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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.

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A method is described for using a limited number (typically 10-50) of low-dose radiographs to reconstruct the three-dimensional (3D) distribution of x-ray attenuation in the breast. The method uses x-ray cone-beam imaging, an electronic digital detector, and constrained nonlinear iterative computational techniques. Images are reconstructed with high resolution in two dimensions and lower resolution in the third dimension. The 3D distribution of attenuation that is projected into one image in conventional mammography can be separated into many layers (typically 30-80 1-mm-thick layers, depending on breast thickness), increasing the conspicuity of features that are often obscured by overlapping structure in a single-projection view. Schemes that record breast images at nonuniform angular increments, nonuniform image exposure, and nonuniform detector resolution are investigated in order to reduce the total x-ray exposure necessary to obtain diagnostically useful 3D reconstructions, and to improve the quality of the reconstructed images for a given exposure. The total patient radiation dose can be comparable to that used for a standard two-view mammogram. The method is illustrated with images from mastectomy specimens, a phantom, and human volunteers. The results show how image quality is affected by various data-collection protocols.

We present a detailed analysis of the electrical and optical properties of amorphous transparent conducting thin films of indium oxide prepared by ion beam sputtering with a wide range of carrier concentrations. We show that the resistivity is dominated by ionised impurity scattering despite the amorphous structure of the films. The weak effect of the structural disorder is confirmed by studies of the interband absorption and is explained by a consideration of the relative length scales of the structural disorder and the Fermi wavelength.

Diffuse scattering data have been collected on two crystal forms of lysozyme, tetragonal and triclinic, using synchrotron radiation. The observed diffraction patterns were simulated using an exact theory for simple model crystals which relates the diffuse scattering intensity distribution to the amplitudes and correlations of atomic movements. Although the mean square displacements in the tetragonal form are twice that in the triclinic crystal, the predominant component of atomic movement in both crystals is accounted for by short-range coupled motions where displacement correlations decay exponentially as a function of atomic separation, with a relaxation distance of approximately 6 A. Lattice coupled movements with a correlation distance approximately 50 A account for only about 5-10% of the total atomic mean square displacements in the protein crystals. The results contradict various presumptions that the disorder in protein crystals can be modeled predominantly by elastic vibrations or rigid body movements.

Inelastic-neutron-scattering studies of glassy, liquid, and polycrystalline selenium have been performed at temperatures between 100 and 600 K. A self-consistent data evaluation, taking careful account of multiple and multiphonon scattering, shows that a complete interpretation is possible in terms of a temperature-dependent density of vibrational states, together with diffusion broadening of the elastic line. This density of states is used to calculate thermodynamic properties and to show that about one-third of the additional entropy of the liquid is vibrational. The results raise a number of questions concerning current theories of the glass transition.PACS numbers: 64.70.-p, 63.50.+X It has long been realized that a fundamental difference between a glass and a supercooled liquid is the presence in the latter of both conformational and vibrational motion. As an idealized example, motion in the liquid can be represented by an atom vibrating about an equilibrium position which is itself changing with time as a result of diffusion. What is not yet understood, however, is the way in which conformational motion freezes out at the glass transition temperature Tg and the way in which it influences thermodynamic properties. To take the particular example of interest here, there have been no direct measurements of the relative importance of conformational and vibrational contributions to the sudden (but not discontinuous) increase in the heat capacity CpiT) on passing from the glass to the supercooled liquid, * one of the most obvious signatures of Tg, That this increase in C{T) is not simply a "release of degrees of freedom" can be seen in Se^ where CpiT) increases from the classical value of 3/? mole ~ ^ K ~ ^ in the glass to a value 50% higher just above Tg. (Se is a glass with a Tg sufficiently low for easy study, but without bonded H atoms or side groups which complicate the analysis in organic polymers.)Recent mode-coupling theories ^' "^ have attempted to understand conformational motion (also known as relaxation) by describing it in terms of a time-dependent particle-density correlation function. The predictions of these theories have encouraged a number of neutron studies of the dynamics of the glass transition: ^"^ Conformational and vibrational motion are distinguished by time scales of typically 1 ns (varying rapidly with temperature but set by the experiment) and 1 ps, respectively, and show up in quasielastic and spin-echo experiments, on the one hand, and in measurements of the Debye-Waller factor and inelastic scattering, on the other. Most experiments have been concerned with conformational motion above Tg and demonstrate the presence of relaxation with a temperature dependence similar to that shown by the bulk viscosity, and in reasonable agreement with theory. Vibrational motion has been examined less carefully, mainly because detailed analysis of the data is complicated by multiple and multiphonon scattering. In this Letter we adopt a self-consistent, model-independent procedure for data analysis an...