bluuebunny/crossref_metadata_2025 huggingface dataset:
Probing the underlying charge ordering: Ruthenium-doped Sm1−xSrxMnO3 perovskite manganites
Authors: Maignan A. , Martin C. , Hervieu M. , Raveau B. , Hejtmanek J.
Published: July 2002

The effect of ruthenium doping upon the appearance of metallicity and ferromagnetism in the series Sm1−xSrxMnO3 is studied and compared with that previously observed for Sm1−xCaxMnO3. It is shown that, in both cases, Ru doping is efficient in the region where charge ordering exists at long or short range, TC increasing significantly with the doping level. In contrast, in the antiferromagnetic electron-rich region of the Sm1−xSrxMnO3 series (x>0.6), which does not show charge ordering, no ferromagnetism is obtained by Ru doping, though TN is decreased. It is concluded that the huge effect of Ru doping in the appearance of ferromagnetism and metallicity originates from its ability to hinder the underlying charge and orbital ordering, allowing double exchange to be restored.
Deformations in (Al,Ga)As epitaxial layers wafer bonded on dissimilar substrates
Authors: Jenichen B. , Kaganer V. M. , Riedel A. , Kostial H. , Gong Q. , Hey R. , Friedland K. , Ploog K. H.
Published: July 2002

Deformations in heteroepitaxial layer stacks of AlGaAs/GaAs grown on GaAs are measured by triple crystal diffractometry before epitaxial liftoff and after subsequent wafer bonding on various substrates (GaAs, glass, Si, and LiNbO3). The tetragonal deformation present in the as-grown layer stack partially relaxes during epitaxial liftoff. The roughness of the as-grown layer stack gives rise to a bending of the atomic planes after wafer bonding. The widths of the x-ray diffraction peaks are used to estimate the misorientation of the lattice planes and compared with the atomic force microscopy measurements of the surface roughness.
Conductivity spectra of sodium fluorozirconate glasses
Authors: Ghosh A. , Sural M.
Published: July 2002

We have reported conductivity spectra at different temperatures for fluoride glasses of compositions 0.50ZrF4–(0.40−x)BaF2–0.10YF3–xNaF in the frequency range from 10 Hz to 2 MHz. We have analyzed the spectra using conductivity formalism and obtained the frequency exponent, concentration, and hopping frequency of charge carriers (F− ions) from the analysis. We have observed that the concentration of charge carriers is almost independent of temperature, but weakly dependent on composition, and the conductivity is essentially determined by the hopping frequency, i.e., the mobility. The frequency exponent is independent of temperature, but it shows very weak composition dependence. We have also presented a scaling of the conductivity spectra, which implies that the relaxation dynamics of charge carriers in fluoride glasses is independent of temperature and composition.
Dynamical semigroup Fokker–Planck equation approach to transient absorption and fluorescence upconversion spectroscopies
Authors: Shuang Feng , Yang Chen , Yan YiJing
Published: July 2002

A dynamical semigroup Fokker–Planck equation, which meanwhile satisfies also the semiclassical detailed-balance relation at arbitrary temperatures, is constructed and further applied to nonlinear spectroscopic processes of two-surface molecular systems in condensed phases. Included in dissipation are the T1-vibrational relaxation and the pure T2-dephasing in both nuclear and electronic degrees of freedom. A mixed Heisenberg–Schrödinger picture of the field-dressed optical response function is proposed to efficient evaluation of pulsed-laser spectroscopies. Numerical simulations are carried out in a model one-dimensional dissipative Morse molecular system. Both the pump–probe absorption and the time-frequency resolved fluorescence spectra are demonstrated and analyzed in detail in terms of the underlying dissipative dynamics.
Stability of a magnetized Hall plasma discharge
Authors: Chesta E. , Meezan N. B. , Cappelli M. A.
Published: July 2002

Using recent experimental data on the time-averaged, spatially varying plasma properties within a Hall discharge plasma, we present in this article, a theoretical study of the response of this plasma to small (linear) perturbations in its properties. As a starting point for this analysis, we assume a two-dimensional fluid description that includes a simplified equation for the electron energy, and constrain the azimuthal wave vector such that we excite only the dominant (m=1) azimuthal modes. The growth rate and frequencies of predominantly axial and azimuthally propagating plasma disturbances are obtained by numerical solution of the resulting eigenvalue problem under a quasiuniform plasma condition, along the entire discharge channel. The results identify the persistence of a low frequency instability that is associated with the ionization process, concentrated largely in the vicinity of the exit plane, where the magnetic field is at its maximum value, consistent with experimental observations for the relatively low operating voltages (∼100 V) considered in this study.
Threading dislocation evolution in mega-electron-volt phosphorus implanted silicon
Authors: Jasper Craig , Banerjee Suman K. , Hoover Allen , Jones Kevin S.
Published: July 2002

