First principles studies of valley splitting in monolayer transition metal dichalcogenides on BiFeO3*

Control of the spin and valley degrees of freedom of monolayer transition metal dichalcogenides (TMDs) via time reversal symmetry breaking at K and K’ has been successfully demonstrated via magnetic substrates [1], which can generate significantly larger valley splitting than applied magnetic fields. Multiferroic substrates offer an avenue to develop valleytronics devices with switchable in-situ polarization. Using first principles density functional theory calculations, we predict that the ferromagnetically-ordered surface of Fe-terminated (111)-BiFeO3 substrates can produce valley splittings in WSe2 an order of magnitude larger than previously proposed magnetic substrates. We discuss the details of the orbital exchange interactions driving the splitting; and we also develop design principles for tuning the valley splitting through lattice alignment of the TMD monolayer with the substrate, as well as the identity and spin state of the magnetic substrate ions.

[1] C. Zhao, et al., Nat. Nanotechnol. 12 757 (2017)