$d$-Wave Polarization-Spin Locking in Two-Dimensional Altermagnets

TL;DR

This paper introduces the novel $d$-wave polarization-spin locking phenomenon in 2D altermagnets, revealing unique spin-polarization effects and potential spintronic applications driven by Berry connections and symmetry considerations.

Contribution

It identifies the $d$-wave PSL phenomenon in 2D altermagnets, proposes a symmetry-based criterion for its identification, and demonstrates its implications for spintronics and magnetoelectric devices.

Findings

$d$-wave PSL occurs in specific 2D altermagnets with tetragonal symmetry.

Monolayer Cr$_2$X$_2$O exhibits $d$-wave PSL driven by charge order.

The phenomenon enables spin accumulation at orthogonal edges, useful for spin filters.

Abstract

We report the emergence of an uncharted phenomenon, termed $d$-wave polarization-spin locking (PSL), in two-dimensional (2D) altermagnets. This phenomenon arises from nontrivial Berry connections, resulting in perpendicular electronic polarizations in the spin-up and spin-down channels. Symmetry-protected $d$-wave PSL occurs exclusively in $d$-wave altermagnets with tetragonal layer groups. To identify 2D altermagnets capable of exhibiting this phenomenon, we propose a symmetry-eigenvalue-based criterion, and a rapid method by observing the spin-momentum locking. Using first-principles calculations, monolayer Cr$_2$X$_2$O (X = Se, Te) characterizes promising candidates for $d$-wave PSL, driven by the unusual charge order in these monolayers. This unique polarization-spin interplay leads to spin-up and spin-down electrons accumulating at orthogonal edges, enabling potential applications as spin filters or splitters in spintronics. Furthermore, $d$-wave PSL introduces an unexpected spin-driven ferroelectricity in conventional antiferromagnets. Such magnetoelectric coupling positions $d$-wave PSL as an ideal platform for fast antiferromagnetic memory devices. Our findings not only expand the landscape of altermagnets, complementing conventional collinear ferromagnets and antiferromagnets, but also highlight tantalizing functionalities in altermagnetic materials, potentially revolutionizing information technology.