Valley polarization in 2D tetragonal altermagnetism

Abstract

The altermagnetism caused by alternating crystal environment provides a unique opportunity for designing new type of valley polarization. Here, we propose a way to realize valley polarization in two-dimensional (2D) tetragonal altermagnetism by regulating the direction of magnetization. The valley polarization along with spin polarization will arise when the orientation of magnetization breaks the $C_{4z}$ lattice rotational symmetry, particularly in the conventional in-plane $x$ or $y$ directions. When the direction of magnetization switches between the $x$ and $y$ direction, the valley and spin polarizations will be reversed. This is different from the widely studied valley polarization, which occurs in out-of-plane hexagonal magnetic materials with valley physics at -K/K point. Followed by first-principles calculations, our proposal is demonstrated in a 2D Janus tetragonal altermagnetic $\mathrm{F}{\mathrm{e}}_2\mathrm{Mo}{\mathrm{S}}_2\mathrm{S}{\mathrm{e}}_2$ monolayer with good stability but very small valley splitting of 1.6 meV. To clearly see the feasibility of our proposal, an unrealistic material $\mathrm{R}{\mathrm{u}}_2\mathrm{Mo}{\mathrm{S}}_2\mathrm{S}{\mathrm{e}}_2$ is used to show large valley splitting of 90 meV. In fact, our proposal can be readily extended to 2D tetragonal ferromagnetic (FM) materials, for example FM $\mathrm{F}{\mathrm{e}}_2{\mathrm{I}}_2$ monolayer. Our findings can enrich the valley physics, and provide new type of valley materials.

7 pages, 6 figures