bluuebunny/crossref_metadata_2025 huggingface dataset:
Spontaneous Raman scattering in [100], [110], and [11-2] directional GaP waveguides
Authors: Saito Takao , Suto Ken , Nishizawa Jun-ichi , Kawasaki Masaki
Published: July 2002

Spontaneous Raman scattering in GaP waveguides with micrometer size cross sections has been investigated for [100], [110], and [11-2] directions. For backward scattering, the result is the same as for the GaP bulk crystals, which depends exactly on the Raman selection rule. However, it is found that mode changes from longitudinal optical (LO) phonon to transverse optical (TO) phonon mode and TO phonon to LO phonon mode occur when the waveguide cross section is of micrometer size for forward scattering. The mode change is determined by the relationship of the waveguide width, thickness and wave vector of phonons. The Raman scattering intensity ratios for different waveguide directions as well as for different light polarization directions obtained from the forward scattering experiments show good agreement with the calculations for this waveguide effect. In particular, the highest LO intensity is obtained for forward scattering in a [11-2] directional waveguide. This effect should be taken into account for the design of semiconductor Raman amplifiers and lasers with micrometer size waveguides.
Edge flow measurements with Gundestrup probes
Authors: Gunn J. P. , Boucher C. , Devynck P. , Ďuran I. , Dyabilin K. , Horaček J. , Hron M. , Stöckel J. , Van Oost G. , Van Goubergen H. , Žáček F.
Published: July 2002

The ion current collected by a probe in a magnetized plasma is sensitive to the angle between its surface and the flow streamlines. This intuitive concept is the basis of the Gundestrup probe, a polar array of planar collectors mounted around an insulating housing. Probe theory for measuring flows has been developed on two fronts: Recent kinetic and fluid models, reviewed here, give similar predictions for the collected current within the range of applicability of the model assumptions. A comparison with measurements by a rotating Mach probe in the CASTOR tokamak (Czech Academy of Sciences Torus) [J. Stöckel, J. Badalec, I. Ďuran et al., Plasma Phys. Controlled Fusion, 41, 577 (1999)] highlights the role of magnetization in ion collection at grazing angles of incidence between the probe surface and the magnetic field lines.
Correlation of contact resistance with microstructure for Au/Ni/Al/Ti/AlGaN/GaN ohmic contacts using transmission electron microscopy
Authors: Bright A. N. , Thomas P. J. , Weyland M. , Tricker D. M. , Humphreys C. J. , Davies R.
Published: July 2002

Ohmic contacts to AlGaN/GaN heterostructures which have low contact resistance and good surface morphology are required for the development of commercial high power, high frequency transistors in the GaN system. The development of such contacts would be helped by a better understanding of the effect of microstructure on electrical behavior, which is studied here. Au/Ni/Al/Ti/AlGaN/GaN ohmic contact structures were rapid thermal annealed in argon for 60 s at temperatures in the range 550–900 °C. The variation of contact resistance with anneal temperature was correlated with the phase distribution observed by transmission electron microscopy (TEM). A combination of TEM techniques was required to determine the resulting microstructure, including energy filtered TEM, high resolution electron microscopy and energy dispersive x-ray spectrum imaging. Contacts with the lowest resistance were formed after 700 °C annealing. Very little consumption of the 30 nm AlGaN layer was observed. An unidentified phase containing Al, Ti and Au is present at the interface in the samples with low specific contact resistance. The identification of the observed thin interfacial phases (including TiN and AlN) is discussed, along with the effects of oxidation and possible mechanisms of ohmic contact formation.
Maskless epitaxial lateral overgrowth of GaN layers on structured Si(111) substrates
Authors: Strittmatter A. , Rodt S. , Reißmann L. , Bimberg D. , Schröder H. , Obermeier E. , Riemann T. , Christen J. , Krost A.
Published: July 2002

