The $\EtMeP$ and $\EtMeSb$ nearly triangular organic salts are distinguished from most other Pd[(dmit)$_2$] based salts, as they display valence bond and no long range order, respectively. Under pressure, a superconducting phase is revealed in EtMe$_3$P near the boundary of valence bond order. We use slave-rotor theory with an enlarged unit cell to study competition between uniform and broken translational symmetry states, offering a theoretical framework capturing the superconducting, valence bond order, spin liquid, and metallic phases on an isotropic triangular lattice. Our finite temperature phase diagram manifests a remarkable resemblance to the phase diagram of the EtMe$_3$P salt, where the re-entrant transitions of the type insulator-metal-insulator can be explained by an entropy difference between metal and the U(1) spin liquid. We find that the superconducting pairing symmetry is $d \pm id$, and predict different temperature dependences of the specific heat between the spin liquid and metal.
The present work studies the quasi one-dimensional $Ni(dmit)_2$-based compounds within a correlated model. More specifically, we focus our attention on the composed influence of the electronic dimerization-factor and the repulsion, on the transport properties and the localization of the electronic density in the ground-state. Those properties are studied through the computation of the charge gaps (difference between the ionization potential and the electro-affinity: IP-EA) and the long- and short-bond orders of an infinite quarter-filled chain within a $t-J(t,U)$ model. The comparison between …
We present ab initio two-dimensional extended Hubbard-type multiband models for EtMe_3Sb[Pd(dmit)_2]_2 and κ-(BEDT-TTF)_2Cu(NCS)_2, after a downfolding scheme based on the constrained random phase approximation (cRPA) and maximally-localized Wannier orbitals, together with the dimensional downfolding. In the Pd(dmit)_2 salt, the antibonding state of the highest occupied molecular orbital (HOMO) and the bonding/antibonding states of the lowest unoccupied molecular orbital (LUMO) are considered as the orbital degrees of freedom, while, in the κ-BEDT-TTF salt, the HOMO-antibonding/bonding states …
We show that the electronic structures of the title compounds predicted by density functional theory (DFT) are well described by tight binding models. We determine the frustration ratio, J'/J, of the Heisenberg model on the anisotropic triangular lattice, which describes the spin degrees of freedom in the Mott insulating phase for a range of Pd(dmit)2 salts. All of the antiferromagnetic materials studied have J'/J < 0.5 or J'/J > 0.9, consistent with predictions for the Heisenberg model. All salts with 0.5 < J'/J < 0.9, where many-body theories find a number of competing ground states, are kno…
Frequency shifts and nuclear relaxations of 13C NMR of the metal-insulator alternating material, (Me-3,5-DIP)[Ni(dmit)2]2, are presented. The NMR absorption lines originating from metallic and insulating layers are well resolved, which evidences the coexistence of localized spins (π_loc) and conduction π-electrons. The insulating layer is newly found to undergo antiferromagnetic long range order at about 2.5 K, suggesting emergence of S=1/2 Mott insulator. In the metallic layer, we found significant suppressions of static and dynamical susceptibilities of conduction electrons below 35 K, where…
The electronic structure of a quantum spin liquid compound, EtMe3Sb[Pd(dmit)2]2, has been studied with angle-resolved photoemission spectroscopy, together with two other Pd(dmit)2 salts in the valence bond solid or antiferromagnetic state. We have resolved several bands that have negligible dispersions and fit well to the calculated energy levels of an isolated [Pd(dmit)2]2 dimer. EtMe3Sb[Pd(dmit)2]2 being a Mott insulator, its lower Hubbard band is identified, and there is a small gap of ~ 50 meV between this band and the chemical potential. Moreover, the spectral features exhibit polaronic b…
