EFFECTS OF MICROSTRUCTURE ON THE VORTEXGLASS-TRANSITION OF YBCO

The linear dynamical conductivity sigma(w) of quenched-melt-grown YBCO has been determined from the ac susceptibility measured between 1 mHz and 1 MHz in fields from 0.4 T to 4 T. Modulus and phase of sigma(w) exhibit dynamical scaling, which signalizes thermodynamic phase transi tions along a line T-g(B). By comparing the critical exponents of the correlation length and relaxation time of the order parameter fluctuat ions, v = z = 3.0(4), to previous results for crystals and films, the dominance of columnar pinning along the edges of twin boundaries withi n the crystalline grains over isotropic pinning by point defects in a conventional vortex-glass is revealed. The large scale grain boundarie s seems to inhibit the formation of a generic homogeneous superconduct ing phase in the present material.