Effect of Pressure on the Interlayer Charge Transport and the Electronic Structure of the Metallic Layers in the Organic Two-Dimensional Bilayer Metal (BETS)4CoBr4(DCB)

Journal of Experimental and Theoretical Physics - The resistance and magnetoresistance of the organic (BETS)4CoBr4(DCB) metal are studied at atmospheric pressure and a hydrostatic pressure of up to...     

Electron and electron-phonon effects in the quasi-two-dimensional molecular conductor θ-(BETS)<Subscript>4</Subscript>HgBr<Subscript>4</Subscript>(C<Subscript>6</Subscript>H<Subscript>5</Subscript>Cl): Optical studies at 300–15 K

This paper reports on a study of the polarized reflectance and optical conductivity spectra of the quasi-two-dimensional molecular conductor θ-(BETS)₄HgBr₄(C₆H₅Cl) within the 700–6500-cm^?1 region at 300–15 K and within the 9000–40 000 cm^?1 region at 300 K performed along two principal directions in the crystal plane parallel to the conducting layers of the BETS molecules. The IR spectra obtained at 300 K follow a close-to-Drude behavior, with strong broad features (1200–1400 cm^?1) due to electron-vibrational (vibronic) coupling (VC) superposed on the high Drude background. As the temperature is lowered in the range 180–80 K, in the spectra there appears a Lorentz term with ω_t=2900 cm^?1, as well as three additional VC-induced bands in the 800–1180-cm^?1 region, which disappear as the temperature is decreased further. The results obtained indicate the existence of unstable structural distortions along the two principal directions in the crystal, which are accompanied by the formation of a commensurate charge-density wave.     

Cyclotron resonance and the de Haas-van Alfven effect in the (BEDT-TTF)8Hg4Cl12(C6H5Cl)2 organic conductor

This paper reports on measurements of the de Haas-van Alfven effect in the quasi-two-dimensional organic conductor (BEDT-TTF)₈Hg₄Cl₁₂(C₆H₅Cl)₂ performed within the temperature range 0.33–1.44 K in magnetic fields of B≤50 T. An analysis of quantum oscillations together with cyclotron resonance data obtained in the 40–120-GHz frequency range revealed that the complex quantum-oscillation spectrum is formed by the fundamental frequencies α～256 T and β～670–610 T, as well as by combination and multiple frequencies. It is shown that the character of the temperature-induced oscillation-spectrum rearrangement can be interpreted within a model taking into account the existence of a magnetic phase transition at T _c～0.9 K and the closeness of the fundamental frequency α with effective mass m*=1.48m ₀ to the spin damping condition.     

Metallic and Mott Insulating Spin‐Frustrated Antiferromagnetic States in Ionic Fullerene Complexes with a Two‐Dimensional Hexagonal C60.− Packing Motif

(MDABCO⁺)(C₆₀^.?)(TPC) ( 1 ), in which MDABCO⁺ is N‐methyldiazabicyclooctanium, TPC is triptycene, and both have threefold symmetry, is a rare example of a fullerene‐based quasi‐2D metal and contains closely packed hexagonal fullerene layers with interfullerene center‐to‐center distances of 10.07 Å at 300 K. Evidence for the metallic nature of 1 was obtained by optical and microwave conductivity measurements on single crystals. The metal is characterized by a nontypical Drude response and relatively large optical mass (m*/m₀=6.7). The latter indicates a narrow‐band nature, which is consistent with the calculated bandwidth of 0.10–0.15 eV. The coexistence of metallic and antiferromagnetic nonmetallic 2D layers was observed in 1 above 200–230 K. It was assumed that the nonmetallic layers undergo a transition to the metallic state below 200 K due to ordering of the fullerene and cationic sublattices. New layered complex (MQ⁺)(C₆₀^.?)(TPC) ( 2 ) with a hexagonal arrangement of C₆₀^.? was obtained by increasing the interfullerene distance with the bulkier N‐methylquinuclidinium cations (MQ⁺) having threefold symmetry. The structure of 2 is characterized by increased interfullerene center‐to‐center distances in the layers (10.124, 10.155, and 10.177 Å at 250 K). Unit‐cell doubling parallel to the 2D layer (along the b axis) was observed at low temperatures. In contrast to metallic 1 , 2 exhibits a nonmetallic spin‐frustrated state with an antiferromagnetic interaction of spins (the Weiss temperature is ?27 K) and no magnetic ordering down to 1.9 K. It was supposed that the expanded interfullerene distances in the triangular arrangement decrease the bandwidth and suppress metallic conductivity in 2 , and thus a Mott–Hubbard insulating state with antiferromagnetically frustrated spins results.     

The nature of enhanced linear and nonlinear optical effects in fullerene solutions

The “blue” emission from fullerene C₆₀ and its derivatives in frozen toluene solution is discovered and analyzed in the framework of the electromagnetic theory of enhanced optical effects. It is shown that the emission, combining enhanced spectra of Raman scattering and one-photon luminescence, is due to clustering of fullerene molecules in solution. Photoexcitation of charge-transfer excitons in clusters provides the polarization required for the enhancement. A direct relationship is established between the observed phenomenon and nonlinear optical properties of the medium. Empirical and computational tests are proposed to select matrices with various nonlinear optical properties.     

Crystal and molecular structure of organic semiconductor (ET)8[Hg4Cl12] · 2C6H6

A new organic semiconductor, (ET)₈[Hg₄C1₁₂] · 2C6H6, obtained in the ET⁺-HgCl ₃ ^– -PhF system has been studied by X-ray structural analysis. Radical cations of bis(ethylenedithio)tctrathiafulvalene (ET) in the organic layer of the structure are packed in stacks ofa-type. The average angle between the planes of ET cations from adjacent stacks is 50.1°. The anionic layer is formed by four-charge centrosymmetric [Hg₄Cl₁₂]^4– complexes and benzene solvate molecules. A comparative crystal-chemical study of the salts obtained by the reaction in the ET⁺-HgX ₃ ^– -PhY system (where X = Cl, Br, and I; Y = F, Cl, and Br) made it possible to reveal a substantial effect of the sizes of the X and Y atoms on the composition of the salts and on the structural characteristics of the layers, which are responsible for the various conductivities of these salts.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 386–391, February, 1996     

Crystal and molecular structure of (BEDT-TTF)4Cd2I6

The new radical-cation salt (BEDT-TTF)₄Cd₂I₆ has been studied by x-ray diffraction (a=19.284(6), b=12.84(1), c=7.869 (2) Å; α=72.77(4), β=94.44(2), γ= 103.97(3)^o, space group P1, Z=1, d_calc=2.32 g/cm³). The radical cation (BEDT-TTF)^1/2+ and the Cd₂I₆ ^2? anions form organic and inorganic layers alternating along the a axis. In the organic layer, (BEDT-TTF)^1/2+ are arranged in stacks and ribbons. The radical-cations in the ribbon are packed “side to side” and are joined to one another through shortened intermolecular contacts S...S 3.346(5)-3.657(4) Å. The Cd₂I₆ ^2? anion is dimeric. The Cd atom has a distorted tetrahedral bond configuration, with bond lengths Cd-I 2.722(l)-2.886(1) Å and valence angles I-Cd-I 93.8(1)-114.1(1)°.