Chronicle

At the Session of the Department of Physical, Mathematical and Chemical Sciences of the Academy of Sciences of the Ukrainian SSR

On June 20–23, 1949, a Session of the Department of Physical, Mathematical and Chemical Sciences of the Academy of Sciences of the Ukrainian SSR was held in Kiev.

At the plenary meetings, physics was represented by reports by V. E. Lashkarev, Full Member of the Academy of Sciences of the Ukrainian SSR, “Studies of the Kinetics of Photoconductivity of Semiconductors,” and by G. D. Latyshev, Corresponding Member of the Academy of Sciences of the Ukrainian SSR, “Research in the Field of the Physics of the Atomic Nucleus.” The main theme of the first report was the questions of nonlinear photoconductivity, which is not the result of any side or secondary processes, but corresponds to the very essence of the phenomenon of photoconductivity. According to the theory developed by the speaker, the kinetics of photoconductivity is characterized by the probability of disappearance of photocurrent carriers, the inverse of their lifetime, and by the “yield of photocurrent,” i.e., the probability of creation of photocurrent carriers under the action of light. For the experimental determination of these quantities, several different methods were employed, of which the method of the displaced photocurrent and the method of analyzing oscillograms of single pulses were developed in the speaker’s laboratory. A detailed study by G. A. Fedorus and I. R. Potapenko of the kinetics of photoconductivity of lead-sulfide photoresistances over a wide temperature range led to a number of interesting results. It turned out that in this case the local levels at which photoelectrons are fixed are the same levels that determine the hole conductivity of the semiconductor. Thus, the kinetics of photoconductivity proved in this case to be the kinetics of the establishment of thermal equilibrium between the band and the local levels. Recombination of the trapped photoelectrons with free holes occurs by means of a two-stage process, of which one does not require activation energy, while the other does require it. It turned out that near the lower band there exist levels filled with electrons that energetically capture the free holes created by the action of light and then give them up by thermal excitation.

In addition to lead-sulfide photoresistances, copper oxide was also subjected to investigation; in it groups of various levels were found and studied.

One of the most interesting reports at the session was that of G. D. Latyshev, in which the speaker dwelt on three questions connected with the recent work of the laboratory he heads. These were the investigation of the fine structure of β-spectra, the discovery of monochromatic positrons of internal conversion, and investigations that led to

toward the discovery of angular correlation between $\gamma$-quanta and $\alpha$-particles emitted in radioactive decay.

One of the most fundamental results of G. D. Latyshev’s work was his discovery of the effect of nuclear rotation. G. D. Latyshev succeeded in presenting his remarkable achievements in the field of nuclear physics in a very clear and engaging form. As is known, for these works G. D. Latyshev was awarded the Stalin Prize, first class, for 1948.

The joint meeting of the sections of physical and chemical sciences was devoted to the properties of liquids. Lively debate was provoked by the report of Corresponding Member of the Academy of Sciences of the Ukrainian SSR S. S. Urusovsky, “On Features in the Temperature Dependence of Certain Properties of Liquids.”

Having increased the accuracy of measurements of the surface tension of liquids by one order of magnitude through improvement of the capillary method, the author studied the temperature dependence of the surface tension of certain liquids as they enter the region of the supercooled state. For certain substances that exist in the solid crystalline state in several phases, he found deviations from the general course of the temperature dependence of the surface tension of supercooled liquids precisely at those temperature points that correspond to the temperatures of appearance of the various modifications of this substance. On this basis, in the speaker’s opinion, one may conclude that different phases of the liquid state exist, corresponding to the phases of the solid state. At the same time, however, the speaker noted that he observed similar special points also for liquid benzene, for which in the solid state only one phase is known.

In the remarks on the report, a number of physicists noted that transformations in liquids as substantial as the transition from one phase state to another proposed by the speaker should entail, in addition to changes in surface tension, a whole series of other physical changes of macroscopic scale, for example optical, X-ray-structural, and others.

