Evolution of multigap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB_2

摘要

Starting from first principles, we show the formation and evolution of superconducting gaps in MgB_2 at its ultrathin limit. Atomically thin MgB_2 is distinctly different from bulk MgB_2 in that surface states become comparable in electronic density to the bulklike σ and π bands. Combining the ab initio electron-phonon coupling with the anisotropic Eliashberg equations, we show that monolayer MgB_2 develops three distinct superconducting gaps, on completely separate parts of the Fermi surface due to the emergent surface contribution. These gaps hybridize nontrivially with every extra monolayer added to the film owing to the opening of additional coupling channels. Furthermore, we reveal that the three-gap superconductivity in monolayer MgB_2 is robust over the entire temperature range that stretches up to a considerably high critical temperature of 20 K. The latter can be boosted to >50 K under biaxial tensile strain of ~4%, which is an enhancement that is stronger than in any other graphene-related superconductor known to date.