FREQUENCY AND FIELD VARIATION OF VORTEX DYNAMICS IN YBA2CU3O7-DELTA

We have measured the complex ac resistivity <(rho)over tilde> (= rho(1 ) + i rho(2)) of YBa2Cu3O7-delta thin films in the frequency range of 45 MHz to 50 GHz. The microwave conductivity sigma(1), obtained from < (rho)over tilde> in zero field, manifests considerable frequency depen dence in the vicinity of T-c, showing that fluctuation and inhomogenei ty effects are significantly reduced at high frequencies. With an appl ied dc magnetic field, the vortex dynamics changes at a crossover freq uency f(x), showing scaling behavior p(i) proportional to omega(alpha i) at low frequencies (f < f(x)) and mean-field behavior at high frequ encies (f > f(x)).