Nonlinear dynamic conductivity in structrurally chiral Weyl semimetals

WSMs are 3D versions of graphene, characterized by isolated band crossings which act as monopoles of the Berry curvature field. From the properties of the Berry curvature under time-reversal and inversion, it follows that breaking either T or I is necessary for a crystal to exhibit a WSM phase. I-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 second-order nonlinear conductivity, defined by J_i = σ_ijk.E_iE_j. For monochromatic electric fields the nonlinear response generates dc currents whose direction is dependent on the polarization state of the electric field, giving rise to phenomena known as photogalvanic effects (PGEs). In this talk I will present results on PGEs in response to linear and circular polarized light (LPGE and CPGE, respectively) as probes of both the symmetry and topology of the WSM phase. RhSi is an ideal candidate for such a study, as point group symmetry predicts the simplest possible structure of σ_ijk , in which the only nonvanishing tensor elements are even and odd permutations of xyz. Furthermore, RhSi is structurally chiral (or handed), and the absence of mirror planes breaks the degeneracy of Weyl nodes of opposite Berry monopole charge. I will present results on the polarization selection rules and spectra of 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 current is allowed because truncation at the 001 surface breaks the nonsymmorphic (screw) symmetry and therefore may derive from helicoidal surface bands that give rise to Fermi arcs (see Chang et al. 1906.03207).