An analysis of the stress relaxation curves

The shape of the stress relaxation curves is analysed under similar assumptions to those in the case of the creep. The presence of two mechanisms operating independently or sequentially is discussed.The authors wish to thank Prof. P. Luká and Prof. J. adek for valuable discussions and remarks. One of the authors (M. Hamerský) is grateful to Prof. J. C. M. Cotterill for his support of measurements on stress relaxation of Cd-Zn alloy during the author's stay at the Department of Structural properties, Technical University of Denmark, Lyngby.     

The oscillation of liquid molecules and the scatter of the Rayleigh line

The results of an experimental investigation into the intensity of the Rayleigh line scatter are given. Relations are obtained which describe the intensity distribution in the scatter and a model of the oscillation of liquid molecules is discussed.In conclusion the authors express their thanks and gratitude to M. F. Fuchs for his interest and for suggesting the idea of the possible connection between the constants determining the contour of the Kayleigh line scatter and the factor in front of the exponential in Frenkel's formula [9]. The authors are also grateful to A. V. Sechkarev for valuable criticisms and assistance.     

Quantum processes in an intense electromagnetic field. I

The Furry approach for arbitrary external electromagnetic fields that do not generate real pairs is considered. Problems of the existence of a stable vacuum in the field are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 81–86, October, 1976.The author is grateful to F. A. Berezin for refinement of the theorems used in this work and to A. A. Grib, S. G. Mamaev, and V. M. Mostepanenko for valuable critical comments.     

Mechanism of thermoluminescence in AlN:O

Thermoluminescence of AlN: O in the 80–420 K temperature range is described. Two mechanisms are proposed for its explanation: the release of charges due to thermal ionization of the centres and migration of charges on the impurity centres. The probability of the occurrence of impurity migration is discussed.The authors are indebted to Dr. J.Pastrák, CSc for valuable discussions, stimulating hints and his interest in the present work. They are also grateful to Ing. J.Hejduk and his collaborators for preparation of samples and for chemical analyses.     

Phonon structure of Mn4+ activator centres in AIN

The temperature dependence of the phonon structure of Mn⁴⁺ activator centres in AlN was investigated. The spacing of the observed equidistant peaks is –160 cm^–1; phonons of this wave number, however, were not found in the lattice vibration spectra. Pseudo-local vibrations are, therefore, presumed to be involved. The intensity of individual peaks obtained by graphical decomposition of the emission maxima is in agreement with theory for all measured temperatures.We are grateful to Professor M. Trlifaj and Mr. V. Novotný for their valuable discussions, to Ing. J. Hejduk and Mr. J. Veselý for the sample preparation, and to Ing. J. Vondich and Mrs. H. Kuerová for their help in computing the theoretical curves.     

Covariant polarization density matrix for pure states of particles with integral spin

A closed expression is obtained for the covariant polarization density matrix of pure states of particles with integral spin. The application of the results to the decay f⁰ is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 111–115, August, 1977.I am very grateful to A. B. Govorkov for his interest in the work and valuable comments. I am also very grateful to B. Z. Kopeliovich for a helpful discussion.     

Excitation propagation and memory functions in molecular exciton-phonon systems: Inclusion of exciton nonconservation

A tractable model is introduced to discuss the exciton propagation in ideal molecular crystals under the influence of phonons when the number of excitons is (owing to transfer, influence of the surrouding and crude representation used to avoid problems with nonadiabatic effects) not conserved. As compared with standard results with the exciton conservation, excitation memory functions (summed partially to the infinite order in the exciton-phonon coupling) are found to be little changed.This paper is dedicated to Prof. M. Wagner on the occasion of his 60^th birthday. The author is indebted to him for his constant interest and inspiration. He should also like to express his gratitude to Prof. Wagner and the University of Stuttgart for invitation and extending their kind hospitality during a stay in Stuttgart where this work was performed. A grant of the Heraeus Stiftung making this stay possible is also appreciated.     

High-frequency conductivity of films with different surfaces

The high-frequency conductivity of films with different surfaces and spherically symmetric energy band is calculated. General expressions are obtained for the conductivity for arbitrary values of the following quantities: the film thickness, the frequencies of the incident wave, and the specularity parameters with allowance for the bulk scattering mechanisms. A detailed analysis is made of the influence of the boundedness of the sample and the different surfaces on the conductivity in limiting cases with respect to the film thickness and the frequency of the incident wave. In particular, it is shown that in the case of thin films and low frequencies the conductivity decreases with decreasing thickness. The influence of different surfaces on the conductivity is also considered. The dependence of the coefficient of absorption of the wave on the film thickness and the carrier density is analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 88–92, July, 1980.We are sincerely grateful to Professor B. M. Askerov for his interest in the work and a helpful discussion of the obtained results.     

Charged dust spheres in the general theory of relativity

Tolman's problem for charged dust is solved in quadratures. The external solution (Reisner-Nordstrom) is obtained in the natural reference frames of charged test particles. The conditions for matching a charged dust sphere with a vacuum over a fixed Lagrangian sphere are formulated. They are equivalent to boundary conditions on the initial data.Translated from Izvestiya Uchebnykh Zavedenii, Fizika, No. 4, pp. 107–113, April, 1976.The author is deeply grateful to his colleagues K. A. Bronnikov and M. A. Koval'chuk for discussion of this article and a number of valuable remarks.     

