Table of contents

A review of the results of real and numerical experiments characterizing the thermodynamics of the melting of simple substances is given. With the examples of argon and sodium it is demonstrated that the qualitative trends in the behavior of the thermodynamic quantities along the melting curve are independent of the form of the interaction forces. The question of the form of the "characteristic" energy determining the melting or crystallization of a substance is analyzed. It is concluded that melting is the most general example of an order-disorder transition in coordinate space. The question of the nature of maxima in melting curves is discussed. The empirical relations of Lindemann and Simon are considered.

Perfection of the technique of macroscopic physical experiments has recently been proceeding so intensively that we can now inquire naturally under what conditions in macroscopic experiments will an increase in sensitivity be limited by the quantum-mechanical properties of the test objects. In this article we determine the limiting values of the detectable accelerations (or forces) when free particles or oscillators are used as the test objects. The conditions for attaining the limiting sensitivity are discussed. It is shown possible to increase the sensitivity of a converter of mechanical into electrical oscillations by increasing the relaxation time of the electric resonator. The possibility is discussed of nondestructive recording of the n -quantum state of an oscillator. It is shown with the example of a concrete experimental design that one can determine the value of n (including n = 0) in such a way that the probability of transition to the adjacent levels after the measurement will be small.

The review is devoted to the problem of using cyclic electron accelerators and storage rings as sources of high-power ultraviolet radiation and x rays. A brief description is given of the properties of synchrotron radiation and also of the undulatory radiation which can be generated in such installations. Particular cases of use of these radiations are discussed. The parameters of Soviet accelerators and storage rings and the intensities of the synchrotron radiation beams from these installations in the far ultraviolet and x-ray regions of the spectrum are given. These characteristics are compared with the corresponding data for the DESY synchrotron at Hamburg, whose radiation is extensively used for x-ray structural analysis of biological polymers, investigation of the electronic structure of solids and gases, surface phenomena, and many other purposes.

This review gives information on the electric and optical properties of liquid crystals (LC), and discusses in detail various electro-optical effects in nematic, cholesteric, and smectic LC. Dielectric deformations of LC in electric fields, electrodynamic instabilities, the electromechanical effect, etc., are treated. Special attention is paid to explaining the physical reasons for the change in the optical properties of thin LC films when acted on by an electric potential. The practical applications of LC in electro-optical devices are briefly reviewed.

A review is given of the current status of the self-induced transparency effect. The McCall–Hahn theory (2π pulses) is considered and its generalization to the case of phase modulation of radiation pulses, degeneracy of transitions, and two-photon resonance is discussed. The relationship with the theory of nonlinear waves, obeying the Korteweg–de Vries equation, is pointed out. The possibility of similar coherent effects in direct interband transitions in semiconductors is discussed. A summary is given of the experimental results demonstrating a strong fall in the absorption and a considerable slowing down of light pulses, typical of the self-induced transparency. Possible physical applications of the effect, based on these properties, are discussed.

The review reports the main results, obtained by the paraelectric resonance method, of theoretical and experimental investigations of the physical properties of crystals with noncentral ions. The nature of the noncentrality of a number of impurities in cubic crystals is explained. It is shown that the cause of the noncentrality is a Jahn-Teller pseudoeffect. The form of the multiple-well potential in which the impurity moves is obtained. The spectrum of the paraelectric resonance of the noncentral ions and the effect of axial pressures on the spectrum are calculated in the tunnel approximation. Comparison of theory and experiment is used to determine the effective dipole moment, the tunnel splittings, and the coefficients of ion-lattice coupling. The line shape and the ion-lattice relaxation in paraelectric resonance are investigated. It is shown that at low temperatures and low impurity concentrations the principal broadening mechanisms are the internal electric fields and the stresses caused by the defects. Agreement is obtained between the observed and calculated lines shapes and relaxation times. The features of the magnetic resonances in crystal? with noncentral ions in external fields, or in the presence of axial pressures, are considered. It turns out that the characteristic field, temperature, and angular dependences of the frequencies can be used as a precise method of investigating the properties of noncentral ions. The features of the spin-lattice relaxation in crystals with noncentral ions are indicated. New methods for the study of crystals with noncentral ions, namely paraelectric-acoustic resonance and paraelastic resonance, are proposed.

A review is given of the results of theoretical and experimental investigations of electrically excited preionization gas lasers in which the pressure in the working mixture may reach tens of atmospheres. The method of preionization by external agency is considered and it is reported that, under optimal conditions, practically 100% of the electrical pump energy can be converted into the energy of molecular vibrations. Experimental results are quoted to show that the output power of electrically excited preionization lasers increases proportionally to the square of the gas pressure. The results are given of experimental investigations of the threshold and output characteristics as well as of the efficiency and gain of electrically excited preionization (and, particularly, electron-beam-controlled) carbon dioxide lasers. High-pressure lasers of this kind are promising sources for thermonuclear fusion, selective stimulation of chemical reactions, and industrial processing. Some numerical data are given on the various modes of operation of electrically excited preionization carbon dioxide amplifiers and oscillators. The characteristics of some high-power laser units are briefly described. A discussion is given of potential application of the electrical preionization method in the excitation of the vibration-rotational molecular levels (CO₂, CO, N₂O, NO, N₂, HF, and others) and of electronic state (N₂, H₂, CO, Cu, Pb, Xe₂, and so on). The review is based on papers published up to May 1973.

The review is devoted to an analysis of the result of experimental research on equilibrium and kinetic properties of liquids near critical points, and to a comparison of the results with modern theoretical concepts. The work treated mainly is that performed during the last five to six years. An analysis is made of the specific requirements that must be satisfied in experiments under conditions of anomalous susceptibility of the system to external perturbations. The following main conclusion is drawn: At present there are no experimental groups for doubting that the scaling hypothesis is a true reflection of the most essential features of the critical phenomena. Near the critical points, the liquids exhibit a universal behavior, and the universality region turns out to be quite broad (up to temperatures and densities differing by 20-30% from the critical values).

a phenomenon that is not yet fully understood theoretically and has been studied only slightly, and which has a number of distinctive properties as being the strongest manifestation of spatial dispersion. The conclusions of the phenomenological macroscopic theory and their agreement with experiment are discussed briefly. The best-suited substances are summarized, and the nature of the propagation of light waves in crystals is described. A microscopic treatment of the problem is presented with a number of examples of the beststudied crystals. The extent of agreement between theory and experiment, the relation of the gyrotropy to the crystal structure and of its constituent elements to the energy levels (electronic, vibronic, and vibrational) and to exciton effects are analyzed. Molecular, ionic, and covalent crystals and semiconductors are discussed; magnetoelectric media and liquid crystals are covered. Studies published up to the beginning of 1974 are used.