Effect of Ge 4+ and Mg 2+ doping on superconductivity, fluctuation induced conductivity and interplanar coupling of TlSr 2CaCu 2O 7−δ superconductors

Abstract

Substitution of Ge 4+ in place of Cu in Tl 0.85Cr 0.15Sr 2CaCu 2− x Ge x O 7− δ ( x = 0–0.6) showed initial increase in zero critical temperature value, T c zero 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 T c zero is unexpected as tetravalent Ge 4+ substitution is expected to strongly reduce hole concentration in the samples and suppress T c zero . 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, T 2D – 3D observed at x = 0.1. FTIR spectroscopy analysis indicates Ge 4+ substitution cause reduction in CuO 2/GeO 2 interplanar distance while our calculation based on Lawrence–Doniach model revealed highest superconducting coherence length, ξ c (0) and interplanar coupling, J at x = 0.3. On the other hand, substitution of divalent Mg 2+ for Ca 2+ in (Tl 0.5Pb 0.5)(Sr 1.8Yb 0.2)(Ca 1− y Mg y )Cu 2O 7 ( y = 0–1.0), which is not expected to directly vary hole concentration, surprisingly caused T c zero 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 T 2D – 3D was at y = 0.4. Similar calculation revealed highest values of ξ c (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 CuO 2 plane as a result of Mg 2+ substitution. The increased ξ c (0) and J as a result of the Ge 4+ and Mg 2+ substitutions are suggested to contributed to sustenance of superconductivity above 80 K in the samples.