Single anisotropic gap superconductivity and proximity effect in PbTaSe2

PbTaSe2 is a noncentrosymmetric superconductor having topological Dirac surface states as shown by recent experiments. Here, we report calculations on bulk and few-layer PbTaSe2 using the first-principles anisotropic Migdal-Eliashberg theory, to explore the nature of the superconducting gap and the surface-bulk proximity effect in this stoichiometric system. In bulk, the multiband Pb p/Ta d Fermi surface exhibits a single anisotropic full superconducting gap, with larger gap values appearing on in-plane orbitals that can couple effectively to the in-plane phonons. A Pb-terminated few-layer PbTaSe2 is found to still have a single-anisotropic-gap feature, but its superconducting gap is evidently reduced relative to the bulk case. Comparison between our theoretical results and experimental tunneling data suggests a significant proximity-enhanced superconductivity on the realistic surface of PbTaSe2. The present findings would apply to other same-structured ABSe2 (A = Pb, Sn; B = Ta, Nb) and are crucial for studying topological superconductors and Majorana fermions in promising topological metals with intrinsic full-gap superconductivity.