Second Order Nonlinear Responses in Chiral Weyl Semimetal CoSi

The origin of the nonlinear responses in Weyl semimetals has become an increasingly contentious issue in recent years, with some of these effects being attributed to topology and others to symmetry. Part of this contention comes from the fact that the Weyl nodes are required by mirror symmetries in non-chiral Weyl semimetals to appear at exactly the same energy in the band structure, making it impossible to probe a single cone. Recently, chiral Weyl semimetals – with no mirror symmetries – have been observed with paired Weyl cones at significantly different energies (>100meV), allowing for cone discrimination based on choice of laser wavelength. Here, we present our study of the second order nonlinear responses in one such chiral Weyl semimetal, CoSi. In particular, we look at variation of the second harmonic generation (SHG) at multiple crystalline facets of CoSi, and the spatial inhomogeneity and temporal dynamics of its induced SHG. Further, we compare the SHG response strength to a variety of second order nonlinear crystals. Finally, we examine the spontaneous photocurrent response in CoSi.