arXiv subjects

R. T. Clay

Publications and source records attributed to R. T. Clay.

At least 19 recordsLinked to original sources

arXiv2021

Absence of superconductivity in the Hubbard dimer model for kappa-(BEDT-TTF)_2X

In the most studied family of organic superconductors kappa-(BEDT-TTF)_2X, the BEDT-TTF molecules that make up the conducting planes are coupled as dimers. For some anions X, an antiferromagnetic insulator is found at low temperatures adjacent to superconductivity. With an average of one hole carrier per dimer, the BEDT-TTF band is effectively 1/2-filled. Numerous theories have suggested that fluctuations of the magnetic order can drive superconducting pairing in these models, even as direct calculations of superconducting pairing in monomer 1/2-filled band models find no superconductivity. Here we present accurate zero-temperature Density Matrix Renormalization Group (DMRG) calculations of a dimerized lattice with one hole per dimer. While we do find an antiferromagnetic state in our results, we find no evidence for superconducting pairing. This further demonstrates that magnetic fluctuations in the effective 1/2-filled band approach do not drive superconductivity in these and related materials.

cond-mat.str-el↗

arXiv2019

Cooperative enhancement of superconducting correlations by electron-electron and electron-phonon interactions in the quarter-filled band

We present the results of Quantum Monte Carlo calculations for a two dimensional frustrated Hubbard model coupled to bond phonons. The model is known to have a d-wave superconducting ground state in the limit of large phonon frequency for sufficiently strong electron-phonon coupling. In the absence of electron-phonon coupling the Hubbard interaction U enhances superconducting pairing in the quarter-filled (density $ρ$ = 0.5) band. We show here that at $ρ$ = 0.5 electron-electron and electron-phonon interactions cooperatively reinforce d-wave pairing, while competing with each other at all other densities. Cooperative degrees of freedom are found in many phase transitions and are essential to understanding superconductivity in strongly correlated materials.

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arXiv2019

Theory of triangular lattice quasi-one-dimensional charge-transfer solids

Recent investigations of the magnetic properties and the discovery of superconductivity in quasi-one-dimensional triangular lattice organic charge-transfer solids have indicated the severe limitations of the effective 1/2-filled band Hubbard model for these and related systems. Our computational studies of these materials within a 1/4-filled band Hubbard model in which the organic monomer molecules, and not their dimers, constitute the sites of the Hamiltonian are able to reproduce the experimental results. We ascribe the spin gap transition in kappa-(BEDT-TTF)_2B(CN)_4 to the formation of a two-dimensional paired-electron crystal and make the testable prediction that the spin gap will be accompanied by charge-ordering and period doubling in two directions. We find enhancement of the long-range component of superconducting pairing correlations by the Hubbard repulsive interaction for band parameters corresponding to kappa-(BEDT-TTF)_2CF_3SO_3. The overall results strongly support a valence bond theory of superconductivity we have proposed recently.

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arXiv2018

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

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. 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. Uniquely in the quarter-filled band, there is a very strong tendency to form nearest neighbor spin-singlets, in both one and two dimensions. 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 state and the Wigner crystal of single electrons. Further, unlike these classical broken symmetries, the PEC is characterized by a spin gap. The tendency to the PEC in two dimensions is enhanced by lattice frustration. 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 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. 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.

cond-mat.supr-con↗

arXiv2016

Bond patterns and charge order amplitude in 1/4-filled charge-transfer solids

Metal-insulator transition accompanied by charge-ordering has been widely investigated in quasi-one-dimensional conductors, including in particular organic charge-transfer solids. Among such materials the 1/4-filled band charge-transfer solids are of strong interest, because of the commensurate nature of the charge-ordering in these systems. The period-four charge-order pattern ...1100... here is accompanied by two distinct bond distortion patterns, giving rise to bond-charge-density waves (BCDW) of types 1 and 2. Using quantum Monte Carlo methods, we determine the phase diagram within the extended Hubbard Hamiltonian that gives both types 1 and 2 BCDW in the thermodynamic limit. We further investigate the effect of electron-electron and electron-phonon interactions on the amount of charge disproportionation. Our results show that between these two bond patterns, one (BCDW2) in general coexists with a large magnitude charge order, which is highly sensitive to electron-phonon interactions, while the other (BCDW1) is characterized by weak charge order. We discuss the relevance of our work to experiments on several 1/4-filled conductors, focusing in particular on the materials (EDO-TTF)_2X and (DMEDO-TTF)_2X with large amplitude charge-order.

