Language：English   Publishing type：Research paper (scientific journal)   Publisher：Science advances  

Nonlinear transport phenomena in condensed matter reflect the geometric nature, quantum coherence, and many-body correlation of electronic states. Electric currents in solids are classified into (i) ohmic current, (ii) supercurrent, and (iii) geometric or topological current. While the nonlinear current-voltage (I-V) characteristics of the former two categories have been extensive research topics recently, those of the last category remains unexplored. Among them, the quantum Hall current is a representative example. Realized in two-dimensional electronic systems under a strong magnetic field, the topological protection quantizes the Hall conductance in the unit of e2/h (e, elementary charge; and h, Planck constant), of which the edge transport picture gives a good account. Here, we theoretically study the nonlinear I-VH characteristic of the edge transport up to third order in VH. We find that nonlinearity arises in the Hall response from electron-electron interaction between the counterpropagating edge channels with the nonlinear energy dispersions. We also discuss possible experimental observations.

DOI： 10.1126/sciadv.ado2704

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Repository Public URL： https://hdl.handle.net/2324/7361955

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Nature Nanotechnology  

In two-dimensional systems, perpendicular magnetic fields can induce a bulk band gap and chiral edge states, which gives rise to the quantum Hall effect. The quantum Hall effect is characterized by zero longitudinal resistance (Rxx) and Hall resistance (Rxy) plateaus quantized to h/(υe2) in the linear response regime, where υ is the Landau level filling factor, e is the elementary charge and h is Planck’s constant. Here we explore the nonlinear response of monolayer graphene when tuned to a quantum Hall state. We observe a third-order Hall effect that exhibits a nonzero voltage plateau scaling cubically with the probe current. By contrast, the third-order longitudinal voltage remains zero. The magnitude of the third-order response is insensitive to variations in magnetic field (down to ~5 T) and in temperature (up to ~60 K). Moreover, the third-order response emerges in graphene devices with a variety of geometries, different substrates and stacking configurations. We term the effect third-order nonlinear response of the quantum Hall state and propose that electron–electron interaction between the quantum Hall edge states is the origin of the nonlinear response of the quantum Hall state.

DOI： 10.1038/s41565-024-01730-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：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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Publishing type：Research paper (scientific journal)   Publisher：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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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

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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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.107.L201116/fulltext

Total pages：XII, 136   Language：English   Book type：Scholarly book

DOI： 10.1007/978-981-10-3743-6

Event date： 2024.11

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Katahira Campus, Tohoku University, Sendai, JAPAN  

Other Link： https://www.crc-ms.tohoku.ac.jp/en/initiatives/case02/CRCGP-MSSP2024.html

Event date： 2024.8

Language：Japanese   Presentation type：Public lecture, seminar, tutorial, course, or other speech  

Venue：愛知県蒲郡市  

Repository Public URL： https://hdl.handle.net/2324/7361954

Event date： 2024.5

Language：English   Presentation type：Oral presentation (invited, special)  

Venue：Wako, Saitama, Japan  

Authorship：Lead author,　Last author,　Corresponding author   Language：Japanese   Publishing type：Lecture material (seminar, tutorial, course, lecture, etc.)  

DOI： 10.57393/natsugaku.3.0_257

Author：Other 

Author：Other 

Author：Other