Theories on Frustrated Electrons in Two-Dimensional Organic Solids

Anomalously field-susceptible spin clusters emerging in the electric-dipole liquid candidate κ-(ET)2Hg(SCN)2Br

Mutual interactions in many-body systems bring about various exotic phases, among which liquid-like states failing to order due to frustration are of keen interest. The organic system with an anisotropic triangular lattice of molecular dimers, κ-(ET)₂Hg(SCN)₂Br, has been suggested to host a dipole liquid arising from intradimer charge-imbalance instability, possibly offering an unprecedented stage for the spin degrees of freedom. Here, we show that an extraordinary unordered/unfrozen spin state having soft matter-like spatiotemporal characteristics emerges in this system. ¹H nuclear magnetic resonance (NMR) spectra and magnetization measurements indicate that gigantic, staggered moments are nonlinearly and inhomogeneously induced by a magnetic field, whereas the moments vanish in the zero-field limit. The analysis of the NMR relaxation rate signifies that the moments fluctuate at a characteristic frequency slowing down to below megahertz at low temperatures. The inhomogeneity, local correlation, and slow dynamics indicative of middle-scale dynamical correlation length of several nanometers suggest novel frustration-driven spin clusterization.

Charge-Ordering and Structural Transition in the New Organic Conductor δ'-(BEDT-TTF)2CF3CF2SO3

We report structural, transport, and optical properties and electronic structure calculations of the δ'-(BEDT-TTF)₂CF₃CF₂SO₃ (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) organic conductor that has been synthesized by electrocrystallization. Electronic structure calculations demonstrate the quasi-one-dimensional Fermi surfaces of the compound, while the optical spectra are characteristic for a dimer-Mott insulator. The single-crystal X-ray diffraction measurements reveal the structural phase transition at 200 K from the ambient-temperature monoclinic P2₁/m phase to the low-temperature orthorhombic Pca2₁ phase, while the resistivity measurements clearly show the first order semiconductor-semiconductor transition at the same temperature. This transition is accompanied by charge-ordering as it is confirmed by splitting of charge-sensitive vibrational modes observed in the Raman and infrared spectra. The horizontal stripe charge-order pattern is suggested based on the crystal structure, band structure calculations, and optical spectra.

Geometric Frustration and Long-Range Ordering Induced by Surface Pressure Oscillation in a Langmuir-Blodgett Monolayer of Magnetic Soft Spheres

As a step toward the bottom-up construction of magnonic systems, this paper demonstrates the use of a large-amplitude surface-pressure annealing technique to generate 2-D order in a Langmuir-Blodgett monolayer of magnetic soft spheres comprising a surfactant-encapsulated polyoxometalate. The films show a distorted square lattice interpreted as due to geometric frustration caused by 2-D confinement between soft walls, one being the air interface and the other the aqueous subphase. Hysteresis and relaxation phenomena in the 2-D layers are suggested to be due to folding and time-dependent interpenetration of surfactant chains.

Correlated insulating states at fractional fillings of moiré superlattices

Quantum particles on a lattice with competing long-range interactions are ubiquitous in physics; transition metal oxides^1,2, layered molecular crystals³ and trapped-ion arrays⁴ are a few examples. In the strongly interacting regime, these systems often show a rich variety of quantum many-body ground states that challenge theory². The emergence of transition metal dichalcogenide moiré superlattices provides a highly controllable platform in which to study long-range electronic correlations^5-12. Here we report an observation of nearly two dozen correlated insulating states at fractional fillings of tungsten diselenide/tungsten disulfide moiré superlattices. This finding is enabled by a new optical sensing technique that is based on the sensitivity to the dielectric environment of the exciton excited states in a single-layer semiconductor of tungsten diselenide. The cascade of insulating states shows an energy ordering that is nearly symmetric about a filling factor of half a particle per superlattice site. We propose a series of charge-ordered states at commensurate filling fractions that range from generalized Wigner crystals⁷ to charge density waves. Our study lays the groundwork for using moiré superlattices to simulate a wealth of quantum many-body problems that are described by the two-dimensional extended Hubbard model^3,13,14 or spin models with long-range charge-charge and exchange interactions^15,16.

