Faculty of Physics
M.V.Lomonosov Moscow State University
Theoretical and mathematical physics

The method of projection operators in the theory of pulsed NMR processes with selective irradiation

V.S. Tumanov

Moscow University Physics Bulletin 1993. 48. N 6. P. 1

The method of projection operators is applied to the theory of pulsed nuclear magnetic resonance processes with selective irradiation. The use of the projection operator method alone and in combination with the known one-transition operator method is considered. In particular, general formulas are derived that describe the evolution of the density operator in the case of arbitrary first-order spectra.

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Phase spaces of nonzero curvature and the evolution of physical systems

A.G. Popov

Moscow University Physics Bulletin 1993. 48. N 6. P. 5

It is proposed that phase spaces of nonzero cunature (non-Euclidean phase spaces) be used to describe physical phenomena. The singularities of these spaces show good correspondence to the invariant (equilibrium) states of physical systems. Pseudospherical phase spaces are investigated for which the Lobachevski geometry is valid. General principles are stated for the evolution of phenomena described, in particular, by the sin-Gordon, Korteweg-de Vries, Burgers, Liouville, and other equations.

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Image information content in the problem of interpretation of tunnel electron microscope measurements

V.P. Manolov, Yu.P. Pyt'ev

Moscow University Physics Bulletin 1993. 48. N 6. P. 11

The maximum resolving power of a measuring-computer system based on a scanning tunnel microscope (STM) has been investigated in the linear approximation. A sequence of surface classes of increasing dimension is denned. In each of the classes the surface is reconstructed with maximum accuracy from STM measurement data. The dependence of the error in the reconstruction of the surface coordinates on the noise-to-signal ratio is determined. The conditions are found for an optimal discretization of STM measurements that provides maximum accuracy in the reconstruction of measured surface.

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Atomic and nuclear physics

Multistage emission of $\gamma$-quanta in reactions of neutron capture by nuclei at medium energies

F.A. Zhivopistsev, V.A. Ivanov, Hurelsukh Sar-Odyn

Moscow University Physics Bulletin 1993. 48. N 6. P. 18

The possibility of describing a nonequilibrium gamma spectrum in the $(n,\gamma)$ reaction at $\varepsilon_n$ = 14.1 MeV using the quantum model of statistical multistage compound processes (SMCP) and statistical multistage direct processes (SMDP). It is shown that a consistent description of experimental data can be obtained for the $(n,\gamma)$ reactions at $\varepsilon_n$ = 14.1 MeV on $^{93}Nb$ and $^{133}Cs$ nuclei if contributions from lSDP, 2SDP, SMCP, and equilibrium gamma emission are taken into account.

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A model of self-induced transparency in a medium of nonlinear oscillators

V.A. Kubarev

Moscow University Physics Bulletin 1993. 48. N 6. P. 23

The problem of propagation of wave packets in a resonant medium consisting of classical nonlinear oscillators is considered. For the case of transverse propagation in a cold collisionless magnetoactive plasma, solutions of the type of solitary waves are obtained. An analogy with the phenomenon of optical self-induced transparency is noted.

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Diffraction of radio waves by a random phase screen with small-scale and large-scale inhomogeneities

A.G. Vologdin, V.D. Gusev, L.I. Prikhod'ko

Moscow University Physics Bulletin 1993. 48. N 6. P. 30

The problem of diffraction of radio waves by a random phase screen with small-scale and large-scale inhomogeneities (LSI) of the ionosphere is considered. The solution of this problem helps analyze the effect of the LSI focusing action on the statistical characteristics of fast fluctuations of the field reflected by the ionosphere. In particular, it is noted that the focusing (defocusing) action of LSI results in a quasi-periodic variation of the shape and the orientation of the characteristic ellipse of the correlation coefficient of the field on the Earth's surface, which is confirmed experimentally.

