The chemical physics of unconventional superconductivity

Mazumdar, S.; Clay, R. Torsten

doi:10.1002/qua.24637

Cited by 15 publications

(3 citation statements)

References 42 publications

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“…The above results show that the PEC is a distinct phase in the correlated ρ = 1 2 systems and is actually observed in 2D CTS with CO. In analogy with the conjectures built around the concept of a density wave of Cooper pairs [43,44,[502][503][504][504][505][506][507], we have further proposed that SC will occur upon the destabilization of the PEC state [16,124,125,565,574]. Note that this idea also has overlaps with the idea of SC emerging from a 2D SP state (which we have emphasized does not exist at ρ = 1), as the dimerized horizontal stripes of Figs.…”

Section: Paired Electron Crystal In Two Dimensionssupporting

confidence: 57%

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

Clay,

Mazumdar

2018

Preprint

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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...

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Section: Paired Electron Crystal In Two Dimensionssupporting

confidence: 57%

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

Clay,

Mazumdar

2018

Preprint

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“…Empirically, CTS showing the BCDW1 distortion are more likely to show superconductivity (SC) under pressure 4 . We have suggested that unconventional SC in ρ = 0.5 materials arises from the delocalization of the ('1−1') singlet pairs formed in the insulating Paired Electron Crystal (PEC), which has the same CO pattern • • • 1100 • • • as BCDW1 and BCDW2 [27][28][29] . Within such a model, the effective mass of the singlet is smaller in the BCDW1 because of the weaker binding, and hence the mobility of inter-dimer pairs here would be expected to be large, allowing a transition to a paired liquid state under the application of pressure 28 .…”

Section: B Bcdw1mentioning

confidence: 99%

Bond patterns and charge-order amplitude in quarter-filled charge-transfer solids

et al. 2017

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Most quasi-one-dimensional (quasi-1D) quarter-filled organic charge-transfer solids (CTS) with insulating ground states have two thermodynamic transitions: a high-temperature metal-insulator transition followed by a low-temperature magnetic transition. This sequence of transitions can be understood within the 1D Peierlsextended Hubbard (PEH) model. However, in some quasi-1D CTS both transitions occur simultaneously in a direct metal to spin-gapped insulator transition. In this second class of materials the organic stack bond distortion pattern does not follow the pattern of a second dimerization of a dimer lattice. These materials also display charge ordering of a large amplitude below the transition. Using quantum Monte Carlo methods we show that the same PEH model can be used to understand both classes of materials, however, within different parameter regions. We discuss the relevance of our work to experiments on several quarter-filled conductors, focusing in particular on the materials (EDO-TTF) 2 X and (DMEDO-TTF) 2 X.

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“…Two of us have recently pointed out the unusual abundance 34 of seemingly unrelated correlated-electron materials that are superconducting at carrier concentration ρ 0.5. In all cases the superconductor belongs to a family of materials with varying ρ, and no SC is observed for ρ substantially different from 0.5 (as is also true for the CTS).…”

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confidence: 99%

Coulomb-enhanced superconducting pair correlations and paired-electron liquid in the frustrated quarter-filled band

et al. 2016

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A necessary condition for superconductivity (SC) driven by electron correlations is that electron-electron (e-e) interactions enhance superconducting pair-pair correlations, relative to the noninteracting limit. We report high-precision numerical calculations of the ground state within the frustrated two-dimensional (2D) Hubbard Hamiltonian for a wide range of carrier concentration ρ (0 < ρ < 1) per site. We find that long range superconducting pair correlations are enhanced only for ρ 0.5. At all other fillings e-e interactions mostly suppress pair correlations. The enhancement of pair correlations is driven by the strong tendency to local singlet bond formation and spin gap (SG) in ρ = 0.5, in lattices with quantum fluctuation [1][2][3] . We also report determinantal quantum Monte Carlo (DQMC) calculations that are in strong agreement with our ground state results. Our work provides a key ingredient to the mechanism of SC in the 2D organic charge-transfer solids (CTS), and many other unconventional superconductors with frustrated crystal lattices and ρ 0.5, while explaining the absence of SC in structurally related materials with substantially different ρ.The possibility that e-e interactions can be the driving force behind SC in correlated-electron systems has been intensely investigated since the discovery of SC in the high T c cuprates. The minimal requirements for a complete theory are, (i) the superconducting pair correlations are enhanced by e-e interactions, and (ii) pair correlations are long range. For moderate to large e-e interactions, pair correlations are perhaps best calculated numerically, which however can be done only for finite clusters. The simplest model incorporating e-e interactions is usually assumed to be the Hubbard model, which in quite general form can be written asAll terms in Eq. 1 have their standard meaning. The first sum is the kinetic energy of noninteracting electrons within a 2D tight-binding model with hopping matrix elements t ij ; U and V ij are onsite and nearest neighbor (n.n.) Coulomb interactions respectively. Existing numerical calculations within Eq. 1 on 2D lattices have failed to find enhancement of pair-pair correlations relative to the noninteracting model without making severe approximations 4 . It has sometimes been surmised that correlated-electron SC might evolve upon doping a spin-gapped semiconductor, as would occur in toy models such as a 2D lattice consisting of weakly coupled even-leg ladders 5,6 . Finding realistic 2D models with SG and enhanced pair correlations however remains challenging. In the present work we demonstrate from explicit numerical calculations on frustrated 2D lattices enhanced pair correlations evolving from a spin-gapped state at a carrier density ρ 0.5, far from the region most heavily investigated until now (0.7 < ρ < 1.0). We further point out the strong relevance of the resulting theoretical picture to real materials, in particular the 2D CTS superconductors, which were discovered earlier than the cuprates 7 but are still not un...

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The chemical physics of unconventional superconductivity

Cited by 15 publications

References 42 publications

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

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

Bond patterns and charge-order amplitude in quarter-filled charge-transfer solids

Coulomb-enhanced superconducting pair correlations and paired-electron liquid in the frustrated quarter-filled band

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