Observation of two distinct superconducting phases in Ce[Cu.sub.2][Si.sub.2]

We report the presence of two disconnected superconducting domes in the pressure-temperature phase diagram of partially germanium-substituted Ce[Cu.sub.2][Si.sub.2]. The lower density superconducting dome lies on the threshold of antiferromagnetic order, indicating magnetically mediated pairing, whereas the higher density superconducting regime straddles a weakly first-order volume collapse, suggesting a pairing interaction based on spatially extended density fluctuations. Two distinct pairing mechanisms thus appear to operate in the single, wide, superconducting range of stoichiometric Ce[Cu.sub.2][Si.sub.2], both of which might apply more generally to other classes of correlated electron systems.