Showing papers in "Journal of Experimental and Theoretical Physics in 2006"

Update of the e + e − → π + π − cross section measured by the spherical neutral detector in the energy region 400 < √s < 1000 MeV

Abstract: The corrected cross section of the e + e − → π+π− process measured in the spherical neutral detector experiment at the VEPP-2M e + e − collider is presented. The update is necessary due to a flaw in the e + e − → π+π− and e + e − → μ+μ− Monte Carlo event generators used previously in data analysis.

Semiclassical model of a one-dimensional quantum dot

Abstract: A one-dimensional quantum dot at zero temperature is used as an example for developing a consistent semiclassical method. The method can also be applied to systems of higher dimension that admit separation of variables. For electrons confined by a quartic potential, the Thomas-Fermi approximation is used to calculate the self-consistent potential, the electron density distribution, and the total energy as a function of the electron number and the effective electron charge representing the strength of interaction between electrons. Use is made of scaling with respect to the electron number. An energy quantization condition is derived. The oscillating part of the electron density and both gradient and shell corrections to the total electron energy are calculated by using the results based on the Thomas-Fermi model and analytical expressions derived in this study. The dependence of the shell correction on the interaction strength is examined. Comparisons with results calculated by the density functional method are presented. The relationship between the results obtained and the Strutinsky correction is discussed.

High-accuracy calculations of dipole, quadrupole, and octupole electric dynamic polarizabilities and van der Waals coefficients C 6 , C 8 , and C 10 for alkaline-earth dimers

Abstract: The static and dynamic electric dipole, quadrupole, and octupole polarizabilities of the alkaline-earth atoms (beryllium, magnesium, calcium, strontium, and barium) in the ground state were calculated. The dynamic polarizabilities obtained were used to calculate the van der Waals coefficients C 6, C 8, and C 10 of alkaline-earth metal dimers for the interaction of two like atoms in the ground state. The results are compared with other theoretical and experimental data.

Crystal structure and magnetic properties of Ba-ordered manganites Ln0.70Ba0.30MnO3−δ (Ln = Pr, Nd)

Abstract: The structure and magnetic properties of the Ba-ordered state in solid solutions of manganites Ln0.70Ba0.30MnO3−δ (Ln = Pr, Nd) with a cation ratio Ln3+/Ba2+ ≫ 1 are studied experimentally. The samples are obtained by two-stage synthesis. The initial stoichiometric Ba-disordered solid solutions Ln0.70Ba0.30MnO3 synthesized in air according to traditional ceramic technology are characterized by the orthorhombic (Imma, Z = 4) perovskite-like unit cell and are ferromagnets with Curie temperatures T C ≈ 173 and ≈ 143 K for Pr and Nd, respectively. The average size of a crystalline in the initial samples is 5 μm. It is found that annealing of the initial samples in a vacuum of P[O2] = 10−4 Pa leads to their separation into three phases: (1) the anion-deficient ordered LnBaMn2O5 phase described by a tetragonal (P4/mmm, Z = 2) perovskite-like unit cell, as well as the phases (2) Ln2O3 (P $$\bar 3$$ m1, Z = 1) and (3) MnO (Fm $$\bar 3$$ m, Z = 2). Reduction leads to the formation of a nanocomposite with an average crystallite size = 100 nm. Anion-deficient Ba-ordered phases of LnBaMn2O5 exhibit ferrimagnetic properties with Neel temperatures T N ≈ 113 and ≈123 K for Pr and Nd, respectively. Annealing of anion-deficient samples in air at a moderate temperature of T = 800°C does not change the average size of the nanocrystallite, but noticeably alters their phase composition. Stoichiometric nanocomposites consist of two perovskite-like phases: (1) the Ba-deficient ordered stoichiometric phase LnBaMn2O6, which is described by a tetragonal (P4/mmm, Z = 2) unit cell and has the Curie temperatures T C ≈ 313 (Pr) and ≈303 K (Nd), and (2) the Ba-disordered superstoichiometric phase Ln0.90Ba0.10MnO3+δ, which is described by an orthorhombic (Imma, Z = 4) unit cell and has Curie temperatures T C ≈ 138 (Pr) and ≈123 K (Nd). The two magnetic phases of the Ba-ordered nanocomposite are exchange-coupled. For the low-temperature magnetic phase, a temperature hysteresis is observed at ΔT ≈ 22 K in a field of 10 Oe and at ΔT ≈ 5 K in a field of 1 kOe. It is shown that states with different degrees of ordering of cations in the A sublattice can be obtained employing different technological conditions of treatment. The significant changes in the magnetic properties of Ba-ordered nanocomposites are explained on the basis of chemical phase separation taking into account the effect of compression, which is a consequence of the action of chemical (cation ordering) and external (surface tension) pressures.

