BEDT-TTF Layers Research Articles

Persistence of fermionic spin excitations through a genuine Mott transition in κ -type organics

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 $\kappa$-[(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 excitations persist. We propose that the transition can be classified into a genuine Mott transition.

Metal-insulator phase transition in the δ−(BEDT−TTF)4 [2,6-anthracene- bis(sulfonate)] ·4H2O studied by infrared spectroscopy

Temperature dependence of polarized infrared (IR) reflectance spectra of two-dimensional weakly dimerized organic conductor $\ensuremath{\delta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}$[2,6-anthracene-bis(sulfonate)]$\ifmmode\cdot\else\textperiodcentered\fi{}4{\mathrm{H}}_{2}\mathrm{O}$ with $\frac{1}{4}$-filled band has been measured to investigate the metal-insulator phase transition at ${T}_{\mathrm{MI}}=85\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ linked to charge ordering (CO). Vibrational bands related to $\mathrm{C}=\mathrm{C}$ stretching vibrations of the BEDT-TTF donor molecule give evidence of the CO and charge fluctuations both in the metallic and insulating phases above and below 85 K, respectively. Considerable modifications of the vibrational features due to electron-molecular vibration coupling are also observed. The phase transition is associated with the reorganization of hydrogen $\mathrm{OH}\ensuremath{\cdots}\mathrm{O}$ bonds in anion layers between the sulfonate moieties of the anion and the trapped water molecules, which is transmitted to conducting BEDT-TTF layers through weaker C-$\mathrm{H}\ensuremath{\cdots}\mathrm{O}$ bonds. The analysis of electronic bands confirms the presence of strong electronic correlations in the material. Two electronic absorptions are observed in the mid-IR region, attributed to transitions between Hubbard bands and transitions inside BEDT-TTF dimers. A low-energy electronic absorption in the far-IR region is attributed to collective excitations of the short-range charge order. Below 85 K, the opening of a charge gap of \ensuremath{\sim}24 meV is observed both parallel and perpendicular to the BEDT-TTF stacking direction. The spectroscopic data indicate that the CO phase transition is a consequence of both Coulomb interactions between charge carriers and donor-acceptor interactions.

Anion Arrangement Effects on Electronic States of κ-type BEDT-TTF Compounds

We investigate the anion arrangement effect on the electronic states of various bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) compounds composed of BEDT-TTF layers and counter-anion networks via...

Predicted photoinduced topological phases in organic salt α−(BEDT-TTF)2I3

The emergence of photo-induced topological phases and their phase transitions are theoretically predicted in organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$, which possesses inclined Dirac cones in its band structure. By analyzing a photo-driven tight-binding model describing conduction electrons in the BEDT-TTF layer using the Floquet theorem, we demonstrate that irradiation with circularly polarized light opens a gap at the Dirac points, and the system eventually becomes a Chern insulator characterized by a quantized topological invariant. A rich phase diagram is obtained in plane of amplitude and frequency of light, which contains Chern insulator, semimetal, and normal insulator phases. We find that the photo-induced Hall conductivity provides a sensitive means to detect the predicted phase evolutions experimentally. This work contributes towards developing the optical manipulation of electronic states in matter through broadening the range of target materials that manifest photo-induced topological phase transitions.

A new compound in the BEDT-TTF family [BEDT-TTF = bis-(ethyl-enedi-thio)-tetra-thia-fulvalene] with a tetra-thio-cyanato-cuprate(II) anion, (BEDT-TTF)4[Cu(NCS)4

