Strong electronic correlations in superconducting organic charge transfer salts

Abstract

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