Superconductivity in two-dimensional boron allotropes

We use ab initio evolutionary algorithm and first-principles calculations to investigate structural, electronic, vibrational, and superconducting properties of two-dimensional ($2\mathrm{D}$) boron allotropes. Remarkably, we show that conventional BCS superconductivity in the stable $2\mathrm{D}$ boron structures is ubiquitous with the critical temperature ${T}_{c}$ above the liquid hydrogen temperature for certain configurations. Due to the electronic states of the Fermi surface originating from both $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ electrons, the superconductivity of the $2\mathrm{D}$ structures arises from multiple phonon modes. Our results support that $2\mathrm{D}$ boron structure may be a pure single-element material with the highest ${T}_{c}$ on conditions without high pressure and external strain.