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 k-(BEDT-TTF)_2X and Et_nMe_{4-n}Pn[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 there is a direct transition as a function of pressure 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 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, k-(BEDT-TTF)_2Cu_2(CN)_3 and EtMe_3Sb[Pd(dmit)_2]_2, there is strong evidence for a spin liquid ground state, and for EtMe_3P[Pd(dmit)_2]_2, a valence bond solid ground state. We review theoretical attempts to explain these phenomena, arguing that this can be captured by a Hubbard model on the anisotropic triangular lattice at half filling, and that resonating valence bond wavefunctions can capture most of the essential physics. We review evidence that this model can have a spin liquid ground state for a range of parameters that are realistic for the relevant materials. We conclude by summarising the progress made thus far and identifying some of the key questions still to be answered.

Major rewrite. New material added and many typos corrected. 67 pages, 41 figures. Thanks to those who commented on the previous version

References in corpus (79)

• Scanning tunneling spectroscopy of high-temperature superconductors
• Quantum criticality
• Quantum spin-liquid emerging in two-dimensional correlated Dirac fermions
• Quantum magnetism and criticality
• U(1) Gauge Theory of the Hubbard Model : Spin Liquid States and Possible Application to k-(BEDT-TTF)_2 Cu_2 (CN)_3
• Mott Transition from a Spin Liquid to a Fermi Liquid in the Spin-Frustrated Organic Conductor kappa-(ET)2Cu2(CN)3
• Unconventional critical behaviour in a quasi-two-dimensional organic conductor
• Ground State of the Kagome Lattice Heisenberg Antiferromagnet
• Unified explanation of the Kadowaki-Woods ratio in strongly correlated materials
• Quantum phase transition induced by Dzyaloshinskii-Moriya in the kagome antiferromagnet
• Theory of a continuous Mott transition in two dimensions
• Mott Transition, Antiferromagnetism, and d-wave Superconductivity in Two-Dimensional Organic Conductors
• The Nernst effect and the boundaries of the Fermi liquid picture
• Rotationally-invariant slave-boson formalism and momentum dependence of the quasiparticle weight
• Spinons and triplons in spatially anisotropic frustrated antiferromagnets
• Theoretical Aspects of Charge Ordering in Molecular Conductors
• Excitation spectra of the spin-1/2 triangular-lattice Heisenberg antiferromagnet
• Strong electronic correlations in superconducting organic charge transfer salts
• Pseudogap opening and formation of Fermi arcs as an orbital-selective Mott transition in momentum space
• Effective spin model for the spin-liquid phase of the Hubbard model on the triangular lattice
• Amperean Pairing Instability in the U(1) Spin Liquid State with Fermi Surface and Application to $\kappa -(BEDT-TTF{)}_{2}C{u}_2(CN{)}_3$
• The Hubbard model on the triangular lattice: Spiral order and spin liquid
• Emergence of inhomogeneous moments from spin liquid in the triangular-lattice Mott insulator $\kappa$-(ET)${}_2$Cu${}_2$(CN)${}_3$
• Gapless spin liquids on the three dimensional hyper-kagome lattice of Na${}_4$Ir${}_3$O${}_8$
• Bandwidth-controlled Mott transition in $\kappa -(BEDT-TTF{)}_{2}Cu[N(CN{)}_2]B{r}_{x}C{l}_{1-x}$ I. Optical studies of localized charge excitations
• Global phase diagrams of frustrated quantum antiferromagnets in two dimensions: doubled Chern-Simons theory
• Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model
• Finite temperature Mott transition in Hubbard model on anisotropic triangular lattice
• Specific heat measurements of the gap structure of the organic superconductors kappa-(ET)_2Cu[N(CN)_2]Br and kappa-(ET)_2Cu(NCS)_2
• Superconductivity and a Mott Transition in a Hubbard Model on an Anisotropic Triangular Lattice
• Bandwidth-controlled Mott transition in kappa-(BEDT-TTF)2Cu[N(CN)2]Br{x}Cl{1-x}: Optical studies of correlated carriers
• Spin Liquid Phase in Anisotropic Triangular Lattice Heisenberg Model: Exact diagonalization and density-matrix renormalization group calculations
• Electronic Correlations in Oligo-acene and -thiophene Organic Molecular Crystals
• Exact diagonalization study of Mott transition in the Hubbard model on an anisotropic triangular lattice
• Mott Transition in a Valence Bond Solid Insulator with a Triangular Lattice
