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

TL;DR

This paper introduces a new monolayer borophene called super-B with unique bonding, stability, and electronic properties, including high-temperature superconductivity at 25.3 K, offering insights for flexible devices and superconductors.

Contribution

It reports the discovery and characterization of a novel borophene allotrope with unique five-center bonds and high Tc superconductivity at ambient conditions.

Findings

Super-B has a flat structure with good stability.

Super-B exhibits a high Tc of 25.3 K.

Super-B's electronic structure includes five-center three-electron bonds.

Abstract

We report a previously unknown monolayer borophene allotrope and we call it super-B with a flat structure based on the 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 $\pi$ bond, called $\pi$(5c-3e). This particular electronic structure plays a pivotal role in stabilizing the super-B chemically. By extra doping, super-B can be transformed into a Dirac material from pristine metal. Like graphene, it can also sustain tensile strain smaller than 24%, indicating superior flexibility. Moreover, due to the small atomic mass and large density of states at the Fermi level, super-B has the highest critical temperature Tc of 25.3 K in single-element superconductors at ambient conditions. We attribute this high Tc of super-B to the giant anharmonicity of two linear acoustic phonon branches and an unusually low optic phonon mode. These predictions provide new insight into the chemical nature of low dimensional boron nanostructures and highlight the potential applications of designing flexible devices and high Tc superconductors.