Effect of Ge substitution on excess conductivity and infrared absorption of Tl0·85Cr0·15Sr1−xGexCaCu2O7 superconducting ceramics

Abstract

Samples of Tl0·85Cr0·15Sr2−xGexCaCu2O7−δ (x = 0–0·3) were successfully synthesised using the solid state synthesis method. Electrical resistance measurements showed that the zero critical temperature value Tc zero was in the range of 90–98 K for x = 0–0·15 and in the range of 60–78 K for x = 0·2–0·3. Analysis of excess conductivity behaviour based on the Aslamazov–Larkin model for the unsubstituted x = 0 compound revealed a two-dimensional fluctuation behaviour. Substitution of Ge in Tl0·85Cr0·15(Sr2−xGex)CaCu2O7−δ induced two- to three-dimensional transition with the highest transition temperature T2D–3D observed at x = 0·10. In this study, the calculation using the Lawrence–Doniach model showed that the sample with the lowest anisotropy was x = 0·08. Fourier transform infrared absorption together with X-ray diffraction data analyses indicate increasing CuO interplane coupling with Ge substitution, which is suggested to cause sustenance of the high Tc zero and metallic normal state behaviour for x = 0–0·15.