Structure and electronic properties of BEDT-TTF and it's charge transfer salts

We report a detailed computational study, of the organic superconducting charge transfer BEDT-TTF salts, whose electronic properties depend strongly on the packing of the donor radical cations. Electronic structure calculations have been performed at the molecular and periodic level using density functional theory (DFT). These calculations have been used to investigate the magnetic ordering in the BEDT-TTF[FeBr₄] salt, where we obtained an antiferromagnetic solution in agreement with experiment. Detailed geometry optimisations have been carried out for both BEDT-TTF[FeBr₄] salt and the neutral BEDT-TTF crystal.     

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)

Physics of the Solid State - 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...     

Photoinduced transition of [Fe(CN)5NO] nitroprusside anions to metastable states and electron localization in the (BEDT-TTF)4K[Fe(CN)5NO]2 molecular metal

The crystals of (BEDT-TTF)₄K[Fe(CN)₅NO]₂, representing a quasi-two-dimensional organic metal with conducting layers of bis(ethylenedithio)tetrathiofulvalene (BEDT-TTF) and nonconducting layers containing pho-tochromic nitroprusside anions [Fe(CN)₅NO]^2?, were studied by the method of electron spin resonance. Illuminated by light with a wavelength of 514.5 nm, the organic metal crystals feature the formation of localized paramagnetic centers in the conducting cation layers of BEDT-TTF. The phenomenon of electron localization in the BEDT-TTF layers is related to the light-induced formation of long-lived metastable states of nitroprusside anions.     

Femtosecond mid-IR pump-probe spectroscopy of photoinduced insulator to metal transition in dimer Mott insulator κ-(BEDT-TTF)2X

Ultrafast dynamics of the photoinduced insulator (I) to metal (M) transition were investigated using femtosecond mid-infrared pump-probe spectroscopy in two-dimensional organic Mott insulators [bis (ethylenedithio)]-tetrathiafulvalene (BEDT-TTF) salts κ-(BEDT-TTF)₂X (κ-(ET)₂X, where X denotes anion). In κ-(d-ET)₂Cu[N(CN)₂]Br, a metallic state was photogenerated using a phonon-mediated mechanism: the effective bandwidth increases through the photoinduced molecular rearrangement. The mechanism differs fundamentally from the previously reported photoinduced filling control in one-dimensional Mott insulators.     

Critical behavior of a filling-controlled Mott-transition observed at an organic field-effect-transistor interface

An organic Mott-insulator κ-(BEDT-TTF)₂Cu[N(CN)₂] Cl(κ-Cl) on SiO₂/Si substrate showed an ambipolar field-effect-transistor (FET) characteristics without any hysteresis, which means a continuous Mott-transition at filling-controlled regime (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene). In order to extract the critical exponent in the vicinity of the Mott-insulating phase, an analysis based on Efros-Shklovskii approximation is performed. The model fitting well reproduces the device characteristics over wide range of temperature and gate voltage. In this analysis, Coulomb gap temperature that characterizes the ground state of doped Mott-insulator shows inversely proportional critical behavior against the doping concentration.     

Organic superconductors revisited

The surfaces of a ten years aged crystal and a freshly prepared κ-(BEDT-TTF)₂Cu(NCS)₂ crystal were compared by scanning tunneling microscopy (STM). The molecularly-resolved STM images of the bc plane of the crystals agree with each other and with the electronic contrast obtained by new density functional theory (DFT) based simulations. Even after ten years STM images of the molecular stacking of BEDT-TTF display a variation in brightness at the positions of different molecules. We attribute this symmetry breaking concerning the brightness in the STM images of the otherwise equivalent BEDT-TTF dimers to the electronic states of a relaxed surface.     

Molecular designing analysis of a new superconducting metal dithiolene complex

The map of the intermolecular overlap integrals(S) of the lowest unoccupied molecular orbital(LUMO) of Ni(dmit)₂ was calculated by varying the intermolecular configuration parameters in order to examine the dimensionality of the electronic band structure of the molecular conductors based on Ni(dmit)₂. Despite the similarity of the molecular structure between Ni(dmit)₂ and BEDT-TTF, the intermolecular transverse interaction of Ni(dmit)₂ becomes very weak. Although the crystal structure of a new molecular superconductor (TTF)[Ni(dmit)₂]₂ exhibits the three-dimensional (3-D) sulfur network, its tight-binding band calculation gives a strongly 1-D band.     

