Chronicle

At the Session of the Division of Physical-Mathematical and Chemical Sciences of the Academy of Sciences of the Ukrainian SSR

The session, held in Kiev from April 25 to 28, 1950, was devoted to questions of physics, geology, and astronomy. One general meeting was held; the remaining meetings were sectional.

The papers presented reflected the work of physicists of the Ukrainian Academy of Sciences in five areas: low-temperature physics, solid-state physics, X-ray physics and metallophysics, electronics, and absorption spectra of crystals.

As at the previous session (see UFN 39, 319, 1949), a series of interesting reports was presented by the laboratory headed by Corresponding Member of the Academy of Sciences of the Ukrainian SSR B. G. Lazarev.

At the general meeting B. G. Lazarev reported on work carried out by him jointly with B. N. Esel’son and Corresponding Member of the Academy of Sciences of the Ukrainian SSR I. M. Lifshits on the separation of helium isotopes and the study of certain properties of solutions of He³ in He⁴.

The authors succeeded in developing a method for enriching helium in the isotope He³ which, in its effectiveness, considerably surpasses all previous attempts. The enrichment process by this method is two-stage. The primary enrichment, by approximately a factor of 2000, is carried out using a thermomechanical effect, when from liquid helium located at a temperature below the λ-transition point, He⁴ is removed together with the superconducting phase. In the course of 7 days the authors obtained in this way 4.3 liters of helium with a concentration of the light isotope of 0.01%. The secondary enrichment, increasing the concentration of the light isotope by approximately a factor of 150, is carried out very rapidly with the aid of a miniature distillation column. In one experiment, from the indicated 4.3 liters, 22 cm³ of a helium mixture with a concentration of the light isotope of about 1.5% was obtained.

Obtaining appreciable quantities of helium with a rich content of He³ made it possible to carry out a number of experiments with solutions of He³ in He⁴. First of all it was shown that increasing the concentration of He³ in helium shifts the λ-transition point toward lower temperatures. For a concentration of He³ of one and a half percent this shift proved to be equal to 0.03°; for a concentration of 0.34% the shift already lies beyond the limits of the accuracy of measurement, estimated at 0.005°.

It is known that the rate of flow of helium II along a wetting film from a vessel with a high liquid level into a vessel with a low level does not depend on the difference of these levels. This independence for

of solutions of He³ in He⁴, proved to be maintained until a certain small difference of levels is reached, after which the rate of overflow depends almost linearly on this difference. The authors explain this effect on the basis that the change in the concentration of the components in the solution in both vessels, as a result of the overflow of He II, leads, on the one hand, to the occurrence of osmotic pressure, balancing a certain difference of levels; on the other hand, to a difference in the elasticity of He³ vapor over liquids with different concentrations of it, which causes the transfer of He³ through the gas phase and the accompanying overflow of He⁴ through the film.

E. S. Borovik reported on an investigation, in metals, of the Hall effect in strong effective fields. The strong effect of the magnetic field is achieved by deep cooling of the metal. Since in this case the conductivity of the metal increases strongly, which indicates an increase in the mean free path of the electron, the action of the magnetic field on the electron during its free path also correspondingly increases. Thanks to this, in ordinary magnetic fields one can obtain phenomena for which, at ordinary temperatures, extremely strong fields would be required.

The author characterizes the Hall effect by the dimensionless quantity \(E_y\), which is the ratio of the Hall electric field to the electric field in the direction of the current. For one group of metals, represented by zinc and beryllium, this quantity passes through a maximum as the magnetic field increases. For another group, represented by indium, this ratio increases with increasing field, apparently without bound. The observed phenomena agree, to a first approximation, with a theory operating with a bisonal model of the metal with equal concentrations of electrons and holes; however, the field-independence of the Hall constant predicted by the theory is not at all justified by experiment. For metals of the beryllium and zinc type such independence is observed only in the limiting cases of very small and very large fields.

B. I. Verkin, in the report “Magnetic Properties of Metals at Low Temperatures,” presented the results of work carried out jointly with Prof. B. G. Lazarev and M. S. Rudenko. The authors established that the phenomenon observed at low temperatures on single crystals of bismuth and zinc—periodic change of the diamagnetic moment with change in the intensity of the external magnetic field—is apparently a general-metallic property, since, in addition to bismuth, it has been discovered in a number of metals: cadmium, beryllium, magnesium, tin, and indium. For individual metals this phenomenon is observed in different temperature regions, at very different values of the intensity of the external magnetic field, and with very varied values of the period.

