Microwave surface impedance of YBCO thin films in DC applied fields

We measure the microwave surface impedance of YBa2Cu3O7−δ thin films in the mixed state, for films produced using PLD and electron beam co-evaporation techniques. Measurements were made as a function of field (0–4 T) and temperature (4K−Tc) at 8 GHz and 16 GHz. From these measurements we extract the flux line restoring force constant (κp), viscosity (η), and pinning potential well depth (U). In a magnetic field, deviation from an Rs ∝ ω2 indicates a small but significant thermal activation of the flux lines. A field dependent κp is observed for all samples, where κp ℝ B-α, and α>0. Furthermore a variation in the magnitude and field dependence of κp is observed between samples, where the largest values of α (∼0.4) are observed in the samples with the smallest values of κp.