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

Theoretical concepts and experimental evidence of heterogeneity in glass-forming liquids and polymers are reviewed. The main purpose is to provide an introduction to theoretical developments and recent experiments which have led to rapidly increasing knowledge. Realizing that there is no consensus in regard to the various scenarios of the glass transition starting from rather different assumptions we try to give a balanced overview although we also compare and interrelate some of the approaches. The experimental part describes recent nuclear magnetic resonance, dielectric, and optical experiments from which dynamically distinguishable subensembles can be selected thus proving the existence of a well defined dynamical heterogeneity.

Heterogeneity at the glass transition: a review

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

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.

Elastic effects of structural relaxation in glasses at low temperatures

Anomalous Sound Velocity in Vitreous Silica at Very Low Temperatures

The Gardon and Narayanaswami theory of annealing and tempering of glass is rederived in a unifying way using the framework of the theory of thermoviscoelasticity. It is extended by including the effect of isothermal pressure relaxation in addition to the relaxation of shear stress. In the new theory, relaxation of shear stress, isotropic stress (pressure) and thermal stress, which causes thermal expansion, are treated on the same level. The thermal expansion coefficient, a frequency-dependent quantity like bulk and shear modulus, has high- and low-frequency limits identified with the thermal expansion coefficients of glass and liquid. The results of numerical calculation of stresses and strains in a symmetrically cooled infinite plate for the extended theory illustrate the decisive importance of the dispersion of the thermal expansion coefficient.

Thermoviscoelastic theory of freezing of stress and strain in a symmetrically cooled infinite glass plate

To explore the physical content of the intuitive concept of dyanmic molecular cooperativity, as thematized in a famous paper of Adam and Gibbs [1], a class of lattice-gas and kinetic Ising models has been investigated [2–25]. The concept is thought to apply to the non-vibrational diffusive motion of molecules in supercooled liquids near the glass transition. The models made the intuitive idea concrete and allow various of its physical concequences to be studied in detail and to be compared with observed properties of such liquids.

Models of cooperative dynamics near the glass transition

A fundamental mechanism for the transmission of information between living cells is provided by the ionic transport through channels across the cell membranes. These channels, which are proteins imbedded in the membranes, usually fluctuate between open (conducting) and closed (non-conducting) states. A defect-diffusion gating model inspired by the analogy between glasses and proteins has been formulated; its predictions are shown to be in good agreement with the observed distributions, f ( t ), of closed-state durations. In particular, the model predicts that f ( t ) behaves at long times either as a stretched exponential or as a power law, depending on the initial defect configuration. There is also considerable evidence supporting the view that the ionic pathway is lined by membrane-spanning protein segments folded to form α-helices. In this paper, a model is presented for the motion of an α-helix in the framework formed by the rest of the protein. The physically appealing possibility of interpreting the defects diffusing in the protein in terms of kink-like excitations of the helix is examined.

J. Jäckle

Papers

Elastic effects of structural relaxation in glasses at low temperatures

Anomalous Sound Velocity in Vitreous Silica at Very Low Temperatures

Thermoviscoelastic theory of freezing of stress and strain in a symmetrically cooled infinite glass plate

Models of cooperative dynamics near the glass transition

Diffusion and gating in membrane proteins