Relationship of Time Reversal Symmetry Breaking with Optical Kerr Rotation

TL;DR

This paper proves that optical Kerr rotation, a magneto-optical effect, can only occur in materials that break microscopic time reversal symmetry, linking experimental observations in superconductors to fundamental symmetry principles.

Contribution

It establishes a theoretical link between Kerr rotation and time reversal symmetry breaking, applicable in linear response, and clarifies the interpretation of recent experimental results.

Findings

Kerr rotation requires breaking of microscopic time reversal symmetry.

Recent experiments in superconductors show Kerr rotation, implying symmetry breaking.

The reciprocity theorem constrains the conditions under which Kerr effects can occur.

Abstract

We prove an instance of the Reciprocity Theorem that demonstrates that Kerr rotation, also known as the magneto-optical Kerr effect, may only arise in materials that break microscopic time reversal symmetry. This argument applies in the linear response regime, and only fails for nonlinear effects. Recent measurements with a modified Sagnac Interferometer have found finite Kerr rotation in a variety of superconductors. The Sagnac Interferometer is a probe for nonreciprocity, so it must be that time reversal symmetry is broken in these materials.