Mixed-state surface impedance of Nb-Ti: Flux flow, pinning, and creep

We present a thorough study of the surface impedance Zs in the mixed state of sputtered Nb-Ti coatings on insulating substrates, with the aim to assess and quantify the mechanisms governing vortex motion. Measurements are performed by means of a dual mode dielectric-loaded resonator allowing the simultaneous determination of Zs at 16.5 and 26.7 GHz, which enable to derive the various relevant vortex dynamics parameters. A static magnetic field μ0H≤1.25 T is applied normally to the sample surface, in the T=5 K−Tc temperature range. The measured pinning frequency is ≈4.5 GHz at 5 K and 1 T, with a pronounced decrease with field. A scaling analysis of the pinning constant kp(H,T) dependence shows that pinning is dominated by point pins. Thermal creep is negligibly small at the lowest T, possibly becoming finite only near Tc. Flux flow resistivity is found to be consistent with the time-dependent Ginzburg-Landau predictions for a moderately dirty superconductor.