BEDT-TTF Molecules Research Articles

Microchip-calorimetry of organic charge transfer complex which shows superconductivity at low temperatures

We carried out thermodynamic measurements of organic charge transfer complex of κ-(BEDT-TTF)2Cu[N(CN)2]Br, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene by TCG3880 chip device in order to examine capability of the chip calorimeter at low temperature region and under magnetic fields. TCG3880 chip is mounted on a 3He cryostat available in combination with a superconductive magnet up to 7T. Thermal anomalies related to the glass-like freezing of ethylene groups of BEDT-TTF molecules and the superconductive transition were observed. A frequency dependence of the thermal anomaly of the glass formation and a magnetic fields dependence of the thermal anomaly of the superconductive transition are reported. The results presented in this work demonstrate that the TCG3880 is quite useful for thermodynamic investigations of the organic charge transfer complex with much reduced sample quantity as compared with those of relaxation and adiabatic calorimetry.

Charge transport in charge-ordered layered crystalsθ−(BEDT-TTF)2MZn(SCN)4(M=Cs,Rb): Effects of long-range Coulomb interaction and the Pauli exclusion principle

We have measured the current-voltage $(I\text{\ensuremath{-}}V)$ characteristics, dielectric properties, and magnetoresistances of insulating layered organic crystals $\ensuremath{\theta}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}M\text{Zn}{(\text{SCN})}_{4}$ $(M=\text{Cs},\text{Rb})$, in which electron-electron Coulomb interactions are considered to induce charge ordering. The in-plane $I\text{\ensuremath{-}}V$ characteristics follow the power law with a large exponent that exceeds 10 in the low-temperature limit. The nonlinear $I\text{\ensuremath{-}}V$ characteristics are attributed to electric field induced unbinding of pairs of an electron and a hole that are thermally excited and attracted to each other due to two-dimensional long-range Coulomb interaction. The temperature and frequency dependences of the in-plane dielectric constant for $M=\text{Cs}$ are explained by the polarization of the electron-hole bound pairs, consistently with the $I\text{\ensuremath{-}}V$ characteristics. The large dielectric anisotropy ($\ensuremath{\approx}100$ at 0.6 K) observed for $M=\text{Cs}$ suggests two-dimensional long-range Coulomb interaction, which is also consistent with the explanation of the nonlinear $I\text{\ensuremath{-}}V$ curves. The organic crystals have a large positive magnetoresistance ratio, e.g., $\ensuremath{\approx}10\text{ }000\mathrm{%}$ for $M=\text{Cs}$ in a magnetic field of 10 T at 0.1 K. The magnetoresistance is nearly independent of the magnetic field orientation despite the highly two-dimensional charge transport, indicating that it is electron-spin related. The magnetoresistance may be caused by magnetic field induced parallel alignment of spins of mobile and localized electrons, both in the highest occupied molecular orbital of a BEDT-TTF molecule, and by the resulting suppression of conduction due to the Pauli exclusion principle.

Low temperature heat capacity measurements of the spin-liquid states of hydrogenated and deuterated κ-(BEDT-TTF) 2Cu 2(CN) 3

Heat capacity measurements of organic triangular lattice compound κ-(BEDT-TTF) 2Cu 2(CN) 3 were performed to discuss the low energy excitations from the spin-liquid ground states. Existence of the T-linear electronic coefficient with finite electronic heat capacity coefficient γ was confirmed in three different samples from different batches, although small sample dependence was observed in the absolute values of the heat capacities. Concerning the sample in which hydrogen atoms in ethylene group in BEDT-TTF molecule have been substituted by deuterons, we have observed almost similar thermodynamic behavior as the hydrogenated sample. The absence of drastic change of electronic properties of this compound is consistent with the electronic phase diagram given by Kurosaki et al. [11] (Phys. Rev. Lett. 95 (2005) 17001). The obtained data are well consistent with the previous heat capacity experiments. The existence of the γ term demonstrates that the excitations from the quantum spin-liquid states show a gapless behavior at least down to 0.7 K.

