bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear dependence of Maxwell displacement current across chiral phospholipid mixed monolayers on molar ratio
Authors: Iwamoto Mitsumasa , Wu Chen-Xu , Zhao Wei
Published: July 2002

Mixed Langmuir films composed of the left- and right-handed chiral enantiomers are analyzed using a model based on the Bragg–Williams theory for binary mixtures, and the contribution of the mixing effect to the generation of Maxwell displacement current (MDC) and surface pressure-area isotherm is discussed. It is revealed that the formation of the homogeneous and intimate mixture of the left- and right-handed chiral enantiomers gives rise to the additional MDC generation due to the dipole–dipole interaction between the left- and right-handed chiral enantiomer molecules by the molecular twist transition during monolayer compression.
Quasielastic neutron scattering of two-dimensional water in a vermiculite clay
Authors: Swenson J. , Bergman R. , Howells W. S.
Published: July 2002

A well-characterized Na–vermiculite clay, containing zero, one, or two molecular layers of water between the clay platelets, has been studied by quasielastic neutron scattering (QENS). Experiments were carried out at a temperature of 300 K in two different scattering geometries; the clay platelets being at 45 and 135° angles to the incident beam in order to make the elastic Q-vector perpendicular and parallel, respectively, to the clay platelets for a scattering angle of 90° (Q≈1.33 Å−1). The resulting QENS spectra show that almost no hydrogen motion occurs perpendicular to the clay platelets on the experimental time scale (about 2–40 ps). The two-H2O layer vermiculite exhibits a planar rotational motion of water molecules, forming hydration shells around the Na ions, and a basically two-dimensional translational jump-diffusion motion. The translational motion was modeled using the Gaussian jump-length distribution model, resulting in a mean jump length of 1.1 Å and an average residence time of 2.3 ps. Using these values we obtain an effective diffusion coefficient of 8.8*10−10 m2/s, which is only a factor 2–3 lower than for bulk water. The correlation time of the rotational motion was estimated to approximately 27 ps. In the case of the one-H2O layer vermiculite we were only able to observe a planar rotational motion with a rotational correlation time of 16 ps, i.e., faster than in the two-H2O layer vermiculite. This suggests that a smaller number of water molecules are involved in the rotational process in the one-H2O layer vermiculite, and furthermore that the translational motion, if existent, is too slow to be observed on the experimental time scale.
Influence of electron–electron interaction on the mobility of electrons on the surface of liquid helium
Authors: Adamenko I. N. , Zhukov A. V. , Nemchenko K. É.
Published: July 2002

The mobility of a two-dimensional electron gas localized on the surface of liquid helium at low temperatures is found by solving the kinetic equation for the electron distribution function. The result for the mobility contains both the electron–ripplon interaction time and the electron–electron scattering time. The theory is valid for an arbitrary relationship between the electron–ripplon and electron–electron interaction times and gives the previously known expressions in the limiting cases. The results are compared with experimental data, and new experiments are proposed which can yield information about the electron–electron interaction.
Dependence of critical current on field angle in off-c-axis grown Bi2Sr2CaCu2O8 film
Authors: Durrell J. H. , Gibson G. , Barber Z. H. , Evetts J. E. , Rössler R. , Pedarnig J. D. , Bäuerle D.
Published: July 2002

Critical current measurements for varying angles of applied field have been performed on Bi2Sr2CaCu2O8 thin films grown on 10° vicinal (001) substrates. Measurements were performed on current tracks orientated both parallel (L) and transverse (T) to the vicinal steps. No Josephson vortex channeling effect was observed, in contrast to results previously obtained on both oxygenated and deoxygenated YBa2Cu3O7−δ films grown on vicinal substrates. In addition, no force-free peak was observed when the applied field was parallel to the current. This provides experimental evidence that there is a difference between the pancake coupling mechanism in YBa2Cu3O7−δ at small field angles where Josephson vortices are present and that in Bi2Sr2CaCu2O8.
High magnetic field corrections to resistance thermometers for low temperature calorimetry
Authors: Fortune Nathanael , Gossett Gayle , Peabody Lydia , Lehe Katherine , Uji S. , Aoki H.
Published: July 2002

We present a general analytical method of correcting for magnetic-field-induced changes in both the apparent temperature and the sensitivity of resistive thermometers at dilution refrigerator temperatures. With this method, we are able to reduce magnetic field induced errors in temperature to a level limited only by our ability to regulate the temperature in the absence of a magnetic field. We illustrate the application of our method to two resistive sensors in magnetic fields up to 18 T: a custom-made AuxGe1−x thin film sensor used in calorimetry and a commercially available ruthenium–oxide thick film resistor used in thermometry.
Direct observation of the steric effect in Penning ionization reaction of Ar*+CHCl3→CHCl2++Cl+e−+Ar
Authors: Yamato Masanori , Okada Seiki , Wei-Keh Wu Victor , Ohoyama Hiroshi , Kasai Toshio
Published: July 2002

