Showing papers in "Jetp Letters in 2002"

Precession Spin Relaxation Mechanism Caused by Frequent Electron-Electron Collisions

Abstract: It is shown that the D’yakonov-Perel’ spin relaxation mechanism in a two-dimensional electron gas is controlled not only by the electron-momentum relaxation that accounts for the electron mobility but also by the electron-electron collisions. The kinetic equation describing the mixing of electron spin in the k space was solved, and the spin relaxation time τs caused by frequent electron-electron collisions was determined. The time τ s was calculated for a nondegenerate electron gas both with and without allowance for the exchange interaction.

Trans-Planckian particle creation in cosmology and ultrahigh energy cosmic rays

Abstract: We consider observational constraints on the creation of particles induced by hypothetical trans-Planckian effects during the current stage of the Universe’s expansion. We show that compatibility with the diffuse γ-ray background measured by the EGRET experiment strongly restricts this creation. In particular, it rules out the possibility of detecting signatures of such short-distance effects in anisotropies of the cosmic microwave background radiation. On the other hand, the possibility that some part of ultrahigh-energy cosmic rays originates from new trans-Planckian physics remains open.

Critical current in SFIFS junctions

Abstract: A quantitative theory of the Josephson effect in SFIFS junctions (S denotes bulk superconductor, F is metallic ferromagnet, and I is insulating barrier) is presented in the dirty limit. A fully self-consistent numerical procedure is employed to solve the Usadel equations for arbitrary values of the F-layer thicknesses, magnetizations, and interface parameters. In the case of antiparallel ferromagnet magnetizations, the effect of critical current Ic enhancement by the exchange energy H is observed, while in the case of parallel magnetizations the junction exhibits a transition to the π state. In the limit of thin F layers, we study these peculiarities of the critical current analytically and explain them qualitatively; the scenario of the 0-πtransition in our case differs from those studied before. The effect of switching between 0 and π states by changing the mutual orientation of F layers is demonstrated.

Shock compression of solid deuterium

Abstract: Experimental data on the compression of solid deuterium at a pressure of ∼60 GPa are presented. The data were obtained on a generator of powerful converging spherical shock waves. The results are compared with the data of shock experiments obtained on a Nova laser facility and an electrodynamic EPBF-Z plant.

In situ study of the mechanism of formation of pressure-densified Sio 2 glasses

Abstract: The volume of glassy a-SiO2 upon compression to 9 GPa was measured in situ at high temperatures up to 730 K and at both pressure buildup and release. It was established that the residual densification of a-SiO2 glass after high-pressure treatment was due to the irreversible transformation accompanied by a small change in volume directly under pressure. The bulk modulus of the new amorphous modification was appreciably higher (80% more than its original value), giving rise to residual densification as high as 18% under normal conditions. It was shown that the transformation pressure shifted to a lower pressure of about 4 GPa with a rise in temperature. A conclusion was drawn about the existence of at least two pressure-induced phase transitions accompanied by structure rearrangement in a-SiO2. A nonequilibrium phase diagram is suggested for glassy SiO2. It accounts for all the presently available experimental data and is confirmed by the existing modeling data.

Nanotube Devices: A Microscopic Model

Abstract: A microscopic model is developed for calculating electrostatic properties of nanotube devices. It is shown that the quantum-mechanical approach yields the same results as the statistical calculation in the limit of a thin tube suspended over a conducting gate at a distance exceeding the nanotube radius. A closed analytic expression is obtained for the atomistic capacitance of a straight nanotube and for a nanotube with a modest curvature. This method allows the fast and exact calculation of device parameters for the nanotube electromechanical systems and nanotube electronic devices.

High-energy QCD asymptotic behavior of photon-photon collisions

Abstract: The high-energy behavior of the total cross section for highly virtual photons, as predicted by the BFKL equation at next-to-leading order (NLO) in QCD, is discussed. The NLO BFKL predictions, improved by the BLM optimal scale setting, are in good agreement with recent OPAL and L3 data at CERN LEP2. NLO BFKL predictions for future linear colliders are presented.

