  The vortex dynamics in YBa2Cu3O7-d thin films have been studied at microwave
frequencies. A pronounced peak in the surface resistance, Rs, is observed in
these films at frequencies of 4.88 and 9.55 GHz for magnetic fields varying
from 0.2 to 0.8 T. The peak is associated with an order-disorder transformation
of the flux line lattice as the temperature or field is increased. The
occurrence of the peak in Rs is crucially dependent on the depinning frequency,
wp and on the nature and concentration of growth defects present in these
films. Introduction of artificial defects by swift heavy ion irradiation with
200 MeV Ag ion at a fluence of 4x1010 ions/cm2 enhances wp and suppresses the
peak at 4.88 GHz but the peak at 9.55 GHz remains unaffected. A second peak at
lower temperature has also been observed at 9.55 GHz. This is related to twin
boundaries from angular dependence studies of Rs. Based on the temperature
variation of Rs, vortex phase diagrams have been constructed at 9.55 GHz.