Electron-positron pair production by electromagnetic pulses

Abstract: Electron-positron pair production in vacuum by a single focused laser pulse and by two counter-propagating colliding focused pulses is analyzed. A focused laser pulse is described using a realistic three-dimensional model based on an exact solution of Maxwell’s equations. In particular, this model reproduces an important property of focused beams, namely, the existence of two types of waves with a transverse electric or magnetic vector (e-or h-polarized wave, respectively). The dependence of the number of produced pairs on the radiation intensity and focusing parameter is studied. It has been shown that the number of pairs produced in the field of a single e-polarized pulse is many orders of magnitude larger than that for an h-polarized pulse. The pulse-intensity dependence of the number of pairs produced by a single pulse is so sharp that the total energy of pairs produced by the e-polarized pulse with intensity near the intensity I S = 4.65 × 1029 W/cm2 characteristic of QED is comparable with the energy of the pulse itself. This circumstance imposes a natural physical bound on the maximum attainable intensity of a laser pulse. For the case of two colliding circularly polarized pulses, it is shown that pair production becomes experimentally observable when the intensity of each beam is I ∼ 1026 W/cm2, which is one to two orders of magnitude lower than that for a single pulse.

A strange attractor of the Smale-Williams type in the chaotic dynamics of a physical system

Abstract: A nonautonomous nonlinear system is constructed and implemented as an experimental device. As represented by a 4D stroboscopic Poincare map, the system exhibits a Smale-Williams-type strange attractor. The system consists of two coupled van der Pol oscillators whose frequencies differ by a factor of two. The corresponding Hopf bifurcation parameters slowly vary as periodic functions of time in antiphase with one another; i.e., excitation is alternately transferred between the oscillators. The mechanisms underlying the system’s chaotic dynamics and onset of chaos are qualitatively explained. A governing system of differential equations is formulated. The existence of a chaotic attractor is confirmed by numerical results. Hyperbolicity is verified numerically by performing a statistical analysis of the distribution of the angle between the stable and unstable subspaces of manifolds of the chaotic invariant set. Experimental results are in qualitative agreement with numerical predictions.

On the possibility of enrichment of H 2 O nuclear spin isomers by adsorption

Abstract: Experimental data on the enrichment of H2O nuclear spin isomers by means of adsorption, as suggested in several works by other authors, are reported. We were unable to observe any enrichment of isomers. The conclusion was drawn that the adsorption characteristics of water ortho and para isomers were insignificantly different. An analysis of the relaxation of H2O spin isomers induced by the intramolecular mixing of the molecular ortho and para spin states and caused by collisions with paramagnetic oxygen molecules was performed.

Topological characteristics of bonds in SiO2 and GeO2 oxide systems upon a glass-liquid transition

Abstract: Using the Angell model of broken bonds (configurons), configuron clustering in a topologically disordered lattice (network) of amorphous SiO2 and GeO2 upon a glass-liquid transition is considered. It is shown that the glass-liquid transition is accompanied by the formation of a macroscopic (percolation) configuron cluster penetrating the entire bulk of the material and possessing fractal geometry. The glass-liquid (overcooled liquid) percolation phase transition in the amorphous substance is accompanied by a change in the Hausdorff dimension of the bond network structure for configurons from the three-dimensional Euclidean dimension in the glassy state to a fractal dimension of 2.55 ± 0.05 in the liquidlike state. Contrary to the kinetic character of the liquid-glass transition, the glass-transition temperature is a thermodynamic parameter of the amorphous substance, depending parametrically on the cooling rate.

