Boron kagome-layer induced intrinsic superconductivity in a MnB3 monolayer with a high critical temperature

The design of two-dimensional superconductors has attracted great research interest owing to their wide application in nanoscale devices. Here, we combine first-principles calculations with structure searching technology to identify a unique stable hexagonal $h\text{\ensuremath{-}}{\mathrm{MnB}}_{3}$ monolayer, exhibiting a slightly higher energy with respect to the reported tetragonal $t\text{\ensuremath{-}}{\mathrm{MnB}}_{3}$. Interestingly, $h\text{\ensuremath{-}}{\mathrm{MnB}}_{3}$ contains two boron kagome layers sandwiched with Mn atoms. It exhibits metallic properties and has a superconducting transition temperature of 24.9 K, which is much higher than 2.9 K in $t\text{\ensuremath{-}}{\mathrm{MnB}}_{3}$. Its superconductivity mainly originates from the coupling between in-plane vibrational phonons of boron kagome layers and electrons of Mn atoms. $h\text{\ensuremath{-}}{\mathrm{MnB}}_{3}$ exhibits a tunable superconductivity, and reaches a maximum of 34 K at 2% tensile strain resulting from the softening in-plane modes of boron kagome layers. The Si (111) surface may be an ideal substrate for the growth of superconductive $h\text{\ensuremath{-}}{\mathrm{MnB}}_{3}$. The unique superconducting mechanism observed here could inspire the searching of more boron kagome based two-dimensional superconductors.