Dynamic fluctuations in the superconductivity of NbN films from microwave conductivity measurements

TL;DR

This study investigates the microwave frequency and temperature dependence of superconducting fluctuations in NbN films, revealing dimensional crossover behavior near the critical temperature and providing insights into fluctuation conductivity models.

Contribution

It presents detailed measurements of ac conductivity in NbN films, demonstrating scaling behavior in thin films and dimensional crossover in thicker films near Tc.

Findings

Fluctuation conductivity increases rapidly near Tc.

Scaling behavior consistent with 2D Aslamazov-Larkin model in thin films.

Dimensional crossover from 3D to 2D observed in thicker films.

Abstract

We have measured the frequency and temperature dependences of complex ac conductivity, \sigma(\omega)=\sigma_1(\omega)-i\sigma_2(\omega), of NbN films in zero magnetic field between 0.1 to 10 GHz using a microwave broadband technique. In the vicinity of superconducting critical temperature, Tc, both \sigma_1(\omega) and \sigma_2(\omega) showed a rapid increase in the low frequency limit owing to the fluctuation effect of superconductivity. For the films thinner than 300 nm, frequency and temperature dependences of fluctuation conductivity, \sigma(\omega,T), were successfully scaled onto one scaling function, which was consistent with the Aslamazov and Larkin model for two dimensional (2D) cases. For thicker films, \sigma(\omega,T) data could not be scaled, but indicated that the dimensional crossover from three dimensions (3D) to 2D occurred as the temperature approached Tc from above. This provides a good reference of ac fluctuation conductivity for more exotic superconductors of current interest.