Organic Mott Insulator Research Articles

Quantum critical dynamics in the two-dimensional transverse Ising model

In the vicinity of the quantum critical point (QCP), thermodynamic properties diverge toward zero temperature governed by universal exponents. Although this fact is well known, how it is reflected in quantum dynamics has not been addressed. The QCP of the transverse Ising model on a triangular lattice is an ideal platform to test the issue, since it has an experimental realization, the dielectrics being realized in an organic dimer Mott insulator, $\ensuremath{\kappa}\text{\ensuremath{-}}{\mathrm{ET}}_{2}X$, where a quantum electric dipole represents the Ising degrees of freedom. We track the Glauber-type dynamics of the model by constructing a kinetic protocol based on the quantum Monte Carlo method. The dynamical susceptibility takes the form of the Debye function and shows a significant peak narrowing in approaching a QCP due to the divergence of the relaxation timescale. It explains the anomaly of dielectric constants observed in the organic materials, indicating that the material is very near the ferroelectric QCP. We disclose how the dynamical and other critical exponents develop near QCP beyond the simple field theory.

Charge and spin interplay in a molecular-dimer-based organic Mott insulator

Triangular lattice quasi-two-dimensional Mott insulators based on the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecule and its analogies present a possibility to produce exotic phases by coupling charge and spin degrees of freedom. In this work we discuss magnetic properties of one such material, $\ensuremath{\kappa}$-(BEDT-${\mathrm{TTF})}_{2}\mathrm{Hg}{(\mathrm{SCN})}_{2}\mathrm{Cl}$, which is found at the border of the phase transition between a Mott insulator into a charge ordered state. Our magnetic susceptibility and cantilever magnetization measurements demonstrate how the charge degree of freedom defines magnetic properties for few different charge phases observed in this material as a function of temperature. Between ${T}_{\text{CO}}=30\phantom{\rule{0.16em}{0ex}}\text{K}$ and ${T}_{\text{S}}=24\phantom{\rule{0.16em}{0ex}}\text{K}$ we observe charge and spin separation due to one-dimensional charge stripes formed in this material below ${T}_{\text{CO}}=30\phantom{\rule{0.16em}{0ex}}\text{K}$. Below ${T}_{\text{S}}=24\phantom{\rule{0.16em}{0ex}}\text{K}$ charge and spin degrees of freedom demonstrate coupling. Spin-singlet correlations develop below 24 K, however, the melting of charge order below 15 K prevents the spin-singlet-state formation, leaving the system in the inhomogeneous state with charge ordered spin-singlet domains and charge and spin fluctuating ones.

Phonon-dressed states in an organic Mott insulator

In an electronic system of solids excited by a femtosecond laser pulse, new quantum states called photon-dressed states are created via the coupling of original electron wavefunctions and periodic electric fields of light. The study of photon-dressed states is a central issue in nonlinear optical science, as a coherent response associated with a photon-dressed state can cause ultrafast optical-switching phenomena. In general, however, phase-sensitive dynamics of photon-dressed states are difficult to observe due to their short lifetime. Here, we show that excitation of the organic Mott insulator potassium-tetracyanoquinodimethane with a strong mid-infrared pulse induces a couple of intramolecular vibrations, which add temporally periodic potentials to the electronic system, giving rise to phonon-dressed states in the picosecond time domain. Through sub-cycle spectroscopy using a phase-stable mid-infrared pulse as an excitation and an ultrashort visible pulse as a probe, we observe optical radiation consisting of four kinds of coherent oscillations with double, sum, and differential frequencies of two intramolecular vibrations. The probe-energy dependence of each oscillation can be well interpreted with phonon-dressed states in the framework of Floquet theory. These findings open a possibility for Floquet engineering in correlated electron materials with strong charge–phonon couplings.

Simultaneous Control of Bandfilling and Bandwidth in Electric Double-Layer Transistor Based on Organic Mott Insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl

The physics of quantum many-body systems have been studied using bulk correlated materials, and recently, moiré superlattices formed by atomic bilayers have appeared as a novel platform in which the carrier concentration and the band structures are highly tunable. In this brief review, we introduce an intermediate platform between those systems, namely, a band-filling- and bandwidth-tunable electric double-layer transistor based on a real organic Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl. In the proximity of the bandwidth-control Mott transition at half filling, both electron and hole doping induced superconductivity (with almost identical transition temperatures) in the same sample. The normal state under electric double-layer doping exhibited non-Fermi liquid behaviors as in many correlated materials. The doping levels for the superconductivity and the non-Fermi liquid behaviors were highly doping-asymmetric. Model calculations based on the anisotropic triangular lattice explained many phenomena and the doping asymmetry, implying the importance of the noninteracting band structure (particularly the flat part of the band).