13C-NMR study of charge order state of the organic conductor [formula omitted

It is said that the quasi-two dimensional organic conductor ( BEDT-TTF ) 2 X forming two columns structure like α or θ salts, shows charge order (CO) state at low temperature. To confirm the existence of the CO state and the electron magnetism in α ′ phase whose symmetry is different from that of α and θ phases, we investigated 13C-NMR on α ′ ‐ ( BEDT-TTF ) 2 IBr 2 , in which one-sided central carbon on BEDT-TTF molecules is substituted with 13C. As a result, the change of spectrum was observed at 200 K which corresponds to CO transition and the angular dependence of spectrum at 15 K indicated that the charge ordering pattern was intra-column separating type. The spectrum in paramagnetic state can be examined by not only the difference of the charge density on on-site molecule but also the contribution from off-site molecules. This result suggested that the hyperfine coupling tensor changes after CO transition in which the charge separates extremely.

Chiral conductors from BEDT-TTF and related chiral donor molecules

BEDT-TTF salts have been widely studied because of their ability to combine various physical properties in the same lattice. The series of salts containing tris(oxalato)metallate anions has been widely studied and has given rise to paramagnetism, ferromagnetism, superconductivity, semiconductivity, conductivity and proton conductivity. However, one property that has not been exploited fully in this family of materials is chirality.

1/fnoise in the quasi-two-dimensional organic conductorκ−(BEDT-TTF)2Cu[N(CN)2]Cl

Resistance noise spectroscopy is applied to bulk single crystals of the quasi-two-dimensional organic conductor $\ensuremath{\kappa}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}\text{Cu}[\text{N}{(\text{CN})}_{2}]\text{Cl}$ both under moderate-pressure and at ambient-pressure conditions. When pressurized, the system can be shifted to the inhomogeneous coexistence region of antiferromagnetic insulating and superconducting phases, where percolation effects dominate the electronic fluctuations [J. M\uller et al., Phys. Rev. Lett. 102, 047004 (2009)]. Independent of the pressure conditions, at higher temperatures we observe generic $1/{f}^{\ensuremath{\alpha}}$-type spectra, typical for this class of quasi-two-dimensional organic charge-transfer salts. The magnitude of the electronic noise is extremely enhanced compared to typical values of homogeneous semiconductors or metals. This indicates that a highly inhomogeneous current distribution may be an intrinsic property of organic charge-transfer salts. The temperature dependence of the nearly $1/f$ spectra can be very well described by a generalized random fluctuation model [P. Dutta, P. Dimon, and P. M. Horn, Rev. Lett. 43, 646 (1979)]. We find that the number of fluctuators and/or their coupling to the electrical resistance depend on the temperature. The phenomenological model explains a pronounced peak structure in the low-frequency noise at around 100 K, which is not observed in the resistivity itself, in terms of the thermally activated conformational degrees of freedom of the BEDT-TTF molecules' ethylene endgroups.

Charge-transfer processes in radical ion molecular conductors κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x : The superconductor (x = 0.9) and the conductor with the metal-insulator transition (x = 0)

Optical spectral investigations of low-dimensional organic molecular conductors κ-(BEDT-TTF)2Cu[N(CN)2]Br x Cl1 − x with x = 0.9 (the superconductor with T c = 11.3 K) and x = 0 (the metal with the metal-insulator transition at T < 50 K) are performed in the range 50–6000 cm−1 (6 meV–0.74 eV) at temperatures from 300 to 20 K. The optical conductivity spectra are quantitatively analyzed in terms of the proposed model, according to which the charge transfer involves two types of charge carriers, i.e., electrons (holes) localized on clusters (dimers and tetramers formed by BEDT-TTF molecules) and quasi-free charge carriers, with the use of the tetramer “cluster“ model based on the Hubbard Hamiltonian for correlated electrons and the Drude model for quasi-free charge carriers. Physical parameters of the model, such as the energy of Coulomb repulsion between two electrons (holes) in one molecule, the transfer integrals between molecules inside the dimer and between dimers, and the electron-molecular vibration coupling constants, are determined. The anisotropy of the spectra in the conducting plane is explained. The inference is made that only electrons localized on clusters couple with intramolecular vibrations.

