Prediction of quasi-one-dimensional superconductivity in metastable two-dimensional boron

Two-dimensional (2D) boron has been predicted to exhibit various structural polymorphs and interesting physics, including superconductivity, ideal strength, negative Possion's ratio, and a higher thermal conductivity than graphene, etc. However, the difficulty in experimental synthesis of 2D boron has further reduced its exploration and applications. In this work we have used a crystal structure search method and first-principles calculations. We have proposed a scheme of ``synthesize under high pressure and peel off at ambient conditions'' to first predict a bulk phase of $\mathrm{K}{\mathrm{B}}_{4}$, and then peel off free standing 2D boron from it. Based on electron-phonon coupling calculations, we estimated that 2D boron is an intrinsic superconductor with ${T}_{c}=17.9\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In addition we found a strong anisotropy of the electron-phonon coupling strength in 2D boron, thus the superconductivity in 2D boron possesses a quasi-1D feature. Our proposed scheme for the synthesis of free standing 2D boron will stimulate further studies of superconductivity in low dimensional materials, both theoretically and experimentally.