Steric effect in the Penning ionization reaction of Ar*(3P2,0)+CHCl3→Ar+CHCl2++Cl+e− was directly observed at an average collision energy of 0.13 eV using the oriented CHCl3 molecular beam. The product CHCl2+ ions are measured for the H-end, the CCl3-end, and sideways orientations. The obtained steric opacity function reveals that the CCl3-end orientation is more favorable than the H-end orientation, and the sideways approach is found to be more favorable than the collinear approaches from both ends of the molecule. Furthermore, we confirm the good correlation between Penning ionization anisotropy and the electron density distribution of the 2a2 HOMO orbital of the CHCl3 molecule, whose electron cloud is mostly localized around the sideways. These results substantiate the electron exchange mechanism which is commonly accepted for the Penning ionization reaction, where the overlap of projectile atomic and target molecular orbital plays a key role in Penning ionization efficiency.
Quenching the thermal contribution in laser assisted scanning tunneling microscopy
Authors: Landi Sandra Marcela , Martı́nez Oscar Eduardo
Published: July 2002

When irradiating the sample of a scanning tunneling microscope with a modulated light intensity, light absorption results in tip and sample heating and expansion at the modulation frequency, obscuring other possible laser induced mechanisms. This thermal noise limits the use of light modulation when very high spatial resolution is desired in fluorescence or nonlinear optics near field experiments, being an extreme case the measurement of the optical rectification with scanning tunneling microscopes. In this work we describe a method in which the thermal expansion at the modulation frequency can be reduced by orders of magnitude. The method is based on the irradiation of the sample with two interfering laser beams at different frequencies and incidence direction, giving light fringes traveling in the illuminated zone. Solving the heat diffusion equation we predict the thermal behavior of sample and find a good agreement with experimental data.
Ab initio molecular dynamics simulation of LiBr association in water
Authors: Izvekov Sergei , Philpott Michael R.
Published: July 2002

A computationally economical scheme which unifies the density functional description of an ionic solute and the classical description of a solvent was developed. The density functional part of the scheme comprises Car–Parrinello and related formalisms. The substantial saving in the computer time is achieved by performing the ab initio molecular dynamics of the solute electronic structure in a relatively small basis set constructed from lowest energy Kohn–Sham orbitals calculated for a single anion in vacuum, instead of using plane wave basis. The methodology permits simulation of an ionic solution for longer time scales while keeping accuracy in the prediction of the solute electronic structure. As an example the association of the Li+–Br− ion-pair system in water is studied. The results of the combined molecular dynamics simulation are compared with that obtained from the classical simulation with ion-ion interaction described by the pair potential of Born–Huggins–Mayer type. The comparison reveals an important role played by the polarization of the Br− ion in the dynamics of ion pair association.
Sol-gel coated glass cells for spin-exchange polarized He3
Authors: Hsu Ming F. , Cates G. D. , Kominis I. , Aksay I. A. , Dabbs D. M.
Published: July 2002

We have developed a high-purity sol-gel coating for the interior surface of glass cells used for polarizing He3 by spin-exchange optical pumping. The coating is designed to minimize spin relaxation due to wall collisions. A longitudinal spin-relaxation time T1 in a sol-gel coated Pyrex cell of 344±8 h was achieved, the longest T1 we have ever recorded for a gaseous sample. Repeated trials indicated that the coating was quite robust. Results using an uncoated Pyrex cell were also quite good, although inferior to the performance of the coated cell.
Multiple configuration quantum/classical treatments of reaction dynamics
Authors: Wang Lichang , Meurer William J. , McCoy Anne B.
Published: July 2002

The accuracy of quantum/classical approaches for studies of reaction dynamics is investigated through simulations of the collinear and J=0 dynamics of the O(3P)+HCl reaction on two potential surfaces. The results of classical and two types of quantum/classical treatments of this reaction are compared to the results of quantum wave packet simulations. It is found that the accuracy of the single configuration quantum/classical treatment is sensitive to features of the potential surface. Most of this sensitivity is removed when a second configuration is introduced. For collision energies below 0.8 eV, the multiple configuration quantum/classical treatment provides reaction probabilities and product state distributions that are in good agreement with the results of the corresponding quantum simulation. The agreement deteriorates at higher collision energies, but here the agreement between the quantum and classical results is quite good.
Application of atomic-force-microscope direct patterning to selective positioning of InAs quantum dots on GaAs
Authors: Hyon C. K. , Choi S. C. , Song S.-H. , Hwang S. W. , Son M. H. , Ahn D. , Park Y. J. , Kim E. K.
Published: July 2002

The application of atomic-force-microscope (AFM) direct patterning to the selective positioning of InAs quantum dots (QDs) on a (100) GaAs substrate has been proposed and experimentally implemented. The AFM direct patterning was used to generate various patterns of several tens of nanometers in size, and InAs QDs were subsequently grown by a metalorganic chemical vapor deposition technique. A nonuniform distribution of the QDs was observed near the patterns. The detailed shape of the QD distribution and the size of the QDs depended on the geometrical properties such as the sidewall angle, the spacing, and the width of the patterns. We have been able to ascertain, through our work, what growth conditions are necessary for QDs’ alignment along the patterns.
Excited states and photodissociation dynamics of the triiodine radical (I3)
Authors: Choi Hyeon , Taylor Travis R. , Bise Ryan T. , Hoops Alexandra A. , Neumark Daniel M.
Published: July 2002

