bluuebunny/crossref_metadata_2025 huggingface dataset:
Suboptimal control of transient nonisothermal viscoelastic fluid flows
Authors: Marduel X. , Kunisch K.
Published: July 2002

Suboptimal control of transient viscoelastic fluid flows in a 4 to 1 planar contracting channel is investigated. The control mechanism is based on heating or cooling the fluid along a portion of the boundary of the flow domain. In order to perform the control, a nonisothermal model for viscoelastic fluids is used consisting of the Phan-Thien–Tanner model with relaxation time and elastic viscosity depending on temperature [following an exponential dependency, the Williams–Landel–Ferry model]. Moreover, the momentum, mass, and constitutive equations are coupled with the heat equation forming the so-called primal system. The goal of the control is to reduce the large recirculation zones at the corners of the contraction. The suboptimal strategy utilized for this purpose is the so-called “instantaneous control” method.
On the incomplete similarity for turbulent velocity profiles in rough pipes
Authors: Porporato Amilcare , Sordo Sebastiano
Published: July 2002

The possibility of extending Barenblatt’s hypothesis of incomplete similarity for the velocity profile is analyzed for rough-pipe flows. The verification of this hypothesis, which leads to a power law in the intermediate region of the flow, is performed using Nikuradse’s well-known data for pipes with homogeneous sand roughness. As detailed comparison between the log law and the power law would require much higher measurement resolution, no attempt is made here to assess the validity of one or the other law, and the power law profile is simply used as a working hypothesis. The power law fits well for a large part of the pipe section both for the transitionally rough regime and the fully rough regime. The coefficients of the power law show an interesting behavior as a function of the Reynolds number and the relative roughness. For the fully rough regime, it is possible to obtain simple expressions for the power law coefficients and the resistance law.
Low-temperature control of nanoscale molecular dynamics
Authors: Hyldgaard P.
Published: July 2002

A novel in situ probe of the nanoscale molecular dynamics of organic-molecule and fullerene-tube nanostructures is proposed. General and consistent results for the nonlinear-current coupling to the nanostructure excitations are presented to document a frequency-selective electrostatic control of this current stimulation and optimal operation as a local source of current-induced molecular excitations Ωi. The control is possible for temperatures T≪Ωi. Finally, it is explained in detail how Raman measurements of this molecular dynamics would probe the nanoscale excitations within organic and fullerene nanostructures under nonlinear transport conditions.
Impurity states in triangular two-dimensional Heisenberg antiferromagnets
Authors: Abdullaev M. N.
Published: July 2002

A theoretical study is made of the spin excitations in two-dimensional three-sublattice Heisenberg antiferromagnets containing an impurity atom in one of the sublattices. It is assumed that the coupling parameter between the impurity and matrix is positive, so that the impurity atom and the atom of the sublattice containing the impurity have the same spin directions in the ground state. In the Green’s function method those functions are chosen that consist of combinations of sums and differences of the spin deviation operators. From the poles of these functions expressions are obtained for the states A1, B1, E1, and E2 of the group D6h, and these expressions are used to determine their energies.
Experimental study of the mixing transition in a gaseous axisymmetric jet
Authors: Meyer T. R. , Dutton J. C. , Lucht R. P.
Published: July 2002

Instantaneous, quantitative, planar images of molecularly mixed-jet fluid fraction were obtained for the purpose of studying the mixing transition in a gaseous axisymmetric jet from ReD=16 200–29 200. By using a simultaneous nitric oxide and acetone planar laser-induced fluorescence technique, the mixing transition was detected from sudden changes in the molecularly mixed-jet fluid volume fraction, the growth rate of the shear layer, the preferred mixed-jet fluid fraction, and the character of axial/radial probability density functions. The mixing transition for all Reynolds numbers in this regime was found to begin after the first vortex pairing near Rx/λ=6 and was completed by the second vortex pairing near Rx/λ=12, where R=(1−r)/(1+r), r is the low- to high-speed freestream velocity ratio, and λ is the natural instability wavelength. The statistical quantities at all Reynolds numbers were found to collapse when scaled with Rx/λ, with the exception of the mixing layer width. The latter collapsed for all Reynolds numbers when scaled by Rx/λ prior to the mixing transition, and by x/D beyond the mixing transition, as expected for turbulent jets for which r≈0.
Temperature-dependent viscous gravity currents with shear heating
Authors: Vasilyev Oleg V. , Ten Arkady A. , Yuen David A.
Published: July 2002

We have studied the effects of viscous dissipation on gravity current in the Stokes flow regime for both constant volume and constant flux boundary conditions. We have also examined the influence of temperature-dependent viscosity, as well as the relative importance of thermal and chemical buoyant forces. For the constant volume case a three-stage evolution was found. This aspect concerning the existence of the multiple stages is new and was not found previously. This three-stage behavior comes as a result of the interaction between the two quasi-isothermal regimes. The first regime corresponds to an early stage with a uniformly high temperature, whereas the third stage represents the final period, when most part of the current has cooled down to uniformly low temperature. This evolutionary process with three-stages is characteristic of a temperature-dependent viscous fluid and does not depend too much on viscous dissipation, which induces a longer transient period. In contrast to constant volume case, there is only one stage of development for the constant flux current. Although temperature-dependent viscosity influences the current dynamics, the rate of expansion follows a t1/2 asymptote which is close to the prediction for a constant viscosity model with a constant flux condition. Viscous dissipation exerts definitely a stronger influence in the constant flux gravity currents as compared to the constant volume case, because of the faster velocities produced by the constant flux condition.
On the influence of a magnetic field on structural transitions in HTSCs
Authors: Mamaluı̆ A. A. , Sablin I. N.
Published: July 2002