Charge Order–Disorder Phase Transition in α'-[Bis(ethylenedithio)tetrathiafulvalene]2IBr2 [α'-(BEDT-TTF)2IBr2

We present the anomalies at ∼200 K in electrical resistivity, transmittance and reflectance in the far-infrared region, and the magnetic susceptibility of α'-(BEDT-TTF) 2 IBr 2 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene]. This phase transition is investigated by infrared, Raman, and far-infrared spectroscopies. Through the spectroscopic investigation, we found that the positive charges on BEDT-TTF molecules are strongly localized owing to a strong Coulomb interaction in both low-temperature (LT) and high-temperature (HT) phases. We show the spectroscopic data that suggest the charge order in the LT phase and the charge disorder in the HT phase. The phase transition at ∼200 K in α'-(BEDT-TTF) 2 IBr 2 is the charge order–disorder phase transition not accompanied by structural changes. On the basis of the localized picture, we qualitatively interpreted the experimental results above and below the phase transition. The effect of hydrostatic pressure was investigated by electrical resistivity measuremen...

Evidence for charge order in organic superconductors obtained by vibrational spectroscopy

We study charge disproportionation in few quasi-two-dimensional BEDT-TTF-based compounds by following the temperature dependence of a charge-sensitive vibration ν27(B1u) of BEDT-TTF molecule. While in a charge ordered insulator θ-(BEDT-TTF)2RbZn(SCN)4 a difference between charge on the lattice sites is as high as 0.6e, a small charge disproportionation of 0.15–0.2e is found in two metallic compounds that become superconducting at low temperatures β″-(BEDT-TTF)2SF5CH2CF2SO3 and β-(EDT-TTF)4[Hg3I8](1-x). In contrast to these, a pure metallic β″-(BEDT-TTF)2SO3CHFSF5 does not show any presence of charge disproportionation. This study suggests a correlation between a slight charge disproportionation in the metallic state and superconductivity.

New BEDT-TTF/[Fe(C5O5)3]3-Hybrid System: Synthesis, Crystal Structure, and Physical Properties of a Chirality-Induced α Phase and a Novel Magnetic Molecular Metal

The paramagnetic and chiral anion [Fe(C5O5)3]3- (C5O52-=croconate) has been combined with the organic donor BEDT-TTF (=ET=bis(ethylenedithio)tetrathiafulvalene) to synthesize a novel paramagnetic semiconductor with the first chirality-induced alpha phase, alpha-(BEDT-TTF)5[Fe(C5O5)3].5H2O (1), and one of the few known paramagnetic molecular metals, beta-(BEDT-TTF)5[Fe(C5O5)3].C6H5CN (2). Both compounds present layers of BEDT-TTF molecules, with the alpha or beta packing modes, alternating with layers containing the high-spin S=5/2 Fe(III) anions and solvent molecules. In the alpha phase, the alternation of the chiral [Fe(C5O5)3]3- anions along the direction perpendicular to the BEDT-TTF chains induces an alternation of the tilt angle of the BEDT-TTF molecules, giving rise to the observed alpha phase. The alpha phase presents a semiconductor behavior with a high room-temperature conductivity (6 S.cm-1) and an activation energy of 116 meV. The beta phase presents a metallic behavior down to ca. 120 K, where a charge localization takes place with a reentrance to the metallic state below ca. 20 K followed by a metal-semiconductor transition at ca. 10 K. The magnetic properties are dominated by the paramagnetic S=5/2 [Fe(C5O5)3]3- anion with an extra Pauli-type paramagnetism in the metallic beta phase. The ESR spectra confirm the presence of the high-spin Fe(III)-containing anion and show a progressive localization in the organic sublattice along with an antiferromagnetic coupling below ca. 120 K that, in the metallic beta phase, could be at the origin of the transition from the metallic to the activated conductivity regime. The correlation between crystal structure and conductivity behavior has been studied by means of tight-binding band structure calculations which provide a rationalization of the charge distribution and conductivity results.