The electronic spectroscopy and photodissociation dynamics of the I3 radical are investigated with two experimental methods. The ground and several low-lying excited states of the I3 radical are characterized by photoelectron spectroscopy of I3− at 213 nm. Assignments of these states are discussed with reference to recent calculations. In addition, photodissociation of the I3 radical was investigated at selected photon energies (4.59, 4.96, and 5.17 eV) by fast radical beam photofragment translational spectroscopy. Two product channels were observed with mass ratios of 1:2 and 1:1, and translational energy (P(ET)) distributions were measured. The P(ET) distributions for products with mass ratio 1:2 show that this channel corresponds to I2 in various electronic states along with atomic I in its P3/22 or P1/22 state. The 1:1 channel corresponds primarily to concerted three-body dissociation to three I atoms.
Long-lived quantum-confined infrared transitions in CdSe nanocrystals
Authors: Ginger D. S. , Dhoot A. S. , Finlayson C. E. , Greenham N. C.
Published: July 2002

We present quasi-steady-state photoinduced absorption measurements on thin films of CdSe nanocrystals dispersed in a polystyrene matrix. For nanocrystals treated with pyridine we observe an intense, size-dependent absorption peaking in the mid-infrared when the samples are irradiated with visible light. This infrared absorption is associated with a size-dependent bleach in the visible, near the peak of the first excitonic absorption. We attribute the infrared absorption to an intraband electron transition in the quantum dots and measure the lifetime of the absorbing state to be 1.0 ms at 295 K.
Quantitative harmonization of the three molecular orbital, valence bond, and broken symmetry approaches to the exchange coupling constant: Corrections and discussion
Authors: Mouesca Jean-Marie
Published: July 2002

Three current methods, used to evaluate exchange coupling constants in molecular magnetism, i.e., the molecular orbital (MO) model [Hay et al., J. Am. Chem. Soc. 94, 4884 (1975)], the valence bond (VB) model [Kahn and Briat, J. Chem. Soc. Trans. II 72, 268 (1976)], and the broken symmetry (BS) model [Noodleman, J. Chem. Phys. 74, 5737 (1981)], have been revisited. In effect, the three published antiferromagnetic contributions seem mutually inconsistent, as far as their magnitudes are concerned. As it turns out, the VB term −2ΔS, where Δ is the singly occupied MO gap in the triplet state and S the (natural) magnetic orbital overlap, is shown to be overestimated by a factor of 2 (the VB ferromagnetic term, supposedly small, is actually of the order of ΔS>0). Moreover, Noodleman’s explicit condition derived from the variational optimization of the BS state energy results in fact from the implicit neglect of the VB ionic contribution right from the start of his methodology. Alternative (both rigorous and approximate) expressions are proposed in both VB and BS cases. The MO approach, although not being the best physically appropriate for the study of weakly interacting monomers (i.e., defined by S2≪1), is left untouched at this level of the theory.
Evaluation of liquid–vapor density profiles for associating fluids in pores from density-functional theory
Authors: Pizio Orest , Patrykiejew Andrzej , Sokołowski Stefan
Published: July 2002

Using density-functional theory we calculate density profiles of an associating fluid in slit like pores as functions of two variables: The distance from the pore wall and the distance along the pore axis. Attention is focused on evaluation of the profiles characterizing the coexistence between two confined phases. We also calculate changes in the grand canonical potential connected with the formation of an interface between two coexisting confined phases. Specific calculations have been carried out for the associating, chain forming Lennard-Jones fluid adsorbed in a slitlike pore.
Dielectric function of epitaxial ZnSe films
Authors: Koo M. S. , Kim T. J. , Lee M. S. , Oh M. S. , Kim Y. D. , Yoo S. D. , Aspnes D. E. , Jonker B. T.
Published: July 2002

We examine various ZnSe spectra to obtain that which best represents the dielectric response ε of ZnSe. The measured evolution of pseudodielectric function 〈ε〉 data with chemical etching shows that the natural overlayer on ZnSe can be modeled accurately only if we assume that it contains amorphous Se. Hence previous assumptions made in correcting 〈ε〉 mathematically are not correct, and data obtained on stripped samples yield the best representation of ε.
Thermodynamic analysis of hole trapping in SiO2 films on silicon
Authors: Boureau G. , Carniato S. , Capron N. , Garapon J. , Poumellec B.
Published: July 2002

A thermodynamic approach based on the existence of a local equilibrium is used to evaluate the temperature dependence of the number of defects responsible for hole trapping (oxygen vacancies transformed into E′ centers) near the Si–SiO2 interface. This approach eliminates the discrepancies between theoretical calculations of the formation energy of oxygen vacancies and hole trapping modeling.
Adaptive femtosecond optical pulse combining
Authors: Jones R. , Nolte D. D. , Melloch M. R.
Published: July 2002

We combine two nominal 100 fs pulses into a pulse train using an adaptive holographic quantum-well film as an adaptive pulse combiner in a two-wave mixing geometry. The two pulses in the combined pulse train are phase-locked and are immune to drifting optical path differences or delay times between the two input pulses. The phase is controlled by the choice of center wavelength. The spectrum of the pulse train is equivalent to the spectral interferogram between two ultrafast pulses.
Coating flows within a rotating horizontal cylinder: Lubrication analysis, numerical computations, and experimental measurements
Authors: Tirumkudulu Mahesh , Acrivos Andreas
Published: July 2002

