LINEAR AC MAGNETIC RESPONSE NEAR THE VORTEX-GLASS TRANSITION IN SINGLE-CRYSTALLINE YBA2CU3O7

By using a miniature two-coil mutual-inductance method and an exact inversion scheme, the ac penetration depth lambda(ac) was measured in a twinned YBa2Cu3O7 crystal in dc fields parallel to the c axis. The Ohmic dc resistance was also measured on the same crystal. The behavior of these quantities was altogether consistent with the vortex-glass-transition scaling. However, if we calculate the Labusch parameter and the pinning relaxation time from lambda(ac), they show unexpectedly large frequency dependencies, which can hardly be explained by the standard theory of the linear ac response of pinned vortices.