Design of sign-reversible Berry phase effect in 2D magneto-valley material

Manipulating sign-reversible Berry phase effects is both fundamentally intriguing and practically appealing for searching for exotic topological quantum states. However, the realization of multiple Berry phases in the magneto-valley lattice is rather challenging due to the complex interactions from spin–orbit coupling (SOC), band topology, and magnetic ordering. Here, taking single-layer spin-valley RuCl 2 as an example, we find that sign-reversible Berry phase transitions from ferrovalley (FV) to half-valley semimetal (HVS) to quantum anomalous valley Hall effect (QAVHE) can be achieved via tuning electronic correlation effect or biaxial strains. Remarkably, QAVHE phase, which combines both the features of quantum anomalous Hall and anomalous Hall valley effect, is introduced by sign-reversible Berry curvature or band inversion of d x y / d x 2 − y 2 and d z 2 orbitals at only one of the K / Kʹ valleys of single-layer RuCl 2 . And the boundary of QAVHE phase is the HVS state, which can achieve 100% intrinsically valley polarization. Further, a k ⋅ p model unveiled the valley-controllable sign-reversible Berry phase effects. These discoveries establish RuCl 2 as a promising candidate to explore exotic quantum states at the confluence of nontrivial topology, electronic correlation, and valley degree of freedom.