We measure the frequency dependence of the complex ac conductivity of NbN films with different levels of disorder in frequency range 0.4-20 GHz. Films with low disorder exhibit a narrow dynamic fluctuation regime above T_c as expected for a conventional superconductor. However, for strongly disordered samples, the fluctuation regime extends well above T_c, with a strongly frequency-dependent superfluid stiffness which disappears only at a temperature T* close to the pseudogap temperature obtained from scanning tunneling measurements. Such a finite-frequency response is associated to a marked slowing down of the superconducting fluctuations already below T*. The corresponding large length-scale fluctuations suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor.
@article{arxiv.1210.7462,
title = {Frequency dependent superfluid stiffness in the pseudogap regime in strongly disordered NbN thin films},
author = {Mintu Mondal and Anand Kamlapure and Somesh Chandra Ganguli and John Jesudasan and Vivas Bagwe and Lara Benfatto and Pratap Raychaudhuri},
journal= {arXiv preprint arXiv:1210.7462},
year = {2013}
}