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arXiv2016

Coulomb enhancement of superconducting pair-pair correlations in a $\frac{3}{4}$-filled model for $κ$-(BEDT-TTF)$_2$X

We present the results of precise correlated-electron calculations on the monomer lattices of the organic charge-transfer solids $κ$-(BEDT-TTF)$_2$X for 32 and 64 molecular sites. Our calculations are for band parameters corresponding to X = Cu[N(CN)$_2$]Cl and Cu$_2$(CN)$_3$, which are semiconducting antiferromagnetic and quantum spin liquid, respectively, at ambient pressure. We have performed our calculations for variable electron densities $ρ$ per BEDT-TTF molecule, with $ρ$ ranging from 1 to 2. We find that $d$-wave superconducting pair-pair correlations are enhanced by electron-electron interactions only for a narrow carrier concentration about $ρ=1.5$, which is precisely the carrier concentration where superconductivity in the charge-transfer solids occurs. Our results indicate that the enhancement in pair-pair correlations is not related to antiferromagnetic order, but to a proximate hidden spin-singlet state that manifests itself as a charge-ordered state in other charge-transfer solids. Long-range superconducting order does not appear to be present in the purely electronic model, suggesting that electron-phonon interactions also must play a role in a complete theory of superconductivity.

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arXiv2014

The chemical physics of unconventional superconductivity

Attempts to explain correlated-electron superconductivity have largely focused on the proximity of the superconducting state to antiferromagnetism. Yet, there exist many correlated-electron systems that exhibit insulator-superconducting transitions where the insulating state exhibits spatial broken symmetry different from antiferromagnetism. Here we focus on a subset of such compounds which are seemingly very different in which specific chemical stoichiometries play a distinct role, and small deviations from stoichiometry can destroy superconductivity. These superconducting materials share a unique carrier concentration, at which we show there is a stronger than usual tendency to form local spin-singlets. We posit that superconductivity is a consequence of these pseudomolecules becoming mobile as was suggested by Schafroth a few years prior to the advent of the BCS theory.

cond-mat.supr-con↗

arXiv2014

Comment on "Tuning the Magnetic Dimensionality by Charge Ordering in the Molecular TMTTF Salts"

Yoshimi et al. [arXiv:1110.3573] have attempted to explain the pressure(P)-dependent behavior of Fabre salts which exhibit charge order (CO), antiferromagnetic (AFM), and spin-Peierls (SP) phases. Experiments find two AFM phases, AFM1 at large P and AFM2 at small P. Yoshimi et al. suggest that there also exist two distinct zero-temperature SP phases, SP1 and SP2. Here we point out that the occurrence of two distinct SP phases contradicts experiments, and is found because of unrealistic model parameters.

cond-mat.str-el↗

arXiv2013

Monte Carlo simulations of two-dimensional fermion systems with string-bond states

We describe an application of variational Monte Carlo to two-dimensional fermionic systems within the recently developed tensor-network string-bond state (SBS) ansatz. We use a combination of variational Monte Carlo and stochastic optimization to optimize the matrix-product state matrices representing the ground state. We present results for a two-dimensional spinless fermion model including nearest-neighbor Coulomb interactions and determine using finite-size scaling the phase boundary between charge-ordered insulating and metallic phases. This approach can treat frustrated systems and be easily extended to for fermions with spin.

