VORTEX-PAIR NUCLEATION AT DEFECTS - A MECHANISM FOR ANOMALOUS TEMPERATURE-DEPENDENCE IN THE SUPERCONDUCTING SCREENING LENGTH

Low-field ac screening measurements on YBa2Cu3O7-delta-films and (BEDT-TTF)2Cu(SCN)2 crystals (where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene), both thought to contain a high density of defects, reveal a diminution of screening and a common extrinsic temperature dependence of the screening length-lambda. Vortex-core pinning at the defects is shown to give a low-temperature T2 power-law temperature dependence to lambda that, in contrast to the exponential behavior expected from s-wave pairing, can be mistaken as evidence for lines or nodes of the energy gap on the Fermi surface.