Lattice Dynamics Coupled to Charge and Spin Degrees of Freedom in the Molecular Dimer-Mott Insulator κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Cl

Inelastic neutron scattering measurements on the molecular dimer-Mott insulator κ-(BEDT-TTF)_{2}Cu[N(CN)_{2}]Cl reveal a phonon anomaly in a wide temperature range. Starting from T_{ins}∼50-60  K where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature T_{N}=27  K, where also a ferroelectric ordering at T_{FE}≈T_{N} occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below T_{N}, while no change of the lattice symmetry is observed across T_{N} in neutron diffraction measurements. We assign the phonon anomalies to structural fluctuations coupled to charge and spin degrees of freedom in the BEDT-TTF molecules.

Quantum spin liquids unveil the genuine Mott state

The localization of charge carriers by electronic repulsion was suggested by Mott in the 1930s to explain the insulating state observed in supposedly metallic NiO. The Mott metal-insulator transition has been subject of intense investigations ever since^1-3-not least for its relation to high-temperature superconductivity⁴. A detailed comparison to real materials, however, is lacking because the pristine Mott state is commonly obscured by antiferromagnetism and a complicated band structure. Here we study organic quantum spin liquids, prototype realizations of the single-band Hubbard model in the absence of magnetic order. Mapping the Hubbard bands by optical spectroscopy provides an absolute measure of the interaction strength and bandwidth-the crucial parameters that enter calculations. In this way, we advance beyond conventional temperature-pressure plots and quantitatively compose a generic phase diagram for all genuine Mott insulators based on the absolute strength of the electronic correlations. We also identify metallic quantum fluctuations as a precursor of the Mott insulator-metal transition, previously predicted but never observed. Our results suggest that all relevant phenomena in the phase diagram scale with the Coulomb repulsion U, which provides a direct link to unconventional superconductivity in cuprates and other strongly correlated materials.

Pressure dependence of the metal-insulator transition in κ-(BEDT-TTF)2Hg(SCN)2Cl: optical and transport studies

The two-dimensional organic conductor κ-(BEDT-TTF)₂-Hg(SCN)₂Cl exhibits a pronounced metal-insulator transition at [Formula: see text] K. From the splitting of the molecular vibrations, the phase transition can be unambiguously assigned to charge-ordering with [Formula: see text]. We have investigated the pressure evolution of this behavior by temperature-dependent electrical transport measurements and optical investigations applying hydrostatic pressure up to 12 kbar. The data reveal a mean-field like down-shift of [Formula: see text] with a critical pressure of [Formula: see text] kbar and a metallic state above the suppression of the charge-ordered state; no traces of superconductivity could be identified down to T  =  1.5 K. As the charge order [Formula: see text] sets in abruptly with decreasing temperature, its size remains unaffected by pressure. However, the fraction of charge imbalanced molecules decreases until it is completely absent above 1.6 kbar.

Electronic ferroelectricity in molecular organic crystals

Electronic ferroelectricity in molecular organic crystals is reviewed from a theoretical perspective. In particular, we focus on the charge-driven-type electronic ferroelectricity where electronic charge order without inversion symmetry induces a spontaneous electric polarization in quarter-filling systems. Two necessary conditions to realize this type of ferroelectricity are the dimer-type lattice structure and alternate electronic charge alignments. Some prototypical organic compounds are introduced. In particular, κ-type BEDT-TTF organic salts, which are termed the dimer-Mott insulating systems, are focused on. Recent developments in the theoretical researches for dielectric and magnetodielectric properties, a collective dipole excitation and a possibility of superconductivity induced by polar charge fluctuation are reviewed. Some perspectives are presented.

Optical conductivity measurement of a dimer Mott-insulator to charge-order phase transition in a two-dimensional quarter-filled organic salt compound

We report a novel insulator-insulator transition arising from the internal charge degrees of freedom in the two-dimensional quarter-filled organic salt β-(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 appears below Tc, indicating a competition between the two insulating phases. Our infrared imaging spectroscopy has further revealed a spatially competitive electronic phase far below Tc, suggesting a nature of quantum phase transition driven by material-parameter variations.

Magnetic ordering and charge dynamics in κ-(BEDT-TTF)2Cu[N(CN)2]Cl

The Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl consists of molecular dimers arranged on an anisotropic triangular lattice. At low temperatures a pronounced dielectric anomaly is observed, and eventually a canted antiferromagnetic ground state forms. Optical spectroscopy clearly rules out charge imbalance and the existence of quantum electric dipoles with a dipolar-spin coupling. Here we suggest a novel form of spin-charge coupling where the prominent in-plane dielectric response in κ-(BEDT-TTF)2Cu[N(CN)2]Cl is explained by short-range discommensurations of the antiferromagnetic phase in the temperature range 30 K < T < 50 K, and by relaxation of charged domain walls in the ferromagnetic structure at lower temperatures.