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

The complex electrical conductivity sigma(omega) is determined from th e linear ac susceptibility between 3 Hz and 3 MHz measured in fields b etween 0.4 and 12T parallel and perpendicular to the c axis of a twinn ed crystal. Dynamical scaling of Absolute value of sigma and sigma''/s igma' reveals ordering temperatures T(g)(B) and field-independent scal ing functions by using the variable (T(c2) - T(g))/\T - T(g)\. For B p erpendicular-to c, the critical exponents nu and z agree with values o btained on films indicating isotropic vortex-glass fluctuations, where as those for B parallel-to c indicate elongated fluctuations in the c direction mediated by correlated pinning via the edges of twin boundar ies.