Reversible mixed state, specific heat under magnetic field and superconducting fluctuations in the normal state of the high T c Hg-1223

Abstract

The equilibrium properties of the mixed state are investigated by means of the differential susceptibility X' H in the reversible domain up to 4 T for the Hg-1223 superconducting phase. The penetration depth λ ab (0)≈1100 Å is deduced and the role of the thermal fluctuations on vortices is considered within the approach of Bulaevskii et al. This allows an estimation of a crossover field H cr related to the Josephson coupling and hence to the anisotropy for which we give a lower estimated value of 16. The low anisotropy level may point to a less 2D character of this Hg based superconductor as compared to Tl and Bi based cuprates and this can explain why the observation of clear specific heat anomalies at the superconducting transition is possible for similar samples containing the Hg-1223 and the Hg-1212 phases. The height of the specific heat anomaly is comparable to that of the YBCO system and from C p ( H) measurements we can deduce an average H' c2 slope ≈2 T/K for 1223 and 4 T/K for 1212. Finally, from the ac susceptibility we show evidence for diamagnetic fluctuations in the normal state up to 200 K.