The effect of dose and energy on postannealing defect formation for high energy (mega-electron-volt) phosphorus implanted silicon has been studied using etch pit studies and transmission electron microscopy (TEM). Previous work has shown that after annealing there is a strong dependence of dislocation density threading to the surface on the implanted phosphorus dose and energy. A superlinear increase in threading dislocation density (TDD) with implant energy between 180 and 1500 keV is observed for a dose of 1×1014 cm−2. In addition as a function of ion fluence, there is a maximum in the threading dislocation density at a dose of 1×1014 cm−2 followed by a rapid decrease in TDD. Both the superlinear increase in TDD with increasing energy and the rapid decrease with increasing dose have been further investigated by TEM. A TEM study of these higher doses revealed formation of a strong bimodal loop distribution with small loops averaging <1000 Å and large loops averaging around 1 μm in size. Over the dose range of 1×1014 cm−2 to 5×1014 cm−2, the superlinear decrease in TDD from 1×106 cm−2 to <1×104 cm−2 coincides with the superlinear increase in small dislocation loops from below 1×106 cm−2 to above 1×1010 cm−2. It is suggested that the homogeneous nucleation theory can explain many of the results. However, the chemical presence of phosphorus appears to also play an important role in the formation of the small dislocation loops and possibly threading dislocations.
Solutions of relativistic wave equations in superpositions of Aharonov–Bohm, magnetic, and electric fields
Authors: Bagrov V. G. , Gitman D. M. , Tlyachev V. B.
Published: July 2002

We present new exact solutions (in 3+1 and 2+1 dimensions) of relativistic wave equations (Klein–Gordon and Dirac) in external electromagnetic fields of special form. These fields are combinations of Aharonov–Bohm solenoid field and some additional electric and magnetic fields. In particular, as such additional fields, we consider longitudinal electric and magnetic fields, some crossed fields, and some special nonuniform fields. The solutions obtained can be useful to study the Aharonov–Bohm effect in the corresponding electromagnetic fields.
Recombination enhanced defect reactions in 1 MeV electron irradiated p InGaP
Authors: Khan Aurangzeb , Yamaguchi Masafumi , Bourgoin Jacques C. , Ando Koshi , Takamoto Tatsuya
Published: July 2002

Direct recombination enhanced annealing of the radiation-induced defect H2 in p InGaP has been observed by deep level transient spectroscopy (DLTS). Detailed analysis of the annealing data at zero and reverse bias shows that annealing rates are independent of the defect charge state or this defect interacts with the two bands, i.e., is a recombination center trapping alternatively an electron, then a hole. An experiment based on minority carrier capture on a majority trap by the double carrier pulse DLTS technique further supports the evidence that H2 has a large minority carrier capture cross section and is an efficient nonradiative recombination center. Recombination-enhanced defect annealing rates obeys a simple Arrhenius law with an activation enthalpy of 0.51±0.09 eV, in contrast to athermal processes observed in GaP. Detailed analysis of results reveals that the mechanism involved in the minority carrier injection annealing of the H2 defect is energy release mechanism in which enhancement is induced by the energy which is released when a minority carrier is trapped on the defect site. Finally, analysis of the depth profiles data relates that H2 acts as a donor, which partially compensates the acceptors.
Limits on the Curie temperature of (III,Mn)V ferromagnetic semiconductors
Authors: Schliemann John , König Jürgen , Lin Hsiu-Hau , MacDonald Allan H.
Published: July 2002

Mean-field-theory predicts that the Curie temperature Tc of a (III,Mn)V ferromagnet will be proportional to the valence band density-of-states of its host (III,V) semiconductor, suggesting a route toward room-temperature ferromagnetism in this materials class. In this letter, we use theoretical estimates of spin-wave energies and Monte Carlo simulations to demonstrate that long-wavelength collective fluctuations, neglected by mean-field theory, will limit the critical temperature in large density-of-states materials. We discuss implications for high Tc searches.
Cooperative magnetism and the Preisach model
Authors: Skomski R. , Sellmyer D. J.
Published: July 2002

Cooperative and noncooperative magnetization processes in magnetic nanostructures are investigated. Using model calculations it is shown that the Preisach model and related approaches, such as Henkel, ΔM, and ΔH plots, describe magnetism on a mean-field level and cannot account for intra- and inter-granular cooperative effects. For example, the ΔM plot of a nucleation-controlled two-domain particle gives the false impression of a positive intergranular interaction. A simple but nontrivial cooperative model, consisting of two interacting but nonequivalent particles, is used to show that cooperative effects are most pronounced for narrow switching-field distributions, i.e., for nearly rectangular loops. This is unfavorable from the point of magnetic recording, where one aims at combining narrow loops with a noncooperative local switching behavior. A general rule is that the neglect of cooperative effects leads to an overestimation of the coercivity.
Switching dynamics and critical behavior of standard problem No. 4
Authors: McMichael R. D. , Donahue M. J. , Porter D. G. , Eicke Jason
Published: July 2002

We report results for μMAG standard problem No. 4, a 500 nm×125 nm×3 nm rectangle of material with properties to mimic Permalloy. Switching dynamics are calculated for fields applied instantaneously to an initial s state: Field 1 at 170° and Field 2 at 190° (−170°) from the positive long axis. Reversal in Field 1 proceeds by propagation of end domains toward the sample center. Reversal in Field 2 involves rotation of the end domains in one direction while the center of the particle rotates in the opposite direction, resulting in collapsing 360° walls with complex dynamics on fine length scales. Approaching the static coercivity, Hc, in small field steps, we find that the ring down frequency, f, and susceptibility, χ, are in approximate agreement with a single spin model that predicts f∝(Hc−H)1/4 and χ∝(Hc−H)−1/2. We show a correlation between the modes of oscillation that become unstable at the critical field and the switching behavior.
Positive giant magnetoresistance in ferrimagnetic/Cu/ferrimagnetic films
Authors: Lai Chih-Huang , Lin Chao-Cheng , Chen B. M. , Shieh Han-Ping D. , Chang Ching-Ray
Published: July 2002

