Intercalant-independent transition temperature in superconducting black phosphorus

Research on black phosphorus (BP) has been experiencing a renaissance over the last few years, after the demonstration that few-layer BP exhibits high carrier mobility and a thickness-dependent band gap. For a long time, bulk BP is also known to be a superconductor under high pressure exceeding 10 GPa. The superconductivity is due to a structural transformation into another allotrope of phosphorous and accompanied by a semiconductor-metal transition. No superconductivity could be achieved for BP itself (that is, in its normal orthorhombic form) despite several attempts reported in the literature. Here we describe successful intercalation of BP by several alkali metals (Li, K, Rb, Cs) and alkali-earth Ca. All the intercalated compounds are found to be superconducting, exhibiting the same (within our experimental accuracy) critical temperature of 3.8+-0.1 K and practically identical characteristics in the superconducting state. Such universal superconductivity, independent of the chemical composition, is highly unusual. We attribute it to intrinsic superconductivity of heavily-doped individual phosphorene layers, while the intercalated layers of metal atoms mostly play a role of charge reservoirs.