Studies of a light source based on the retardation of supersonic pulse discharge jets

The spectral distribution of the radiated energy of an imploded plasma formed through the collision of a super-sonic plasma jet of a pulse discharge against a stationary obstacle was studied. Its brightness temperature and absorption were measured. It is shown that the plasma can served as a source of a continuous spectrum with an energy distribution similar to that of an absolute blackbody. The brightness temperature of the source was variable over a broad range by appropriate adjustment of the discharge conditions.The authors are very grateful to M. A. El'yashevich for his interest in the present study and for his valuable comments on the results.     

Influence of a magnetic field on the optical orientation of Γ6 × Γ8 hot excitons in cubic semiconductors

A calculation is performed of a hot polarized gG₆ × ₈ exciton luminescence excited in cubic semiconductors by polarized light and emitting longitudinal optical phonons before the emission decay. It is shown that the degree of linear and circular polarization of exciton emission does not, in general, depend monotonically on the field strength.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 85–89, July, 1984.The author is grateful to E. L. Ivchenko for his help in this work, and to G. E. Pikus for a discussion of results.     

Scattering of photons on plasmons in superconductors

Scattering of photons on plasmons in superconductors is discussed in the framework of the Landau-Ginzburg theory of superconductivity. A simple interpretation of the Feynman diagrams for this scattering is given.The author is grateful to Dr. M. Noga for stimulating discussions and valuable advice. He is indebted to Drs. M. Odehnal and J. Hoek for valuable remarks how to interpret the results obtained.     

Some recent developments in cosmic rays

Recent work on the energy spectrum, composition and anisotropy of the primary cosmic ray flux from about 10⁹ eV to 10²⁰ eV is summarized, together with related information on phonons between 1 MeV and 200 MeV. Solar particles are not discussed, the emphasis being on topics bearing on the origin of the radiation, which is still an unsolved problem, although the probabilities are strongly in favour of an entirely galactic origin for the particles. Mechanisms of acceleration are not discussed.     

Exciton effects in disordered semiconductors

Experimental data on optical and photoelectric properties of molecular (polymeric) disordered semiconductors of inorganic and organic nature are analyzed. It is shown that anomalies in optical properties and in the structure of the internal photoeffect quantum-yield spectrum can be understood by assuming the existence of exciton states in disordered semiconductors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 86–101, February, 1976.The author is grateful to Prof. S. G. Grenishin for his interest in this work.     

One- and two-nucleon emission following the nuclear radiative pion capture

Decay channels in the radiative pion capture reactions are discussed and their respective yields calculated. Specific consequences of a recently suggested quadrupole excitation mode in the ( ^–,) reaction are found and their experimental investigation is urged. A yield of about 19% of the total decay was estimated for the two-nucleon decay channel even for the light nuclei like¹⁶O.On leave of absence fromthe Institute of Nuclear Physics, Czechosl. Acad. Sci., 250 68 e, Czechoslovakia.We a re grateful to the members of Lausanne-Münich-Zürich collaboration, particularly to C.Joseph, G.Strassner and P.Trüol for supplying us with the experimental results prior to publication. M. G. also thanks them and R.Wünsch for interesting discussions. We are grateful to R. A.Sakaev for his help at an early stage of this work.     

Pressure and temperature dependence of Raman scattering and optical absorption in crystalline S4N4

Raman scattering and optical absorption in crystalline S₄N₄ have been measured both as a function of pressure at 295 K and low temperatures. Polarized single crystal Raman data were also obtained as an aid in the assignment of the Raman active phonons. The pressure coefficients of the Raman active external and S-S stretching modes show a discontinuity near 7 kbar indicative of a second order phase change. The optical absorption edge at about 2.5 eV of a sublimed film of S₄N₄ shows red shifts of 1.3 × 10^?5 eV bar^?1 and 6.3 × 10^?4 eV K^?1 with pressure and temperature respectively. In the light of these results, the electronic, vibrational and structural properties of the crystal are discussed.     

Seven matched bolometers for millimeter wave observations

We have fabricated a millimeter wave observation system using an array of bolometric detectors. The performance of the system depends largely on obtaining identical performance from each bolometer. We achieved a variance in the responsivity of less than 4% except for one of the seven elements. The bolometers had an electrical NEP of 1.3×10^–16 W/Hz under radiation background loading of 30pW. We used an AC bridge readout circuit to significantly improve the stability of the array and showed that observations can be done without a mechanical beam switch. The bolometer array is now in use on the Nobeyama 45-m telescope for 150GHz observations.H. Matsuo thanks those who helped him work on bolometers during his stay in Berkeley, especially to T. Wilbanks, M. Devlin and C. Inman. The authors are also grateful to S. Sato and T. Watabe at Nagoya University for their advice on the AC bridge circuit. This work is partly supported by the Shimazdu Science Foundation.