Spin valves composed of ferrimagnetic/Cu/ferrimagnetic layers were fabricated with the magnetization perpendicular to the film planes. By changing the composition of ferrimagnetic layers, both negative and positive giant magnetoresistance (GMR) can be observed in ferrimagnetic spin valves. For samples consisting of both transition-metal (TM-) rich TbFeCo and GdFeCo, negative GMR values were obtained. Due to the high resistivity of amorphous ferrimagnetic films, the shunting effect of Cu led to relatively small MR ratio. The negative MR effect was 1% for 1.7 nm Cu. For spin valves consisting of rare-earth (RE-)rich TbFeCo and TM-rich GdFeCo, positive GMR values were observed. A thin layer of Co was inserted between RE-rich TbFeCo and Cu to manipulate the sign of GMR.
Influence of As incorporation on the deviation from Vegard’s law in the AlxGa1−xSb/GaSb system
Authors: Bocchi C. , Franchi S. , Germini F. , Baraldi A. , Magnanini R. , De Salvador D. , Berti M. , Drigo A. V.
Published: July 2002

A possible unintentional incorporation of As in AlxGa1−xSb/GaSb heterostructures grown by molecular beam epitaxy in a chamber where group-III arsenides are also prepared, was investigated by high resolution x-ray diffraction measurements. The incorporation was determined by measuring the lattice mismatch between GaSb substrates and the possibly As-contaminated GaSb buffer layers in several structures containing layers with different Al concentration. The largest As molar fraction value measured is 0.000 43. The effect of the As incorporation on the deviation from Vegard’s law, as previously found in the AlGaSb epitaxial system, was considered. From this analysis it was possible to confirm the nonlinear variation of the lattice constant versus the Al content, as previously determined.
Fabrication and characterization of pulse laser deposited Ni2Si Ohmic contacts on n-SiC for high power and high temperature device applications
Authors: Cole M. W. , Joshi P. C. , Ervin M.
Published: July 2002

Ni 2 Si Ohmic contacts were fabricated via pulsed laser deposition on n-SiC. The contacts electrical, structural, compositional, and surface morphological properties were investigated as a function of annealing temperatures ranging from 700 to 950 °C. The as-deposited and 700 °C annealed contacts were non-Ohmic. Annealing at 950 °C yielded excellent Ohmic behavior, an abrupt void free interface, and a smooth surface morphology. No residual carbon was present within the contact metallization or at the contact-SiC interface and the contact showed no appreciable thickness increase as a result of the annealing process. Our results demonstrate that aside from maintaining the desirable electrical integrity associated with Ni and Ni/Si Ohmic contacts, the Ni2Si Ohmic contacts possessed improved interfacial, compositional, microstructural, and surface properties which are required for reliable high temperature and high power device operation.
The radial structure of a plasma column sustained by a surface wave
Authors: Azarenkov N. A. , Denysenko I. B. , Gapon A. V. , Johnston T. W.
Published: July 2002

The self-consistent radial profiles are obtained for a steady-state, surface-wave-sustained, unmagnetized argon plasma in a cylindrical dielectric tube surrounded by a conductor. The density profiles obtained using the correct constant mean-free-path ion–neutral collision differ considerably from those obtained using the common (but here quite incorrect) approximation of constant mean free time, but are little affected by changes in the profile of wave energy deposition. Various profiles for some typical cases are also compared for three different electron heat transport assumptions. These electron heat transport coefficients are as follows: (i) The correct value, (ii) 102× the correct value, to obtain the essentially isothermal case, and (iii) 10−2× the correct value, to approximate negligible heat transport (the adiabatic case). From the comparison of the temperature profiles, it became clear that the best electron heat transport simplification is to take the electrons to be isothermal [i.e., assumption (ii)], in which case only the total absorption per unit length is important, and not the details of the surface wave behavior.
Rotational hysteresis of torque curves in polycrystalline ferro/antiferromagnetic systems
Authors: Zhang K. , Kai T. , Zhao T. , Fujiwara H. , Hou C. , Kief M. T.
Published: July 2002

Rotational hysteresis of ferromagnetic (F)/antiferromagnetic (AF) exchange coupled systems was studied by using NiFe/IrMn, NiFe/FeMn, and NiFe/NiMn samples sputter deposited under almost the same conditions, although the sample of NiFe/NiMn was annealed later to obtain the antiferromagnetic phase for the NiMn layer. The rotational hysteresis of each sample exhibited quite a different feature from each other, especially in the rotational hysteresis–magnetization angle curves. The NiFe/IrMn bilayer showed a dip at around the antiparallel direction to the applied field direction during sample preparation (pinning direction), while the NiFe/NiMn bilayer exhibited a large peak in that direction. The NiFe/FeMn bilayer did not show any noticeable structure in the rotational hysteresis–magnetization curve. Based on the analysis of those data done by using the model recently proposed by the authors, the following was inferred: (1) The distribution of the anisotropy easy axes of IrMn of the NiFe/IrMn bilayer grains is limited in some range of angles around the pinning direction. Some direct exchange coupling exists between the IrMn grains. (2) The distribution of the anisotropy easy axes of FeMn grains of the NiFe/FeMn bilayer is almost random and the direct exchange coupling between the FeMn grains is negligible. (3) In the NiFe/NiMn bilayer, substantial strength of direct exchange coupling exists between some of the grains which switch and the ones which do not switch with the rotation of the magnetization of the NiFe layer, although the volume fraction of the former is much smaller than the latter.
Electrode roughness and interfacial mixing effects on the tunnel junction thermal stability
Authors: Cardoso S. , Freitas P. P. , Zhang Z. G. , Wei P. , Barradas N. , Soares J. C.
Published: July 2002

