MgB4 trilayer film: A four-gap superconductor

Abstract

Since the discovery of two-gap superconductivity in bulk MgB2, many efforts have been devoted to searching for multigap superconductors. Recently, a monolayer of MgB2 and a trilayer film of AlB4 were predicted to be robust three-gap superconductors. Here we investigate using the fully anisotropic Migdal-Eliashberg theory the superconducting behaviors in a series of boron-based monolayers and trilayer films that are dynamically stable but not synthesized experimentally heretofore. Remarkably, we find that the trilayer film of MgB4 has an exciting four-gap superconducting nature with a high transition temperature Tc∼52 K. We demonstrate that this high-Tc four-gap superconductivity is closely related to the Fermi surface exhibiting an unusually strong electron-phonon coupling with evident four-region distribution characteristics. The electron-lattice coupling is dominated by the acoustic modes of a softening kink and the high-energy optical phonons around the Brillouin-zone center, of which the former ones place the system on the verge of a latent charge density wave state. To our knowledge, no material with more than three superconducting gaps has been reported. At the same time, we also show that the GaB4 trilayer film has a robust three-gap nature with Tc∼51 K, and the trilayer films of BeB4 and LiB4 are two-gap superconductors with Tc's of ∼39 and 32 K, respectively.

Source-reported materials — not catalogue approval

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