Michel Evain

Structural Characterization and Band Electronic Structure of α-(BEDT-TTF)2I3 below its 135 K Phase Transition

Abstract The nature of the 135 K metal-insulator (MI) transition in α-(BEDT-TTF)2l3, abbreviated α-(ET)2l3, was examined by determining the crystal structures above (298 K) and below (120 K) the phase transition and also by calculating the band electronic structures at both temperatures. This study demonstrates that both the crystal and band electronic structures of α-(ET)2l3 change only slightly upon passing through the MI transition. Within each sheetlike network of ET radical cations, the magnitudes of interactions between adjacent pairs of ET molecules (i-j) above and below the MI transition temperature (T MI) were evaluated by calculating their interaction energies, β ij = ⟨Ψ, H off|Ψ⟩, where Ψ1); and Ψ1 are the HOMOs of ET molecules i and j, respectively. The band electronic structures calculated by using single-zeta Slater type orbitals show that α-(ET)2l3 is a semiconductor with band gaps of 13 meV (298 K) and 35 meV (120 K). Thus, within the one-electron model, the apparently metallic properties...

(MDT-TTF)2 AuI2: An ambient pressure organic superconductor (Tc = 4.5 K) based on an unsymmetrical electron donor

Abstract R f penetration depth measurements show (MDT-TTF) 2 AuI 2 to be an ambient pressure superconductor with a T c ∼4.5 K. The effectof hydrostatic pressure on T c was found to be −0.92 K/kbar, which suggests that anions longer than AuI 2 - may puff up the lattice and may yield cation-radical salts of MDT-TTF with even higher superconducting transition temperatures. (MDT-TTF) 2 AuI 2 has a room temperature ESR linewidth of ∼ 100 G, which gradually increases as the temperature is lowered. The structure and properties of (MDT-TTF) 2 AuI 2 bear a close resemblance to those of the 10 K organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 . The band electronic structure of (MDT-TTF) 2 AuI 2 is two-dimensional, with a closed Fermi surface. The MDT-TTF donor molecule prossesses only four CH bonds to form short contacts with anions, as compared to eight for BEDT-TTF. Thus, the translational and/or librational modes of vibration are expected to be substantially different in (MDT-TTF) 2 AuI 2 and κ-(BEDT-TTF) 2 Cu(NCS) 2 .

Observation of two different -C-C-H bending modes in the Ambient-Pressure organic superconductor κ-(ET)2Cu(NCS)2 by polarized infrared reflectance measurements and implications on its structural and physical properties

Abstract We have carried out micro-FT-IR polarized reflectance measurements on the (100) faces of the ambient pressure organic superconductors κ-(ET) 2 Cu(NCS) 2 and d 8 -κ-(ET) 2 Cu(NCS) 2 , T c ⋍ 10.4 K and 10.8 K, respectively. The spectrum of κ-(ET) 2 Cu(NCS) 2 exhibits absorptions due to -C-C-H bending at 1280 and 1302 cm -1 . The frequency difference of these -C-C-H bending modes correlates with the fact that κ-(ET) 2 Cu(NCS) 2 contains two types of ET molecules widely different in the “softness” of their hydrogen…anion interactions which in turn affects T c . Large shifts in the vibronic envelope are observed for d 8 -κ-(ET) 2 Cu(NCS) 2 , and these may be due to the softer -C-D…anion interactions in the non-equivalent d 8 -ET molecules.

Recent Progress in the Development of Structure-Property Correlations for κ-Phase Organic Superconductors

Abstract An examination of the molecular packing arrangements of the electron-donor molecules in κ-phase organic conductors and superconductors [κ-(BEDT-TTF)2X (X ˭ I3−, Cu(NCS)2−), κ-(MDT-TTF)2AuI2, and κ-(BMDT-TTF)2Au(CN)2] reveal that the main predictors of superconductivity appear to be (i) bond-over-ring intradimer molecular packing and (ii) relatively short (∼ 3.35A) intradimer separations. These structural features in newly discovered κ-phase organic conductors may be requirements for the occurrence of ambient pressure superconductivity.

The Interaction of Light with Rb0.73Cr5Te8 and Polymer/Nanoferrite Composites

Abstract The magnetic and photomagnetic characterizations of two materials are reported: ploymer/nanoferrite composites and the non-stoichiometric crystalline materials Rb0.73Cr5Te8. In the inorganic-organic composite material, the inorganic component is iron oxide and the organic component is the polymer poly(p-ethylphenol) (PEP). The Rb0.73Cr5Te8 material is a new non-stoichiometric formulation of the ACr5Te8 system that exhibits spin glass type magnetic behavior with a spin glass freezing temperature of 175K. The photomagnetism of both compounds has been measured at several temperatures below the spin blocking temperature.

Study of the formal oxidation state of iron in Fe1+xNb3−xSe10(x = 13) by band electronic structure calculations

Tight-binding band electronic structures were calculated for two ordered model structures of Fe1+xNb3−xSe10(x=13). The electronic structure of Fe1+xNb3−xSe10(x=13) is essentially given by superposing the band electronic structures of the NbSe3 double prismatic chains and the Fe/Nb double octahedral chains. The charge density wave of Fe1+xNb3−xSe10(x=13), which occurs in the NbSe3 double trigonal prismatic chains, is explained when localized d-levels associated with Fe3+ are considered to exist in the Fe/Nb double octahedral chains.

Band electronic structures of unsymmetrical electron donor salts (MET)X, (MPT)2X·(THF), and (MET)3(ReO4)2 (X = C104 -, PF6 -)

Abstract Tight-binding band electronic structures were calculated on unsymmetrical electron donor salts (MET)X, (MPT) 2 X·(THF), and (MET) 3 (ReO 4 ) 2 , where MET ( 1 ) is methylenedithio-ethylenedithio-tetrathiafulvalene, MPT ( 2 ) is methylenedithio-propylenedithio-tetrathiafulvalene, and X = c 10 4 - , PF 6 - . Both (MPT) 2 X·(THF) and (MET) 3 (ReO 4 ) 2 have band gaps, but (MET)X has a half-filled band. The semiconducting property of (MET)X stems from the nesting of its Fermi surface, which gives rise to a spin density-wave state.