The question of the influence of mechanical stresses, induced by pinning of magnetic flux, on low-temperature structural phase transitions in HTSC systems is examined. A model is applied to a specific structural transition in YBa2Cu3O6+x and makes it possible to describe the experimentally observed anomalies of various physical properties exhibiting hysteretic behavior in the temperature range 50–200 K. The magnetostriction induced change, studied in this work, in the position of the boundaries of the hysteretic region is estimated.
Conductance quantization in metal nanowires produced by chemical anodization
Authors: Fisun V. V. , Yanson Yu. I.
Published: July 2002

Measurements of the conductance of a metal nanowire are performed. A niobium nanowire is produced during etching and subsequent anodization of a macroscopic wire at the location where the wire is preconstricted. At the final stage, before the conductor ruptures, the dependence of the conductivity on the decreasing diameter shows a distinct step structure with horizontal sections approximately corresponding to integral conductance quanta 2e2/h=7.75⋅10−5 S. This phenomenon is explained as a discrete passage of electron wave functions through an atomic-size metal contact between two massive electrodes.
Xenon excimer compounds with oxygen in inert-gas crystal matrices
Authors: Belov A. G. , Yurtaeva E. M.
Published: July 2002

Xenon excimer compounds with oxygen in low-temperature inert-gas (R) crystal matrices are investigated. The transformation of the excimer emission spectrum as a reslt of a change in the Xe concentration is analyzed. It is shown that the luminescence band with a peak near 1.7 eV could be due to the superposition of two bands with Emax=1.8 and 1.72 eV. The higher-energy band with Emax=1.8 eV is observed at low xenon concentration, and it can be interpreted as a transition from the 11Π state of the complex XeO* to the lowest state XeO X3Π. The band with Emax=1.72 eV, which predominates in the intrinsic Xe matrix with oxygen impurity and high-concentrated solutions R+Xe+O2 is attributed to emission of the triatomic excimer Xe2O*. A comparative analysis of the experimental data and theoretical calculations, obtained for analogous compounds of xenon halides and hydrides as well as the xenon compound with the isoelectronic S atom, is performed. The results suggest that the 3.33 and 3.58 eV bands observed in the xenon matrix belong to emission of charge-transfer complexes Xe2+O− and Xe2+S−.
Oscillatory acoustoelectronic effect in quasi-two-dimensional conductors in a quantizing magnetic field
Authors: Kirichenko O. V. , Peschanskiı̆ V. G.
Published: July 2002

It is shown that the quasi-two-dimensional character of the electron energy spectrum of a layered conductor results in a large decrease of the absorption of longitudinal sound propagating in a direction normal to the layers in a quantizing magnetic field.
Global instabilities in countercurrent jets
Authors: Grinstein F. F. , DeVore C. R.
Published: July 2002

Self-excited instabilities in supersonic, countercurrent (CC) jets are demonstrated based on numerical simulations, and it is argued that they can be used to explain unresolved discrepancies between laboratory and theoretical analysis. Self-excitation is based on upstream feedback mechanisms acting on the subsonic outer jet regions, and associated with underexpanded initial conditions or partial confinement effects introduced by a collar surrounding the CC jet shear layer. Recognition of these instabilities provides new insights on the role played by the collar in the laboratory CC jet systems, suggesting that practical approaches to the active control of CC jets might be based on suitable direct excitation of the shear layer within the region of influence of the collar.
Bound states of spatially separated electrons in crossed electric and magnetic fields
Authors: Vol E. D. , Shevchenko S. I.
Published: July 2002

A new mechanism is proposed for the binding of two carriers of the same sign in a two-layer system in crossed electric E and magnetic B fields of a special form. A field configuration for which the electric and magnetic fields in the layers are equal in magnitude and opposite in direction is considered. In this geometry of the fields an additional integral of the motion arises: the pair momentum P. For the case when the masses of the carriers in the layers are considerably different, a method is proposed for calculation the states of a pair without making use of the assumption that the Coulomb interaction between carriers is small. The character of the dependence of the energy of a pair on its momentum, E(P), is determined by the ratio of the fields |E/B|. For |E/B|⩾Λcr∝e2/ℏc the energy of a pair is a monotonic function of |P| and there are no bound states in the system. For Λ⩽Λcr, however, the function E(P) is nonmonotonic. A local maximum and local minimum appear on this function, indicating the appearance of bound states of two carriers of the same sign. The physical mechanism leading to the appearance of bound states in crossed fields is investigated, and their main characteristics are calculated. Ways of realizing these bound states in concrete systems and their possible experimental manifestations are discussed.
Low-temperature mechanical hysteresis in superelastic In–Pb alloys
Authors: Fomenko L. S. , Natsik V. D. , Lubenets S. V.
Published: July 2002

