Proximity Effect at the Superconductor-Topological Insulator Interface from First Principles

A three-dimensional topological insulator (TI) film overlaid on an s-wave superconductor (SC) has emerged as one of good candidates for hosting a Majorana zero-mode at each vortex core in the proximity-induced topological SC state. Experiments on TI-SC heterostructures showed an intriguing range of proximity-induced SC gap sizes arising from TI bulk or topological surface states when the Fermi level lies in the TI bulk conduction band or in the topological surface state. Theoretical studies have been, so far, carried out based on effective models. Here we present the proximity-induced SC gap features, quasiparticle spectra, and effective TI pairing potential as a function of film thickness and chemical potential for Bi₂Se₃ films on SC PdTe, by solving the Bogoliubov-de Gennes equations within the screened Korringa-Kohn-Rostoker method in the framework of density-functional theory. We find multiple proximity-induced SC gaps where the gap sizes highly depend on characteristics of the TI states. The smallest gap arises from the topological surface state with its gap size depending on the TI pairing potential. Our results are compared to experiment.