Through resistivity measurements of an organic crystal hosting massless Dirac fermions with a charge-ordering instability, we reveal the effect of interactions among Dirac fermions on the charge transport. A low-temperature resistivity upturn appears robustly irrespectively of pressure and is enhanced while approaching the critical pressure of charge ordering, indicating that the insulating behavior originates from short-range Coulomb interactions. Observation of apparently vanishing gap in the charge-ordered phase accords with the theoretical prediction of the non-topological edge states.
We consider the possibility that the electrons injected into organic field-effect transistors are strongly correlated. A single layer of acenes can be modelled by a Hubbard Hamiltonian similar to that used for the kappa-(BEDT-TTF)(2)X family of organic superconductors. The injected electrons do not necessarily undergo a transition to a Mott insulator state as they would in bulk crystals when the system is half-filled. We calculate the fillings needed for obtaining insulating states in the framework of the slave-boson theory and in the limit of large Hubbard repulsion, U. We also suggest that t…
We report the influence of the field effect on the dc resistance and Hall coefficient in the strain-induced Mott insulating state of an organic superconductor $κ$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. Conductivity obeys the formula for activated transport $σ_{\Box} = σ_{0}\exp(-W/k_{B}T)$, where $σ_{0}$ is a constant and $W$ depends on the gate voltage. The gate voltage dependence of the Hall coefficient shows that, unlike in conventional FETs, the effective mobility of dense hole carriers ($\sim1.6\times 10^{14}$ cm$^{-2}$) is enhanced by a positive gate voltage. This implies that carrier doping…
We investigated the interlayer magnetoresistance in an organic massless Dirac electron system $α$-(BEDT-TTF)$_2$I$_3$ under pressure. We experimentally demonstrate that the width of the zero mode owing to carrier scattering is much narrower than that of the other Landau levels.
Quantum entanglement effects between the electronic spin and charge degrees of freedom are examined in an organic molecular solid, termed a dimer-Mott insulating system, in which molecular dimers are arranged in a crystal as fundamental units. A low energy effective model includes an antisymmetric exchange interaction, as one of the dominant magnetic interactions. This interaction favors a 90 degree spin configuration, and competes with the Heisenberg-type exchange interaction. Stabilities of the magnetic ordered phases are examined by using the spin-wave theory, as well as the Schwinger-boson…
Magnetic properties of the conduction pi-electron system of kappa-BETS)2Mn[N(CN)2]3 have been probed using 13C NMR. At ambient pressure, the metal-insulator transition observed in the resistivity measurements below T~23K is shown to be accompanied by ordering of the pi-spins in a long-range staggered structure. As the metal-insulator transition is suppressed by applying a small pressure of ~0.5 kbar, the pi-spin system maintains the properties of the metallic state down to 5K.
We report two surprising results regarding the nature of the spatial broken symmetries in the two-dimensional (2D), quarter-filled band with strong electron-electron interactions. First, in direct contradiction to the predictions of one-electron theory, we find a coexisting ``bond-order and charge density wave'' (BCDW) insulating ground state in the 2D rectangular lattice for all anisotropies, including the isotropic limit. Second, we find that the BCDW further coexists with a spin-density wave (SDW) in the range of large anisotropy. Further, in contrast to the interacting half-filled band, in…
We investigate the effect of strong electronic correlation on the massless Dirac fermion system, $α$-(BEDT-TTF)$_2$I$_3$, under pressure. In this organic salt, one can control the electronic correlation by changing pressure and access the quantum critical point between the massless Dirac fermion phase and the charge ordering phase. We theoretically study the electronic structure of this system by applying the slave-rotor theory and find that the Fermi velocity decreases without creating a mass gap upon approaching the quantum critical point from the massless Dirac fermion phase. We show that t…
We have fabricated a metallic structure in an organic Mott insulator $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. The periodic metallic domains of approximately 90$\times$90 $μ$m$^2$ obtained by X-ray irradiation through a molybdenum mesh mask are visualized by scanning microregion infrared reflectance spectroscopy technique. No deterioration by irradiation is found in a range of nanometer to micrometer scales. We demonstrate that the present processing method is applicable for the fabrication of molecular electronic devices.
