  First-principles calculations predicted monolayer blue phosphorus to be an
alternative two-dimensional allotrope of phosphorus, like the recently
discovered monolayer black phosphorus. Due to its unique crystalline and
electronic structure, blue phosphorus may be a promising candidate as a
BCS-superconductor after proper intercalation. In this study, using
first-principles calculations, the favorable intercalation sites for some
alkali metals and alkaline earths have been identified for Blue-P bilayer and
the stacking configuration of bilayer is changed. Then the blue phosphorus
bilayer transforms from a semiconductor to a metal due to the charge transfer
from metal to phosphorus. Own to the strong electron-phonon coupling, isotropic
superconducting state is induced and the calculated transition temperatures are
$20.4$, $20.1$, and $14.4$K for Li-, Na-, and Mg-intercaltion, respectively,
which is superior to other predicted or experimentally observed two-dimensional
BCS-superconductors.
