Reversible magnetic properties of superconducting (Tl,Pb)Sr2Ca2Cu3O9 and Tl2Ba2Ca2Cu3O10

Abstract
We have carried out magnetization measurements on superconducting (Tl,Pb)Sr2 Ca2 Cu3 O9 and Tl2 Ba2 Ca2 Cu3 O10 in the magnetically reversible region, with the applied field parallel to the c axis. By employing a model suggested by Hao et al [Phys. Rev. B 43, 2844 (1991)], the values of the penetration depth, the coherence length, and the critical fields are obtained along with the Ginzburg-Landau parameter κc. The results show that, below 100 K, κc of (Tl,Pb)Sr2 Ca2 Cu3 O9 decreases slowly as expected by the theoretical calculations whereas κc of Tl2 Ba2 Ca2 Cu3 O10 increases slightly. The difference in the temperature dependence of κc suggests that the fluctuation effect is more significant and extended in Tl2 Ba2 Ca2 Cu3 O10 due to its higher anisotropy. Additionally, we have extracted the penetration depth λ near Tc again from linear relations of magnetization versus lnH. The λ(T) results are consistent with the behavior expected from BCS theory.