Multiple Topological Phases Controlled via Strain in Two-Dimensional Altermagnets

TL;DR

This paper explores how strain can control multiple topological phases in two-dimensional altermagnets, revealing new quantum states and transitions with potential applications in topological spintronics.

Contribution

It introduces a strain-controlled mechanism to switch between topological phases in 2D altermagnets and demonstrates this in monolayer CrO using first-principles calculations.

Findings

Identification of a type-II quantum spin Hall state.

Strain-induced transition to a quantum anomalous Hall state.

Demonstration of topological phase control in monolayer CrO.

Abstract

Altermagnets (AMs) are an emergent class of magnetic materials that combine properties of ferromagnets and antiferromagnets, exhibiting spin-polarized Fermi surfaces and zero net magnetic moment due to combined time-reversal and crystal symmetry. Here, we construct a Kondo-lattice model on a two-dimensional square Lieb lattice to investigate the topological properties of AMs. We identify a type-II quantum spin Hall state characterized by spin-polarized counterpropagating edge states. Breaking the $C_{4z}\mathcal{T}$ symmetry, which connects magnetic sublattices, induces a transition to a quantum anomalous Hall state. We further establish a strain-induced mechanism to control these topological phase transitions and present the corresponding phase diagram. Finally, we demonstrate the predicted transitions in monolayer CrO, a realistic altermagnetic candidate, using first-principles calculations. Our findings highlight the potential of 2D AMs as a versatile platform for topological spintronics, enabling strain-tunable helical and chiral edge states within a single system.