__Keywords:__

topological insulator

orbital current

nonlinear response

THz light

Nonlinear orbital response in topological insulators

Nonlinear optical responses are often used as a probe for studying electronic properties. For the topological materials, studies so far focused on the photogalvanic electric current, which requires the breaking of inversion symmetry. In this work we present the theory for orbital current response in inversion-symmetric topological insulators. We find a symmetry-allowed orbital current response that occurs in centrosymmetric materials under illumination by the linearly polarized light. The sign of the dc nonlinear conductivity reflects the Z2 index and the conductivity changes the sign at the phase transition. We derive a general result, and discuss its application to Bernevig-Hughes-Zhang model and 1T' phase of transition metal dichalcogenides. Experimental setups for the observation of the orbital current are also discussed.

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twisted bilayer graphene

Purcell effect

electron-lattice cooling

resistivity

Purcell effect in moire graphene

The efficiency of optical emitters can be dramatically enhanced by reducing the effective mode volume (the Purcell effect). Here we predict an analogous enhancement for electron-phonon (el-ph) scattering, achieved by compressing the electronic Wannier orbitals. Reshaping of Wannier orbitals is a prominent effect in graphene moire superlattices (SLs) where the orbitals are tunable by the twist angle. A reduction of the orbital effective volume leads to an enhancement in the effective el-ph coupling strength, yielding the values considerably bigger than those known for pristine monolayer graphene. The enhanced coupling boosts the el-ph scattering rates, pushing them above the values predicted from the enhanced spectral density of electronic excitations. The enhanced phonon emission and scattering rates are manifest in the observables such as electron-lattice cooling and the linear-T resistivity, both of which are directly tunable by the moire twist angle.

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nonlinear response

spintronics

shift current

THz light

Optical rectification of spin current in quantum spin systems

We theoretically propose a method of rectifying spin current with a linearly polarized electromagnetic wave in inversion-asymmetric magnetic insulators. To demonstrate the proposal, we consider quantum spin chains as a simple example; these models are mapped to fermion (spinon) models via Jordan-Wigner transformation. Using a nonlinear response theory, we find that a dc spin current is generated by the linearly polarized waves. The spin current shows rich anisotropic behavior depending on the direction of the electromagnetic wave. This is a manifestation of the rich interplay between spins and the waves; inverse Dzyaloshinskii-Moriya, Zeeman, and magnetostriction couplings lead to different behaviors of the spin current. The resultant spin current is insensitive to the relaxation time of spinons, a property of which potentially benefits a long-distance propagation of the spin current. An estimate of the required electromagnetic wave is given.

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electrical magnetochiral effect

nonreciprocal current

magnetic scattering

spin chirality

helical magnet

Nonreciprocal current by magnetic fluctuation

The non-collinear spin configurations give rise to many nontrivial phenomena related to the Berry phase. They are often related to the vector and scalar spin chiralities. The scalar spin chirality leads to the topological Hall effect in metals, while the vector spin chirality to the ferroelectricity of spin origin, i.e., multiferroics in insulators. However, the role of the vector spin chirality in conducting systems has not yet been elucidated. Here we show theoretically that the spin correlation with vector spin chirality in chiral magnets scatters electrons asymmetrically, resulting in nonreciprocal transport phenomena, i.e., electrical magnetochiral effect. This asymmetric scattering appears in the leading-order scattering term, implying a large nonreciprocity in the charge and spin currents. We find that the temperature and magnetic field dependence of the eMCE reproduces that observed in MnSi. Our results reveal the microscopic mechanism of eMCE and its potential in producing a large nonreciprocal response.

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anomalous Hall effect

skew scattering

skyrmion

chiral magnet

spin Hall effect

Large anomalous Hall effect by spin-cluster scattering

Noncoplanar magnetic orders in magnetic metals give rise to an anomalous Hall effect of unconventional origin, which, by the spin Berry phase effect, is known as the topological Hall effect. This effect is pronounced in the low temperature limit, where the fluctuation of spins is suppressed. In contrast, we here discuss that the fluctuating but locally correlated spins close to the phase boundary give rise to another anomalous Hall effect, thatwith the opposite sign to the topologicalHall effect. Using the Born approximation, we show that the anomalous Hall effect is attributed to the skew scattering induced by the local correlation of spins. The relation of the scalar spin chirality to the skew scattering amplitude is given, and the explicit formula for the Hall conductivity is derived using a semiclassical Boltzmann transport theory. Our theory potentially accounts for the sign change of the anomalous Hall effect observed in chiral magnets in the vicinity of the phase boundary.