Symmetry breaking and circular photogalvanic effect in epitaxial Cd$_x$Hg$_{1-x}$Te films

TL;DR

This paper reports the observation of the circular photogalvanic effect in epitaxial CdHgTe films, indicating symmetry breaking possibly due to strain, with a developed theory explaining the quantum interference mechanisms involved.

Contribution

It demonstrates the CPGE in bulk CdHgTe films, suggesting symmetry reduction and providing a theoretical model for the effect's origin.

Findings

CPGE observed in bulk CdHgTe films with non-inverted band structure

Symmetry reduction likely caused by strain in the material

Theoretical explanation based on quantum interference in free-carrier absorption

Abstract

We report on the observation of symmetry breaking and the circular photogalvanic effect in Cd$_x$Hg$_{1-x}$Te alloys. We demonstrate that irradiation of bulk epitaxial films with circularly polarized terahertz radiation leads to the circular photogalvanic effect (CPGE) yielding a photocurrent whose direction reverses upon switching the photon helicity. This effect is forbidden in bulk zinc-blende crystals by symmetry arguments, therefore, its observation indicates either the symmetry reduction of bulk material or that the photocurrent is excited in the topological surface states formed in a material with low Cadmium concentration. We show that the bulk states play a crucial role because the CPGE was also clearly detected in samples with non-inverted band structure. We suggest that strain is a reason of the symmetry reduction. We develop a theory of the CPGE showing that the photocurrent results from the quantum interference of different pathways contributing to the free-carrier absorption (Drude-like) of monochromatic radiation.