Kramers degeneracy in a magnetic field and Zeeman spin-orbit coupling in antiferromagnets

TL;DR

This paper investigates how symmetries in antiferromagnets lead to degeneracies in electron states, revealing a Zeeman spin-orbit coupling effect that varies with momentum and impacts various materials.

Contribution

It introduces the concept of Zeeman spin-orbit coupling in antiferromagnets arising from symmetry-protected and accidental degeneracies.

Findings

Symmetry-protected Kramers degeneracy exists at specific momenta.

Accidental degeneracies can change shape under perturbations.

Zeeman coupling becomes a momentum-dependent spin-orbit interaction.

Abstract

In this article, I analyze the symmetries and degeneracies of electron eigenstates in a commensurate collinear antiferromagnet. In a magnetic field transverse to the staggered magnetization, a hidden anti-unitary symmetry protects double degeneracy of the Bloch eigenstates at a special set of momenta. In addition to this `Kramers degeneracy' subset, the manifold of momenta, labeling the doubly degenerate Bloch states in the Brillouin zone, may also contain an `accidental degeneracy' subset, that is not protected by symmetry and that may change its shape under perturbation. These degeneracies give rise to a substantial momentum dependence of the transverse g-factor in the Zeeman coupling, turning the latter into a spin-orbit interaction. I discuss a number of materials, where Zeeman spin-orbit coupling is likely to be present, and outline the simplest properties and experimental consequences of this interaction, that may be relevant to systems from chromium to borocarbides, cuprates, hexaborides, iron pnictides, as well as organic and heavy fermion conductors.