Mechanical hysteresis caused by pseudotwinning in In–Pb alloys with concentrations of 6, 8, and 11.6 at.% Pb is investigated in the temperature range 1.7–180 K. The parameters of the hysteresis are estimated: the thermodynamic stress τT responsible for the reversibility of plastic deformation (superelasticity) and the friction stress τf characterizing the resistance exerted by the crystal lattice and its defects to the motion of twin boundaries. It is shown that the parameters of the mechanical hysteresis are determined by athermal processes: the fraction of the sample that has gone into the twin (or, on unloading, to the parent) orientation depends solely on the value of the applied stress. With increasing lead concentration the value of τT increases and τf decreases. One of the main conditions for the appearance of superelasticity is the inequality τT>τf. The hardening of superelastic alloys under cyclic loading is investigated (up to 300 cycles). In alloys with 8 at.% Pb the cyclic loading leads to a slight expansion of the hysteresis loop. For the alloy with 6 at.% Pb there is typically a strong distortion of the hysteresis loop, with the appearance of a transition-hardening part with a large coefficient of work hardening. An analytical description of the cyclic deformation diagrams of a superelastic alloy is proposed.
Temperature-dependent resistance of a finite one-dimensional Josephson junction array
Authors: Engström K. , Kinaret J. M.
Published: July 2002

We study theoretically the temperature and array-length dependences of the resistance of a finite one-dimensional array of Josephson junctions. We use both analytic approximations and numerical simulations, and conclude that within the self-charging model, all finite arrays are resistive in the low-temperature limit. A heuristic analysis shows qualitative agreement with the resistance obtained from Monte Carlo simulations, establishing a connection between resistance and the occurrence of vortices in the corresponding 1+1D XY model. We compare our results with recent experiments and conclude that while the self-charging model reproduces some of the experimental observations, it underestimates the superconducting tendencies in the experimental structures.
EPR spectrum of the Fe3+ ion in bromcresol green (C21H14Br4O5S) and features in the dynamics of the surrounding molecules
Authors: Chabanenko V. V. , Vasyukov V. N. , Kochkanjan R. O. , Nechitailov M. M. , Szymczak H. , Piechota S. , Nabialek A.
Published: July 2002

The EPR spectrum of an Fe3+ impurity in the organic substance bromcresol green (C21H14Br4O5S) is investigated in the temperature interval 4.2–295 K. At a temperature of 4.2 K the spectrum is a superposition of two lines with effective g factors g1=4.39 and g2=2.03. As the temperature is lowered a redistribution of the intensity between line occurs. The integral intensity of the first line decreases with increasing temperature, while that of the second line increases. Another feature of the temperature dependence of the EPR spectrum of the Fe3+ ion in C21H14Br4O5S is manifested in the exponential growth of the width of the second resonance line with decreasing temperature. The observed properties of the EPR spectrum in C21H14Br4O5S are typical for systems with a multiwell potential and permit one to characterize the dynamics of the molecules of the environment of the magnetic ion.
Electromagnetic surface waves in layered conductors
Authors: Gokhfeld V. M.
Published: July 2002

An integral dispersion relation is derived and used to investigate the spectral and relaxation properties of surface plasma oscillations in layered conductors with a quasi-two-dimensional character of the charge-carrier motion. The results demonstrate significant differences from the case of an isotropic metal.
Flux-line pinning by columnar magnetic defects in a type-II superconductor
Authors: Krivenko S. A. , Suleı̆manov N. M.
Published: July 2002

The interaction of a magnetic vortex in a type-II superconductor with a cylindrical (columnar) defect filled with a nonconducting magnetic material is investigated for the first time. Analytical expressions are obtained for the vortex pinning force at such a defect. It is shown that if the material filling the cylinder has a small magnetization, then the attractive force it exerts on a vortex is determined mainly by the interaction of the vortex with the surface of the defect. In the case of a material with a large magnetization the interaction of the magnetic field of the vortex with the magnetic moments throughout the volume of the defect becomes substantial. This can serve as a good resource for enhancing the pinning efficiency.
Galvanomagnetic effects in the normal state of high-Tc metal oxides in a model two-band superconductor with a narrow band (level) near the Fermi boundary
Authors: Galaı̆ko V. P. , Bratus’ E. N.
Published: July 2002

The research of V. P. Galaiko, Fiz. Nizk. Temp. 19, 123 (1993) [Low Temp. Phys. 19, 87 (1993)] on electronic orbital effects in samples of a high-Tc superconductor (Y–Ba–Cu–O) in the normal state is continued for the case of dc galvanomagnetic effects. The conductivity tensor, resistivity, and Hall coefficient are calculated. The results agree qualitatively with experiment.
Computer modeling in nonlinear optics: 4W-mixing theory
Authors: Weiszmann A. N.
Published: July 2002

