From charge- and spin-ordering to superconductivity in the organic charge-transfer solids

Abstract

We review recent progress in understanding the different spatial broken symmetries that occur in the normal states of the family of charge-transfer solids (CTS) that exhibit superconductivity (SC), and discuss how this knowledge gives insight to the mechanism of the unconventional SC in these systems. A great variety of spatial broken symmetries occur in the semiconducting states proximate to SC in the CTS, including charge ordering, antiferromagnetism and spin-density wave, spin-Peierls state and the quantum spin liquid. We show that a unified theory of the diverse broken symmetry states necessarily requires explicit incorporation of strong electron-electron interactions and lattice discreteness, and most importantly, the correct bandfilling of one-quarter, as opposed to the effective half-filled band picture that is often employed. Uniquely in the quarter-filled band, there is a very strong tendency to form nearest neighbor spin-singlets, in both one-and two-dimension. The spin-singlet in the quarter-filled band is necessarily charge-disproportionated, with charge-rich pairs of nearest neighbor sites separated by charge-poor pairs of sites in the insulating state. Thus the tendency to spin-singlets, a quantum effect, drives a commensurate charge-order in the correlated quarter-filled band. This charge-ordered spin-singlet, which we label as a paired-electron crystal (PEC), is different from and competes with both the antiferromagnetic (AFM) state and the Wigner crystal (WC) of single electrons. Further, unlike these classical broken symmetries, the PEC is characterized by a spin gap. The tendency to the PEC in two dimension is enhanced by lattice frustration. The concept of the PEC mirrors parallel development of the idea of a density wave of Cooper pairs in the superconducting high T c cuprates, where also the existence of a charge-ordered state in between the antiferromagnetic and the superconducting phase has now been confirmed. Following this characterization of the spatial broken symmetries, we critically reexamine spin-fluctuation and resonating valence bond theories of frustration-driven SC within half-filled band Hubbard and Hubbard-Heisenberg Hamiltonians for the superconducting CTS. We present numerical evidence for the absence of SC within the half-filled band correlated-electron Hamiltonians for any degree of frustration. We then develop a valence-bond theory of SC within which the superconducting state is reached by the destabilization of the PEC by additional pressure-induced lattice frustration that makes the spin-singlets mobile. We present limited but accurate numerical evidence for the existence of such a charge order-SC duality. Our proposed mechanism for SC is the same for CTS in which the proximate semiconducting state is antiferromagnetic instead of charge-ordered, with the only difference that SC in the former is generated via a fluctuating spin-singlet state as opposed to static PEC. In Appendix B we point out that several classes of unconventional superconductors share the same band-fil...

Valence transition model of the pseudogap, charge order, and superconductivity in electron-doped and hole-doped copper oxides

“…4(a) the "horizontal stripe" structure demonstrated numerically for the anisotropic triangular lattice 174 . The occurrence of such paired Wigner crystal structures have been experimentally confirmed in a number of ρ = 0.5 2D organic chargetransfer solids, α-, β-and θ-(BEDT-TTF) 2 X, some of which are superconducting under pressure 51 .…”

“…This is simply a consequence of smaller ∆E(M, n) in thin films with weak 3D contribution to the Madelung energy, such that the configuration with Cu 1+ is lower in energy even without doping. Highest T c at zero doping is a signature that the paired Wigner crystal which is a precursor to SC, is most stable 51,173 at exactly ρ = 0.5 or very close to this filling. This is where the superconducting pair-pair correlations are the strongest 49,50 (see below).…”

“…Although the valence transition to the PG state in the cuprates (as well as homogenous population of Bi 3+ in doped BaBiO 3 ) was actually predicted within the valence transition model 31 , theoretical work along this direction was not continued, as there appeared to be no obvious explanation of SC within the theory. The motivation for the present work comes from, (a) the recent demonstration of the enhancement of superconducting pair-pair correlations by Coulomb interactions within the theoretical model appropriate for the post-valence transition cuprates [49][50][51] , and (b) experimental discoveries (see section II) over the intervening decades that strongly justify the theory. The goal of the present paper is to demonstrate that the model gives simultaneously the simplest yet most comprehensive explanations of the peculiar features of the cuprate families, both electron-and hole-doped.…”

“…The goal of the present paper is to demonstrate that the model gives simultaneously the simplest yet most comprehensive explanations of the peculiar features of the cuprate families, both electron-and hole-doped. SC in both families (and in several other correlated-electron superconductors, see Appendix) can be understood within a valence bond (VB) theory [49][50][51] that is influenced by Anderson's resonating valence bond (RVB) 52 theory but is substantially different. Indeed, our theory is at the interface of the RVB theory and the oldest version of the so-called bipolaron theory of superconductivity 53 , that can be thought of as a precursor to the RVB theory.…”

Quantum Monte Carlo study of lattice polarons in the two-dimensional three-orbital Su–Schrieffer–Heeger model

“…There is an urgent need to address off-diagonal e-ph interactions in higher dimensions, because such couplings are not only relevant to many materialse.g. the organic charge-transfer solids [3,23,29], the rare-earth nickelates [30][31][32], and high-T c superconductors like the cuprates [33,34] and bismuthates [35] -but several recent studies in the few-particle limit have shown that the new physics can occur in such models. For example, strong off-diagonal interactions can produce highly mobile polarons with light effective masses [24], generate robust phonon-mediated pairing [25], and even stabilize and control the location of a type-II Dirac point [36].…”