Extremely slow charge fluctuations in the metallic state of the two-dimensional molecular conductor theta-(BEDT-TTF)2RbZn(SCN)4

Large charge disproportionation has been confirmed in the metallic state of a 1/4-filled organic conductor theta-(BEDT-TTF)2RbZn(SCN)4 by means of 13C-NMR analysis on a selectively 13C-enriched single crystal sample. By comparing the homogeneous and inhomogeneous linewidths, the temperature dependence of the extremely slow dynamics of charge fluctuations has been determined first. The exotic nature of the metallic state of this salt is discussed.     

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)₂I₃, θ-(ET)₂RbZn(SCN)₄, and κ-(d-ET)₂Cu[N(CN)₂Br] were investigated using ultrafast spectroscopy in the near, mid-infrared and terahertz (THz) regions. In charge ordered salts α-(ET)₂I₃ and θ-(ET)₂RbZn(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)₂I₃. However, in θ-(ET)₂RbZn(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)₂Cu[N(CN)₂Br], photo-generation 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.     

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.     

A Double Isotopic Labelling Study of the Infrared Spectra of the Linkage Isomers [Pd(bipy)(SCN)2], [Pd(bipy)(NCS)2] and Related Complexes

Abstract

The IR spectra of the linkage isomers [Pd(bipy)(SCN)₂] and [Pd(bipy)(NCS)₂] have been determined in the C≡N stretching region (2200–2000 cm^?1) and below 500 cm^?1. The band shifts resulting from deuteration of the 2,2′-bipyridine (bipy) ring and ¹⁵NCS-labelling are shown to provide a ready means for distinguishing between the internal ligand modes, the μPd-N(bipy) and μPd-SCN/μPd-NCS vibrations. The assignment technique has been further extended to the complexes [Pt(bipy)(SCN)₂] and [Pd(phen)(SCN)₂] (phen = 1,10-phenanthroline). Finally, a comparison between the IR spectra of [Pd(bipy)(NCO)₂], [Pd(bipy)(NCS)₂] and [Pd(bipy)(SCN)₂] reveals that the frequencies μM-NCO, μM-NCS and μM-SCN decrease in the sequence NCO > NCS > SCN.     

A new stable organic metal: -(BETS)C(CN). The first -type radical cation salt with a planar-triangular discrete organic anion

The -(BETS)₂C(CN)₃ radical cation salt was prepared by electrocrystallization, and its crystal structure was determined by single crystal X-ray diffraction. The electronic structure of -(BETS)₂C(CN)₃ was studied by means of the extended Hückel tight binding method. The electrical conductivity of this salt as a function of temperature shows a metallic behaviour down to 1.3 K. Shubnikov-de Haas oscillations reveal both the classical and magnetic breakdown orbits on the Fermi surface typical of the -type organic conductors. In addition, a low frequency oscillation (250 T) which is not predicted by the band structure calculations has been found in the oscillation spectrum. Received: 11 March 1998 / Revised: 9 June 1998 / Accepted: 11 June 1998     

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     

Magnetic field effect on the pairing state competition in quasi-one-dimensional organic superconductors (TMTSF)2X

We study the effect of the magnetic field on the pairing state competition in organic conductors (TMTSF)₂X by applying random phase approximation to a quasi-one-dimensional extended Hubbard model. We show that the singlet pairing, triplet pairing and the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) superconducting states may compete when charge fluctuations coexist with spin fluctuations. This rises a possibility of a consecutive transition from singlet pairing to FFLO state and further to S_z = 1 triplet pairing upon increasing the magnetic field. We also show that the singlet and S_z = 0 triplet components of the gap function in the FFLO state have “d-wave” and “f-wave” forms, respectively, which are strongly mixed.     

The thermal metal-insulator phase transition in (EDO-TTF)2PF6

ABSTRACT

The thermal metal-insulator phase transition in the π-stacked (EDO-TTF)₂PF₆ charge transfer salt is of the Peierls type. It is related to geometrical reorganisations and charge ordering phenomena. We report that dimerising displacements are involved in the mechanism of this transition. By using periodic quantum chemical calculations, we find a double well potential in which dimerisation and charge localisation become manifest. By analysing the nuclear wavefunctions we discuss the mechanism of the phase transition in terms of thermal fluctuations.     