The ground state of $λ$-(BEDT-TTF)$_2$GaCl$_4$, which has the same structure as the organic superconductor $λ$-(BETS)$_2$GaCl$_4$, was investigated by magnetic susceptibility and $^{13}$C NMR measurements. The temperature dependence of the magnetic susceptibility revealed an antiferromagnetic (AF) correlation with $J/k_{\rm B} \simeq$ 98 K. NMR spectrum splitting and the divergence of $1/T_1$ were observed at approximately 13 K, which is associated with the AF transition. We found that the AF structure is commensurate according to discrete NMR peak splitting, suggesting that the ground state o…
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…
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 …
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 superconductivity is absent within the 1/2-filled band triangular lattice repulsive Hubbard model that has been proposed for organic charge-transfer solids. We posit that organic superconductivity is rather reached from a Bond-Charge Density Wave that either constitutes the insulating state proximate to superconductivity, or is extremely close in energy to the antiferromagnetic state, and replaces the latter under pressure. The Bond-Charge Density Wave can be described within an effective attractive $U$ extended Hubbard Hamiltonian with repulsive nearest neighbor interaction $V$. …
The charge ordering phenomena in quasi two-dimensional 1/4-filled organic compounds (ET)_2X (ET=BEDT-TTF) are investigated theoretically for the $θ$ and $α$-type structures, based on the Hartree approximation for the extended Hubbard models with both on-site and intersite Coulomb interactions. It is found that charge ordered states of stripe-type are stabilized for the relevant values of Coulomb energies, while the spatial pattern of the stripes sensitively depends on the anisotropy of the models. By comparing the results of calculations with the experimental facts, where the effects of quantu…
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.
Observations of $ν=1$ Quantum Hall Effect and Inter-Band Effects of Magnetic fields on Hall Conductivity in Organic Massless Dirac Fermion System $α$-(BETS)$_2$I$_3$ under Pressure
本文在压力下的 α-(BETS)₂I₃ 有机无质量 Dirac 费米子体系中观测到 ν=1 量子 Hall 态，并通过化学势跨越 Dirac 点揭示了磁场对 Hall 电导率的带间效应。
We have investigated the magnetic field-induced metal-insulator transition in the tau-phase organic conductors, which occurs in fields above 35 T, and below 14 K, by magnetization, thermoelectric, and pressure dependent transport methods. Our results show that the transition is a bulk thermodynamic process where a magnetic field-dependent gap opens upon entry into the insulating state. We argue that the transition involves a magnetic field-induced change in the electronic structure.
Recent experiments have demonstrated that the performances of organic FETs strongly depend on the dielectric properties of the gate insulator. In particular, it has been shown that the temperature dependence of the mobility evolves from a metallic-like to an insulating behavior upon increasing the dielectric constant of the gate material. This phenomenon can be explained in terms of the formation of small polarons, due to the polar interaction of the charge carriers with the phonons at the organic/dielectric interface. Building on this model, the possible consequences of the Coulomb repulsion …
Quasi one-dimensional organic conductors: from Fröhlich conductivity and Peierls insulating state to magnetically-mediated superconductivity, a retrospective
$^{13}$C NMR observation of a nonmagnetic charge-ordered state in the organic conductor $κ$-(ET)$_{2}$Hg(SCN)$_{2}$Cl
University of Tokyo（东京大学）；The Institute for Solid State Physics, University of Tokyo（东京大学固体物理研究所）；Institute of Problems of Chemical Physics RAS（俄罗斯科学院化学物理问题研究所）；The Institute for Quantum Matter and the Department of Physics and Astronomy, The Johns Hopkins University（约翰斯·霍普金斯大学量子物质研究所及物理与天文学系）；Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；University of Stuttgart（斯图加特大学）；Department of Advanced Materials Science, University of Tokyo（东京大学先进材料科学系）
We report the observation of nonlinear anomalous Hall effect (NLAHE) in the multilayered organic conductor $α$-(BEDT-TTF)$_2$I$_3$ in the charge order (CO) insulating phase just under the critical pressure for transition into two-dimensional (2D) massless Dirac fermion (DF) phase. We successfully extracted the finite nonlinear Hall voltage proportional to square current at zero magnetic field. The observed NLAHE features, current direction dependence and correlation with CO, are consistent with the previous estimation assuming 2D massive DF with a pair of tilted Dirac cones. This is the first …
