Reliable determination of vortex parameters from measurements of the microwave complex resistivity

We discuss and propose a complete data treatment, in close contact to typical microwave experimental data, in order to derive vortex parameters, such as pinning constant and viscous drag coefficient ͑also referred to as “vortex viscosity”͒, in a way as model independent as possible. We show that many of the accepted models for the complex resistivity can be described by a single, very general analytical expression. Using typical mea- surements of real and imaginary resistivity as a function of the applied field, we show that, even for single- frequency measurements, it is always possible to obtain ͑a͒ estimates of viscous drag coefficient and pinning constant with well-defined upper and lower bounds and ͑b͒ quantitative information about thermal creep. It turns out that neglecting thermal creep, in particular and counterintuitively at low temperatures, might result in a severe overestimation of the viscous drag coefficient. We also discuss the impact of thermal creep on the determination of the pinning constant. The present results might lead to a reconsideration of several estimates of the vortex parameters.