Proximity of pseudogapped superconductor and commensurate antiferromagnet in a quasi-two-dimensional organic system

We performed the single-crystal 13C NMR studies on a quasi-two-dimensional system, deuterated kappa-(BEDT-TTF)2Cu[N(CN)(2)]Br, which is just on the border of the Mott transition. The NMR spectra are separated into two parts coming from the metallic (superconducting) and insulating phases due to the phase separation at low temperature. The examination of the separated spectra revealed that the Mott transition in this system is characterized by the first-order transition between the pseudogapped superconductor and the simplest commensurate antiferromagnet with a moment of 0.26 mu(B)/dimer.     

Charge fluctuation,charge ordering,and zero-gap state in organic conductors

We have carried out a series of measurements of angular dependence of solid-state NMR spectrum using single crystal samples on various organic molecular conductors, in order to investigate the natures of the electronic states at low temperatures. We confirmed a charge ordered insulating state in α-(BEDT-TTF)₂I₃ and large charge disproportionation in the metallic state of this salt. In another charge ordered system, θ-(BEDT-TTF)₂RbZn(SCN)₄, we observed unusual NMR line broadening, proportional to resonance shift, in the metallic state above the transition. We found that this broadening is due to charge disproportionation, or more correctly, due to the inhomogeneity of local susceptibility at nuclear sites and analyzed its dynamics. We observed similar broadening in various organic molecular conductors as well, such as θ-(BEDT-TTF)₂CsZn(SCN)₄, an exotic Bechgaad salt, (TMTSF)₂FSO₃, and λ-type BETS salts, λ-(BETS)₂(Fe,Ga)Cl₄. We found the mechanism of CD in each system is different, respectively.     

Magnetic-field-induced Mott transition in a quasi-two-dimensional organic conductor

We investigated the effect of magnetic field on the highly correlated metal near the Mott transition in the quasi-two-dimensional layered organic conductor, kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl, by the resistance measurements under control of temperature, pressure, and magnetic field. It was demonstrated that the marginal metallic phase near the Mott transition is susceptible to the field-induced localization transition of the first order, as was predicted theoretically. The thermodynamic consideration of the present results gives a conceptual pressure-field phase diagram of the Mott transition at low temperatures.     

Temperature Dependence of the Critical Field of the Organic Superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

JETP Letters - We present upper critical magnetic fields data for κ(BEDT-TTF)2Cu[N(CN)2]Br (hereafter k-Br), single crystals with the magnetic field applied either or perpendicular and...     

Metallic pattern fabrication in organic Mott insulating crystal by local X-ray irradiation

We have fabricated a metallic structure in an organic Mott insulator κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. The periodic metallic domains of approximately 90×90 μm² obtained by X-ray irradiation through a molybdenum mesh mask are visualized by scanning microregion infrared reflectance spectroscopy technique. No deterioration by irradiation is found in a range of nanometer to micrometer scales. We demonstrate that the present processing method is applicable for the fabrication of molecular electronic devices.     

Knight shift tensors and π-spin densities in the organic metals αt-(BEDT-TTF)2I3 and (BEDT-TTF)2Cu(NCS)2

¹³C-MASS spectra of pure BEDT-TTF and of the organic metals α_t-(BEDT-TTF)₂I₃ and (BEDT-TTF)₂Cu(NCS)₂ were recorded atν _L = 68 MHz. Isotropic shifts and the principal components of the shift tensors were determined, respectively, from the center and spinning side bands. For pure BEDT-TTF which is a diamagnetic insolator, the measured shifts arechemical shifts while for the organic metals they are the sum of chemical and Knight shifts. In each of the compounds the shifts are assigned ingroups to theinner, middle andouter carbons of the BEDT-TTF molecule. For the organic metals the separation of the experimental shifts into chemical and Knight shifts is discussed. From the anisotropic part of the Knight shift tensors the π-spin densities at the carbon and sulphur positions of the BEDT-TTF molecule are inferred. The result is that the π-spin density of the unpaired hole is concentrated on the center part of the BEDT-TTF molecule, i.e., on the inner and middle carbons, and on the inner sulphurs. It is argued that the current density is concentrated on this part of the BEDT-TTF molecule as well.     

