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Register a new userWhen chiral photons meet chiral fermions - Photoinduced anomalous Hall effects in Weyl semimetals
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The Weyl semimetal is characterized by three-dimensional linear band touching points called Weyl nodes. These nodes come in pairs with opposite chiralities. We show that the coupling of circularly polarized photons with these chiral electrons generates a Hall conductivity without any applied magnetic field in the plane orthogonal to the light propagation. This phenomenon comes about because with all three Pauli matrices exhausted to form the three-dimensional linear dispersion, the Weyl nodes cannot be gapped. Rather, the net influence of chiral photons is to shift the positions of the Weyl nodes. Interestingly, the momentum shift is tightly correlated with the chirality of the node to produce a net anomalous Hall signal. Application of our proposal to the recently discovered TaAs family of Weyl semimetals leads to an order-of-magnitude estimate of the photoinduced Hall conductivity which is within the experimentally accessible range.
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We report on exotic response properties in 3D time-reversal invariant Weyl semimetals with mirror symmetry. Despite having a vanishing anomalous Hall coefficient, we find that the momentum-space quadrupole moment formed by four Weyl nodes determines the coefficient of a mixed emph{electromagnetic charge-stress} response, in which momentum flows perpendicular to an applied electric field, and electric charge accumulates on certain types of lattice defects. This response is described by a mixed Chern-Simons-like term in 3 spatial dimensions, which couples a rank-2 gauge field to the usual electromagnetic gauge field. On certain 2D surfaces of the bulk 3D Weyl semimetal, we find what we will call rank-2 chiral fermions, with $omega =k_x k_y$ dispersion. The intrinsically 2D rank-2 chiral fermions have a mixed charge-momentum anomaly which is cancelled by the bulk of the 3D system.
Chiral anomaly or Adler-Bell-Jackiw anomaly in Weyl semimetals (WSMs) has a significant impact on the electron transport behaviors, leading to remarkable longitudinal or planar electrical and thermoelectric transport phenomena in the presence of electromagnetic gauge fields. These phenomena are consequences of the imbalanced chiral charge and energy induced by chiral anomaly in the presence of parallel electric ($mathbf{E}$) and magnetic ($mathbf{B}$) fields ($mathbf{E cdot B } eq 0$) or $(mathbf{B cdot abla }T eq 0)$ ($mathbf{ abla}T$ is the thermal gradient). We here propose another two fascinating transport properties, namely, the nonlinear planar Nernst effect and nonlinear planar thermal Hall effect induced by chiral anomaly in the presence of $mathbf{B cdot abla}T eq 0$ in WSMs. Using the semiclassical Boltzmann transport theory, we derive the analytical expressions for the chiral anomaly induced nonlinear Nernst and thermal Hall transport coefficients and also evaluate the fundamental mathematical relations among them in the nonlinear regime. The formulas we find in this current work are consistent with that predicted for the nonlinear anomalous electrical and thermoelectric effects induced by Berry curvature dipole recently. Additionally, in contrast to the recent work, by utilizing the lattice Weyl Hamiltonian with intrinsic chiral chemical potential, we find that the chiral anomaly induced nonlinear planar effects can exist even for a pair of oppositely tilted or non-tilted Weyl cones in both time reversal and inversion broken WSMs. The chiral anomaly induced nonlinear planar effects predicted here along with the related parameter dependencies are hence possible to be realized in realistic WSMs in experiment.
After the experimental realization, the Berry curvature dipole (BCD) induced nonlinear Hall effect (NLHE) has attracted tremendous interest to the condensed matter community. Here, we investigate another family of Hall effect, namely, chiral anomaly induced nonlinear Hall effect (CNHE) in multi-Weyl semimetal (mWSM). In contrast to the BCD induced NLHE, CNHE appears because of the combination of both chiral anomaly and anomalous velocity due to non-trivial Berry curvature. Using the semiclassical Boltzmann theory within the relaxation time approximation, we show that, in contrast to the chiral anomaly induced linear Hall effect, the magnitude of CNHE decreases with the topological charge n. Interestingly, we find that unlike the case of n=1, the CNHE has different behaviors in different planes. Our prediction on the behavior of CNHE in mWSM can directly be checked in experiments.
In this paper, the chiral Hall effect of strained Weyl semimetals without any external magnetic field is proposed. Electron-phonon coupling emerges in the low-energy fermionic sector through a pseudogauge potential. We show that, by using chiral kinetic theory, the chiral Hall effect appears as a response to a real time-varying electric field in the presence of structural distortion and it causes spatial chirality and charges separation in a Weyl system. We also show that the coupling of the electrons to acoustic phonons as a gapless excitation leads to emerging an optical absorption peak at $omega=omega_{el}$, where $omega_{el}$ is defined as a characteristic frequency associated with the pseudomagnetic field. We also propose the strain-induced planar Hall effect as another transport signature of the chiral-anomaly equation.
Insulating honeycomb ferromagnet CrI$_3$ has recently attracted considerable attention due to its potential use for dissipationless spintronics applications. Recently, topological spin excitations have been observed experimentally in bulk CrI$_3$ by L. Chen, et al. [Phys. Rev. X ${bf 8}$, 041028 (2018)] using inelastic neutron scattering. This suggest that bulk CrI$_3$ has strong spin-orbit coupling and its spin Hamiltonian should include a next-nearest neighbour Dzyaloshinskii-Moriya (DM) interaction. Inspired by this experiment, we study non-equilibrium emergent photon-dressed topological spin and thermal Hall transports in laser-irradiated CrI$_3$ with and without the DM interaction. We show that the spin excitations can be manipulated into different topological phases with different Chern numbers. Most importantly, we show that the emergent photon-dressed spin and thermal Hall response can be switched to different signs. Hence, the generated magnon spin photocurrents can be manipulated by the laser field, which is of great interest in ultrafast spin current generation and could pave the way for future applications of CrI$_3$ to topological opto-spintronics and opto-magnonics.
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