DYNAMIC CONDUCTIVITY NEAR T-C OF YBA2CU3O7 FILMS - VORTEX-GLASS-LIKE CRITICALITY IN THE ABSENCE OF EXTERNALLY INDUCED VORTICES

Using a contactless method, we investigated the electrical ac conducti vity sigma' + i sigma '' in the CuO2 planes of thin Y-Ba-Cu-O films de posited on (001)-SrTiO3 and -MgO substrates at zero magnetic field. Th e temperature variation of the phase angles, phi(T;omega)=arctan(sigma ''/sigma'), measured between 30 Hz and 2 GHz, allows a precise defini tion of the superconducting transition T-c. Remarkably, the frequency- independent value phi(T-c)=74 degrees, obtained for both films, agrees with those observed previously for high vortex densities (0.1-19 T), where this fact was associated with the universality near the so-calle d vortex-glass transition line. Since the shape of the present scaling function for sigma(omega) also proves to be identical with that found in a high field, we argue that the same fluctuations destroy supercon ductivity in zero field and in the presence of a dense vortex lattice.