Superconductivity in the Presence of Strong Pauli Paramagnetism: CeCu2Si2

A comparison was made between four low-temperature properties of La${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ and Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$. Whereas La${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ behaves like a normal metal, Ce${\mathrm{Cu}}_2$ ${\mathrm{Si}}_2$ shows (i) low-temperature anomalies typical of "unstable $4f$ shell" behavior and (ii) a transition into a superconducting state at $T_c\simeq 0.5$ K. Our experiments demonstrate for the first time that superconductivity can exist in a metal in which many-body interactions, probably magnetic in origin, have strongly renormalized the properties of the conduction-electron gas.