Published in Journal of Alloys and Compounds 195 (1993) 455-458. 1 Reversibility of the mixed state of the 90K Tl 2 Ba 2 CuO 6+x superconductor C. Opagiste a , M. Couach a , A.F. Khoder a , F. Monnier a , A. Junod b , G. Triscone b , J. Muller b , J.L. Jorda c , T.K. Jondo c and M.Th. Cohen-Adad c . a Centre d'Etude Nucléaire de Grenoble / SPSMS / LCP, BP 85 X, F-38041 Grenoble Cedex (France). b D.P.M.C., Université de Genève, CH-1211 Genève 4 (Switzerland). c Laboratoire de Physico-Chimie Minérale II, Université Claude Bernard Lyon I, F-69622 (France). Abstract The reversible mixed state of the Tl 2 Ba 2 CuO 6+x high Tc superconductor with Tc of 90K has been investigated by DC SQUID measurements of the magnetization M(H, T) and by direct measurement of the AC differential susceptibility χ' H which reduces to the equilibrium dM/dH when, as it is actually the present case in the reversible regime, the flux flow viscous contribution to χ' H could be neglected. The two methods give quite similar results: a strong (field enhanced) contribution from diamagnetic fluctuation above Tc, while below Tc one observes deviation from the Abrikosov mean field theory of the mixed state which is presumably generated by thermal fluctuation enhanced effect on the vortex density and distortions in the layered structure. This behavior appears to be common to all the high Tc superconductors, as we have shown in an earlier work [1, 2, 3] for the YBa 2 Cu 3 O 7- x , 2223-Tl and 2212-Bi based compounds. Moreover, these deviations prevent any reliable determination of the Hc 2 critical field line by the standard method of linear extrapolation of M(H, T) curves or by the step anomaly on X'H expected to occur at Tc(H) if the mean field theory was to be followed in the mixed state. 1. Introduction Much experimental work has been and is still devoted to the irreversible behavior of the vortex state of the high Tc superconductors because of its evident practical implications on the critical currents enhancement and more important the usability of these compounds at high temperature, say liquid N 2 temperature, by the control of the location of the so called irreversibility line H*(T) (by irradiation, or chemical and metallurgical defects). Meanwhile, irreversibility in the mixed state makes too much complicated the study of the basic parameters of the superconductivity which can be extracted from equilibrium thermodynamical properties such as magnetization M(H, T) and specific heat Cp(H, T). In this paper, we will focus on the equilibrium magnetic properties of the mixed state, accessible in the "reversible" regime of the Tl 2 Ba 2 CuO 6+x 90K superconductor and will show that, like in the other high Tc layered superconductors [1, 2, 3, 4, 5], the mixed state magnetic behavior of these compounds deviates seriously from what can be expected theoretically within the framework of the Abrikosov mean field theory both in the linear (close to Tc) regime as well in the London regime (Hc l << H << Hc 2 ). 2. Considerations on measurements on ceramics (Hc 1 << H << Hc 2 ) The investigated Tl 2 Ba 2 CuO 6+x samples were prepared by using a high pressure, high temperature route as described in [6, 7]. As this compound belongs to a class of highly anisotropic materials, our measurements give an angular average of M and dM/dH. Micrograhs of the investigated samples revealed no preferable texture [6], thus a random polycrystalline approximation within the framework of an effective mass model to account for anisotropy allows the calculation of an angular averaging factor g(γ) which gives the ceramic to single crystal (H // c axis) ratio of the measured M or