L. Pietronero

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.

Surface theory of melting

Abstract We demonstrate that melting is a surface initiated process. The surface becomes unstable before the bulk and the process of melting consists in the unstable surface that proceeds into the otherwise stable bulk.

Dynamics of collective excitations in one dimensional superionic conductors

Abstract We study the dynamics of collective excitations of a chain of interacting ions in a periodic potential including the effects of discreteness and of thermal damping. We describe a method to discriminate between the various possible cases ranging from soliton like propagation to the hopping of the collective coordinate. The frequency dependent mobility is computed and specific applications to materials are discussed.

The microwave conductivity of solid electrolytes

Abstract We investigate the frequency dependent conductivity σ(ω) for ω in the vicinity of the jump rate Γ 0 . The most general model based on hopping diffusion predicts a σ(ω) consistent with experiment for β-Al 2 O 3 but qualitatively at odds for Ag + and Cu + conductors. We consider the coupling of the transition rate to an external variable such as a long lived, low frequency lattice mode and show that this mechanism can give structure in σ(ω) of the form observed in Ag + and Cu + conductors.

Nonlinear screening in layered semimetals

Abstract The screening charge distribution ϱ for a semimetal is nonlinear. Within the Thomas Fermi approximation we compute ϱ analytically for the case of layered semimetals. The screening charge perpendicular to the layers is shown to decay as a power law whose exponent depends on the form of the band structure. Possible breakdowns of the Thomas Fermi approximation are analyzed especially in relation to intercalation compounds. It is shown that they might give rise to spatial oscillations in ϱ.

Microscopic models for conductivity in solid electrolytes

We discuss microscopic models for ionic conductivity with emphasis on material independent general results. Correlated 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.

Dispersion relations of the intercalate modes in graphite intercalation compounds

Abstract We study the dynamics of the charged intercalate molecules. The model for the intercalate is a two dimensional plasma with the appropriate screening due to the charge carriers in the graphite layers. The dependence of the dispersion relation on stage and charge transfer is discussed and the relation of these modes to conductivity is pointed out.

Theory of the electrical conductivity of acceptor intercalation compounds of graphite

Abstract We present a microscopic calculation of the electrical conductivity of acceptor intercalation compounds of graphite. In our model the electron-phonon scattering has two contributions:(a) Scattering with the phonons of the graphite layers; (b) scattering with the modes of the charged intercalate molecules. Specific results are derived for C 8 n AsF 5 and compared with experiments.

About the conductivity anomaly in (Na, K)-β″-Ga2O3 mixed crystals

Abstract We propose a model to explain the anomaly in the conductivity of (Na, K)-β″-gallate mixed crystals in terms of site occupancies for the two ionic species and compute the associated entropy change.