Layered Organic Conductors Research Articles

Magnetic oscillation spectrum of surface resistance in layered organic compounds

The magnetic quantum oscillations of surface resistance in quasi-two dimensional layered organic conductors have been obtained numerically for a magnetic field parallel to the surface. The resonance fields, which were theoretically calculated in a previous work on surface states, are found to coincide closely to the positions of the maxima in the ∂R/∂B curves. We find that, in quasi-two dimensional organic conductors, the transitions between the adjacent surface states are the most present transitions in the sum curve for surface resistance derivative oscillation spectrum. Our results, obtained from the calculations of the oscillation spectra of the individual series and the sum of six series, confirm that the theoretical description of surface states in the anisotropic organic conductors is essentially correct in its numerical aspects. These studies will be very helpful for analyzing and explaining the experimental curves and for their comparison with the theoretical results. Since there are yet no experimental data on surface impedance oscillations in quasi-two dimensional organic conductors with this work we would like to shed new light on this problem in order to motive new research in that direction mostly because of a need of such studies for a possible utilization of surface effects in the existing organic devices.

Near-surface thermal generation of longitudinal bulk acoustic waves in organic conductors

The linear thermal generation of near-surface longitudinal bulk acoustic waves (NSLBAWs) in organic conductors with layered structure is considered. These are waves that are generated and propagate at a distance of order or less than the thermal source attenuation length, i.e., are confined to the thermal skin layer of the conductor. The Green’s functions approach is applied to obtain the integral form for the solutions of the thermal conduction equation and wave equation. Analytical expressions for the temperature distribution and the wave amplitude are obtained in case when a current is applied along the least conducting direction of the quasi-two-dimensional organic conductor $ \beta - ({\rm BEDT}-{\rm TTF})_{2}{\rm IBr}_{2}$ as a thermal source. The behavior of both thermal and acoustic field is investigated near the conductor’s surface. In addition, the angular and magnetic field dependencies of the NSLBAWs amplitude are obtained and analyzed in order to emphasize the specific features of these waves. Furthermore, a comparison of the properties of both NSLBAWs and the longitudinal bulk acoustic waves that are generated and attenuate in the depth of the conductor, i.e., deep longitudinal bulk acoustic waves (DLBAWs) reveals some similarities and important differences in the process of thermal generation of the waves. We discuss the possible application of the thermal generation of NSLBAWs in organic conductors and why these waves are more preferable than the DLBAWs for studying some of the properties and/or states in layered organic conductors as well as for determining the parameters that shape the Fermi surface that cannot be obtained through the DLBAWs or transport measurements.

Using AMRO Quantum Oscillations to Probe the Fermi Surface of Quasi-2d Layered Organic Conductors in Low Temperatures and High Magnetic Fields

Using AMRO Quantum Oscillations to Probe the Fermi Surface of Quasi-2d Layered Organic Conductors in Low Temperatures and High Magnetic Fields

Anisotropic Superconductivity and Dimensional Crossover in a Layered Organic Conductor with Strong Correlation

Researchers have observed magnetotransport phenomena due to the interplay of dimensionality and gap symmetry in a highly anisotropic and strongly correlated organic superconductor.

Phase transitions in θ-(ET)4MnBr4(C6H6−nCln) (n = 1, 2) driven by ordering in anion and/or cation layers

Newly synthesized layered organic conductors θ-(ET)4MnBr4(1,2-C6H4Cl2) (1) and θ-(ET)4MnBr4(C6H5Cl) (2) show metal-like conductivity along conducting layers down to 4.3 and 60 K, respectively, while showing semiconducting properties in the perpendicular direction. Phase transitions driven by ordering in anion layers in 1 and ordering in a half of cation layers in 2 were found to occur at 297 and ~240 K, respectively. Crystal structures above and below the phase transitions were determined for 1 at 260 and 333 K and for 2 at 260 and 220 K. In the above-transition phases of both compounds, attributed to the tetragonal crystal system, all ET layers are similar in structure. In the below-transition phases, attributed to triclinic and monoclinic crystal systems, the neighboring ET layers of each phase differ in periods along the stacks and in dihedral angles θ between radical cations of the adjacent stacks.

Layered organic conductors κ-(BEDT-TTF)2X: magnetic and superconducting properties

κ-(ET)2X are described by a two bands model. We investigate the superconductivity pairing due to spin fluctuations. The superconductivity coupling constants of various pairing symmetries are calculated. They strongly depend on the nesting properties and on the species of the anion X. These nesting properties may mimic either chemical or hydrostatic pressure. The found results provide a plausible explanation for the available experiments.

