NONLINEAR AC SUSCEPTIBILITY, SURFACE AND BULK SHIELDING

We calculate the nonlinear AC response of a thin superconducting strip in perpendicular field, shielded by an edge current due to the geomet rical barrier. A comparison with the results for infinite samples in p arallel field, screened by a surface barrier, and with those for scree ning by a bulk current in the critical state, shows that the AC respon se due to a barrier has general features that are independent of geome try, and that are significantly different from those for screening by a bulk current in the critical state. By consequence, the nonlinear (g lobal) AC susceptibility can be used to determine the origin of magnet ic irreversibility. A comparison with experiments on a Bi2Sr2CaCu2O8+d elta crystal shows that in this material, the low-frequency AC screeni ng at high temperature is mainly due to the screening by an edge curre nt, and that this is the unique source of the nonlinear magnetic respo nse at temperatures above 40 K.