Bose Condensation of Interwell Excitons in Double Quantum Wells

Abstract: The luminescence of interwell excitons in double quantum wells GaAs/AlGaAs (n-i-n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells were studied as functions of density and temperature within the domains on the scale less than one micron. For this purpose, the surfaces of the samples were coated with a metallic mask containing specially prepared holes (windows) of a micron size an less for the photoexcitation and observation of luminescence. For weak pumping (less than 50 μW), the interwell excitons are strongly localized because of small-scale fluctuations of a random potential, and the corresponding photoluminescence line is inhomogeneously broadened (up to 2.5 meV). As the resonant excitation power increases, the line due to the delocalized excitons arises in a thresholdlike manner, after which its intensity linearly increases with increasing pump power, narrows (the smallest width is 350 μeV), and undergoes a shift (of about 0.5 μeV) to lower energies, in accordance with the filling of the lowest state in the domain. With a rise in temperature, this line disappears from the spectrum (T c ≤ 3.4 K). The observed phenomenon is attributed to Bose-Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature range studied (1.5–3.4 K), the critical exciton density and temperature increase almost linearly with temperature.

Transformation of the orbital angular momentum of a beam with optical vortex in an astigmatic optical system

Abstract: Double sign inversion of the topological charge of an optical vortex was predicted and observed experimentally for a beam focused by a cylindrical lens. Beam evolution after passing through the lens is analyzed by the decomposition of the orbital angular momentum into the “vortical” and “mechanical” components. Topological reactions in the beam wave resulting in the sign inversion of the optical vortex upon the intersection of the wave-front edge dislocation are considered.

On the Kelvin-Helmholtz instability in superfluids

Abstract: The Kelvin-Helmholtz instability in superfluids is discussed on the basis of the first experimental observation of such an instability at the interface between superfluid 3He-A and superfluid 3He-B (R. Blaauwgeers, V. B. Eltsov, G. Eska et al., cond-mat/0111343). We discuss why the Kelvin-Helmholtz criterion, the Landau critical velocity for nucleation of ripplons, and the free-energy consideration all give different values for the instability threshold.

Mass of the Higgs versus Fourth Generation Masses

Abstract: The predicted value of the higgs mass m H is analyzed assuming the existence of the fourth generation of leptons (N, E) and quarks (U, D). The steep and flat directions are found in the five-dimensional parameter space: m H, m U, m D, m N, m E. The LEPTOP fit of the precision electroweak data is compatible (in particular) with m H∼ 300 GeV, m N∼50 GeV, m E∼100 GeV, m U+m D∼500 GeV, and |m U−m D|∼75 GeV. The quality of fits drastically improves when the data on b-and c-quark asymmetries and new NuTeV data on deep inelastic scattering are ignored.

Waveguide modes of hollow photonic-crystal fibers

Abstract: Waveguide modes of microstructure fibers with a hollow core and a two-dimensionally periodic cladding are studied experimentally and theoretically. The spectrum of modes guided in the hollow core of these fibers displays isolated maxima, indicating that waveguiding is supported due to the high reflectivity of the fiber cladding within photonic band gaps. The main properties of the spectrum of modes guided in a hollow core of a photonic-crystal fiber and radiation intensity distribution in these modes are qualitatively explained in terms of the model of a periodic coaxial waveguide.

Plateau Effects in the Spectra of Electrons Scattered from Atoms in a Strong Laser Field

Abstract: An exact quantum solution of the problem of electron scattering from a short-range potential in the presence of a strong elliptically polarized laser field is obtained. The differential scattering cross section as a function of the number of absorbed (or emitted) photons exhibits a plateau caused by the rescattering of electrons from the scattering center. Numerical results for a linearly polarized laser field are presented, and it is shown that the plateau boundaries agree well with classical estimates.