Anisotropic mobility and carrier dynamics in the β-type BEDT-TTF salts as studied by inter-layer transverse magnetoresistance

A new method to estimate an in-plane conduction anisotropy in a quasi-two-dimensional (q2D) layered conductor by measuring the inter-layer transverse magnetoresistance is proposed. We applied this method to layered organic conductors β-(BEDT-TTF)2X (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene, C10H8S8; X = IBr2, I2Br) by applying magnetic field rotating within the basal plane at 4.2 K. We found the anisotropic behaviour of carrier mobility μ. From this, anomalous distribution of carrier lifetime τ on the Fermi surface is derived, by the use of Fermi surface data reported for the materials. Calculations of the non-uniform susceptibility χ0(q) suggest that carrier scattering is enhanced at specific k-points related to partial nesting of the Fermi surface. The present method is thus demonstrated to be an efficient experimental tool to elucidate anisotropic carrier dynamics in q2D conductors.

Strong Coupling Superconductivity, Pseudogap, and Mott Transition

An intricate interplay between superconductivity, pseudogap, and Mott transition, either bandwidth driven or doping driven, occurs in materials. Layered organic conductors and cuprates offer two prime examples. We provide a unified perspective of this interplay in the two-dimensional Hubbard model within cellular dynamical mean-field theory on a 2×2 plaquette and using the continuous-time quantum Monte Carlo method as impurity solver. Both at half filling and at finite doping, the metallic normal state close to the Mott insulator is unstable to d-wave superconductivity. Superconductivity can destroy the first-order transition that separates the pseudogap phase from the overdoped metal, yet that normal state transition leaves its marks on the dynamic properties of the superconducting phase. For example, as a function of doping one finds a rapid change in the particle-hole asymmetry of the superconducting density of states. In the doped Mott insulator, the dynamical mean-field superconducting transition temperature T(c)(d) does not scale with the order parameter when there is a normal-state pseudogap. T(c)(d) corresponds to the local pair formation temperature observed in tunneling experiments and is distinct from the pseudogap temperature.

Phase transition at 320 K in a new layered organic metal conductor (BEDT-TTF)4CoBr4(C6H4Cl2)

A new layered organic conductor θ-(BEDT-TTF)4CoBr4(1,2-C6H4Cl2) with different resistivity behaviour that has metallic conducting layers and is semiconducting perpendicular to the layers has been prepared. A phase transition near 320 K was found in this salt by temperature variable resistivity and X-ray diffraction measurements. Precise crystal structure data, below transition (triclinic phase) and above transition (tetragonal phase), was determined. The crystal structures of both phases are composed of θ-type radical cation layers alternating with layers involving [CoBr4]2− anions and 1,2-dichlorobenzene molecules. The lower symmetry in the crystal structure of the triclinic phase is due to a non-symmetrical arrangement of the solvent molecules within the anion layers. In each structure, the neighbouring conducting layers of the unit cell differ in stacking directions. In the tetragonal phase, all conducting layers are similar in structure, and in the low-temperature triclinic phase, the neighbouring conducting layers differ in periods along the stacks (4.703 A and 4.993 A) and in dihedral angles θ between the BEDT-TTF radical cations of the adjacent stacks. The finding of compounds of the θ-(BEDT-TTF)4CoBr4(1,2-C6H4Cl2) type with phase transitions above room temperature provides a wide possibility of investigating magnetoresistance, quantum oscillations, and the mechanism of interlayer transport in layered conductors.

Ultrafast IR and THz spectroscopy of photo-induced insulator to metal transition in highly correlated organic system

Photo-induced insulator to metal transitions (PIMT) in quarter filled layered organic conductors ET [bis(ethylenedithio)tetrathiafulvalene]-based salts α-(ET) 2 I 3 , θ-(ET) 2 RbZn(SCN) 4 , and κ-(d-ET) 2 Cu[N(CN) 2 Br] were investigated using ultrafast spectroscopy in the near, mid-infrared and terahertz (THz) regions. In charge ordered salts α-(ET) 2 I 3 and θ-(ET) 2 RbZn(SCN) 4 , an immediate ( < 30 fs) generation of a microscopic metallic state is driven by the electronic process. Subsequently, condensation of the microscopic metallic domain to the macroscopic scale is accompanied by a small molecular rearrangement in α-(ET) 2 I 3 . However, in θ-(ET) 2 RbZn(SCN) 4 , a large structural difference between the insulator and metallic phases prevents stabilization of the macroscopic metallic state. In a dimer Mott insulator κ-(d-ET) 2 Cu[N(CN) 2 Br], photogeneration of the metallic state shows a finite rise time of ca. 1 ps, which is attributable to the photo-induced change of on-site Coulomb energy on each dimer (U dim ) through dimeric molecular rearrangement. Thus, the ultrafast dynamics of PIMT depend strongly on the molecular arrangement in the layer of ET salts.

