Microwave observation of the vortex locked-in state in YBa2Cu3O7 thin films with columnar defects

We report the angular dependence of microwave magnetoabsorption of both pristine and 270-MeV-Ag-ion-irradiated YBa2Cu3O7-δ (YBCO) c-axis-oriented films. At these ion energies, columnar defects are introduced into YBCO, and Nelson and Vinokur [Phys. Rev. B. 68, 2398 (1992)] have predicted a state in which vortices are locked to such columnar defects. This has been confirmed by Budhani et al. [Phys. Rev. Lett. 72, 566 (1994)] using dc measurements. Such dc measurements effectively measure the depth of the pinning potential. We show evidence of a similar effect in the microwave-frequency regime. Here the bottom rather than the edge of the pinning potential is being probed. A simple analysis suggests a distribution of accommodation angles.