We investigate the spin-orbit coupling (SOC) effects in $α$- and $κ$-phase BEDT-TTF and BEDT-TSF organic salts. Contrary to the assumption that SOC in organics is negligible due to light C, S, H atoms, we show the relevance of such an interaction in a few representative cases. In the weakly correlated regime, SOC manifests primarily in the opening of energy gaps at degenerate band touching points. This effect becomes especially important for Dirac semimetals such as $α$-(ET)$_2$I$_3$. Furthermore, in the magnetic insulating phase, SOC results in additional anisotropic exchange interactions, wh…
To determine how electron correlations open a gap in two-dimensional massless Dirac electrons in the organic compounds $α$-(BEDT-TTF)$_2$I$_3$ [$α$-(ET)$_2$I$_3$] and $α$-(BEDT-TSeF)$_2$I$_3$ [$α$-(BETS)$_2$I$_3$], we derive and analyze $ab$ $initio$ low-energy effective Hamiltonians for these two compounds. We find that the horizontal stripe charge ordering opens a gap in the massless Dirac electrons in $α$-(ET)$_2$I$_3$, while an insulating phase without explicit symmetry breaking appears in $α$-(BETS)$_2$I$_3$. We clarify that the combination of the anisotropic transfer integrals and the el…
The organic charge-transfer salt $κ$-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Br is a quasi two-dimensional metal with a half-filled conduction band at ambient conditions. When cooled below $T=80$ K it undergoes a pronounced transition to an insulating phase where the resistivity increases many orders of magnitude. In order to elucidate the nature of this metal-insulator transition we have performed comprehensive transport, dielectric and optical investigations. The findings are compared with other dimerized $κ$-(BEDT-TTF) salts, in particular the Cl-analogue, where a charge-order transition takes place a…
An inductive method is used to follow the magnetic field-dependent susceptibility of the coupled charge density wave (CDW) and spin-Peierls (SP) ordered state behavior in the dual chain organic conductor Perylene$_2$[Pt(mnt)$_2$]. In addition to the coexisting SP-CDW state phase below 8 K and 20 T, the measurements show that a second spin-gapped phase appears above 20 T that coincides with a field-induced insulating phase. The results support a strong coupling of the CDW and SP order parameters even in high magnetic fields, and provide new insight into the nature of the magnetic susceptibility…
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…
We theoretically investigate charge order and nonlinear conduction in a quasi-two-dimensional organic conductor β-(meso-DMBEDT-TTF)2PF6 [DMBEDT-TTF=dimethylbis(ethylenedithio)tetrathiafulvalene]. Within the Hartree-Fock approximation, we study effects of structural distortion on the experimentally observed checkerboard charge order and its bias-induced melting by using an extended Hubbard model with Peierls- and Holstein-types of electron-lattice interactions. The structural distortion is important in realizing the charge order. The current-voltage characteristics obtained by a nonequilibrium …
We explore the nature of the metal-insulator transition in $β^{\prime\prime}$-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Cl by x-ray, electrical transport, ESR, Raman and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction $a$, setting in already at the crossover temperature of 150K from the metallic to the insulating state. A spin gap $Δ_σ= 47$meV is extracted. From the activated resistivity behavior below 55K a charge gap $Δ_ρ=60$meV is derived. At $T_{\rm CO} = 72$K the C=C vibrational modes reveal the development of a cha…
The thermoelectric effect in a two-dimensional (2D) massless Dirac fermion (DF) system at the quantum limit is discussed to verify the prediction of high-performance thermopower in an organic conductor α-(BEDT-TTF)2I3. Because of relatively large Zeeman splitting in α-(BEDT-TTF)2I3, the boundless increase of thermopower at high magnetic fields, predicted without the Zeeman effect, is hardly expected, whereas there appears to be a broad local maximum. This is characteristic of 2D DF systems with Zeeman splitting and is recognized in the previous experiment. In contrast to 3D Dirac/Weyl semimeta…
In-plane magnetoresistance for organic massless Dirac electron system (OMDES) $α$-(BEDT-TTF)$_2$I$_3$ and $θ$-(BEDT-TTF)$_2$I$_3$ in addition to possible candidates of the OMDES $α$-(BETS)$_2$I$_3$ and $α$-(BEDT-STF)$_2$I$_3$, was investigated under hydrostatic pressure. We have found the universal behavior of the in-plane magnetoresistance under a low magnetic field perpendicular to two-dimensional plane. As for $α$-(BEDT-TTF)$_2$I$_3$, the universality was examined with the parameters of temperature, magnetic field and its direction. We suggest that the universal magnetoresistance behavior i…
