Unraveling the evolution of multigap superconductivity in layered Na–B–C films: An additional energy gap induced by the internal B–C layer

Abstract

Multigap superconductors provide a platform to confirm rich new physics such as time-reversal symmetry breaking, giant paramagnetic response, and hidden criticality. However, an obstacle hindering the experimental validation of these phenomena lies in the lack of superconductors with three or more gaps and critical temperatures higher than the liquid nitrogen temperature. In this work, we predicted NaB₂C₂ and Na₂B₃C₃ films with high-temperature superconductivity (beyond 90 K) using the fully anisotropic Migdal-Eliashberg theory. The multigap behaviors of Na–B–C films with three and five atomic layers were analyzed in detail, revealing two typical configurations: three-gap (NaB₂C₂) and four-gap (Na₂B₃C₃) superconductors. Compared with the NaB₂C₂ film, the additional gap observed in the Na₂B₃C₃ film originates from the in-plane covalent state of the internal B–C layer. This research offers valuable insights into the evolution of multigap superconductivity in layered B–C films.