Circular Photocurrent in Weyl Semimetals with Mirror Symmetry
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ONIC PROPERTIES OF SOLID
Circular Photocurrent in Weyl Semimetals with Mirror Symmetry N. V. Leppenena,*, E. L. Ivchenkoa, and L. E. Goluba aIoffe
Physical–Technical Institute, Russian Academy of Sciences, St. Petersburg, 194021 Russia *e-mail: [email protected] Received February 15, 2019; revised February 15, 2019; accepted February 27, 2019
Abstract—We have considered theoretically Weyl’s semimetals, for which the point symmetry group contains reflection planes and which have equivalent valleys with opposite chiralities. These materials include the most often studied compounds, viz., monopnictides TaAs, NbAs, TaP, NbP of transition metals, as well as Bi1 ‒ xSbx alloys. Circular photogalvanic current inverting its direction upon the sign reversal of the circular polarization has been calculated for light absorption under direct optical transitions near Weyl points. The total contribution of all valleys to the photocurrent differs from zero beyond the limits of Weyl’s model when the spin-independent tilt linear in wavevector k or spin-dependent terms cubic in k are taken into account additionally in the effective electron Hamiltonian. When the tilt of the energy dispersion curve in a Weyl semimetal of symmetry C4v is considered, the photogalvanic current can be expressed in terms of the components of rank-two symmetric tensor determining the energy spectrum of carriers near a Weyl’s node. At low temperatures, this contribution to the photocurrent is excited in a certain limited frequency interval Δ. The photocurrent associated with cubic corrections in the optical absorption region is proportional to the square of light frequency and is generated both within window Δ and beyond its limits. DOI: 10.1134/S1063776119070070
1. INTRODUCTION Three-dimensional systems with a linear spectrum, viz., Weyl’s semimetals, have been discovered recently and are actively investigated (see recent reviews [1, 2]). These systems with a nondegenerate energy spectrum of quasiparticles (except double degeneracy at a Weyl node) attract considerable attention owing to their peculiar electrical and optical properties in the bulk and on the surface and extend the concepts of the topological theory in solid state physics. In such systems, charge carriers are described in the simplest model by the effective Hamiltonian in the form of Weyl’s Hamiltonian used for describing neutrino, which explains their name. In Weyl’s semimetals, the circular photocurrent (i.e., electric current appearing upon absorption of light without the application of external voltage and reversing its direction upon a change in the sign of the circular polarization of light) behaves in a remarkable way [3]. Namely, it was found that in the absence of reflection planes, the circular photocurrent is directed along the photon momentum, and its generation rate is determined (apart from the electric field strength of the wave) by world constants [4]. Actual Weyl’s semimetals TaAs, TaP, NbAs, NbP [5–11] and Bi1 – xSbx [12] have point symmetry groups C4v and C3v , respe
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