Search Results (17)

The metal–insulator transition induced by the gate electric field in the charge order phase of the $\alpha$-(BEDT-TTF)${}_2$I${}_3$ single-crystal field-effect transistor (FET) structure was clearly observed near the phase transition temperature. An abrupt increase in the electrical conductance induced by the applied gate electric field was evident, which corresponds to the partial dissolution of the charge order phase triggered by the gate electric field. The estimated nominal dissolved charge order region (i.e., the gate-induced metallic region) was overestimated in 130–150 K, suggesting additional effects such as Joule heating. On the other hand, in the lower temperature region below 120 K, the corresponding dissolved charge order was several monolayers of BEDT-TTF, suggesting that it is possible to dissolve the charge order phase within the bistable temperature region. Full article

We have detected a large Nernst effect in the charge density wave state of the multiband organic metal α-(BEDT-TTF)₂KHg(SCN)₄. We find that apart from the phonon drag effect, the energy relaxation processes that govern the electron–phonon interactions and the momentum relaxation processes that determine the mobility of the q1D charge carriers have a significant role in observing the large Nernst signal in the CDW state in this organic metal. The emphasised momentum relaxation dynamics in the low field CDW state (${\mathrm{CDW}}_0$) is a clear indicator of the presence of a significant carrier mobility that might be the main source for observation of the largest Nernst signal. The momentum relaxation is absent with increasing angle and magnetic field, i.e., in the high-field CDW state (${\mathrm{CDW}}_{\mathrm{x}}$) as evident from the much smaller Nernst effect amplitude in this state. In this case, only the phonon drag effect and electron–phonon interactions are contributing to the transverse thermoelectric signal. Our findings advance and change previous observations on the complex properties of this organic metal. Full article

A technique to establish electrical contact and perform multi-probe electrical measurements (e.g., four-probe measurements), even at low temperatures, is presented in this work. The natural adhesion contact (NAC) is applicable to the wide range of dimensions of organic crystals. Furthermore, the precise electrode patterns required to carry out multi-probe measurements are guaranteed, in contrast to fine conductive paste painting methods. We demonstrate four-probe electrical measurements of $\alpha$-(BEDT-TTF)${}_2$I${}_3$ (where BEDT-TTF = bis(ethylenedithio) tetrathiafulvalene) at temperatures down to 100 K. The obtained temperature dependence showed a steep meta l–insulator transition and exhibited zero hysteresis throughout several measurement sequences. Full article

We theoretically study the structural and electronic properties of a molecular conductor, $\alpha$-(BEDT-TTF)${}_2$I${}_3$, using first-principles density-functional theory calculations, especially in its low-temperature charge-ordered state at ambient pressure. We apply a hybrid functional approach and compare the results with a conventional exchange-correlation functional within the generalized gradient approximation. By performing structural optimization, we found a stable charge-ordered solution for the former, in contrast to the latter approach where the magnitude of the charge imbalance becomes considerably small compared to that when the experimental structure is adopted. The electronic band structure near the Fermi level, with and without structural optimization, as well as the molecule-dependent local density of states of the charge-ordered state are discussed. Full article

Field-effect transistors based on strongly correlated insulators are an excellent platform for studying the electronic phase transition and simultaneously developing phase transition transistors. Molecular conductors are suitable for phase transition transistors owing to the high tunability of the electronic states. Molecular Mott transistors show field-induced phase transitions including superconducting transitions. However, their application to charge-ordered insulators is limited. In this study, we fabricated electric double layer transistors based on quarter-filled charge-ordered insulators α-(BEDT-TTF)₂I₃ and α-(BETS)₂I₃. We observed ambipolar field effects in both compounds where both electron and hole doping (up to the order of ${10}^{13} {\mathrm{cm}}^{-2}$) reduces the resistance by the band filling shift from the commensurate value. The maximum field-effect mobilities are approximately 10 and 55 ${\mathrm{cm}}^2/\mathrm{Vs}$, and the gate-induced conductivities are 0.96 and $3.6 e^2/h$ in α-(BEDT-TTF)₂I₃ and α-(BETS)₂I₃, respectively. However, gate-induced metallic conduction does not emerge. The gate voltage dependence of the activation energy in α-(BEDT-TTF)₂I₃ and the Hall resistance in α-(BETS)₂I₃ imply that the electric double layer doping in the present experimental setup induces hopping transport rather than band-like two-dimensional transport. Full article

New bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)-, bis(ethylenediseleno)tetrathiafulvalene (BEST)- and bis(ethylenedithio)tetraselenafulvalene (BETS)-based organic charge-transfer (CT) salts—α-(BEDT-TTF)₃(HOC₂H₄SO₃)₂ (1), β-(BEST)₃(HOC₂H₄SO₃)₂·H₂O (2) and α-(BETS)₂(HOC₂H₄SO₃)·H₂O (3)—have been prepared. Salts 1 and 2 show semiconducting behaviour. Salt 3, which is almost isostructural to α-(BETS)₂I₃, shows metallic behaviour down to 70 K and then shows a broader metal–insulator transition than that of α-(BETS)₂I₃. The reason for the difference in behaviour is estimated by the comparison of the Madelung energies of the full set of patterns of possible donor’s charge-ordered and anion’s disordered states. Full article

The effect of the Coulomb interaction in interlayer magnetoresistance is elucidated in collaboration with theory and experiments for the Dirac electron system in organic conductor $\alpha$ -(BEDT-TTF) ${}_2$I${}_3$ under a strong magnetic field. It is found that the effective g-factor enhanced by Coulomb interaction depends on the angle of the magnetic field, resulting in the field-angle dependence of a characteristic magnetic field in which interlayer resistance has a minimum due to spin splitting $N=0$ Landau levels. The qualitative agreement between the theory and experimental results for the field-angle dependence of interlayer magnetoresistance is obtained. Full article

