Orbital gyrotropic magneto-electric effect and its strain engineering in monolayer Nb$X_2$

TL;DR

This paper demonstrates that strain can induce a large orbital gyrotropic magneto-electric effect in monolayer NbX2, driven by valley orbital moments and symmetry breaking, without requiring spin-orbit coupling, enabling potential device control.

Contribution

It reveals strain-induced orbital GME in monolayer NbX2 driven by valley orbital moments, without spin-orbit coupling, and provides a theoretical model for this effect.

Findings

Strain induces a large GME in monolayer NbX2.

The effect is driven by valley orbital moments and symmetry breaking.

The GME can be switched by changing strain conditions.

Abstract

Electrical control of the orbital degrees of freedom is an important area of research in the emerging field of "orbitronics." Orbital {\it gyrotropic} magneto-electric effect (OGME) is the generation of an orbital magnetization in a nonmagnetic metal by an applied electric field. Here, we show that strain induces a large GME in the monolayer Nb$X_2$ ($X =$ S, Se) normal to the plane, primarily driven by the orbital moments of the Bloch bands as opposed to the conventional spin magnetization, without any need for spin-orbit coupling. The key physics is captured within an effective two-band valley-orbital model and it is shown to be driven by three key ingredients: the intrinsic valley orbital moment, broken $C_{3z}$ symmetry, and strain-induced Fermi surface changes. The effect can be furthermore switched by changing the strain condition, with potential for future device applications.