LOCAL AC MAGNETIC RESPONSE IN BI2SR2CACU2O8 SINGLE-CRYSTALS

The local in- and out-of-phase ac magnetic response B'(ac) and B ''(ac ) was measured in Bi2Sr2CaCu2O8 (BSCCO) single crystals at elevated te mperatures using microscopic arrays of Hall sensors. The local B'(ac) is found, under certain experimental conditions, to attain negative va lues, i.e. the local induction decreases when external field increases . Such anti-phase behavior is inconsistent with existing models that p redict an in-phase variation of the local magnetic induction with appl ied field over the entire sample. Further, the in-phase component B'(a c) has a maximum value in the center of the sample this contradicts th e usual expectation of maximum screening in the center. The dissipatio n peak B ''(ac) is found to be caused by the ''collapse'' of the geome trical barrier and does not reflect a change in the bulk resistivity. A new model fot the ac response in the presence of geometrical barrier is derived and is shown to describe the measured behavior very well.