Electronic structure and unconventional non-linear response in double Weyl semimetal SrSi$_2$

TL;DR

This paper investigates the electronic structure and unconventional non-linear optical response of the double Weyl semimetal SrSi$_2$, revealing unique topological features and non-quantized circular photogalvanic effects depending on Fermi level positioning.

Contribution

It provides a detailed analysis of the topological surface states, spin orbit coupling effects, and the non-quantized CPGE in SrSi$_2$, highlighting differences from time-reversal breaking Weyl semimetals.

Findings

Fermi surface behavior classifies SrSi$_2$ as a type-I WSM.

Surface Fermi arcs evolve with energy and are affected by spin orbit coupling.

CPGE is quantized only near specific Fermi levels, contrasting with time-reversal WSMs.

Abstract

Considering a non-centrosymmetric, non-magnetic double Weyl semimetal (WSM) SrSi$_2$, we investigate the electron and hole pockets in bulk Fermi surface behavior that enables us to characterize the material as a type-I WSM. We study the structural handedness of the material and correlate it with the distinct surface Fermi surface at two opposite surfaces following an energy evolution. The Fermi arc singlet becomes doublet with the onset of spin orbit coupling that is in accordance with the topological charge of the Weyl Nodes (WNs). A finite energy separation between WNs of opposite chirality in SrSi$_2$ allows us to compute circular photogalvanic effect (CPGE). Followed by the three band formula, we show that CPGE is only quantized for Fermi level chosen in the vicinity of WN residing at higher value of energy. Surprisingly, for the other WN of opposite chirality in the lower value of energy, CPGE is not found to be quantized. Such a behavior of CPGE is in complete contrast to the time reversal breaking WSM where CPGE is quantized to two opposite plateau depending on the topological charge of the activated WN. We further analyze our finding by examining the momentum resolved CPGE. Finally we show that two band formula for CPGE is not able to capture the quantization that is apprehended by the three band formula.