GaN layers are laterally overgrown by metalorganic chemical vapor deposition on structured Si(111) substrates in a single growth process. The substrates are structured with parallel grooves along the Si 〈1–10〉 or perpendicular to the Si 〈1–10〉 direction by standard photolithography and subsequent dry etching. Due to the anisotropic chemical dry etch process, the remaining Si ridges are underetched. The GaN layer grows nearly exclusively on the bottom of the grooves and on the top of the ridges between the grooves. These two growth fronts are completely separated from each other. As a consequence, the GaN layer growing from the ridge area between grooves can extend over the grooves. This process is similar to the so called pendeo-epitaxy process, but is completely mask free during growth and does not require any growth interruption. The improvement of the crystalline and the optical quality of the GaN layer is demonstrated by atomic force microscopy and cathodoluminescence spectroscopy.
Monte Carlo studies of hysteresis curves in magnetic composites with fine magnetic particles
Authors: Xu C. , Li Z. Y. , Hui P. M.
Published: July 2002

The hysteresis of composite systems consisting of nanosized ferromagnetic particles embedded in a nonmagnetic metal matrix is studied using the Monte Carlo method. In the model, the nanosized particles are assumed to be single domain and to possess random uniaxial anisotropy. The particles are well dispersed so that the interactions between them are weak and can be neglected. These particles may be superparamagnetic (unblocked) or not (blocked) depending on the temperature T, energy barrier ΔE, and measurement time τm. It is found that the hysteresis loop, which is obtained even in the absence of interparticle interactions, is mainly caused by the blocked particles as a result of the irreversible transitions between states of minimum energy. The remanence and coercivity of such systems decrease as T increases and disappear at high T. The results are in agreement with recent experimental observations.
Exploring the idea of self-guided dynamics
Authors: Lahiri Ansuman , Nilsson Lennart , Laaksonen Aatto
Published: July 2002

We have examined the recently proposed concept of self-guiding in the context of both stochastic and deterministic dynamics of a test particle in a double-well potential. In contrast to the original suggestion, results from our stochastic dynamics simulations indicate that enhancement of the systematic part of the force by introduction of a self-guiding term actually decreases the sampling efficiency of the system. It was found that only by substantially reducing the correlation between the actual force and the self-guiding force an enhancement in sampling could be achieved. In a deterministic analog of the same problem, constructed by coupling the test particle to a bath of a number of harmonic oscillators, an analogous reduction in the barrier crossing rate could be observed for the case when the actual force and the biasing force were strongly correlated. However, even a moderate decorrelation resulted in appreciable enhancement in barrier crossings. For the deterministic dynamics of the test particle uncoupled to the bath, no decrease in sampling was observed. We suggest that depending on the inertial memory of the system, the degree of correlation between the actual and the biasing force determines whether sampling will increase or not. This provides a unified picture and gives us insight about the applicability of the method under different simulation conditions.
Monte Carlo simulations of symmetric and asymmetric angular model liquids
Authors: Calero S. , Garzón B. , Mejı́as J. A. , Lago S.
Published: July 2002

Monte Carlo simulations of fluids made of nonpolar angular αβγ molecules in the NVT ensemble have been performed. They provide quantitative information on the influence of αβ and βγ distances (Lαβ and Lβγ), temperature and density on the pressure, internal energy, and structural functions of these fluids. Three different models have been considered. The first one is a symmetric angular model of propane with Lαβ=Lβγ=0.4123 in reduced units. A detailed investigation of the thermodynamic and structural properties of this alkane is presented here. The results of the present study are in agreement with those obtained by previous molecular dynamics simulation and with experimental data. The second model consists of nonsymmetrical molecules in which Lαβ=0.4123 and Lβγ=0.6 (reduced units). The third model is a symmetrical one with Lαβ=Lβγ=0.6, a larger distance than for propane. Differences in the critical properties for each model are estimated and differences on the variation of the critical temperature with the molecular shape are discussed. Some preliminary comparisons among simulations and x-ray diffraction experiments in liquid propane are also presented.
Magnetic signature of compositional gradient in exchange-spring bilayer films of CoPt/Co
Authors: Crew D. C. , Lewis L. H. , Kim J. , Barmak K.
Published: July 2002

