On the Heavy Fermion Road

Abstract

The heavy fermion properties are reviewed on selected examples often studied in Grenoble. The physical problems are presented as example of exotic matter at low temperature with three major insights: the localisation of the 4f electrons, the collapse of the long range magnetic order and the entrance in an intermediate valence regime. The question on the first or second order nature of the quantum phase transition will be discussed. In the first example, the Ising Kondo lattice of CeRu2Si2, experiments seem to be described with the picture of 4f itinerant electron dominated by their spin fluctuations; the strong feedback, induced in magnetic field, between magnetostriction and switch in the nature of the magnetic interactions illustrates the proximity of a quantum critical field. Comparison is made with CeCu6 or YbRh2Si2 series where the idea of local criticality seems to emerge. Basic arguments are discussed for the breakdown of the idea that Yb heavy fermion systems are the the hole analogue of the cerium ones. Special focus is given on the interplay between unconconventional superconductivity and magnetism either for dominant antiferromagnetic or ferromagnetic correlations. Huge electronic Gruneisen parameter leads often to strong concomitant 4f density and spin fluctuations >.The main boost given by the discovery of new materials as quasi-bidimensional Ce 115 compounds or recently by CePt3 Si or skutterudites is spectacular for the theoretical developments. Finally, the now canonical cases of the UPt3, URu2Si2, UPd2Al3 and UBe13 give now a sound basis in the rich domain of unconventional superconductors.