Evidence for Type II - Clean Behavior in YBa sub 2 Cu sub 3 O sub x Thin-Film Superconductors

Measurements of the penetration length lambda of c-axis oriented thin films of YBa sub 2 Cu sub 3 O sub x in magnetic fields H as high as 14 T enable a determination of the transition T sub c (H) by extrapolation to lambda = infinity. For fields in the range 1 T H 14 T, lambda (H,T) is found to have a square root dependence on H and a temperature dependence near T sub c which reduces to that expected for an array of Josephson- coupled proximity junctions. The observed non-linear dependence of the upper critical field H sub c2 on temperature is well described in the context of a pair-breaking formalism in which the dominant pair-breaking rate is proportional to H sup 1/2, a prediction of Type II clean-limit theory, rather than proportional to H as predicted by similar theory in the dirty-limit. For high-T sub c oxide superconductors, this result imposes a heretofore unrecognized and uniquely different interpretation of the parameters derived from the temperature dependence of H sub c2.