Determining ethylene group disorder levels inκ−(BEDT−TTF)2Cu[N(CN)2]Br

We present a detailed structural investigation of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br at temperatures $T$ from 9 to 300 K. Anomalies in the $T$ dependence of the lattice parameters are associated with a glass-like transition previously reported at $T_g$ = 77 K. From structure refinements at 9, 100 and 300 K, the orthorhombic crystalline symmetry, space group {\it Pnma}, is established at all temperatures. Further, we extract the $T$ dependence of the occupation factor of the eclipsed conformation of the terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2) %, with an increase to 97(3) % at 9 K. We conclude that the glass-like transition is not primarily caused by configurational freezing-out of the ethylene groups.

EPR Study of the Electronic States of β’-(BEDT-TTF)(TCNQ)

β′-(BEDT-TTF)(TCNQ) is a compound of BEDT-TTF (=ET) and TCNQ molecules aligned orthogonally with each other, forming two-dimensional sheets and one-dimensional columns of 1/4-filled π band, respectively. It is known that the metal-insulator transition occurs at 330 K at ambient pressure. We have measured the electronic spin susceptibility by means of the EPR-NMR method at 50 MHz, and the angular dependence of g-factor and line width of EPR both at Q (34 GHz) and W (94 GHz) band. We successfully confirmed that the antiferromagnetic transition occurs in ET sheets and TCNQ columns, independently.

Electrical Inhomogeneity at the Mott Transition in the Band Width Controlled κ-(BEDT-TTF)2Cu[N(CN)2Br

The electronic phase separation on macroscopic scale is studied in the organic Mott system κ-(BEDT-TTF)2Cu[N(CN)2]Br by means of scanning micro-region infrared spectroscopy using the synchrotron radiation. The phase separation appears in the vicinity of the Mott boundary in the single crystals of which the band width is controlled by partly substituting the BEDT-TTF molecule with the deuterated one. The transport properties under the phase separation are considered to be influenced by the percolation process of the domains.

Pressure dependence of Shubnikov–de Haas oscillation spectra in the quasi-two-dimensional organic metalβ″(BEDT−TTF)4(NH4)[Fe(C2O4)3]∙DMF

The pressure dependence of the interlayer magnetoresistance of the quasi-two dimensional organic metal ${\ensuremath{\beta}}^{\ensuremath{''}}{\ensuremath{-}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{4}(\mathrm{N}{\mathrm{H}}_{4})[\mathrm{Fe}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}]∙\mathrm{DMF}$ [where BEDT-TTF represents bis(ethylenedithio)tetrathiofulvalene and DMF is dimethylformamide] has been investigated up to $1\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in pulsed magnetic fields up to $55\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The Shubnikov--de Haas oscillation spectra can be interpreted on the basis of three compensated orbits in the whole pressure range studied, suggesting that the Fermi surface topology remains qualitatively the same as the applied pressure varies. In addition, all the observed frequencies, normalized to their value at ambient pressure, exhibit the same sizable pressure dependence. Despite this behavior, which is at variance with that of numerous charge-transfer salts based on the BEDT-TTF molecule, nonmonotonic pressure-induced variations of parameters such as the scattering rate linked to the various detected orbits are observed.

Disorder effect on the superconductivity of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2partly substituted by the deuterated molecules

The effect of the disorder induced by the deuterated BEDT-TTF (d-BEDTTTF) molecule substitution on the superconductivity has been investigated in the quasi two dimensional organic superconductor κ-[(h-BEDT-TTF)1−x(d-BEDT-TTF)x]2Cu(NCS)2. The scattering time τ obtained by the de Haas-van Alphen effect changes in the form of 1/τ ∝ x(1 − x), which is known as Nordheim relation. Although the scattering time takes minimum around x = 0.5, the superconducting transition temperature Tc increases linearly with x. The relation between the scattering time and Tcis not simply explained by the nonmagnetic impurity effect in the unconventional superconductivity expected in this organic superconductor.

