Polarity-Driven Atomic Displacements at the 2D Mg2TiO4-MgO (001) Oxide Interface for Hosting Potential Interlayer Excitons

Interlayer excitons in solid-state systems have emerged as candidates for realizing novel platforms ranging from excitonic transistors and optical qubits to exciton condensates. Interlayer excitons have been discovered in 2D transition metal dichalcogenides, with large exciton binding energies and the ability to form various van der Waals heterostructures. Here, an oxide system consisting of a single unit cell of Mg2TiO4 on MgO (001) is proposed as a platform for hosting interlayer excitons. Using a combination of density functional theory (DFT) calculations, molecular beam epitaxy growth, and in situ crystal truncation rod measurements, it is shown that the Mg2TiO4-MgO interface can be precisely controlled to yield an internal electric field suitable for hosting interlayer excitons. The atoms in the polar Mg2TiO4 layers are observed to be displaced to reduce polarity at the interface with the non-polar MgO (001) surface. Such polarity-driven atomic displacements strongly affect electrostatics of the film and the interface, resulting in localization of filled and empty band-edge states in different layers of the Mg2TiO4 film. The DFT calculations suggest that the electronic structure is favorable for localization of photoexcited electrons in the bottom layer and holes in the top layer, which may bind to form interlayer exciton states.