Second Harmonic Generation Spectroscopy of the Charge Density Wave in the Weyl Semimetal CoSi

The low-energy electronic behavior of topologically ordered Weyl semimetals is dominated by quasilinearly dispersing bands whose intersections in momentum serve as sources and sinks of the Berry curvature. The presence of charge density wave (CDW) ordering can gap out these nodal points and transform the material into a novel, and as-yet unobserved form of topological order known as an axion insulator that obeys axion electrodynamics and can host a half-quantized surface anomalous Hall conductivity. The material CoSi is a promising candidate for observing an axion insulator due to the putative presence of a CDW instability, however there is disagreement on the CDW transition temperature and whether it is present only on the surface or if it also orders in the bulk. Here, we discuss our temperature dependent rotational anisotropy second harmonic generation (RA-SHG) study of the (001) surface of CoSi near 100 K and describe our data pointing to the existence of a surface-only CDW.