Superconductivity and topological properties of MgB2 -type diborides from first principles

The superconductivities and topological properties of ${\mathrm{MgB}}_{2}$-type diborides are investigated by means of first-principles calculations with different exchange-correlation functionals. Functionals with the van der Waals (vdW) correction (such as OptB88-vdW) may predict critical temperature (${T}_{c}$) comparable with experimental results for several ${\mathrm{MgB}}_{2}$-type superconductors, particularly for the pristine ${\mathrm{MgB}}_{2}$ (39.3 vs 39 K). Interestingly, the spin-fluctuation is found to play a significant role in the superconducting behavior of diborides with transition metal elements, and their ${T}_{c}$ can be enhanced monotonically by applying tensile strains. Furthermore, Dirac surface states of $\mathrm{Ta}\phantom{\rule{0.16em}{0ex}}{\mathrm{B}}_{2}$ and ${\mathrm{NbB}}_{2}$ are revealed, suggesting their potential use as topological superconducting materials. This paper provides a useful guideline for ab initio studies of superconductivities and topological properties of vdW layered materials.