Ablated matter expansion and crater formation under the action of ultrashort laser pulse

Abstract: The action of a subpicosecond laser pulse on a target made of an absorbing condensed substance is considered. The propagation of an electron heat conduction wave and the crystal lattice heating prior to the hydrodynamic expansion of the target are analyzed. In these initial interaction stages, a heated layer with a thickness of d T is formed at the target surface. The dependence of d T on the absorbed laser energy density F[J/cm2] is evaluated. The motion of ablated matter in the expansion stage is described using a numerical solution of the equations of gasdynamics and the results of molecular dynamics (MD) simulations. The MD simulations are performed using a large number (∼103) of parallel processors, which allows the number of model atoms to be increased up to a level (about 3.5 × 107) close to that encountered under real experimental conditions.

Anomalies in the optical properties of nanocrystalline copper oxides CuO and Cu2O near the fundamental absorption edge

Abstract: A 0.1–0.15-eV displacement of the fundamental absorption edge in the optical absorption spectra of nanocrystalline oxide n-CuO (relative to the position of the fundamental absorption edge in the spectra of CuO single crystals) towards lower energies (red shift) is observed against the background of strong blurring. Nanocrystalline n-Cu2O exhibits a displacement of the fundamental absorption edge towards higher energies (blue shift) by approximately 0.35 eV. The size of crystallites in n-CuO and n-Cu2O ranges from 10 to 90 nm. The blue shift of the fundamental absorption edge of n-Cu2O is typical of classical wide-gap semiconductors and can be explained by size quantization upon a change in the particle size. The anomalous red shift of the fundamental absorption edge of the strongly correlated nanocrystalline oxide n-CuO can be attributed to the highly defective structure of n-CuO, anomalies in the electronic structure of strongly correlated compounds based on 3d metals, and their tendency to electronic phase separation with the formation of metal-like inclusions.

Collective dynamics in liquid aluminum near the melting temperature: Theory and computer simulation

Abstract: The microscopic collective dynamics of liquid aluminum near the melting temperature has been studied using two independent methods: first, using a theoretical approach developed in terms of the Zwanzig-Mori formalism and based on Bogolyubov’s idea of reduced description of relaxation processes in liquids; second, using molecular dynamics simulation. The X-ray inelastic scattering spectra obtained with the theoretical approach and computer simulation are compared with experimental data. The high-frequency acoustic excitations that appear on microscopic spatial scales in liquid aluminum are found to be mainly caused by two-, three-, and four-particle interactions.

Effect of Magnetic Field on Mechanics of Nonmagnetic Crystals: The Nature of Magnetoplasticity

Abstract: The magnetoplastic effect in mechanics of nonmagnetic crystals is attributed to spin evolution in the spin-selective nanoscale reactor created by electron transfer from a dislocation to a stopper. In this “dislocation + stopper” system, dislocation depinning is facilitated because the Coulomb attraction between the dislocation and the stopper is switched off. Since magnetic field stimulates the singlet-to-triplet conversion of the nanoscale reactor (the reverse electron transfer is forbidden), the nanoscale reactor with switched-off Coulomb interaction has a longer lifetime. The resulting increase in depinning rate and dislocation mobility provides a physical explanation for magnetoplasticity.

Two-dimensional S-N-S junction with Rashba spin-orbit coupling

Abstract: The effect of Rashba spin-orbit coupling on the supercurrent in S-2DEG-S proximity junctions is investigated in the clean limit. A generalization of Beenakker’s formula for Andreev levels to the case of spin-orbit scattering is presented. Spin-orbit induced splitting of Andreev bound states is predicted for an infinite-width junction with nonvanishing normal backscattering at S-N interfaces. However, a semiclassical average of the Josephson current is insensitive to the Rashba coupling as long as the electron-electron interaction in 2DEG is neglected.

