Investigation on the reported superconductivity in intercalated black phosphorus

Superconductivity intrinsic to the intercalated black phosphorus (BP) with a transition temperature T_c of 3.8 K, independent of the intercalant, whether an alkali or an alkaline earth element, has been reported by R. Zhang et al. (2017). However, the reported T_c and the field effect on the superconducting (SC) transition both appear to be similar to those for the pure Sn, which is commonly used for BP synthesis under the vapor transport (VP) method. We decided to determine if a minute amount of Sn is present in the starting BP and if it is the culprit for the small SC signal detected. EDS results confirmed the existence of Sn in the starting BP purchased from the same company as in R. Zhang et al. We have reproduced the SC transition at 3.8 K in Li- and Na-intercalated BP that contain minute amounts of Sn when prepared by the VP method but not in BP that are free of Sn when prepared by the high-pressure method. We therefore concluded the SC transition reported by R. Zhang et al. is associated with the Sn but not intrinsic to the intercalated BP crystals.