Superconducting fluctuations in Tl-based single crystals: Evidence of universal features and derivation of basic parameters

The critical fluctuations of the magnetization near the ${\mathit{H}}_{\mathit{c}2}$(T) line have been studied in four different single crystals of Tl-based compounds (${\mathrm{Tl}}_{0.5}$ ${\mathrm{Pb}}_{0.5}$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_7$, ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$, ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$, and ${\mathrm{TlBa}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_9$) with the magnetic field applied along the c axis. It is shown that the magnetization can be scaled in reduced units according to the two-dimensional scaling developed by Ullah-Dorsey and Tesanovic et al., exhibiting the universality of the fluctuations. From this analysis, we deduce the values of the Ginzburg parameter (${\mathit{G}}_{\mathit{i}}$), the coherence length [${\xi }_{\mathit{a}\mathit{b}}$(0)], and the penetration depth [${\lambda }_{\mathit{a}\mathit{b}}$(0)] of each phase. The values of the Ginzburg fluctuation parameter in the different compounds are put in relation with the relative location of their irreversibility lines.