Experimental verification for a scaling law relating amplitude and frequency in the complex susceptibility of type-II superconductors

In this work we present complex susceptibility measurements chi(h,v), for a large range of field amplitudes h and frequencies v, taken in two Nb sampl es (one single crystal and one polycrystal) and one melt-textured YBa2Cu3O7 -delta sample. A predicted scaling law for chi(h,v) is verified for several sets of data, plotted against the scaling variable h(v(ref)/v)(1/(n-1)), w here v(ref) is an arbitrary reference frequency and n is a creep exponent. Data taken closer to T-c revealed a useful scaling applied directly to the critical current density J(c), evaluated through the peak position of the i maginary component chi ". An unexpected frequency dependence of n was obser ved for the YBa2Cu3O7-delta sample. This is associated with a failure of th e logarithmic approximation for the activation energy U = U(c)ln(J(c)/J), w hich is assumed in the scaling formalism. A more accurate description in th is case is obtained with U = U-c(J/J(c))(-mu), from the collective creep th eory. This type of analysis led us to propose an effective creep exponent n (eff), in an attempt to keep the original scaling law. (C) 2000 Elsevier Sc ience B.V. All rights reserved.