Angular dependence of the current-voltage characteristics and voltage fluctuation spectrum in ceramic superconductors

Measurements were made of the current-voltage characteristics and voltage fluctuation spectrum as functions of the angle between the magnetic field applied in the plane of the sample and the direction of the transport current in a YBa₂Cu₃O_7−δ ceramic superconductor. The angular dependence of the fluctuation spectrum was measured first. It is shown that the current-voltage characteristics are not described using the vortex line cutting model and the voltage fluctuations are not caused by independent motion of vortices in the bulk of the sample. The results are consistent with the model of a self-organized critical state. Pis’ma Zh. Tekh. Fiz. 25, 8–13 (February 12, 1999)     

Long-range influence of weak photon irradiation (0.95 μm wavelength) on the mechanical properties of metals

It has been established that when 18 μm thick permalloy foils are exposed to 0.95 μm light for 0.2–100 s, the microhardness increases on both sides. This effect is not caused by radiation heating and is discussed in terms of a model proposed earlier to explain a low-dose long-range effect under ion bombardment. Pis’ma Zh. Tekh. Fiz. 24, 9–13 (December 12, 1998)     

g-Jitter mixed convection adjacent to a vertical stretching sheet

This paper studies the effect of periodical gravity modulation, or g-jitter induced mixed convection, on the flow and heat transfer characteristics associated with a stretching vertical surface in a viscous and incompressible fluid. The velocity and temperature of the sheet are assumed to vary linearly withx, wherex is the distance along the sheet. It is assumed that the gravity vector modulation is given byg*(t)=g_o [1+ɛ cos(πωt)]k, and the resulting non-similar boundary layer equations are solved numerically using an implicit finite-difference scheme. The effects of the amplitude of modulation, frequency of the single-harmonic component of oscillation, mixed convection parameter and Prandtl number on the skin friction and Nusselt number are discussed in detail.     

Angular momentum of the fields of a few-mode waveguide: Conversion of the angular momentum

An analysis is made of the transformation of the angular momentum density in the field of an unstable IV vortex of a few-mode optical fiber. It is shown that the effect of mode dispersion of IV vortices is observed as the conversion of the polarization and orbital components of the electrodynamic angular momentum. The angular momentum defect may be recorded experimentally as a mechanical twist of the optical few-mode fiber. Formally the dispersion process resembles the conversion of the signs of the orbital and polarization components of the angular momentum density. A complex pseudopotential, whose real and imaginary parts characterize the field lines and lines of equal pseudopotential, is introduced to describe the energy flux density of the fiber vortex. The conversion of field states with equivalent partial ê ⁺ F ₁(R)exp{−iφ} and ê2⁻ F ₁(R)exp{+iφ} vortices was investigated experimentally. Pis#x2019;ma Zh. Tekh. Fiz. 23, 58–65 (November 26, 1997)     

Heating of a current filament and formation of constrictions in a pulsed vacuum discharge

It is shown that a rapid current rise in a pulsed vacuum discharge is accompanied by enhanced compression of the current filament by its self-induced magnetic field. As a result, a constriction forms at a distance L≃1 mm from the cathode and the electron temperature increases to 10²–10³ eV at currents of order 1 kA. This behavior explains the observed increase in the degree of ion charge and the appearance of x-rays as the current pulse length decreases. The criterion for a rapid rise is the condition τ<L/V≃10⁻²⁷ s, where τ is the characteristic current amplification time and V≃10⁶ is the velocity of the cathode plasma. Pis’ma Zh. Tekh. Fiz. 24, 50–56 (September 26, 1998)     

Functional approach to the Rayleigh-Bénard problem for a nematic liquid crystal

The derivation of a functional analog of the free energy—the generalized thermodynamic potential— for an open system such as a nematic liquid crystal in a temperature gradient field is proposed. Near the threshold of thermal convection all the hydrodynamic variables can be described by a single complex amplitude w, the order parameter of the system, which enables one to determine the spatial position and the intensity of rotation of the convection rolls that arise. The orthogonality condition for the solutions is shown to yield an Euler equation from which one can recover the generalized thermodynamic potential, the extrema of which correspond to the most probable realizations of the dissipative structures. Pis’ma Zh. Tekh. Fiz. 24, 6–12 (June 26, 1998)     

Angular momentum of the fields of a few-mode fiber. III. Optical Magnus effect, Berry phase, and topological birefringence