We consider the flow within a rotating horizontal cylinder containing a small amount of a very viscous liquid which completely coats the cylinder surface. We show that, under creeping flow conditions, the addition of the hydrostatic pressure term to the standard lubrication equation leads to film thickness profiles which, over a broad range of parameters, are in close agreement with those obtained experimentally, as well as via the solution to the full Stokes equations.
A diagrammatic theory of time correlation functions of facilitated kinetic Ising models
Authors: Pitts Steven J. , Andersen Hans C.
Published: July 2002

We present a diagrammatic formulation of the kinetic theory of time correlation functions for facilitated kinetic Ising models with directed constraints. Such models have been studied because their relaxation properties are similar to those of glass forming liquids and because some of them undergo ergodic–nonergodic transitions. Using a Hilbert space formulation of the dynamics of Markov processes that satisfy a detailed balance condition, we derive a diagrammatic series for the spin autocorrelation function, its memory function, and its irreducible memory function. Using graphical methods we derive various approximations for the irreducible memory function, including approximations equivalent to various versions of the simplified and extended mode coupling theory of Götze and co-workers. An extended mode coupling theory for the “East model” is compared with simulation data. The theory is accurate at short and intermediate times for all up-spin concentrations and for all times at high up-spin concentrations. The major failing of the extended mode coupling theory for this model is the prediction of long time decay that is too rapid and too exponential, as compared with the simulation results.
Electronic structures and dynamics of the excited triplet states of α,ω-diphenylpolyynes
Authors: Nagano Yasutomo , Ikoma Tadaaki , Akiyama Kimio , Tero-Kubota Shozo
Published: July 2002

The lowest excited triplet (T1) states of diphenylacetylene and several α,ω-diphenylpolyynes (DPY) having two, three, four and six triple bonds were studied using continuous wave time-resolved electron paramagnetic resonance (CW-TREPR), pulsed EPR, and phosphorescence spectroscopy. Linear and planar molecular structures in the T1 states of DPY were suggested from the magnetophotoselection experiments and observation of a strong 0-0 band in the well-resolved phosphorescence spectra. The spin density distributions, which were obtained by electron spin echo envelope modulation measurements and semi-empirical molecular orbital calculations, and the phosphorescence polarization normal to the long axis of molecule for the 0-0 bands suggested that the T1 states of DPY have a B1u3(πxπx*) electronic configuration. DPY showed an abnormally large |D| value of the zero-field splitting parameters that increased with increasing molecular size. Such an unusual behavior of the D values is interpreted in terms of the spin-orbit interaction between the T1 and Au3(πxπy*) states mainly due to their energy proximity which is probably characteristic of the excited states for the polyynes. The estimated energy gap between the πx3πy* and πx3πx* states for DPY ranges from 2900 cm−1 to 1400 cm−1. There existed a good correlation between the acceleration of the nonradiative decay rate constant from the T1 state and appearance of a ring twisting vibronic band with b1g symmetry in the phosphorescence spectra for DPY. Therefore, we concluded that the vibronic interaction between the πx3πy* and πx3πx* states promoted by the b1g vibration leads to a broadening of the potential surface of the T1 state which results in the fast radiationless decay to the ground state.
Optimal bounds on field fluctuations for random composites: Three dimensional problems
Authors: Lipton Robert
Published: July 2002

The electric field fluctuation inside a two-phase anisotropic composite is studied when the composite sample is subjected to a constant applied electric field. Upper and lower bounds on the covariance tensor of the electric field are found in terms of the effective dielectric properties of the composite. The lower bounds are shown to be optimal for two well known families of microgeometries. Lower bounds on the covariance tensor are found when only the phase volume fractions and the two-point correlation function are available. For statistically isotropic composites optimal lower bounds are derived when only the phase volume fractions are known.
Observation of terahertz electric pulses generated by nearly filled-gap nonuniform illumination excitation
Authors: Lu Shi-Hsiang , Li Jun-Liang , Yu Jian-Shen , Horng Sheng-Fu , Chi C. C.
Published: July 2002

Terahertz (THz) electric pulses generated by nonuniform illumination excitation in a nearly filled-gap configuration were observed. In this scheme, the excitation beam is focused to a spotsize only slightly smaller than the spacing between the transmission lines and is located symmetrically within the gap. With 100 fs laser pulses excitation on sliding-contact photoconductive switches fabricated on low-temperature-grown GaAs, electric pulse correlation with 190±20 fs full width at half maximum, which corresponds to a 3 dB bandwidth of 1.1 THz, was observed. Moreover, electric pulses with three times larger peak amplitude than those generated with filled-gap illumination from In coplanar striplines were observed. Bias, wavelength, and pump power dependencies were investigated.
Low-energy electron-induced processes in condensed CF2Cl2 films
Authors: Hedhili M. N. , Lachgar M. , Le Coat Y. , Azria R. , Tronc M. , Lu Q. B. , Madey T. E.
Published: July 2002