Experimental studies were performed on model bilayers of fully ordered L10 CoPt with thicknesses of 25, 50, or 100 nm, capped by Co of varying thicknesses from 2.6–75 nm. The bilayers were subjected to different annealing conditions; transmission electron microscopy and x-ray diffraction supplied data concerning the crystallographic structure, microstructure, and texture of the constituent phases in the bilayers. The degree of exchange coupling was investigated primarily with magnetic measurement and magnetic force microscopy. Samples subjected to annealing treatments at 300 °C showed evidence of grain growth and increased texture of the hexagonally close-packed (hcp) cobalt layer relative to the as-deposited condition. The degree of exchange coupling in these materials improved only slightly with annealing, as evidenced by both magnetic recoil curve measurement and magnetic force microscopy. Samples subjected to a higher-temperature annealing treatment of 550 °C evidenced dramatic improvement in the magnetic coupling, an improvement attributed to interdiffusion of CoPt and Co and the formation of alloy phases, specifically face-centered-cubic and hcp Co–Pt solid solutions.
Assessment of W1 and W2 theories for the computation of electron affinities, ionization potentials, heats of formation, and proton affinities
Authors: Parthiban Srinivasan , Martin Jan M. L.
Published: July 2002

The performance of two recent ab initio computational thermochemistry schemes, W1 and W2 theory [J. M. L. Martin and G. de Oliveira, J. Chem. Phys. 111, 1843 (1999)], is assessed for an enlarged sample of thermochemical data consisting of the ionization potentials and electron affinities in the G2-1 and G2-2 sets, as well as the heats of formation in the G2-1 and a subset of the G2-2 set. We find W1 theory to be several times more accurate for ionization potentials and electron affinities than commonly used (and less expensive) computational thermochemistry schemes such as G2, G3, and CBS-QB3: W2 theory represents a slight improvement for electron affinities but no significant one for ionization potentials. The use of a two-point A+B/L5 rather than a three-point A+B/CL extrapolation for the self-consistent field (SCF) component greatly enhances the numerical stability of the W1 method for systems with slow basis set convergence. Inclusion of first-order spin–orbit coupling is essential for accurate ionization potentials and electron affinities involving degenerate electronic states: Inner-shell correlation is somewhat more important for ionization potentials than for electron affinities, while scalar relativistic effects are required for the highest accuracy. The mean deviation from experiment for the G2-1 heats of formation is within the average experimental uncertainty. W1 theory appears to be a valuable tool for obtaining benchmark quality proton affinities.
Vortex dynamics and magnetic anisotropy in RuSr2GdCu2O8
Authors: Srikanth H. , Spinu L. , Kodenkandath T. , Wiley J. B. , Tallon J.
Published: July 2002

Polycrystalline samples of RuSr2GdCu2O8 with a magnetic ordering temperature (TM=135 K) and a resistively determined superconducting onset temperature (Tc=45 K) were synthesized using a solid-state reaction. Transport and magnetic measurements indicated coexistence of weak ferromagnetic-like order and superconductivity at low temperatures. A novel radio-frequency (rf) resonant technique was employed to probe the mixed state response. The results clearly indicate that in the superconducting state (T<Tc), the rf response is dominated by the flux lattice dynamics and in the normal state (Tc<T<TM), the data are consistent with transverse susceptibility exhibiting sharp peaks at characteristic switching and anisotropy fields.
Semiclassical treatment of charge transfer in molecule-surface scattering
Authors: Bach Christian , Groß Axel
Published: July 2002

