Effect of Re substitution on the formation and fluctuation-induced conductivity of a Tl Sr2(Ca1−xRex)Cu2O7−δ (x=0.05−0.30) superconductor

Samples with the nominal starting compositions of TlSr2(Ca1−xRex)Cu2O7−δ (x=0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) were prepared using a solid state reaction. The samples were studied using powder X-ray diffraction, energy dispersive X-ray analysis and electrical resistance measurements. An excess conductivity analysis was carried out using the Aslamazov–Larkin (AL) theory, and the dimension of fluctuation induced conductivity (λ) was determined. 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 superconductive samples. Substituting the Re in TlSr2(Ca1−xRex)Cu2O7−δ (Tl-1212) caused an initial increase in the zero-resistance-temperature (Tc zero). The sample where x=0.20 showed the highest Tc onset and Tc zero. The XRD patterns indicated that this sample was only Tl-1212 phase. The excess conductivity analyses showed that Re substitution induced 2D-to-3D conductivity transitions at the highest transition temperature (T2D−3D) observed at x=0.15. The calculations based on the Lawrence–Doniach model revealed that the shortest coherence length and inter-plane coupling (J) occurred when x=0.30. Re in the Tl-1212 phase is in a mixed valence state (4+/5+) with an optimal value of 4.5+.