We report measurements of electron stimulated desorption yields and kinetic energy distributions for F− and Cl− anions from thin films of CF2Cl2 condensed on polycrystalline platinum at 20 K. Dissociative electron attachment processes cause a peak in the F− yield at an electron energy of ∼4 eV and a peak in the Cl− yield at ∼7 eV. After electron irradiation, a new dominant structure appears in the Cl− yield at 5 eV incident electron energy, and Cl− ions having high kinetic energy (>1 eV) are observed in the kinetic energy distributions. These changes are attributed to the synthesis of Cl2 in the condensed phase induced by electron irradiation of the CF2Cl2 film. This Cl2 formation is demonstrated by the observation in the Cl− yield of the Cl2−/Cl2 2Πu resonance state, and by Cl− kinetic energy distributions that are similar to those obtained from condensed pure Cl2 films.
Reflectance difference spectroscopy of mixed phases of indium phosphide (001)
Authors: Begarney M. J. , Li C. H. , Law D. C. , Visbeck S. B. , Sun Y. , Hicks R. F.
Published: July 2002

Reflectance difference spectra of mixed (2×1) and (2×4) phases of indium phosphide (001) have been recorded and benchmarked against scanning tunneling micrographs of the surface. The line shapes are found to be linear combinations of the spectra of the pure (2×1) and (2×4) structures, Δr/rmixed=xΔr/r(2×4)+(1−x)Δr/r(2×1), where x is the weighting factor. Thus, in the absence of adsorbates, the reflectance difference spectra can be used to estimate the surface composition, i.e., the fractional coverage of phosphorous is ΘP=1−0.81x±0.06x.
Wetting of methanol on the n-alkanes: Observation of short-range critical wetting
Authors: Ross David , Bonn Daniel , Meunier Jacques
Published: July 2002

We present results of the wetting properties of methanol at the liquid–vapor interface of different n-alkanes ranging from hexane to undecane. Measurement of the contact angle shows that, as the bulk critical point is approached, wetting transitions occur for long alkanes, whereas drying is found for short alkanes. Measurement of the wetting layer thickness, as well as precise contact angle measurements close to the wetting temperature Tw, reveal a change in the order of the transition. First order (discontinuous) wetting occurs for Tw far from the bulk critical point Tc. Critical (continuous) wetting is observed for Tw close to Tc. The observed critical wetting transition has all the features of the long-sought short-range critical wetting transition. We argue that it is possible to observe short-range critical wetting in this system, because the long-range van der Waals interactions are negligible for wetting transitions close to the bulk critical point.
Photoinduced light absorption and dichroism of Ca3Mn2Ge3O12 garnet as a probe of electronic processes and intrinsic electric fields
Authors: Eremenko V. V. , Gnatchenko S. L. , Kachur I. S. , Piryatinskaya V. G. , Ratner A. M. , Shapiro V. V. , Fally M. , Rupp R. A.
Published: July 2002

Measurements of photoinduced light absorption in Ca3Mn2Ge3O12 revealed some unusual features: a saturation with the pumping intensity and a broad straggling of relaxation times with a predominance of very long times. These experimental facts cannot be understood in terms of photoinduced absorption centers associated with impurities or lattice defects, but are naturally explained within the notion of random electric fields of active charges. Active charges are produced by light pumping via the dissociation of coupled pairs of charges (consisting of a Mn-hole coupled with a compensating negative impurity or a negatively charged vacancy) which exist in the ground state. Such active charges create electric fields in a larger volume than coupled pairs, thus enhancing the probability for forbidden optical transitions. On the other hand, the random fields of active charges promote hopping of holes and hence the relaxation of photoinduced effects. A broad distribution of random-field magnitudes gives rise to a very broad range of hole hopping rates. There is also a much faster annihilation process immediately conditioned by light pumping. The simultaneous action of these relaxation channels, depending on the number of active charges, pumping intensity, and temperature, explains the entire experimental picture qualitatively and in part quantitatively. Photoinduced dichroism as well as birefringence, observed under polarized pumping, are caused by an anisotropic distribution of photoproduced holes over polarization directions.
Modeling of bombardment induced oxidation of silicon
Authors: De Witte H. , Vandervorst W. , Gijbels R.
Published: July 2002

Secondary ion mass spectrometry has become the preferred tool for impurity profiling primarily due to its excellent depth resolution and high detection sensitivity. Prerequisite in obtaining high detection sensitivity for positive secondary ions is the use of oxygen as primary ions. This leads to a high degree of oxidation of the sample surface, which is essential for a high secondary ion ionization efficiency. Unfortunately, this oxygen bombardment not only leads to the transformation of the original target surface into an oxidized layer but, as the latter requires a certain fluence before stationary state is reached, inherently causes some nonlinearities and transients in the secondary ion signal and the fluence-eroded depth relation. In this work a computer code implantation, sputtering, replacement/relocation, and diffusion (ISRD) has been optimized to predict the compositional changes of the sample surface (or altered layer formation), the sputter yields and the surface regression as a result of the interaction of oxygen beams with Si-targets. This article describes a careful reevaluation of the previously used version of ISRD (and the parameters contained in the program) in order to obtain a systematic agreement with experimental data on sputter yields, altered layer formation, and surface recession, and with other theoretical predictions.
Geometric invariant theory approach to the determination of ground states of D-wave condensates in isotropic space
Authors: Gufan Yu. M. , Popov Al. V. , Sartori G. , Talamini V. , Valente G. , Vinberg E. B.
Published: July 2002