We have treated the ionization probability of iodine molecules scattered from diamond by a semiclassical surface hopping scheme, namely Tully’s fewest-switches algorithm [J. Chem. Phys. 93, 1061 (1990)]. The interaction is described by a model potential that has been adjusted to empirical data. We start with a one-dimensional two-state model in which just the molecular distance from the surface and the neutral and negatively charged state of I2 are considered. We determine the ionization probability within the adiabatic and diabatic representation and compare it with exact quantum calculations. For this particular problem we find that the diabatic picture shows too little coherence, while the adiabatic representation yields satisfactory results. In the second part we have successively increased the complexity of the simulation by additionally taking a surface oscillator coordinate, the molecular rotation and vibration into account. Including more degrees of freedom damps out the Stückelberg oscillations present in the one-dimensional model. Our results qualitatively reproduce the observed dependence of the ionization probability on the incident energy of the molecules. This dependence is not given by the electronic coupling per se, but rather due to energy transfer to substrate and internal degrees of freedom during the scattering event. Finally, we are also able to reproduce the measured dissociation probability which can be explained in a centrifugal model.
Application and methodological improvements to the floating-wire technique to characterize the magnetic properties of a spectrometer dipole
Authors: Stein H. J. , Krol G. , Barsov S. , Koptev V.
Published: July 2002

The floating wire technique was applied to characterize the focusing properties of the D2 spectrometer magnet to be used in the ANKE installation of the COSY Jülich accelerator facility. Trajectories corresponding to 140–600 MeV/c ejectiles emerging from a target in front of the rectangular-type D2 magnet were measured in order to determine the location of foci where detectors for the physics experiments were to be placed. The focus for trajectories of the same momentum was localized by varying the trajectory start angle at the target. To overcome the problem of instability, which arises when the pulley has to be located behind the focus, an unbalanced pulley was used. Other improvements include methodological features such as the particular design of our air-bearing pulley enabling a “freezing-in” of the wire movement and therefore yielding a very high accuracy of the wire position measurement, the use of a 40 μm gold-plated tungsten wire best fulfilling the mechanical and electrical requirements, a fast-regulating power supply in combination with an absolutely calibrated current meter, and the excellent accuracy of modern surveying instrumentation. An overall momentum accuracy of Δp/p⩽10−3 and a position accuracy of the trajectories of ±1 mm in the vicinity of the foci have been demonstrated. The precision and accuracy of the floating-wire measurements are discussed and compared with ray-tracing calculations based on measured and calculated field maps.
Intermolecular bending levels in an open-shell diatom–diatom complex: Infrared spectroscopy and model calculations of the OH–N2 complex
Authors: Marshall Mark D. , Pond Bethany V. , Hopman Shawn M. , Lester Marsha I.
Published: July 2002

A theoretical framework has been developed to describe the bending levels associated with an intermolecular potential of moderate anisotropy between an open-shell diatom and a diatom partner, such as OH–CO or OH–N2. The model explicitly allows for coupling between the electronic and spin angular momenta of the open-shell OH radical and the vibrational angular momentum arising from intermolecular bending motion of the complex. The energies and wave functions of the intermolecular bending levels for the OH–N2 complex have been computed based on a dipole–quadrupole interaction. The model is used to interpret the infrared spectrum of the linear OH–N2 complex in the OH overtone region, which has been recorded by detecting the OH fragments from vibrational predissociation. The pure OH overtone band at 6973.54(2) cm−1 and several combination bands, which involve the simultaneous excitation of OH stretching and geared bending modes, have been observed, analyzed, and assigned within the context of the model. In addition, the time evolution and quantum state distribution of the OH fragments yield the lifetime for vibrationally activated OH–N2 of 30±4 ns and an upper limit for the ground state binding energy of OH–N2, D0⩽277 cm−1.
Thermal stability of magnetron and ion beam sputtered top and bottom spin-valve films
Authors: Mao Ming , Cerjan Charlie , Hung Stephanie , Miloslavsky Lena , Chien Chester , Sant Sudhi
Published: July 2002

