Experimental study of scaling laws for the complex susceptibility of type-II superconductors

Measurements of the nonlinear complex susceptibility chi, taken as a function of amplitude h and frequency nu of the ac held, can collapse into a single universal curve if they are plotted against the scaling variable hJ(v(ref))/J(nu), where J is a shielding current density and V-ref is an arbitrary reference frequency. In this work we present chi(h,nu) data for a large range of amplitudes and frequencies, measured in two Nb samples (one single crystal and one polycrystal) and one melt-textured YBa2Cu3O7-delta sample. The predicted scaling relation is verified for several sets of data taken at different temperatures below T-c and applied magnetic fields well above the first critical field H-c1. Data taken closer to T-c revealed a useful scaling applied directly to the critical current density, evaluated from the peak positions of the imaginary component chi ". The occurrence of different regimes of collective vortex creep is also discussed for the YBa2Cu3O7-delta sample.