Strong electronic correlations in superconducting organic charge transfer salts

Abstract

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.

Chirald-wave superconductivity in doped graphene

“…Experimental results seem ambiguous with regards to if the superconducting state is fully gapped or contains nodes, see e.g. [199,195] and references therein. The interplay between antiferromagnetism, the Mott transition, and d-wave superconductivity has been studied by different theoretical methods [200,201,202].…”

Antiferromagnetic spin fluctuations in the metallic phase of quasi-two-dimensional organic superconductors

“…8,[15][16][17]47 This suggests that these systems undergo a crossover from a bad metal regime for T Ͼ T 0 to a renormalized Fermi liquid below T 0 . However, we will show below ͑also see Fig.…”

“…Similar physics occurs in the other dimerized polymorphs, such as the ␤, ␤Ј, and phases. 8 These materials display a wide variety of unconventional behaviors 8 including antiferromagnetic and spin liquid insulating states, unconventional superconductivity, and the metallic phase which we focus on in this paper. They also share highly anisotropic crystal and band structures.…”

“…However, the importance of spin fluctuations for the superconducting -͑ET͒ 2 X has been pointed out by several groups. 8,[31][32][33][34][35][36] Since superconductivity arises from an instability of the metallic phase, it is important to understand the strength of the spin fluctuations in the metallic phase.…”

Ionic Hubbard model on a triangular lattice forNa0.5CoO2,Rb0.5Powell1, Merino2, McKenzie3 2009Phys. Rev. B9020View full textAdd to dashboardCite

“…Measurements of the relaxation rates for 59 Co NMR found two distinct Co sites consistent with a charge ordered state. 25 However, the authors noted that the degree of charge disproportionation is rather small.…”