The thermal stability of magnetic tunnel junctions with ultrathin (<8 Å) Al2O3 barriers was studied and compared with 15 Å barriers. The tunnel magnetoresistance (TMR) decay cannot be explained only by Mn diffusion into the pinned CoFe layer since this diffusion starts above 300 °C independently of the barrier thickness, while the TMR degradation already occurs at 250–270 °C for the thinner barriers. The thermal stability is probably controlled by changes at the CoFe/Al2O3 interfaces and/or barrier structure. Structural analysis of 15 Å barriers after annealing at 435 °C, shows the existence of an interface region (8–12 Å thick) where CoFe and Al2O3 are found. This interfacial region can be explained by the increased roughness in the bottom electrode after annealing, as measured by atomic-force microscopy (from 1.5 to 4 Å). Ultrathin barriers show a similar trend. The use of low-resistance junctions using thin barriers requires good control of the roughness of the low-resistance bottom electrodes. This is done by preannealing and low-angle ion-beam smoothing 500-Å-thick Cu or Al films, which will then keep a roughness <2 Å during processing temperatures up to 400 °C. Low-resistance junctions (R×A∼40–60 Ω μm2) with 7 Å barriers grown on 600 Å Al buffers after the surface treatment show 25% TMR after annealing at 270 °C.
Magnetization dynamics in NiFe thin films induced by short in-plane magnetic field pulses
Authors: Gerrits Th. , Hohlfeld J. , Gielkens O. , Veenstra K. J. , Bal K. , Rasing Th. , van den Berg H. A. M.
Published: July 2002

The magnetization dynamics in a thin NiFe film was investigated by applying short in-plane magnetic field pulses while probing the response using a time-resolved magneto-optical Kerr effect setup. In-plane magnetic field pulses, with duration shorter than the relaxation of the system, were generated using a photoconductive switch and by subsequent propagation of current pulses along a waveguide. The field pulses with typical rise and decay times of 10–60 and 500–700 ps, respectively, have a maximum field strength of 9 Oe, by which Permalloy elements of 16 nm thickness and lateral dimensions of 10×20 μm were excited. The observed coherent precession of a ferromagnetic NiFe system had precession frequencies of several GHz and relaxation times on a nanosecond time scale. The dynamic properties observed agree well the Gilberts’s precession equation and the static magnetic properties of the elements
An alternative derivation of the equation for small angle scattering from pores with fuzzy interfaces
Authors: McMahon Phillip J. , Snook Ian , Smith Edgar
Published: July 2002

An alternative derivation of the small angle scattering from fuzzy interfaces is presented. We treat the divergent case of β=1/2 not examined in the original derivation and show that a solution to the small angle integral can be obtained. This approach to evaluate small angle problems may also be applicable to other small angle models where divergence exists in integral equation within a physical range.
Soft pseudopotentials for efficient quantum Monte Carlo calculations: From Be to Ne and Al to Ar
Authors: Ovcharenko Ivan , Aspuru-Guzik Alán , Lester William A.
Published: July 2002

We have developed “soft” ab initio Hartree–Fock pseudopotentials for Be–Ne and Al–Ar that avoid singularities at the electron-nuclear cusp. The absence of these singularities is a desired feature for quantum Monte Carlo (QMC) calculations, because it considerably reduces local energy fluctuations when sampling the QMC wave function. A modified procedure for obtaining soft pseudopotentials is introduced. Stability of QMC calculations with the developed pseudopotentials is demonstrated by the ability to use larger time steps for comparable accuracy with the previously available pseudopotentials. As expected, ionization potentials and electron affinities computed using the present approach compare very favorably with other Hartree–Fock pseudopotential methods.
Collisionless kinetic-fluid closure and its application to the three-mode ion temperature gradient driven system
Authors: Sugama H. , Watanabe T.-H. , Horton W.
Published: July 2002

A novel closure model is presented to give a set of fluid equations which describe a collisionless kinetic system. In order to take account of the time reversal symmetry of the collisionless kinetic equation, the new closure model relates the parallel heat flux to the temperature and the parallel flow in terms of the real-valued coefficients in the unstable wave number space. Effects of the closure model on turbulence saturation and anomalous transport are investigated based on kinetic and fluid entropy balances. When the closure model is applied to the three-mode ion temperature gradient (ITG) driven system, the fluid system of equations reproduces the exact nonlinear kinetic solution found by Watanabe, Sugama, and Sato [Phys. Plasmas 7, 984 (2000)]. Oscillatory behaviors and initial amplitude dependence of other numerical kinetic solutions of the three-mode ITG problem can also be accurately described by the fluid system.
Ab initio zero electron kinetic energy spectroscopy of the ArCl− and KrCl− anions
Authors: Buchachenko A. A. , Szczȩśniak M. M. , Chałasiński Grzegorz
Published: July 2002

