Effect of composition on the superconductivity of CeCu2Si2

The superconductivity in fifteen off-stoichiometric samples of the compound Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ was investigated, and a strong dependence of $T_c$ on the concentration of all three constituents was found. The variation of $T_c$ was particularly pronounced for the series of $\mathrm{Ce}{\mathrm{Cu}}_2\pm {}_{\delta }{}^{}{}_{}{}^{}\mathrm{Si}_2^{}{}_{}{}^{}$; the highest (0.67 K) and the sharpest $T_c$ was found in copper-rich samples, while $T_c$ of the sample containing only 1 at.% less copper was lower than 0.07 K. Metallographic and crystallographic studies, combined with differential thermal analysis on some of these samples allowed us to discuss the limit of the homogeneity range and the type of formation of Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$. Supplementary results on other compounds in the ternary system Ce-Cu-Si are also presented. Superconductivity was not found above 0.07 K in the compounds isostructural to Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$: $\mathrm{Ce}{T}_2{\mathrm{Si}}_2$ ($T=\mathrm{C}\mathrm{o},\mathrm{P}\mathrm{d},\mathrm{R}\mathrm{h},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{R}\mathrm{u}$) and $\mathrm{Ce}{T}_2{\mathrm{Ge}}_2$ ($T=\mathrm{Co}\mathrm{and}\mathrm{Ni}$).