In crystals there appears magnetic anisotropy, expressed in the angular dependence of the magnetic susceptibility when the crystal is rotated relative to the direction of the magnetic field. According to existing theories, such anisotropy should be absent in the basal plane of the crystal. Therefore measurements were made with single crystals of bismuth, zinc, and beryllium, oriented in such a way that the field vector, lying in their basal plane, could form different angles with the binary axes. In this case as well, at sufficiently low temperatures, both anisotropy of the magnetic properties of the crystals and a periodic dependence of the magnetic susceptibility on the field intensity are manifested.

A. A. Galkin and P. A. Bezuglyi clarified the question “On the frequency dependence of the detection of alternating current by a superconducting—”

It was shown in due course in the laboratory of B. G. Lazarev that the phenomenon of destruction of the superconducting state by a current can be used for detecting alternating current, including radio frequencies. The frequency characteristics of the detector properties of a superconductor were studied on thallium. Repeating this work on tin in the region of audio frequencies, foreign authors came to the conclusion that at frequencies greater than 20,000 hertz the detector properties of tin improve with increasing frequency—a result not consistent with the data obtained for thallium.

Galkin and Bezuglyi showed that these results for tin may be explained by the nonisothermal conditions under which the experiment was carried out. Because of the smallness of the coefficient of heat conduction of helium I, in those experiments where it is used, inertialess dissipation of the heat of transition is not ensured. As a result, the transition process approaches an adiabatic one, which leads to an increase of the critical currents and to a deterioration of the detector characteristics.

Corresponding Member of the Academy of Sciences of the Ukrainian SSR I. M. Lifshits, considering the question “On the kinetics of the destruction of the superconducting state by a magnetic field,” proceeded from the premise that the relaxation time of the phenomena of the superconducting transition is apparently very small. Therefore, for not too high frequencies of variation of the magnetic field one may neglect them and carry out a macroscopic consideration of the problem.

This reduces to the formulation of certain electrodynamic and thermodynamic conditions, namely: a change of the field leads to the appearance in the specimen of Foucault currents, which retard the advance of the boundary between the normal and superconducting phases. On the other hand, the heat of transition produced by these currents causes the opposite acceleration of the motion of the boundary. Writing the corresponding equations of electrodynamics and heat conduction leads, under definite conditions, to the solution of the problem for the isothermal regime at low frequencies.

Solid-state physics was represented by four reports. The first of them concerned a newly developing field, advanced by Soviet theoretical physics (I. E. Tamm) and now acquiring great practical importance, namely the question of electronic states on the surface of semiconductors. The authors—Full Member of the Academy of Sciences of the Ukrainian SSR V. E. Lashkarev and V. I. Lyashenko—analyzed some cases of the direct manifestation of surface charges caused by the presence of surface levels, and indicated methods for detecting and studying these levels, in particular with the aid of techniques they had developed.

First of all, the surface charge is reflected in the work function of an electron from a semiconductor. This latter circumstance leads to a distinctive method for detecting changes in the surface charge through changes in the contact potential, measured with the aid of a sensitive vibrating electrometer developed in the laboratory. Changes in the filling of surface levels are manifested in a number of phenomena, for example: in the screening of an external field, which is observed in contact phenomena at the boundary between a semiconductor and a metal; in the investigation of photo-electromotive forces in semiconductors by the capacitor method upon application of an external field; in the study of the influence of an external field on the conductivity of a thin semiconductor film. The surface charge also has a significant effect on the surface conductivity of the semiconductor. The authors’ investigations led to the conclusion that surface levels of acceptor type exist on copper oxide. The authors showed that

adsorption of molecules of certain substances (for example, water or alcohol) on the surface of the oxide sharply reduces the degree of filling of surface states by electrons, which is immediately manifested both in a change in the work function and in a change in the surface conductivity of the semiconductor.

Two other reports were a development of the ideas of S. I. Pekar’s polaron theory.

In his presentation of the theory of $F$-centers, Prof. S. I. Pekar considers a model of an $F$-center, which is an electron localized near a positive point charge in an inertially polarizable dielectric medium. This problem, previously solved by classical methods, is now solved quantum-mechanically. The energy spectra and $\psi$-function of the electron in the $F$-center are calculated. By considering phototransitions of the electron from the ground state to the first excited state, all the features of the $F$-absorption band are explained. The calculated energy of thermal dissociation of the $F$-center with the formation of a free polaron turns out to be considerably less than the energy of excitation by light. The theory contains one arbitrary parameter, after determination of which, from comparison of one theoretical result with experiment, all the others are computed. The author illustrated the excellent agreement of the conclusions of the theory with experimental data for an entire series of crystals. The results of the work show that the dielectric-continuum model is capable of explaining the properties of $F$-centers in sufficient detail.