A complete and rigorous determination of the possible ground states for D-wave pairing Bose condensates is presented, using a geometrical invariant theory approach to the problem. The order parameter is argued to be a vector, transforming according to a ten-dimensional real representation of the group G=O3⊗U1×〈T 〉. We determine the equalities and inequalities defining the orbit space of this linear group and its symmetry strata, which are in a one-to-one correspondence with the possible distinct phases of the system. We find 15 allowed phases (besides the unbroken one), with different symmetries, that we thoroughly determine. The group–subgroup relations between bordering phases are pointed out. The perturbative sixth degree corrections to the minimum of a fourth degree polynomial G-invariant free energy, calculated by Mermin, are also determined.
Above-room-temperature mid-infrared lasing from vertical-cavity surface-emitting PbTe quantum-well lasers
Authors: Heiss W. , Schwarzl T. , Springholz G. , Biermann K. , Reimann K.
Published: July 2002

Above-room-temperature operation of vertical-cavity surface-emitting lasers emitting in the mid-infrared is reported. The stimulated emission is generated in PbTe quantum wells embedded in two-wavelength microcavities by optically pumping with fs laser pulses. The spectrum of the laser modes is broadened and blue-shifted due to dynamic band filling. The intensity of the mid-infrared emission and the laser threshold depends on the energy of the microcavity resonance. At a wavelength of 3.1 μm, laser operation is obtained up to a temperature of 65 °C, limited by nonradiative recombination processes.
Electron attachment to CF3Cl and CH3Cl on the surface and in the bulk of solid Kr
Authors: Nagesha K. , Fabrikant I. I. , Sanche L.
Published: July 2002

The formation of stable anions induced by the impact of 0–1.6 eV electrons on CF3Cl and CH3Cl condensed at the surface, and embedded within the bulk of solid Kr is investigated by measuring charging of the doped Kr films. Effects of the Pt substrate and under- and over-layer coverage of these molecules on the dissociative electron attachment and stable anion formation (SAF) are studied in detail by varying the film thickness and the position of the molecules within the film. Due to recent advances, we provide new and more reliable values of the absolute cross sections, σSAF, for SAF. σSAF reaches a maximum of 2.1×10−16 cm2 at 0.6 eV for CF3Cl embedded in solid Kr; for CH3Cl, this maximum is ∼10−16 cm2 at 0.2 eV. These values correspond to an enhancement of factors of 5 and 10, respectively, relative to σSAF measured at the surface. The electron energy dependence of σSAF is also evaluated theoretically from calculations performed with the R-matrix formalism extended to include the influence of condensed-matter environments. By incorporating into the theory the band structure of the solid and different values of the polarization induced into the medium by electron capture, we study the effect of these parameters on σSAF. Comparison between experimental and theoretical σSAF’s allows us to analyze the gas-phase parameters that must be modified to generate condensed-phase cross sections from gas-phase data and to discuss the effect of phenomena which are absent in the gas-phase (e.g., caging).
Novel laser flash technique to measure thermal effusivity of highly viscous liquids at high temperature
Authors: Ohta Hiromichi , Shibata Hiroyuki , Suzuki Atsushi , Waseda Yoshio
Published: July 2002

A simple but useful experimental technique featuring a novel configuration for a sample cell, high-speed infrared detector and a heating pulse laser has been developed to obtain reliable thermal effusivity of a highly viscous liquid at high temperature. The sample is melted in a platinum crucible and heated to the desired temperature. A single laser pulse is flashed on the bottom of the platinum crucible and the infrared ray irradiated from the same bottom surface is measured to obtain the temperature decay, from which the thermal effusivity of the liquid sample can be estimated. The usefulness and validity of this new laser flash technique were successfully confirmed by measuring the thermal effusivity of silicate glass in the temperature range between 773 and 1673 K.
Spatial theory of optimal disturbances in a circular pipe flow
Authors: Reshotko Eli , Tumin Anatoli
Published: July 2002

A spatial theory of linear transient growth for disturbances in a circular pipe is presented. Following from the consideration of a signaling problem, the spatial development of disturbances downstream of a source may be presented as a sum of decaying eigenmodes. Therefore, the problem of optimal disturbances in the pipe flow may be considered as an initial value problem on the subset of the downstream decaying eigenmodes, and a standard optimization procedure may be applied for evaluation of the optimal transient growth. Examples are presented for spatial transient growth of axisymmetric and nonaxisymmetric disturbances. It is shown that stationary disturbances may achieve more significant transient growth than nonstationary ones. The maximum of the transient growth exists at azimuthal index m=1 for stationary perturbations, whereas nonstationary perturbations may achieve their maxima at higher azimuthal indices.
Nonideality in the composition dependence of viscosity in binary mixtures
Authors: Srinivas Goundla , Mukherjee Arnab , Bagchi Biman
Published: July 2002

In this work we introduce two models to understand the anomalous composition dependence of viscosity of binary mixtures. Both models consist of a mixture of two molecular species (A and B) with the same diameter and mass but varying solute–solvent Lennard-Jones interaction. In model I, the two different species are strongly attractive while in model II, the attraction is weaker than that between the pure components. We have carried out extensive computer simulations of the two models. In addition, we study mode coupling theory for the viscosity of binary mixtures. Both the molecular dynamics simulations and the microscopic theory show the emergence of strong nonideality even in such simple systems. Model I shows a positive departure from ideality while model II shows the reverse behavior. The reason can be traced to the enhanced mean square stress fluctuations (MSSF) in the model I but decreased MSSF in the model II. The models show deviations (from ideality) very similar to the ones observed in experiments.
On the concentration of the spectrum of integral-difference collision operator with Gaussian equilibrium distribution function in a vicinity of zero
Authors: Melnikov Yu.
Published: July 2002

