## 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, {vert bar}{ital Z}{vert bar}{infinity}{omega}{sup 0.83}, and a frequency-independent phase {phi}=74{degree}. 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}{similar to}({ital T}{minus}{ital T}{sub {ital g}}){sup 5,8}. Within experimental error, all of these results are in excellent agreementmore »