Kramers Dichroism in PT Symmetric Magnets

TL;DR

This paper reveals a novel optical phenomenon called Kramers dichroism in PT symmetric magnets, enabling control and detection of quantum coherence between Kramers pairs through polarized light and nonlinear responses.

Contribution

It uncovers Kramers dichroism and associated nonlinear effects, providing new methods to manipulate and measure the Kramers degree of freedom in magnetic materials.

Findings

Circularly polarized light induces coherent superpositions in Kramers pairs.

Linearly polarized light leaves Kramers pairs in a mixed state.

Large second-order nonlinear responses are activated by Kramers degeneracy.

Abstract

Superpositions between states in doubly degenerate Kramers pairs can act as an internal degree of freedom. Here we uncover a Kramers dichroism in PT symmetric magnets: interband transitions induced by circularly polarized light irradiation produce a coherent superposition between Kramers partnered states. This allows to optically control the Kramers degree of freedom. In contrast, Kramers pairs optically excited by linearly polarized light remain in a completely mixed state. Strikingly, we find a class of second-order nonlinear responses that directly track the coherence between Kramers partnered states. Such Kramers nonlinearities can be pronounced producing large second-order nonlinear layer polarization responses activated by Kramers degeneracy in layered antiferromagnets. Together with Kramers dichroism, these render optical responses a novel means for accessing the Kramers degree of freedom and diagnosing their quantum coherent state.