Phase-Transition Devices Based on Organic Mott Insulators

Abstract Organic Mott-insulators are abundant among molecular conductors and are relevant to many emerging properties such as insulator-to-metal transitions, superconductivity, multiferroics, spin-liquids, and so on. In this review, phase transitions controlled by electric-field-induced carrier doping, strain-induced pressure effect, and light-induced carrier generation observed at a thin film single crystal surface of organic Mott-insulators are described. The control of electronic phases by external stimuli can give rise to a possibility of future application. At the same time, thermodynamic parameters can be tuned by those input signals to determine the system phase diagram, providing basic knowledge indispensable for Mott physics. Therefore, devices made of organic Mott-insulators can contribute to obtaining much information about Mott-transition and associated superconductivity, as well as the feasibility of application in future electronics.

Nonequilibrium optical response of a one-dimensional Mott insulator

We define, compute and analyze the nonequilibrium differential optical conductivity of the one-dimensional extended Hubbard model at half-filling after applying a pump pulse, using the time-dependent density matrix renormalization group method. The melting of the Mott insulator is accompanied by a suppression of the local magnetic moment and ensuing photogeneration of doublon-holon pairs. The differential optical conductivity reveals $(i)$ mid-gap states related to parity-forbidden optical states, and $(ii)$ strong renormalization and hybridization of the excitonic resonance and the absorption band, yielding a Fano resonance. We offer evidence and interpret such a resonance as a signature of nonequilibrium optical excitations resembling excitonic strings, (bi)excitons, and unbound doublon-holon pairs, depending on the magnitude of the intersite Coulomb repulsion. We discuss our results in the context of pump and probe spectroscopy experiments on organic Mott insulators.

Multifunctional molecular spintronics device based on neutral π-radicals predicted by first-principles study

Recently, planar neutral π-radicals were synthesized based on the insertion of tricoordinate B atoms in the triphenylmethyl framework (Kushida et al. (2017) [24]). This radical shows well balanced ambipolar carrier transport characteristics and has been used for the preparation of organic Mott-insulator transistors that work at room temperature. Here, using a first-principles method which combined non-equilibrium Green's function with spin-polarized density functional theory, we explore the transport properties of a molecular junction where a neutral π-radical dimer is sandwiched between two Au electrodes. Obvious spin current polarization, dual spin polarized rectification, magnetoresistance and low bias negative differential conductance are obtained. Moreover, the molecular device shows strong magnetoelectric coupling that the bias not only diminishes and eventually eliminates radical's magnetic moment but also manipulates magnetic configuration. Our investigations may help design a new generation of transistors with multifunction, super-miniaturization and low-power consumption.

Dihedral-Angle Dependence of Intermolecular Transfer Integrals in BEDT-BDT-Based Radical-Cation Salts with θ-Type Molecular Arrangements

We report the structural and physical properties of a new organic Mott insulator (BEDT-BDT)AsF6 (BEDT-BDT: benzo[1,2-g:4,5-g′]bis(thieno[2,3-b][1,4dithiin). This AsF6 salt has the same structure as the PF6 salt. Although the anions are disordered, the donor molecules form a θ-type arrangement. The temperature dependence of the resistivity exhibits semiconducting behavior. The static magnetic susceptibility follows Curie–Weiss law over a wide temperature range; however, below 25 K, the magnetic susceptibility is in agreement with a one-dimensional chain model with the exchange coupling J = 7.4 K. No structural phase transition was observed down to 93 K. At 270 K, the Fermi surface calculated by the tight-binding approximation is a two-dimensional cylinder; however, it is significantly distorted at 93 K. This is because the dihedral angles between the BEDT-BDT molecules become larger due to lattice shrinkage at low temperatures, which results in a smaller transfer integral (t1) along the stack direction. This slight change in the dihedral angle gives rise to a significant change in the electronic structure of the AsF6 salt. Radical-cation salts using BEDT-BDT, in which the highest occupied molecular orbital does not have a dominant sign throughout the molecule, are sensitive to slight differences in the overlap between the molecules, and their electronic structures are more variable than those of conventional θ-type conductors.

