UC Irvine

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 UBe13 demonstrate the breakdown of a description in terms of a single energy scale TK. Here TK becomes temperature dependent below ≃ 10 K, and the canonical relationship between R(H) and M(H) does not hold. ii) Superconductivity in UPt3 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 UPt3. iii) The possibilities of noncubic strain induced by anisotropic superconductivity in UBe13 is investigated by ultrasound experiments. The suggestion of tetragonal strain in (U, Th)Be13 is found to be not applicable, and observed frequency and amplitude dependent absorption anomalies at T ≪ Tc might reflect domain wall motion. © 1987.