Insight into Two-Dimensional Borophene: Five-Center Bond and Phonon-Mediated Superconductivity*

We report a previously unknown monolayer borophene allotrope and we call it super-B with a flat structure based on ab initio calculations. It has good thermal, dynamical, and mechanical stability compared with many other typical borophenes. We find that super-B has a fascinating chemical bond environment consisting of standard sp, sp² hybridizations, and delocalized five-center three-electron π bond, called π(5c-3e), relying on the natural bond orbital analysis. This exceptional distribution of electron orbitals plays a pivotal role in stabilizing the super-B chemically. Moreover, super-B has a critical temperature T_c of 20.8 K at ambient condition. We attribute this high T_c of super-B to the giant anharmonicity of two linear acoustic phonon branches and an unusually low optic phonon mode. These discoveries provide new insight into the chemical nature of low dimensional boron nanostructures and highlight the potential applications of designing flexible devices and high T_c superconductors.
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