The theory for calculating the energies and relative intensities of the photodetachment transitions of the weakly bound diatomic anions is implemented for simulations of the zero electron kinetic energy spectra of the ArCl− and KrCl− anions using high-quality ab initio potentials. Its key features are the reduction of the molecular electronic photodetachment transition dipole moment matrix elements to the combination of atomic ones within the atoms-in-molecule model and estimation of the latter by well-developed approaches to the atomic photodetachment processes. The difference in the electronic photodetachment transition dipole moments for distinct electronic states of the neutral, importance of vibrational–rotational coupling, and rotational structure of vibronic bands are analyzed. It is shown that ab initio simulations provide almost the same level of agreement with measured spectra as did the empirical potentials adjusted to reproduce the experimental data. The predictive power of the theory is demonstrated by the discovery of the hidden transitions to the I3/2 state of KrCl which strongy overlap with those to X1/2 state.
Scalar relativistic effects on energies of molecules containing atoms from hydrogen through argon
Authors: Kedziora Gary S. , Pople John A. , Ratner Mark A. , Redfern Paul C. , Curtiss Larry A.
Published: July 2002

Stationary direct perturbation theory is used to calculate a scalar relativistic correction to the species in the G3/99 test set. We observe that the relativistic energy almost always reduces atomization energies, electron affinities, and ionization potentials. Exceptions occur when s orbitals play a predominant role in the energy differences between reactants and products. The scalar relativistic energy, when added to G3 theory and the empirical parameters are reoptimized, gives slightly worse agreement with experiment.
Million-atom molecular dynamics simulation of flat InAs overlayers with self-limiting thickness on GaAs square nanomesas
Authors: Su Xiaotao , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya , Madhukar Anupam
Published: July 2002

Large-scale molecular dynamics simulations are performed to investigate the mechanical stresses in InAs/GaAs nanomesas with {101}-type sidewalls. The in-plane lattice constant of InAs layers parallel to the InAs/GaAs(001) interface starts to exceed the InAs bulk value at the twelfth monolayer (ML) and the hydrostatic stresses in InAs layers become tensile above ∼12 ML. As a result, it is not favorable to have InAs overlayers thicker than 12 ML. This may explain the experimental findings of the growth of flat InAs overlayers with self-limiting thickness of ∼11 ML on GaAs nanomesas.
Microcathodoluminescence of impurity doping at gallium nitride/sapphire interfaces
Authors: Goss S. H. , Sun X. L. , Young A. P. , Brillson L. J. , Look D. C. , Molnar R. J.
Published: July 2002

We have used low-temperature cathodoluminescence spectroscopy (CLS) to probe the spatial distribution and energies of electronic defects near GaN/Al2O3 interfaces grown by hydride vapor phase epitaxy (HVPE). Cross sectional secondary electron microscopy CLS shows systematic variations in impurity/defect emissions over a wide range of HVPE GaN/Sapphire electronic properties. These data, along with electrochemical capacitance–voltage profiling and secondary ion mass spectrometry, provide a consistent picture of near-interface doping by O diffusion from Al2O3 into GaN, over a range 100–1000 nm.
The molecular structures and nature of interactions in CH3+Arn(n=1–8) complexes
Authors: Gora Robert W. , Roszak Szczepan , Leszczynski Jerzy
Published: July 2002

The CH3+Arn complexes are investigated for n up to 8. The molecular structures, dissociation energies, and vibrational frequencies of those complexes are studied, and the nature of the interactions is discussed. All possible structural isomers were considered. The formation of clusters is based on the consecutive filling of four distinct shells. The theoretically determined consecutive dissociation energies are in excellent agreement with experimental data derived from infrared photodissociation spectra and pulsed electron beam high-pressure mass spectrometry. The influence of an argon environment on the electron affinity of the CH3+ ion is also discussed.
Monte Carlo simulations of free chains in end-linked polymer networks
Authors: Gilra Nisha , Panagiotopoulos Athanassios Z. , Cohen Claude
Published: July 2002

The structural properties of end-linked polymer networks prepared in the presence of inert linear chain solvent were investigated with Monte Carlo simulations using the three-dimensional bond fluctuation model on a simple cubic lattice. Networks of 50-mer precursor chains were prepared in a solvent of 50-mer inert linear chains with a series of concentrations and two ratios, r, of cross-link sites to chain ends. The networks were formed under both stoichiometric (r=1) and optimal (r=1.2) conditions for minimizing the network imperfections and soluble material and maximizing the elastic material. A maximum is observed in the fraction of elastic material at small degrees of dilution and is explained in terms of entanglement effects. The conformational behavior of a small concentration of linear 50-mer probe chains trapped in end-linked networks of mesh sizes ranging from 10- to 50-mer was also studied. The radius of gyration of the linear chains was found to decrease with decreasing mesh size of the host network, in agreement with a theoretical scaling relationship; but the magnitude of the effect is small.
Enhanced sampling in simulations of dense systems: The phase behavior of collapsed polymer globules
Authors: Paul W. , Müller M.
Published: July 2002

Slow relaxation due to dense packing encumbers the simulation of a variety of many particle systems like, for instance, collapsed polymers (folded proteins) or structural glasses. We propose to overcome this problem by a new algorithm, assigning each particle a fourth space coordinate and treating an expanded ensemble of systems of which the one with all fourth coordinates equal to zero is the physical system. An application of this simulation strategy to the coil–globule transition of homopolymers using the bond-fluctuation model reveals a hitherto unfound first-order liquid to solid transition of the collapsed globule.
Photoelectron spectroscopy of C3Si and C4Si2 anions
Authors: Davico Gustavo E. , Schwartz Rebecca L. , Lineberger W. Carl
Published: July 2002