The optical conductivity in the charge order phase is calculated in the two-dimensional extended Hubbard model describing an organic Dirac electron system $\alpha$ -(BEDT-TTF) ${}_2$ I ${}_3$ using the mean field theory and the Nakano-Kubo formula. Because the interband excitation is characteristic in a two-dimensional Dirac electron system, a peak structure is found above the charge order gap. It is shown that the peak structure originates from the Van Hove singularities of the conduction and valence bands, where those singularities are located at a saddle point between two Dirac cones in momentum space. The frequency of the peak structure exhibits drastic change in the vicinity of the charge order transition. Full article

A zero-gap state with a Dirac cone type energy dispersion was discovered in an organic conductor α-(BEDT-TTF)₂I₃ under high hydrostatic pressures. This is the first two-dimensional (2D) zero-gap state discovered in bulk crystals with a layered structure. Moreover, the Dirac cones are highly tilted in a k-space. This system, thus, provides a testing ground for the investigation of physical phenomena in the multilayered, massless Dirac electron system with anisotropic Fermi velocity. Recently, the carrier injection into this system has been succeeded. Thus, the investigations in this system have expanded. The recent developments are remarkable. This effect exhibits peculiar (quantum) transport phenomena characteristic of electrons on the Dirac cone type energy structure. Full article

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 [K_0.8(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 (T_c = 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. Full article

This paper reviews charge ordering in the organic conductors, β″-(BEDT-TTF) (TCNQ), θ-(BEDT-TTF)₂X, and α-(BEDT-TTF)₂X. Here, BEDT-TTF and TCNQ represent bis(ethylenedithio)tetrathiafulvalene and 7,7,8,8-tetracyanoquinodimethane, respectively. These compounds, all of which have a quarter-filled band, were evaluated using infrared and Raman spectroscopy in addition to optical conductivity measurements. It was found that β″-(BEDT-TTF)(TCNQ) changes continuously from a uniform metal to a charge-ordered metal with increasing temperature. Although charge disproportionation was clearly observed, long-range charge order is not realized. Among six θ-type salts, four compounds with a narrow band show the metal-insulator transition. However, they maintain a large amplitude of charge order (Δρ~0.6) in both metallic and insulating phases. In the X = CsZn(SCN)₄ salt with intermediate bandwidth, the amplitude of charge order is very small (Δρ < 0.07) over the whole temperature range. However, fluctuation of charge order is indicated in the Raman spectrum and optical conductivity. No indication of the fluctuation of charge order is found in the wide band X = I₃ salt. In α-(BEDT-TTF)₂I₃ the amplitude of charge order changes discontinuously from small amplitude at high temperature to large amplitude (Δρ_max~0.6) at low temperature. The long-range charge-ordered state shows ferroelectric polarization with fast optical response. The fluctuation of multiple stripes occurs in the high-temperature metallic phase. Among α-(BEDT-TTF)₂MHg(SCN)₄ (X = NH₄, K, Rb, Tl), the fluctuation of charge order is indicated only in the X = NH₄ salt. α′-(BEDT-TTF)₂IBr₂ shows successive phase transitions to the ferroelectric state keeping a large amplitude of charge order (Δρ_max~0.8) over the whole temperature range. It was found that the amplitude and fluctuation of charge order in these compounds is enhanced as the kinetic energy (bandwidth) decreases. Full article

A zero-gap state with a Dirac cone type energy dispersion was discovered in an organic conductor α-(BEDT-TTF)₂I₃ under high hydrostatic pressures. This is the first two-dimensional (2D) zero-gap state discovered in bulk crystals with a layered structure. In contrast to the case of graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermion system with anisotropic Fermi velocity. This system exhibits remarkable transport phenomena characteristic to electrons on the Dirac cone type energy structure. Full article

Up to five different crystalline radical salts have been prepared with the organic donor BEDT-TTF and three different polynitrile anions. With the polynitrile dianion tcpd²⁻ (=C[C(CN)₂]₃²⁻), two closely related radical salts: α'-(ET)₄tcpd·THF (1) (THF = tetrahydrofurane) and α'-(ET)₄tcpd·H₂O (2) have been prepared, depending on the solvent used in the synthesis. With the mono-anion tcnoetOH⁻ (=[(NC)₂CC(OCH₂CH₂OH)C(CN)₂]⁻) two polymorphs with similar physical properties but different crystal packings have been synthesized: θ-(ET)₂(tcnoetOH) (3) and β''-(ET)₂(tcnoetOH) (4). Finally, with the mono-anion tcnoprOH⁻ (=[(NC)₂CC(OCH₂CH₂CH₂OH)C(CN)₂]⁻) we have prepared a metallic radical salt: β''-(ET)₂(tcnoprOH)(CH₂Cl₂CH₃Cl)_0.5 (5). Salts 1‑4 are semiconductors with high room temperature conductivities and activation energies in the range 0.1–0.5 eV, whereas salt 5 is metallic down to 0.4 K although it does not show any superconducting transition above this temperature. Full article

A new mixed-anion crystal composed of BEDT-TTF radical cation salt [BEDT-TTF]₂(CuBr₂)_0.4(CuCl₂)_0.6 with an α'-type donor arrangement with a formal charge of +0.5 per BEDT-TTF was prepared by using a chemical oxidation method and characterized by using X-ray diffraction, four-probe electrical resistivity measurements (semiconductor: ρ_rt = 2 × 10² Ω cm, E_a = 0.2 eV), and energy band calculations. The results showed that this system had a quasi-one dimensional Fermi surface. Full article