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arXiv2013

Organic superconductors: the need to go beyond effective 1/2-filled band models

A frustrated, effective 1/2-filled band Hubbard-Heisenberg model has been proposed to describe the strongly dimerized charge-transfer solid families kappa-(ET)_2X and Z[Pd(dmit)_2]_2. In addition to unconventional superconductivity these materials also exhibit antiferromagnetism, candidate spin-liquid phases, and in the case of Z=EtMe_3P, a so-called valence-bond solid phase. We show that neither superconductivity nor the valence-bond solid phase occurs within the Hubbard-Heisenberg model, indicating that the effective 1/2-filled band model is unsuitable for these materials.

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arXiv2012

Ground state and finite temperature behavior of 1/4-filled band zigzag ladders

We consider the simplest example of lattice frustration in the 1/4-filled band, a one-dimensional chain with next-nearest neighbor interactions. For this zigzag ladder with electron-electron as well as electron-phonon interactions we present numerical results for ground state as well as thermodynamic properties. In this system the ground state bond distortion pattern is independent of electron-electron interaction strength. The spin gap in the ground state of the zigzag ladder increases with the degree of frustration. Unlike in one-dimension, where the spin-gap and charge ordering transitions can be distinct, we show that in the ladder they occur simultaneously. We discuss spin gap and charge ordering transitions in 1/4-filled materials with one, two, or three dimensional crystal structures. We show empirically that regardless of dimensionality the occurrence of simultaneous or distinct charge and magnetic transitions can be correlated with the ground state bond distortion pattern.

cond-mat.str-el↗

arXiv2012

Absence of long-range superconducting correlations in the frustrated 1/2-filled band Hubbard model

We present many-body calculations of superconducting pair-pair correlations in the ground state of the half-filled band Hubbard model on large anisotropic triangular lattices. Our calculations cover nearly the complete range of anisotropies between the square and isotropic triangular lattice limits. We find that the superconducting pair-pair correlations decrease monotonically with increasing onsite Hubbard interaction U for inter-pair distances greater than nearest neighbor. For the large lattices of interest here the distance dependence of the correlations approaches that for noninteracting electrons. Both these results are consistent with the absence of superconductivity in this model in the thermodynamic limit. We conclude that the effective 1/2-filled band Hubbard model, suggested by many authors to be appropriate for the kappa-(BEDT-TTF)-based organic charge-transfer solids, does not explain the superconducting transition in these materials.

cond-mat.str-el↗

arXiv2011

Beyond the quantum spin liquid concept in frustrated two dimensional organic superconductors

The occurrence of antiferromagnetism in kappa-(ET)_2X can be understood within an effective 1/2-filled band with dimers of ET molecules containing one hole each. We argue that while this effective model can describe the presence of antiferromagnetism, a complete description for these materials requires the correct carrier density of one-half per molecule. For dimerized and strongly frustrated 1/4-filled lattices we show that a singlet-paired state coexisting with charge ordering occurs that we have termed the Paired Electron Crystal (PEC). Here we investigate the 1/4-filled model on a dimerized lattice, showing regions where AFM, PEC, and the Wigner-crystal occur. We point out the need to go beyond quantum spin liquid concepts for highly frustrated materials such as kappa-(ET)_2Cu_2(CN)_3 and beta'-EtMe_3Sb[Pd(dmit)_2]_2 which we believe are PECs at low temperatures.

cond-mat.str-el↗

arXiv2011

Theory of carrier concentration-dependent electronic behavior in layered cobaltates

A natural explanation for the carrier concentration-dependent electronic behavior in the layered cobaltates emerges within correlated-electron Hamiltonians with finite on-site and significant nearest neighbor hole-hole Coulomb repulsions. The nearest neighbor repulsion decreases hole double-occupancy below hole density 1/3, but increases the same at higher hole densities. Our conclusion is valid for both single-band and three-band extended Hubbard Hamiltonians, and sheds light on concentration-dependent $e_g^\prime$ hole occupancy within the latter.

