Abstract
Using a microwave (13 GHz) technique which is sensitive to field-induced changes in the surface resistance, we have observed twin-boundary pinning in zone-melt-textured crystals of YBa2Cu3O7-δ. With the rotation of a static 1.9-T field applied in thea-b plane, the surface resistanceR s exhibits minima at 90° intervals. The twin-boundary minima are shown to be connected to the orientation of the crystalline axes of the sample. The resistance minima are observed superimposed on a flux-flow response over the temperature range ofT c to the lowest temperature attempted. Substantial flux mobility is observed to the lowest temperatures. The angular dependence of the flux-flow response at low temperatures is consistent with the “brick” model for local intergranular conductivity, which provides for an in-plane isotropic component.
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