P. Day

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.

Collective excitations in the 1-D-ferromagnet CsFeCl3 with singlet ground state

Abstract By means of inalastic neutron scattering we have determined the dispersion relation of the magnetic excitations in CsFeCl3 at different temperatures. The dispersion in c-direction, along the Fe-chains is typically ferromagnetic and in the hexagonal plane antiferromagnetic. Due to the lack of an applicable theory the data were parametrized by the simple heuristic formula:ħω = [2J[1 - cos πqc] [A + 2J(1 - cos π)qc)] + [C + J (1.5 + γ(q))]2] 1 2 The gap was found to be C = 0.148 THz, the easy plane anisotropy A = 0.308 THz, the ferromagnetic interaction J = 0.148 THz and the antiferromagnetic interaction J = -0.04 THz. At 1.25 K all excitations had a width smaller than the instrumental resolution ΔE = 0.025 THz. These results can be interpreted as follows: CsFeCl3 is a singlet ground state system with strong ferromagnetic interaction J along the crystallographic c-axis and weak antiferromagnetic interaction J in the plane perpendicular to c. In addition we have measured the influence of a magnetic field along the hexagonal c-axis. The splitting found agrees with the assumed level scheme yielding g = 2.5 for the first excited level.

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.

Inelastic neutron scattering study of magnetic excitations in the helimagnetic and antiferromagnetic phases of NiBr2

Abstract The spin wave dispersion in NiBr 2 has been studied by medium and long wavelength inelastic neutron scattering in the [1 1 0], [1 0 0] and [0 0 1] directions at 4.2 and 30 K, i.e. in the incommensurate helical and collinear antiferromagnetic phases. The values of the intralayer Heisenberg exchange constant J ij and XY anisotropy constant D at 4.2(30) K are J 01 0.379(1)(0.379(1)), J 02 0.0036(50)(0.0036(50)), J 03 - 0.105(5) (−0.105(5)), J ′ - 0.0423(50)(−0.389(50)) D 0.0364(50)(0.0290(50)), where J ′; is the interlayer exchange constant. In fitting the 4.2 K data account is taken of the co-existence of three equivalent domains and of intensity arising from ω( q ) and ω( q ± k 0 ) where k 0 is the wavevector of the helix. In the low frequency region of the dispersion curve such peaks are resolved. The results reinforce the hypothesis that in zero-field the commensurate-incommensurate phase transition is driven by fluctuations.

High-field Magnetotransport Studies of (et)2mhg(scn)4 Salts (m=k, Nh4)

The angle dependence of the magnetoresistance (MR) of (ET)2(SCN)4 (M=K,NH4) has been measured in steady magnetic fields up to 50 T. In (ET)2KHg(SCN)4 We observe two SdH frequencies in the MR below the resistive kink feature at 22 T, and different values of g*m* above and below the kink; we propose that these effects result from antiferromagnetic ordering between the conducting planes. In (ET)2NH4Hg(SCN)4 we have found that both the frequency of the Shubnikov-de Haas (SdH) oscillations and the effective mass of the carriers vary to a high degree of accuracy as 1/cos(theta) (theta is the angle between the field and the normal to the conducting layers) as expected for a quasi two-dimensional system, and in contrast to a previous study by others. Finally we comment on the validity of the Harmonic Ratio method for measuring effective masses.

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.