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Abstract

The protected helical surface states in thin films of topological insulators (TIs) are subject to intersurface hybridization. This leads to gap opening and spin texture changes as witnessed in photoemission and quasiparticle interference investigations. Theoretical studies show that universally, the hybridization energy exhibits exponential decay as well as sign oscillations as a function of film thickness, depending on the effective band parameters of the material. When a step is introduced in the TI film, e.g., by profiling the substrate such that the hybridization has different signs on both sides of the step, one-dimensional bound states appear within the hybridization gap which decay exponentially with distance from the step. The step bound states have linear dispersion and inherit the helical spin locking from the surface states and are therefore nondegenerate. When the substrate becomes an s-wave superconductor, Majorana zero modes located at the step ends are created inside the superconducting gap. The proposed scenario involves just a suitably stepped interface of a superconductor and a TI and therefore may be the simplest device able to host Majorana zero modes.