Quantized circular photogalvanic effect in Weyl semimetals

Fernando De Juan,Adolfo G Grushin,Joel E Moore,Takahiro Morimoto

doi:10.1038/ncomms15995

Abstract

The circular photogalvanic effect (CPGE) is the part of a photocurrent that switches depending on the sense of circular polarization of the incident light. It has been consistently observed in systems without inversion symmetry and depends on non-universal material details. Here we find that in a class of Weyl semimetals (for example, SrSi2) and three-dimensional Rashba materials (for example, doped Te) without inversion and mirror symmetries, the injection contribution to the CPGE trace is effectively quantized in terms of the fundamental constants e, h, c and with no material-dependent parameters. This is so because the CPGE directly measures the topological charge of Weyl points, and non-quantized corrections from disorder and additional bands can be small over a significant range of incident frequencies. Moreover, the magnitude of the CPGE induced by a Weyl node is relatively large, which enables the direct detection of the monopole charge with current techniques.

Highlights

The circular photogalvanic effect (CPGE) is the part of a photocurrent that switches depending on the sense of circular polarization of the incident light
While it is clear that there is no equilibrium current[16], a finite current is possible in the transport limit with the frequency o-0 after the transferred momentum q 1⁄4 0. This current has the same origin as natural optical activity[20,21]. It is determined by orbital moments rather than the chiral anomaly and its magnitude depends on a non-universal material-dependent property: the energy splitting between Weyl points
The main finding of this paper is that in a Weyl semimetal where nodes of opposite chirality lie at different energies, the circular photogalvanic effect (CPGE) becomes a truly quantized response that depends only on fundamental constants and the monopole charge of a Weyl node

Summary

Introduction

The circular photogalvanic effect (CPGE) is the part of a photocurrent that switches depending on the sense of circular polarization of the incident light. The typical magnitude of the CPGE at low frequency corresponds to an observed switchable photocurrent jB10–100 pA for incident intensity of IB1 W over a cm-sized sample in quantum wells that have time-reversal symmetry but low spatial symmetry[29] It has been obtained theoretically as a Berry phase effect[33,34,35], possibly the first in nonlinear optics, but there is no quantization: the effect measures the strength of the leading allowed Berry curvature term, which in three-dimensional (3D) materials[35] can be viewed as the dipole moment of Berry curvature. We find that that the CPGE induced current for a Weyl point is quantized and given by djU dt

Methods

Results

Conclusion