Abstract
We have investigated time resolved microwave absorption at 10 GHz of an epitaxial YBCO thin film placed in a magnetic field perpendicular to the plane of the film. When a small modulation field with frequencies up to 100 kHz is applied parallel or perpendicular to a large DC field the Fourier transformation of the absorption signal shows a large number of higher harmonics. We interpret the signal in terms of flux lines oscillating in their pinning potential. The measurements are discussed in the framework of the Gittleman-Rosenblum-equation. The shape of the pinning potential is found to be anharmonic. By this approach the irreversibility line can be defined as the line where the pinning potential vanishes. This manifests itself experimentally as the vanishing of all the harmonics.
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