A fully-quantized nonperturbative coherent four-wave (4W) mixing theory and a three-dimensional computer program to solve the model are presented here. A detailed algorithm for the solution, program extension for extra resonances, and plotting techniques are also given.
Dual implants in InGaAs
Authors: Rao Mulpuri V. , Moore F. , Dietrich H. B.
Published: July 2002

P/Be and Ar/Be dual implantations were performed into In0.53Ga0.47As. Significantly higher Be dopant activation was obtained for P/Be dual implantation compared to Be implantation. Lower dopant activation was obtained for Ar/Be dual implantation. Be in-diffusion during annealing is reduced for both P/Be and Ar/Be dual implantations.
Impact-ionization model consistent with the band structure of semiconductors
Authors: Sano Nobuyuki , Yoshii Akira
Published: July 2002

A new formula for calculating the impact-ionization probability for electrons in semiconductors is derived in terms of the density of states of semiconductors and thus takes into account the details of the realistic band structure. Applying this formula to Si, GaAs, InAs, and In0.53Ga0.47As yields ionization probabilities similar to those derived from the first principles under the constant matrix element approximation, and at high energies (ε≥3 eV) the magnitude and the energy dependence of the calculated ionization probability are similar for each of these materials.
Magnetization reversal properties near the reorientation phase transition of ultrathin Fe/Ag(100) films
Authors: Berger A. , Hopster H.
Published: July 2002

We have measured the magnetization reversal properties of ultrathin Fe/Ag(100) films in the vicinity of the reorientation phase transition using the magneto-optical Kerr effect. Near the reorientation temperature Tr, we observe a characteristic change in the perpendicular magnetization curves M(H) from a strongly nonlinear behavior for T<Tr to a simple linear field dependence above Tr. The experimental observations are discussed within the context of the expected temperature-dependent magnetization structure and the corresponding magnetization reversal processes.
Simulation of void and oxygen precipitation processes during high temperature annealing of silicon wafers
Authors: Romanowski A. , Rozgonyi G. , Tamatsuka M.
Published: July 2002

The kinetics for dissolution/growth of defects in Czochralski silicon wafers during a 1 h high temperature annealing at 1100 °C has been investigated. The size and distribution of point defects such as vacancy, self-interstitial and oxygen interstitial, are simulated for oxygen and hydrogen ambient annealing. The boundary conditions are analyzed separately for hydrogen and oxygen annealing. A deterministic homogeneous model is used for describing the defect kinetics. The self-interstitial injection rate during oxide annealing is calculated from the Deal-Grove model. Simulated void and oxygen size distributions are compared to B- and C-mode capacitor failure distribution functions. Experimental and theoretical data show that voids can be dissolved during either oxygen or hydrogen annealing, while oxygen precipitates are dissolved during hydrogen annealing and only partially dissolved during oxygen annealing.
Precursor decay anomaly in single-crystal lithium fluoride
Authors: Sano Yukio
Published: July 2002

The purpose of this study is to reduce the precursor decay anomaly in single-crystal lithium fluoride (LiF) using a macroscopic approach. To this end, a method of predicting the evolving unsteady plane wave fronts created in the crystal upon impact is developed. Parameters included in modeled strain waves in the fronts are determined such that the predicted particle velocity-time history at the impact surface fits the detector current at the LiF-quartz interface measured by Asay et al. [J. Appl. Phys. 43, 2132 (1972)]. Another condition used is that the particle velocity-time histories at and near the surface are initially parallel. It is assumed that when the amplitude of a near-steady precursor in the predicted unsteady wave front, which increases from a static yield stress, becomes a maximum, a kink occurs at the rear of the precursor and then it begins to decay. The precursor decay curves estimated, based on this assumption, are much lower than Asay’s decay curve. These lower curves are expected to reduce significantly the precursor decay anomaly in this crystal.
Optical grating recording in anisotropic photorefractive thin film: Dimensional resonance
Authors: Kalinin V. , Solymar L.
Published: July 2002

A differential equation and boundary conditions are derived for the amplitude of the first harmonic of the space-charge field in the case of a lossy thin photorefractive film with anisotropic conductivity that is illuminated by a moving interference pattern. It is shown that for certain values of the anisotropy parameter, detuning, and film thickness, a new resonance phenomenon may occur. This resonance is distinct from and additional to the one that relies on the agreement between the speed of the interference pattern and that of the space charge wave, and has been known for a long time.
Relaxation dynamics of nonequilibrium electron-phonon gases in a polar semiconductor
Authors: Nahm J. C. , Hong H. R. , Kim D. Y. , Kim C. S.
Published: July 2002

The thermalization of nonequilibrium electrons and phonons is investigated theoretically within the framework of a semiclassical description of the Boltzmann equation. The electrons are assumed to be excited in the intraband absorption in a polar semiconductor and subsequently undergo a Fröhlich interaction to generate hot phonons. The coupled nonlinear Boltzmann equations for the electron-phonon system are directly solved numerically utilizing a discretization scheme. Consequently, a detailed analysis is given of the relaxation dynamics of the distribution functions and the decay of the hot electron and phonon temperatures.
Two-photon spectroscopy of Rydberg states of molecular oxygen
Authors: Pratt S. T. , Dehmer J. L. , Dehmer P. M.
Published: July 2002

