Reversibility of the mixed state of the 90K Tl2Ba2CuO6+x superconductor (original) (raw)

The reversible mixed state of the Tl2Ba2CuO6+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 YBa2Cu3O7-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.

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