The 364 nm photoelectron spectra of the linear C3Si− and C4Si2− anions are reported. Accurate adiabatic electron affinities are determined: EA(3Σ C3Si)=2.827±0.007 eV and EA(C4Si2)=2.543±0.006 eV. Several vibrational frequencies for both neutral molecules are also obtained. The term energy for the first linear excited state of C3Si (either Δ1 or Σ)1 is 0.274±0.015 eV. For C4Si2, the term energy is substantially lower than in C3Si and vibronic interactions between the two states become stronger. Experimental results are compared with high-level ab initio calculations for C3Si (see Rintelman and Gordon, following paper) and with our own calculations for C4Si2 and its anion.
Direct observation of longitudinal spatial hole burning in semiconductor optical amplifiers with injection
Authors: Fehr J.-N. , Dupertuis M.-A. , Hessler T. P. , Kappei L. , Marti D. , Selbmann P. E. , Deveaud B. , Pleumeekers J. L. , Emery J.-Y. , Dagens B.
Published: July 2002

Measurements of spontaneous emission from InGaAsP semiconductor optical amplifiers provide information on both the carrier density and temperature. By spatially resolving the light emitted along the active layer of the device, we find evidence of longitudinal spatial hole burning which results from amplified spontaneous emission in the structure and is modified by the injected optical signal. Under injection, we also observe pronounced asymmetry of the amplified spontaneous emission intensity from the two facets which we relate to the carrier density profile. The experimental results are in good agreement with numerical simulations. An analysis of the measured spectra reveals an unexpected very high temperature (400 K) and its decrease by at least 35 K in the middle of the device when light is injected.
Anomalies and Schwinger terms in NCG field theory models
Authors: Langmann E. , Mickelsson J. , Rydh S.
Published: July 2002

We study the quantization of chiral fermions coupled to generalized Dirac operators arising in NCG Yang–Mills theory. The cocycles describing chiral symmetry breaking are calculated. In particular, we introduce a generalized locality principle for the cocycles. Local cocycles are by definition expressions which can be written as generalized traces of operator commutators. In the case of pseudodifferential operators, these traces lead in fact to integrals of ordinary local de Rham forms. As an application of the general ideas we discuss the case of noncommutative tori. We also develop a gerbe theoretic approach to the chiral anomaly in the Hamiltonian quantization of NCG field theory.
Shock wave interaction with a spiral vortex
Authors: Barbosa F. J. , Skews B. W.
Published: July 2002

Previous studies on shock–vortex interactions have either been numerical in nature with idealized boundary conditions and prescribed vortex flows, or in complex experimental flow fields, such as where the vortex is shed at the trailing edge of an aerofoil. In this study a bifurcated shock tube facility has been constructed where two plane waves arrive sequentially at the trailing edge of a wedge. The first shock wave results in a spiral vortex being shed, which is then impacted by the second wave. Accurate control of the delay between the two shock waves was achieved using a highly repeatable piston actuated shock tube driver. A number of interesting new features of this interaction have been identified. The work specifically examines the development of a transient pressure spike, physically occupying an area less than 0.5 mm in diameter and having a duration of 15 μs, with a pressure nearly two-and-a-half times that of the surrounding fluid. This has been done both numerically using an adapting mesh Euler code, and experimentally, the latter with the careful use of fast response miniature pressure transducers. Numerically generated holographic interferograms and shadowgraph images have been generated for direct visual comparisons with the equivalent experimental results of the whole flow field, from which the reason for the production of the pressure spike is established as being due to local shock wave focusing resulting from part of the shock being pulled around the vortex to impact on itself. The generation of a second pressure peak is also examined, as is the wave field emanating from the interaction and the influence on the vortex. The use of both experimental images and numerical flow visualization algorithms were found to provide complimentary information, which allowed for detailed investigation and understanding of the shock wave–vortex interaction.
Formation, structural and magnetic properties of Gd3(Co,Cr)29 compounds
Authors: Wang W. Q. , Wang J. L. , Tang N. , Wu G. H. , Yang F. M. , Jin H. M.
Published: July 2002

Co-based Gd3Co29−xCrx compounds with a Nd3(Fe,Ti)29-type of structure have been synthesized successfully for x=6.5 and 7.0. The crystal structure and magnetic properties of the Gd3Co29−xCrx compounds have been investigated by x-ray diffraction analysis and magnetic measurements. It has been found that Gd3(Co,Cr)29 with x=6.5 and 7.0 are of ferrimagnetic compounds with Curie temperature of 412 K for x=6.5 and 359 K for x=7.0, respectively. Temperature dependence of the magnetization shows a compensation point of the magnetization at the certain temperature below the Curie temperature. The intersublattice molecular field coefficient, nRT, was derived to be 3.3 T f.u./μB for Gd3Co22.5Cr6.5 on the basis of the magnetization curve at the compensation temperature. It is noteworthy that the Gd3(Co,Cr)29 compounds exhibit favorable easy-axis anisotropy in the whole temperature range below Curie temperature.
Collision-induced electronic energy transfer from v=0 of the E(0g+) ion-pair state in I2: Collisions with I2(X)
Authors: Fecko Christopher J. , Freedman Miriam A. , Stephenson Thomas A.
Published: July 2002

The collision-induced electronic energy transfer that occurs when I2 in the E(0g+) ion-pair electronic state collides with ground electronic state I2 has been investigated. We prepare I2 in single rotational levels in v=0 of the E state using two-color double resonance laser excitation. The resulting emission spectrum shows that the nearby (ΔTe=−385 cm−1) D(0u+) electronic state is populated. The cross section for collision-induced E→D energy transfer is found to be 18±3 Å2. A range of D state vibrational levels are populated, consistent with a model in which overlap between the initial and final vibrational wave functions is important, but modulated by propensities for small vibrational energy gaps and those energy gaps that are closely matched to the v=0→v=1 energy separation in the I2(X) collision partner.
Instantons in four-dimensional gauged O(5) Skyrme models
Authors: Brihaye Y. , Paturyan V. , Piette B. M. A. G. , Tchrakian D. H.
Published: July 2002