Exact solutions in the dynamics of alternating open chains of spins s = 1/2 with the XY Hamiltonian and their application to problems of multiple-quantum dynamics and quantum information theory

Abstract: A method for exactly diagonalizing the XY Hamiltonian of an alternating open chain of spins s = 1/2 has been proposed on the basis of the Jordan-Wigner transformation and analysis of the dynamics of spinless fermions. The multiple-quantum spin dynamics of alternating open chains at high temperatures has been analyzed and the intensities of multiple-quantum coherences have been calculated. The problem of the transfer of a quantum state from one end of the alternating chain to the other is studied. It has been shown that the ideal transfer of qubits is possible in alternating chains with a larger number of spins than that in homogeneous chains.

Second moment of multiple-quantum NMR and a time-dependent growth of the number of multispin correlations in solids

Abstract: The time evolution of multispin correlations (the growth of the number of correlated spins as a function of time) can be observed directly using the multiple-quantum nuclear magnetic resonance spectroscopy of solids. A quantity related to this number, namely, the second moment 〈n 2(t)〉 of the intensity distribution of coherences of different orders in the multiple-quantum spectrum can be calculated using the theory proposed in this work. An approach to the calculation of the four-spin time correlation function through which this moment is expressed is developed. The main sequences of contributions in the expansion of this function into a time power series are summed using the approximation of a large number of neighbors both for systems with a secular dipole-dipole interaction and for systems with a nonsecular effective interaction. An exponential dependence of 〈n 2(t)〉 is obtained. The value of 〈n 2(t)〉 is additionally calculated using an expansion in terms of orthogonal operators for three model examples corresponding to different limiting realizations of spin systems. It is shown that the results of the microscopic theory at least qualitatively agree with both the results obtained for model examples and experimental results obtained recently for adamantane.

On the high conductivity of nonconjugated polymers

Abstract: The mechanism of charge transfer in a metal-electroactive polymer-metal structure has been experimentally studied near the threshold of the uniaxial-pressure-induced transition into a high-conductivity state in the polymer. The dynamics of the I-V curve is investigated as a function of the applied pressure. The data obtained are analyzed in terms of the model of injection currents using the concepts of possible scanning of a quasi-Fermi level near an injection level. Our estimates suggest that a narrow band made of deep trap states located near the Fermi level forms in the polymer film in the pretransition pressure range. In the immediate vicinity of the transition range, a narrow band of coherent charge transfer appears from these states; this band can be responsible for the high metal-type conductivity of thin polymer films, which has been repeatedly observed by many researchers.

Relativistic Theory of Tunnel and Multiphoton Ionization of Atoms in a Strong Laser Field

Abstract: Relativistic generalization is developed for the semiclassical theory of tunnel and multiphoton ionization of atoms and ions in the field of an intense electromagnetic wave (Keldysh theory). The cases of linear, circular, and elliptic polarizations of radiation are considered. For arbitrary values of the adiabaticity parameter γ, the exponential factor in the ionization rate for a relativistic bound state is calculated. For low-frequency laser radiation , an asymptotically exact formula for the tunnel ionization rate for the atomic s level is obtained including the Coulomb, spin, and adiabatic corrections and the preexponential factor. The ionization rate for the ground level of a hydrogen-like atom (ion) with Z ≲ 100 is calculated as a function of the laser radiation intensity. The range of applicability is determined for nonrelativistic ionization theory. The imaginary time method is used in the calculations.

Tunneling ionization of a hydrogen atom in short and ultrashort laser pulses

Abstract: The ionization of a hydrogen atom in a linearly polarized low-frequency electromagnetic field is investigated by direct numerical integration of the time-dependent Schrodinger equation. The data obtained for various ionization regimes and various initial atomic states are compared with the Keldysh and Perelomov-Popov-Terent’ev (PPT) theories. The validity ranges for the quasi-static model of tunneling ionization and the PPT theory in laser intensity and frequency are determined. The tunneling ionization of the excited 2s and 2p states is discussed. The ionization of a hydrogen atom in an ultrashort (on the order of one optical period) pulse is investigated.

