Dynamic conductivity near Tc of YBa>2Cu3O7 films: Vortex-glass-like criticality in the absence of externally induced vortices

Using a contactless method, we investigated the electrical ac conductivity σ'+iσdouble-prime in the CuO2 planes of thin Y-Ba-Cu-O films deposited on (001)-SrTiO3 and -MgO substrates at zero magnetic field. The temperature variation of the phase angles, var-phi(T;ω)=arctan(σdouble-prime/σ'), measured between 30 Hz and 2 GHz, allows a precise definition of the superconducting transition Tc. Remarkably, the frequency-independent value var-phi(Tc)=74 degree, obtained for both films, agrees with those observed previously for high vortex densities (0.1 endash 19 T), where this fact was associated with the universality near the so-called vortex-glass transition line. Since the shape of the present scaling function for σ(ω) also proves to be identical with that found in a high field, we argue that the same fluctuations destroy superconductivity in zero field and in the presence of a dense vortex lattice. copyright 1997 The American Physical Society