The thermal stability of top and bottom IrMn exchange-biased spin-valve films prepared by ion beam deposition (IBD) and magnetron sputtering physical vapor deposition (PVD) is compared. These films exhibit identical temperature dependence for the exchange bias field Hex, with a blocking temperature of TB=250 °C, that is independent of preparation technique. Isothermal annealing at temperatures below TB led to a ln(t) dependent degradation in Hex, suggesting a thermal activation process. The high crystallographic quality of the IBD films leads to a superior stability compared to PVD films. Top spin-valve films are also found to be more stable than bottom spin-valve films.
Model for stress generated upon contact of neighboring islands on the surface of a substrate
Authors: Freund L. B. , Chason Eric
Published: July 2002

In the early stage of growth of a metal film on a substrate by the Volmer–Weber mechanism, a tensile stress in the film is observed to arise at about the point in the process when islands of deposited material begin to coalesce. The mechanism commonly proposed as the origin of this tensile stress is that the coalescing islands deform in order to form a relatively low energy grain boundary, at the expense of some surface energy by surface area reduction, and that this proceeds until a stress is generated that has magnitude sufficient to prevent further area reduction. Several models have been proposed for this process, but the inferred tensile stress estimates have been much larger than observed stress magnitudes in many cases. The purpose here is to introduce a model for the process based on the theory of contact of elastic solids with cohesion. A description of the process is developed on this premise for one-dimensional, two-dimensional, and three-dimensional states of deformation of coalescing islands. It is found that the latter case leads to an estimate of film stress generally consistent with observations. The features of the model for different dimensionalities are compared and contrasted with each other, as well as with other models which have been proposed for this process.
Synthesis, self-assembly, and magnetic behavior of a two-dimensional superlattice of single-crystal ε-Co nanoparticles
Authors: Puntes Victor F. , Krishnan Kannan M. , Alivisatos Paul
Published: July 2002

A method of producing high-quality magnetic colloidal dispersions by the rapid pyrolysis of cobalt carbonyl in an inert atmosphere was employed to produce monodispersed, stabilized, defect-free ε-cobalt nanocrystals, with spherical shapes and sizes ranging from 3 to 17 nm. The size distribution and the shape of the nanocrystals were controlled by varying the surfactant (oleic acid, phosphonic oxides and acids, etc.), its concentration, and the reaction temperature. These particles have been observed to produce two-dimensional self-assemblies when evaporated at low rates in a controlled atmosphere. A collective behavior due to dipolar interactions has been observed in the low susceptibility measurements corresponding to a highly ordered fine particles system.
Supercooling in a two-dimensional Lennard-Jones mixture
Authors: Sim Eunji , Patashinski Alexander Z. , Ratner Mark A.
Published: July 2002

A mixture of large and small Lennard-Jones particles is studied; in the study we focus on the relations among temperature, packing, structure, and transport in the supercooled state. As the temperature decreases, one sees increasing component separation, and local ordering. Clusters consisting of only small particles grow in size with time and cooling; the dominant local structure in those clusters is hexagonal. The rest of the system, including almost all large and some small particles, remains amorphous, with local order dictated by the geometry of dense packing of mixed large and small particles. To study these effects independently, we modified the conventional model by managing the relaxation of large particles. At low temperatures, the diffusion of small particles dramatically decreases when the large particles are frozen. An interpretation of these findings based on the local rearrangement kinetics is proposed.
Polymer chain dynamics in dilute solution under Couette flow. II. High molecular weight poly(α-methylstyrene) in good solvent
Authors: Tsunashima Yoshisuke
Published: July 2002

