The Enigma of Superconductivity in ε-NbN crystal
Tsu Lien Hung1*, Fan Yu Chiu2, Ya Hsin Pai3, Cheng Yen Liu4, Min Nu Ou1, Ting Kuo Lee1, Yang Yuan Chen1
1Institute of Physics, Academia Sinica, Taipei, Taiwan
2Department of Physics, National Taiwan University, Taipei, Taiwan
3Department of Applied Physics and Chemistry, University of Taipei, Taipei, Taiwan
4Department of Physics, National Central University, Taoyuan, Taiwan
* Presenter:Tsu Lien Hung,
NbN has attracted intensive attention of scientists and engineers due to its multi-structure phases and diverse physical properties, including hardness, superconductivity and topological properties. For example, superconductivity near 9-17 K was found in cubic phase NbN, in contrast, no superconductivity has ever been reported in hexagonal structural ε-NbN either in experiment or theory till the superconductivity observed by You et al in 2016. However, the consequence was derived from the magnetic susceptibility of a powder specimen including multi phases of cubic- and ε-NbN phases. Since no similar result was attested by others subsequently, thus the assertion is debatable. To clarify the issue, we have measured the specific heat and magnetic susceptibility of powder specimens with main phase of cubic and ε-NbN individually; however, only superconductivity found in cubic NbN but not in ε-NbN. To exclude the influence on measurements by the appearance impurity phase, we developed an innovative technique to measure the electrical conductivity of a single crystal with micrometer size. Once again only superconductivity found in the cubic NbN crystal but not in ε-NbN. Based on the manifest electric conductivity measurements on pure single crystal of ε-NbN, we thus resolve the mysterious issue that the ε-NbN is a non-superconductor that is in consistent with all earlier theoretical works.

Keywords: Niobium Nitrite, Superconductivity