The various origins of a frequency-dependent conductivity in amorphous semiconductors are reviewed, stressing particularly recent advances and the influences that factors such as correlation and non-random spatial distributions of electrically active centres can have on the a.c. conductivity. A comprehensive survey is given of the experimental a.c. data for two types of amorphous semiconductor, namely chalcogenide and pnictide materials. It is concluded that the a.c. behaviour at intermediate to high temperatures is well accounted for by the correlated-barrier-hopping model, whereas the low-temperature behaviour is probably due to atomic tunnelling.

A.c. conduction in amorphous chalcogenide and pnictide semiconductors

REVIEW ARTICLE: Glassy metals

This review covers a wide range of experimental and theoretical studies of two-level or tunnelling states in glasses. Emphasis is on fundamental physics rather than a detailed comparison of experiment and theory. Sections cover the static and dynamic properties of tunnelling states, their contribution to thermal properties and their response to weak and strong electric and acoustic fields, both steady state and pulsed. A section on metallic glasses focuses on the importance of electron tunnelling-state interactions, and a final section illustrates approaches to a microscopic description by means of selected examples.

https://iopscience.iop.org/article/10.1088/0034-4885/50/12/003/pdf

Two-level states in glasses

Thermal conductivity of polymers

In order to test whether the low-energy excitations explored extensively in amorphous solids are indeed universal, all measurements published on the low-temperature thermal conductivity and acoustic attenuation in these solids have been reviewed, on a total of over 60 different compositions. The ratio of the phonon wavelength \ensuremath{\lambda} to the phonon mean free path l has been found to lie between ${10}^{\ensuremath{-}3}$ and ${10}^{\ensuremath{-}2}$ in almost all cases, independent of chemical composition and frequency (wavelength) of the elastic waves, which varied by more than nine orders of magnitude in the different experiments. When the data were fitted with the tunneling model, which is based on the assumption of atomic or molecular tunneling states with a certain spectral distribution, the tunneling strength C, which describes their coupling to the lattice, was found to range from ${10}^{\ensuremath{-}4}$ to ${10}^{\ensuremath{-}3}$ in almost all cases. The only exceptions reported so far are certain films of amorphous silicon, germanium, and carbon. In these films, low-temperature acoustic attenuations over two orders of magnitude smaller have been observed compared to all other amorphous solids. Another remarkable observation is that a large number of disordered crystals, and even a thermally equilibrated quasicrystal, have low-energy lattice vibrations that are quantitatively indistinguishable from those of amorphous solids. Their tunneling strengths also range from ${10}^{\ensuremath{-}4}$ to ${10}^{\ensuremath{-}3}.$ These measurements have also been reviewed. It is concluded that the absence of long-range order is neither sufficient nor necessary for the existence of the low-energy excitations.

Low-temperature thermal conductivity and acoustic attenuation in amorphous solids

A complete theoretical description of the effect of the relaxation of two-state structural defects on the elastic properties of glasses at low temperatures is given. New experimental data on the ultrasonic attenuation in vitreous silica and borosilicate glass in the temperature range between 0.3 and 4 K and at frequencies between 30 and about 500 MHz are presented and explained by this theory. We propose that over the entire temperature range from our lowest temperature of 0.3 to about 100 K the attenuation is due to the relaxation of defects with similar atomic structure.

L. Piché

Papers

Elastic effects of structural relaxation in glasses at low temperatures

Anomalous Sound Velocity in Vitreous Silica at Very Low Temperatures