Transport properties of strongly correlated metals

Full text: The metallic state of strongly correlated materials is still poorly understood. There is a wide range of materials such as heavy-fermions, transition metal oxides and quasi-two-dimensional molecular crystals, k-(BEDT-TTF)2X that have electronic transport properties that are quite different from conventional metals. For example, a drude peak is only present in the optical conductivity at low temperatures and the resistance can have values corresponding to a mean-free path much less than a lattice constant. One way the effect of electronic correlations present in these systems are included is to study a Hubbard model in the limit of large dimensions. In this limit, dynamical correlations in each lattice site are treated exactly, while non-local spatial correlations are neglected. We find non-monotonous and strong temperature dependence of the different transport quantities including the resistance, thermopower, Hall coefficient and optical conductivity. Fermi liquid behaviour is found below a low energy scale or Kondo temperature, for which low-energy excitations are coherent excitations of quasi-particles. At higher temperatures, Fermi liquid behaviour is no longer present. We show that many of the observed properties of the organic superconductors of the type k-(BEDT-TTF)2X with X = Cu[N(CN)2]CI, Cu[N(CN)2]Br and X = Cu(NCS)2, and ,β-(BEDT-TTF)2I3 can be explained from the existence of this crossover between coherent and incoherent states. The thermopower is particularly sensitive to this crossover