Intrinsic Valley Splitting and Direct-to-Indirect Band Gap Transition in Monolayer HfZrSiCO 2 .

Both a reasonably large valley splitting (VS) and a sufficiently long valley exciton lifetime are crucial in valleytronics device applications. Currently, no single system possesses both attributes simultaneously. Herein, we demonstrate that a Janus monolayer HfZrSiCO 2 concurrently hosts a giant intrinsic VS and excitonic quasi-particles with long valley lifetime due to valley-sublayer coupling and built-in electric field. In addition, the band structure of the monolayer HfZrSiCO 2 can be continuously manipulated by either an external electric field or a biaxial strain, giving rise to a tunable VS and driving a direct-to-indirect band gap transition. Moreover, the system exhibits valley-contrasting linear dichroism in exciton absorption. These results suggest that the Janus monolayer HfZrSiCO 2 is a promising candidate for information applications.