Dynamics of poly(α-methylstyrene) of high molecular weight Mw=6.85×106 in solution of good solvent was investigated at 25 °C by dynamic light scattering under Couette flow. The applied shear rate was ranging from 0 to 4.5 s−1. The present purpose was to inspect the theoretical way of descriptions of flexible-linear-chain dynamics in dilute solution. At the shear rate below 2.8 s−1, two modes of motions coexisted, i.e., the diffusion motion of the single chain and the intramolecular modes of motions. The decay rate for the diffusion mode increased with increasing the shear rate but that for the internal modes was nearly constant. Whereas, above 2.8–4.5 s−1, the internal modes were suppressed exclusively and only the center-of-mass translational diffusion of the chain was detected. The observed diffusion was found to be the Brownian motion conducted by a quasi-ideal chain, while the internal motions were suffering the Zimm-type of nondraining hydrodynamic interactions. Moreover, the universal ratio Ω/D0q2 under Couette flow was examined closely as a function of qRG because this is a critical measure to inspect how the polymer chain behaves dynamically and how the dynamics can be affected by the hydrodynamic interactions in solution. Here q and RG are the scattering vector and the radius of gyration of the chain, respectively. The obtained result showed that the ratio was located on the theoretical curve for flexible linear chains in Θ state, where the theory was performed under the microscopic descriptions on both chain segments and solvents with full hydrodynamic interactions. This fact confirms strictly that the coupled kinetic equations for chain segments and solvent in the same dynamic level are indispensable to describe rigorously the dynamics of flexible polymers in dilute solution.
Stoichiometry-induced roughness on antimonide growth surfaces
Authors: Bracker A. S. , Nosho B. Z. , Barvosa-Carter W. , Whitman L. J. , Bennett B. R. , Shanabrook B. V. , Culbertson J. C.
Published: July 2002

Phase shifts in the intensity oscillation of reflection high-energy electron diffraction spots provide evidence for monolayer island formation on AlSb that is caused by sudden changes in surface stoichiometry. High-resolution scanning tunneling microscopy confirms the interpretation of the phase shift. These results are consistent with a previous structural assignment of the AlSb β(4×3) and α(4×3) surface reconstructions and provide guidelines for producing smooth interfaces in antimonide-based heterostructures.
Pattern formation induced by co-deposition of Ni and C60 on MgO(100)
Authors: Vacik Jiri , Naramoto Hiroshi , Narumi Kazumasa , Yamamoto Shunya , Miyashita Kiyoshi
Published: July 2002

We report on the unusual phenomenon of pattern formation in the thin film of Ni+C60 mixture deposited on the MgO(100) substrate. Under certain deposition kinetics a periodic system of stripe domains was formed. The domains consist of sub-half-micrometer large droplets encompassed with a thin C60—based rind with a possible polymeric structure. The stripes are embedded in the double-layer platform falling into the epitaxial Ni and amorphous-C (a-C) thin films. During co-deposition the C60 molecules partly disintegrate and transform towards a-C. Due to the accumulation of a-C (with limited solubility in Ni) thermodynamic instability arises in the system and reaching a certain level, it triggers spontaneous partitioning (and self-organization) of the deposited material. As a principal mechanism of the pattern formation, a sequential drift and coordinate release of the thermodynamic instability is proposed.
Zone-folding effect on optical phonon in GaN/Al0.2Ga0.8N superlattices
Authors: Chen C. H. , Chen Y. F. , Shih An , Lee S. C. , Jiang H. X.
Published: July 2002

Optical properties of GaN/Al0.2Ga0.8N superlattices have been investigated by Raman scattering and photoluminescence measurements. It is found that the A1(LO) phonon decreases in frequency with decreasing quantum-well width. The frequency shift is attributed to the effect of phonon zone folding. Through the study of photoluminescence, we show that our observation of the zone-folding effect on optical phonon in GaN/Al0.2Ga0.8N superlattices is due to the sharpness of the interfaces between barrier and well layers. The sharp interfaces prevent the appearance of mixed interface modes which mask the phonon effect of zone folding in previous reports.
Vibrational state distribution and relaxation of vinoxy radicals
Authors: Su Hongmei , Bersohn Richard
Published: July 2002

