Thermodynamic fluctuation under high pressure in Hg-1223 superconductors

Excess conductivities of polycrystalline Hg-1223 superconductor have been studied under high-pressure conditions. The critical temperature was chosen as the mean field transition temperature T_c^mf Above this temperature, the normal state resistivity decreases with increasing pressure. The relative change in resistivity w.r.t. pressure, namely d In ρ/dp, is about -21% GPa^-1. It is much smaller than the typical value of -10% GPa^-1 for most high-T_c superconductors. As a consequence of the change in the normal state resistivity due to the applied pressure, the fluctuation-induced conductivity is also changed. The experimental results were analysed within the framework of Aslamazov-Larkin theory. The logarithmic plots of the excess conductivity versus reduced temperature ε(ε = T/T_c^mf - 1) reveal that there is a cross-over temperature T₀ which determines the transition from two- to three-dimensional conductivity of charge carrier transportation. With the help of the cross-over temperature T₀, the values of the coherence length for the Hg-1223 superconductor could be estimated. The change in the fluctuation conductivity is discussed in terms of the coherence length and the thermodynamical critical field H_c.