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The range of applicability of the geometrical optics approach for a refractive medium with random inhomogeneities

V.D. Gusev, S.M. Golynskii, Wu Xiaoping

Moscow University Physics Bulletin 1993. 48. N 6. P. 34

Based on the criterion of nonintersection of rays, a new condition is found that determines the range of applicability of the geometrical optics approximation for oblique incidence of radiation on a refractive scattering layer. It is shown that the area of validity of the geometrical optics approach substantially depends on the "intensity" of fluctuations of the medium permittivity. The derived condition considerably extends the applicability of the ray interpretation of the wave field in the medium in question compared to the standard determination of the region of geometrical optics.

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Excitation and dynamics of strongly nonlinear Trivellpece-Gould modes

I.M. Aleshin, L.S. Kuz'menkov, 0.0. Trubachev

Moscow University Physics Bulletin 1993. 48. N 6. P. 38

Conditions are found for the excitation of strongly nonlinear waves in magnetized plasma in a cylindrical waveguide by an ion bunch. It is shown that both periodic and solitary waves can be excited and the form of the waves strongly depends on the plasma temperature.

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Optics and spectroscopy

A mode-locked solid-state ring laser with intracavity generation of the second harmonic

M.R. Paleev

Moscow University Physics Bulletin 1993. 48. N 6. P. 44

The intracavity generation of the second harmonic makes it possible to substantially improve the amplitude-frequency characteristics of a solid-state ring laser operating in the active mode-locking regime: to eliminate the competitive suppression of counterrunning waves, to extend the region of the beating regime, and to remove anomalies in the frequency characteristic, which are typical of the free running laser mode. The effect of the loss modulation frequency on the width of the mode-coupling region and the beat frequency is analyzed.

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Acoustics and molecular physics

Thermal explosion during laser dissociation of molecular gas

V.E. Baklashova, B.F. Gordiets, A.I. Osipov

Moscow University Physics Bulletin 1993. 48. N 6. P. 50

The kinetics of diatomic molecule photodisaociation under laser irradiation is studied theoretically for the case where heat exchange exists between the gas and the ambient medium. The mode of thermal explosion in such a system is discovered and investigated. The behavior of the entropy production function in the photodissociation process is analyzed.

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Acoustic nonlinearity of nonuniform flow of an oscillating liquid

A.A. Zaikin, 0.V. Rudenko

Moscow University Physics Bulletin 1993. 48. N 6. P. 56

The efficiency of nonlinearity production by a localized geometrical inhomogeneity is estimated using as an example of the latter a particle layer. The Bernoulli equation for the near field and a Lighthill-type equation in the wave zone are employed in the calculation. Use is also made of the equivalent nonlinearity parameter.

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Solid state physics

Cluster approximation for the Heisenberg ferromagnetic with nonmagnetic impurities

A.K. Arzhnikov, A.V. Vedyaev

Moscow University Physics Bulletin 1993. 48. N 6. P. 59

An expression for the stiffness coefficient D of spin waves as a function of impurity concentration was derived for the Heisenberg ferromagnetic substance with nonmagnetic impurities in the limit of low temperatures using a simple cluster method of the Bethe-Peierls type. The pattern of the dependence was shown to change with varying concentration from $D\simeq J_0bSx/2$ at $x\sim 1$ to $D\simeq J_0bS|x—х_0|^{2\tau}$ at $x\sim x_0$.

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Microstresses induced by hydrogenation and plastic defomiation in palladium and its alloys with copper, platinum, hafnium, and samarium

G.P. Revkevich, M. Mitkova, A.A. Katsnel'son, V. Khristov, G.P. Zhmurko, I.N. Avertseva

Moscow University Physics Bulletin 1993. 48. N 6. P. 63

The positions of maxima in the X-ray diffraction patterns were measured and the lattice parameters $a$ were calculated for pure palladium and its alloys with Cu, Pt, Hf, and Sm. Microstresses developed in the sample surface subjected to plastic deformation by polishing increase the lattice parameter of the cubic crystal, the effect being dependent on the crystallographic orientation of the surface. Saturation of the samples with hydrogen decreases the level of microstresses and may even change their sign.

