Bandwidth tuning triggers interplay of charge order and superconductivity in two-dimensional organic materials

Optical Conductivity Spectra of Charge-Crystal and Charge-Glass States in a Series of θ-Type BEDT-TTF Compounds

In the 3/4-filled band system θ-(BEDT-TTF)2X with a two-dimensional triangular lattice, charge ordering (CO) often occurs due to strong inter-site Coulomb repulsion. However, the strong geometrical frustration of the triangular lattice can prohibit long-range CO, resulting in a charge-glass state in which the charge configurations are randomly distributed. Here, we investigate the charge-glass states of orthorhombic and monoclinic θ-type BEDT-TTF salts by measuring the electrical resistivity and optical conductivity spectra. We find a substantial difference between the charge-glass states of the orthorhombic and monoclinic systems. The charge-glass state in the orthorhombic system with an isotropic triangular lattice exhibits larger low-energy excitations than that in the monoclinic one with an anisotropic triangular lattice and becomes more metallic as the isotropy of the triangular lattice increases. These results can be understood by the different charge-glass formation mechanisms in the two systems: in the orthorhombic system, the charge-glass state originates from geometric frustration due to the equilateral triangular lattice, leading to metallic 3-fold COs, whereas in the monoclinic system, the charge-glass formation originates from geometric frustration of the isosceles triangular lattice, in which the charge-glass state is described by the superposition of insulating 2-fold stripe COs.

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.

Tuning Charge Order in (TMTTF)2X by Partial Anion Substitution

In the quasi-one-dimensional (TMTTF)2X compounds with effectively quarter-filled bands, electronic charge order is stabilized from the delicate interplay of Coulomb repulsion and electronic bandwidth. The correlation strength is commonly tuned by physical pressure or chemical substitution with stoichiometric ratios of anions and cations. Here, we investigate the charge-ordered state through partial substitution of the anions in (TMTTF)2[AsF6]1−x[SbF6]x with x≈0.3, determined from the intensity of infrared vibrations, which is sufficient to suppress the spin-Peierls state. Our dc transport experiments reveal a transition temperature TCO = 120 K and charge gap ΔCO=430 K between the values of the two parent compounds (TMTTF)2AsF6 and (TMTTF)2SbF6. Upon plotting the two parameters for different (TMTTF)2X, we find a universal relationship between TCO and ΔCO yielding that the energy gap vanishes for transition temperatures TCO≤60 K. While these quantities indicate that the macroscopic correlation strength is continuously tuned, our vibrational spectroscopy results probing the local charge disproportionation suggest that 2δ is modulated on a microscopic level.

Modern History of Organic Conductors: An Overview

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

Superconductivity emerging from a stripe charge order in IrTe2 nanoflakes

Superconductivity in the vicinity of a competing electronic order often manifests itself with a superconducting dome, centered at a presumed quantum critical point in the phase diagram. This common feature, found in many unconventional superconductors, has supported a prevalent scenario in which fluctuations or partial melting of a parent order are essential for inducing or enhancing superconductivity. Here we present a contrary example, found in IrTe₂ nanoflakes of which the superconducting dome is identified well inside the parent stripe charge ordering phase in the thickness-dependent phase diagram. The coexisting stripe charge order in IrTe₂ nanoflakes significantly increases the out-of-plane coherence length and the coupling strength of superconductivity, in contrast to the doped bulk IrTe₂. These findings clarify that the inherent instabilities of the parent stripe phase are sufficient to induce superconductivity in IrTe₂ without its complete or partial melting. Our study highlights the thickness control as an effective means to unveil intrinsic phase diagrams of correlated van der Waals materials.

Superconductivity often appears due to suppression of competing electronic orders. Here, the authors present a contrary example showing a superconducting dome inside the parent phase with a stripe charge order in IrTe₂ nanoflakes and identify their unusual superconducting properties.

The metal-insulator transition in the organic conductor β″-(BEDT-TTF)2Hg(SCN)2Cl

We explore the nature of the metal-insulator transition in the two-dimensional organic compound β^″-(BEDT-TTF)₂Hg(SCN)₂Cl by x-ray, electrical transport, ESR, Raman, and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction (a-b), setting in already at the crossover temperature of 150 K from the metallic to the insulating state. A spin gap of Δ_σ=47 meV is extracted. From the activated resistivity behavior below T = 55 K, a charge gap of Δ_ρ=60 meV is derived. At T_CO = 72 K, the C=C vibrational modes reveal the development of a charge-ordered state with a charge disproportionation of 2δ_ρ=0.34e. In addition to a slight structural dimerization, charge-order causes stripes most likely perpendicular to the stacks.

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.

Charge fluctuations and ethylene-group-ordering transition in β''-(BEDT-TTF)4[(H3O)Fe(C2O4)3]⋅Y molecular charge-transfer salts

The polarized infrared reflectance and Raman spectra of the three quasi-two-dimensional β''-(BEDT-TTF)4[(H3O)Fe(C2O4)3]⋅Y bifunctional charge-transfer salts, where BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene and Y = C6H5Br, (C6H5CN)0.17(C6H5Br)0.83, (C6H5CN)0.4(C6H5F)0.6, have been measured as a function of the temperature. Signatures of charge inhomogenity have been found in both Raman and infrared spectra of the β''-(BEDT-TTF)4[(H3O)Fe(C2O4)3]⋅Y superconductors. A 100 K transition to a mixed insulating/metallic state is clearly seen for the first time in the temperature dependence of the electronic spectra of superconducting β''-(BEDT-TTF)4[(H3O)Fe(C2O4)3]⋅C6H5Br. We suggest that this phase transition is due to subtle changes in the ethylene groups ordering, which are related to a structural phase transition in the anionic layer. The infrared and Raman spectra of quasi-two-dimensional metal α-'pseudo-κ'-(BEDT-TTF)4[(H3O)Fe(C2O4)3]C6H4Br2 are also investigated.

Quantum criticality in organic conductors? Fermi liquid versus non-Fermi-liquid behaviour

Organic metals exhibit unusual electronic properties in their charge and spin degrees of freedom that have puzzled physicists for decades. By now this behaviour is established as intrinsic and related to electronic interactions. Like other correlated electron systems, such as heavy fermions or transition-metal oxides, organic conductors are located next to some ordered phase in the spin or charge sectors. Theory predicts quantum fluctuations to become important at low temperatures and quantum critical behaviour present in most physical properties. Here we survey the experimental evidence of quantum criticality in well-established organic model compounds and look for indications of non-Fermi-liquid behaviour.