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.     

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.     

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.     

Hydrostatic-Pressure-Induced Reentrance of the Metallic State in the κ-(ET)2Hg(SCN)2Cl Quasi-Two-Dimensional Organic Conductor

On cooling below 30 K, the κ-(ET)₂Hg(SCN)₂Cl quasi-two-dimensional organic metal, which is in the quantum spin liquid state at liquid helium temperatures, undergoes a transition to the Mott insulator state. The application of a hydrostatic pressure p = 0.7 kbar stabilizes the metallic state and makes it possible to study the behavior of the interlayer magnetoresistance at liquid helium temperatures. The field dependence of the magnetoresistance exhibits an unlimited power-law growth, which indicates that the polaron mechanism contributes to the interlayer transport. The spectrum of observed magnetoresistance oscillations corresponds to the Fermi surfaces characteristic of conducting layers with the κ-type structure.

     

The origin of the phase separation in partially deuterated κ-(ET)2Cu[N(CN)2]Br studied by infrared magneto-optical imaging spectroscopy

The direct observation of the phase separation between the metallic and insulating states of 75%-deuterated κ-(ET)₂Cu[N(CN)₂]Br (d33) using infrared magneto-optical imaging spectroscopy is reported, as well as the associated temperature, cooling rate, and magnetic field dependencies of the separation. The distribution of the center of spectral weight (〈ω〉) of d33 did not change under any of the conditions in which data were taken and was wider than that of the non-deuterated material. This result indicates that the inhomogenity of the sample itself is important as part of the origin of the metal-insulator phase separation.     

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.     

Control of electronic state in organic conductors by systematic band filling

Organic superconductors have been usually obtained by the bandwidth control of antiferromagnetic insulator. In order to find another parameter to control the electronic state, the band filling control has been applied to obtain the carrier-controlled organic conductors. As a result, the systematic band filling control for four kinds of organic conductors has been succeeded. The obtained crystals were (a) 2:1 salts, λ-ET₂(GaCl₄¹⁻)_1−x(CoCl₄²⁻)_x [ET=bis(ethylenedithio)tetrathiafulvalene], doping to the antiferromagnetic insulator, (b) 2:1 salts, δ′-ET₂(GaCl₄)_1−x(CoCl₄)_x, doping to the non-magnetic insulator, (c) 3:1 salts, α-ET₃(CoCl₄)_1−x(GaCl₄)_x(TCE), doping to the charge ordered salt, and (d) 3:2 salts, β′-ET₃(CoCl₄)_2−x(GaCl₄)_x, doping to the half-filled band insulator. For both 2:1 salts, (a) and (b), the semiconducting behaviors have transferred to relatively conductive semiconducting ones by doping. The largest change of transport property is observed for (d) β′-phase from semiconducting behavior of parent material to metallic ones [β′-ET₃(CoCl₄)_2−x(GaCl₄)_x (x=0.88, 0.66)] down to 140 and 100 K, respectively.These systematic band filling control suggests that the doping to the 1/2-filled band insulator is most effective.     

Superconductivity at normal pressure in κ-(BEDT-TTF)<Subscript>2</Subscript>Cu[N(CN)<Subscript>2</Subscript>]Cl crystals

Single crystals of the κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl cation-radical salt are obtained. These crystals exhibit metallic properties and pass to a superconducting state with T_c = 11.5 K at ambient pressure.     

Vibrating reed magnetometer studies of superconducting and magnetic materials

ABSTRACT

Applications of sensitive vibrating reed magnetometry to a variety of superconducting and magnetic materials are reviewed. The advantages of conducting vibrating reed studies of small single crystals, ceramics, thin films and multilayers with high anisotropy, composition gradients, strain and nonlinear magnetic response are documented. The equilibrium and dynamic properties of magnetic flux in superconductors, and coexistence of magnetic and superconducting states are emphasised. Vibrating reed experiments on Nb, NbSe₂, (Ba_0.6K_0.4)BiO₃, HoNi₂B₂C, {Nb(x)/Ni(y)}_Z multilayer films, and κ-(ET)₂Cu{N(CN)₂}Br have revealed subtle, exotic superconducting behaviours that remain unexplained. Future prospects for vibrating reed measurements of ferromagnetic thin films with patterned nanostructures are considered.     