The vinoxy radical ⋅CH2CHO is a product of the reaction of O(3P) atoms with terminal alkenes and can also be made by photodissociation of an alkyl vinyl ether. In either case it is formed in a vibrationally excited state. The nascent radical displays a rich electronic spectrum to the red of its X→B band origin consisting of bands originating from vibrationally excited states. Some transitions, true “hot bands,” terminate on the vibrationless B state; others, sequence bands, terminate on vibrationally excited B states. The spectra become unobservably weak at a certain energy. The difference between that energy and the energy of the band origin is roughly the maximum vibrational energy in the radical. This is 5600 cm−1 for the vinoxy produced by photodissociation of ethyl vinyl ether at 193 nm and 3200 cm−1 for the product of the reaction of O(3P) with ethylene, propene, 1-butene, and 1-pentene. There is a remarkable cooling of the vibrations as the hydrocarbon chain lengthens. The average vibrational energy of the vinoxy product of the reaction O(3P) with ethylene, propene, 1-butene, and 1-pentene is 2100, 1800, 1570, and 1180 cm−1, respectively. This cooling implies that the reaction complex lives long enough for internal vibrational relaxation to occur. The average vibrational energy in the reaction-produced vinoxy is small, which implies that there is considerable kinetic energy. The time dependence of the intensity of the hot bands measures the relaxation rates of different energies, some of which are the energies of a single vibrational state. The ground-state population increases monotonically to an asymptote. The population of most states grows with time and then decays. The growth is due to a cascading from upper states. The populations of the highest energy states decay monotonically; the still higher energy states are almost unpopulated. These results prove that the relaxation proceeds stepwise. The magnitude of the step, ∼200–300 cm−1, can be inferred from the growth rate of the ground-state population.
Polymer motions from localization to Rouse dynamics in supercooled melts
Authors: Ilan Boaz , Loring Roger F.
Published: July 2002

Laboratory and simulation studies of relaxation in supercooled polymer melts provide a view of the approach to the glass transition that is complementary to that obtained from study of small molecule liquids, because of the additional length scales characteristic of macromolecules. Recent molecular dynamics simulations of a supercooled melt of bead–spring polymers have shown that the motions of a coarse-grained polymer bead on length scales smaller than the bead diameter closely resemble dynamics in a supercooled simple liquid, in which molecules are spatially localized in accordance with the predictions of mode coupling theory. On longer length scales, the connectivity of the polymer becomes significant and molecular motions may be described by the Rouse model. We present calculations of chain dynamics in a supercooled melt from the dynamically disordered Rouse model. This dynamical mean field model shows qualitative agreement with the simulation data in describing both the short time regime of spatial localization at low temperature and the longer time regimes of Rouse dynamics.
The infrared spectra of OCS+ and OCS− trapped in solid neon
Authors: Lugez Catherine L. , Thompson Warren E. , Jacox Marilyn E.
Published: July 2002

When a Ne:OCS sample was codeposited at approximately 5 K with a beam of neon atoms that were excited in a microwave discharge, the infrared spectrum of the resulting deposit included absorptions which can be assigned to OCS+, OCS−, and one or more dimer ions. Detailed isotopic substitution studies support the assignments offered for OCS+ and OCS−. The CO- and CS-stretching absorptions observed for OCS+ isolated in a neon matrix lie close to the corresponding gas-phase band centers. The CO- and CS-stretching fundamentals of OCS− isolated in solid neon were observed for the first time at 1646.4 and 718.2 cm−1, respectively. These frequencies are in good agreement with values previously calculated using ab initio and density functional methods. The least-squares force constant fit to the data obtained in isotopic substitution experiments supports the proposed assignment. Dimer ions were also stabilized, but infrared data for these species were insufficient to provide definitive assignments.
Displaced-points path integral method for including quantum effects in the Monte Carlo evaluation of free energies
Authors: Mielke Steven L. , Truhlar Donald G.
Published: July 2002

