Superconductivity in CeCu2Si2Single Crystals

Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ single crystals prepared with an excess of Cu exhibit low residual resistivity ${\rho }_0$, low susceptibility $\chi$, small unit-cell volume $V$, and bulk "heavy-fermion" superconductivity below 0.65 K, while crystals grown from stoichiometric melt show higher ${\rho }_0$ and $\chi$, larger $V$, but no superconductivity. Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ behaves as an $s$-state superconductor with strongly temperature-dependent pair breaking and almost isotropic Fermi surface.