Phase transitions in the Hubbard model and ionic Hubbard model at half-filling on the honeycomb lattice are investigated in the strong coupling perturbation theory which corresponds to an expansion in powers of the hopping $t$ around the atomic limit. Within this formulation we find analytic expressions for the single-particle spectrum, whereby the calculation of the insulating gap is reduced to a simple root finding problem. This enables high precision determination of the insulating gap that does not require any extrapolation procedure. The critical value of Mott transition on the honeycomb …
Electronic phase separation consisting of the metallic and insulating domains with 50 -- 100 $μ$m in diameter is found in the organic Mott system $κ$-[($h$8-BEDT-TTF)$_{1-x}$($d$8-BEDT-TTF)$_{x}$]$_{2}$Cu[N(CN)$_{2}$]Br by means of scanning micro-region infrared spectroscopy using the synchrotron radiation. The phase separation appears below the critical end temperature 35 -- 40 K of the first order Mott transition. The observation of the macroscopic size of the domains indicates a different class of the intrinsic electronic inhomogeneity from the nano-scale one reported in the inorganic Mott …
The low-temperature electronic state of the layered organic charge-transfer salt $κ$-(BETS)$_2$FeCl$_4$ was probed by interlayer electrical resistance measurements under magnetic field. Both above and below $T_{\mathrm{N}}=0.47\,$K, the temperature of antiferromagnetic ordering of $3d$-electron spins of Fe$^{3+}$ localized in the insulating anion layers, a non-saturating linear $R(T)$ dependence has been observed. A weak superconducting signal has been detected in the antiferromagnetic state, at temperatures $\leq 0.2\,$K. Despite the very high crystal quality, only a tiny fraction of the samp…
We study properties of Wigner crystal in snaked nanochannels and show that they are characterized by a conducting sliding phase at low charge densities and an insulating pinned phase emerging above a certain critical charge density. We trace parallels between this model problem and the Little suggestion for electron transport in organic molecules. We also show that in presence of periodic potential inside the snaked channel the sliding phase exists only inside a certain window of electron densities that has similarities with a pressure dependence of conductivity in organic conductors. Our stud…
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 phase diagram of the layered organic superconductor $κ$-(ET)$_{2}$Cu[N(CN)$_{2}$]Cl has been accurately measured from a combination of $^{1}$H NMR and AC susceptibility techniques under helium gas pressure. The domains of stability of antiferromagnetic and superconducting long-range orders in the pressure {\it vs} temperature plane have been determined. Both phases overlap through a first-order boundary that separates two regions of inhomogeneous phase coexistence. The boundary curve is found to merge with another first order line related to the metal-insulator transition in the paramagnet…
The nature of the insulating ground state in quasi-two-dimensional organic conductor lambda-(BETS)_2GaX_zY_{4-z}, where the existence of a spin gap is suggested by susceptibility measurement, has been studied theoretically. Hartree-Fock calculations at absolute zero temperature show that if the on-site Coulomb interaction exceeds some critical value, then antiferromagnetic spin ordering emerges and eventually leads to an insulating state which can be considered as a two-dimensional localized spin system. Based on the quantum Monte Carlo simulations of Katoh and Imada, we will show that this sy…
The strong long-range Coulomb interaction between massless Dirac fermions in graphene can drive a semimetal-insulator transition. We show that this transition is strongly suppressed when the Coulomb interaction is screened by such effects as disorder, thermal fluctuation, doping, and finite volume. It is completely suppressed once the screening factor $μ$ is beyond a threshold $μ_{c}$ even for infinitely strong coupling. However, such transition is still possible if there is an additional strong contact four-fermion interaction. The differences between screened and contact interactions are als…
The mutual influence of the conduction electron system provided by organic donor layers and magnetic system localized in insulating layers of the molecular charge transfer salt $κ$-(BETS)$_2$FeBr$_4$ has been studied. It is demonstrated that besides the high-field re-entrant superconducting state, the interaction between the two systems plays important role for the low-field superconductivity. The coupling of normal-state charge carriers to the magnetic system is reflected in magnetic quantum oscillations and can be evaluated based on the angle-dependent beating behaviour of the oscillations. …
The highly insulating antiferromagnetic phase of the magnetic- organic conductor lambda-(BETS)2FeCl4 has been probed by resonant mm wave methods vs. magnetic field, temperature, and frequency. Our results show evidence for magnetic sub-phases within the temperature-field phase diagram as previously predicted by Brossard et al. [Eur. Phys. J. B 1, 439(1998)].
