Strain effects on the topological and valley properties of the Janus monolayer VSiGeN4

Abstract

Strain is an effective method to tune the electronic properties of two-dimensional (2D) materials and can induce novel phase transition. Recently, the 2D $M{A}_{2}{Z}_{4}$ family of materials has attracted interest because of their emerging topological, magnetic, and superconducting properties. Here, we investigate the impact of strain effects ($a/{a}_{0}$ = 0.96--1.04) on the physical properties of the Janus monolayer ${\mathrm{VSiGeN}}_{4}$ as a derivative of ${\mathrm{VSi}}_{2}{\mathrm{N}}_{4}$ or ${\mathrm{VGe}}_{2}{\mathrm{N}}_{4}$, which possesses dynamical, mechanical, and thermal stabilities. For out-of-plane magnetic anisotropy, with increasing strain, ${\mathrm{VSiGeN}}_{4}$ undergoes a transition between a ferrovalley semiconductor (FVS), half-valley metal (HVM), valley-polarized quantum anomalous Hall insulator, HVM, and FVS. These effects imply twice topological phase transitions, which are related to the sign-reversible Berry curvature and band inversion between ${d}_{xy}+{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{{z}^{2}}$ orbitals for the $K$ or $\ensuremath{-}K$ valley. The band inversion also leads to transformation of valley splitting strength between the valence and conduction bands. However, for in-plane magnetic anisotropy, no special quantum anomalous Hall (QAH) states and valley polarization exist within the considered strain range. The actual magnetic anisotropy energy shows no special QAH and HVM states in monolayer ${\mathrm{VSiGeN}}_{4}$. Fortunately, these can easily be achieved by an external magnetic field, which adjusts the easy magnetization axis of ${\mathrm{VSiGeN}}_{4}$ from an in-plane one to an out-of-plane one. Our findings shed light on how strain can be employed to engineer the electronic states of ${\mathrm{VSiGeN}}_{4}$, which may lead to new perspectives on multifunctional quantum devices in valleytronics and spintronics.