Two-photon resonant, three-photon ionization spectrum of O2X 3Σ−g, v″=0 is presented in the energy region between the 5s–4d complex and the O+2X 2Π1/2g, v+=0 ionization threshold. The spectrum exhibits a number of new transitions to ns and nd Rydberg states with n=5-9 and v′=0 and 1 as well as transitions to the previously unobserved 5s–4d, v′=3 levels. For a given value of n and v′, the observed bands split into two groups separated by approximately the O+2X 2Π1/2g−2Π3/2g spin–orbit splitting. All of the bands show at least some sharp rotational structure, although in most cases this structure is limited to a few lines. In addition, the direct two-photon ionization spectrum of O2X 3Σ−g, v″=0 is presented in the energy region between the O+2X 2Π1/2g, v+=0 and 1 thresholds; the spectrum shows transitions to Rydberg states with n=8-12 and v′=1. These states are analogous to the v′=0 states observed below the ionization threshold, and they decay by vibrational autoionization into the v+=0 continuum. Taken together, the v′=0 and 1 Rydberg states observed here provide a significant addition to the knowledge of Rydberg series converging to the ground electronic state of O+2.
The effect of temperature on the luminescence from electron-irradiated H2O ice
Authors: Quickenden T. I. , Matich A. J. , Bakker M. G. , Freeman C. G. , Sangster D. F.
Published: July 2002

The effect of temperature on the 385 nm luminescence band emitted by electron-irradiated H2O ice has been determined between 79 and 117 K. From 79 to 101 K, the luminescence intensity did not change greatly with increasing temperature, but fell steeply between 101 and 117 K, paralleling the behavior of trapped OH. Kinetic analysis of the luminescence decay was possible in the 79–103 K region and revealed a long-lived pseudo-first-order decay and a superimposed short-lived decay with respective activation energies of 0.036±0.005 and 0.021±0.005 eV. The long-lived emission is attributed to the migration of H+ to OH− formed from trapped OH, subsequent reaction producing H2O(C 1B1) which emits excimer luminescence when it falls to the dissociative A 1B1 state. This mechanism was tested by using it to derive a kinetic expression which relates the observed temperature dependencies of both the luminescence intensity and the decay rate. This mechanism suggests that the radiolytic yield of OH− in ice is in the vicinity of G(OH−)=0.2/100 eV.
Optimal control of selectivity of unimolecular reactions via an excited electronic state with designed lasers
Authors: Shi Shenghua , Rabitz Herschel
Published: July 2002

A molecular system is steered on the ground electronic surface from the initial state to a desired target state at time T via an excited electronic state by using optimally designed coherent laser fields. A new algorithm based on the SU(2) algebra is developed to solve the time-dependent Schrödinger equation for the systems involving two electronic states with time-dependent Hamiltonians. For the design of optimal fields with restricted functional forms the rotating wave approximation is introduced for significantly reducing the computational effort. As a model of unimolecular reactions, a double-well switching problem is studied. The objective is to move the system from one well to the other. It is found that the unrestricted optimal fields which successfully move the system from one well to the other at the target time T are complicated. The objective is achieved through the cooperative interaction between the system and the driving field. The optimal fields with restricted functional forms, such as a train of Gaussian pulses with a single carrier frequency, can also lead to the satisfactory achievement of the objective. However, except for some propitious cases a simple two-pulse pump–dump scheme does not achieve the control objective satisfactorily. Possible further potential applications are discussed briefly.
Wetting of alkanes on water from a Cahn-type theory
Authors: Ragil K. , Bonn D. , Broseta D. , Meunier J.
Published: July 2002

It is shown in this paper that the wetting behavior of n-alkanes on a water surface can be predicted using the wetting theory of Cahn, together with experimental results for n-alkane vapor adsorption on water which are readily available in the literature. The evolution of the wetting behavior with the carbon number of linear alkanes and with temperature is determined and wetting temperatures are calculated. The results agree with experimental observations at room temperature, except for pentane which appears to be a limiting case. The determination of the wetting temperature demonstrates that in order to account for the wetting behavior, additional effects of long-range forces have to be taken into account.
Nucleation in superheated liquid argon–krypton solutions
Authors: Baidakov V. G. , Kaverin A. M. , Boltachev G. Sh.
Published: July 2002

We report nucleation-rate measurements in metastable liquid argon–krypton solutions at pressures of 1.0 and 1.6 MPa over a wide temperature and concentration range. These measurements were performed with the use of a superheated liquid lifetime measurement method. The experimental results are compared with the homogeneous nucleation theory data both using a macroscopic (capillary) approach and taking into account the dependence of critical bubble surface tension on interface curvature. The size effect in nucleation is considered in the framework of the Van-der-Waals, Cahn–Hilliard method. The experimental data indicate that the homogeneous nucleation theory quantitatively describes the kinetics of a first order phase transition in binary solutions of simple liquids if the size effect is taken into account and nucleation rates are J≳106 m−3 sec−1. At J≲106 m−3 sec−1 there is initiated nucleation. A diffusion spinodal of a solution is approximated. The attainable superheating temperature data are presented.
Diffusion of mass-selected carbon atoms and molecules in argon and neon matrices
Authors: Grutter Michel , Freivogel Patrick , Forney Daniel , Maier John P.
Published: July 2002