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…
A few organic conductors show a diversity of exciting properties like Mott insulating behaviour, spin liquid, antiferromagnetism, bad metal or unconventional superconductivity controlled by small changes in temperature, pressure or chemical substitution. While such a behaviour can be technologically relevant for functional switches, a full understanding of its microscopic origin is still lacking and poses a challenge in condensed matter physics since these phases may be a manifestation of electronic correlation. Here we determine from first principles the microscopic nature of the electronic p…
Low-energy properties of the one-dimensional Kondo lattice model are investigated by using bosonization and renormalization group methods. The formation of the spin and charge excitation gaps at half-filling is discussed in detail both for antiferromagnetic and ferromagnetic Kondo couplings. Also, the effect of the interaction between conduction electrons on the gap formation is studied both for the Kondo insulating phase and for the Haldane gap phase. It is found that while the repulsive interaction changes the dependence of the excitation gap on the Kondo exchange coupling, the attractive in…
A finite residual linear term in the thermal conductivity at zero temperature in insulating magnets indicates the presence of gapless excitations of itinerant quasiparticles, which has been observed in some candidate materials of quantum spin liquids (QSLs). In the organic triangular insulator beta'-EtMe3Sb[Pd(dmit)2]2, a QSL candidate material, the low-temperature thermal conductivity depends on the cooling process and the finite residual term is observed only in samples with large thermal conductivity. Moreover, the cooling rate dependence is largely sample dependent. Here we find that, whil…
Low-dimensional organic conductors could establish themselves as model systems for the investigation of the physics in reduced dimensions. In the metallic state of a one-dimensional solid, Fermi-liquid theory breaks down and spin and charge degrees of freedom become separated. But the metallic phase is not stable in one dimension: as the temperature is reduced, the electronic charge and spin tend to arrange themselves in an ordered fashion due to strong correlations. The competition of the different interactions is responsible for which broken-symmetry ground state is eventually realized in a …
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 …
Submillimeter and millimeter wave electron spin resonance (ESR) measurements of the pi-d correlated organic conductor lambda-(BETS)2Fe0.6Ga0.4Cl4 have been performed. Antiferromagnetic resonance (AFMR) has been observed in the insulating antiferromagnetic phase, and its frequency-field dependence can be reproduced by the biaxial anisotropic AFMR theory. We find that in this alloy system, the easy-axis is near the b-axis, unlike previous results for the pure lambda-(BETS)2FeCl4 salts where it is closer to the c*-axis. We have also observed electron paramagnetic resonance (EPR) in the metallic p…
$^{77}$Se-NMR spectrum and frequency shift measurements in the paramagnetic metal (PM) and antiferromagnetic insulating (AFI) phases are reported for a small single crystal of the organic conductor $λ-$(BETS)$_{2}$FeCl$_{4}$ as a function of temperature ($T$) and field alignment for an applied magnetic field $B_{0}$ = 9 T. The results show that in the low $T$ limit, where the localized Fe$^{3+}$ spins ($S_{d}$ = 5/2) are almost fully polarized, the conduction electrons (Se $π$-electrons, spin $s_π$ = 1/2) in the BETS molecules experience an exchange field ($\bf{B}$$_{πd}$) from the Fe$^{3+}$ s…
We examine critically two different recently proposed models of charge ordering in the nominally 1/4-filled organic charge transfer solids (CTS). In one dimension, the two models are characterized by site charge densities of the form ...1010... and ...1100..., respectively. We establish the following theoretical results: 1) there exists a critical nearest-neighbor Coulomb interaction (V_c > 2|t|) only above which does the ...1010... state become the ground state; 2) Hartree-Fock mean field theory predicts V_c incorrectly; 3) accurate quantum Monte Carlo calculations indicate that V_c increases…
The spatial charge arrangement of a typical quasi-two-dimensional organic conductor alpha-(BEDT-TTF)2I3 is revealed by single crystal structure analysis using synchrotron radiation. The results show that the horizontal stripe type structure, which was suggested by mean field theory, is established. We also find the charge disproportion above the metal-insulator transition temperature and a significant change in transfer integrals caused by the phase transition. Our result elucidates the insulating phase of this material as a 2k_F charge density localization.