Organic charge transfer salts based on the molecule Pd(dmit)2 display strong electronic correlations and geometrical frustration, leading to spin liquid, valence bond solid, and superconducting states, amongst other interesting phases. The low energy electronic degrees of freedom of these materials are often described by a single band model; a triangular lattice with a molecular orbital representing a Pd(dmit)2 dimer on each site. We use ab initio electronic structure calculations to construct and parametrize low energy effective model Hamiltonians for a class of Me(4-n) EtnX[Pd(dmit)2]2 (X=As…
Magnetic structures of organic Mott insulators X[Pd(dmit)2]2 (X=Me4P, Me4Sb), of which electronic states are located near quantum spin liquid (X=EtMe3Sb), are demonstrated by 13C NMR. Antiferromagnetic spectra and nuclear relaxations show two distinct magnetic moments within each Pd(dmit)2 molecule, which cannot be described by single band dimer-Mott model and requires intramolecular electronic correlation. This unconventional fragmentation of S = 1/2 electron spin with strong quantum fluctuation is presumably caused by nearly degenerated intramolecular multiple orbitals, and shares a notion o…
The organic charge-transfer salt EtMe$_3$P[Pd(dmit)$_2$]$_2$ is a quasi-two-dimensional Mott insulator with localized spins $S$ = 1/2 residing on a distorted triangular lattice. Here we report measurements of the uniaxial thermal expansion coefficients $α_i$ along the in-plane $i$ = $a$- and $c$-axis as well as along the out-of-plane $b$-axis for temperatures 1.4\,K $\leq $T$ \leq$ 200\,K. Particular attention is paid to the lattice effects around the phase transition at $T_{VBS}$ = 25\,K into a low-temperature valence-bond-solid phase and the paramagnetic regime above where effects of short-r…
In the field of interacting electron systems the Mott metal-to-insulator (MI) transition represents one of the pivotal issues. The role played by lattice degrees of freedom for the Mott MI transition and the Mott criticality in a variety of materials are current topics under debate. In this context, molecular conductors of the $κ$-(BEDT-TTF)$_2$X type constitute a class of materials for unraveling several aspects of the Mott physics. In this review, we present a synopsis of literature results with focus on recent expansivity measurements probing the Mott MI transition in this class of material…
A molecular Mott insulator $β$'-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ is a quantum spin liquid candidate. In 2010, it was reported that thermal conductivity of $β$'-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ is characterized by its large value and gapless behavior (a finite temperature-linear term). In 2019, however, two other research groups reported opposite data (much smaller value and a vanishingly small temperature-linear term) and the discrepancy in the thermal conductivity measurement data emerges as a serious problem concerning the ground state of the quantum spin liquid. Recently, the cooling rate was propo…
We report the discovery of a correlated insulator with a bulk gap at two-thirds filling in a geometrically frustrated Hubbard model that describes the low-energy physics of Mo$_3$S$_7$(dmit)$_3$. This is very different from the Mott insulator expected at half-filling. We show that the insulating phase, which persists even for very weak electron-electron interactions ($U$), is adiabatically connected to the Haldane phase and is consistent with experiments on Mo$_3$S$_7$(dmit)$_3$
EtMe$_3$Sb[Pd(dmit)$_2$]$_2$, an organic Mott insulator with nearly isotropic triangular lattice, is a candidate material for a quantum spin liquid, in which the zero-point fluctuations do not allow the spins to order. The itinerant gapless excitations inferred from the thermal transport measurements in this system have been a hotly debated issue recently. While the presence of a finite linear residual thermal conductivity, $κ_0/T \equiv κ/T (T \rightarrow 0)$, has been shown [M. Yamashita {\it et al.} Science {\bf 328}, 1246 (2010)], recent experiments [P. Bourgeois-Hope {\it et al.}, Phys. R…