Femtosecond carrier relaxation dynamics and photoinduced phase separation in κ-(BEDT-TTF)2Cu[N(CN)2]X (X=Br,Cl)

We investigate the relaxation dynamics of nonequilibrium carriers in organic conductors κ-(BEDT-TTF)(2)Cu[N(CN)(2)]X (X=Br and Cl) using ultrafast time-resolved optical spectroscopy. The dynamics for both salts show similar temperature dependences, which is well characterized by the carrier relaxation across the pseudogap (PG) of the magnitude Δ(PG) ≈ 16 meV for Br salt and 7.0 meV for Cl salt. On the other hand, only the Br salt shows an abrupt increase of the decay time at low temperature, indicating an additional decay component associated with the superconducting (SC) gap below T(c). The fluence dependent dynamics at low temperature evidences the superposition of the SC component onto the PG component. These results indicate a metallic-insulating phase separation in the Br salt triggered by photoexcited nonequilibrium carriers.     

Circular Dichroism and Conformation of Bilirubin Bis N,N, N-Trimethyl α-Phenethylammonium Salts

Analysis of the bisignate circular dichroism spectra of bilirubin-IXα bis-propionate salts of (S)-(-)-N, N, N-trimethyl α-phenethylammonium hydroxide reveals a preference in CHC1₃, (CH₃)₂ CO and CH₃ CN solvents for folded conformations in which the propionate groups are intramolecularly H-bonded to the opposing pyrromethenone units.     

Staggered spin order of localized π-electrons in the insulating state of the organic conductor κ-(BETS)<Subscript>2</Subscript>Mn[N(CN)<Subscript>2</Subscript>]<Subscript>3</Subscript>

The magnetic properties of the conduction π-electron system of κ-(BETS)₂Mn[N(CN)₂]₃ have been probed using ¹³C NMR. At ambient pressure, the metal-insulator transition observed in the resistivity measurements below T ? 23 K is shown to be accompanied by ordering of the π-spins in a long-range staggered structure. As the metal-insulator transition is suppressed by applying a small pressure of ～0.5 kbar, the π-spin system maintains the properties of the metallic state down to 5 K.     

Superconductivity of κ-(BEDT-TTF)2Cu[N(CN)2]I under pressure

The insulating state of κ-(BEDT-TTF)₂Cu[N(CN)₂]I salt appearing at ambient pressure at low temperatures is suppressed by hydrostatic pressure. The resistive measurements showed that the emerging metallic state reveals superconductivity in high-quality crystals. The superconducting state with the transition temperature of about 8 K is stable at pressures higher than 0.1 GPa.     

铝-铜-镓三元系合金相图

铝-铜-镓三元系合金相图的室温截面已经用X射线方法测定出来了。室温固相截面包含11个单相(即α, γ₂, γ′, δ, ζ₁, ζ₂, η₂, θ, θ′,α_铝和镓)相区,14个双相(即α + ζ₁, α+ γ₂, α + γ′, γ₂ + γ′, γ′+ ζ₁, γ₂+δ, γ′+δ,δ +ζ₂, ζ₂+η₂, η₂+θ,η₂+ θ′, γ′ + θ′, θ′+ 镓和θ+α_铝)相区和9个三相(即α + γ′+ζ₁, α+ γ₂+ γ′, γ₂+ γ′ + δ,γ′ + δ + θ′, δ+ζ₂+θ′, ζ₂ + η₂ + θ′, η₂+ θ′ + 镓, η₂ +θ + 镓和θ +镓+ α_铝)相区。所有单相和三个二元系内室温存在的单相相同,没有新相出现。 关键词：     

A family of angle-moments proportional to r-n,n = 1,2,…, in free space

The moments M_n(r) ≡ 1/2 ∝^2π₀ dθ sinⁿ θ I(r,θ) of the intensity I(r, θ) in free space surrounding a spherical object emitting radiation with an arbitrary directional dependence are shown to be exactly proportional to r^-(n+1), n = 0, 1,….     

