Microwave absorption of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films with columnar defects

We have measured the microwave magnetoabsorption of both pristine and irradiated c-axis-oriented YBa/sub 2/Cu/sub 3/O/sub 7/ thin films. The irradiation was done using 270-MeV Ag ions and produces columnar defects along the film normal. The angular dependence of the absorption for pristine films can be described by sin/spl theta/; where /spl theta/ is the inclination of the field from the film plane. However, there is a minimum in the magnetoabsorption of the irradiated films when the applied field is parallel to the columns. This minimum is indicative of vortex localization as predicted by Nelson and Vinokur. We believe that this is the first high-frequency measurement of this phenomenon.>