Low-frequency electrodynamics in the mixed state of superconducting NbN and a-MoGe films using two-coil mutual inductance technique

We investigate the low-frequency electrodynamics in the vortex state of two type-II superconducting films, namely, a moderate-to-strongly pinned Niobium Nitride (NbN) and a very weakly pinned amorphous Molybdenum Germanium (a-MoGe) film. We employ a two-coil mutual inductance technique to extract the complex penetration depth, . The sample response is studied through the temperature variation of in the mixed state, where we employ a model developed by Coffey and Clem (the CC model) to extract different vortex lattice (VL) parameters, such as the restoring pinning force constant (Labusch parameter), VL drag coefficient and pinning potential barrier. We observe that a consistent description of the inductive and dissipative part of the response is only possible when we take the viscous drag on the vortices to be several orders of magnitude larger than the viscous drag estimated from the Bardeen–Stephen model.