Tunable Spontaneous Valley Coherence in Atomically Thin Semiconductors

An interesting feature of monolayer transition metal dichalcogenides (TMDCs) is the existence of two degenerate, but disparate, band extrema valleys in k space with different optical selection rules. In order to manipulate this “valley degree of freedom”, we should be able to control, and manipulate the coherence between the two valleys.
In this work we study the spontaneous valley coherence generated in a van der Waals heterostructure created by combining a TMDC with other two dimensional (2D) materials. First, we discuss general results on exploring the parameter space of the optical conductivity tensor of such a system to get the highest values of valley coherence and coherence times. Second, we analyze the spontaneously generated valley coherence for “hybrid” (a mixture of interlayer and intralyer) excitons in a system involving an anisotropic photonic metasurface and a TMDC heterostructure. We study the dependence of the coherence on external tuning knobs such as an electric field, and interlayer coupling in the heterostructure. Our analysis should help the community in designing heterostructures for maximum valley coherence, and help solidify the foundation of the valley degree of freedom as a qubit.