Superconducting Fluctuations and Excess Conductivity Analysis of Te-Substituted TlSr₂(Ca_1−x Te _x )Cu₂O_7−δ

Abstract

Samples with nominal starting compositions TlSr₂(Ca_1−x Te _x )Cu₂O_7−δ (x = 0.05–0.30) were prepared by the solid-state reaction method and studied by powder X-ray diffraction and electrical resistance measurements. The purpose of this work was to determine the valence state of Te in the TlSr₂CaCu₂O_7−δ (Tl-1212) phase. It is also to determine the dimension of fluctuation-induced conductivity λ, and the superconducting fluctuations and excess conductivity using the Aslamazov–Larkin (AL) theory. The Lawrence–Donaich (LD) theory was used to calculate the coherence length ξ _c(0), interlayer coupling J, and anisotropy γ = (ξ _ab(0)/ ξ _c(0)). X-ray powder diffraction patterns indicated that the samples were dominantly Tl-1212 phase. Temperature-dependent resistance measurements showed metallic normal-state behavior for all samples. The onset transition temperature, T _{c onset}, was between 32 K (x = 0.05) and 61 K (x = 0.25). The valence of Te in Tl-1212 is suggested to be Te⁶⁺ from the analysis of the highest T _c in the x = 0.25 sample. Excess conductivity analyses showed that Te substitution induced 2D-to-3D conductivity transition with the highest transition temperature, T _2D−3D observed for x = 0.25. The calculations based on Lawrence–Doniach model showed the shortest coherence length, ξ _c(0), and highest anisotropy (γ = 10.24) for the x = 0.25 sample.