The Enigma of Superconductivity in ε-NbN crystal

Tsu Lien Hung^1*, 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, email:tlhung@gate.sinica.edu.tw

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