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Recent theoretical and experimental studies suggest that van der Waalsheterostructures with n- and p-doped bilayers of transition metaldichalcogenides are promising facilitators of exciton superfluidity. Excitonsuperfluidity in such bilayer systems is often modelled by solving a mean-fieldgap equation defined for only the conduction and valence band of the electronand hole material respectively. A key quantity entering the gap equation is theeffective Coulomb potential acting as the bare interaction in the subspace ofthe model. Since the model only includes a few bands around the Fermi energythe effective model interaction is partially screened. Although the screeningis a material dependent quantity it has in previous works been accounted for inan ad hoc manner, by assuming a static dielectric constant of 2 for a widerange of different materials. In this work we show that the effective modelinteraction can be derived from first principles using open source codeframeworks. Using this novel ab initio downfolding procedure we show that thematerial dependent screening is essential to predict both magnitude and trendsof exciton binding energies and superfluid properties. Furthermore, we suggestnew material platforms of both transition metal oxides and dichalcogenides withsuperior properties compared to the standard devices with two transition metaldichalcogenide layers.

Exciton Superfluidity in 2D Heterostructures from First Principles: The importance of material specific screening