  Based on first-principles lattice dynamics and electron-phonon coupling
calculations, B2C sheet is predicted to be a two-dimensional (2D)
phonon-mediated superconductor with a relatively high transition temperature
(Tc). The electron-phonon coupling parameter calculated is 0.92, and it is
mainly contributed by low frequency out-of-plane phonon modes and electronic
states with a {\pi} character. When the Coulomb pseudopotential is set to 0.10,
the estimated temperature Tc is 19.2 K. To be best of our knowledge, B2C is the
first pristine 2D superconductor with a Tc higher than the boiling point of
liquid helium.
