Emergence of Surface Superconductivity through Interference in Superconducting-proximity Topological Insulators

Abstract

Superconducting-proximity topological insulators (STIs) have garnered significant research attention over the past two decades. In this Letter, we demonstrate that a low-dimensional STI in the topological-nontrivial phase (TP) exhibits an interference-induced surface (boundary) superconductivity with the surface critical temperature $T_{cs}$ significantly higher than the bulk one $T_{cb}$. Such a surface superconductivity is built due to the interference of the scattering quasiparticle states, rather than due to the presence of the topological bound states (TBSs). As the system goes deeper into the TP, the surface exhibits a crossover from the interference- to TBS-induced phase, where the surface enhancement of superconductivity is governed by the TBSs. Our study unveils a substantial variation in the maximal $T_{cs}$ along this crossover, attaining values being twice the maximal bulk critical temperature of the STI. Beyond shedding light on the nature of surface superconductivity in STIs, our study introduces a tangible method for experimentally manipulating their critical superconducting temperatures.