The recently proposed multiferroic state of the charge-transfer salt κ-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl [P. Lunkenheimer et al., Nature Mater., vol. 11, pp. 755-758, Sept. 2012] has been studied by dc-conductivity, magnetic susceptibility and measurements of the dielectric constant on various, differently prepared single crystals. In the majority of crystals we confirm the existence of an order-disorder-type ferroelectric state which coincides with antiferromagnetic order. This phenomenology rules out scenarios which consider an inhomogeneous, short-range-ordered ferroelectric state. Measurements…
Systematic pressure- and temperature-dependent infrared studies on the two-dimensional organic quantum spin-liquid $β^{\prime}$-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ disclose the electronic and lattice evolution across the Mott insulator-metal transition. Increasing hydrostatic pressure continuously suppresses the insulating ground state; for $p>0.6$~GPa, a Drude-like component develops indicating the appearance of coherent quasiparticles at the Fermi level. In the vicinity of the Mott transition, not only the electronic state changes rapidly, but also the vibration modes exhibit a jump both in frequen…
We have investigated the Mott transition in a quasi-two-dimensional Mott insulator EtMe$_3$P[Pd(dmit)$_2$]$_2$ with a spin-frustrated triangular lattice in hydrostatic pressure and magnetic field. In the pressure-temperature ($P$-$T$) phase diagram, a valence bond solid phase is found to neighbor the superconductor and metal phases at low temperatures. The profile of the phase diagram is common to those of Mott insulators with antiferromagnetic order. In contrast to the antiferromagnetic Mott insulators, the resistivity in the metallic phase exhibits anomalous temperature dependence, $ρ= ρ_0 +…
We investigated the electronic states of the quasi-one-dimensional organic conductors $δ'_{P}$-(BPDT-TTF)$_2$ICl$_2$ and $δ'_{C}$-(BPDT-TTF)$_2$ICl$_2$, both of which are insulating at room temperature owing to strong electron correlations. Through measurements of electrical resistivity, optical conductivity, and magnetic susceptibility, as well as band-structure calculations, we have revealed that the two materials possess completely different ground states, even though they have the same chemical composition and stacking configuration of the donor molecules. We have found that the $δ_P'$-typ…
We report a novel insulator-insulator transition arising from the internal charge degrees of freedom in the two-dimensional quarter-filled organic salt beta-(meso-DMBEDT-TTF)2PF6. The optical conductivity spectra above Tc = 70 K display a prominent feature of the dimer-Mott insulator, characterized by a substantial growth of a dimer peak near 0.6 eV with decreasing temperature. The dimer-peak growth is rapidly quenched as soon as a peak of the charge order shows up below Tc, indicating a competition between the two insulating phases. Our infrared imaging spectroscopy has further revealed a spa…
The band structure and magnetic properties of organic charge-transfer salt $λ$-(BEDSe-TTF)$_2$GaCl$_4$ (BEDSe-TTF: bis(ethylenediseleno)tetrathiafulvalene; abbreviated as $λ$-BEDSe) are investigated. The reported crystal structure is confirmed using X-ray diffraction measurements, and the transfer integrals are calculated. The degree of electron correlation $U/W$ ($U$: on-site Coulomb repulsion, $W$: bandwidth) of $λ$-BEDSe is larger than one and comparable to that of the isostructural Mott insulator $λ$-(ET)$_2$GaCl$_4$ (ET: bis(ethylenedithio)tetrathiafulvalene, abbreviated as $λ$-ET), where…
The infrared response of the organic conductor (TMTSF)$_2$PF$_6$ and the Mott insulator (TMTTF)$_2$PF$_6$ are investigated as a function of temperature and pressure and for the polarization parallel and perpendicular to the molecular stacks. By applying external pressure on (TMTTF)$_2$PF$_6$, the Mott gap rapidly diminishes until the deconfinement transition occurs when the gap energy is approximately twice the interchain transfer integral. In its deconfined state (TMTTF)$_2$PF$_6$ exhibits a crossover from a quasi-one-dimensional to a higher-dimensional metal upon reducing the temperature. Fo…
