M. Kurmoo

Structure and properties of a new conducting organic charge-transfer salt β-(BEDT-TTF) 2AuBr 2

Abstract A new conducting organic charge-transfer salt, β-(BEDT-TTF) 2AuBr 2 has been synthesised and its crystal structure determined. Single crystal four-probe conductivity has been measured at ambient and applied pressures from 300–1.4 K and EPR spectroscopy from 300–4.2 K. β-(BEDT-TTF) 2AuBr 2 crystallises in P 1 (Z=1) with a = 9.020(1), b = 5.707(9), c = 16.320(1) A ; ∝ = 97.60(1), β = 92.12(1), γ = 102.89(1)o; V = 811.8(2) A 3 . While the electronic properties are two-dimensional, the molecular packing of the BEDT-TTF molecules is slightly different from that found in other β-phase BEDT-TTF salts with triatomic counterions. β-(BEDT-TTF) 2AuBr 2 is metallic to 1.4K at pressures up to 5.6 kbar although at all pressures studied there is appreciable hysteresis in the cooling and warming cycles of the conductivity between 6 and 14K. Abrupt transitions in the temperature dependence of both the peak-to-peak EPR linewidth and integrated signal intensity occur near 20K. The possibility of a low temperature structural modification is discussed.

Cyclotron resonance studies of electron dynamics in BEDT-TTF salts

Summary form only given. Cyclotron resonance (CR) measurements have been performed on quasi-two dimensional (Q2D) systems of electrons in the BEDT-TTF family of organic charge transfer salts. The use of a resonant cavity in conjunction with various microwave sources of radiation has recently given rise to much more detail than has previously been reported. Structure in the main CR line shape is observed, as well as a series of harmonics. Although the observation of these harmonics may be expected from our experimental geometry, these features do not occur at simple fractions of the fundamental resonance field. It is thought that the strong correlation between carriers in these materials leads to a distortion of the energy band close to the Fermi edge. A simple model of the renormalised Landau levels suggests that the field position of the CR harmonics may be much more influenced by the electron-electron interactions than is the fundamental CR. An extensive study of the energy and temperature dependence of the CR has been used to test the validity of the model and to assess whether the CR harmonics may provide us with a greater. understanding of the types of interaction giving rise to the novel ground state properties in this class of material.

Magnetotransport studies of the organic metals (BEDT-TTF)2AuBr2 and (BEDT-TTF)2KHg(SCN)4

Abstract We report magnetoresistance measurements on ET 2 KHg(SCN) 4 and β″ET 2 AuBr 2 . Both show Shubnikov-de Haas oscillation frequencies additional to those expected from band structure calculations, and other magnetoresistance anomalies occur (e.g. hysteresis, “kink” structures). These effects are explained qualitatively by band structure modifications produced by magnetic ordering.

Optical and electrical properties of quasi-bidimensional molecular semi-conductors: (TTF)14(MCl4)4, M = Co, Mn or Zn

Abstract A new class of radical-cation has been investigated: (TTF)14(MCl4)4 where M = Co, Mn, Zn. These compounds crystallize in a monoclinic system with two sets of orthogonal triads in a crystallographic plane. D.c. electrical conductivity measurements exhibit a quasi 2d isotropic behaviour in agreement with the structural investigation. Furthermore the optical properties, both reflectivity and absorption measurements, reveal several attractive features: i) a polarized electron-molecular vibrations coupling giving rise to vibronic lines which are specific for each sort of triads. ii) electronic absorption bands and a plasma resonance which indicate that we are in presence of a 2d conductor with strong Coulomb interactions.

Magnetoresistance and reflectance studies of β2AuBr2

Abstract Polarised infrared reflectance measurements at room temperature and low temperature magnetoresistance studies are reported for the organic metal β″-ET 2 AuBr 2 (ET is bis(ethylenedithio)-tetrathiofulvalene). Some band parameters are estimated from fitting of the reflectance to the Drude model. A large anisotropic transverse magnetoresistance has been measured in this material and the temperature dependence of the magnetoresistance has been studied. Shubnikov-de Haas oscillations are observed in the magnetoresistance at 1K and below and three distinct periodicities are observed. A negative component is observed in the magnetoresistance for j∂b at low temperature which becomes dominant as the temperature is reduced. An explanation of this effect based on magnetic-field-induced alignment of the carrier motion in a quasi-one-dimensional band is proposed.