Considering “Recombination of electrons in colored crystals,” Prof. S. I. Pekar, together with the speaker Yu. E. Perlin, point out important cases in which the recombination coefficient can be calculated theoretically. This is done for the case when the probability of localization of an electron approaching the recombination center is large, and the recombination time is determined by the diffusion of the electron to the center under the influence of the latter’s force field. Considering electron recombination in colored alkali-halide crystals, the authors show that here recombination at $F$-centers plays the dominant role. The electron is attracted to the neutral $F$-center, polarizing it by its Coulomb field. The polarizability of the latter can be calculated by considering the quantum-mechanical problem of an $F$-center in an external electric field. After this, a formula is obtained for the electron displacement which contains no unknown parameters and makes it possible to compare with experiment, which leads to quite satisfactory results.

The third theoretical report by Corresponding Member of the Academy of Sciences of the Ukrainian SSR I. M. Lifshits and L. S. Gulida, delivered by the latter, concerned “The theory of local phase transformations in solids.” The work considers the possibility of the emergence of local nuclei of a new (liquid) phase inside a solid for arbitrary values of the deformation tensor in the latter. In this case the calculation can be carried out only for isotropic cases, and therefore the theory does not claim quantitative conclusions, but gives qualitative explanations. In this sense, the conditions are formulated under which the nucleus of a new phase that has arisen leads to a phase transition throughout the entire volume, as well as the conditions under which the growth of nuclei ceases upon reaching critical sizes.

Of the following group of reports, two were delivered by Stalin Prize laureate, Corresponding Member of the Academy of Sciences of the Ukrainian SSR V. I. Danilov.

In the work of V. I. Danilov and A. F. Skryshevsky, “Determination of the structure of liquid molecules from X-ray diffraction patterns,” the fundamental possibility of such a determination on the basis of ...

...that in a liquid the intramolecular distances (also taking thermal vibrations into account) between neighboring atoms are always smaller than the distances between the nearest atoms of neighboring molecules. Therefore, on electron-density distribution curves the “intramolecular” maxima, both in their form and in topology, can be clearly distinguished from intermolecular ones, which may be used for the stated purpose. From an analysis of the electron-distribution curves available in the literature for molecular liquids (CCl₄, phosphorus), data are obtained for intramolecular distances that are not inferior in accuracy to those from electron- and X-ray-diffraction measurements on gas molecules. X-ray diffraction study of liquid para- and orthodichlorobenzene showed that analysis of electron-distribution curves gives sufficient data for a complete determination of the structure of the complex molecules of these substances.

V. I. Danilov and D. E. Ovsienko, in the paper “On the nucleation of crystallization centers on solid surfaces,” distinguish two kinds of mechanical impurities in liquids: active ones, which reduce the supercooling of the liquid at which it begins to crystallize, and inactive ones, whose presence does not affect the nucleation of crystallization centers in the liquid. Experiments show that one and the same crystal, when in contact with a supercooled liquid, may prove active with respect to one modification and inactive with respect to another (NaCl crystals—α- and β-salol). Crystalline oxides of a number of metals exhibit great activity in the crystallization of a supercooled liquid metal. Investigation of the kinetics of the nucleation of crystallization centers indicates a fluctuational character of the appearance of centers on oxides. A number of experimental facts indicate that the activity of crystalline surfaces is connected with the phenomenon of orientational separation. When centers arise on active surfaces, phenomena of additional activation are observed.

The laboratory of Prof. B. Ya. Pines was represented by the next two reports.

Two years ago (see UFN 35, 287, 1948) the laboratory set itself the task of directly determining experimentally the local composition in alloys of variable concentration. Now V. S. Kagan and B. Ya. Pines propose, for this purpose, an X-ray method based on measuring the absorption of a monochromatic beam of X-rays at a given point of the alloy film. For a two-component system, in order to determine the thickness and concentrations it is sufficient to measure the absorption for two different wavelengths. This method is especially expedient for film thicknesses of the order of the half-absorption thickness. The method requires the use of very narrow X-ray beams of different wavelengths. The authors obtain them by means of a specially constructed four-anode sharply focused tube equipped with a light-powerful monochromator (see UFN 39, 322, 1949). The proposed method is also very advantageous for determining small quantities of heavy elements in the presence of light ones.

