Quasiparticle scattering time in NbN superconducting thin films

Abstract

A deep understanding of the quasiparticle inelastic scattering rate in superconducting materials is beneficial for the further development of ultrasensitive and rapid superconducting detectors. From the flux flow instability in current–voltage characteristics of NbN superconducting films, we have derived the critical vortex velocity (ʋ*) and quasiparticle inelastic scattering time (τE) of the 10- and 15-nm-thick NbN films at different magnetic fields and temperatures within the framework of the Larkin–Ovchinnikov model. The critical vortex velocity drove the transition from the power law ʋ* ∝ B−1/2 dependent to field-independent behavior where the nonequilibrium quasiparticles are uniformly distributed. The inelastic scattering time of quasiparticles obeys T−n temperature dependence, where the exponents are 2.5 and 2.3 for the 10 and 15-nm-thick NbN films, respectively. As the thickness of the film decreases from 15 to 10 nm, a reduction of τE from 88 to 12 ps is observed at 8 K. Moreover, the small value of τE for ultra-thin films can be associated with the low-average value of the superconducting gap, which is precisely what the detector needs.