Prediction of above 20 K superconductivity of blue phosphorus bilayer with metal intercalations

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.