Valley-polarized quantum anomalous Hall insulator in monolayer $\mathrm{RuBr_2}$

TL;DR

This paper predicts a valley-polarized quantum anomalous Hall insulator in monolayer RuBr2 driven by electron correlation and strain, with potential for room-temperature spintronic applications.

Contribution

It demonstrates the emergence of VQAHI in monolayer RuBr2 via first-principles calculations, highlighting the role of electron correlation, strain, and magnetic anisotropy.

Findings

VQAHI occurs under perpendicular magnetic anisotropy.

Strain can induce VQAHI even with realistic U values.

Edge states exhibit chiral-spin-valley locking, enabling spin and valley polarization.

Abstract

Coexistence of intrinsic ferrovalley (FV) and nontrivial band topology attracts intensive interest both for its fundamental physics and for its potential applications, namely valley-polarized quantum anomalous Hall insulator (VQAHI). Here, based on first-principles calculations by using generalized gradient approximation plus $U$ (GGA+$U$) approach, the VQAHI induced by electronic correlation or strain can occur in monolayer $\mathrm{RuBr_2}$. For perpendicular magnetic anisotropy (PMA), the ferrovalley (FV) to half-valley-metal (HVM) to quantum anomalous Hall (QAH) to HVM to FV transitions can be driven by increasing electron correlation $U$. However, there are no special QAH states and valley polarization for in-plane magnetic anisotropy. By calculating actual magnetic anisotropy energy (MAE), the VQAHI indeed can exist between two HVM states due to PMA, a unit Chern number/a chiral edge state and spontaneous valley polarization. The increasing $U$ can induce VQAHI, which can be explained by sign-reversible Berry curvature or band inversion between $d_{xy}$/$d_{x^2-y^2}$ and $d_{z^2}$ orbitals. Even though the real $U$ falls outside the range, the VQAHI can be achieved by strain. Taking $U$$=$2.25 eV as a concrete case, the monolayer $\mathrm{RuBr_2}$ can change from a common ferromagentic (FM) semiconductor to VQAHI under about 0.985 compressive strain. It is noted that the edge states of VQAHI are chiral-spin-valley locking, which can achieve complete spin and valley polarizations for low-dissipation electronics devices. Both energy band gap and valley splitting of VQAHI in monolayer $\mathrm{RuBr_2}$ are higher than the thermal energy of room temperature (25 meV), which is key at room temperature for device applications.