Based on dynamical cluster approximation (DCA) quantum Monte Carlo simulations, we study the interaction-driven Mott metal-insulator transition (MIT) in the half-filled Hubbard model on the anisotropic two-dimensional triangular lattice, where the degree of frustration is varied between the unfrustrated case and the fully frustrated, isotropic triangular lattice. Upon increasing the DCA cluster size, we analyze the evolution of the MIT phase boundary as a function of frustration in the phase diagram spanned by the interaction strength and temperature, and provide a quantitative description of …
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 quantu…
Experiments performed over the last twenty years have revealed striking similarities between several two-dimensional (2D) fermion systems, including diluted two-dimensional electron liquids in semiconductors, 3He monolayers, and layered organic charge-transfer salts. These experimental results, together with recent theoretical advances, provide compelling evidence that strong electronic correlations - Wigner-Mott physics - dominate the universal features of the corresponding metal-insulator transitions (MIT). Here we review the recent theoretical work exploring quantum criticality of Mott and …
The electrodynamic response of organic spin liquids with highly-frustrated triangular lattices has been measured in a wide energy range. While the overall optical spectra of these Mott insulators are governed by transitions between the Hubbard bands, distinct in-gap excitations can be identified at low temperatures and frequencies which we attribute to the quantum spin liquid state. For the strongly correlated $β^{\prime}$-EtMe$_3$\-Sb\-[Pd(dmit)$_2$]$_2$, we discover enhanced conductivity below $175~{\rm cm}^{-1}$, comparable to the energy of the magnetic coupling $J\approx 250$ K. For $ω\rig…
Intense experimental and theoretical studies have demonstrated that the anisotropic triangular lattice as realized in the kappa-(BEDT-TTF)2X family of organic charge transfer (CT) salts yields a complex phase diagram with magnetic, superconducting, Mott insulating and even spin liquid phases. With extensive density functional theory (DFT) calculations we refresh the link between manybody theory and experiment by determining hopping parameters of the underlying Hubbard model. This leads us to revise the widely used semiempirical parameters in the direction of less frustrated, more anisotropic t…
We have measured and analyzed the dielectric constant of the dimer Mott insulator $κ-$(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, which is known as a playground for a spin-liquid state. Most unexpectedly, this particular organic salt has nontrivial charge degrees of freedom, being characterized by a dipole-glass-like dielectric relaxation below around 60 K. This is ascribed to the charge disproportionation within the dimer due to the intersite Coulomb repulsion. In this respect, the dipole observed here is purely quantum mechanical, and thereby exhibits unexpected responses.
We use midinfrared pulses with stable carrier-envelope phase offset to drive molecular vibrations in the charge transfer salt ET-F2TCNQ, a prototypical one-dimensional Mott insulator. We find that the Mott gap, which is probed resonantly with 10 fs laser pulses, oscillates with the pump field. This observation reveals that molecular excitations can coherently perturb the electronic on-site interactions (Hubbard U) by changing the local orbital wave function. The gap oscillates at twice the frequency of the vibrational mode, indicating that the molecular distortions couple quadratically to the …
The phase diagrams of isotropic and anisotropic triangular lattices with local Coulomb interactions are evaluated within cluster dynamical mean field theory. As a result of partial geometric frustration in the anisotropic lattice, short range correlations are shown to give rise to reentrant behavior which is absent in the fully frustrated isotropic limit. The qualitative features of the phase diagrams including the critical temperatures are in good agreement with experimental data for the layered organic charge transfer salts kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl and kappa-(BEDT-TTF)_2Cu_2(CN)_3.
