Singular or non-Fermi liquids

An introductory survey of the theoretical ideas and calculations and the experimental results which depart from Landau Fermi liquids is presented. The common themes and possible routes to the singularities leading to the breakdown of Landau Fermi liquids are categorized following an elementary discussion of the theory. Soluble examples of singular or non-Fermi liquids include models of impurities in metals with special symmetries and one-dimensional interacting fermions. A review of these is followed by a discussion of singular Fermi liquids in a wide variety of experimental situations and theoretical models. These include the effects of low-energy collective fluctuations, gauge fields due either to symmetries in the Hamiltonian or possible dynamically generated symmetries, fluctuations around quantum critical points, the normal state of high-temperature superconductors and the two-dimensional metallic state. For the last three systems, the principal experimental results are summarized and the outstanding theoretical issues are highlighted.