Ya. E. Geguzin reported on work carried out jointly with Prof. B. Ya. Pines: “The energy of mixing of binary metallic alloys.” The systems Bi-Cd, Pb-Sn, Bi-Sb, Bi-Sn, and Bi-Pb were studied. The energies of mixing were determined: 1) from the course of the solubility curves on equilibrium diagrams; 2) on the basis of data obtained from measurements of the vapor pressure as a function of alloy concentration; and 3) from the values of the “jumps” in heat capacity at the boundaries of the regions of separation. These determinations were carried out on

on the basis of the calculation performed of the simplest equilibrium diagrams of binary alloys, from which it follows that heats and heat capacities are determined by the same parameters as the equilibrium diagrams (mixing energies, entropy jumps at the phase transition, and the phase-transition temperatures of the pure components). Confirmation of the correctness of the calculation is seen in the fact that the mixing energies determined from data on the melting temperature and from the course of the solubility curves are in good agreement. Data on heat capacity in some cases, especially for Bi-Cd, do not agree with the calculation, which is probably due to the nonequilibrium realization of the transition through the region of stratification.

In the work of Prof. R. I. Garber, V. I. Starstev, I. A. Gindin, and M. G. Konstantinovskii, the subject of the study was “Annealing of Twinned Iron.” Twins were obtained by tearing strips of coarse-grained Armco iron at the temperature of liquid nitrogen. Annealing was carried out at a temperature of 800° C. Annealing led to the disappearance of the twin interlayer simultaneously in several places located along its length. After prolonged annealing, the single-crystal nature of the grain was restored. The duration of annealing necessary for this increases with increasing thickness of the twin interlayers. Since the results of annealing twin interlayers in iron crystals completely coincide with the results of analogous experiments by R. I. Garber with table salt, the authors conclude that single crystals are applicable as models for the study of the mechanical properties of metals. The authors also discovered twinning in $\alpha$-iron at room temperature.

Electronics was represented at the session by two theoretical and two experimental reports. The first two include: “Free Oscillations of Plasma in a Magnetic Field” by Prof. A. I. Akhiezer and L. E. Pargamanik, and “On Charge-Density Waves” by Prof. A. I. Akhiezer and Ya. B. Fainberg.

In the first work, the influence of a magnetic field on free longitudinal oscillations of plasma was investigated. By the method of the kinetic equation, a dispersion formula was obtained for longitudinal oscillations of plasma in an external constant homogeneous magnetic field. The external magnetic field substantially changes both the frequency and the damping of the free oscillations of the plasma. The oscillations split by frequency into two branches. The process proves to be anisotropic and depends essentially on the angle between the wave vector and the direction of the magnetic field. Waves of a definite frequency and direction of propagation prove to be undamped in a magnetic field.

In the second work, the interaction of plasma with an unmodulated beam of charged particles was considered. It is shown that the state of the beam becomes unstable, and the density fluctuations existing in it at all times propagate in the form of charge-density waves with increasing amplitude. This is especially sharply manifested if the unperturbed velocity of the beam exceeds the thermal velocity of the plasma particles. In their character these waves are analogous to waves with increasing amplitude excited in a waveguide or periodic structure when a beam of charged particles passes through them, if only the velocity of the beam exceeds some critical value. The authors obtained the dispersion formula for plasma in the presence of a beam and determined the law of increase of the electric field.

In their report “On the Nature of Thermoelectron Emission of an Oxide Cathode,” Corresponding Member of the Academy of Sciences of the Ukrainian SSR N. D. Morgulis and Ya. P. Zingerman presented the results of experimental studies

of the oxide cathode. Measurements of the thermoelectromotive forces, volume resistance, and thermoelectron emission during activation of the cathode show that in the initial stage the oxide layer of the cathode is characterized by a “hole sign” of the thermoelectric emf, which gradually changes over to an electronic one. This transformation begins at the core and, gradually advancing toward the surface, takes over the entire thickness of the cathode. Later, when the main increase in emission and the fall of the volume resistance occur, the thermoelectric emf already changes hardly at all, having reached a value of the order of mV/deg. It was further shown that, in an emission regime reaching up to \(22\ \mathrm{a/cm^2}\), the oxide cathode is a “thick” semiconductor film, whose emission properties should not be affected by the properties of the contact barrier layer at the core. With the aid of the combined method of combined use of a probe and an electrostatic analyzer, the authors made possible an experimental comparison of the cathode emission separately with its near-contact and near-surface properties. It was shown thereby that it is precisely the near-surface layer that determines the emission properties of the oxide cathode. The authors’ data agree with the hypothesis that cathode self-heating in a short-time switching regime is caused by electrolytic liberation of oxygen ions.

