Table of contents

This article reviews the experimental and theoretical studies of the properties of hydrogen included in metals. The problem is discussed of diffusion of hydrogen atoms in a metallic matrix, and the theory of coherent and incoherent diffusion. Phase transitions caused by elastic interactions are considered in a system of hydrogen atoms included in the interstices of a metallic matrix. The electronic properties of compounds of hydrogen with different metals, the experimental studies of the electronic band structure and of phase transitions in the electronic subsystem (metal-dielectric in cerium hydride, magnetic transitions) are reviewed. Special attention is paid to superconductivity in hydrogen-metal systems. Detailed results are presented of experimental and theoretical studies of the superconducting properties of compounds of palladium with hydrogen and deuterium, and also compounds of palladium with the noble metals and hydrogen. The effect of inclusion of hydrogen on the superconductivity of other transition metals is discussed.

The review analyzes the physical mechanisms of energy dissipation in type-II superconducting alloys in the mixed state. A very simple microscopic theory is presented for the dissipative processes in alloys with paramagnetic impurities. The main premises of the microscopic theory are described and its results are presented for the resistivity in the case of ordinary alloys (without paramagnetic impurities) in weak magnetic fields smaller than the upper critical field, at the low temperatures and at temperatures close to critical, and also in the case of strong magnetic fields, on the order of the upper critical field, in the entire temperature interval. The theory is compared with experiment.

The electrical, optical, and magnetic properties of layered compounds of dichalcogenides of transition metals and intercalated crystals are considered. Particular attention is paid to the anisotropy of these properties and to the analysis of the changes produced by intercalation. Experimental data are presented on the structural transitions in layered crystals and are examined from the point of view of the ideas concerning the charge-density wave. The bulk of the review is devoted to the superconducting properties of layered compounds (the critical temperature, the specific-heat discontinuity in the transition, the gap, the fluctuations above T_c, and the magnetic properties of ordinary and intercalated layered superconductors). The theoretical concepts that make it possible to describe the distinguishing features of the superconducting properties of layered and intercalated compounds are discussed (the effective-mass model and the model of Josephson interaction between layers).

Magnetic dipole-dipole interactions in a nuclear-spin system can lead to ordering of the spins only at temperatures ∼10^–6 °K. For nuclear antiferro- and ferromagnetic states to occur in this case, however, only the nuclear-spin system must be cooled to this temperature. This becomes possible in a number of cases, since the spin-lattice relaxation time becomes large enough already at a crystal-lattice temperature ∼0.1–0.3 °K, and the spin system is well insulated from the lattice. We discuss dynamic methods of obtaining near-zero positive and negative spin temperatures in a solid by adiabatic demagnetization of the nuclear-spin system. A simple theory is described, which yields the possible ordered configurations of the spins in the crystals. Methods of observing ordered states are described, together with the results of recent experiments that confirm the onset of nuclear antiferro- and ferromagnetic states.

The article discusses the history of the discovery of intense bursts of cosmic gamma rays in the Vela group of satellites. A summary is given of the data on observation of gamma-ray bursts in various space instruments. The possibilities of terrestrial observation of phenomena due to the gamma bursts are discussed. Various models of the gamma bursts are analyzed: stellar flares, the activity of neutron stars, nonstationary accretion into compact objects, explosions of supernovae, and the collapse of supermassive bodies in the nuclei of active galaxies. The prospects for futher study of cosmic gamma-ray bursts are discussed.