cond-mat.str-el↗

arXiv2011

The Paired Electron Crystal: order from frustration in the quarter-filled band

We present a study of the effects of simultaneous charge- and spin-frustration on the two-dimensional strongly correlated quarter-filled band on an anisotropic triangular lattice. The broken-symmetry states that dominate in the weakly frustrated region near the rectangular lattice limit are the well known antiferromagnetic state with in-phase lattice dimerization along one direction, and the Wigner crystal state with the checkerboard charge order. For moderate to strong frustration, however, the dominant phase is a novel spin-singlet paired-electron crystal (PEC), consisting of pairs of charge-rich sites separated by pairs of charge-poor sites. The PEC, with coexisting charge-order and spin-gap in two dimension, is the quarter-filled band equivalent of the valence bond solid (VBS) that can appear in the frustrated half-filled band within antiferromagnetic spin Hamiltonians. We discuss the phase diagram as a function of on-site and intersite Coulomb interactions as well as electron-phonon coupling strength. We speculate that the spin-bonded pairs of the PEC can become mobile for even stronger frustration, giving rise to a paired-electron liquid. We discuss the implications of the PEC concept for understanding several classes of quarter-filled band materials that display unconventional superconductivity, focusing in particular on organic charge transfer solids. Our work points out the need to go beyond quantum spin liquid (QSL) concepts for highly frustrated organic charge-transfer solids such as kappa-(BEDT-TTF)_2Cu_2(CN)_3 and EtMe_3Sb[Pd(dmit)_2]_2, which we believe show frustration-induced charge disproportionation at low temperatures. We discuss possible application to layered cobaltates and 1/4-filled band spinels.

cond-mat.str-el↗

arXiv2010

Cooperative orbital ordering and Peierls instability in the checkerboard lattice with doubly degenerate orbitals

It has been suggested that the metal-insulator transitions in a number of spinel materials with partially-filled t_2g d-orbitals can be explained as orbitally-driven Peierls instabilities. Motivated by these suggestions, we examine theoretically the possibility of formation of such orbitally-driven states within a simplified theoretical model, a two-dimensional checkerboard lattice with two directional metal orbitals per atomic site. We include orbital ordering and inter-atom electron-phonon interactions self-consistently within a semi-classical approximation, and onsite intra- and inter-orbital electron-electron interactions at the Hartree-Fock level. We find a stable, orbitally-induced Peierls bond-dimerized state for carrier concentration of one electron per atom. The Peierls bond distortion pattern continues to be period 2 bond-dimerization even when the charge density in the orbitals forming the one-dimensional band is significantly smaller than 1. In contrast, for carrier density of half an electron per atom the Peierls instability is absent within one-electron theory as well as mean-field theory of electron-electron interactions, even for nearly complete orbital ordering. We discuss the implications of our results in relation to complex charge, bond, and orbital-ordering found in spinels.

cond-mat.str-el↗

arXiv2010

The Paired-Electron Crystal in the Two-Dimensional Frustrated Quarter-Filled Band

The competition between antiferromagnetic and spin-singlet ground states within quantum spin models and the 1/2-filled band Hubbard model has received intense scrutiny. Here we demonstrate a frustration-induced transition from Néel antiferromagnetism to spin-singlet in the interacting 1/4-filled band on an anisotropic triangular lattice. While the antiferromagnetic state has equal charge densities 0.5 on all sites, the spin-singlet state is a paired-electron crystal, with pairs of charge-rich sites separated by pairs of charge-poor sites. The paired-electron crystal provides a natural description of the spin-gapped state proximate to superconductivity in many organic charge-transfer solids. Pressure-induced superconductivity in these correlated-electron systems is likely a transition from the 1/4-filled band valence bond solid to a valence bond liquid.

cond-mat.str-el↗

arXiv2009

Bipolaron Density-Wave Driven By Antiferromagnetic Correlations and Frustration in Organic Superconductors

We describe the Paired Electron Crystal (PEC) which occurs in the interacting frustrated two-dimensional 1/4-filled band. The PEC is a charge-ordered state with nearest-neighbor spin singlets separated by pairs of vacant sites, and can be thought of as a bipolaron density wave. The PEC has been experimentally observed in the insulating state proximate to superconductivity in the organic charge-transfer solids. Increased frustration drives a PEC-to-superconductor transition in these systems.

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