Low-frequency transition radiation from a short laser pulse at the plasma boundary

Abstract: The transition radiation that appears when a short laser pulse crosses the vacuum-plasma boundary and that is attributable to an vortex electric current produced at the plasma boundary by averaged ponderomotive forces has been considered for the first time. The spectral, angular, and spatiotemporal characteristics of the transition radiation in a vacuum are analyzed. It is shown that under typical conditions of present-day laser plasma experiments, the frequency of the transition radiation lies in the terahertz range and its power can reach several megawatts.

Clausius-mossotti-type relation for planar monolayers

Abstract: The dielectric properties of a quasi-two-dimensional molecular monolayer are analyzed. The dielectric function of the monolayer is expressed in terms of molecular polarizability and monolayer characteristics. The expression is analogous to the well-known Clausius-Mossotti equation for three-dimensional systems. The response of the monolayer to an external field is calculated. The case of a planar array of nanoparticles is also considered. The solution is obtained in the framework of a local-field theory.

Lasing in liquid crystal thin films

Abstract: A lasing condition is formulated in matrix form for optically anisotropic thin films. Lasing behavior of liquid-crystal slabs is analyzed. In particular, it is shown that if the spatial extent of a liquid crystal slab is much larger than its thickness, then laser emission is feasible not only along the normal to the slab, but also in the entire angular sector. The generated laser light can be observed experimentally as a spot or as concentric rings on a screen. The lowest lasing threshold corresponds to in-plane sliding modes leaking into the substrate. The feedback required for lasing is provided by reflection from the interfaces, rather than edges, of the liquid-crystal slab operating as a planar Fabry-Perot cavity. For cholesteric liquid crystals, it is shown that energy loss to the sliding modes leaking into the substrates and escaping through their edges is a key factor that limits the efficiency of band-edge emission along the normal to the slab.

Experimental study of the reaction e + e - → K S K L in the energy range √ S = 1.04-1.38 GeV

Abstract: We present a measurement of the e + e − → K S K L cross section in the energy range √s = 1.04−1.38 GeV. For an energy of √s ≥ 1.2 GeV the cross section exceeds vector meson dominance model predictions with only ρ(770), ω(783), and ϕ(1020) mesons taken into account. The measured cross section agrees well with previous measurements.

The atomic and electron structure of ZrO 2

Abstract: The atomic structure of amorphous and crystalline zirconium dioxide (ZrO2) films is studied using X-ray diffraction and extended X-ray absorption fine structure techniques. The electron structure of ZrO2 is experimentally determined using X-ray and UV photoelectron spectroscopy, and the electron energy band structure is theoretically calculated using electron density functional method. According to these data, the valence band of ZrO2 consists of three subbands separated by an ionic gap. The upper subband is formed by the O2p states and Zr4d states; the medium subband is formed by the O2s states; and the narrow lower subband is formed predominantly by the Zr4p states. The bandgap width in amorphous ZrO2, as determined using the electron energy loss spectroscopy data, amounts to 4.7 eV. The electron band structure calculations performed for a cubic ZrO2 phase point to the existence of both light (0.3m 0) and heavy (3.5m 0) holes, where m 0 is the free electron mass. The effective masses of band electrons in ZrO2 fall within (0.6–2.0)m 0.

Crystalline and magnetic structures of La1−x BixMnO3+δ manganites

Abstract: The crystalline and magnetic structures and magnetic properties of La1−x BixMnO3+δ (0.4 ≤ x ≤ 0.6, 0 ≤ δ ≤ 0.06) manganites have been studied. The solid solutions having the stoichiometric oxygen content are shown to be orbitally ordered A-type antiferromagnets. An increase in the oxygen content above the stoichiometric value is found to cause Mn4+ ions in the perovskite lattice, to remove the cooperative Jahn-Teller distortions, and to form a long-range ferromagnetic order. This order becomes broken as the concentration of the tetravalent manganese ions increases further. The tendency toward breaking the ferromagnetic order increases with the bismuth content. The magnetic properties are interpreted in terms of superexchange interactions on the assumption of local lattice distortions induced by anisotropy of the 6s 2(Bi3+)-2p 6(O2−) chemical bonds.

