Two-gap and three-gap superconductivity in AlB2-based films

Abstract

In contrast to MgB2 superconducting at 39K, the isostructural AlB2 has no superconductivity due to the absence of metallization driven by the covalent state of boron in MgB2. In this paper, we investigate from first principles the superconducting behavior in ultrathin limit of AlB2, and reveal a two-gap superconducting nature with transition temperatures Tcs in the range of 17−26.5K. Our results indicate that the superconductivity in AlB2 films originates mainly from the surface-boron-plane-driven covalent state which crosses the Fermi level because of the quantum confinement. Therefore, the hyperstoichiometric AlB2+x films with the surface boron layer existing on each side of the systems also promise admirable superconductivity. In particular, a high Tc∼47K with an exciting three-gap nature is captured in the thinnest AlB2+x films that possess a stoichiometry of AlB4 and a sandwich configuration of B2−Al−B2. In addition, the effect of external strain on superconducting behavior in the thinnest AlB2 film and sandwich AlB4 film is also involved, which shows that a small compressive strain can further boost the Tcs in both films and a three-gap nature can be also observed in the thinnest AlB2 film under a biaxial tensile strain of 2.0%−2.5%. These findings pave the way not only for study of superconductivity in AlB2-based materials but also the search for multigap superconductors.

Source-reported materials — not catalogue approval

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