A.C. Susceptibility and Specific Heat in Low B.C. Fields of La2-xSrxCuO4 and YBa2Cu3O7 High Temperature Superconductors

Abstract

We report the giant effect of small d.c. fields on a.c. diamagnetic transitions in La2-xSrxCuO4, and YBa2Cu3O7. An unexpected frequency effect is observed. In order to elucidate the apparent conflict Between the high critical field slope as measured by magnetoresistance experiments and the large depression of a.c. diamagnetic transitions, we locate the bulk transition by high resolution specific heat measurements in small fields. The results, together with d.c. magnetization measurements near Tc and SEM micrographs, suggest a model where superconducting regions with a high critical field are separated by weak links.