We consider a family of four-dimensional nonlinear sigma models based on an O(5) symmetric group, whose fields take their values on the 4-sphere S4. An SO(4)-subgroup of the model is gauged. The solutions of the model are characterized by two distinct topological charges, the Chern–Pontryagin charge of the gauge field and the degree of the map, i.e., the winding number, of the S4 field. The one-dimensional equations arising from the variation of the action density subjected to spherical symmetry are integrated numerically. Several properties of the solutions thus constructed are pointed out. The only solution with unit Chern–Pontryagin charge are the usual BPST instantons with zero S4 winding number, while solutions with unit S4 winding number have zero Chern–Pontryagin charge.
An instrument for simultaneous kinetic measurements of microscopic infrared dichroism and stress of inhomogeneous polymer thin films at constant elongation rate
Authors: Shigematsu Yasuyoshi , Takada Akihiko , Nemoto Norio , Nitta Koh-Hei
Published: July 2002

A new instrument has been built for a study on deformation mechanisms of inhomogeneous polymer thin films from simultaneous in situ kinetic measurements of microscopic infrared dichroism and stress at constant elongation rate. During elongation, the center position of the film is slightly adjusted manually by direct observation of the sample using the microscope so as to measure IR interferogram from the same rectangular area with a side from 20 to 600 μm. The average orientation angle θ(t) of the main chain to the draw direction is estimated from the function A(t)=(A∥−A⊥)/(A∥+A⊥) using a photoelastic modulator. Here A∥ and A⊥ are the absorbances measured with radiation polarized parallel and perpendicular to the draw direction, respectively. True stress is calculated from nominal stress by correcting changes in film thickness as well as width accompanied by large deformation. Capability and limit of the instrument are examined in detail using two polypropylene films with different morphologies, one consisting of crystallites showing no gross morphology such as spherluites and another including only a huge spherulite embedded in the matrix.
Graphical representation of the partition function of a one-dimensional δ-function Bose gas
Authors: Kato Go , Wadati Miki
Published: July 2002

The one-dimensional repulsive δ-function bose system is studied. By only using the Bethe ansatz equation, n-particle partition functions are exactly calculated. From this expression for the n-particle partition function, the n-particle cluster integral is derived. The results completely agree with those of the thermal Bethe ansatz (TBA). This directly proves the validity of the TBA. The theory of partitions and graphs is used to simplify the discussion.
An algebraic approach to Macdonald–Koornwinder polynomials: Rodrigues-type formula and inner product identity
Authors: Nishino Akinori , Komori Yasushi
Published: July 2002

We study Macdonald–Koornwinder polynomials in the context of double affine Hecke algebras. Nonsymmetric Macdonald–Koornwinder polynomials are constructed by use of raising operators provided by a representation theory of the double affine Hecke algebra associated with A2l(2)-type affine root system. This enables us to evaluate diagonal terms of scalar products of the nonsymmetric polynomials algebraically. The Macdonald–Koornwinder polynomials are expressed by linear combinations of the nonsymmetric counterparts. We show a new proof of the inner product identity of the Macdonald–Koornwinder polynomials without Opdam–Cherednik’s shift operators.
The sufficient conditions under which the S-matrix orthogonal part is real and even
Authors: Muravyev Dmitry I.
Published: July 2002

The sufficient conditions under which the S-matrix orthogonal part U(k) is real and even are proved. These conditions are required for generalization of the projection matrices method in the N×N Riemann–Hilbert boundary value problem, N=2,3,…,N<+∞. The dependence of the analytical properties of the orthogonal matrix U(k) on the S-matrix diagonal part is thoroughly considered. The examples in which the sufficient conditions are not realized and Re U(k)≠0, U(−k)=−U(k) are demonstrated.
Radiating sources in higher-dimensional gravity
Authors: Liu Hongya , Wesson Paul S.
Published: July 2002

We study a time-dependent five-dimensional (5D) metric which contains a static four-dimensional (4D) sub-metric whose three-dimensional (3D) part is spherically symmetric. An expansion in the metric coefficient allow us to obtain close-to Schwarzschild approximation to a class of spherically symmetric solutions. Using Campbell’s embedding theorem and the induced-matter formalism we obtain two 4D solutions. One describes a source with the stiff equation of state believed to be applicable to dense astrophysical objects, and the other describes a spherical source with a radial heat flow.
Modification of Co/Pt multilayers by gallium irradiation—Part 2: The effect of patterning using a highly focused ion beam
Authors: Warin P. , Hyndman R. , Glerak J. , Chapman J. N. , Ferré J. , Jamet J. P. , Mathet V. , Chappert C.
Published: July 2002

The local and collective behavior of magnetic arrays fabricated by focused ion beam (FIB) patterning of a Co/Pt multilayer is described. The arrays comprised 1 μm nonirradiated square elements separated by narrow lines which were written using the FIB. While the square elements supported perpendicular magnetization, the ion fluence used to write the lines was chosen to make the local magnetization there lie in-plane. Lorentz microscopy showed that lines were approximately 60 nm wide and that the magnetization had the expected orientation. Application of fields perpendicular and parallel to the array showed that the magnetization in the square elements and in the lines could be controlled essentially independently of each other. Magneto-optic microscopy was used to study the behavior of the arrays as a whole. Frustrated checkerboard patterns were observed, whose detailed properties depended to an extent on the fluence used to write the lines.
Chains of Frobenius subalgebras of so(M) and the corresponding twists
Authors: Lyakhovsky Vladimir D. , Stolin Alexander , Kulish Petr P.
Published: July 2002

