Within the last three decades, an increasing number of rare-earth (RE) and actinide-based intermetallic compounds has been found to exhibit anomalous physical properties: Whereas at high temperatures they normally show clear indications of f-derived local magnetic moments, the low-temperature properties can be best explained by the existence of itinerant, albeit very heavy, fermions.

There are numerous manifestations of these heavy-fermion (HF) phenomena to derive from the local Kondo interaction which, at high temperatures, implies a tiny energetical preference of the conduction electrons to align their spin antiparallel to the localized f-spins.

Upon cooling to well below the characteristic Kondo temperature T_K, typically between a few K and about 100 K, this antiferromagnetic (AF) coupling becomes very strong such that, eventually, the local moment is fully screened by the conduction electrons. The ground state of this "Kondo singlet" is an entangled state, (|↑>_f|↓>_c - |↓>_f|↑>_c). In a periodic lattice of Kondo ions, the "Kondo lattice", Bloch's theorem causes these Kondo singlets to couple coherently and act as "composite" charge carriers with internal quantum numbers which are identical to those of the bare electron, i.e., spin ħ/2 and charge e. The local Coulomb repulsion which prevents double occupancy of the f sites causes the mobility of these carriers to be extremely low. Therefore, the composite charge carriers exhibit a small Fermi velocity v_F* (being of the order of the velocity of sound) or, correspondingly, a large effective mass (of inertia), m* ≈ 100 - 1000 m_el.

Heavy fermion (HF) metals have served in the 1980s and early 1990s as model systems for Fermi liquids and unconventional superconductivity. More recently they have become prototype materials for non-Fermi-liquid (NFL) phenomena, quantum criticality and the interrelation of quantum criticality and unconventional superconductivity. These latter topics are in the focus of our current research activities.