Optical and Tunneling Studies of Energy Gap in Superconducting Niobium Nitride Films

  • Yoshinori UzawaEmail author
  • Shingo Saito
  • Wei Qiu
  • Kazumasa Makise
  • Takafumi Kojima
  • Zhen Wang


We have prepared an epitaxial niobium nitride (NbN) film and NbN/AlN/NbN tunnel junctions to investigate the energy gaps. By measuring the optical conductivity spectrum of a 41-nm-thick film with the critical temperature (TC) of about 14 K by terahertz time-domain spectroscopy, we obtained the gap frequency of about 1.2 THz. An ill-defined gap was suggested due to the broadening of the onset of absorption as the temperature increases. On the other hand, the current–voltage curve measurement of the tunnel junctions showed the current rise at the gap voltage of 5.6 mV (corresponding to 1.4 THz) with a smeared shape as the temperature increases. We found that both gap broadenings can be explained by introducing a temperature-dependent imaginary energy gap part into the superconducting energy gap, corresponding to a finite quasi-particle lifetime in the NbN films.


Epitaxial NbN Gap broadening Quasi-particle lifetime 



This work was supported by JSPS KAKENHI Grant Numbers 18H03881 and 19H02205.


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Authors and Affiliations

  1. 1.National Astronomical Observatory of JapanMitakaJapan
  2. 2.National Institute of Information and Communications TechnologyKoganeiJapan
  3. 3.National Institute of Advanced Industrial Science and TechnologyTsukubaJapan
  4. 4.Shanghai Institute of Microsystem and Information TechnologyShanghaiChina

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