P. G. Borzyak reported on attempts to apply the wedge method to the investigation of oxygen–cesium and silver–oxygen–cesium photocathodes. The authors developed a method for obtaining wedge-shaped films of alkali metals and demonstrated the applicability of the micro-weighing method for determining the composition of oxygen–cesium films. For the silver–oxygen–cesium cathode, values of the optical constants were obtained, and it was shown that the spectral characteristic of the photoeffect of this cathode, as also of the oxygen–cesium cathode, is not in any connection with its optical properties described by the optical constants. Therefore the principal optical absorption in this case is not photoelectric, and the photoeffect must be ascribed an impurity nature.

The last group of reports reflected the work of Corresponding Member of the Academy of Sciences of the Ukrainian SSR A. F. Prikhotko’s laboratory on the study of the absorption spectra of crystals at low temperatures and the continuation of the corresponding theoretical investigations of A. S. Davydov.

At the general session A. F. Prikhotko presented the results of work carried out by her jointly with V. L. Broude, V. S. Medvedev, N. E. Neustroeva, and O. P. Kharitonova on the study of molecular crystals, chiefly of the aromatic series. Having improved the experimental technique, the authors succeeded in a short time in collecting abundant experimental material. Especially detailed studies were made of the absorption spectra of crystals—homologs of benzene. Crystals at low temperatures reveal, as was already shown for naphthalene (UFN 35, 289, 1948), a series of bands connected with electronic transitions of two kinds. Some of them coincide completely with the gaseous ones; others are present only in crystals. It turned out that, with an increase in the number of benzene rings in the molecule, the absorption spectrum of the crystal changes substantially.

In her report at the section A. F. Prikhotko communicated on two other works of the laboratory.

The well-known Obreimov diffraction method for obtaining dispersion curves with the aid of thin crystals gives good results in regions of transparency, but fails in the region of appreciable absorption. To expand the possibilities of this method, Corresponding Member of the Academy of Sciences of the Ukrainian SSR I. V. Obreimov and A. F. Prikhotko applied, for the indicated ...

higher goals the interferometric method. They constructed a miniature quartz interferometer with two plates, making it possible to work also at low temperatures, and used it to study anthracene. By this method the course of the dispersion curves can be traced considerably farther into the absorption region and even, for not very intense lines, in the absorption bands themselves. The use of data from the diffraction method facilitates, by the procedure developed, the processing of the results of the interferometric method. A very substantial result of the authors’ study of anthracene is the determination of the polarization of electronic transitions. The polarization of the transition corresponding to the far ultraviolet proved to be different from the other two.

A. Yu. Eichis, using Obreimov’s diffraction method, determined the dispersion curves for crystals of anthracene and phenanthrene at room temperature and at \(-140^\circ\)C. On the basis of the curves obtained, she calculated, by the usual dispersion formula, the oscillator strengths for the first electronic transitions of both crystals. They proved to be substantially different for the two measurements.

In the paper “Theory of the Dispersion and Absorption of Molecular Crystals,” A. S. Davydov, using the model of a one-dimensional molecular crystal, investigated the influence of the interaction of intramolecular vibrations with acoustic lattice vibrations and with rotational vibrations of the molecule. Owing to the interaction with the acoustic vibrations, an additional absorption maximum arises in the crystal. However, the dispersion curve is not the sum of the curves corresponding to the two absorption maxima; it also loses its usual symmetric form. Within a large part of the additional absorption band a normal dispersion is observed. Owing to the transfer of the energy of intramolecular vibrations to rotational vibrations of the molecules, absorption occurs not only in the region of the resonance frequency corresponding to the case of fixed molecules, but also in the region to the right of it, over a distance equal to the limiting frequency of the rotational vibrations (\(10\)—\(150\ \mathrm{cm}^{-1}\)). A characteristic feature of the absorption curve in the region of this frequency is its abrupt fall on one side. The dispersion curve in this case also differs from the sum of the dispersion curves for the two resonance frequencies.

P. Borzyak