Interference of spin splittings in magneto-oscillation phenomena in two-dimensional systems

Abstract: The spin splitting caused by the terms linear in wavevector in the effective Hamiltonian containing can give rise to the new magneto-oscillation phenomena in two-dimensional systems. It is shown that the joint action of the spin-dependent contributions due to the heterostructure asymmetry and to the lack of inversion center in the bulk material suppresses beats that arise in the magneto-oscillation phenomena in the presence of the terms of only one of these types. © 2002 MAIK “Nauka/Interperiodica”.

Two-dimensional ordering of inclusions in smectic C phase

Abstract: The organization of inclusions nucleated in free standing films of smectic C (SmC) liquid crystal is investigated using polarized light microscopy. Anchoring on the inclusion boundaries induces distortions of the in-plane orientational order of the SmC phase, which drive the elastic interactions between inclusions. Such interactions show a quadrupolar character. At low concentration, the inclusions self-organize in linear or branched chain structures, while at high concentrations two-dimensional patterns appear in the film.

Magnetization-induced second harmonic generation in magnetophotonic microcavities based on ferrite garnets

Abstract: Magnetization-induced second harmonic generation was observed in magnetophotonic microcavities consisting of a ferromagnetic yttrium-iron garnet layer surrounded by nonmagnetic photonic crystals (Bragg reflectors). At resonance between the fundamental radiation and the microcavity mode in the geometry of polar magnetooptical Kerr effect, the polarization rotation for the second harmonic was found to be (18.5±0.5)°/μm for the fundamental radiation with a wavelength of 825 nm.

The strong effect of three-center interactions on the formation of superconductivity with \(d{x^2-y^2}\) symmetry in the t-J* model

Abstract: The effect of three-center interactions on the formation of a superconducting phase with \(d{x^2-y^2} \) symmetry is considered using the diagram technique for Hubbard operators and irreducible Green’s functions. It is shown that these interactions lead to a decrease in Tc by a factor of several tens.

Frictional shear cracks

Abstract: We discuss crack propagation along the interface between two dissimilar materials. The crack edge separates two states of the interface, “stick” and “slip.” In the slip region, we assume that the shear stress is proportional to the sliding velocity; i.e., the linear viscous friction law is valid. In this picture, the static friction appears as the tile Griffith threshold for crack propagation. We calculate the crack velocity as a function of the applied shear stress and find that the main dissipation comes from the macroscopic region and is mainly due to the friction at the interface. The relevance of our results to recent experiments, Baumberger et al., Phys. Rev. Lett. 88, 075509 (2002), is discussed.

Entanglement of Atomic States upon Collective Radiative Decay

Abstract: It is shown in the Markovian approximation that the relaxation of two atoms noninteracting with each other in the field of a common thermostat results in the entanglement of atomic states. With time, this entanglement either vanishes or takes a stationary value depending on the initial conditions.

Untwisting of the helical structure in a plane layer of chiral liquid crystal

Abstract: The untwisting of the helical structure of a chiral liquid crystal (CLC) in a thin plane layer exposed to an external action (temperature or field) and its dependence on the molecular adhesive forces at the layer boundaries are studied theoretically. It is shown that the critical electric (magnetic) field for complete untwisting in a thin layer may be appreciably lower than in the corresponding bulk CLC sample, and, contrary to the latter, the untwisting proceeds jumpwise. The expressions relating the jump temperature (field), i.e., the magnitude of untwisting action, to the CLC material parameters, layer thickness, and surface adhesive potential are given. The jump temperature (field) hysteresis is studied. In particular, it is shown that, for certain parameters, the untwisted helix remains untwisted after the removal of external action. The revealed qualitative regularities of untwisting are illustrated by numerical computations with the use of particular parameters of a CLC layer.

