Chiral Optical Response of Multifold Fermions

TL;DR

This paper explores the unique optical responses of multifold fermions, revealing their potential for enhanced gyrotropic magnetic effects and quantized circular photogalvanic effects, supported by theoretical models and material predictions.

Contribution

It introduces experimental probes for multifold fermions based on GME and CPGE, highlighting their distinct properties and identifying new candidate materials.

Findings

Multifold fermions can have zero Berry curvature but still exhibit finite GME.

CPGE in multifold fermions is quantized and frequency-independent under certain conditions.

Identified new materials capable of hosting chiral multifold fermions, including compounds in the AsBaPt and Gd3Cl3C families.

Abstract

Multifold fermions are generalizations of two-fold degenerate Weyl fermions with three-, four-, six- or eight-fold degeneracies protected by crystal symmetries, of which only the last type is necessarily non-chiral. Their low energy degrees of freedom can be described as emergent particles not present in the Standard Model of particle physics. We propose a range of experimental probes for multifold fermions in chiral symmetry groups based on the gyrotropic magnetic effect (GME) and the circular photo-galvanic effect (CPGE). We find that, in contrast to Weyl fermions, multifold fermions can have zero Berry curvature yet a finite GME, leading to an enhanced response. The CPGE is quantized and independent of frequency provided that the frequency region at which it is probed defines closed optically-activated momentum surfaces. We confirm the above properties by calculations in symmetry-restricted tight binding models with realistic density functional theory parameters. 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.