FLUX-DENSITY IN THE MIXED STATE IN THE HIGH-TC SUPERCONDUCTORS AND THE PREDICTIONS OF ABRIKOSOV THEORY

The high-Tc superconductors are known to exhibit a large domain of reversibility in the (H,T) plane. In this domain, measurements of H, the differential ac susceptibility in a static magnetic field H, can yield equilibrium values for dM/dH where, as expected, H=dM/dH>0, provided that the diffusion time of the vortices into the bulk of the sample is short compared to the experimental time scale E=f-1, where f is the frequency [A. F. Khoder and M. Couach (unpublished)]. An irreversibility line (H,T)f is obtained for each frequency. In the reversible domain above this line, the measured H for our Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O samples are shown to agree with the predictions of the Abrikosov-Ginsburg-Landau theory. This indicates the absence of any phase transition along the irreversibility line and in the reversible domain. © 1990 The American Physical Society.