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

Measurements of the nonlinear complex susceptibility chi, taken as a functi on of amplitude h and frequency nu of the ac held, can collapse into a sing le universal curve if they are plotted against the scaling variable hJ(v(re f))/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 ra nge of amplitudes and frequencies, measured in two Nb samples (one single c rystal and one polycrystal) and one melt-textured YBa2Cu3O7-delta sample. T he predicted scaling relation is verified for several sets of data taken at different temperatures below T-c and applied magnetic fields well above th e first critical field H-c1. Data taken closer to T-c revealed a useful sca ling applied directly to the critical current density, evaluated from the p eak positions of the imaginary component chi ". The occurrence of different regimes of collective vortex creep is also discussed for the YBa2Cu3O7-del ta sample.