Quantum transport theory for unconventional magnets: Interplay of altermagnetism and p-wave magnetism with superconductivity

TL;DR

This paper develops a comprehensive quantum transport theory for unconventional magnetic materials, including altermagnets and p-wave magnets, applicable in both normal and superconducting states, revealing new effects like spin-dependent diffusion and proximity-induced magnetization.

Contribution

The authors introduce a unified low-energy quantum transport framework for various unconventional magnets, incorporating their unique symmetries and predicting novel phenomena in superconducting hybrid systems.

Findings

Complete description of spin-splitting in diffusive altermagnets.

Prediction of proximity-induced magnetization in superconductor-altermagnet systems.

Identification of spin-galvanic effects specific to p-wave magnets in superconducting states.

Abstract

We present a quantum transport theory for generic magnetic metals, in which magnetism occurs predominantly due to exchange interactions, such as ferromagnets, antiferromagnets, altermagnets and p-wave magnets. Our theory is valid both for the normal and the superconducting state. We derive the effective low-energy action for each of these materials, where the spin space groups are used to determine the form of the tensor coefficients appearing in the action. The transport equations, which are obtained as the saddle point equations of this action, describe a wider range of phenomena than the usual quasiclassical equations. In ferromagnets, in addition to the usual exchange field and spin relaxation effects, we identify a spin-dependent renormalization of the diffusion coefficient, which provides a description of spinpolarized currents in both the normal and superconducting equal spin-triplet states. In the normal state, our equations provide a complete description of the spin-splitting effect in diffusive systems, recently predicted in ideal clean altermagnets. In the superconducting state, our equations predict a proximity induced magnetization, the appearance of a spontaneous magnetic moment in hybrid superconductor-altermagnet systems. The distribution and polarization direction of this magnetic moment depend on the symmetry of the structure, thus measurements of such polarization reveal the underlying microscopic symmetry of the altermagnet. Finally, for inversionsymmetry broken antiferromagnets, such as the p-wave magnet, we show that spin-galvanic effects which are distinguishable from the spin-galvanic effect induced by spin-orbit coupling only in the superconducting state. Besides these examples, our model applies to arbitrary magnetic systems, providing a complete theory for nonequilibrium transport in diffusive nonconventional magnets at arbitrary temperatures.