Absence of superconductivity in micrometer-sized epsilon-NbN single crystals

It is important to study the properties of high quality single crystal in order to resolve the issue of an interesting material in which certain debatable fundamental properties exist. However, it is unfortunate that a sizable single crystal for experimental measurements is not always available. NbN is one of the examples; it has attracted scientific and engineering interest due to its diverse physical properties and a variety of structural phases. Until now superconductivity is only observed in cubic delta- and tetragonal gamma-NbN but not in hexagonal epsilon-NbN. Recently, Zou et al. reported the observation of superconductivity with T-c similar to 11.6 K in a hexagonal epsilon-NbN based on the measurement on a multiphase powder specimen. In order to resolve the issue, the work used the electron backscattering diffraction technique to characterize phases of micron-size NbN crystals from commercial powders and measure their transport properties. Our results unambiguously confirm that the hexagonal epsilon-NbN phase is not superconducting.