To verify the conclusions that the speaker had drawn on the basis of measurements of surface tension, it was considered desirable to employ other methods of physical investigation of the changes occurring in a liquid.

I. M. Lifshits, having analyzed from the theoretical side certain variants of explaining the observed special points by phase transformations in liquids, showed that they cannot be consistently derived on the basis of thermodynamic or any other physical considerations.

B. I. Verkin and N. S. Rudenko, in the report “Certain Regularities in the Viscosity of Simple Liquids (Liquefied Gases),” reported on their work to elucidate general physical regularities for simple liquids, by which are meant nonpolar and nonassociating substances built of molecules with spherical or nearly spherical symmetry. To this end, first of all, the dependence of viscosity on temperature was investigated for liquid oxygen, nitrogen, argon, hydrogen, deuterium, chlorine, bromine, carbon dioxide, methane, and ethylene at equilibrium pressures. This dependence proved to be linear. At equal reduced temperatures

$$ \theta = \frac{T}{T_{\mathrm{cr}}} $$

the reduced fluidities of simple liquids turn out to be equal to one another. The ratio of viscosity to the molecular weight of the liquid, for any value of reduced temperature, is a constant. To isolate and study the purely temperature factor, relative measurements were made of the viscosity of nitrogen and argon in a definite

in a temperature interval at constant density values, using a falling-body viscometer technique developed for this purpose. It turned out that at constant density the influence of temperature on the fluidity of a liquid is less than at equilibrium pressures, and this influence is the smaller, the lower the density of the liquid. It is interesting that the curve of the temperature dependence of fluidity changes continuously in passing through the critical temperature. The authors come to the conclusion that, in a liquid of constant density, the mechanism by which fluidity changes with temperature is the same as the mechanism of this change in a strongly compressed gas.

B. E. Gordon and V. L. Broude were interested in the nature of the color of solutions of alkali and alkaline-earth metals in ammonia. To elucidate this nature they introduced electrons from a platinum point into liquid ammonia. In liquid ammonia containing a small amount of electrolyte (NaCl, NaNH₂), a dark-blue stream flows from the platinum pointed cathode toward the anode. Investigating the behavior of this stream in a magnetic field, the authors concluded that electrons do indeed flow from the cathode into the liquid dielectric, and that the blue color of the stream is caused by their presence. From measurements of the time of disappearance of the color after the current was switched off at various temperatures, the activation energy of the decolorization reaction was determined; the authors regard this reaction as electron recombination.

At the meetings of the Section of Physical Sciences, a whole group of reports was presented by the laboratory headed by Corresponding Member of the Academy of Sciences of the Ukrainian SSR B. G. Lazarev. Continuing their investigations of the properties of superconductors under hydrostatic compression, L. S. Kan, B. G. Lazarev, and A. I. Sudovtsev, in addition to indium and tin, studied lead, mercury, thallium, tantalum, Bi₃Ni, the lead–tin and tin–bismuth alloys. Since, according to previous ideas, hydrostatic compression of a superconductor should lower the critical temperature and the magnetic field of the superconducting transition, the most interesting result of the investigations was the discovery, in thallium and Bi₃Ni, of changes of these critical quantities in the opposite direction.

Having constructed a long bimetallic spiral, one of whose metals was a superconductor, the authors measured the precisely calculated change in volume of tin at the superconducting transition and even determined the temperature dependence of this change. The results obtained agree with the interpretation of the superconducting transition under these conditions as a first-order transition. Thus the opinion of Canadian physicists, who had asserted that the magnitude of the effect lies beyond the limits of experimental attainability, was refuted.

V. I. Khotkevich and V. R. Golik studied the influence of plastic deformation on the superconductivity of metals: tin, indium, thallium, and mercury. It turned out that plastic deformation considerably changes the superconducting properties of the metals studied, and its influence differs from that of hydrostatic compression or stretching. For tin, indium, and thallium, plastic deformation leads to a considerable increase in the critical temperature, caused by the distortions of the lattice that it produces. The change of the critical temperature with the degree of deformation is described by curves with maxima, owing to the presence of two phenomena: an increase of the critical temperature as a result of plastic deformation and its decrease as a result of hydrostatic compression.