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Geophysics

Model description of a plane turbulent mixing layer in a viscous fluid

E.P. Anisimova, V.V. Petrov, A.A. Speranskaya

Moscow University Physics Bulletin 1993. 48. N 6. P. 70

A model description is proposed for the average velocity profile in a plane turbulent mixing layer. The model takes into account the experimental fact of formation of coherent structures in such layers. The model was tested using data of laboratory experiments and field observations.

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Optics and spectroscopy

Interpretation of the 180 nm emission band observed in the spectra of discharge plasma of Kr + iodine donor composition

I.S. Lakoba, S.P. Chernov, P.B. Essel'bakh

Moscow University Physics Bulletin 1993. 48. N 6. P. 74

A continuum band with the intensity maximum at 180 nm was detected in the emission spectrum of a discharge plasma of krypton with an admixture of iodine-containing compounds. An analysis of the experimental data allows the band to be explained by a quasi-intersection of the potential curves of excited states of the KrI molecule, not observed earlier in monohalides of inert gases.

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Acoustics and molecular physics

The effect of subsidiary waves on the acoustic field structure in a liquid layer

A.V. Kondrashova, K.A. Pestov, 0.S. Tonakanov

Moscow University Physics Bulletin 1993. 48. N 6. P. 77

The horizontal and vertical structure of the acoustic field of the subsidiary wave, and also the integrated field of the first normal and the subsidiary waves, are considered theoretically. It is shown that under certain experimental conditions it is necessary to take into account the inftuence of the subsidiary wave on the acoustic field.

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Solid state physics

Defect production in gallium phosphide induced by weakly absorbed laser pulses

I.Yu. Viskovatykh, P.K. Kashkarov, V.Yu. Timoshenko

Moscow University Physics Bulletin 1993. 48. N 6. P. 81

Defect production has been studied in gallium phosphide crystals irradiated by ruby laser pulses corresponding to the region of transparency for interband transitions. It has been shown that the defect production efficiency increases with an increase in the original defect density in the near-surface area resulting from the peculiarities of physical and chemical treatment of specimens. The process of defect rearrangement has been found to be athermal. The concentration of laser-induced centers of photoluminescence quenching depends nonlinearly on the laser pulse energy.

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Electron-diffraction determination of phase structure in Pr-Fe alloys

A.S. Ilyushin$^1$, N.A. Khatanova$^1$, E.A. Rykova$^1$, N.B. Kol'chugina$^2$, 0.D. Chistyakov$^2$, G.S. Burkhanov$^2$

Moscow University Physics Bulletin 1993. 48. N 6. P. 84

Electron diffraction of samples of the alloy Pr-77 at.% Fe, preliminarily annealed for 100 hat 600°C, revealed the following phases: $\alpha-Pr$, $Pr_2Fe_{17}$, $PrFe_2 (С14)$, and $PrFe_2 (С15)$. The lattice parameters were determined for the hitherto unobserved intermetallic compounds $PrFe_2$ of two types: (i) $MgZn_2 (С14)$, space group $P6_3/mmc$, $а = 5,26 А$, $с = 8,62 А$ and (ii) $MgCu_2 (C15)$, space group $Fd3m$, $а = 6,76 А$. Structural-orientation correlations between these compounds and $Pr_2Fe_{17}$ were established, which shows that the three compounds can be transformed into each other.

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On the theory of hopping conductivity of quasi-one-dimensional disordered systems

I.P. Zvyagin

Moscow University Physics Bulletin 1993. 48. N 6. P. 87

The anisotropy of hopping conductivity of quasi-one-dimensional systems related to the convoluted shape of optimal carrier paths is considered. It is demonstrated that for systems described in the framework of the R-percolation model the specific features of the carrier path shape do not lead to exponential anisotropy of the conductivity.

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