記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Nano Letters  

We study the polarization response to a temperature gradient in insulators, known as the thermopolarization effect. We show that this response can be understood through the free energy response function to an electric field gradient, which we call the Q tensor. By using the Q tensor, we present a unified description of the polarization responses to both electric fields and temperature gradients and derive the generalized Mott relation. Additionally, we draw an analogy with the anomalous Hall and Nernst effects. These effects are observable as the Seebeck effect when the system’s linear size is shorter than the screening length.

DOI： 10.1021/acs.nanolett.4c05870

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掲載種別：研究論文（学術雑誌）   出版者・発行元：npj Quantum Materials  

The incommensurate charge density wave states (CDWs) can exhibit steady motion in the flow limit after depinning, behaving as a nonequilibrium system with time-dependent states. Since the moving CDW, like an electric current, breaks both time-reversal and inversion symmetries, one may speculate the emergence of nonreciprocal nonlinear responses from such motion. However, the moving CDW order parameter is intrinsically time-dependent in the lab frame, and it is known to be challenging to evaluate the responses of such a time-varying system. In this work, following the principle of Galilean relativity, we resolve this time-dependent hard problem in the lab frame by mapping the system to the comoving frame with static CDW states through the Galilean transformation. We explicitly show that the nonreciprocal nonlinear responses would be generated by the movement of CDW states through violating Galilean relativity.

DOI： 10.1038/s41535-024-00695-7

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review B  

Non-Hermitian descriptions often model open or driven systems away from the equilibrium. Nonetheless, in equilibrium electronic systems, a non-Hermitian nature of an effective Hamiltonian manifests itself as unconventional observables such as a bulk Fermi arc and skin effects. We theoretically reveal that spin-dependent quasiparticle lifetimes, which signify the non-Hermiticity of an effective model in the equilibrium, induce the anomalous Hall effect, namely, the Hall effect without an external magnetic field. We first examine the effect of nonmagnetic and magnetic impurities and obtain a non-Hermitian effective model. Then, we calculate the Kubo formula from the microscopic model to ascertain a non-Hermitian interpretation of the longitudinal and Hall conductivities. Our results elucidate the vital role of the non-Hermitian equilibrium nature in the quantum transport phenomena.

DOI： 10.1103/PhysRevB.107.L201116

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その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.107.L201116/fulltext

記述言語：日本語   出版者・発行元：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.78.2.0_1038

CiNii Research

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Physical Review Letters  

The relativistic Dirac equation covers the fundamentals of electronic phenomena in solids and as such it effectively describes the electronic states of the topological insulators like Bi2Se3 and Bi2Te3. Topological insulators feature gapless surface states and, moreover, magnetic doping and resultant ferromagnetic ordering break time-reversal symmetry to realize quantum anomalous Hall and Chern insulators. Here, we focus on the bulk and investigate the mutual coupling of electronic and magnetic properties of Dirac electrons. Without carrier doping, spiral magnetic orders cause a ferroelectric polarization through the spin-orbit coupling. In a doped metallic state, the anisotropic magnetoresistance arises without uniform magnetization. We find that electric current induces uniform magnetization and conversely an oscillating magnetic order induces electric current. Our model provides a coherent and unified description of all those phenomena. The mutual control of electric and magnetic properties demonstrates implementations of antiferromagnetic spintronics. We also discuss the stoichiometric magnetic topological insulator MnBi2Te4.

DOI： 10.1103/PhysRevLett.129.216601

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その他リンク： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevLett.129.216601/fulltext

記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Journal of the Physical Society of Japan  

We propose the second-order response of metals in an electric field induced by magnetic impurities which locally break inversion symmetry. The impurities with toroidal moments scatter conduction electrons in the presence of the spin–orbit coupling, leading to nonreciprocal response. This mechanism is ubiquitous when a magnetic impurity is placed off an inversion center such as an interstitial site and a surface of a two-dimensional system.

DOI： 10.7566/JPSJ.91.115001

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記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：Nature Nanotechnology  

Graphene-based samples have shown a plethora of exotic characteristics and these properties may help the realization of a new generation of fast electronic devices. However, graphene’s centrosymmetry prohibits second-order electronic transport. Here, we show giant second-order nonlinear transports in graphene moiré superlattices at zero magnetic field, both longitudinal and transverse to the applied current direction. High carrier mobility and inversion symmetry breaking by hexagonal boron nitride lead to nonlinear conductivities five orders of magnitude larger than those in WTe2. The nonlinear conductivity strongly depends on the gate voltage as well as on the stacking configuration, with a giant enhancement originating from the moiré bands. Longitudinal nonlinear conductivity cannot originate from Berry curvature dipoles. Our theoretical modelling highlights skew scattering of chiral Bloch electrons as the physical origin. With these results, we demonstrate nonlinear charge transport due to valley-contrasting chirality, which constitutes an alternative means to induce second-order transports in van der Waals heterostructures. Our approach is promising for applications in frequency-doubling and energy harvesting via rectification.

DOI： 10.1038/s41565-021-01060-6

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開催年月日： 2024年5月

記述言語：英語   会議種別：口頭発表（招待・特別）  

開催地：Wako, Saitama, Japan