Three-gap superconductivity in two-dimensional InB2/InB4 films

In recent years, multigap superconductors have attracted much attention since the discovery of novel two-gap superconductivity with transition temperature ${T}_{c}\ensuremath{\sim}39\phantom{\rule{4pt}{0ex}}\text{K}$ in bulk ${\mathrm{MgB}}_{2}$. Based on the first-principles calculation and anisotropic Migdal-Eliashberg theory, we conduct a study on a series of two-dimensional (2D) boron-based materials with doped metal atoms to search for multigap superconductors. We find that ${\mathrm{InB}}_{2}$ monolayer films and ${\mathrm{InB}}_{4}$ trilayer films are dynamically stable but not synthesized experimentally yet. An evident three-gap superconductor with high ${T}_{c}\ensuremath{\sim}$ 41.5 K is obtained in a ${\mathrm{InB}}_{2}$ monolayer film. Similarly, ${\mathrm{InB}}_{4}$ trilayer film is a novel superconductor with three distinct superconducting gaps and a high ${T}_{c}\ensuremath{\sim}$ 53 K. The superconductivity in both 2D films originates mainly from the covalent-state-driven metallization. In addition, the effect of biaxial strain on the superconducting behavior of $\mathrm{In}{\text{B}}_{4}$ trilayer films is also involved. The ${\mathrm{InB}}_{4}$ trilayer films stay dynamically stable under biaxial tensile strain of $\ensuremath{-}3%--6%$, and the highest ${T}_{c}$ boosts to 64 K under the biaxial tensile strain of $\ensuremath{\sim}4%$. Meanwhile, we also find that the ${\mathrm{GeB}}_{4}$ and ${\mathrm{ZnB}}_{4}$ trilayer films are two-gap superconductors with high ${T}_{c}\ensuremath{\sim}$ 48.5 and 32 K, respectively.