Superconductivity in Niobium Nitride

Niobium nitride is a well-known superconductor with many excellent physical properties, such as high hardness, high shear rigidity, high bulk modulus. It has several structure phases, including cubic, hexagonal, and tetragonal. Among these phases, the cubic phase has been extensively studied the superconductivity with a wide range of T_c, 9 ~ 17 K. In our work, the cubic phase specimens of NbN were been measured by X-ray diffraction, magnetic susceptibility, resistivity, and specific heat. We observed that the T_c of cubic phase appeared 13.6 K from magnetic and specific heat measurements. Very recently the hexagonal phase of epsilon-NbN (ε-NbN) has been reported to have the superconducting transition temperature about 11 K [1]. A micron sized NbN crystal was obtained from commercial product, and confirmed to be epsilon phase by electron back-scattering diffraction (EBSD) technique. However, no superconductivity was found in the micron sized epsilon-NbN crystal. We inferred that the superconductivity of epsilon-NbN reported in literature may have shorter Nb-N bond length as that of the cubic NbN phase which is about 0.02 Å shorter than our epsilon-NbN crystal.
[1]. Zou, Y., et. al . Sci Rep, 6, 22330. (2016).