Direct Measurement of Helicoid Surface States in RhSi using Nonlinear Optics

TL;DR

This study directly measures the surface states of RhSi Weyl semimetal using nonlinear optics, revealing photogalvanic effects that align with predictions but also show unexpected symmetry properties.

Contribution

First experimental observation of Fermi arc surface states in RhSi via nonlinear optical measurements, confirming theoretical predictions and uncovering emergent symmetries.

Findings

Measured circular photogalvanic effect consistent with theory

Observed linear photogalvanic effect with unexpected symmetry

Surface photocurrent direction deviates from predictions

Abstract

Despite the fundamental nature of the edge state in topological physics, direct measurement of electronic and optical properties of the Fermi arcs of topological semimetals has posed a significant experimental challenge, as their response is often overwhelmed by the metallic bulk. However, laser-driven currents carried by surface and bulk states can propagate in different directions in nonsymmorphic crystals, allowing for the two components to be easily separated. Motivated by a recent theoretical prediction \cite{chang20}, we have measured the linear and circular photogalvanic effect currents deriving from the Fermi arcs of the nonsymmorphic, chiral Weyl semimetal RhSi over the $0.45 - 1.1$ eV incident photon energy range. Our data are in good agreement with the predicted magnitude of the circular photogalvanic effect as a function of photon energy, although the direction of the surface photocurrent departed from the theoretical expectation over the energy range studied. Surface currents arising from the linear photogalvanic effect were observed as well, with the unexpected result that only two of the six allowed tensor element were required to describe the measurements, suggesting an approximate emergent mirror symmetry inconsistent with the space group of the crystal.