P. Brüesch

Phase phonons and intramolecular electron-phonon coupling in the organic linear chain semiconductor TEA(TCNQ)2

Abstract The vibrational excitations responsible for the remarkable series of infrared absorption bands observed in the organic linear chain semiconductor TEA(TCNQ)2 are identified to be the phase phonons which result from the coupling of the conduction electron molecular orbital to the totally symmetric vibrations of the TCNQ molecule.

Phonon dispersion and β→α transition in silver iodide

Abstract The phonon dispersion of β-AgI single crystals has been measured by inelastic neutron scattering at room temperature. The most interesting feature is a low lying optical branch at 2 meV. This mode shows a flat dispersion and has an unusually high amplitude of vibration favouring the promotion of a cation to an interstitial site. It is shown that this mode is essential for the occurence of a cation order-disorder transition into a highly ionic conducting phase (α-AgI) at 147°C.

Evidence for the fröhlich collective mode in the one-dimensional conductor K2[Pt(CN)4] Br0.3 · 3H2O from far infrared reflection

Abstract Far infrared and infrared reflection measurements on single crystals of the one dimensional conductor K2[Pt(CN)4] Br0.3 · 3H2O have been performed at 4.2K, 62K and 300K for light polarized parallel (E‖z) and perpendicular (E ⊊ z) to the platinum chains. At 4.2K and 62K a strong structure is observed in the E‖z spectrum near 40 cm-1 which is not observable at 300K. This structure is interpreted as due to the Frohlich collective 2qF-phonon mode.

Optical excitation of the electronic plasma oscillation in the one-dimensional conductor K2Pt(CN)4Br0.33·3H2O

Abstract Reflectivity measurements with p -polarized light under non-normal incidence at a (001) surface of the highly anisotropic conductor K 2 Pt(CN) 4 Br 0.33 ·3H 2 O show a resonance near the electronic plasma frequency. The results are discussed using the Fresnel formula for anisotropically absorbing crystals and classical dispersion theory for the description of the plasma oscillation.

Optical properties of α-Ag2 and β-Ag2S in the infrared and far-infrared

Abstract Reflectivity measurements in the far infrared and infrared (20–1500 cm−1) of polycristalline β-Ag2S at 4.2, 62 and 300K and of α-Ag2S at 473K are presented. The observed phonon structure of β-Ag2S is discussed. The reflectance of α-Ag2S is high in the far infrared and shows a plasma edge near 1000 cm−1. The optical constants of α-Ag2S as calculated with a Kramers-Kronig analysis are compared with the predictions of the Drude theory.

Confirmation of one-dimensional metallic behaviour in K2Pt(CN)4Br0.3. (H2O)n by far infrared reflection

Abstract Far infrared reflection measurements on single crystals of K2Pt(CN)4Br0.3. (H2O)n show an unusually high anisotropy of the reflectivity and confirm the nearly one-dimensional metallic behaviour of this compound.

Far infrared optical excitation in metallic uranium sulfide

Abstract Previous near normal incidence reflectivity measurements on US single crystals from 12 to 0.03 eV have been extended down to 0.0018 eV (15 cm −1 ). A broad plateau with a reflectivity of 90±2% is observed between 40 and 400 cm −1 with a further increase of the reflectivity below 40 cm −1 . A Kramers-Kronig transformation of the data shows the existence of a resonance at 315 cm −1 . From a comparison with recent neutron data and other arguments we deduce that this resonance is due to the excitation of a transverse optical phonon coupled to an f→ d or d →f interband transition.

Microscopic models for conductivity in solid electrolytes

We discuss microscopic models for ionic conductivity with emphasis on material independent general results. n nCorrelated motion of particles can be experimentally investigated by studying the frequency dependent conductivity σ(ω). We start the discussion with the simplest single particle jump diffusion model and then proceed to more advanced descriptions. All models which derive σ(ω) from the equilibrium transition rates contain no memory effects by construction. It can be shown that the correlated motion of interacting particles with no memory effects is always backward and [dσ(ω)]/dω ≥ 0. Experimentally [dσ(ω)/dω < 0 is found in Ag+ and Cu+ conductors and we discuss a model which allows for memory effects by coupling between diffusion and long lived vibrational fluctuations.

Lattice Dynamics and Ionic Motion in Superionic Conductors

Superionic conductors are characterized by an electrical conductivity comparable to that of a liquid electrolyte 1. Classically the approach in discussing the d.c. conductivity σ(0) has been to treat it in terms of defect concentration, defect mobility and their respective activation energies 2.

Effect of Relaxation on some Thermodynamic Properties of Lattice Gas Models with Applications to AgCrS2 and Hollandite

Recently, various aspects of interacting particles moving in a periodic substrate potential have been investigated. Many papers have considered the commensurate-incommensurate transition within various restricted assumptions. (Restriction to T=0 [1–3], continuum approximation [3–5], low temperatures [6–7]). Studies of the order-disorder regime have been restricted mainly to one dimensional models [8–10]. Here we want to discuss some aspects of this problem for one and two dimensional models. In order to discuss the order-disorder regime, we have chosen to relate the original Hamiltonian to an effective Cluster Hamiltonian. Then, standard methods can be applied. In the following we show how this can be achieved for two specific examples.