A new phase combining BEDT-TTF and [Cu(NCS)4]2- as the counter-anion, namely bis-[bis(ethyl-enedi-thio)-tetra-thia-fulvalenium] tetra-thio-cyanato-cuprate(II) bis-[bis(ethyl-ene-di-thio)-tetra-thia-fulvalene], (C10H8S8)2[Cu(NCS)4]·2C10H8S8 or (BEDT-TTF)4[Cu(NCS)4] was obtained during a galvanostatic electrocrystallization process. As previously observed with BEDT-TTF-based compounds with oxalatometallate anions, the BEDT-TTF mol-ecules in (BEDT-TTF)4[Cu(NCS)4] exhibit the so-called pseudo-κ arrangement, with two BEDT-TTF mol-ecules being positively charged and two electronically neutral. The bond lengths and angles in the two unique BEDT-TTF mol-ecules differ slightly. The crystal structure consists of layers of BEDT-TTF mol-ecules extending parallel to (001). The width of this layer corresponds to the length of the a axis [16.9036 (17) Å]. The BEDT-TTF layers are separated by layers of centrosymmetric square-planar [Cu(NCS)4]2- dianions.

Specific Structural Disorder in an Anion Layer and Its Influence on Conducting Properties of New Crystals of the (BEDT-TTF)4A+[M3+(ox)3]G Family, Where G Is 2-Halopyridine; M Is Cr, Ga; A+ Is [K0.8(H3O)0.2

New crystals (1–4) of organic conductors based on the radical cation salts of the bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) with paramagnetic and diamagnetic tris(oxalato)metallate anions {A+[M3+(ox)3]3−G}2−, where M is Cr, Ga; G is 2-chloropyridine, 2-bromopyridine; and A+ is [K0.8(H3O)0.2]+ have been prepared and their crystal structure and transport properties were studied. All crystals belong to the monoclinic group of the (BEDT-TTF)4A+[M3+(ox)3]G family with β″-packing type of conducting BEDT-TTF layers. In contrast to the known superconducting crystals with M3+ = Fe3+ and G = 2-chloro- or 2-bromopyridine (Tc = 4.0–4.3 K), crystals with Cr3+ and Ga3+ ions exhibit metallic properties down to 0.5 K without superconducting transition. Upon cooling these crystals, the incommensurate superstructure appears, which has never been observed before in the numerous β″-salts of the family. In addition, orthorhombic (sp. group Pbca) semiconducting crystals α″-(BEDT-TTF)5[Ga(ox)3]·3.4·H2O·0.6 EtOH (5) were obtained. It is a new compound in the family of BEDT-TTF crystals with tris(oxalato)metallate anions.

Ambient-pressure molecular superconductor with a superlattice containing layers of tris(oxalato)rhodate enantiomers and 18-crown-6.

We report a novel multilayered organic-inorganic hybrid material, β″-(BEDT-TTF)2[(H2O)(NH4)2Rh(C2O4)3]·18-crown-6. This is the first molecular superconductor to have a superlattice with layers of both BEDT-TTF and 18-crown-6 and also the first with the anion tris(oxalato)rhodate. This is the 2D superconductor with the widest gap between conducting layers, where only a single donor packing motif is observed (β″). The strong 2D nature of this system strongly suggests that the superconducting transition is a Kosterlitz-Thouless transition. A superconducting Tc of 2.7 K at ambient pressure was found by transport measurements and 2.5 K by magnetic susceptibility measurements.

A strongly polarized organic conductor

The electrically conducting salt α-(BEDT-TTF)2(PO-CON(CH3)CH2SO3)·3H2O crystallized in the polar space group, Pna21. 2D conducting BEDT-TTF layers and anion layers alternate along the crystallographic c axis. In the crystal, permanent dipoles of all anions point along the c axis, providing a net crystal dipole moment.

Efficiency Enhancement of Tandem Organic Light-Emitting Devices Fabricated Utilizing an Organic BEDT-TTF and HAT-CN Charge Generation Layer

The electrical and optical properties of tandem organic light-emitting devices (OLEDs) fabricated utilizing an organic bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) and 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) charge generation layer (CGL) were investigated to enhance their efficiency. While the operating voltage of the tandem OLEDs with a BEDT-TTF and HAT-CN CGL at 50 mA/cm2 was 11.2 V lower than that of the tandem OLEDs without a CGL, the current efficiency of the tandem OLEDs with a BEDT-TTF and a HAT-CN CGL at 50 mA/cm2 was 0.8 cd/A higher than that of the tandem OLEDs without a CGL. An increase in the current efficiency and a decrease in the operating voltage of the tandem OLEDs with a BEDT-TTF and an HAT-CN CGL were attributed to the enhancement of the electron injection due to its existence in the highest occupied molecular orbital level of the BEDT-TTF between the HAT-CN and the tris-(8-hydroxyquinoline)aluminum layer.

