F-Electron materials: a reservoir of novel electronic states and phenomena

Abstract

f-Electron materials exhibit a rich variety of strongly correlated electron states and phenomena. Two examples, non-Fermi liquid (NFL) behavior at low temperatures and superconductivity in the vicinity of quantum critical points, are discussed in this article. Non-Fermi liquids are characterized by weak power law or logarithmic temperature dependences of the electrical resistivity, specific heat, and magnetic susceptibility at low temperatures, as well as a dynamical susceptibility whose imaginary part scales with frequency ω divided by T. These NFL characteristics were first established in chemically substituted f-electron materials and later observed in stoichiometric f-electron compounds, suggesting that the NFL behavior is a general phenomenon whose underlying mechanism(s) is (are) not primarily associated with atomic disorder. In certain antiferromagnetic and ferromagnetic f-electron intermetallic compounds, superconductivity has been observed in the vicinity of the critical pressure P c where the Néel or Curie temperature vanishes. We discuss recent experiments on the ferromagnetic compound UGe 2 and suggest that superconductivity, which is found to occur within the ferromagnetic phase below P c in this compound, may have s-wave character and coexist in a spatially inhomogeneous manner with the ferromagnetism.