Chains of extended Jordanian twists are studied for the universal enveloping algebras U(so(M)). The carrier subalgebra of a canonical chain FB0≺pmax cannot cover the maximal nilpotent subalgebra N+(so(M)). We demonstrate that there exist other types of Frobenius subalgebras in so(M) that can be large enough to include N+(so(M)). The problem is that the canonical chains FB0≺p do not preserve the primitivity on these new carrier spaces. We show that this difficulty can be overcome and the primitivity can be restored if one changes the basis and passes to the deformed carrier spaces. Finally, the twisting elements for the new Frobenius subalgebras are explicitly constructed. This gives rise to a new family of universal R-matrices for orthogonal algebras. For a special case of g=so(5) and its defining representation we present the corresponding matrix solution of the Yang–Baxter equation.
Symmetries in the Hubbard model with n-fold orbital degeneracy
Authors: Ying Zu-Jian , Li You-Quan , Gu Shi-Jian
Published: July 2002

The present paper studies the symmetries of the Hubbard model of electrons with generally n-fold orbital degeneracy. It is shown that SUd(2n) and SUc(2n) symmetries hold, respectively, for the model with completely repulsive or attractive on-site interaction and that with partly attractive interactions. An extended Lieb–Mattis transformation is given to map these two symmetries into each other. The subsymmetry SUd(e)(n)⊗SUd(o)(n) is found to be shared by the two models with arbitrary chemical potential μ. By assuming at most two electrons on each site it is found that SUd(2n)P and SUc(2n)P both exist in each kind of the two models and consequently lead to a larger symmetry SUd(2n)P×SUc(2n)P. Another underlying symmetry (SUc(e)(2)P×⋯×SUc(e)(2)P)⊗(SUc(o)(2)P×⋯×SUc(o)(2)P) is also revealed for the unified U model under the excluding. The symmetry is valid for the partially attractive model with chemical potential μ=−U.
Generalized model for all-optical light modulation in bacteriorhodopsin
Authors: Roy Sukhdev , Singh C. P. , Reddy K. P. J.
Published: July 2002

We present a generalized model for the photochemical cycle of bacteriorhodopsin (bR) protein molecule. Rate equations have been solved for the detailed light-induced processes in bR for its nine states: B→K↔L↔MI→MII↔N↔O↔P→Q→B. The complete steady-state intensity-induced population densities in various states of the molecule have been computed to obtain a general, exact, and analytical expression for the nonlinear absorption coefficient for multiple modulation pump laser beams. All-optical light modulation of different probe laser beam transmissions by intensity induced population changes due to one and two modulation laser beams has been analyzed. The proposed model has been shown to accurately model experimental results.
Conditions conducive to the chemi-ionization reaction O(3P)+CH(X 2Π,a 4Σ−)→HCO+(X 1Σ+)+e−
Authors: Metropoulos Aristophanes , Mavridis Aristides
Published: July 2002

We have determined the favorable geometries and intermediate HCO states that allow the chemi-ionization reaction O(3P)+CH→HCO+(X 1Σ+)+e− to take place in low-energy collisions, when CH is either in the ground X 2Π or in the first excited a 4Σ− state.
Cluster embedding in an elastic polarizable environment: Density functional study of Pd atoms adsorbed at oxygen vacancies of MgO(001)
Authors: Nasluzov Vladimir A. , Rivanenkov Vladimir V. , Gordienko Alexey B. , Neyman Konstantin M. , Birkenheuer Uwe , Rösch Notker
Published: July 2002

Adsorption complexes of palladium atoms on Fs, Fs+, Fs2+, and O2− centers of MgO(001) surface have been investigated with a gradient-corrected (Becke–Perdew) density functional method applied to embedded cluster models. This study presents the first application of a self-consistent hybrid quantum mechanical/molecular mechanical embedding approach where the defect-induced distortions are treated variationally and the environment is allowed to react on perturbations of a reference configuration describing the regular surface. The cluster models are embedded in an elastic polarizable environment which is described at the atomistic level using a shell model treatment of ionic polarizabilities. The frontier region that separates the quantum mechanical cluster and the classical environment is represented by pseudopotential centers without basis functions. Accounting in this way for the relaxation of the electronic structure of the adsorption complex results in energy corrections of 1.9 and 5.3 eV for electron affinities of the charged defects Fs+ and Fs2+, respectively, as compared to models with a bulk-terminated geometry. The relaxation increases the stability of the adsorption complex Pd/Fs by 0.4 eV and decreases the stability of the complex Pd/Fs2+ by 1.0 eV, but it only weakly affects the binding energy of Pd/Fs+. The calculations provide no indication that the metal species is oxidized, not even for the most electron deficient complex Pd/Fs2+. The binding energy of the complex Pd/O2− is calculated at −1.4 eV, that of the complex Pd/Fs2+ at −1.3 eV. The complexes Pd/Fs and Pd/Fs+ exhibit notably higher binding energies, −2.5 and −4.0 eV, respectively; in these complexes, a covalent polar adsorption bond is formed, accompanied by donation of electronic density to the Pd 5s orbital.