BEDT-TTF-based organic conductors with [MBr4]2− anions, M = Cd, Co, Zn, Mn, Hg. Effect of solvent

New layered conductors of (BEDT-TTF)4(Anion)(solvent) and (BEDT-TTF)3(Anion) stoichiometery (BEDT-TTF=bis(ethylenedithio) tetrathiafulvalene) with double-charged anions of tetrahedral geometry, [MBr4]2−, M=Cd, Co, Zn, Mn, Hg, have been synthesized using 2-halogenotoluenes, fluorobenzene and chlorocycloalkanes as solvents. The effect of solvent on composition and electroconducting properties of the compounds has been traced. Crystal structure of θ-(BEDT-TTF)4CdBr4(C7H7Br) and (BEDT-TTF)3CoBr4 has been determined. In the first compound, conducting layers show similar structure but differ in the direction of the BEDT-TTF stacks. In the second one, two crystallographically independent BEDT-TTF layers are considerably different from each other. In layer I, radical cations are linked in ribbons by side-to-side type. In layer II, BEDT-TTF forms dimers stacked with a fan-shaped offset. The ESR linewidths for (BEDT-TTF)4(MBr4)(solvent), solvent=C7H7Cl, C6H5F are close to the linewidth for θ-(BEDT-TTF)4CdBr4(C7H7Br) that allows one to assume similar structural packing motifs.

New crystal packing arrangements in radical cation salts of BEDT-TTF with [Cr(NCS)6]3− and [Cr(NCS)5(NH3)]2−

BEDT-TTF forms three packing arrangement styles in its radical cation salts with [Cr(NCS)6]3− in two of which two trans-oriented isothiocyanate ligands penetrate the BEDT-TTF layers either at the point where a solvent (nitrobenzene) is incorporated in a stack of donors or by four donor molecules forming a “tube” motif to accept a ligand at each end along with a small solvent molecule in between (acetonitrile). The [Cr(NCS)5NH3]2− ion forms a related crystal packing arrangement with BEDT-TTF with a reduction in the number of “tube” motifs needed to accept an isothiocyanate ligand.

Measurement of the Nonlinear Conducting States ofα−(BEDT−TTF)2I3Using Electronic Raman Scattering

Nonlinear conducting states in a strongly correlated organic electronic system α-(BEDT-TTF)2I3 [BEDT-TTF=bis(ethylenedithio)-tetrathiafulvalene] are studied by Raman spectroscopy. Wide-range Raman spectra of nonlinear conducing states provide direct information about conducting properties through the electronic Raman process. A comparison between the behaviors of the electronic modes of BEDT-TTF layers and the vibrational mode of I3 molecules reveals the formation of nonequilibrium states in which only the electronic parts show the change of states. We obtained a spatial map of the conducting regions of the nonlinear conducting states by utilizing the electronic Raman intensity as a measure of the highly conducting states. A spatially inhomogeneous formation of nonlinear conducting states was observed.

