Excess conductivity analysis of Cr substituted TlSr2CaCu2O7-δ superconductor at Tl and Ca sites

Samples with nominal starting compositions (Tl1−xCrx)Sr2CaCu2O7−δ and TlSr2(Ca1−xCrx)Cu2O7−δ (x = 0.05-0.30) had been prepared by the solid state reaction method and studied by powder X-ray diffraction method, electrical resistance measurements and energy dispersive X-ray analysis (EDX). Cr2S3 was used as the Cr source. Excess conductivity analysis using the Aslamazov-Larkin (AL) theory to determine the dimension of fluctuation induced conductivity λ was carried out. The Lawrence-Donaich (LD) theory was used to calculate the coherence length ξc(0), Josephson coupling J, and γ = (ξab(0)/ξc(0)) of the superconducting samples. Partial substitution of Cr in place of Tl in (Tl1−xCrx)Sr2CaCu2O7−δ (Tl-1212) showed initial increase in zero-resistance-temperature, Tc zero. Sample with x = 0.15 showed the highest superconducting onset temperature Tc onset (105 K) and the lowest normal state electrical resistivity. XRD patterns indicated that the samples were dominantly Tl-1212 phase. Excess conductivity analyses showed that Cr substitution induced 2D-to-3D conductivity transition with the highest transition temperature, T2D-3D observed at x = 0.15. The calculations based on Lawrence-Doniach model revealed shortest coherence length, ξc(0) and lowest interplanar coupling, J for x = 0.20. Substitution of Cr in place of Ca in TlSr2(Ca1−xCrx)Cu2O7−δ showed Tc onset between 90 K (x = 0.3) and 104 K (x = 0.2). Excess conductivity analyses for TlSr2(Ca1−xCrx)Cu2O7−δ showed 2D-to-3D transition of fluctuation induced conductivity and the highest T2D-3D was for x = 0.2. The maximum value of ξc(0) and J was also for x = 0.2. This work showed strong relation between Tc onset and T2D-3D.