Spin-orbit enabled unconventional Stoner magnetism

TL;DR

This paper uncovers a new type of pseudospin symmetry in materials with spin-orbit coupling that leads to unconventional magnetic and superconducting behaviors, distinct from traditional ferromagnetism.

Contribution

It identifies a novel 'spinless' pseudospin symmetry in non-symmorphic space groups that alters magnetic and superconducting properties, expanding understanding of Stoner instability.

Findings

Spinless pseudospin forbids coupling to Zeeman field.

Spinless-pseudospin ferromagnets break time-reversal symmetry without net magnetization.

Extinguishes paramagnetic limiting in superconductors.

Abstract

The Stoner instability remains a cornerstone for understanding metallic ferromagnets. This instability captures the interplay of Coulomb repulsion, Pauli exclusion, and two-fold fermionic spin degeneracy. In materials with spin-orbit coupling, this fermionic spin is generalized to a two-fold degenerate pseudospin which is typically believed to have symmetry properties as spin. Here we identify a distinct symmetry of this pseudospin that forbids it to couple to a Zeeman field. This `spinless' property is required to exist in five non-symmorphic space groups and has non-trivial implications for superconductivity and magnetism. With Coulomb repulsion, Fermi surfaces composed primarily of this spinless pseudospin feature give rise to Stoner instabilities into magnetic states that are qualitatively different than ferromagnets. These spinless-pseudospin ferromagnets break time-reversal symmetry, have a vanishing magnetization, are non-collinear, and exhibit altermagnetic-like energy band spin-splittings. In superconductors, for all pairing symmetries and field orientations, this spinless pseudospin extinguishes paramagnetic limiting. We discuss applications to superconducting UCoGe and magnetic NiS$_{2-x}$Se$_x$.