• Dynamics and transport of the Z_2 spin liquid: application to kappa-(ET)2Cu2(CN)3
• Universality Classes of Metal-Insulator Transitions in Strongly Correlated Electron Systems and Mechanism of High-Temperature Superconductivity
• Absence of superconductivity in the half-filled band Hubbard model on the anisotropic triangular lattice
• Quasiparticles at the verge of localization near the Mott metal-insulator transition in a two-dimensional material
• The Dependence of the Superconducting Transition Temperature of Organic Molecular Crystals on Intrinsically Non-Magnetic Disorder: a Signature of either Unconventional Superconductivity or Novel Local Magnetic Moment Formation
• Transport Properties of a spinon Fermi surface coupled to a U(1) gauge field
• Symmetry of the superconducting order parameter in frustrated systems determined by the spatial anisotropy of spin correlations
• Power-law Conductivity inside the Mott gap: application to $\kappa -(BEDT-TTF{)}_{2}C{u}_2(CN{)}_3$
• Magnetism and d-wave superconductivity on the half-filled square lattice with frustration
• The Origin of Anomalous Low-Temperature Downturns in the Thermal Conductivity of Cuprates
• Electronic Correlations in CoO2, the Parent Compound of Triangular Cobaltates
• Spin-triplet pairing instability of the spinon Fermi surface in a U(1) spin liquid
• Real space imaging of the metal - insulator phase separation in the band width controlled organic Mott system $\kappa$-(BEDT-TTF)${}_2$Cu[N(CN)${}_2$]Br
• Neel order, ring exchange and charge fluctuations in the half-filled Hubbard model
• Possibility of Gapless Spin Liquid State by One-dimensionalization
• Study of the Hubbard model on the triangular lattice using dynamical cluster approximation and dual fermion methods
• Scaling theory of the Mott transition and breakdown of the Grüneisen scaling near a finite-temperature critical end point
• Universality of liquid-gas Mott transitions at finite temperatures
• Coulomb parameters and photoemission for the molecular metal TTF-TCNQ
• Towards the parameterisation of the Hubbard model for salts of BEDT-TTF: A density functional study of isolated molecules
• Onset of a boson mode at superconducting critical point of underdoped YBa2Cu3Oy
• Universal scaling relations in molecular superconductors
• Magnetic order in coupled spin-half and spin-one Heisenberg chains in anisotropic triangular-lattice geometry
• Insulator-metal transition on the triangular lattice
• Electronic Properties of the Hubbard Model on a Frustrated Triangular Lattice
• Excitation spectra and ground state properties of the layered spin-1/2 frustrated antiferromagnets Cs_2CuCl_4 and Cs_2CuBr_4
• Sensitivity of the interlayer magnetoresistance of layered metals to intralayer anisotropies
• Quantum critical transition from charge-ordered to superconducting state in the triangular lattice negative-U extended Hubbard model
• Spectroscopic evidence for preformed Cooper pairs in the pseudogap phase of cuprates
• Superconducting Pairing Symmetries in Anisotropic Triangular Quantum Antiferromagnets
• Quantum critical "opalescence" around metal-insulator transitions
• Studies of non-magnetic impurities in the spin-1/2 Kagome Antiferromagnet
• Mixed order parameters, accidental nodes and broken time reversal symmetry in organic superconductors: a group theoretical analysis
• On the Relationship Between the Critical Temperature and the London Penetration Depth in Layered Organic Superconductors
• Analytical solution of a Hubbard model extended by nearest neighbour Coulomb and exchange interaction on a triangle and tetrahedron
• Antiferromagnetic Spin Fluctuations in the Metallic Phase of Quasi-Two-Dimensional Organic Superconductors
• Tensor networks-a new tool for old problems
• Apparent Violation of the Wiedemann-Franz law near a magnetic field tuned metal-antiferromagnetic quantum critical point
• Quantum Mott Transition and Superconductivity
• Unconventional Superconducting Symmetry in a Checkerboard Antiferromagnet
• A phenomenological model of the superconducting state of the Bechgaard salts
• Vertex Corrections and the Korringa Ratio in Strongly Correlated Electron Materials
• Variational wave functions for frustrated magnetic models
• Bilayer Hubbard model for 3He: a cluster dynamical mean-field calculation