Conduction mechanisms of Chiral-like spin solitons with Carriers in the Spin-frustrated and layered organic conductor

We have proposed the conduction mechanism of the hole-induced chiral-like spin solitons in the layered organic conductor with triangular lattice, and have discussed the pressure efiect of difiusivity of those hole-induced solitons.

Anomalous Pressure Dependence of Superconductivity in Layered Organic Conductor, κ-(BEDT-TTF)4Hg2.89Br8

The P – T phase diagram and transport properties of a layered organic superconductor, κ-(BEDT-TTF) 4 Hg 2.89 Br 8 , which can be viewed as a nealy half-filled band system, are reported. Through careful resistivity measurements under pressures of up to 4.5 GPa, we uncovered anomalous pressure dependence of T c of this superconductor, including a peak structure at approximatly 0.7 GPa and a minimum structure at higher pressure. The normal-state resistivity showed T -linear behavior at low pressures and this behavior gradually transformed into T -squared behavior with increasing pressure. The pressure, P c ≈2 GPa, at which T c has a turning value, corresponds to the onset of the T -squared behavior.

Antiferromagnetism and Superconductivity in Layered Organic Conductors: Variational Cluster Approach

The kappa-(ET)2X layered conductors (where ET stands for BEDT-TTF) are studied within the dimer model as a function of the diagonal hopping t' and Hubbard repulsion U. Antiferromagnetism and d-wave superconductivity are investigated at zero temperature using variational cluster perturbation theory (VCPT). For large U, Néel antiferromagnetism exists for t' < t(c2)', with t(c2)' approximately 0.9. For fixed t', as U is decreased (or pressure increased), a d(x2-y2) superconducting phase appears. When U is decreased further, then a d(xy) order takes over. There is a critical value of t(c1)' approximately 0.8 of t' beyond which the AF and dSC phases are separated by the Mott disordered phase.

Gossamer Superconductivity near Antiferromagnetic Mott Insulator in Layered Organic Conductors

Layered organic superconductors are on the verge of the Mott insulator. We use the Gutzwiller variational method to study a two-dimensional Hubbard model including a spin exchange coupling term as a minimal model for the compounds. The ground state is found to be a Gossamer superconductor at small on-site Coulomb repulsion U and an antiferromagnetic Mott insulator at large U, separated by a first order phase transition. Our theory is qualitatively consistent with major experiments reported in organic superconductors.

Shubnikov–de Haas Effect and Angular-dependent Magnetoresistance in New Layered Organic Conductors ET3Cl(DFBIB) and ET3Br(pBIB)

This paper reports the measurements of Shubnikov–de Haas (SdH) and angular-dependent magnetoresistance oscillations (AMROs) for new layered organic conductors ET 3 Cl(DFBIB) and ET 3 Br( p BIB), where ET, DFBIB, and p BIB stand for bis(ethylenedithio)tetrathiafulvalene, 1,4-difluoro-2,5-bis(iodoethynyl)benzene, and p -bis(iodoethynl)benzene, respectively. Each of them has only a two-dimensional Fermi surface (FS), whose cross sectional area corresponds to about 50% of the first Brillouin zone. No significant difference between them is found in the size and shape of the cross-section of the FS. In spite of the similarity of the electronic state, the sharp peaks in the magnetoresistance are clearly observed for ET 3 Cl(DFBIB) under in-plane magnetic fields, while anomalous broad peaks for ET 3 Br( p BIB). The results suggest that the anion structure play an important role in the interlayer transport.

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.

Fermi surface in new layered organic conductors (BEDT-TTF) 3Br( pBIB) and (BEDT-TTF) 3Cl(DFBIB)

We report the measurements of Shubnikov–de Haas (SdH) oscillation and angular dependent magnetoresistance oscillation (ADMRO) in a new layered organic conductor (BEDT-TTF) 3Br( pBIB), where pBIB stands for p-bis(iodoethynyl)benzene. When magnetic fields are applied perpendicular to the layers, two distinct SdH oscillations are clearly observed, which arise from the two extremal cross-sectional areas of the slightly warped cylindrical Fermi surface (FS). The cross-sectional areas correspond to 50.9 and 51.6% of the first Brillouin zone. From the analysis of the ADMRO, the area of FS is estimated to be 52.5% of the first Brillouin zone. The ADMRO data also shows that the anisotropy of the FS is enhanced by change of Br( pBIB) for Cl(DFBIB), where DFBIB stands for 1,4-difluoro-2,5-bis(iodoethynyl)benzene. These results are consistent with the band calculation.

High-frequency impedance of organic metals in a strong magnetic field

Propagation of electromagnetic waves is studied theoretically in layered organic conductors with metal-type conductivity, whose Fermi surface (apart from a slightly corrugated cylinder) a priori contains two sheets in the form of weakly corrugated planes. It is shown that the presence of such a group of charge carriers significantly affects the magnitude of wave attenuation.