The families of organic charge-transfer salts $κ$-(BEDT-TTF)$_2X$ and $κ$-(BETS)$_2X$ have proven to serve as a powerful playground for the investigation of the physics of frustrated Mott insulators. These materials have been ascribed model character, since dimerization of the organic molecules allows to map these materials onto a single band Hubbard model, in which the dimers reside on an anisotropic triangular lattice. By changing the inorganic unit $X$ or applying physical pressure, the correlation strength and anisotropy of the triangular lattice can be varied. This has lead to the discove…
We review the role of strong electronic correlations in quasi--two-dimensional organic charge transfer salts such as (BEDT-TTF)$_2X$, (BETS)$_2Y$ and $β'$-[Pd(dmit)$_2$]$_2Z$. We begin by defining minimal models for these materials. It is necessary to identify two classes of material: the first class is strongly dimerised and is described by a half-filled Hubbard model; the second class is not strongly dimerised and is described by a quarter filled extended Hubbard model. We argue that these models capture the essential physics of these materials. We explore the phase diagram of the half-fille…
Finding microscopic models for metallic states that exhibit quantum critical properties such as $ω/T$ scaling is a major theoretical challenge. We calculate the local dynamical spin susceptibility $χ(T,ω)$ for a Hubbard model at half filling using Dynamical Mean-Field Theory, which is exact in infinite dimensions. Qualitatively distinct behavior is found in the different regions of the phase diagram: Mott insulator, Fermi liquid metal, bad metal, and a quantum critical region above the finite temperature critical point. The signature of the latter is $ω/T$ scaling where $T$ is the temperature.…
We investigate the dynamics of correlated charge carriers in the vicinity of the Mott metal-to-insulator transition in various of the title quasi-two-dimensional organic charge-transfer salts by means of fluctuation (noise) spectroscopy. The observed $1/f$-type fluctuations are quantitatively very well described by a phenomenological model based on the concept of non-exponetial kinetics. The main result is a correlation-induced enhancement of the fluctuations accompanied by a substantial shift of spectral weight to low frequencies close to the Mott critical endpoint. This sudden slowing down o…
In Mott insulators, the strong electron-electron Coulomb repulsion prevents metallicity and charge excitations are gapped. In dimensions greater than one, their spins are usually ordered antiferromagnetically at low temperatures. Geometrical frustrations can destroy this long-range order, leading to exotic quantum spin liquid (QSL) states. However, their magnetic ground states have been a long-standing mystery. Here we show that a QSL state in the organic Mott insulator EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ with two-dimensional triangular lattice has Pauli-paramagnetic-like low-energy excitations, whic…
The Bechgaard salts are made of weakly coupled one dimensional chains. This particular structure gives the possibility to observe in these systems a dimensional crossover between a high temperature (or high energy) one dimensional phase and a two or three dimensional system. Since the filling of the chains is commensurate the system thus undergoes a deconfinement transition from a one dimensional Mott insulator to a two (or three) dimensional metal. Such a transition has of course a strong impact on the physical properties of these compounds, and is directly seen in transport measurements. In …
An investigation of the P/T phase diagram of the quarter-filled organic conductors, [EDT-TTF-CONMe2]2X, is reported on the basis of transport and NMR studies of two members, X=AsF6 and Br of the family. The strongly insulating character of these materials in the low pressure regime has been attributed to a remarkably stable charge ordered state confirmed by 13C NMR and the only existence of 1/4 Umklapp e-e scattering favoring a charge ordering instead of the 1D Mott localization seen in (TM)2X which are quarter-filled compounds with dimerization. A non magnetic insulating phase instead of the …
We have performed in-plane transport measurements on the two-dimensional organic salt $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl. A variable (gas) pressure technique allows for a detailed study of the changes in conductivity through the insulator-to-metal transition. We identify four different transport regimes as a function of pressure and temperature (corresponding to insulating, semi-conducting, ''bad metal'', and strongly correlated Fermi liquid behaviours). Marked hysteresis is found in the transition region, which displays complex physics that we attribute to strong spatial inhomogeneities. A…