Vortex system dynamics and energy losses in a current-carrying 2D superconducting wafer

Abstract: The dynamic processes occurring in the vortex system of a 2D superconducting wafer carrying transport current are investigated using the model of the vortex system of high-temperature superconductors Calculations are performed by the Monte Carlo method For the first time, the dynamics of magnetic field penetration in a current-carrying HTSC wafer is demonstrated and the energy losses associated with a change in transport current are calculated It is shown that changes in the transport current amplitude and in the number of defects lead to a change in the energy liberation mechanism: hysteresis energy losses are replaced by the losses in the saturated layer

Motion of clusters of weakly coupled two-dimensional cavity solitons

Abstract: An analysis of clusters of weakly coupled two-dimensional spatial optical solitons in a large-aperture class A laser with a saturable absorber is developed. The symmetries that control the transverse motion of the clusters are described. Numerical solutions of the governing generalized complex Ginzburg-Landau equation demonstrate the existence of four types of clusters of weakly coupled cavity solitons that correspond to symmetries of transverse intensity distributions and energy flows: (1) stationary (with two mirror symmetry axes), (2) rotating about a stationary center of mass (invariant under rotation), (3) translating without rotation (with a single mirror symmetry axis), and (4) asymmetric ones rotating about a center of mass that moves around a circle (with equal periods of rotation and circular motion).

Chaos in Composite Billiards

Abstract: The mechanisms and features of the chaotic behavior in billiards with ray splitting (refraction) are considered. In contrast to ordinary billiards, the law of motion in composite billiards that is coded with a sequence of ray visits to different media is shown to be deterministically chaotic. The analysis is performed in terms of a geometrical-dynamical approach in which a symmetric phase space is used instead of the ordinary Hamiltonian phase space. The chaotization elements in composite billiards of a general position are studied. The dynamics of rays in ring billiards consisting of two concentric media with different refractive indices is considered.

On the structure of wave fields in the region of linear interaction between ordinary and extraordinary waves in two-dimensionally inhomogeneous magnetoactive plasmas

Abstract: The linear interaction between ordinary and extraordinary waves in two-dimensionally inhomogeneous magnetoactive plasmas in the electron-cyclotron frequency range is considered. Exact solutions of the reduced wave equation that describes the wave fields in the region of linear interaction are obtained. A procedure is determined for recalculating the fields that pass through and reflect from the transformation region. The field distributions in the transformed and reflected beams are studied analytically for various incident beams. The results may be considered as a solution of a new standard problem with a wide range of astrophysical and laboratory applications.

Spin correlations and a mesoscopic structure in Ni-Mn-Ga

Abstract: The mesoscopic structures of the Heusler alloys Ni49.1Mn29.4Ga21.5 and Ni2MnGa are studied by small-angle polarized neutron scattering in the temperature range 15 < T < 400 K. The characteristic temperatures of phase transformations (ferromagnetic, martensitic, and premartensitic transformations) and the characteristic sizes of mesoscopic inhomogeneities in them have been determined. Differences in the spin dynamics of these phases and magnetic-nuclear interference upon neutron scattering have been revealed. The evolutions of the mesoscopic structures in the nonstoichiometric and stoichiometric alloys are found to be substantially different.

Electron and hole injection in metal-oxide-nitride-oxide-silicon structures

Abstract: The kinetics of electron and hole accumulation in metal-oxide-nitride-oxide-semiconductor structures is studied. Experimental data are compared with a theoretical model that takes into account tunnel injection, electron and hole capture by traps in amorphous silicon nitride SiNx, and trap ionization. Agreement between experimental and calculated data is obtained for the bandgap width E g = 8.0 eV of amorphous SiO2, which corresponds to the barrier for holes Φh = 3.8 eV at the Si/SiO2 interface. The tunneling effective masses for holes in SiO2 and SiNx are estimated at m h * ≈ (0.4–0.5)m 0. The parameters of electron and hole traps in SiNx are determined within the phonon-coupled trap model: the optical energy W opt = 2.6 eV and the thermal energy W T = 1.3 eV.