DIVERGING TIME SCALES FOR ONSET OF IRREVERSIBILITY IN HIGH-TEMPERATURE SUPERCONDUCTORS

The onset temperature, T(irr) for magnetic irreversibility in high-temperature superconductors has been investigated as a function of frequency (0.2 Hz less-than-or-equal-to f less-than-or-equal-to 65 000 Hz) and DC magnetic field (0<H< 4000 Oe). In Y-Ba-Cu-O crystals T(irr) is essentially independent of the frequency, while in Bi-Sr-Ca-Cu-O and TI-Ba-Ca-Cu-O crystals T(irr) exhibits a strong frequency dependence. For all three systems T(irr)(H) converges to a finite temperature as the frequency approaches zero. Analysis of the experimental data shows that conventional theories of thermally-activated vortex motion cannot explain the measured frequency dependence of T(irr) Rather, the data indicate that T(irr) for f --> 0 can be interpreted as a transition temperature separating the reversible vortex-liquid and the irreversible vortex-glass phases.