Photocurrent generation probes bulk and surface responses chiral Weyl semimetals

Weyl semimetals (WSMs)s are characterized by isolated band crossings which act as monopoles of the Berry curvature field. Inversion breaking WSMs exhibit response functions that are forbidden in systems that possess a center of symmetry. We have investigated an important example of such responses: the 2^nd nonlinear conductivity, defined by J_i.=σ_ijkE_jE_k. For monochromatic electric fields the nonlinear response generates currents whose direction is dependent on the polarization state of the field, giving rise to photogalvanic effects (PGEs). In this talk I present results on PGEs in response to linear and circular polarized light (LPGE and CPGE, respectively). RhSi is an ideal candidate for such a study, as symmetry predicts a simple structure of σ_ijk, in which the only nonvanishing tensor elements are permutations of xyz. Furthermore, RhSi is structurally chiral, and the absence of mirror planes breaks the degeneracy of Weyl nodes of opposite Berry charge. I will describe the PGEs for light incident on 111 and 001 surfaces. On 111 we observe a CPGE current whose direction is parallel to the wavevector of the light and whose spectrum is consistent with photoexcitation across a Weyl cone. More surprising is that we observe in-plane CPGE and LPGE currents at normal incidence on the 001 surface, where the bulk point group predicts a null effect. I discuss the possibility that the observed current becomes allowed by the breaking of nonsymmorphic (screw) symmetry by the surface and is related to the response function of the surface bands that give rise to Fermi arcs.