CRITICAL-STATE FLUX-PENETRATION AND LINEAR MICROWAVE VORTEX RESPONSE IN YBA2CU3O7-DELTA

The vortex. contribution to the de magnetic field (H) dependence of th e microwave surface impedance (Z) over tilde(s)(H)=R-s(H)+iX(s)(H) of YBa2Cu3O7-delta thin films was measured using suspended patterned reso nators. (Z) over tilde(s)(H) is shown to be a direct measure of the fl ux density B(H) enabling a very precise test of models of flux penetra tion. Three regimes of field-dependent behavior were observed: (1) Ini tial flux penetration occurs on very low-field scales H-i(4.2 K)simila r to 10 mT, (2) at moderate fields the flux penetration into the virgi n state is in excellent agreement with calculations based upon the fie ld-induced Bean critical state for thin-film geometry, parametrized by a field scale H-s(4.2 K)similar to J(c)d similar to 0.5 T, and (3) fo r very high fields H much greater than H-s, the flux density is unifor m and the measurements enable direct determination of vortex parameter s such as pinning force constants alpha(p) and vortex viscosity eta. H owever, hysteresis loops are in disagreement with the thin-film Bean m odel, and instead are governed by the low-field scale H-i, rather than by H-s. Geometric barriers are insufficient to account for the hyster esis loops. [S0163-1829(97)08641-4].