Polar Kerr Effect and Time Reversal Symmetry Breaking in Bilayer Graphene

TL;DR

This paper predicts optical signatures, including Kerr rotation and reflection anisotropy, as indicators of symmetry-breaking states like quantum anomalous Hall and nematic phases in bilayer graphene, detectable with current experimental methods.

Contribution

It introduces a method to identify symmetry-breaking states in bilayer graphene through optical responses, emphasizing the role of inter-band matrix elements at optical frequencies.

Findings

Optical responses are highly sensitive to symmetry-breaking states.

Kerr rotation and reflection anisotropy serve as fingerprints for specific phases.

Resonant enhancement occurs in the near-infrared regime.

Abstract

The unique sensitivity of optical response to different types of symmetry breaking can be used to detect and identify spontaneously ordered many-body states in bilayer graphene. We predict a strong response at optical frequencies, sensitive to electronic phenomena at low energies, which arises because of nonzero inter-band matrix elements of the electric current operator. In particular, the polar Kerr rotation and reflection anisotropy provide fingerprints of the quantum anomalous Hall state and the nematic state, characterized by spontaneously broken time reversal symmetry and lattice rotation symmetry, respectively. These optical signatures, which undergo a resonant enhancement in the near-infrared regime, lie well within reach of existing experimental techniques.