Electron-phonon coupling in a honeycomb borophene grown on Al(111) surface

Abstract

Recently, a honeycomb borophene was reported to grow successfully on Al(111) surface. Since the metallic $\ensuremath{\sigma}$-bonding bands of honeycomb boron sheet play a crucial role in the 39 K superconductivity of ${\mathrm{MgB}}_{2}$, it is physically interesting to examine whether a similar property exists in this material. We have calculated the electronic structures and the electron-phonon coupling for honeycomb borophene by explicitly considering the substrate effect using first-principles density functional theory in conjunction with the Wannier interpolation technique. We find that the $s{p}^{2}$-hybridized $\ensuremath{\sigma}$-bonding bands of honeycomb borophene are metallized due to moderate charge transfer from the Al substrate, similar to ${\mathrm{MgB}}_{2}$. However, the electron-phonon coupling in honeycomb borophene is much weaker than in ${\mathrm{MgB}}_{2}$ due to the hardening of the bond-stretching boron phonon modes and the reduction of phonon density of states. Nevertheless, the interlayer coupling between Al-associated phonons and electrons in borophene is strong. Based on this observation, we predict that a superconducting transition temperature of 6.5 K can be observed in a free-standing borophene decorated by a single Al layer, namely monolayer ${\mathrm{AlB}}_{2}$. Accordingly, a similar superconducting transition temperature could be expected in honeycomb borophene on Al(111).