Non-Fermi-liquid behavior in heavy-fermion systems

While many heavy-fermion systems can be described as Fermi liquid at very low temperatures (T→0), remarkable deviations in the thermodynamic (specific heat, magnetization) and transport properties (resistivity) have been observed recently in a number of systems. We review the experimental evidence for this 'non-Fermi-liquid' behavior which can be brought about by different microscopic origins. In particular, single-ion phenomena such as the two-channel quadrupolar Kondo effect or a distribution of Kondo temperatures, or collective phenomena such as the proximity to magnetic order at a zero-temperature quantum phase transition are discussed. These different routes to non-Fermi-liquid behavior are illustrated by examples of different systems. ((orig.))