## Large and quantized non-linear responses in chiral topological metals.

**Speaker:**Adolfo Grushin, Institut Neel, CNRS and Universite Grenoble Alpes, F-38042 Grenoble, France

**When:**July 25, 2018 (Wed), 12:30PM to 01:30PM (add to my calendar)

**Location:**SCI 328

**Hosted by:**Claudio Chamon

*This event is part of the Condensed Matter Theory Seminar Series. *

In this talk I will discuss our recent results concerning nonlinear and linear responses of topological chiral metals. We have recently predicted 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). Although not topologically protected, this result extends beyond two band models to all chiral metals with multifold fermions, which opens up the search to new materials. We identify a range of previously-unidentified ternary compounds able to exhibit chiral multifold fermions of all types (including a range of materials in the families AsBaPt and Gd$*3$Cl$*3$C), and provide specific predictions for the known multifold material RhSi. As a side result we also calculate the gyrotropy of all chiral multifold fermions, which we find also to be enhanced compared to the Weyl semimetal case.

Finally, if time permits I will discuss briefly the measurement and modeling of the second harmonic generation in TaAs, the frequency doubling of an incident pulse upon reflection or transmission. Experimentally it is found 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.

Refs:

F. de Juan, AGG, T. Morimoto, J. E. Moore Nat. Comm. 8, 15995 (2017)

F. Flicker, F. de Juan, T. Morimoto, B. Bradlyn, M. Vergniory, AGG arXiv:1806.09642

S. Pakantar et al. arXiv: 1804.06973