ANISOTROPIC DYNAMICAL SCALING NEAR THE VORTEX-GLASS TRANSITION OF TWINNED YBA2CU3O7-DELTA

The complex electrical conductivity sigma(omega) is determined from the linear ac susceptibility between 3 Hz and 3 MHz measured in fields between 0.4 and 12T parallel and perpendicular to the c axis of a twinned crystal. Dynamical scaling of Absolute value of sigma and sigma''/sigma' reveals ordering temperatures T(g)(B) and field-independent scaling functions by using the variable (T(c2) - T(g))/\T - T(g)\. For B perpendicular-to c, the critical exponents nu and z agree with values obtained on films indicating isotropic vortex-glass fluctuations, whereas those for B parallel-to c indicate elongated fluctuations in the c direction mediated by correlated pinning via the edges of twin boundaries.