Effects of Ni-to-Cu substitution on the properties of the high-$T_c$superconductor ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$

We report experimental data about the influence of Ni substitution on the properties of the high-$T_c$ superconductor ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ni}}_{\mathit{x}}$ ${\mathrm{O}}_{4\mathrm{-}\mathit{y}}$, 0≤x≤0.05. Superconductivity is totally suppressed above x=0.04 and the Curie law observed at x=0.05 reveals an effective magnetic moment of 0.7${\mu }_B$ instead of √3 ${\mu }_B$ as for spin (1/2. This result is explained by the existence of a singly &) level. Possibilities for non-s-wave superconductivity are also discussed in conjunction with our experimental data.