Table of contents

A review is given of the experimental results on high-temperature superconducting ceramics (oxides) published in the form of papers and preprints. A brief account is given of the history of the discovery of a new classical superconducting ceramics, and the methods of their preparation as well as the main physical properties are described. The mechanism of superconductivity of these materials is discussed.

The most important results of theoretical studies of crystallization of a three-dimensional electron gas are presented. The crystallization criterion, magnetic field effects, viscous-liquid model, and possibility of experimental realization of Wigner ordering are considered. The results of experimental studies in compensated semiconductors published during the last two decades are analyzed in detail using the concepts of Wigner crystallization. It is shown that there is not a single convincing experimental proof of the existence of Wigner crystallization, and that all known experimental results can be explained in a noncontradictory way by the magnetic-field-induced localization of electrons in potential wells created by randomly distributed impurities.

The basic ideas and a number of results of the theory of low-energy excitations and localized electronic states, which are responsible for the universal low-temperature anomalies in glasses and unusual electronic properties of nonmetallic glasses, primarily glassy semiconductors, are reviewed. The basic hypotheses and models, previously proposed in this connection in order to explain the numerous empirical data, are discussed. Recently developed concepts regarding soft atomic configurations as a significant feature of the structure of glasses, determining new types of low-energy excitations in such anharmonic systems and of localized electronic states—self-trapped electron pairs with negative correlation energy, with very strong interelectronic attraction—are examined. The above-mentioned properties of glasses are analyzed on the basis of this theory and briefly compared with experiment. Some important problems in the theory of glasses are indicated.

A review is presented of two original methods of calibrating sources and detectors of optical radiation. Their main feature is that they are absolute, i.e., no calibrated devices are employed. Readings are taken in dimensionless units, i.e., number of pulses per photon, when detectors are calibrated, and number of photons per field mode, when sources are calibrated. A new name is proposed for the latter quantity, namely, the planck, since it is a natural quantum unit of the basic photometric variable, i.e., spectral radiance of radiation. Both methods are based on a nonlinear-optics effect, namely, parametric scattering of light, observed in birefringent piezoelectric crystals such as lithium niobate. The necessary data on this effect are presented together with the theoretical bases of the methods and the results of experimental investigations and comparisons with traditional methods.

A review is presented of the experimental data on the high-temperature superconductivity recently discovered in metal-oxide ceramics.

Neutron stars are compact objects whose properties depend strongly on the equation of state of the highly compressed nuclear matter. Analysis of the emission from several extraterrestrial sources whose activity is linked with neutron stars made it possible to refine the characteristics of these stars. This refinement in turn made it possible to obtain additional information on the properties of nuclear matter.