OBSERVATION OF CRITICAL SCALING BEHAVIOR IN THE AC IMPEDANCE AT THE ONSET OF SUPERCONDUCTIVITY IN A LARGE MAGNETIC-FIELD

We have measured the ac impedance of Y-Ba-Cu-O thin films in a magnetic field of 0.55 T and in the frequency range from 100 kHz to 500 MHz. At the temperature of the dc vortex-glass transition but in the ac linear regime, we find scaling behavior of the impedance amplitude, \Z\ varies as omega-0.83, and a frequency-independent phase-phi = 74-degrees. At higher temperatures, we find a crossover from a frequency-independent to a frequency-dependent impedance. This crossover frequency-OMEGA varies with temperature like OMEGA approximately (T - T(g))5.8. Within experimental error, all of these results are in excellent agreement with predictions for the ac impedance of the vortex-glass phase of a superconductor in a large magnetic field.