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

Although the idea of achieving topological superconductivity (TSC) in hybrid heterostructures of superconductors and systems with topologically nontrivial band structures has been proposed, the realization of TSC in a single stoichiometric material is most desired for fundamental experimental exploration of TSC and its device applications. In this context, based on electronic structure calculations, it was recently proposed that YRuB₂ is a candidate TSC that also hosts special hourglass-type Dirac rings protected by symmetry. Bulk measurements on YRuB₂ detect a single superconducting gap of ∼ 1 meV. 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. Therefore, we present YRuB₂ as a unique system where the interaction between topological surface states and bulk superconductivity leads to novel physical insights in understanding the candidate topological superconductors.