Effect of Ge4+ and Mg2+ doping on superconductivity, fluctuation induced conductivity and interplanar coupling of TlSr2CaCu2O7−δ superconductors

Substitution of Ge4+ in place of Cu in Tl0.85Cr0.15Sr2CaCu2−xGexO7−δ (x = 0–0.6) showed initial increase in zero critical temperature value, Tczero from 98 K (x = 0) to 100 K (x = 0.1) and in the range of 85–86 K for x = 0.2–0.3. The slow decrease in Tczero is unexpected as tetravalent Ge4+ substitution is expected to strongly reduce hole concentration in the samples and suppress Tczero. Excess conductivity analyses of resistance versus temperature data based on Asmalazov–Larkin (AL) theory revealed that the substitution induced 2D-to-3D transition of fluctuation induced conductivity with the highest transition temperature, T2D–3D observed at x = 0.1. FTIR spectroscopy analysis indicates Ge4+ substitution cause reduction in CuO2/GeO2 interplanar distance while our calculation based on Lawrence–Doniach model revealed highest superconducting coherence length, ξc(0) and interplanar coupling, J at x = 0.3. On the other hand, substitution of divalent Mg2+ for Ca2+ in (Tl0.5Pb0.5)(Sr1.8Yb0.2)(Ca1−yMgy)Cu2O7 (y = 0–1.0), which is not expected to directly vary hole concentration, surprisingly caused Tczero to increase from 89.6 K (y = 0) to an optimum value of 95.9 K (y = 0.6) before decreasing with further increase in y. Excess conductivity analyses showed 2D-to-3D transition of fluctuation induced conductivity for all samples where the highest T2D–3D was at y = 0.4. Similar calculation revealed highest values of ξc(0) and J also at y = 0.4. FTIR analysis of the samples indicates inequivalent Cu(1)O(2)Pb/Tl lengths and possible tilting of CuO2 plane as a result of Mg2+ substitution. The increased ξc(0) and J as a result of the Ge4+ and Mg2+ substitutions are suggested to contributed to sustenance of superconductivity above 80 K in the samples.

► Substitution of Ge in Tl0.85Cr0.15Sr2CaCu2−xGexO7−δ induced 2D-to-3D transition in superconducting fluctuation.
► FTIR indicates Ge4+ substitution caused reduction in CuO2/GeO2 interplanar distance.
► The reduction suggests increase in CuO2/GeO2 interplanar coupling.
► For Tl0.5Pb0.5Sr1.8Yb0.2Ca1−yMgyCu2O7, 2D-to-3D transition in fluctuation was observed.
► FTIR indicates inequivalent Cu(1)-O(2)-Pb/Tl lengths and possible tilting of CuO2 plane.