A. A. Galkin, L. S. Kan, and B. G. Lazarev continued investigations on the kinetics of the superconducting transition. The peculiar variation of the resistance of a specimen near the transition temperature (see UFN 35, issue 2, 285, 1948) can be explained if it is assumed that, in the process of transition,

in the specimen there arise nuclei of the superconducting phase, having a finite growth rate (\(\simeq 1\ \mathrm{cm/sec}\) without a magnetic field and above \(10\ \mathrm{cm/sec}\) in a field). However, because of their smallness these growth rates of the nuclei cannot be used to explain the observed inertia of the destruction of superconductivity by an alternating high-frequency current. Experiments were undertaken to determine the velocity of displacement of the magnetic-field front inside a superconductor by means of a transformer with a superconducting core. These experiments led the authors to the conclusion that there exists a new kind of intermediate state, connected with an adiabatic transition of the specimen in a magnetic field. The observed transition velocities of the specimen are limited by the time required for the field to change from the critical isothermal value to the critical adiabatic one. In a specimen in the intermediate state the wave front moves with a speed of approximately \(10^3\ \mathrm{cm/sec}\).

Studying the resistance of bismuth single crystals in a strong magnetic field, E. S. Borovik and B. G. Lazarev found that the character of the increase in the resistance of plates whose thickness is of the order of the electron mean free path is affected by their orientation relative to the field. The increase in resistance is minimal when the plane of the plate is parallel to the field. The sharp dependence of the magnitude of this effect on temperature makes it possible to estimate the order of the electron mean free path in bismuth. In trying to understand the mechanism of this phenomenon, the authors came to the conclusion that it was necessary to investigate the mechanism by which the resistance of metals increases in a magnetic field. This work led them to the conclusion that the existing theoretical ideas about galvanomagnetic phenomena in metals are completely unsatisfactory.

A. I. Akhiezer, having characterized the difficulties arising in the classical and quantum theories of galvanomagnetic phenomena, pointed out that all the difficulties are removed if one admits a bisonal scheme of conductivity for a metal (electron conductivity in one zone and hole conductivity in another, with equal concentrations of electrons and holes). D. I. Blokhintsev, in his remarks, joined A. I. Akhiezer’s point of view and noted that this idea had already been put forward in 1933 in a paper by Blokhintsev and Nordheim.

B. I. Verkin, B. G. Lazarev, and N. S. Rudenko found for a tin single crystal at a temperature of \(4.2^\circ \mathrm{K}\) a phenomenon known since 1930 for bismuth, and later also found for zinc—namely, a periodic change of magnetic susceptibility with change of the magnetic field. The period of the change in magnetic susceptibility decreases with increasing field, being, for example, \(100\ \mathrm{gs}\) at a field of \(7500\ \mathrm{gs}\), \(80\) at \(9500\), and \(60\) at \(11000\). A. I. Akhiezer noted that a free-electron gas under Fermi statistics gives magnetic susceptibility with oscillations; however, upon further consideration of the question, this model of the electron gas does not stand up to scrutiny, since the consideration leads to the conclusion that not all electrons participate in this phenomenon.

Corresponding Member of the Academy of Sciences of the Ukrainian SSR N. D. Morgulis, in his report “Elementary processes in emission from a surmão-cesium cathode,” summarized the results of studies being carried out in the laboratory he heads on the photoeffect and secondary electron emission of the above-mentioned cathode.

