The band electronic structures of (BEDTTTF)2X,X− = BrO4− and ReO4−

The strictly isostructural organic metals, (BEDTTTF)₂ReO₄ and (BEDTTTF)₂BrO₄ consist of sheets of BEDT-TTF (abbreviated ET) donor molecules which form a novel two-dimensional network. Despite the similar structures these salts possess different physical properties. (ET)₂ReO₄ is a highly anisotropic metal down to 80 K but becomes insulating below this temperature. An applied pressure greater than 4.5 kbar reportedly suppresses the MI transition and gives superconductivity at 2 K. However, no superconductivity has yet been observed for (ET)₂BrO₄. Band electronic structure calculations derived from their ET molecule network geometries at 298 and 120 K, and intrastack and interstack energies between nearest neighbor ET molecules, show that (ET)₂ReO₄ and (ET)₂BrO₄ are highly anisotropic metals, and both possess very similar band structures.     

Transport property of a newly synthesized organic conductor, β″-(BEDT-TTF)2AuBr2

Transport phenomena of a newly synthesized organic conductor, β″-(BEDT-TTF)₂AuBr₂ were studied. All the crystals examined were found to be metallic down to very low temperatures. For samples which contain impurities, an increase in the resistivity with decreasing temperature was observed below 13 K.Galvanomagnetic effect was measured at liquid helium temperatures. Negative magnetoresistance at low magnetic field and positive magnetoresistance at high field were found to coexist in this temperature region. Kochler's rule applies to the positive component of the magnetoresistance.     

Resonant Raman scattering in organic conductors α and β-(BEDT-TTF)2X (X=I3 and IBr2)

Molecular vibrations in the crystals of organic conductors α and β-(BEDT-TTF)₂X, (X=I₃, IBr₂), a neutral donor BEDT-TTF, and acceptors n-Bu₄N·I₃ and n-Bu₄N·IBr₂ are investigated by Raman scattering. Strong Resonant effects are observed in the I₃ compounds, when the incident laser wavelength approaches the optical transition energy of the I₃ anion. More than ten overtones of the symmetric stretching mode of anions appear. For the modes related to the C=C vibrations the decrease of peak energies and their broadening are observed in the charge-transfer salts.     

Spatial and electronic structures of the germanium-tantalum clusters TaGe n − (n = 8–17)

The results of optimizing the spatial structure and calculated electronic spectra of the TaGe _n ^? anion clusters (n = 8–17) have been presented. The calculations have been performed in terms of the density functional theory. The most probable spatial structures of clusters detected in the experiment have been determined by comparing the calculated and available experimental data.     

Diamagnetic evidence of superconductivity in the organic conductors β- (BEDT-TTF) 2IBr2 and β- (BEDT-TTF) 2AuI2

Replacement of the triiodide linear anion I₃^- by the symmetric linear anions IBr₂^- and AuI₂^- in the organic superconducting metal β- (BEDT-TTF) ₂I₃ results in an increase of the transition temperature from 1.2 to 1.9 and to 3.2 K respectively. Meissner effect measurements indicate that in all cases superconductivity is a volume property.     

Magnetic field-temperature phase diagram of the organic conductor α-(BEDT-TTF)2KHg(SCN)4

We present systematic magnetic torque studies of the “magnetic field-temperature” phase diagram of the layered organic conductor α-(BEDT-TTF)₂KHg(SCN)₄ at fields nearly perpendicular and nearly parallel to the highly conducting plane. The shape of the phase diagram is compared to that predicted for a charge-density-wave system in a broad field range.     

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.     

Crystal structure and magnetic properties of pseudoternary TbRh2−xMxSi2(M = Ru,Ir) compounds

The structural and magnetic characteristics of the pseudoternary TbRh_2−xM_xSi₂(M = Ru, Ir) compounds were studied. The compounds crystallize in the tetragonal ThCr₂Si₂-type structure. The magnetic data were collected in the temperature range 70–300 K. Their magnetic susceptibilities satisfy the Curie-Weiss law in the temperatures higher than 130 K. The magnetic moment of the rare earth atom is larger than of the free Tb³⁺ ion. A modified RKKY theory with included interaction between the conduction electrons was applied to explain the variation of properties of the compounds.     

Study of the critical current density and the thermodynamic critical field in deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br organic superconductor

ABSTRACT

We have studied the reversible and irreversible part of the hysteresis loops as a function of slow cooling rate through the order–disorder transformation near 80?K for the deuterated (κ-D₈-Br) κ-(BEDT-TTF)₂Cu[N(CN)₂]Br organic superconductor. We estimated the critical current density J_C and the thermodynamic critical field H_C from the magnetic hysteresis loops. Temperature dependence of the critical current density derived from the irreversible part using Bean’s model. The thermodynamic critical field H_C has been obtained from the reversible part of the hysteresis loops.     

