Quantum frustration in organic Mott insulators: from spin liquids to unconventional superconductors

Abstract

We review the interplay of frustration and strong electronic correlations in quasi-two-dimensional organic charge transfer salts, such as (BEDT-TTF) 2 X and EtnMe 4−n P n[Pd(dmit) 2 ] 2 . These two forces drive a range of exotic phases including spin liquids, valence bond crystals, pseudogapped metals, and unconventional superconductivity. Of particular interest is that in several materials with increasing pressure there is a first-order transition from a spin liquid Mott insulating state to a superconducting state. Experiments on these materials raise a number of profound questions about the quantum behaviour of frustrated systems, particularly the intimate connection between spin liquids and superconductivity. Insights into these questions have come from a wide range of theoretical techniques including first principles electronic structure, quantum many-body theory and quantum field theory. In this review we introduce some of the basic ideas of the field by discussing a simple frustrated Heisenberg model with four spins. We then describe the key experimental results, emphasizing that for two materials, κ-(BEDT-TTF) 2 Cu 2 -(CN) 3 and EtMe 3 Sb[Pd(dmit) 2 ] 2 , there is strong evidence for a spin liquid ground state, and for another, EtMe 3 P[Pd(dmit) 2 ] 2 , there is evidence of a valence bond crystal ground state. We review theoretical attempts to explain these phenomena, arguing that they can be captured by a Hubbard model on the anisotropic triangular lattice at half filling, and that Resonating Valence Bond (RVB) wavefunctions capture most of the essential physics. We review evidence that this Hubbard model can have a spin liquid ground state for a range of parameters that are realistic for the relevant materials. In particular, spatial anisotropy and ring exchange are key to destabilising magnetic order. We conclude by summarising the progress made thus far and identifying some of the key questions still to be answered. (S,S13,S24)= J J' J J J 1 2 3 4

Staggered Flux State in Two-Dimensional Hubbard Models

“…[73][74][75] In these compounds, SC arises for T c 12 K, and a pseudogap behavior similar to that of cuprates has been observed. Therefore, we need to check whether its origin is identical to that of cuprates.…”

Shear viscosity of strongly interacting fermionic quantum fluids

“…For cuprates [61], the hopping integral t 0.18 eV, a = 3.9Å and for organic charge-transfer salts [36] From Fig. 5, we can clearly see that for all U < 3.0 and for 3 He parameters ζ (T ) is above the KSS limit.…”

“…Furthermore, DMFT has also been found to provide quantitative description of the resistivity [34] and the frequency-dependent optical conductivity [35] for organic charge-transfer salts that can be described by a half-filled two-dimensional Hubbard model on an anisotropic triangular lattice [36]. DMFT combined with electronic-structure calculations based on density functional theory (DFT) has given an excellent description of a large class of transition-metal and rare-earth compounds [37].…”

Power-law dependence of the optical conductivity observed in the quantum spin-liquid compoundκ-(BEDT-TTF)2Cu2(CN)Elsässer1, Wu2, Dressel3 et al. 2012Phys. Rev. B

“…2,46,47 Recently two approaches were brought forward both based on a quarter-filled extended Hubbard model that includes the interaction V between molecules on the same but also different dimers. In general, within Hubbard-type models there is no contribution from spins to optics; however, besides single electron conductivity, singlets might form which move around leading to an additional contribution to the optical conductivity.…”

“…Among the two-dimensional organic charge-transfer compounds, the κ-(BEDT-TTF) 2 X family [where BEDT-TTF stands for bis-(ethylenedithio)tetrathiafulvalene] is of particular interest, because the constituting cationic dimers are arranged in an anisotropic triangular lattice ( Fig. 1) with a delicate interplay between electronic correlations (given by the on-site Coulomb repulsion U ), the effects of low dimensionality and spin frustration.…”