The Coulomb interaction among massless Dirac fermions in graphene is unscreened around the isotropic Dirac points, causing a logarithmic velocity renormalization and a cone reshaping. In less symmetric Dirac materials possessing anisotropic cones with tilted axes, the Coulomb interaction can provide still more exotic phenomena which have not been experimentally unveiled yet. Here, using site-selective nuclear magnetic resonance, we find a non-uniform cone reshaping accompanied by a bandwidth reduction and an emergent ferrimagnetism in tilted Dirac cones that appear on the verge of charge order…
We present an ab initio analysis for the ground-state properties of a correlated organic compound $κ$-(BEDT-TTF)2Cu(NCS)2. First, we derive an effective two-dimensional low-energy model from first principles, having short-ranged transfers and short-ranged Coulomb and exchange interactions. Then, we perform many-variable variational Monte Carlo calculations for this model and draw a ground-state phase diagram as functions of scaling parameters for the onsite and off-site interactions. The phase diagram consists of three phases; a paramagnetic metallic phase, an antiferromagnetic (Mott) insulati…
Tight binding electrons on a honeycomb lattice are described by an effective Dirac theory at low energies. Lowering symmetry by an alternate ionic potential ($Δ$) generates a single-particle gap in the spectrum. We employ the dynamical mean field theory (DMFT) technique, to study the effect of on-site electron correlation ($U$) on massive Dirac fermions. For a fixed mass parameter $Δ$, we find that beyond a critical value $U_{c1}(Δ)$ massive Dirac fermions become massless. Further increasing $U$ beyond $U_{c2}(Δ)$, there will be another phase transition to the Mott insulating state. Therefore …
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…
Two-dimensional (2D) strongly correlated electron systems underlie many of the most important phenomena in contemporary condensed matter physics, including the Quantum Hall Effect (QHE), ``high T_c'' superconductivity, and possible exotic conducting states in silicon MOSFETs. We demonstrate the existence of yet another exotic ground state in strongly correlated, 2D electronic materials: a novel, insulating bond-order/charge density wave state (BCDW) in the commensurate 1/4-filled band that persists for all anisotropies within the 2D lattice, in contradiction to the non-interacting electron pre…
In order to understand the physical properties of the series of organic conductors $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]$X$ with $X$ = Cl, Br, and I, not only electronic correlations but also the effect of disorder has to be taken into account. While for Cl- and Br-containing salts the influence of both parameters were investigated and a universal phase diagram was proposed, the position of $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I is still not settled. Here we have conducted transport, infrared, and dielectric measurements on single crystals of the title compound to clarify its electronic state at low tempe…
We show that there are strong similarities in the spin lattice relaxation of non-magnetic organic charge transfer salts, and that these similarities can be understood in terms of spin fluctuations. Further, we show that, in all of the kappa-phase organic superconductors for which there is nuclear magnetic resonance data, the energy scale for the spin fluctuations coincides with the energy scale for the pseudogap. This suggests that the pseudogap is caused by short-range spin correlations. In the weakly frustrated metals k-(BEDT-TTF)_2Cu[N(CN)_2]Br, k-(BEDT-TTF)_2Cu(NCS)_2, and k-(BEDT-TTF)_2Cu…
Theoretical studies on charge ordering phenomena in quarter-filled molecular (organic) conductors are reviewed. Extended Hubbard models including not only the on-site but also the inter-site Coulomb repulsion are constructed in a straightforward way from the crystal structures, which serve for individual study on each material as well as for their systematic understandings. In general the inter-site Coulomb interaction stabilizes Wigner crystal-type charge ordered states, where the charge localizes in an arranged manner avoiding each other, and can drive the system insulating. The variety in t…