Annealing reactions of carbon atoms, molecules, and their anions, trapped in noble gas matrices, were studied spectroscopically. To this aim Cn− (n=1–4) anions were mass-selected and codeposited with argon or neon to form matrices at low temperatures. The charged species were neutralized by uv irradiation. Carbon chain elongation is observed only if neutral or charged atomic carbon is present in the matrix. Spectra of linear carbon clusters up to C11 were observed after C− deposition, electron detachment, and subsequent annealing. The reaction between two Cn (n⩾2) species in solid argon or neon was studied and can be disregarded even at temperatures near the point of matrix sublimation.
Semiclassical picture of collision-induced Λ-doublet transitions in diatomic molecules
Authors: Kovalenko Laurie J. , Delos John B.
Published: July 2002

We investigate collision-induced Λ-doublet transitions in a system similar to NO+Ar, based on a semiclassical model in which nuclear motion is treated classically and electronic motion quantum mechanically. We present a picture of this process by monitoring 〈Λ〉, the expectation value of the projection of electronic orbital-angular momentum onto the molecular NO axis, over the duration of the collision. In a typical collision, the interaction with Ar would cause the electronic orbital-angular momentum to precess about the rotating NO–Ar vector. However, since this angular momentum is locked tightly to the diatomic axis, it is restricted to oscillation along this axis. This oscillation leads to transitions between Λ-doublet states. In addition to providing this physical picture of the collision process, we calculate an alignment effect of 1.2 for a hypothetical three-vector correlation experiment, neglecting spin.
A local understanding of the quantum chemical geometric phase theorem in terms of diabatic states
Authors: Atchity Gregory J. , Ruedenberg Klaus
Published: July 2002

A global understanding of the geometric phase theorem associated with conical intersections is gained in terms of local properties of the Hamiltonian along the path actually traversed by means of a resolution of adiabatic states in terms of diabatic states. The analysis also answers certain questions that are left open by formulations in terms of intersection seams. It moreover leads to a method for determining the location of the adiabatic intersections.
Perturbations in the 3sσg 1,3Πg Rydberg states of O2: Bound–bound interactions with the second Πg1 and Δg1 valence states
Authors: Lewis B. R. , Gibson S. T. , Morrill J. S. , Ginter M. L.
Published: July 2002

Existing experimental (2+1) REMPI spectra for transitions into rotationally resolved levels of the 3sσg d 1Πg and 3sσg C 3Πg Rydberg states of O2 have been rotationally analyzed, resulting in the first characterization of rotational perturbations in d(v=1–3) and C(v=2). In addition, the results of this analysis have been interpreted with the aid of a coupled-channel Schrödinger-equation (CSE) model of the interacting electronic states. The identification and characterization of perturbations in the d and C states have allowed the nature of the interactions between the 3sσg 1,3Πg Rydberg states and the II 1Πg valence state to be clarified and a realistic empirical potential-energy curve for the II 1Πg state to be determined. While it is found that first- and second-order interactions with the II 1Πg valence state are responsible for the strongest perturbations observed in d(v=1–3) and C(v=2), additional weak perturbations found in d(v=2 and 3) are shown to result from a second-order interaction with the II 1Δg valence state. These weak perturbations, including an extra level observed for d(v=3, J=17), appear to be the first experimental evidence for the II 1Δg state, long predicted theoretically. Finally, detailed comparisons between experimental spectra and d 1Πg←←a 1Δg(1,0), (2,0) and (3,0) (2+1) REMPI spectra calculated using the CSE model are presented which support and illustrate these conclusions.
Molecular dynamics simulations of simple dipolar liquids in spherical cavity: Effects of confinement on structural, dielectric, and dynamical properties
Authors: Senapati Sanjib , Chandra Amalendu
Published: July 2002

The equilibrium and dynamical properties of Stockmayer liquids confined in a spherical cavity are investigated by means of molecular dynamics simulations. The simulations are carried out at varying density and cavity size. Various equilibrium and time dependent quantities such as the spatial and orientational density profiles, dielectric constants, average energies, pressures, components of translational diffusion tensors parallel and perpendicular to the cavity surface, rotational diffusion coefficients and several time correlation functions are calculated and the effects of confinement on the above properties are discussed. The density profiles are found to be highly inhomogeneous near the cavity wall, and the dielectric constant of the liquids in cavity is found to be significantly smaller than that of the bulk phases. The diffusion along the surface normal and also the dipolar orientational relaxation of solvent molecules in cavity are found to slow down because of confinement. The dynamics of solvation of a newly created charge distribution in the cavity is also studied and the results are compared with the dynamics of solvation in bulk solvent. The solvation in the cavity is found to occur at a much slower rate.
Conservation of angular momentum for systems of charged particles
Authors: Willis Barton L.
Published: July 2002

