All optical ultrafast pure spin current in the altermagnet Cr$_2$SO

TL;DR

This paper demonstrates an all-optical method to generate nearly 100% pure spin currents in the altermagnet Cr$_2$SO using infrared valley excitation and THz pulses, enabling lightwave control of spin in low spin-orbit materials.

Contribution

It introduces a novel all-optical approach for generating pure spin currents in 2D altermagnets, leveraging valley selection rules and THz pulses, which was previously thought unlikely due to symmetry constraints.

Findings

Large, nearly 100% pure spin currents achieved in Cr$_2$SO.

Infrared valley excitation combined with THz pulses enables spin current generation.

Provides a practical route for lightwave-controlled spintronics in low spin-orbit materials.

Abstract

All-optical generation of pure spin current -- the flow of spin in the absence of a corresponding charge flow -- relies on a symmetry based compensation of valley charge. The 2d $d$-wave altermagnets, ideal spintronics materials due to a very low spin-orbit coupling, possess a magnetic point group and highly anisotropic valley manifolds that would appear to preclude such current compensation, excluding them as materials for the ultrafast generation of pure spin current. Here we show that infra-red valley excitation combined with a THz pulse envelope allows the generation of large and nearly 100\% pure spin currents in the altermagnet Cr$_2$SO. Our approach is based on a valley selection rule coupling linearly polarized light to spin opposite valleys, along with the intrinsic momentum shift that a co-occurring THz pulse imbues a valley spin excitation with. These results thus provide a practical and all-optical route to the generation of pure spin current in $d$-wave 2d altermagnets, opening a route to lightwave control of spin in an environment with very low intrinsic spin mixing.