Phonon bottleneck limits vortex-induced quasiparticle relaxation in epitaxial NbN thin films

Abstract

We present a comprehensive experimental analysis of vortex-driven quasiparticle dynamics in epitaxial NbN thin films under high magnetic fields. By tracking the Larkin–Ovchinnikov instability velocity as a function of temperature and field, we use the well-known field-independent plateau regime in vortex velocity to extract the quasiparticle relaxation time τ. From this plateau, we extract characteristic recombination times ranging from 80 to 10 ps between 8 K and T_c. Comparing our results with previously reported values for NbN and other superconducting systems, we propose a unified scenario where the superconducting gap and temperature jointly govern the phonon bottleneck limiting recombination. The systematic growth of τ at low temperature, consistent with the expected T³ dependence of phonon-limited heat evacuation through the substrate, suggests a universal trend across different materials and device architectures. These findings provide insights into non-equilibrium superconductivity and establish a fundamental limit for quasiparticle relaxation that constrains the speed of next-generation cryogenic superconducting technologies.