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+.