The first molecular superconductor based on BEDT-TTF radical cation salt with paramagnetic tris(oxalato)ruthenate anion

The first molecular superconductor based on BEDT-TTF radical cation salt with the paramagnetic tris(oxalato)ruthenate anion, β′′-(BEDT-TTF)4Kx(H3O)1−x[RuIII(ox)3]C6H5Br (x ∼ 0.8), is synthesized and its crystal and electronic structure as well as transport and magnetotransport properties are studied. The single crystals have monoclinic C2/c symmetry and β′′-packing of the conducting BEDT-TTF layer. The samples were found to be superconductors with Tc = 2.8–6.3 K (depending on the sample). Well pronounced Shubnikov–de Haas oscillations are observed in the field range 7–17 T. The Fourier spectrum of the oscillations mainly consists of two frequencies, which correspond to the Fermi surface cross sections 6.3% and 0.85% of the Brillouin zone. The first one with good accuracy corresponds to the calculated cross section 7.5%, the origin of the second one is discussed. Semiconducting orthorhombic phase ‘pseudo-κ’-(BEDT-TTF)4Kx(H3O)1−x[RuIII(ox)3]C6H5CN is also synthesized and studied.

Angle-Dependent Evolution of the Fulde-Ferrell-Larkin-Ovchinnikov State in an Organic Superconductor

We report magnetic-field and angular-dependent high-resolution specific-heat measurements of the organic superconductor β''-(BEDT-TTF)2SF5CH2CF2SO3. When the magnetic field is aligned precisely within the conducting BEDT-TTF layer, at low temperatures a clear upturn of the upper critical field beyond the Pauli limit of 9.73 T is observed, hinting at the emergence of a Fulde-Ferrell-Larkin-Ovchinnikov state. This upturn disappears when the field is oriented out of plane by more than ∼0.5 deg. For smaller out-of-plane angles, the specific-heat anomaly at T(c) sharpens and a second peaky phase transition appears within the superconducting state.

Single-Crystal-to-Single-Crystal Transformation from δ-(BEDT-TTF)4[OsNOCl5]1.33(C6H5NO2)0.67 to β"-(BEDT-TTF)3[OsNOCl5

We report on the single-crystal-to-single-crystal transformation occurring over time in a layered organic molecular conductor based on BEDT-TTF. The process is connected with removal of solvent molecules from the complex anion layer resulting in concomitant partial irreversible conversion of the δ-(BEDT-TTF)4[OsNOCl5]1.33(C6H5NO2)0.67 structure to the β"-(BEDT-TTF)3[OsNOCl5] structure. Along with symmetry lowering from I2/a to Р, huge, drastic changes in the conducting BEDT-TTF layer as well as in the anion arrangement are observed, meanwhile crystallinity of the sample is retained. Coexistence of two phases, parent δ and daughter β" in the same crystal helps in the study of their mutual orientation as well as to formulate a mechanism for the structural transformation.

Electron correlations in the quasi-two-dimensional organic conductorθ-(BEDT-TTF)2I3investigated by13C NMR

We report a ${}^{13}$C-NMR study on the ambient-pressure metallic phase of the layered organic conductor $\ensuremath{\theta}$-(BEDT-TTF)${}_{2}$I${}_{3}$ (BEDT-TTF: bisethylenedithio-tetrathiafulvalene), which is expected to connect the physics of correlated electrons and Dirac electrons under pressure. The orientation dependence of the NMR spectra shows that all BEDT-TTF molecules in the unit cell are to be seen equivalent from a microscopic point of view. This feature is consistent with the orthorhombic symmetry of the BEDT-TTF sublattice and also indicates that the monoclinic ${I}_{3}$ sublattice, which should make three molecules in the unit cell nonequivalent, is not practically influential on the electronic state in the conducting BEDT-TTF layers at ambient pressure. There is no signature of charge disproportionation in opposition to most of the $\ensuremath{\theta}$-type BEDT-TTF salts. The analyses of NMR Knight shift $K$ and the nuclear-spin-lattice relaxation rate $1/{T}_{1}$ revealed that the degree of electron correlation, evaluated by the Korringa ratio [$\ensuremath{\propto}1/({T}_{1}T{K}^{2}$)], is in an intermediate regime. However, NMR relaxation rate $1/{T}_{1}$ is enhanced above $\ensuremath{\sim}$ 200 K, which possibly indicates that the system enters into a quantum critical regime of charge-order fluctuations as suggested theoretically.