The dimer Mott insulator $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ exhibits unusual electrodynamic properties. Numerical investigations of the electronic ground state and the molecular and lattice vibrations reveal the importance of the Cu$_2$(CN)$_3^-$ anion network coupled to the BEDT-TTF molecules: The threefold cyanide coordination of copper and linkage isomerism in the anion structure cause a loss of symmetry, frustration, disorder, and domain formation. Our findings consistently explain the temperature and polarization-dependent THz and infrared measurements, reinforce the understanding of dielec…
The competition between electron localization and de-localization in Mott insulators underpins the physics of strongly-correlated electron systems. Photo-excitation, which re-distributes charge between sites, can control this many-body process on the ultrafast timescale. To date, time-resolved studies have been performed in solids in which other degrees of freedom, such as lattice, spin, or orbital excitations come into play. However, the underlying quantum dynamics of bare electronic excitations has remained out of reach. Quantum many-body dynamics have only been detected in the controlled en…
The Mott insulator $κ$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$ forms a highly-frustrated triangular lattice of $S=1/2$ dimers with a possible quantum-spin-liquid state. Our experimental and numerical studies reveal the emergence of a slight charge imbalance between crystallographically inequivalent sites, relaxor dielectric response and hopping dc transport. In a broader perspective we conclude that the universal properties of strongly-correlated charge-transfer salts with spin liquid state are an anion-supported valence band and cyanide-induced quasi-degenerate electronic configurations in the relaxed s…
The physics of the Mott metal-insulator transition (MIT) has attracted huge interest in the last decades. However, despite broad efforts, some key theoretical predictions are still lacking experimental confirmation. In particular, it is not clear whether the large coherent Fermi surface survives in immediate proximity to the bandwidth-controlled first-order MIT. A quantitative experimental verification of the predicted behavior of the quasiparticle effective mass, renormalized by many-body interactions, is also missing. Here we address these issues by employing organic $κ$-type salts as exempl…
The Mott insulator $β'$-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ belongs to a class of charge transfer solids with highly-frustrated triangular lattice of $S=1/2$ molecular dimers and a quantum-spin-liquid ground state. Our experimental and ab initio theoretical studies show the fingerprints of strong correlations and disorder, important role of cation-dimer bonding in charge redistribution, no sign of intra- and inter-dimer dipoles, and the decisive van der Waals contribution to inter-dimer interactions and the ground state structure. The latter consists of quasi-degenerate electronic states related to t…
The dynamics of charge carriers close to the Mott transition is explored theoretically and experimentally in the quasi two-dimensional organic charge-transfer salt $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br$_x$Cl$_{1-x}$, with varying Br content. The frequency dependence of the conductivity deviates significantly from simple Drude model behavior: there is a strong redistribution of spectral weight as the Mott transition is approached and with temperature. The effective mass of the quasiparticles increases considerably when coming close to the insulating phase. A dynamical mean-field-theory treatment of…
Motivated by experiments on non-magnetic triangular-lattice Mott insulators, we study one candidate paramagnetic phase, the columnar dimer (or valence-bond) phase. We apply variants of the bond-operator theory to a dimerized and spatially anisotropic spin-1/2 Heisenberg model and determine its zero-temperature phase diagram and the spectrum of elementary triplet excitations (triplons). Depending on model parameters, we find that the minimum of the triplon energy is located at either a commensurate or an incommensurate wavevector. Condensation of triplons at this commensurate-incommensurate tra…