It is shown that the interaction part of the electromagnetic angular momentum density of a system of charged particles is integrable over any forward light cone. A covariant definition of the mechanical momentum is then given and it is shown that angular momentum is conserved.
Solution of convergence difficulties in the Madelung-sum problem: An extrapolation scheme for sawtooth sequences
Authors: Sarkar Biplab , Bhattacharyya Kamal
Published: July 2002

An extrapolation scheme is presented to handle successfully partial Madelung-sum sequences with zig–zag, both convergent and divergent. Estimates so obtained are found to agree reasonably with the exact results, providing, in cases, even values of superior quality compared to those obtained by invoking the more popular methods of sequence acceleration. The strategy is extremely simple, quite general, and, above all, capable of physically interpreting the limit points in case of divergent sequences. Possibilities of further improvements of the parent scheme are discussed. Remarks on the notion of summability of divergent series in the present context have also been made with a brief analysis. Demonstrative calculations involve a two-dimensional square lattice and a few three-dimensional cubic lattices of practical concern.
Bose fluctuations and paramagnetic susceptibility of normal 2D metal with attraction between carriers: spin gap?
Authors: Loktev V. M. , Sharapov S. G.
Published: July 2002

The paramagnetic susceptibility of a 2D metal with attraction between charge carriers is obtained, proceeding from the simple theoretical field model. The temperature dependence of the magnetic susceptibility is calculated for various concentrations of fermions. It is shown that the paramagnetic susceptibility can be reduced significantly in view of the presence of a finite number of decoupling fermion pairs in the normal phase. The relation of the proposed model to the marginal behavior of the Fermi liquid in high-Tc superconductors, in particular, with the effects associated with the formation of a spin gap, is considered.
Solute segregation at grain boundaries in polycrystals with dominating mass transport by complexes
Authors: Slezov V. V. , Osmayev O. A. , Rogozhkin V. V.
Published: July 2002

Isothermal segregation of an impurity from a finite-size grain to the interface or the external free surface under predominant mass transfer by (vacancy–impurity atom) complexes is studied at low temperatures. A modified equation is obtained for an impurity with the effective diffusion coefficient containing the diffusion coefficients of the impurity and complexes with different weights. The temporal evolution of the impurity concentration is determined for planar, spherical, and cylindrical grains (which are close in shape to conventional grains) in the case of a dilute solution at an arbitrary temperature. Simple algebraic equations are obtained for the impurity concentration at the interface as a function of time. These equations are also valid for a concentrated solution of the impurity at the interface. The kinetics of impurity redissolution, i.e., the enrichment or depletion of the interface with the impurity (the departure of impurity to the bulk of the grain) is considered.
One exactly solvable random spin-1/2 XY chain
Authors: Derzhko O. V. , Verkholyak T. M.
Published: July 2002

Considering one-dimensional spin-1/2 isotropic XY model with Dzyaloshinski–Moriya interaction in random Lorentzian transverse field we have calculated exactly thermodynamic quantities of the model. We used the derived average single-particle density of states to examine the validity of coherent potential approximation.
Jump-like deformation of single crystals of Sn–Cd alloys at temperatures ≲1 K
Authors: Kirichenko G. I. , Natsik V. D. , Pustovalov V. V. , Soldatov V. P. , Shumilin S. E.
Published: July 2002

The low-temperature jump-like deformation of Sn–Cd single crystals is studied systematically for various concentrations. The effect of temperature (in the range 0.5– 4.2 K), Cd concentration (in the interval 0.01–0.53 at.%), strain, and superconducting transition on manifestations of jump-like deformation is analyzed. The obtained results are considered in the light of modern concepts about the nature of low- temperature instability. The observed basic regularities in macroscopic jump-like deformation contradict the hypothesis of thermomechanical instability.
Orbital anisotropy of magnetically distorted superfluid He3-B
Authors: Gongadze A. , Kharadze G. , Vachnadze G.
Published: July 2002

The orbital anisotropy of the magnetized superfluid B-phase of liquid He3 is investigated theoretically for arbitrary fields and temperatures. The behavior of freely rotating B-phase under the action of magnetic field is considered.
Growth kinetics of He4 crystal with a low He3 impurity concentration
Authors: Tsymbalenko V. L.
Published: July 2002

The kinetic growth coefficient is measured for an atomically rough surface of a helium crystal grown from a solution with a low He3 impurity concentration (x=8⋅10−5 and 2⋅10−4). The impurity does not affect the growth rate of the surface in the temperature range from 1.2 to 1.4 K. A comparison of the experimental results with the theory leads to the conclusion that the additional contribution of impurity diffusion to the growth kinetics is equal to zero.
Superconducting properties of YBaCuO thin films obtained by thermocycling
Authors: Yugay K. N. , Seropjan G. M. , Skutin A. A. , Yugay K. K.
Published: July 2002

YBaCuO films fabricated by laser ablation method were subjected to a thermocycling. Our investigations showed that the degradation process of the current transport properties of films is accompanied by the appearance of a maximum on the curve of the critical current density, depending on the number of thermocycles. This effect manifests itself most strongly in the films with degraded superconducting properties. The critical temperature and the transition width vary only slightly in this case.
On the lowest magnetocapacity of a 2D electron system
Authors: Shikin V. B. , Nazin S. S.
Published: July 2002