It is shown that, on the one hand, the evolution of the angular rotation of the lines of nodes of the CP₁₁ mode is a manifestation of the optical Magnus effect in a few-mode fiber with a parabolic refractive index profile, and, on the other hand, the additional phase γ _b =±δβ ₂₁ z in CV and IV vortices is the Berry topological phase, which arises as a result of the cyclic change in the orientations of the orthogonal axes of dislocations. The splitting of the propagation velocities of orthogonal circularly polarized CV⁺ and IV⁻ modes in an LV vortex in a parabolic fiber is a manifestation of the phenomenon of topological birefringence of a few-mode fiber. The azimuth of the linear polarization of a vortex undergoes continuous angular rotation. In an optical fiber with a stepped index profile the CP₁₁ mode forms circularly polarized edge dislocation over lengths which are multiples of half the beat length, and over lengths which are odd multiples of the quarter beat length it forms linearly polarized fields with a purely screw dislocation. This transformation of edge and screw dislocations can be regarded formally as conversion of the polarizational angular momentum into orbital angular momentum. The conversion of angular momentum is a reflection of the dynamical unity of the optical Magnus effect and the Berry topological phase in the fields of a few-mode fiber. Pis’ma Zh. Tekh. Fiz. 23, 59–67 (December 12, 1997)     

Influence of thermomagnetic treatment on the propagation velocity of magnetoelastic oscillations and the ΔE effect in disordered ferromagnetics

An investigation was made of the propagation velocity of magnetoelastic oscillations and the ΔE effect as a function of the magnetic annealing temperature and the external magnetic field in iron-based amorphous metal alloys. It is shown that this dependence is nonmonotonic. The extreme values of the propagation velocity of the magnetoelastic oscillations and the ΔE effect only coincide in a specific range of annealing temperatures. As the annealing temperature increases, the extreme values of the magnetoelastic characteristics shift toward larger magnetic fields. Pis’ma Zh. Tekh. Fiz. 24, 79–83 (August 26, 1998)     

Mathematical model of an angular velocity transducer utilizing a laser with a fiber-optic ring resonator

An angular velocity transducer based on a laser with a fiber-optic ring resonator is considered. A relationship is obtained linking the amplitude and phase characteristics of the radiation with the parameters of the resonator and the rotational velocity of the transducer. Translated from Izmeritel'naya Tekhnika, No. 3, pp. 6–8, March, 1996.     

Intensity and phase of reflected radiation in a vanadium dioxide structure with surface plasmons

Results of theoretical calculations are presented for the angular dependences of the reflection coefficient and phase of reflected p-polarized radiation at wavelengths of 6328 Å and 11 500 Å in vanadium dioxide layered structures with surface plasmons. Results of calculations are presented for the two phases of the vanadium dioxide films — semiconducting and metallic. Pis’ma Zh. Tekh. Fiz. 23, 32–36 (July 12, 1997)     

Photoluminescence properties of AlxGa1−x As epitaxial layers grown under conditions of ultrafast flux cooling

Results are presented of studies of the photoluminescence properties of epitaxial layers of Al_xGa_1−x As solid solutions grown by liquid-phase epitaxy with nonequilibrium crystallization achieved by ultrafast rates of cooling of the flux (V∼10²–10³ °C/s). The photoluminescence characteristics obtained indicate that the epitaxial layers are of high quality. It is also observed that when samples with x _buff=0.5–0.55 are exposed to laser radiation of power density ∼1 kW/cm² at a temperature of 77 K, the spectral composition of the radiation undergoes irreversible changes caused by the formation of an arsenic vacancy (V _As)-donor impurity complex. Pis’ma Zh. Tekh. Fiz. 23, 8–13 (March 12, 1997)     

Study of forced convection in the presence of a liquid-porous-medium interface

Exact solutions of the problem of forced convection of an incompressible fluid (gas) are obtained for a number of geometries with the presence of an interface between an "ideal" fluid and a saturated porous medium. A generalization of the Darcy law allowing for viscous effects that are substantial at the boundaries of a porous body is used to model flow in a porous layer. The flow velocity as a function of the Darcy number is studied. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 6, pp. 895–901, November–December, 1997.     