We investigate the continuous variation of electronic states from a Fermi liquid to a quantum spin liquid induced by chemical substitution in the organic dimer-Mott system of $κ$-[(BEDSe-TTF)$_x$(BEDT-TTF)$_{1-x}$]$_2$Cu[N(CN)$_2$]Br. Electrical transport measurements reveal that the mixing of BEDSe-TTF into the BEDT-TTF layers induces a quantum Mott transition around $x$=0.10. Although a charge gap disappears at this point, magnetic susceptibility and heat capacity measurements indicate that fermionic low-energy excitations remain in the insulating salts, suggesting that fermionic spin excita…
We present a study of the influence of disorder on the Mott metal-insulator transition for the organic charge-transfer salt $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. To this end, disorder was introduced into the system in a controlled way by exposing the single crystals to x-ray irradiation. The crystals were then fine-tuned across the Mott transition by the application of continuously controllable He-gas pressure at low temperatures. Measurements of the thermal expansion and resistance show that the first-order character of the Mott transition prevails for low irradiation doses achieved by irradiati…
Here we present experimental and computational evidences to support that rock-salt cubic VO is a strongly correlated metal with Non-Fermi-Liquid thermodynamics and an unusually strong spin-lattice coupling. An unexpected change of sign of metallic thermopower with composition is tentatively ascribed to the presence of a pseudogap in the density of states. These properties are discussed as signatures of the proximity to a magnetic quantum phase transition. The results are summarized in a new electronic phase diagram for the 3d monoxides, which resembles that of other strongly correlated systems…
Utilizing infrared vibrational spectroscopy we have investigated dimerized two-dimensional organic salts in order to search for possible charge redistribution that might cause electronic dipoles and ferroelectricity: the quantum spin liquid κ-(BEDT-TTF)2Cu2(CN)3, the antiferromagnetic Mott insulator κ-(BEDT-TTF)2$Cu[N(CN)2]Cl, and the superconductor κ-(BEDT-TTF)2$Cu[N(CN)2]Br. None of them exhibit any indication of charge disproportionation upon cooling down to low temperatures. No modification in the charge distribution is observed around T = 6 K where a low-temperature anomaly has been repor…
We utilize a glass-like structural transition in order to induce a Mott metal-insulator transition in the quasi-two-dimensional organic charge-transfer salt $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br. In this material, the terminal ethylene groups of the BEDT-TTF molecules can adopt two different structural orientations within the crystal structure, namely eclipsed (E) and staggered (S) with the relative orientation of the outer C$-$C bonds being parallel and canted, respectively. These two conformations are thermally disordered at room temperature and undergo a glass-like ordering transition at $T_g \…
Organic charge transfer salts exhibit thermal expansion anomalies similar to those found in other strongly correlated electron systems. The thermal expansion can be anisotropic and have a non-monotonic temperature dependence. We show how these anomalies can arise from electronic effects and be significantly enhanced, particularly at temperatures below 100 K, by strong electronic correlations. For the relevant Hubbard model the thermal expansion is related to the dependence of the entropy on the parameters ($t$, $t'$, and $U$) in the Hamiltonian or the temperature dependence of bond orders and …
Metal-insulator transitions and different ground-state phases in quasi-one- dimensional materials, (R_1R_2-DCNQI)_2M (R_1=R_2=CH_3, I and M=Ag, Cu), are studied with a renormalization-group method. We use one-dimensional continuum models with backward scatterings, umklapp processes and couplings with 2k_F and 4k_F phonons (not static lattice distortion). We take a quarter- filled band for M=Ag and a sixth-filled band coupled with a third-filled band for M=Cu. Depending on electron-electron and electron-phonon coupling strengths, the ground-state phase becomes a Tomonaga-Luttinger liquid or a s…
We review some properties of quasi-one-dimensional organic conductors, such as the Bechgaard salts, with an emphasis on aspects related to the crossovers between a Mott insulating state to a metallic state, and crossovers between different metallic behaviors. We discuss why a theoretical description of these issues is a particularly challenging problem, and describe a recent non-perturbative approach designed to deal with systems of coupled chains. This method, dubbed "chain-DMFT", is a generalization of dynamical mean field theory that treats both, one-dimensional and higher dimensional physi…