Raman and IR spectra of new organic metals (ET)8[Hg4X12(C6H5Y)2] (X, Y=Cl, Br)

The Raman and IR reflectivity spectra of the new organic metals (ET)₈[Hg₄X₁₂(C₆H₅Y)₂] (X, Y=Cl, Br) based on the molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF or ET in abbreviated form) have been investigated. These metals differ from the previously studied compounds, particularly, in the type of ET molecular packing in the quasi-two-dimensional conducting layers. A high reflectivity and a plasma minimum observed in the IR reflectivity spectrum indicate the presence of quasi-free charge carriers (holes), as in other conducting ET salts. However, the A _g vibrational modes in the Raman spectrum are not activated in the IR spectrum, as is the case of conducting ET salts with other packing types. The Raman lines are assigned to the normal vibrations in the ET molecule, and their ionization shifts are determined. It is demonstrated that the frequencies of the most intense line ν₃(A _g) show a linear dependence on the cation charge, which is characteristic of different ET salts. No correlations are revealed between the ν₃(A _g) frequencies and the packing type. The strong background with a broad maximum at a Raman shift of about 3000 cm^?1 is observed in the Raman spectra upon excitation with the 2.54-and 2.41-eV lines of an Ar⁺ laser. The assumption is made that such a background can be associated with the scattering by one-particle and collective electronic excitations.     

Photoinduced melting of a stripe-type charge-order and metallic domain formation in a layered BEDT-TTF-based organic salt

Photoinduced melting of charge-order (CO) in [bis(ethylenedithiolo)]-tetrathiafulvalene (BEDT-TTF) salts was investigated by femotosecond spectroscopy. Comparative studies on two polytypes exhibiting large [theta-(BEDT-TTF)2RbZn(SCN)_{4}] and small [alpha-(BEDT-TTF)2I3] molecular rearrangements through the CO transition were performed. Ultrafast melting of CO for both compounds demonstrates the major contribution of the electronic instability which is due to Coulomb interaction. The roles of the molecular rearrangements on the formation of the CO and the metallic domain are discussed on the basis of low-frequency lattice dynamics.     

Anisotropic giant magnetoresistance originating from the π-d interaction in a molecule

We synthesized TPP[Fe^III(Pc)(CN)₂]₂, PTMA_x[Fe^III(Pc)(CN)₂]·y(MeCN), and PXX [Fe^III(Pc)(CN)₂], a new series of charge-transfer salts containing the axially-substituted phthalocyanine (Pc), [Fe^III(Pc)(CN)₂]⁻. In this molecular unit, the π conduction electron derived from the Pc-ring coexists with the d electron which is a potential source of a local magnetic moment. Therefore various phenomena associated with the interplay between local magnetic moments and conduction electrons are expected. We observed the giant negative magnetoresistance (GNMR) in all the three salts. The GNMR is highly anisotropic for the magnetic-field direction, and reflects the g-tensor anisotropy of the local magnetic moment in the [Fe^III(Pc)(CN)₂]⁻ unit. This indicates that the GNMR in these salts originates from the strong π-d interaction in the [Fe^III(Pc)(CN)₂]⁻ unit.     

On the transfer of line radiation in random magnetic fields

An analysis is made of the influence of a magnetic field of constant magnitude but with random orientations, described by the spherical coordinates θ and φ, on the polarization transfer in spectral lines. The assumption that the statistical properties of the field are determined by the one- and two-point probability densities P₁(x, θ, φ) and P₂(x₁, θ₁, φ₁; x₂, θ₂, φ₂) where x is the coordinate along the ray, implies that the variations of θ and φ can be considered as being due to a Markovian process in x. The expectation values of the Stokes parameters 〈Iγ〉, γ = 1, …4, are calculated functions of x or the optical depth τ. The formalism includes the limiting cases of infinite and zero correlation length. Assuming an artificial triplet and simple conditions, the results suggest that isotropic magnetic fields of the type considered have no influence on the polarization characteristics during the transfer.     

He(I) photoelectron spectroscopy of anions in adiponitrile solution

He(I) photoelectron spectra are reported of solutions of salts in adiponitrile in which peaks characteristic of the anions are visible; the vertical ionisation energies of these peaks are 6.91 and 7.85 (I^?), 7.6 (Br^?), 8.1 (Cl^?), 7.0 (CNS^?), 8.1 and 6.8 (NO₂^?), and 7.6eV (SO₄^2?). The salts examined were tetra-n-butyl ammonium (NO₂^?, I^?, Br^?, Cl^?, CNS^?, SO₄^2?), N-methylpyridinium I^?, trimethylphenylammonium I^?, choline I^?, methyltrioctylammonium I^? and methyltriphenylphosphonium Br^?. The relationship between these spectra and the charge transfer to solvent spectra of the anions is discussed.