Spin Multiplicity and Solid-State Electrochemical Behavior in Charge-Transfer Co-crystals of DBTTF/F4TCNQ

Charge-transfer crystals exhibit unique electronic and magnetic properties with interesting applications. The charge-transfer single crystal formed by dibenzotetrathiafulvalene (DBTTF) together with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) presents a long-range ordered supramolecular structure of segregated stacks, with a unitary degree of charge transfer. Thus, the crystal structure is composed of dimerized radical molecules with unpaired electrons. The energy levels and the spin degrees of freedom of this material were investigated by solid-state electrochemistry and electron paramagnetic resonance (EPR) spectroscopy. The electrochemical data, supported by density functional theory calculations, show how this organic Mott insulator has an electronic gap in the range of hundreds of meV. EPR experiments show the presence of a ground-state S = 1 triplet spin state along with localized S = 1/2 spins. The calculations also predict a ground-state triplet configuration, with the singlet configuration at 170 meV higher energy. DBTTF/F4TCNQ seems to be a candidate material for organic electronic and spintronic applications.

Triangular-Lattice Organic Mott Insulator with a Disorder-Free Polyanion.

We report the synthesis, crystal and band structures, and transport properties of organic conductor κ-(ET)2Cu[Au(CN)2]Cl [ET = bis(ethylenedithio)tetrathiafulvalene], which has a triangular spin-lattice (S = 1/2) composed of (ET)2•+ dimers and polyanions with no disorder. The anisotropy of triangular lattice t'/t = 1.19 and physical properties indicate that this material is the first ET-based quantum-spin-liquid candidate having a nearly regular triangular lattice with a disorder-free anion.

Uniaxial Strain Induced Superconductivity in Quantum Spin Liquid κ-(ET)2Ag2(CN)3

We investigate the uniaxial strain effects on the quantum spin liquid κ-(ET)2Ag2(CN)3 which is an organic Mott insulator with a nearly regular S = 1/2 triangular lattice. The uniaxial compressive s...

Presence and absence of itinerant gapless excitations in the quantum spin liquid candidate EtMe3Sb[Pd(dmit)2]2

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, $\kappa_0/T \equiv \kappa/T (T \rightarrow 0)$, has been shown [M. Yamashita {\it et al.} Science {\bf 328}, 1246 (2010)], recent experiments [P. Bourgeois-Hope {\it et al.}, Phys. Rev. X {\bf 9}, 041051 (2019); J. M. Ni {\it et al.}, Phys. Rev. Lett. {\bf 123}, 247204 (2019)] have reported the absence of $\kappa_0/T$. Here we show that the low-temperature thermal conductivity strongly depends on the cooling process of the sample. When cooling down very slowly, a sizable $\kappa_0/T$ is observed. In contrast, when cooling down rapidly, $\kappa_0/T$ vanishes and, in addition, the phonon thermal conductivity is strongly suppressed. These results suggest that possible random scatterers introduced during the cooling process are responsible for the apparent discrepancy of the thermal conductivity data in this organic system. The present results provide evidence that the true ground state of EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ is likely to be a quantum spin liquid with itinerant gapless excitations.

Quantum Disordering of an Antiferromagnetic Order by Quenched Randomness in an Organic Mott Insulator.

The behavior of interacting spins subject to randomness is a longstanding issue and the emergence of exotic quantum states is among intriguing theoretical predictions. We show how a quantum-disordered phase emerges from a classical antiferromagnet by controlled randomness. ^{1}H NMR of a successively x-ray-irradiated organic Mott insulator finds that the magnetic order collapses into a spin-glass-like state, immediately after a slight amount of disorder centers are created, and evolves to a gapless quantum-disordered state without spin freezing, spin gap, or critical slowing down, as reported by T. Furukawa etal. [Phys. Rev. Lett. 115, 077001 (2015)]PRLTAO0031-900710.1103/PhysRevLett.115.077001 through sequential reductions in the spin freezing temperature and moment.

