In this talk I will discuss two of our recent results concerning nonlinear responses of topological metals. The first is the prediction of a quantized circular photogalvanic effect, the part of the photocurrent which changes sign when the light’s polarization flips. We find it is quantized in units of a large universal constant e^3/h^2 times the Weyl monopole charge in mirror free Weyl semimetals (e.g. SrSi2) and three-dimensional Rashba materials (e.g. doped Te). The second is the measurement and modeling of the second harmonic generation in TaAs, the frequency doubling of an incident pulse upon reflection or transmission. We find it experimentally to be extremely large and anisotropic compared to materials in the same symmetry class such as GaAs. We show that these features are well captured phenomenologically in terms of a model of coupled ferroelectric chains, which suggests that TaAs is close to saturating a photocurrent bound that we derive.
Quantized circular photogalvanic effect in Weyl semimetals,
Fernando de Juan, Adolfo G. Grushin, Takahiro Morimoto, Joel E. Moore
Nature Communications 8, 15995 (2017)