VORTEX DEPINNING FREQUENCY IN YBA2CU3O7-X SUPERCONDUCTING THIN-FILMS - ANISOTROPY AND TEMPERATURE-DEPENDENCE

We have used the parallel-plate resonator technique to study the micro wave surface resistance R(s) and the penetration depth lambda of YBa2C u3O7-x thin epitaxial films at 5.5 GHz in the presence of magnetic fie ld H less-than-or-equal-to 0.8 T and at 30 K < T < 80 K. The magnetic- field dependence of R(s) and lambda allows determination of the vortex viscosity eta, pinning constant alpha, and depinning frequency omega0 for different field orientations. To the best of our knowledge, this is among the first measurements of the angular dependence of eta, alph a, and omega0. We find that the angular dependences of eta, alpha, and omega0 are fairly well described by the scaling model of Blatter, Ges hkenbein, and Larkin. We demonstrate that (i) the temperature dependen ce of eta is well described by the Bardeen-Stephen model with reduced normal-state resistivity, (ii) the pinning constant is close to its up per limit at lowest temperatures and decreases exponentially with incr easing temperature, and (iii) the depinning frequency is of the order of 10 GHz, almost orientation independent and weakly depends on temper ature. We analyze the values of the depinning frequency for different type-II superconductors and demonstrate that it is directly related to the quasiparticle scattering rate.