Oscillatory processes at phase boundaries in zone refining with a temperature gradient in a stationary temperature field

A study is made of the effect of the threshold of kinetic processes on the character of motion of the phase boundaries in a crystal-melt. Numerical results are presented and an analysis made of the conditions under which oscillations arise in the rate of growth of epitaxial layers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 68–72, March, 1995.     

Structural distortions of semiconductor silicon crystals caused by a permanent electric field

Structural distortions in nonpolar crystals, in particular, in semiconductor silicon caused by a permanent electric field have been revealed by means of X-ray interferometry. It is shown that the field changes the frequency (period) and direction of moiré fringes and at the voltage exceeding 1.5 kV the moiré patterns disappear. Moiré pattern obtained under action of the electrostatic field does not depend on the direction (polarity) of the field and is completely the same for both polarities.     

Crystallization of a eutectic Pb-Sn melt in the thermal field of a temperature gradient

The crystallization of a eutectic Pb-Sn melt is investigated under nonuniform conditions in vacuum, in air, and in water. It is found that the structure of the solid-state systems formed depends on the cooling conditions. A structural inhomogeneity in volume and a correlation between the microstructure and the coordinate in a thermal field are revealed. Results of technological, electron-microscopic, and statistical investigations are reported. It is shown that the experiment is consistent with the crystallization theory of eutectic melts.     

Effect of a temperature gradient on the polarization distribution in the surface layer of TGS crystals

Effect of variable temperature gradients on the polarization of the surface layers of triglycine-sulphate (TGS) crystals has been analyzed. A fundamental difference in the impact of these gradients on the sample sides corresponding to the positive (“+” P _s ) and negative (“?” P _s ) yield of the polarization vector has been discovered. In particular, it has been found that repolarization of the surface layer under the impact of thermal radiation occurs only on the “?” P _s side.     

Mechanisms of dendrites occurrence during crystallization: Features of the ice crystals formation

Dendrites formation in the course of crystallization presents very general phenomenon, which is analyzed in details via the example of ice crystals growth in deionized water. Neutral molecules of water on the surface are combined into the double electric layer (DEL) of oriented dipoles; its field reorients approaching dipoles with observable radio-emission in the range of 150 kHz. The predominant attraction of oriented dipoles to points of gradients of this field induces dendrites growth from them, e.g., formation of characteristic form of snowflakes at free movement of clusters through saturated vapor in atmosphere. The constant electric field strengthens DELs' field and the growth of dendrites. Described phenomena should appear at crystallization of various substances with dipole molecules, features of radio-emission can allow the monitoring of certain processes in atmosphere and in technological processes. Crystallization of particles without constant moments can be stimulated by DELs of another nature with attraction of virtual moments of particles to gradients of fields and corresponding dendrites formation.     

Interference absorption coefficient of X-rays in crystals in the presence of temperature gradient

We investigate experimentally and theoretically the behavior of the interference coefficient of absorption of X-ray radiation of a quartz single crystal in Laue geometry in the presence of temperature gradient. The total intensity of transmitted X-ray radiation and that reflected from different families of reflecting atomic planes of the quartz single crystal has been recorded. It was shown that with the increase in temperature gradient the coefficient of absorption of X-ray radiation decreases abruptly reaching its minimal value at a certain value of the temperature gradient.     

Hard X-ray diffraction in quartz single crystals in the presence of temperature gradient

The diffraction of hard X-rays in quartz single crystals is considered in the Laue geometry in the presence of temperature gradient. Spectral and angular characteristics of the reflected beam were experimentally studied versus the magnitude of temperature gradient. It is shown that as the temperature gradient applied perpendicular to reflecting (10ī1) atomic planes of quartz single crystal increases, the focus of reflected beam approaches the crystal, the angular and spectral widths increase, and intensity increases by orders of magnitude.     

The temperature dependence of the electric field gradient in ruthenium metal

The electric field gradient in ruthenium metal was studied between 15 K and 863 K. In contradiction to a recently proposed theory [1] the EFG was found to decrease with increasing temperature.     