Influence of the conditions of formation on the structure and phase composition of subsurface layers of uranium coated with the high-temperature oxide

It is established by x-ray structure analysis that the oxide film formed on the surface of uranium during oxidation by dry oxygen at pressures of 0.1–0.001 Pa and temperatures of 500–700 °C for several hours consists of UO₂ with U₄O₉ inclusions. Near the surface of the uranium the temperature of the α→β phase transition is lowered by 150–160 °C in comparison with the transition in the bulk of the metal, and the low-temperature stabilization of the β phase of uranium is observed. Pis’ma Zh. Tekh. Fiz. 24, 7–11 (June 12, 1998)     

Derivation and numerical solution of a closed equation for the specific isoscalar-surface area in a turbulent reactive flow

Based on the equation obtained earlier for the joint probability density function of the fluctuations of an isotropic turbulent scalar field of a reagent and its gradient[Inzh.-Fiz. Zh., 71, No. 5, 827–849 (1998)] the authors derived and numerically solved an equation for the specific isoscalar-surface area Σ_t(Γ) in a turbulent reactive flow. The equation for Σ_t(Γ) contains the single-point probability density function for pulsations of a reactive scalar and the time function that depend on the distribution of the energy of turbulent velocity pulsations and the intensity of scalar reagent pulsations by different length scales. The corresponding equations are written for all these functions. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 73, No. 3, pp. 585–599, May–June, 2000.     

Influence of Na+, Ca2+, and D2O ions on the kinetics of heterophase density fluctuations in a lipid bilayer

Experimentally measured temperature dependences of the velocity and absorption of ultrasound (7 MHz) were used to calculate the temperature dependences of the effective relaxation time τ _eff of the nucleation process near the phase-transition temperature T _t in phospholipid bilayers. It is observed that τ _eff is weakly sensitive to the ion composition of the medium and highly sensitive to the curvature of the bilayer and the replacement of H₂O by D₂O. Pis’ma Zh. Tekh. Fiz. 25, 7–13 (November 26, 1999)     

Investigation of a laser interferometer for measuring deformations of rotational platforms

A laser interferometer intended for measuring linear and angular deformations of high-accuracy rotational platforms has been constructed. The interferometer was investigated when accelerations in the range 0–100 m/sec² were reproduced and also in a regime measuring changes in the platform radius with temperature. Translated from Izmeritel'naya Tekhnika, No. 5, pp. 25–26, May, 1998.     

Negative turbulent heat conduction and its role in the formation of large-scale structures

A mechanism is proposed for the establishment of negative heat conduction under conditions of advanced turbulent convection, which leads to the formation of large-scale structures. A semi-empirical model of turbulent convection and experimental results are used to give some quantitative estimates. Pis’ma Zh. Tekh. Fiz. 24, 67–75 (April 26, 1998)     

Effect of the rise rate of nanosecond high-voltage pulses on the breakdown of air gaps

The breakdown velocity increases to 10–20 cm/ns when the leading edge of the negative voltage pulse decreases to 0.5–2.5 ns. The sharp increase in the velocity can be explained by the appearance of a short-pulse beam of high-energy electrons which produce in the gap a sufficient number of initial electrons. Pis’ma Zh. Tekh. Fiz. 25, 74–80 (September 26, 1999)     

Magnetic-field-induced superconductivity?

It is shown that a giant negative magnetoresistance effect in doped manganites can be explained using a model developed earlier to explain high-temperature superconductivity, based on allowance for the covalence of the transition metal-oxygen bond. This effect is caused by delocalization of π-electrons along asymmetric π-orbitals belonging to chains of covalently coupled Mn and O ions. Unlike high-temperature superconductors, in manganites this delocalization is induced by a magnetic field which ferromagnetically orients unpaired π-electrons and thereby lifts the restriction against their collective motion along the π-orbital when the manganese ions are ferromagnetically ordered. Pis’ma Zh. Tekh. Fiz. 25, 1–3 (July 12, 1999)     

Influence of degradation on the spectrum of voltage fluctuations in thick films of high-temperature superconductors

The noise parameters of high-temperature superconductors (yttrium barium cuprites) have been studied before and after degradation. The samples were thick films—10.5 mm×5.5 mm×8 μm—fabricated by magnetron sputtering on a ZrO substrate. A model is proposed for the onset of voltage fluctuations. The results of the measurements confirm that the observed noise spectrum is caused by fluctuations in the resistivity. Pis’ma Zh. Tekh. Fiz. 23, 91–94 (December 26, 1997)