Topological surface states host superconductivity induced by the bulk condensate in YRuB2

While the possibility of topological superconductivity (TSC) in hybrid heterostructures involving topologically nontrivial band structure and superconductors has been proposed, the realization of TSC in a single stoichiometric material is most desired for fundamental experimental investigation of TSC and its device applications. Bulk measurements on YRuB₂ detect a single superconducting gap of ~1 meV. This is supported by our electronic structure calculations, which also reveal the existence of topological surface states in the system. We performed surface-sensitive Andreev reflection spectroscopy on YRuB₂ and detected the bulk superconducting gap as well as another superconducting gap of ~0.5 meV. From our analysis of electronic structure, we show that the smaller gap is formed in the topological surface states in YRuB₂ due to the proximity of the bulk superconducting condensate. Thus, in agreement with the past theoretical predictions, we present YRuB₂ as a unique system that hosts superconducting topological surface states.