REVERSIBILITY OF THE MIXED STATE OF THE 90K TL2BA2CUO6+X SUPERCONDUCTOR

The reversible mixed state of the Tl2Ba2CuO6+x high Tc superconductor with Tc of 90K has been investigated by DC SQUID measurements of the m agnetization M(H, T) and by direct measurement of the AC differential susceptibility chi(H)' which reduces to the equilibrium dM/dH when, as it is actually the present case in the reversible regime, the flux fl ow viscous contribution to chi(H)' could be neglected. The two methods give quite similar results: a strong (field enhanced) contribution fr om diamagnetic fluctuation above Tc, while below Tc one observes devia tion from the Abrikosov mean field theory of the mixed state which is presumably generated by thermal fluctuation enhanced effect on the vor tex density and distorsions in the layered structure. This behavior ap pears to be common to all the high Tc superconductors as we have shown in an earlier work [1, 2, 3] for the YBa2Cu3O7-x, 2223 Tl and 2212 Bi based compounds. Moreover, these deviations prevent any reliable dete rmination of the H(C2) critical field line by the standard method of l inear extrapolation of M(H, T) curves or by the step anomaly on chi(H) ' expected to occur at Tc(H) if the mean field theory was to be follow ed in the mixed state.