Observation of the rotational doppler effect for optical beams with helical wave front using spiral zone plate

Abstract: A rotating spiral zone plate was used to implement and measure the rotational Doppler effect for plane-polarized optical beams with helical wave front (optical vortices). The frequency shift was analyzed in terms of energy exchange between the beams and moving optical elements.

Phase Diagram of the Bose Condensation of Interwell Excitons in GaAs/AlGaAs Double Quantum Wells

Abstract: The luminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied at low temperatures down to 0.5 K. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells by a tunneling barrier were studied as functions of density and temperature. The studies were performed within domains about one micron in size, which played the role of macroscopic traps for interwell excitons. For this purpose, the sample surface was coated with a metal mask containing special openings (windows) of a micron size or smaller. Both photoexcitation and observation of luminescence were performed through these windows by the fiber optic technique. At low pumping powers, the interwell excitons were strongly localized because of the residual charged impurities, and the corresponding photoluminescence line was nonuniformly broadened. As the laser excitation power increased, a narrow line due to delocalized excitons arose in a threshold-like manner, after which its intensity rapidly increased with growing pumping and the line itself narrowed (to a linewidth less than 1 meV) and shifted toward lower energies (by about 0.5 meV) in accordance with the filling of the lowest exciton state in the domain. An increase in temperature was accompanied by the disappearance of the line from the spectrum in a nonactivation manner. The phenomenon observed in the experiment was attributed to Bose-Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature interval studied (0.5–3.6) K, the critical exciton density and temperature were determined and a phase diagram outlining the exciton condensate region was constructed.

Polarization optics of biphotons

Abstract: The works devoted to studying the polarization properties of a two-photon light generated upon spontaneous parametric down-conversion in the collinear frequency-degenerate regime are briefly overviewed, with emphasis on the studies carried out by us over a period from 1999 to 2001 within the framework of the project “Polarization Optics of Biphotons” of the Russian Foundation for Basic Research. In particular, the polarization state of a two-photon light was analyzed and its pictorial mapping onto the Poincare sphere was proposed. The experiments on polarization transformations of a two-photon light were performed; based on these transformations, a method was suggested for ternary quantum information coding. A two-photon state with the orthogonal photon polarizations was synthesized experimentally from the two beams of identically polarized correlated photons, and the spectral properties of this state were investigated. Finally, a method was suggested for measuring the polarization state of a two-photon light in the collinear frequency-degenerate case (“tomography”).

Interplay between the Structure and Properties of New Metastable Carbon Phases Obtained under High Pressures from Fullerite C 60 and Carbyne

Abstract: A brief review of structural, electrotransport, optical, elastic, and mechanical properties of carbon phases synthesized under pressure by heating fullerite C60 and carbynoid materials is given. A large variety of carbon modifications with a variable bonding type, a variable mean coordination number, a variable molecular or atomic structural type, a variable characteristic dimensionality (from zero-to three-dimensional structures), a variable degree of covalence, etc., were prepared. Emphasis in the review is given to the elucidation of the interplay between the structural and topological characteristics of carbon phases and their key electronic and mechanical properties. A version of the kinetic phase diagram of fullerite C60 transformations on heating under pressure is also suggested. This version is modified with respect to the interpretations known in the literature.

Zero Modes in a Periodic System of Aharonov-Bohm Solenoids

Abstract: The existence of infinitely degenerate zero modes is proved for a quantum-mechanical two-dimensional charged particle with spin 1/2 moving in the field of an infinite system of Aharonov-Bohm solenoids. The condition for appearance of these modes is found and their explicit form is obtained.

