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Most superconductors exhibit spin-singlet pairing within a single band. In multiband systems with strong spin-orbit coupling, more exotic scenarios can emerge, including Cooper pairs between bands with distinct symmetries. Here, we present evidence of the formation of Cooper pairs between spin-nondegenerate helical surface bands in the superconductor β-Bi₂Pd. Scanning tunneling microscopy reveals anisotropic Yu-Shiba-Rusinov (YSR) resonances induced by vanadium impurities, with long-range amplitude modulations attributed to spin-conserving Bogoliubov quasiparticle interference (BQPI). Analysis of BQPI at the subgap YSR energy shows that only a selective subset of normal-state scattering processes contributes to Cooper pair formation, indicating interband mixing. We trace this selectivity to the hybridization of a helical surface band with others via the impurity.
Zaldı́var et al. (Tue,) studied this question.