Altermagnetism and magnetic groups with pseudoscalar electron spin

TL;DR

This paper revises group-theoretical methods for collinear magnetic systems, showing that spin can be treated as a pseudoscalar, leading to a classification of altermagnetic classes and insights into spin splitting and conductivity.

Contribution

It introduces a simplified magnetic group framework for collinear magnets, linking altermagnetic classes to magnetic Laue groups and analyzing spin splitting mechanisms.

Findings

Altermagnetic classes correspond to nontrivial magnetic Laue groups.

Four Laue classes are compatible with nonzero spin conductivity.

A simple model addresses physical mechanisms of spin splitting.

Abstract

We revise existing group-theoretical approaches for a treatment of nonrelativistic collinear magnetic systems with perfect translation invariance. We show that full symmetry groups of these systems, which contain elements with independent rotations in the spin and configuration spaces (spin groups), can be replaced by magnetic groups consisting of elements with rotations acting only on position vectors. This reduction follows from modified transformation properties of electron spin, which in the considered systems becomes effectively a pseudoscalar quantity remaining unchanged upon spatial operations but changing its sign due to an operation of antisymmetry. We introduce a unitary representation of the relevant magnetic point groups and use it for a classification of collinear magnets from the viewpoint of antiferromagnetism-induced spin splitting of electron bands near the center of Brillouin zone. We prove that the recently revealed different altermagnetic classes correspond in a unique way to all nontrivial magnetic Laue classes, i.e., to the Laue groups containing the operation of antisymmetry only in combination with a spatial rotation. Four of these Laue classes are found compatible with a nonzero spin conductivity. Subsequent inspection of a simple model allows us to address briefly the physical mechanisms responsible for the spin splitting in real systems.