Abstract
The pinning of flux lines on point-like defects in superconductors depends on both the magnitude and the symmetry of the order parameter. We study the anisotropy of pinning in untwinned single crystals of YBa2Cu3O $${_{7-\delta}}$$ and crystals of Bi2Sr2CaCu2O $${_{8+\delta}}$$ employing a modified vibrating reed technique with two degrees of freedom. The pinning potential is calculated for an anisotropic d-wave superconductor in the approximation of a single pancake vortex interacting with a single point defect. The response of the reed to pinning potentials with two- and fourfold symmetry is modeled. The appearance of a twofold symmetry is demonstrated for YBa2 Cu3O $${_{7-\delta}}$$ crystals. The effect of the fourfold symmetric component is masked by flux creep effects in our experiments.
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