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.     

1H NMR Spectral Simplification with Achiral and Chiral Lanthanide Shift Reagents. Methastyridone, 2,2-Dimethyl-5-(2-Phenylethenyl)-4-Oxazolidinone

Abstract

The 60 MHz ¹H NMR spectra of methastyridone, 2,2-dimethyl-5-(2-phenylethenyl)-4-oxazolidinone, 1, have been studied at 28° in CDCl₃ solution with the achiral reagent tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato)europium(III), 2, Eu(FOD)₃, and the chiral reagent tris[3-(heptafluoropropylhydroxy-methylene)-d?camphorato]europium(III), 3, Eu(HFC)₃.     

Mixed density wave state in quasi-2D organic conductor

The density wave phase of α-$\alpha \mathrm{-}$(BEDT-TTF)₂KHg(SCN)₄ was investigated by transport properties and magnetic susceptibility. The density wave transition was observed as a broad increase at T_DW$T_{\mathrm{DW}}$=9 K by resistance measurement. Temperature dependence of the static magnetic susceptibility χ$\chi$ shows a large Curie tail below 100 K. By subtracting the Curie component, we found that the magnetic susceptibility increases like weak ferromagnetism with decreasing temperature below 7.4 K. The gradual increase of χ$\chi$ below T_DW$T_{\mathrm{DW}}$ is not expected in simple CDW or SDW, where the magnetic susceptibility decreases with decreasing temperature due to the reduction of Pauli paramagnetic component. To explain the weak ferromagnetic behavior, we consider the coexistence of CDW and SDW. We propose a model of the mixed density wave, where CDW exists with antiferromagnetically coupled canting spins.     

Chemical quenching of positronium in Fe2O3/Al2O3 catalysts

Fe₂O₃/Al₂O₃ catalysts were prepared by solid state reaction method using α-Fe₂O₃ and γ-Al₂O₃ nano powders. The microstructure and surface properties of the catalyst were studied using positron lifetime and coincidence Doppler broadening annihilation radiation measurements. The positron lifetime spectrum shows four components. The two long lifetimes τ₃ and τ₄ are attributed to positronium annihilation in two types of pores distributed inside Al₂O₃ grain and between the grains, respectively. With increasing Fe₂O₃ content from 3 wt% to 40 wt%, the lifetime τ₃ keeps nearly unchanged, while the longest lifetime τ₄ shows decrease from 96 ns to 64 ns. Its intensity decreases drastically from 24% to less than 8%. The Doppler broadening S parameter shows also a continuous decrease. Further analysis of the Doppler broadening spectra reveals a decrease in the p-Ps intensity with increasing Fe₂O₃ content, which rules out the possibility of spin-conversion of positronium. Therefore the decrease of τ₄ is most probably due to the chemical quenching reaction of positronium with Fe ions on the surface of the large pores.     

ZnFe2O4的固相法和水热法制备及其电化学性能研究

本文用固相反应法和水热法制备了ZnFe₂O₄材料,X射线衍射(X-ray diffraction, XRD)表明制备出来的ZnFe₂O₄为尖晶石结构,表面形貌测试 (scanning electron microscopy, SEM) 显示两种方法制备的材料的平均粒径分别为500 nm和200 nm.比表面积测试结果表明,两种方法制备的样品的比表面积分别为136.7 m² g^{-1关键词： 2O₄')" href="#">ZnFe₂O₄ 尖晶石结构 电化学性能 锂离子电池}     

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.     

