Recent trends in heavy-fermion physics

We discuss recent results obtained for the heavy-fermion metals UPd2Al3 and YbRh2Si2. UPd2Al3 is the first among all superconductors for which tunneling and inelastic neutron-scattering data highlight a non-phononic, i.e., magnetic-exciton mediated, pair state. YbRh2Si2 represents a model system exhibiting pronounced non-Fermi liquid effects above a weak antiferromagnetic phase transition at T N =70 mK . Upon approaching the quantum critical point (TN→0), by low doping with Ge, one observes for T K disparate behavior in the temperature dependences of both the electrical resistivity and the electronic specific heat as well as a Curie–Weiss law in the uniform magnetic susceptibility, implying uncompensated large 4f moments. These observations indicate a break up of the composite quasiparticles into their local f-spin and itinerant conduction-electron parts.