Anisotropic dynamical scaling near the vortex-glass transition of twinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The complex electrical conductivity σ(ω) is determined from the linear ac susceptibility between 3 Hz and 3 MHz measured in fields between 0.4 and 12 T paralle and perpendicular to the c axis of a twinned crystal. Dynamical scaling of ‖σ‖ and σ’ ’/σ’ reveals ordering temperatures ${\mathit{T}}_{\mathit{g}}$(B) and field-independent scaling functions by using the variable (${\mathit{T}}_{\mathit{c}2}$-${\mathit{T}}_{\mathit{g}}$)/‖T-${\mathit{T}}_{\mathit{g}}$‖. For B⊥c, the critical exponents ν and z agree with values obtained on films indicating isotropic vortex-glass fluctuations, whereas those for B∥c indicate elongated fluctuations in the c direction mediated by correlated pinning via the edges of twin boundaries.