Large triplet pairing mixing in the FFLO state of spin fluctuation mediated superconductivity in Q1D systems

We study the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state of spin fluctuation mediated superconductivity and focus on the effect of coexisting charge fluctuations. We find that (i) consecutive transitions from singlet pairing to FFLO and further to S_z=1 triplet pairing can generally take place upon increasing the magnetic field when strong charge fluctuations coexist with spin fluctuations and (ii) the enhancement of the charge fluctuations lead to a significant increase of the parity mixing in the FFLO state, where the triplet/singlet component ratio in the gap function can be close to unity. We propose that such consecutive pairing state transition and strong parity mixing in the FFLO state may take place in a quasi-one-dimensional organic superconductor (TMTSF)₂X.     

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.     

Study of structural phase transitions in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+δ</Subscript> by x-ray photoelectron spectroscopy

We have used x-ray photoelectron spectroscopy to investigate the charge state of oxygen found in the basal structural plane of YBa₂Cu₃O₆+γ.. We have observed a change in this state after thermal treatment, with a transition to the adjacent structural phase region. We have shown that changes in the charge state of oxygen can be used as an indicator of structural changes occurring in YBa₂Cu₃O₆+δ.. We have found that the rate of structural relaxation yttrium barium cuprate depends on the amount of structural water it contains. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 195–198, March–April, 2007.     

1H NMR Spectral Simplification with Achiral and Chiral Lanthanide Shift Reagents. Mexiletine

The 60 MHz ¹H NMR spectra of mexiletine, 1-(2,6-dimethylphenoxy)-2-propanamine, 1, have been studied at 28° in CDCl₃ solution with the achiral reagent, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium(III), 2, Eu(FOD)₃, and the chiral reagents tris[3-(trifluoromethylhydroxymethylene)-d-camphorato]europium(III), 3, Eu(FACAM)₃, and tris[3-(heptafluoropropylhydroxymethylene)-d-camphorato]europium(III), 4, Eu(HFC)₃. Substantial lanthanide-induced shifts were seen for the proton signals of 1 with each reagent. Appreciable enantiomeric shift differences were seen for both methyl signals and for each of the CH₂CH proton signals using 3 and 4 that should permit direct determinations of enantiomeric excess for samples of 1. A predominant conformation for 1 is suggested based on observed splittings of the CH₂ proton signals and their relative lanthanide-induced shifts.     

Investigation of 4H—SiC metal—insulation—semiconductor structure with Al2O3/SiO2 stacked dielectric

Atomic layer deposited (ALD) Al₂O₃/dry-oxidized ultrathin SiO₂ films as high-k gate dielectric grown on the 8° off-axis 4H-SiC (0001) epitaxial wafers are investigated in this paper. The metal-insulation-semiconductor (MIS) capacitors, respectively with different gate dielectric stacks (Al₂O₃/SiO₂, Al₂O₃, and SiO₂) are fabricated and compared with each other. The I-V measurements show that the Al₂O₃/SiO₂ stack has a high breakdown field ( ≥ 12 MV/cm) comparable to SiO₂, and a relatively low gate leakage current of 1× 10^-7 A/cm² at electric field of 4 MV/cm comparable to Al₂O₃. The 1-MHz high frequency C-V measurements exhibit that the Al₂O₃/SiO₂ stack has a smaller positive flat-band voltage shift and hysteresis voltage, indicating less effective charge and slow-trap density near the interface.     

Structural, elastic, electronic and optical properties of a new layered-ternary Ta4SiC3 compound

We propose a new layered-ternary Ta₄SiC₃ with two different stacking sequences (α- and β-phases) of the metal atoms along c axis and study their structural stability. The mechanical, electronic and optical properties are then calculated and compared with those of other compounds M₄AX₃ (M=V, Nb, Ta; A=Al, Si and X=C). The predicted compound in the α-phase is found to possess higher bulk modulus than these compounds. The independent elastic constants of the two phases are also evaluated and the results discussed. The electronic band structures for α- and β-Ta₄SiC₃ show metallic conductivity. Ta 5d electrons are mainly contributing to the total density of states (DOS). We see that the hybridization peak of Ta 5d and C 2p lies lower in energy and the Ta 5d-C 2p bond is stronger than Ta 5d-Si 3p bond. Further an analysis of the different optical properties shows the compound to possess improved behavior compared to similar types of compounds.