M. Couach et al., REVERSIBLE MIXED-STATE, SPECIFIC-HEAT UNDER MAGNETIC-FIELD AND SUPERCONDUCTING FLUCTUATIONS IN THE NORMAL-STATE OF THE HIGH T(C)HG-1223, Physics letters. A, 188(1), 1994, pp. 85-90
The equilibrium properties of the mixed state are investigated by mean
s of the differential susceptibility chi(H)' in the reversible domain
up to 4 T for the Hg-1223 superconducting phase. The penetration depth
lambda(ab)(0) approximately 1100 angstrom 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 c
ompared to Tl and Bi based cuprates and this can explain why the obser
vation of clear specific heat anomalies at the superconducting transit
ion 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 approximately 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.