshimasa Hidaka

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS （OXFORD UNIV PRESS INC） 2017 （ 7 ） 2017年07月

筆頭著者, 責任著者, 査読有り, ISSN 2050-3911

We study anomalous transport effects induced by the nonabelian Berry curvature. For that purpose we construct the kinetic theory in (2d)-dimensional phase space with abelian and nonabelian phase-space Berry curvatures. Then we calculate anomalous transport effects on the basis of the kinetic theory. As an example, we study anomalous charge and spin transport effects induced by the SU(2) Berry curvature such as the adiabatic spin pumping, the nonabelian chiral magnetic effect, and the spin analogue of the chiral magnetic effect. We also derive the topological effective theory to reproduce transport effects in insulators calculated from the kinetic theory. Such an effective theory is given by the nonabelian phase-space Chern-Simons theory.

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Quantum Monte Carlo simulation of a two-dimensional Majorana lattice model

Tomoya Hayata, Arata Yamamoto

PHYSICAL REVIEW B （AMER PHYSICAL SOC） 96 （ 3 ） 2017年07月

査読有り, ISSN 2469-9950

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Momentum-space electromagnetic induction in Weyl semimetals

Hiroaki Ishizuka, Tomoya Hayata, Masahito Ueda, Naoto Nagaosa

PHYSICAL REVIEW B （AMER PHYSICAL SOC） 95 （ 24 ） 2017年06月

査読有り, ISSN 2469-9950

We theoretically study the effect of the Berry curvature on the transport properties of Weyl semimetals using a semiclassical Boltzmann transport theory, which results in nonlinear optical responses. In the adiabatic process, the Berry curvature, which involves the time derivative of the Bloch states, contributes to the transport properties such as the adiabatic Thouless pump. Although this effect is very weak in usual solids, it is enhanced in Weyl semimetals, where the Berry curvature contributes to observable nonlinear optical responses due to its nodal structure. In this paper, using the semiclassical Boltzmann theory, we show that a dc photocurrent induced by the Berry curvature robustly persists even in the limit of short scattering time. We also show that the photocurrent is well explained as a consequence of the electromagnetic induction in momentum space. The results indicate that the electromagnetic induction gives rise to a nondissipative photocurrent that is robust against decoherence within a time scale shorter than the periodicity of the incident electromagnetic field. We also discuss the second harmonic response of an ac current when the electron distribution is displaced from the ground state by an external field.

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Classification of sign-problem-free relativistic fermions on the basis of the Majorana positivity

Tomoya Hayata, Arata Yamamoto

PROGRESS OF THEORETICAL AND EXPERIMENTAL PHYSICS （OXFORD UNIV PRESS INC） 2017 （ 4 ） 2017年04月

筆頭著者, 責任著者, 査読有り, ISSN 2050-3911

We classify the sign-problem-free relativistic fermion actions on the basis of the Majorana representation. In the Majorana representation, the sign-problem-free condition is given by the semi-positivity of a Pfaffian. We show that the known sign-problem-free actions of the Dirac fermions, which are usually understood from the semi-positivity of a determinant, e.g., the action of quantum chromodynamics with nonzero chiral chemical potential or nonzero isospin chemical potential, can also be understood from the semi-positivity of a Pfaffian. We also derive new classes of the sign-problem-free relativistic fermion actions with Majorana-type source terms.

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Kinetic derivation of generalized phase space Chern-Simons theory

Tomoya Hayata, Yoshimasa Hidaka

PHYSICAL REVIEW B （AMER PHYSICAL SOC） 95 （ 12 ） 2017年03月

筆頭著者, 責任著者, 査読有り, ISSN 2469-9950

We study anomalous transport phenomena induced by phase space Berry curvature. For that purpose we construct a kinetic theory in 2d phase space when all Abelian Berry curvatures are nonzero. We derive anomalous currents by calculating the complete form of the Poisson brackets of phase space coordinates. Then we construct the low-energy effective theory to reproduce the anomalous currents obtained from the kinetic theory. Such an effective theory is given by the Chern-Simons theory in 1 + 2d dimensions. Some implications of the Chern-Simons theory are also discussed.