It is noted that the existing theory of magnetocapacity of screened 2D electron systems becomes meaningless in the vicinity of magnetocapacity minima. A modification of this theory eliminating these limitations is proposed. The derived equations are used for calculating the lowest magnetovcapacity of a parallel-plate capacitor in which one plate is a 2D electron system.
Mobility of electrons in a quasi-one-dimensional conducting channel on the liquid helium surface in the presence of a magnetic field
Authors: Sokolov S. S. , Hai Guo-Qiang , Studart N.
Published: July 2002

The influence of the transverse magnetic field on the electron mobility in a quasi-one-dimensional channel along the liquid-helium surface is investigated. The mobility calculations are carried out by using the Boltzmann kinetic equation and the criteria for the validity of this approach, which are different from those for two-dimensional systems, are established. Two different limiting regimes corresponding to different roles of the electron-electron interaction in the quasi-one-dimensional electron system are considered. The mobility is shown to be a decreasing function of the magnetic field. It is shown that the temperature dependence of the mobility in the presence of the magnetic field, as in the case of zero field, is a nonmonotonic function.
Hydrodynamic theory of magnets with strong exchange interaction
Authors: Isayev A. A. , Kovalevsky M. Yu. , Peletminsky S. V.
Published: July 2002

A microscopic approach to the description of multisublattice magnets with strong exchange interaction is proposed. Low-frequency dynamics of such magnets is characterized by the appearance of an additional dynamical variable, i.e., the orthogonal matrix of rotation, which corresponds to the total breaking of spin invariance [broken SO(3) symmetry]. The structure of the source that breaks the symmetry of the equilibrium Gibbs distribution is established. The quasiaverage representation is generalized to weakly anisotropic, locally equilibrium states. The thermodynamics of such states is constructed. The method of reduced description is formulated and in its framework the hydrodynamic equations for the density of total spin and the matrix of rotation are obtained. The spectra of spin waves are found and the number of Goldstone and activation modes is determined. Two-sublattice ferrimagnet is considered as a special case of the magnet with broken SO(3) symmetry, which corresponds to the special dependence of thermodynamic functions from the matrix of rotation.
Bloch–Siegert type shift and indirect interaction between nuclear spins
Authors: Buishvili L. L. , Buishvili T. L.
Published: July 2002

The Bloch–Siegert-type shift of resonant frequency associated with cross-relaxation interaction is investigated. It is shown that indirect interaction between spins with a low concentration is determined by the polarization of spins with a higher concentration at low temperatures.
Scaling of critical current in granular HTS materials
Authors: Bogoliubov N. A.
Published: July 2002

The temperature and size dependences of critical current of three bismuth-based and one yttrium-based ceramic samples of rectangular cross sections in zero magnetic field are studied by a contactless method. It is shown that the critical current in ceramic HTS can be presented as the product of temperature- and size-dependent factors. The temperature-dependent factor describes the individual properties of Josephson net for each sample, while the size-dependent factor is a universal homogeneous function whose index is independent of the ratio of the sides of the cross section, temperature, and individual properties of the HTS sample.
Relaxation and effects of potential gradient in a Bi point contact
Authors: Andrievskii V. V. , Komnik Yu. F. , Rozhok S. V.
Published: July 2002

It is shown that electron relaxation in an emitter-type point contact strongly affects the position of the first line of transverse electron focussing (EF) in Bi on the magnetic field scale. As a result, the energy of electrons leaving the point contact region is lower than the energy eV determined by the voltage applied to the point contact. In the case of strong currents, the intrinsic field of the current also affects the position of the EF line. The additional shift of the EF line under the action of this factor depends on V nonlinearly in view of strong nonlinearity of the current–voltage characteristics of Bi point contacts. It is shown that this nonlinearity can be explained by an increase in the concentration of charge carriers in the point contact region under the action of the gradient of potential distribution and interband tunneling. These mechanisms give an accurate description of the nonlinearity of the current–voltage characteristics of Bi point contacts.
Peculiarities of acoustic energy transmission from liquid helium to metals: A Review
Authors: Zinov’eva K. N.
Published: July 2002

Experimental and theoretical studies of the coefficient α of phonon energy transmission from liquid helium to cubic-symmetry metals (tungsten, copper, and aluminum single crystals and gold polycrystals) are reviewed briefly. It is shown that the transmission coefficients for single crystals with a perfect surface are correctly described by the theory of acoustic impedance mismatch model, taking into account absorption of phonons by conduction electrons. Andreev’s theory of electron resonant absorption of a Rayleigh wave by the surface of single crystal with α∼1 is confirmed. In a strongly anisotropic copper single crystal, a resonant pseudo-surface wave absorption peak is also observed. It is shown for aluminum that phonon dissipation decreases sharply upon a transition from the normal to superconducting state, and the height of the Rayleigh peak decreases accordingly. It is found that the main mechanism of phonon scattering in a polycrystal is the Rayleigh scattering at grain boundaries, which is proportional to ω4 and is much stronger than scattering at electrons. The form of angular dependence of the absorption coefficient α(θ) changes significantly.