Study of X-ray diffraction processes in ADP crystals under the influence of temperature gradient

The influence of temperature gradient on the intensity of X-ray beam reflected from atomic planes of antiferroelectric crystal of ammonium dihydro-phosphate (ADP) was studied. It was found that under the influence of temperature gradient on ADP crystal, the intensity of diffracted X-ray beam initially decreases at small values of gradient and then monotonously increases with increasing gradient. It is assumed that in the initial period of the action of temperature gradient the crystal domains have no time to orient in the same direction, and the intensity of beam decreases owing to the scattering of X-rays in different directions when reflected from the walls of boundaries of domains. After the alignment of domains, their arrangement in the same direction, separate areas of unidirectional domains are formed under the action of temperature gradient and the intensity of diffracted X-ray beam increases, as confirm the experimental data. The specified mechanism is supposed to occur also in other crystals having the domain structure.     

Controlling the spectrum of a two-photon field: Inhomogeneous broadening due to a temperature gradient

A method is proposed and implemented for controlling the spectrum of spontaneous parametric down-conversion. The method is based on generating a temperature gradient along a nonlinear crystal in which biphotons are generated. In this case, the phase-synchronism conditions are modified due to the temperature dependence of the refractive indices. As a result, the spectral composition of the emitted two-photon field is changed.     

Effect of temperature and magnetic field on disorder in semiconductor structures

We present the results of consistent theoretical analysis of various factors that may lead to influence of temperature and external magnetic field on disorder in semiconductor structures. Main attention is paid to quantum well (QW) structures in which only QWs or both QW and barriers are doped (the doping level is assumed to be close to the value corresponding to the metal–insulator transition). The above factors include (i) ionization of localized states to the region of delocalized states above the mobility edge, which is presumed to exist in the impurity band; (ii) the coexistence in the upper and lower Hubbard bands (upon doping of QWs as well as barriers); in this case, in particular, the external magnetic field determines the relative contribution of the upper Hubbard band due to spin correlations at doubly filled sites; and (iii) the contribution of the exchange interaction at pairs of sites, in which the external magnetic field can affect the relation between ferromagnetic and antiferromagnetic configurations. All these factors, which affect the structure and degree of disorder, lead to specific features in the temperature dependence of resistivity and determine specific features of the magnetoresistance. Our conclusions are compared with available experimental data.     

Charge grating kinetics in photorefractive crystals subjected to an electric field

The kinetics of charge gratings induced by a standing light wave in a photorefractive crystal in the presence of an electric field is described correctly. It obeys an ordinary differential equation of the fourth order that has two pairs of complex conjugate solutions. A general solution to this equation that satisfies a given initial condition is derived. The field-related mechanism causing charge oscillations and extending the time the gratings take to pass into the steady state is revealed. A relation between these oscillations and space-charge waves is found.     

On the effect of a magnetic field on the yield point and kinetics of macroplasticity in LiF crystals

A strong effect of a static magnetic field B on active deformation kinetics ( =const) in LiF crystals is observed. This is a threshold effect with respect to B and ; it is observed only for B> B _c (B≃0.4 T) and . Magnetic sensitivity is exhibited by the yield stress τ _y , which decreases by approximately a factor of 1.5 for B=0.48 T, and by the stage-II and stage-III hardening coefficients θ_II and θ _III, the former decreasing and the latter increasing in a magnetic field. A physical interpretation is proposed for the observed behavior. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 6, 470–474 (25 March 1997)     

On the sensitivity to temperature of the threshold current in a strained quantum-well semiconductor laser under a magnetic field

The sensitivity to temperature of the threshold current in a strained quantum-well semiconductor laser under a perpendicular magnetic field is determined quantitatively by means of a suitable merit parameter. The values taken on by this parameter are discussed in terms of changes in the temperature and in the magnetic-field strength.     

Sign determination of the57Fe electric field gradient in Al-based crystals and icosahedral quasicrystals

The icosahedral quasicrystals i-AlMn isomorphically suostituted by 28 at.% Fe or by (CrFe) are studied for the first time by in-field Mössbauer spectroscopy (MS) in order to determine the sign and asymmetry parameter of the dominant EFG term. In addition, the orthorhombic o-Al(MnFe) and cubic α- and hexagonal β-Al(MnFe)Si phases are studied. We show that previous MS results are inadequate to determine if there are two sites in the i-AlMn structure in the ratio of the golden number. Our results for i-Al(MnFe) show that the dominant EFG is negative, with asymmetry parameter of about 0.6. For i-Al(CrFe), essentially no deviations are found from the model of Czjzek or the Gaussian isotropic model. The β phase is found to have a very similar quadrupole effect as that in i-Al(MnFe).