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Register a new userGiant photocurrent in asymmetric Weyl semimetals from the helical magnetic effect
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We propose a new type of photoresponse induced in asymmetric Weyl semimetals in an external magnetic field. In usual symmetric Weyl semimetals in a magnetic field, the particles and holes produced by an incident light in different Weyl cones have opposite helicities and hence move in opposite directions, canceling each otherss contributions to the photocurrent. However this cancelation does not occur if the Weyl semimetal possesses both a broken particle-hole symmetry and a broken spatial inversion symmetry. We call the resulting generation of photocurrent the helical magnetic effect because it is induced by the helicity imbalance in a magnetic field. We find that due to the large density of states in a magnetic field, the helical magnetic effect induces a remarkable large photocurrent for incident THz frequency light. This suggests a potential application of asymmetric Weyl semimetals for creating THz photosensors.
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We describe a new type of the Chiral Magnetic Effect (CME) that should occur in Weyl semimetals with an asymmetry in the dispersion relations of the left- and right-handed chiral Weyl fermions. In such materials, time-dependent pumping of electrons from a non-chiral external source generates a non-vanishing chiral chemical potential. This is due to the different capacities of the left- and right-handed (LH and RH) chiral Weyl cones arising from the difference in the density of states in the LH and RH cones. The chiral chemical potential then generates, via the chiral anomaly, a current along the direction of an applied magnetic field even in the absence of an external electric field. The source of chirality imbalance in this new setup is thus due to the band structure of the system and the presence of (non-chiral) electron source, and not due to the parallel electric and magnetic fields. We illustrate the effect by an argument based on the effective field theory, and by the chiral kinetic theory calculation for a rotationally invariant Weyl semimetal with different Fermi velocities in the left and right chiral Weyl cones; we also consider the case of a Weyl semimetal with Weyl nodes at different energies. We argue that this effect is generically present in Weyl semimetals with different dispersion relations for LH and RH chiral Weyl cones, such as SrSi2 recently predicted as a Weyl semimetal with broken inversion and mirror symmetries, as long as the chiral relaxation time is much longer than the transport scattering time.
Theory of light absorption and circular photocurrent in Weyl semimetals is developed for arbitrary large light intensities with account for both elastic and inelastic relaxation processes of Weyl fermions. The direct optical transition rate is shown to saturate at large intensity, and the saturation behaviour depends on the light polarization and on the ratio of the elastic and inelastic relaxation times. The linear-circular dichroism in absorption is shown to exceed 10~% at intermediate light wave amplitudes and fast energy relaxation. At large intensity $I$, the light absorption coefficient drops as $1/sqrt{I}$, and the circular photogalvanic current increases as $sqrt{I}$.
We have studied theoretically the Weyl semimetals the point symmetry group of which has reflection planes and which contain equivalent valleys with opposite chiralities. These include the most frequently studied compounds, namely the transition metals monopnictides TaAs, NbAs, TaP, NbP, and also Bi$_{1-x}$Sb$_x$ alloys. The circular photogalvanic current, which inverts its direction under reversal of the light circular polarization, has been calculated for the light absorption under direct optical transitions near the Weyl points. In the studied materials, the total contribution of all the valleys to the photocurrent is nonzero only beyond the simple Weyl model, namely, if the effective electron Hamiltonian is extended to contain either an anisotropic spin-dependent linear contribution together with a spin-independent tilt or a spin-dependent contribution cubic in the electron wave vector $bf{k}$. With allowance for the tilt of the energy dispersion cone in a Weyl semimetal of the $C_{4v}$ symmetry, the photogalvanic current is expressed in terms of the components of the second-rank symmetric tensor that determines the energy spectrum of the carriers near the Weyl node; at low temperature, this contribution to the photocurrent is generated within a certain limited frequency range $Delta $. The photocurrent due to the cubic corrections, in the optical absorption region, is proportional to the light frequency squared and generated both inside and outside the $Delta$ window.
We suggest the possibility of a linear magnetochiral effect in time reversal breaking Weyl semimetals. Generically the magnetochiral effect consists in a simultaneous linear dependence of the magnetotransport coefficients with the magnetic field and a momentum vector. This simultaneous dependence is allowed by the Onsager reciprocity relations, being the separation vector between the Weyl nodes the vector that plays such role. As a side consequence, we find a non vanishing positive longitudinal magnetoconductivity at Fermi energies above the point where the chirality of the Weyl nodes is globally lost.
The spin Nernst effect describes a transverse spin current induced by the longitudinal thermal gradient in a system with the spin-orbit coupling. Here we study the spin Nernst effect in a mesoscopic four-terminal cross-bar Weyl semimetal device under a perpendicular magnetic field. By using the tight-binding Hamiltonian combining with the nonequilibrium Greens function method, the three elements of the spin current in the transverse leads and then spin Nernst coefficients are obtained. The results show that the spin Nernst effect in the Weyl semimetal has the essential difference with the traditional one: The z direction spin currents is zero without the magnetic field while it appears under the magnetic field, and the x and y direction spin currents in the two transverse leads flows out or flows in together, in contrary to the traditional spin Nernst effect, in which the spin current is induced by the spin-orbit coupling and flows out from one lead and flows in on the other. So we call it the anomalous spin Nernst effect. In addition, we show that the Weyl semimetals have the center-reversal-type symmetry, the mirror-reversal-type symmetry and the electron-hole-type symmetry, which lead to the spin Nernst coefficients being odd function or even function of the Fermi energy, the magnetic field and the transverse terminals. Moreover, the spin Nernst effect in the Weyl semimetals are strongly anisotropic and its coefficients are strongly dependent on both the direction of thermal gradient and the direction of the transverse lead connection. Three non-equivalent connection modes (x-z, z-x and x-y modes) are studied in detail, and the spin Nernst coefficients for three different modes exhibit very different behaviors. These strongly anisotropic behaviors of the spin Nernst effect can be used as the characterization of magnetic Weyl semimetals.
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