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Emergent Electromagnetic Induction and Adiabatic Charge Pumping in Noncentrosymmetric Weyl Semimetals

Hiroaki Ishizuka, Tomoya Hayata, Masahito Ueda, Naoto Nagaosa

PHYSICAL REVIEW LETTERS （AMER PHYSICAL SOC） 117 （ 21 ） 2016年11月

査読有り, ISSN 0031-9007

The photovoltaic effect due to the adiabatic quantum phase in noncentrosymmetric Weyl semimetals is studied. We particularly focus on the case in which an external ac electric field is applied. By considering a generalized Weyl Hamiltonian with nonlinear terms, we show that the photocurrent is induced by circularly, rather than linearly, polarized light. This photovoltaic current can be understood as an emergent electromagnetic induction in momentum space; the Weyl node is a magnetic monopole in momentum space, the circular motion of which induces the electric field. This result is distinct from conventional photovoltaic effects, and the estimated photocurrent is similar to 10(-1)-10(1) nA, which can be detected experimentally.

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Complex saddle points and the sign problem in complex Langevin simulation

Tomoya Hayata, Yoshimasa Hidaka, Yuya Tanizaki

NUCLEAR PHYSICS B （ELSEVIER SCIENCE BV） 911 94 - 105 2016年10月

筆頭著者, 責任著者, 査読有り, ISSN 0550-3213

We show that complex Langevin simulation converges to a wrong result within the semiclassical analysis, by relating it to the Lefschetz-thimble path integral, when the path-integral weight has different phases among dominant complex saddle points. Equilibrium solution of the complex Langevin equation forms local distributions around complex saddle points. Its ensemble average approximately becomes a direct sum of the average in each local distribution, where relative phases among them are dropped. We propose that by taking these phases into account through reweighting, we can solve the wrong convergence problem. However, this prescription may lead to a recurrence of the sign problem in the complex Langevin method for quantum many-body systems. (C) 2016 The Author(s). Published by Elsevier B.V.

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Lefschetz-thimble analysis of the sign problem in one-site fermion model

Yuya Tanizaki, Yoshimasa Hidaka, Tomoya Hayata

NEW JOURNAL OF PHYSICS （IOP PUBLISHING LTD） 18 （ 3 ） 2016年03月

査読有り, ISSN 1367-2630

The Lefschetz-thimble approach to path integrals is applied to a one-site model of electrons, i.e., the one-site Hubbard model. Since the one-site Hubbard model shows a non-analytic behavior at the zero temperature and its path integral expression has the sign problem, this toy model is a good testing ground for an idea or a technique to attack the sign problem. Semiclassical analysis using complex saddle points unveils the significance of interference among multiple Lefschetz thimbles to reproduce the non-analytic behavior by using the path integral. If the number of Lefschetz thimbles is insufficient, we found not only large discrepancies from the exact result, but also thermodynamic instabilities. Analyzing such singular behaviors semiclassically, we propose a criterion to identify the necessary number of Lefschetz thimbles. We argue that this interference of multiple saddle points is a key issue to understand the sign problem of the finite-density quantum chromodynamics.

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Lefschetz-thimble approach to the Silver Blaze problem of one-site fermion model

Tanizaki Y, Hidaka Y, Hayata T

Proceedings of Science Part F128557 2016年

査読有り

The sign problem of finite-density QCD at the zero temperature becomes very
severe if the quark chemical potential exceeds half of the pion mass. In order
to understand its property, we consider the sign problem of the one-site
fermion model appearing in its path-integral expression by using the
Lefschetz-thimble method. We show that the original integration cycle becomes
decomposed into multiple Lefschetz thimbles at a certain value of the fermion
chemical potential, which would correspond to half of the pion mass of
finite-density QCD. This triggers a fictitious phase transition on each
Lefschetz thimble, and the interference of complex phases among them plays an
important role for the correct description of the system. We also show that the
complex Langevin method does not work in this situation.

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論文 - 早田　智也