Superconductivity of Hard Hexagonal ε-NbN Epitaxial Films

Hsiao-Wen Chang^1*, Vankayala Krishna Ranganayakulu^2,3,4, Syu-You Guan², Min-Nan Ou², Yang-Yuan Chen², Tien-Ming Chuang², Chia-Seng Chang², Maw-Kuen Wu², Ming-Jye Wang^1,2

¹Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
²Institute of Physics, Academia Sinica, Taipei, Taiwan
³Department of Engineering and System Science, National Tsing-Hua University, Hsinchu, Taiwan
⁴Taiwan International Graduate Program, Academia Sinica, Taipei, Taiwan

* Presenter:Hsiao-Wen Chang, email:hwchang@asiaa.sinica.edu.tw

Recently, superconducting transition at 11.6 K was firstly reported in hard hexagonal ε-NbN polycrystalline sample by high pressure and high temperature synthesis, which attracts great attentions in condensed matter physics. In the paper, we report the success of growing hard hexagonal ε-NbN ultrathin film on 4H-SiC substrate by magnetron sputtering method for the first time. The high resolution transmission electron microscope images and selected area electron diffraction patterns confirm the hexagonal symmetry of deposited films. The cross-sectional electron dispersion spectroscopy spectrum suggests the Nb:N ratio of 1:1. The deposited film has a shortened c-axis due to the enlarged a-axis by substrate effect. Sharp superconducting transition was observed in both magnetic and transport measurements. The superconducting transition temperature of 21.4 nm is around 12.3 K. The ε-NbN film is a strong coupling superconductor with 2Δ/kBTc ~ 4.8 and its estimated superconducting coherent length at zero temperature in ab-plane is about 4.5 nm. The success of preparing high quality ε-NbN film realizes the possibility of preparing samples for further studies on their fundamental physical property and device applications of ε-NbN in the future.

Keywords: epitaxial film, ε-NbN, magnetron sputtering deposition, superconductivity