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The universal anomalies in the normal state of Cu-0 high-temperature superconductors follow from a single hypothesis: There exist chargeand spin-density excitations with the absorptive part of the polarizability at low frequencies co proportional to co/T, where T is the temperature, and constant otherwise. The behavior in such a situation may be characterized as that of a marginal Fermi liquid. The consequences of this hypothesis are worked out for a variety of physical properties including superconductivity.PACS numbers: 74.70.VyThe normal-state properties of the Cu-0 superconductors are as perplexing as their high transition temperatures. The electrical resistivity p(T), ' the thermal conductivity x(T), the optical conductivity cr(co), the Raman scattering intensity S(ro), the tunneling conductance as a function of voltage g(V), the nuclear relaxation rate T~' (T), and the Hall coefficient RH(T) (Ref.

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For about twenty years, it has been the prevailing view that there can be no metallic state or metal-insulator transition in two dimensions in zero magnetic field. In the last several years, however, unusual behavior suggestive of such a transition has been reported in a variety of dilute two-dimensional electron and hole systems. The physics behind these observations is presently not understood. We review and discuss the main experimental findings and suggested theoretical models.Comment: To be published in Rev. Mod. Phy

We consider the iron pnictides in terms of a proximity to a Mott insulator. The superexchange interactions contain competing nearest-neighbor and next-nearest-neighbor components. In the undoped parent compound, these frustrated interactions lead to a two-sublattice collinear antiferromagnet (each sublattice forming a Néel ordering), with a reduced magnitude for the ordered moment. Electron or hole doping, together with the frustration effect, suppresses the magnetic ordering and allows a superconducting state. The exchange interactions favor a d-wave superconducting order parameter; in the notation appropriate for the Fe square lattice, its orbital symmetry is dxy. A number of existing and future experiments are discussed in light of the theoretical considerations.