Metastable superconductivity in niobium diborides

A metastable superconducting phase of NbB2 with the transition temperature (Tc) around 9 K has been found in this study, although no confirmed superconductivity in the stoichiometric NbB2 has been reported since 1970s. This phase was induced by a rapid-quenching process and disappeared by annealing at 1100 deg. C. Since the superconducting transition was not sharp, the rapidly quenched NbB2 might contain some phases with different superconducting transition temperatures (Tcs). Niobium-deficient and -rich diborides of NbxB2 (x=0.67-1.11) were also prepared for the investigation of the compositional dependence of their Tcs