Frequency and Field Variation of Vortex Dynamics in YBa₂Cu₃O_7-δ

We have measured the complex ac resistivity ρ̃ (  =  ρ₁+iρ₂) of YBa₂Cu₃O_7-δ thin films in the frequency range of 45 MHz to 50 GHz. The microwave conductivity σ₁, obtained from ρ̃ in zero field, manifests considerable frequency dependence in the vicinity of T_c, showing that fluctuation and inhomogeneity effects are significantly reduced at high frequencies. With an applied dc magnetic field, the vortex dynamics changes at a crossover frequency f_x, showing scaling behavior ρ_i∝ω^α_i at low frequencies (f<f_x) and mean-field behavior at high frequencies (f>f_x).