4 J ul 2 00 6 1 Strong electronic correlations in superconducting organic charge transfer salts

We review the role of strong electronic correlations in quasi–twodimensional organic charge transfer salts such as (BEDT-TTF)2X, (BETS)2Y and β′-[Pd(dmit)2]2Z. We begin by defining minimal models for these materials. It is necessary to identify two classes of material: the first class is strongly dimerised and is described by a half-filled Hubbard model; the second class is not strongly dimerised and is described by a quarter filled extended Hubbard model. We argue that these models capture the essential physics of these materials. We explore the phase diagram of the half-filled quasi–two-dimensional organic charge transfer salts, focusing on the metallic and superconducting phases. We review work showing that the metallic phase, which has both Fermi liquid and ‘bad metal’ regimes, is described both quantitatively and qualitatively by dynamical mean field theory (DMFT). The phenomenology of the superconducting state is still a matter of contention. We critically review the experimental situation, focusing on the key experimental results that may distinguish between rival theories of superconductivity, particularly probes of the pairing symmetry and measurements of the superfluid stiffness. We then discuss some strongly correlated theories of superconductivity, in particular, the resonating valence bond (RVB) theory of superconductivity. We conclude by discussing some of the major challenges currently facing the field. These include: parameterising minimal models; the evidence for a pseudogap from nuclear magnetic resonance (NMR) experiments; superconductors with low critical temperatures and extremely small superfluid stiffnesses; the possible spin-liquid states in κ-(ET)2Cu2(CN)3 and β′-[Pd(dmit)2]2Z; and the need for high quality large single crystals.