  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.