We study integral-difference collision operators with a truncated Gaussian equilibrium distribution function. We prove that the number of eigenvalues in an arbitrary small vicinity of zero goes to infinity when the truncation parameter goes to infinity.
Multiple hopping conduction in interpenetrating polymer network composites of poly pyrrole and poly (styrene-co-butyl acrylate)
Authors: Bhattacharyya S. , Saha S. K. , Mandal T. K. , Mandal B. M. , Chakravorty D. , Goswami K.
Published: July 2002

Interpenetrating polymer network composites of poly pyrrole and FeCl3 impregnated poly (styrene-co-butyl acrylate) have been synthesized by chemical route. ac conductivity measurements have been carried out on these specimens in the frequency range 100 to 2 MHz over temperatures varying from 110 to 350 K. Detailed analyses of the results show that a multiple hopping conduction mechanism is operative in all the samples. Two activation energies viz., ∼(0.05–0.08) eV and ∼0.02 eV are found which are believed to arise due to interchain and intrachain hopping of polarons, respectively.
Stress-induced metallic behavior under magnetic field in Pr1−xCaxMnO3 (x=0.5 and 0.4) thin films (invited)
Authors: Prellier W. , Simon Ch. , Mercey B. , Hervieu M. , Haghiri-Gosnet A. M. , Saurel D. , Lecoeur Ph. , Raveau B.
Published: July 2002

We have investigated the role of the stress induced by the presence of the substrate in thin films of colossal magnetoresistive manganites on structural, resistive, and magnetic properties. Because of the strong coupling between the small structural distortions related to the charge ordering (CO) and the resistive properties, the presence of the substrate prevents the full development of the charge ordering in Pr0.5Ca0.5MnO3, especially in the very thin films. For thicker films, the CO state exists, but is not fully developed. Correlatively, the magnetic field which is necessary to suppress the CO is decreased drastically from 25 T to about 5 T on SrTiO3 substrates. We have also investigated the influence of the doping level by studying the case of Pr0.6Ca0.4MnO3.
Electronic sensing of vapors with organic transistors
Authors: Crone B. , Dodabalapur A. , Gelperin A. , Torsi L. , Katz H. E. , Lovinger A. J. , Bao Z.
Published: July 2002

We show that organic thin-film transistors have suitable properties for use in gas sensors. Such sensors possess sensitivity and reproducibility in recognizing a range of gaseous analytes. A wealth of opportunities for chemical recognition arise from the variety of mechanisms associated with different semiconductor–analyte interactions, the ability to vary the chemical constitution of the semiconductor end/side groups, and also the nature of the thin-film morphology.
Quantitative optical determination of the shape of Cu nanocrystals in a composite film
Authors: Gonzalo J. , Serna R. , Afonso C. N. , Bosbach J. , Wenzel T. , Stietz F. , Träger F. , Babonneau D. , Hole D. E.
Published: July 2002

We demonstrate that optical extinction spectroscopy can be used to determine the effective shape of Cu nanocrystals (NCs) embedded in a transparent amorphous Al2O3 host both produced by pulsed laser deposition. The axial ratio of the NCs was extracted from the positions of the surface plasmon modes of the optical extinction spectra of the nanocomposite film. Comparison to the results obtained by grazing incidence small angle x-ray scattering shows excellent agreement. Thus, optical spectroscopy can be used as a simple, easily accessible, and versatile tool for the characterization of the NCs that form nanocomposite films.
Algorithms for fast axisymmetric drop shape analysis measurements by a charge coupled device video camera and simulation procedure for test and evaluation
Authors: Busoni Lorenzo , Carlà Marcello , Lanzi Leonardo
Published: July 2002

A set of fast algorithms for axisymmetric drop shape analysis measurements is described. Speed has been improved by more than 1 order of magnitude over previously available procedures. Frame analysis is performed and drop characteristics and interfacial tension γ are computed in less than 40 ms on a Pentium III 450 MHz PC, while preserving an overall accuracy in Δγ/γ close to 1×10−4. A new procedure is described to evaluate both the algorithms performance and the contribution of each source of experimental error to the overall measurement accuracy.
Influence of Rayleigh scattering on the ion feature of Thomson scattering spectrum from a partially stripped plasma
Authors: Zheng Jian , Yu C. X.
Published: July 2002

Studied in this paper is the influence of Rayleigh scattering on the ion feature of the Thomson scattering spectrum from a partially stripped plasma. The result shows that the profile of the scattering spectrum is nearly the same as that of Thomson scattering with inclusion of Rayleigh scattering, but the intensity of the scattering signal may be remarkably different from that of Thomson scattering.
Resonant Compton scattering of photons from hydrogenic ions in nonideal plasmas
Authors: Song Mi-Young , Jung Young-Dae
Published: July 2002