Pressure-Induced Zero-Gap Semiconducting State in Organic Conductor α-(BEDT-TTF)2I3Salt

We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 salt, which emerges under uniaxial pressure along the a -axis (the stacking axis of the BEDT-TTF molecule). The ZGS state is the state in which a Dirac cone with the band spectrum of a linear dispersion exists around the Fermi point connecting an unoccupied (electron) band with an occupied (hole) band. The spectrum exhibits a large anisotropy in velocity, which depends on the direction from the Fermi point. By varying the magnitude of several transfer energies of a tight-binding model with four sites per unit cell, it is shown that the ZGS state exists in a wide pressure range, and is attributable to the large anisotropy of the transfer energies along the stacking axis.

Two-Band Fluctuation Exchange Study on Superconductivity of β'-(BEDT-TTF)2ICl2under High Pressure

We study the pressure dependence of the superconducting transition temperature of an organic superconductor $\beta'$-(BEDT-TTF)$_2$ICl$_2$ by applying the fluctuation exchange method to the Hubbard model on the original two-band lattice at 3/4-filling rather than the single band model in the strong dimerization limit. Our study is motivated by the fact that hopping parameters evaluated from a first-principles study suggest that the dimerization of the BEDT-TTF molecules is not so strong especially at high pressure. Solving the linearized Eliashberg's equation, a d$_{xy}$-wave-like superconducting state with realistic values of $T_c$ is obtained in a pressure regime somewhat higher than the actual experimental result. These results are similar to those obtained within the single band model in the previous study by Kino {\it et al}. We conclude that the resemblance to the dimer limit is due to a combination of a good Fermi surface nesting, a large density of states near the Fermi level, and a moderate dimerization, which cooperatively enhance electron correlation effects and also the superconducting $T_c$.

Localization of conduction electrons and the magnetic properties of the molecular metals β″-(BEDT-TTF)4NH4[M(C2O4)3] · DMF (M = Cr3+, Fe3+)

Quasi-two-dimensional organic metals β″-(BEDT-TTF)4NH4[M(C2O4)3] · DMF containing the oxalate complexes of Cr3+ or Fe3+ ions between the conducting organic layers of the BEDT-TTF molecules are studied by EPR spectroscopy, and the contributions of these metallic complexes, conduction electrons, and non-equilibrium lattice defects to the magnetic susceptibility are determined. An analysis of the temperature dependence of the magnetic susceptibility and the EPR line shape has revealed partial localization of conduction electrons at T < 20 K in the crystals with Cr3+ ions. The size of the localization region is close to the size of an individual BEDT-TTF molecule. The localization effect weakens as nonequilibrium defects disappear during long-term storage at room temperature. The localization of conduction electrons is found to be accompanied by the appearance of weak antiferromagnetic interaction between the Cr3+ ions at T < 20 K, which disappears when Fe3+ ions substitute for Cr3+ ions.

μSR study of an antiferromagnetic Mott insulator β′-(BEDT-TTF) 2ICl 2

Zero-field μSR measurements on the antiferromagnetic Mott insulator β′-(BEDT-TTF) 2ICl 2 were performed in order to study the microscopic magnetic properties of this organics. The volume fraction of the antiferromagnetic phase was found to be temperature independent. Muon precession frequency, which is proportional to the magnitude of the magnetic moment at the dimer of two BEDT-TTF molecules, does not follow the simple model based on the mean-field approximation.

Electrical Conduction in Divalent BEDT-TTF Salts

Transport and ESR measurements have been performed on organic semiconductors synthesized by the diffusion method. For (BEDT-TTF)Cu 2Br 4, a pressure of 1 GPa raises the conductivity by up to one order of magnitude, but does not alter the semiconducting nature. By the ESR measurements, we find no Cu 2+ signals, indicating that the BEDT-TTF molecules are divalent. The Hall carrier concentration increases exponentially with temperature, approaching the spin concentration evaluated by the low-temperature ESR from π-electrons. Thermopower measurements also support the contribution of the thermally activated electronic carriers. For (BEDTTTF) 2Cu 3Br 7(H 2O), the conductivity increases twice by applying a pressure of 0.8 GPa.