Geometric and electronic structure of epitaxial NbxTi1−xO2 on TiO2(110)

We have investigated the detailed geometric and electronic structure of MBE-grown Nb_xTi_1−xO₂ on TiO₂(110) by means of high-resolution transmission electron microscopy, X-ray photoelectron diffraction, ultraviolet and X-ray photoemission and electron energy loss spectroscopy. We find no measurable change in the NbO bond length relative to that for TiO bonds in TiO₂ in the dilute limit (x = 0.05), and that the epitaxial layers remain strained and coherent with the substrate for x ⩽ ≈ 0.3. However, significant dislocation generation occurs for x > ≈ 0.3. Nb substitution for Ti in the lattice introduces an additional valence electron per atom. The resulting density of states falls in the valence band region, but no new state density occurs in the either the band gap or conduction band. This result is in contrast to what occurs in the very dilute limit (parts per thousand), where Nb electrons occupy a shallow donor level near the conduction band minimum. Based on the electron counting rule, the extra Nb electrons form a non-bonding band which is degenerate with the valence band. The significance of these results for enhanced thermal and photochemistry on Nb_xTi_1−xO₂ surfaces vis a vis TiO₂ is discussed.     

Atomic and electronic structures of Ag/Si(100)-c(6 × 2) surface: A first-principles study

Using the experimental data obtained mainly with the scanning tunneling microscopy observations, density functional theory calculations have been applied to examine an atomic structure of the Ag/Si(100)-c(6 × 2) reconstruction. A set of structural models has been proposed having a similar Si(100) substrate reconstruction which incorporates rows of top Si atom dimers and troughs in between the rows. Stability of about twenty models with various Ag coverage ranging from 1/6 to 1 ML has been tested, that allows reducing the number of plausible models to four. Two of these four models have been attributed to the “regular” intrinsic Ag/Si(100)-c(6 × 2) reconstruction, while the other two to its defect-induced modification. The latter is observed in the local areas near defects and domain boundaries and exhibits 3 × 2 periodicity. Comparing the results of calculations with the experimental STM images, it has been concluded that while the Si(100) substrate reconstruction is solid, the Ag subsystem is flexible due to the presence of the lightly bonded mobile Ag atoms.     

Determination of the electron phonon coupling and the superconducting gap in β-(BEDT-TTF)2X crystals (X=I3, IAuI)

We performed point-contact and tunneling measurements on the organic superconductors -(BEDT-TTF)₂I₃ and -(BEDT-TTF)₂IAuI, in the normal and superconducting states. The point-contact measurement in the normal state provides the Eliashberg function ²()F (). This function has maxima at 4 and 15 mV, as well as a sharp peak at 1 mV which seems to indicate a very soft phonon strongly coupled to the electrons. The measurements in the superconducting state provide the superconducting gap, which for the tunneling measurements has a value 2/k_B T_c4, just slightly above the BCS value.     

Novel (Li2X)+(LiX2)− supersalts (X = F,Cl) with aromaticity: a journey towards the design of a new class of salts

LiF₂ and Li₂F can be considered as representatives of main group superhalogens and superalkali species, respectively. For the first time, we present a study on the interactions between LiX₂ and Li₂X, for X = F, Cl. Our findings show that this interaction leads to the formation of ring-shaped Li₃X₃ supersalts. The quantum theory of atoms in molecule (QTAIM) approach is used to discuss superatomic bonding in these novel species. The aromatic character of Li₃X₃ rings is established by QTAIM in addition to various chemical-reactivity-based measures. Thus, the present work opens up an avenue to further investigate these new classes of aromatic species, theoretically as well as to synthesise them, experimentally.     

Structure and properties of the anions MF4−, MCl4− and MBr4− (M = C,Si, Ge)

Density functional theory (DFT), Møller–Plesset (MP2) and coupled cluster with single and double substitutions including non-iterative triple excitations (CCSD(T)) calculations on the anions MX₄^?, with M = C, Si, Ge and X = F, Cl, Br, show that GeF₄^?, SiCl₄^?, GeCl₄^? and SiBr₄^? prefer a C_2v conformation, but CCl₄^? is an elongated C_3v structure. CBr₄^? has T_d symmetry in MP2, but is slightly more stable in elongated C_3v form with DFT and CCSD(T). GeBr₄^? has T_d symmetry. CF₄^? and SiF₄^? are unstable with respect to loss of an electron. Vertical electron affinities (EAs) are negative also for CCl₄ and SiCl₄, and close to zero for GeF₄ and SiBr₄. Adiabatic EAs range from 0.47 eV for SiCl₄ to 1.78 eV for GeBr₄. The lowest excited states at T_d symmetry are ²T₂ resonances with energies of 2.1–3.5 eV, resulting from excitation of the a₁ singly occupied molecular orbital to vacant t₂ orbitals. Vertical excitation energies (VEEs) and vibrational frequencies are given for the most stable anionic geometries. Comparison with experimental VEEs for CCl₄^? is made. From dissociation energies of MX₄, MX₄^?, MX₃ and MX₃^?, appearance energies of X^?, MX₃^?, X₂^? and MX₂^? were calculated. Most were found to be in reasonable agreement with experimental values. Theoretical spin densities and g-factors have been compared with experimental results available for CCl₄^?, SiCl₄^? and GeCl₄^?.     

The local and electronic structures of Al90FexCe10−x alloys (x=3, 5, 7): XANES analysis

The x-ray absorption near-edge structure (XANES) of Al₉₀Fe_xCe_10?x (x=3, 5, 7) is measured above the Fe K edge and analyzed theoretically by the multiple scattering method. By comparing the experimental data with the theoretical findings, the most adequate model for describing the neighbor environment of Fe atoms is chosen. Since the dipole transition matrix element is a smoothly varying function of energy near the Fe K edge, the x-ray absorption fine structure in this energy range characterizes the density of free p states of Fe in the conduction band of Al₉₀Fe_xCe_10?x alloys.