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‖∞${\mathrm{\omega }}^{0.83}$, and a frequency-independent phase φ=74°. At higher temperatures, we find a crossover from a frequency-independent to a frequency-dependent impedance. This crossover frequency Ω varies with temperature like Ω∼(T-${\mathit{T}}_{\mathit{g}}$ ${)}^{5,8}$. 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.