Superconductivity and heavy fermions.

Experimental studies are discussed which shed light on the nature of the heavy fermion normal state, the occurrence of heavy fermion superconductivity and anomalous properties of the superconducting state: i)Magnetization and magnetoresistance of the Kondo lattice UBe(13) demonstrate the breakdown of a description in terms of a single energy scale T(K). Here T (K) becomes temperature dependent below ~10K, and the canonical relationship between R(H) and M(H) does not hold. ii) Superconductivity in UPt(3) occurs at the borderline to magnetism: Spin-density wave-like instabilities are induced by a few percent of Th or Au, whereas mass enhancement and susceptibility are rapidly reduced by Ir substitution. This suggests that spin fluctuations, rather than phonons, are important in mediating superconducting pairing of heavy fermions in UPt(3).