Early-stage dynamics of light-matter interaction leading to the insulator-to-metal transition in a charge ordered organic crystal

Ultrafast dynamics of the light-matter interaction in a charge-ordered molecular insulator α-(BEDT-TTF)2I3 were studied by pump-probe spectroscopy using few-optical-cycle infrared pulses (pulse width 12 fs). Coherent oscillation of the correlated electrons and subsequent Fano destructive interference with intramolecular vibration were observed in time domain; the results indicated a crucial role for electron-electron interplay in the light-matter interaction leading to the photoinduced insulator-to-metal transition. The qualitative features of this correlated electron motion were reproduced by calculations based on exact many-electron-phonon wave functions.     

Vibrational spectra of two BEDT-TTF-based organic conductors: charge order

Infrared and Raman investigations of two phases of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) based organic conductors with the same CF₃CF₂SO₃⁻ anion: β′-(BEDT-TTF)₂CF₃CF₂SO₃ and δ′-(BEDT-TTF)₂CF₃CF₂SO₃, are shortly reviewed and compared with the most typical infrared properties of the family of (BEDT-TTF)₂RR′SO₃ organic conductors, where R = SF₅, CF₃, and R′ are CH₂, CF₂, CHF, CHFCF₂, and CH₂CF₂. The role of the molecular structur and spatial organization of the counterions is discussed. Presented at 2-nd International Conference on Functional Materials and Devices, ICFMD 2008, June 16–19, 2008, Kuala Lumpur, Malaysia     

Ambient pressure superconductivity at 4–5 K in β-(BEDT-TTF)2AuI2

Crystals of β-(BEDT-TTF)₂AuI₂, derived from the sulfur-based organic donor bis(ethylenedithio) tetrathiafulvalene [BEDT-TTF or “ET”] have been synthesized by electrocrystallization and studied by rf penetration depth measurements at ambient pressure and at temperatures down to 0.45 K. The crystals were found to be superconducting at ambient pressure with T_c = 3.93–4.98 K, which represent the highest values of T_c thus far observed at ambient pressure for an organic superconductor. Measurements of the upper critical field anisotropy are reported.     

Structural and electronic properties of a new molecular conductor, α-(BEDT-TTF)2CsCd(SCN)4

Structural and electronic properties of a new BEDT-TTF based radical cation salt, α-(BEDT-TTF)₂CsCd(SCN)₄, are presented. In the measurements of the electrical resistivity and the magnetic susceptibility, this new α-type salt shows metallic behavior down to low temperature. Measurements of the resistivity under in-plane c-axial strain reveal a newly observed insulating phase, suggesting that α-(BEDT-TTF)₂CsCd(SCN)₄ is placed near an insulating phase which might have close relationships with a superconducting phase realized in α-type salts.     

Half-filled layered organic superconductors and the resonating-valence-bond theory of the hubbard-heisenberg model

We present a resonating-valence-bond theory of superconductivity for the Hubbard-Heisenberg model on an anisotropic triangular lattice. Our calculations are consistent with the observed phase diagram of the half-filled layered organic superconductors, such as the beta, beta', kappa, and lambda phases of (BEDT-TTF)2X [bis(ethylenedithio)tetrathiafulvalene] and (BETS)2X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order transition from a Mott insulator to a dx2-y2 superconductor with a small superfluid stiffness and a pseudogap with dx2-y2 symmetry.     

Anomalous lattice response at the Mott transition in a quasi-2D organic conductor

Discontinuous changes of the lattice parameters at the Mott metal-insulator transition are detected by high-resolution dilatometry on deuterated crystals of the layered organic conductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br. The uniaxial expansivities uncover a striking and unexpected anisotropy, notably a zero effect along the in-plane c axis along which the electronic interactions are relatively strong. A huge thermal expansion anomaly is observed near the end point of the first-order transition line enabling us to explore the critical behavior with very high sensitivity. The analysis yields critical fluctuations with an exponent alpha approximately 0.8+/-0.15 at odds with the novel criticality recently proposed for these materials [Kagawa et al., Nature (London) 436, 534 (2005)]. Our data suggest an intricate role of the lattice degrees of freedom in the Mott transition for the present materials.     

Band structure of β-(BEDT-TTF) 2PF6. One-dimensional metal along the side-by-side molecular array

The energy-band structures of isostructural organic conductors, β-(BEDT-TTF) ₂PF₆ and (BEDT-TTF) ₂AsF₆ are calculated (BEDT-TTF: bis (ethylenedithio) tetrathiafulvalene). The strong dimerization of the face-to-face array makes the Fermi surface open perpendicularly to the side-by-side array. This dimer model indicates that the phase transition around room temperature is associated with a 2k_F charge density wave.