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The fluctuation induced conductivity (FIC) of (Cu0.5Tl0.5−xKx)Ba2Ca3Cu4O12−y (x=0,0.25) superconductor has been studied after potassium substitution at thallium site in the charge reservoir layer (Cu0.5Tl0.5Ba2O4−y). The samples were also post-annealed in oxygen to change the carrier density in CuO2 planes. The zero resistivity critical temperature Tc(0) was found to increase after K+1 substitution at Tl+3 sites in the Cu0.5Tl0.5Ba2O4−y charge reservoir layer. However, Tc(0) has been suppressed after post-annealing the samples in oxygen. The FIC analysis was performed in the frame work of Aslamasov–Larkin theory. The results of FIC analysis have shown three-dimensional (3D), two dimensional (2D) and short wavelength fluctuations (SWFs) in the order parameter. It has been also observed that there is an increase in the 3D-2D and 2D-SWF cross-over temperatures after potassium substitution along with an increase in Tc(0). However, after post-annealing in oxygen, the dimensional cross-over has been shifted to lower temperatures. A direct correlation between the cross-over temperatures, the Tc(0) as well as carrier doping level has been observed.

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