Flux density in the mixed state in the high-Tcsuperconductors and the predictions of Abrikosov theory

The high-${\mathrm{T}}_{\mathrm{c}}$ superconductors are known to exhibit a large domain of reversibility in the (H,T) plane. In this domain, measurements of ${\mathrm{\chi }}_{\mathrm{H}}^{\prime }$, the differential ac susceptibility in a static magnetic field H, can yield equilibrium values for dM/dH where, as expected, ${\mathrm{\chi }}_{\mathrm{H}}^{\prime }$=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 ${\mathrm{\tau }}_{\mathrm{E}}$=${\mathrm{f}}^{\mathrm{-}1}$, where f is the frequency [A. F. Khoder and M. Couach (unpublished)]. An irreversibility line (H,T${)}_{\mathrm{f}}$ is obtained for each frequency. In the reversible domain above this line, the measured ${\mathrm{\chi }}_{\mathrm{H}}^{\prime }$ 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.