Frequency dependence of the irreversibility temperature in Y-Ba-Cu-O

We study the frequency dependence of the irreversibility temperature T _irr in single crystal, thin film and ceramic Y-Ba-Cu-O samples. T _irr, measured by the onset of third harmonic signal, depends weakly on frequency for crystals, moderately for thin films and strongly for ceramic samples. The data is analyzed in the framework of the flux creep and the vortex-glass models. Although we cannot rule out the flux creep scenario, the data is better described by the vortex-glass phase transition model.