Non-Fermi-liquid behavior and doping asymmetry in an organic Mott insulator interface

High-${T}_{\mathrm{C}}$ superconductors show anomalous transport properties in their normal states, such as the bad-metal and pseudogap behaviors. To discuss their origins, it is important to speculate whether these behaviors are material-dependent or universal phenomena in the proximity of the Mott transition by investigating similar but different material systems. An organic Mott transistor is suitable for this purpose owing to the adjacency between the two-dimensional Mott insulating and superconducting states, simple electronic properties, and high doping/bandwidth tunability in the same sample. Here we report the temperature dependence of the transport properties under electron and hole doping in an organic Mott electric-double-layer transistor. At high temperatures, the bad-metal behavior widely appears except at half filling, regardless of the doping polarity. At lower temperatures, the pseudogap behavior is observed only under hole doping, while the Fermi-liquid-like behavior is observed under electron doping. The bad-metal behavior seems a universal high-energy-scale phenomenon, while the pseudogap behavior is based on lower-energy-scale physics that can be influenced by details of the band structure.

Polarization-resolved Ultrafast Spectroscopy in an Organic Mott Insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl

In this work, femtosecond pump probe spectroscopy is conducted in the organic Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl with different probe polarizations. We found that as the temperature decreased...

Pump-probe study of β’-(BEDT-TTF)(TCNQ) crystal near antiferromagnetic transition

The photo-excited state properties of an organic Mott-insulator β’-(BEDT-TTF)(TCNQ) crystal were investigated by pump-probe measurements. Considerable change in the pump-probe reflection signal with a wide spectral range was observed near the antiferromagnetic (AF) transition temperature of the TCNQ layers. In the time response, approach to the AF temperature induced the appearance of a slow decay signal whose lifetime was estimated to be about 3 msec maximum. These results indicate that a sustained mechanism exists for photo-excited states near the AF transition, which is probably related to a critical slowing down phenomenon.

Fragmented Electronic Spins with Quantum Fluctuations in Organic Mott Insulators Near a Quantum Spin Liquid.

Magnetic structures of organic Mott insulators X[Pd(dmit)_{2}]_{2} (X=Me_{4}P, Me_{4}Sb), of which electronic states are located near a quantum spin liquid (X=EtMe_{3}Sb), are demonstrated by ^{13}C nuclear magnetic resonance. 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 of quantum liquids where electronic excitations are fractionalized and S=1/2 spin is no longer an elementary particle.

Spectroscopic signature of trimer Mott insulator and charge disproportionation in BaIrO3

We have measured the reflectivity spectra of the barium iridate $9R$ BaIrO$_3$, the crystal structure of which consists of characteristic Ir$_3$O$_{12}$ trimers. In the high-temperature phase above the transition temperature $T_c\simeq180$ K, we find that the optical conductivity involves two temperature-dependent optical transitions with an ill-defined Drude response. These features are reminiscent of the optical spectra in the organic dimer Mott insulators, implying a possible emergence of an unusual electronic state named trimer Mott insulator in BaIrO$_3$, where the carrier is localized on the trimer owing to the strong Coulomb repulsion. Along with a pronounced splitting of the phonon peak observed below $T_c$, which is a hallmark of charge disproportionation, we discuss a possible phase transition from the trimer Mott insulator to a charge-ordered insulating phase in BaIrO$_3$.

In Which Directions Do Spins Point? —Unexpected Orientations and Unusual Turns in an Organic Mott Insulator

Researchers have precisely determined the configuration of spins in a representative organic Mott insulator and found them to be flopping eccentrically and rotating upon being acted by applied magn...

Quantum-disordered state of magnetic and electric dipoles in an organic Mott system

Strongly enhanced quantum fluctuations often lead to a rich variety of quantum-disordered states. Developing approaches to enhance quantum fluctuations may open paths to realize even more fascinating quantum states. Here, we demonstrate that a coupling of localized spins with the zero-point motion of hydrogen atoms, that is, proton fluctuations in a hydrogen-bonded organic Mott insulator provides a different class of quantum spin liquids (QSLs). We find that divergent dielectric behavior associated with the approach to hydrogen-bond order is suppressed by the quantum proton fluctuations, resulting in a quantum paraelectric (QPE) state. Furthermore, our thermal-transport measurements reveal that a QSL state with gapless spin excitations rapidly emerges upon entering the QPE state. These findings indicate that the quantum proton fluctuations give rise to a QSL—a quantum-disordered state of magnetic and electric dipoles—through the coupling between the electron and proton degrees of freedom.