Plasma screening and collective effects on the resonant Compton scattering of photons by bound atomic electrons from the hydrogenic ions in classical nonideal plasmas are investigated. An effective pseudopotential model taking into account the plasma screening and collective effects is applied to describe the interaction potential in nonideal plasmas. The screened wave functions and energy eigenvalues for the ground and excited states of the target ion are obtained by the Ritz variational method. The nonperturbative expression for the transition matrix element near resonance is obtained by the basic lowest-order photon-perturbation Hamiltonian in the electronic dipole representation. It is found that the resonant Compton scattering cross section including the plasma screening effect is greater than the cross section neglecting the plasma screening effect. It is also found that the collective effect increases the Compton cross section at the resonance.
Improved system for measurement of the magnetization characteristics of high-temperature superconductors by the compensation method in weak magnetic fields
Authors: Zrubec Vladimı́r
Published: July 2002

This article follows the previous publication (1999) of the author where the theoretical sensitivity limits and the basic analysis of the parameters of the superconducting quantum interference device systems for measurement of the magnetization characteristics of high-temperature (HTc) superconductors in ac low-frequency magnetic field with the help of a compensation method were given. It contains the theoretical base for the optimization of such systems and the results achieved by its application in an experimental model of the measuring system. The article deals with the dependence of the spectral sensitivity and the signal-to-noise ratio of the electronic unit of superconducting quantum gradiometer (EU SQG) output on the volume and the susceptibility of the HTc superconducting material sample. It concluded that the chosen dimensions of the pellet shaped sample (diameter 12 mm, thickness 2.5 mm) were optimum for the given measuring system in magnetically unshielded ambient. A method is presented of how to reduce the magnetization current source noise component influence on the sensitivity of the measuring system using a resistance current divider. The concept of spectral sensitivity components of the measuring system has been introduced. These components correspond to the noise spectral densities of three considered noise sources (magnetization current noise component, magnetic noise of the environment, and the EU SQG intrinsic noise). Derived theoretical relations have been applied in the computation and optimization of the experimental system with the standard EU SQG with the input energy sensitivity 1×10−28 J/Hz. In the magnetically unshielded Faraday chamber originally intended for biomagnetic fields measurement, it achieves the spectral sensitivity 8×10−10 Am2 Hz−1/2. Examples of typical magnetization characteristics measured using the triangle alternating magnetization field with a frequency of 0.1 Hz are given. The method and the system are basically also suitable for nonsuperconducting materials.
Bifurcation of the radial electric field in the presence of edge polarization in tokamaks
Authors: Heikkinen J. A. , Jachmich S. , Kiviniemi T. P. , Kurki-Suonio T. , Peeters A. G.
Published: July 2002

In guiding-center simulations of a tokamak plasma edge, bifurcations of both the radial electric field and the electrode current are observed for a threshold value of the electrode bias voltage, in qualitative agreement with experiments. The simulations are neoclassical and Monte Carlo based, also containing the perpendicular viscosity effect. Above the voltage threshold, an inward moving, solitary radial electric field structure is generated with a concomitant drop in the electrode current. The position, width, and height of the soliton depend on the magnitude and radial variation of the conductivity (and, thus, on local plasma parameters), finite orbit effects, and also on the perpendicular viscosity and neutral density. The obtained Er structures are narrower and higher than the ones measured in the edge polarization experiments in the Jülich tokamak TEXTOR-94 [S. Jachmich et al., Plasma Phys. Controlled Fusion 40, 1105 (1998)].
Multiple pulse electron beam converter design for high power radiography
Authors: Pincosy P. A. , Back N. , Bergstrom P. M. , Chen Yu-Jiuan , Poulsen P.
Published: July 2002

The typical response of the x-ray converter material to the passage of a high-powered relativistic electron beam is vaporization and rapid dispersal. The effect of this dispersal on subsequent pulses for multi-pulse radiography is the collective effects on the propagation of the electron beam through the expanding plasma and the reduced number of electron to photon interactions. Thus, for the dual-axis radiographic hydrodynamic test facility, the converter material must either be replaced or confined long enough to accommodate the entire pulse train. Typically the 1-mm-thick high Z and full density converter material is chosen to give peak dose and minimum radiographic spot. For repeated pulses we propose a modified converter, constructed of either low density, high Z material in the form of foam or of foils spaced over ten times the axial thickness of the standard 1 mm converter. The converter material is confined within a tube to impede outward motion in radius outside the beam interaction region. We report single-pulse experiments which measure the dose and spot size produced by the modified converter and compare them to similar measurements made by the standard converter. For multiple pulses over a microsecond time scale, we calculate the radial and axial hydrodynamic flow to study the material reflux into the converter volume and the resultant density decrease as the electron beam energy is deposited. Both the electron transport through the expanding low density plasma and beam in the higher density material are modeled. The x-ray source dose and spot size are calculated to evaluate the impact of the changing converter material density distribution on the radiographic spot size and dose. The results indicate that a multiple-pulse converter design for three or four high-power beam pulses is feasible.
Anisotropic self-focusing of intense lasers in undercritical plasma
Authors: Chen Li-Ming , Lin Hai
Published: July 2002

A variational principle approach to study the anisotropic self-focusing of ultrarelativistic laser beams in the undercritical plasma is performed in this paper. The presence of the anisotropic self-focusing laser intensity profile is viewed as a result of unstabilization of the isotropic profile. The isotropic laser profile is solved and then its stability relative to the anisotropic one is discussed. Numerical results show that parameters of the isotropic profile are important for its possibility of anisotropic unstabilization.