Quantum Geometry-Driven Nonlinear Spin Currents in Floquet Non-Hermitian Altermagnets

TL;DR

This paper introduces a quantum geometric framework for controlling nonlinear spin currents in Floquet non-Hermitian altermagnets via optical driving, highlighting the dominance of quantum metric and tunability of spin responses.

Contribution

It derives an analytical expression for nonlinear spin currents in non-Hermitian phases and demonstrates optical control and reversal of spin currents in a Floquet altermagnet.

Findings

Nonlinear spin conductivity is dominated by the quantum metric.

Optical polarization can tune and reverse spin current directions.

The formalism separates responses into quantum metric, Berry curvature, and Berry connection dipole.

Abstract

Altermagnets are rapidly emerging as a highly promising platform for spintronics, yet dynamically controlling their spin responses remains a fundamental challenge. In this work, we demonstrate that introducing periodic optical driving and non-Hermiticity provides a powerful route to achieve tunable control over these systems. We derive a general analytical expression for nonlinear spin currents in non-Hermitian phases with a spectral line gap, revealing that the intrinsic response cleanly separates into quantum metric, Berry curvature, and Berry connection dipole contributions. Applying this formalism to a Floquet non-Hermitian $d$-wave altermagnet, we uncover that the nonlinear spin conductivity is overwhelmingly dominated by the bare quantum metric. Furthermore, we show that the optical field's polarization can actively tune -- and even strictly reverse -- the direction of both longitudinal and transverse spin currents. Our work establishes a quantum geometric framework for the optical manipulation of nonlinear spin transport in advanced magnetic materials.