In the report “Light-intensity monochromators for sharply focused X-ray tubes,” Prof. B. Ya. Pines reported on experiments to improve X-ray structural-analysis apparatus. In order to increase the power of X-ray tubes, a tube with a rocking anticathode has been constructed, possessing a number of advantages in simplicity, dimensions, and reliability in operation as compared with ...

compared with tubes having rotating anticathodes. To increase the luminosity of monochromators, the author uses doubly bent crystals (a toroidal surface) as the latter, with a ratio of radii of curvature determined by the wavelength of the radiation. These crystals focus the monochromatized beam not in one but in two dimensions; they are obtained from single crystals by plastic deformation. The improvements made make it possible to obtain radiographs of microcrystalline objects with exposures from one to several hours. Monochromatic photography carried out with the apparatus developed makes it possible to observe, on radiographs, effects not distinguishable in ordinary photography. Visible, for example, are the magnesium carbide lines in low-carbon steel (steel 3); the broadening of the lines in radiographs of deformed metal is much clearer, etc.

Prof. S. D. Gertsriken and I. Ya. Degtyar reported the results of their work on the study of the “Influence of alloying impurities on the process of diffusion of chromium in iron—chromium alloys.” The authors studied the diffusion of chromium in iron—chromium alloys with additions of nickel, beryllium, silicon, titanium, tin, niobium, and tungsten in the temperature interval 950–1070° C. They found that the effective activation energy for the diffusion of chromium in an iron—chromium alloy is

\[ 112 \frac{\text{kcal}}{g \cdot atm}. \]

The change in the activation energy, and also in the activation entropy, of a ternary alloy relative to a binary one depends in a definite way on the valence of the impurity. Using the data obtained, one can calculate the homogenization time of each of the mentioned ternary alloys if the homogenization time of the binary alloy is known.

Theoretical physics was represented by three papers. Prof. S. I. Pekar presented the “Quantum theory of polarons.” As in the previously developed theory, an ionic dielectric crystal with an “extra” electron introduced from outside is considered. But the motion of the electron and ions is now treated quantum-mechanically. The ground state of the system is a polaron: the crystal proves to be polarized by the field of the electron, as a result of which it forms for the electron a potential well with a discrete energy spectrum. The effective mass of the polaron has a magnitude on the order of hundreds of free-electron masses. The quantum stationary states of the system in the zero approximation are plane polaron waves; in the first approximation these waves are scattered by polarization oscillations of the ions. A calculation has been made of the scattering probability, the free path, and the mobility of polarons. The latter turned out to be greater than the mobility of a “band” electron.

Such a lively discussion developed around the paper, chiefly on questions concerning the calculation of scattering and the free path, that it had to be transferred to a special session of the theoretical group.

K. B. Tolpygo gave a report on the “Theory of vibrations of an ionic lattice taking into account the ability of the ions to deform,” allowing for the overlap of the electron shells of a pair of ions and the dependence of the exchange integral on their dipole moments. He obtained an expression for the energy of a pair of ions as a function of their displacements and dipole moments, containing quantum terms. On the basis of this, the equations of motion are formulated and solved in the first and second approximations, for a rock-salt-type lattice, allowing for retardation of the interaction. The parameters of the theory are determined for six crystals from comparison of the dispersion formula obtained with experiment; after this, the moduli of elasticity and compressibility are determined theoretically. Tolpygo next found the law of dispersion in the next approximation and established a decrease of frequency with increasing wave vector. This illustra-

refutes L. I. Mandelstam’s assertion about the possibility of a negative group velocity in a crystal. The report also showed that double refraction in crystals of this type lies beyond the limits of experimental accuracy.

Prof. A. I. Akhiezer, in the report “On the Diffraction of Charged Rays,” presented a theory of the scattering of fast charged particles by heavy nuclei, regarded as an absorbing black sphere. If, for considering neutron scattering, one may apply the methods of calculation developed in the theory of light diffraction, then for charged particles a different consideration is required, one that takes into account the action of the nuclear field, which was carried out by the author. He obtained general expressions for the cross section for the scattering of charged particles by absorbing heavy nuclei.

An optical analogue of the case considered, as D. I. Blokhintsev pointed out, is the scattering by a black screen placed in a dielectric medium with a variable refractive index.

P. Borzyak