Vortex dynamics in a NbN film studied by terahertz spectroscopy

Abstract

We have investigated the effect of vortex dynamics on high-frequency conductivity of a superconducting NbN film by using terahertz time-domain spectroscopy. The complex conductivity of the mixed state up to 7 T is determined without using Kramers-Kronig analysis. The experimentally obtained conductivity spectra are analyzed by considering the contribution from quasiparticles in vortices and also from the vortex dynamics. To include the local-field effect in the high-frequency electromagnetic responses of mixed-state superconductors, we combined the Maxwell-Garnett theory with a self-consistent two-fluid model and obtained the expression for the effective conductivity. The volume fraction of the vortices shows the linear dependence on the applied magnetic field, as expected for $s$-wave superconductors, by taking into account the flux-flow resistivity.