Superconductivity of various borides: The role of stretched c-parameter

Abstract

The superconductivity of MgB 2 , AlB 2 , NbB 2+x , and TaB 2+x is intercompared. The stretched c-lattice parameter ͑c = 3.52 Å͒ of MgB 2 in comparison to NbB 2.4 ͑c = 3.32 Å͒ and AlB 2 ͑c = 3.25 Å͒ decides empirically the population of their and bands and as a result their transition temperature T c values, respectively, at 39 and 9.5 K for the first two and no superconductivity for the later. The nonstoichiometry induces an increase in c parameter with Boron excess both in NbB 2+x and TaB 2+x . Magnetization ͑M-T͒ and resistivity measurements ͑ -T͒ in case of niobium boride samples show the absence of superconductivity in stoichiometric NbB 2 sample ͑c = 3.26 Å͒ while a clear diamagnetic signal and a = 0 transition for boron excess NbB 2+x samples. On the other hand, superconductivity is not achieved in TaB 2+x case. The probable reason behind is the comparatively lesser or insufficient stretching of c parameter.

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