Giant Third-Harmonic in Porous Silicon Photonic Crystals and Microcavities

Abstract: In recent years, the nonlinear optics of photoniccrystals (PCs) and PC-based microcavities (MCs) hasbeen progressing intensively [1]. Giant nonlinear-opti-cal phenomena caused by giant effective dispersion atthe edges of the photonic band gap and in the vicinityof the cavity mode, and also by the enhancement ofoptical fields inside PCs and MCs under optimal fre-quency–angular conditions, can be observed in suchmicrostructures. In particular, multiple reflection inter-ference in PCs can compensate the phase mismatch forfundamental and second-harmonic waves when one ofthe waves falls on the edge of the photonic band gap inthe space of frequencies or wave vectors [2]. Such aneffective fulfillment of the phase matching conditionsleads to a resonant enhancement of second-harmonicgeneration in PCs composed, for example, of dielectricpolystyrene spheres [3], alternate GaAs–AlGaAs [4] orZnS–SrF

Droplet spot as a new object in physics of vacuum discharge

Abstract: It is established experimentally that the burning of a low-current (several and tens of amperes) pulsed (microseconds) vacuum discharge is accompanied by the formation of plasma microbunches around some of the droplets leaving the cathode spot. The parameters of these bunches (electron concentration n e∼1026 m−3 and equilibrium temperature T e∼1 eV) are close to the parameters of cathode-spot plasma. The data obtained suggest that the initial temperature of droplets and the thermionic emission from them play a key role in the formation of such plasma microbunches. By analogy with the well-known cathode and anode spots in vacuum discharges, these droplet plasma formations are classified as “droplet spots.” This work reports the first results on studying the formation dynamics and the characteristics of the droplet spots. It is noted that the concept of droplet spots will require a certain refinement of the plasma formation mechanism in vacuum discharges.

Spin-injection mechanism of magnetization reversal and hysteresis of current in magnetic junctions

Abstract: A novel mechanism is proposed for magnetization reversal by the current of magnetic junctions with two metallic ferromagnetic layers and thin separating nonmagnetic layer. The spin-polarized current flows perpendicularly to the interfaces between the ferromagnetic layers, in one of which the spins are pinned and in the other they are free. No domain structure is formed in the ferromagnetic layers. The current breaks spin equilibrium in the free layer, which manifests itself in the injection or extraction of spins. The nonequilibrium spins interact with the magnetization of the lattice due to the effective field of s-d exchange, which is current dependent. At currents exceeding a certain threshold value, this interaction leads to magnetization reversal. Two threshold currents for magnetization reversal have been obtained theoretically, which are reached as the current increases or decreases, respectively. Thus, the phenomenon of current hysteresis is found. The calculated results are in good agreement with experiments on magnetization reversal by current in three-layer junctions of composition Co(I)/Cu/Co(II) prepared in a pillar form.

On the theory of X-ray refractive optics: Exact solution for a parabolic medium

Abstract: The exact solution is obtained for a propagator describing x-ray propagation through a refractive parabolic medium. Such a medium arises in compound many-element refractive x-ray lenses that are used in synchrotron radiation sources. The solution obtained allows one to analyze such lenses in detail to predict their operation in particular applications (beam focusing, microobject imaging, and Fourier transform).

Magnetic Collapse and the Change of Electronic Structure of FeBO 3 Antiferromagnet under High Pressure

Abstract: The effect of high pressures up to 60 GPa on single-crystal and polycrystalline samples of iron borate 57FeBO3 was studied by Mossbauer absorption spectroscopy (57Fe nuclei) in a diamond anvil cell. Magnetic field H hf at the 57Fe nuclei increases with pressure but abruptly drops to zero at 46±2 GPa, indicating the crystal transition from the antiferromagnetic to nonmagnetic state. This is accompanied by an abrupt change in the isomer shift and quadrupole splitting. Their values in the high-pressure phase are evidence for the transition of Fe+3 ions from a high-spin (S=5/2, 6 A 1g ) to low-spin (S=1/2, 6 T2g) state (spin crossover). This correlates with an abrupt decrease in the unit-cell volume (by ∼9%) and optical gap. The change of the magnetic and electronic structures is explained by Mott’s transition with rupturing of strong d-d-electron correlations.