Weak valley-layer coupling and valley polarization in centrosymmetric $\mathrm{FeCl_2}$ monolayer

TL;DR

This study reveals weak valley-layer coupling in centrosymmetric FeCl2 monolayer, showing how valley polarization can be controlled via magnetization and electric fields, with potential for advanced valleytronic devices.

Contribution

It uncovers the existence of weak valley-layer coupling in FeCl2 monolayer and demonstrates control of valley polarization through magnetization and electric field manipulation.

Findings

Weak valley-layer coupling exists only with out-of-plane magnetization.

Valley polarization can be switched by reversing magnetization or electric field direction.

Bilayer FeCl2 can spontaneously exhibit valley polarization without external fields.

Abstract

Using the valley degree of freedom as a carrier of information for storage and processing, valley polarization plays a crucial role. A variety of mechanisms for valley polarization have been proposed, among which the valley-layer coupling mechanism involves the induction of valley polarization by an out-of-plane electric field. Here, through first-principles calculations, it is found that the weak valley-layer coupling can exist in centrosymmetric $\mathrm{FeCl_2}$ monolayer. It is crucial to note that valley-layer coupling only occurs with out-of-plane magnetization and vanishes with in-plane magnetization. Compared to monolayers with strong valley-layer coupling, $\mathrm{FeCl_2}$ requires an extremely strong electric field to achieve the same magnitude of valley splitting. Valley polarization switching can be achieved by manipulating the directions of magnetization and electric field. Reversing only one of these directions switches the valley polarization, whereas reversing both simultaneously leaves it unchanged. Moreover, the simply stacked bilayer $\mathrm{FeCl_2}$, as a $PT$-antiferromagnet, can spontaneously achieve valley polarization without an external electric field, highlighting its potential for miniaturization, ultradensity, and ultrafast performance. Our work provides guidelines for identifying materials with weak valley-layer coupling, and further enables the regulation of valley polarization through electric field and stacking engineering.