Mott-Anderson Transition in Molecular Conductors: Influence of Randomness on Strongly Correlated Electrons in the κ-(BEDT-TTF)2X System

The Mott-Anderson transition has been known as a metal-insulator (MI) transition due to both strong electron-electron interaction and randomness of the electrons. For example, the MI transition in doped semiconductors and transition metal oxides has been investigated up to now as a typical example of the Mott-Anderson transition for changing electron correlations by carrier number control in concurrence with inevitable randomness. On the other hand, molecular conductors have been known as typical strongly correlated electron systems with bandwidth controlled Mott transition. In this paper, we demonstrate our recent studies on the randomness effect of the strongly correlated electrons of the BEDT-TTF molecule based organic conductors. X-ray irradiation on the crystals introduces molecular defects in the insulating anion layer, which cause random potential modulation of the correlated electrons in the conductive BEDT-TTF layer. In combination with hydrostatic pressure, we are able to control the parameters for randomness and correlations for electrons approaching the Mott-Anderson transition.

Charge Transport in Charge-Ordered States of Two-Dimensional Organic Conductors, α-(BEDT-TTF)2I3 and α'-(BEDT-TTF)2IBr2

Electric conductance and dielectric constant have been measured in the charge-ordered states of the quasi two-dimensional organic conductors α-(BEDT-TTF) 2 I 3 and α'-(BEDT-TTF) 2 IBr 2 to understand the charge transport mechanism. The current–voltage ( I – V ) characteristics show a strong nonlinear behavior, which is qualitatively explained by the transport of individual carriers thermally excited from the charge-ordered states in BEDT-TTF layers. The discrepancy with the simulation based on the thermal activation model at low temperatures is likely due to quantum tunneling through the potential barrier. The dielectric constants show a large difference between the in-plane and out-of-plane directions of the samples, showing a highly two-dimensional nature of Coulomb potential. The temperature dependence of dielectric constant is ascribed to the polarization effect of the thermally excited electron–hole pairs, consistent with the model of nonlinear I – V characteristics. All these features are qualitativ...

Charge Distribution in the Surface BEDT-TTF Layer of α-(BEDT-TTF)2I3at Room Temperature with Scanning Tunneling Microscopy

The charge distribution in the surface BEDT-TTF layer of the α-(BEDT-TTF) 2 I 3 crystal with the charge disproportionation state at room temperature was studied in detail by a scanning tunneling microscope (STM), where BEDT-TTF (abbreviated as ET) is bis(ethylenedithio)tetrathiafulvalene. The obtained molecular charge distribution in the surface ET layer suggests that the electronic states of the surface ET layer at room temperature is the charge-ordered state, stable below 135 K in bulk crystals, rather than the charge disproportionation state above 135 K. The most probable mechanism underlying this remarkable finding is the particular freedom to the surface ET layer. The missing of the I 3 layer partially removes the structural constraint of the steric interaction between the thermally vibrating ethylene groups and the neighboring two I 3 layers. This mechanism would have something in common with the layered ET complexes.

Structural phase transition in the β′′-(BEDT-TTF)4H3O[Fe(C2O4)3]·G crystals (where G is a guest solvent molecule)

A structural phase transition from monoclinic C2/c to triclinic P symmetry has been found by X-ray diffraction in a number of single crystals of the known family of organic metals and superconductors β′′-(BEDT-TTF)4H3O[Fe(C2O4)3]·G where G stands for halogenated benzene derivatives and their mixtures with benzonitrile. The transition occurs upon lowering the temperature at 180–230 K. Comparison of the crystal and electronic structure of the monoclinic and triclinic phases reveals details of the structural transformations in the (PhCl + PhCN)-containing superconducting β′′-crystal, as an example. It is shown that the transition concerns mainly the anion layer and has a weak influence on the structure of the BEDT-TTF layer and, consequently, on the conducting properties of the single crystals.