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

Abstract

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 (105K) 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 90K (x=0.3) and 104K (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.