To elucidate the pressure evolution of the electronic structure in an antiferromagnetic dimer-Mott (DM) insulator $β^{\prime}$-(BEDT-TTF)$_2$ICl$_2$, which exhibits superconductivity at 14.2 K under 8 GPa, we measured the polarized infrared (IR) optical spectra under high pressure. At ambient pressure, two characteristic bands due to intra- and interdimer charge transfers have been observed in the IR spectra, supporting that this salt is a typical half-filled DM insulator at ambient pressure. With increasing pressure, however, the intradimer charge transfer excitation shifts to much lower ener…
Motivated by recent experimental suggestions of charge-order-driven ferroelectricity in organic charge-transfer salts, such as $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl, we investigate magnetic and charge-ordered phases that emerge in an extended two-orbital Hubbard model on the anisotropic triangular lattice at $3/4$ filling. This model takes into account the presence of two organic BEDT-TTF molecules, which form a dimer on each site of the lattice, and includes short-range intramolecular and intermolecular interactions and hoppings. By using variational wave functions and quantum Monte Carlo techniq…
We theoretically investigate the effects of charge order and spin frustration on the spin ordering in TMTTF salts. Using first-principles band calculations, we find that a diagonal inter-chain transfer integral $t_{q1}$, which causes spin frustration between the inter-chain dimers in the dimer-Mott insulating state, strongly depends on the choice of anion. Within the numerical Lanczos exact diagonalization method, we show that the ferroelectric charge order changes the role of $t_{q1}$ from the spin frustration to the enhancement of the two-dimensionality in spin sector. The results indicate t…
Inspired by the recent theoretical suggestion that the random-bond $S=1/2$ antiferromagnetic Heisenberg model on the triangular and the kagome lattices might exhibit a randomness-induced quantum spin liquid (QSL) behavior when the strength of the randomness exceeds a critical value, and that this "random-singlet state" might be relevant to the QSL behaviors experimentally observed in triangular organic salts $κ{\rm -(ET)_2 Cu_2 (CN)_3}$and ${\rm EtMe_3 Sb[Pd(dmit)_2]_2}$ and in kagome herbertsmithite ${\rm CuZn_3(OH)_6Cl_2}$, we further investigate the nature of the static and the dynamical sp…
Spin and charge fluctuations at vicinity of metal-to-Mott insulator transitions are studied in an organic solid with molecular dimers. The extended Hubbard model taking account of the internal electronic degree of freedom in a molecular dimer is analyzed using the variational Monte Carlo method. Three kinds of the electronic phases, i.e. a metallic phase, an antiferromagnetic insulating phase and a polar charge ordered phase, compete with each other in the ground state. It is found that the polar-charge fluctuation is dominant in a wide range of the molecular dimerization and Coulomb interacti…
In the organic charge transfer salt k-(BETS)2Mn[N(CN)2]3 the metallic conductivity is provided by itinerant pi-electrons in the layers of BETS molecules, whereas magnetization is largely dominated by the localized d-electrons of the Mn2+ ions in the insulating anionic layers. We study magnetic properties of the compound in its low-temperature, Mott-insulating state by means of magnetic torque technique. The complex behavior of the torque can be qualitatively explained by the coexistence of two weakly interacting magnetic subsystems associated with paramagnetic d-electron spins and antiferromag…
We show that the anisotropy of the effective spin model for the dimer Mott insulator phase of $κ$-(BEDT-TTF)$_2X$ salts is dramatically different from that of the underlying tight-binding model. Intra-dimer quantum interference results in a model of coupled spin chains, where frustrated interchain interactions suppress long-range magnetic order. Thus, we argue, the "spin liquid" phase observed in some of these materials is a remnant of the Tomonaga-Luttinger physics of a single chain. This is consistent with previous experiments and resolves some outstanding puzzles. An erratum [Phys. Rev. Let…
We present a detailed theoretical study of the electronic, magnetic, and structural properties of magnesiowüstite Fe$_{1-x}$Mg$_x$O with $x$ in the range between 0$-$0.875 using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we compute the electronic structure and phase stability of the rock-salt B1-structured (Fe,Mg)O at high pressures relevant for the Earth's lower mantle. We obtain that upon compression paramagnetic (Fe,Mg)O exhibits a spin-state transition of Fe$^{2+}$ ions from a high-spin t…