A semiclassical approximation is proposed for the Monte Carlo quantum mechanical path integral approach to the calculation of equilibrium properties (partition functions, free energies) of molecules or generalized transition states. The approximation is based on replacing the sum over paths by a sum over points that are displaced from the classically sampled points. The displacements are adjusted to ensure the correct results for harmonic potentials, and thus the method is akin to perturbation theory with a harmonic reference. In its simplest form, the approximation takes the form of a classical mechanical calculation with double sampling for every configuration point; thus it can be readily adapted to a variety of applications. Calculations by several other approximate methods are also presented, and the results with the new method compare very favorably with them.
Influence of the morphology on the magneto-transport properties of laser-ablated ultrathin La0.7Ba0.3MnO3 films
Authors: Das A. , Gordon I. , Wagner P. , Cannaerts M. , Moshchalkov V. V. , Bruynseraede Y. , Schuddinck W. , Van Tendeloo G. , Borghs G.
Published: July 2002

We investigate the thickness dependent properties of manganite films characterized by colossal negative magnetoresistance. Ultrathin, wedge-type films (0–120 Å) of La0.7Ba0.3MnO3 were deposited by laser ablation onto SrTiO3 and LaAlO3 substrates. The films were patterned into strips of different thickness and magneto-transport measurements were performed at temperatures between 5 and 290 K and in magnetic fields up to 5 T. Atomic force- and transmission electron microscopy were done to correlate the microstructure with the transport data. The resistivity of the films increases slightly with decreasing thickness due to substrate-induced compressive strain. Below 50 Å, the resistivity rises abruptly indicating a crossover to discontinuous and finally island-like film growth as confirmed by the microstructural techniques. At thicknesses slightly above the threshold for percolative conduction (≈30 Å), an enhanced low-field magnetoresistance was observed as a signature of spin-dependent tunneling.
Ab initio study of the electrochemical polymerization mechanism of ω–diamines
Authors: Lakard Boris , Herlem Guillaume , Fahys Bernard
Published: July 2002

The anodic oxidation of liquid ω–diamine based-electrolyte leads to the passivation of the electrode surface by an insulating film as shown by using an electrochemical quartz crystal microbalance (EQCM) coupled with cyclic voltammetry (CV) technique. These films were identified by infrared-attenuated total reflectance (IR-ATR) as polymeric films: linear polyethylenimine (L-PEI) film for EDA based-electrolyte and linear propylenimine (L-PPI) for 1,3 DAP. We also performed computations of energy and thermochemical values with the quantum-chemical Onsager self-consistent reaction field (SCRF) method at the Hartree–Fock level for modeling the reaction mechanisms leading to the polymeric films.
Cumulant-based expressions for the multibody terms for the correlation between local and electrostatic interactions in the united-residue force field
Authors: Liwo Adam , Czaplewski Cezary , Pillardy Jarosław , Scheraga Harold A.
Published: July 2002

A general method to derive site-site or united-residue potentials is presented. The basic principle of the method is the separation of the degrees of freedom of a system into the primary and secondary ones. The primary degrees of freedom describe the basic features of the system, while the secondary ones are averaged over when calculating the potential of mean force, which is hereafter referred to as the restricted free energy (RFE) function. The RFE can be factored into one-, two-, and multibody terms, using the cluster-cumulant expansion of Kubo. These factors can be assigned the functional forms of the corresponding lowest-order nonzero generalized cumulants, which can, in most cases, be evaluated analytically, after making some simplifying assumptions. This procedure to derive coarse-grain force fields is very valuable when applied to multibody terms, whose functional forms are hard to deduce in another way (e.g., from structural databases). After the functional forms have been derived, they can be parametrized based on the RFE surfaces of model systems obtained from all-atom models or on the statistics derived from structural databases. The approach has been applied to our united-residue force field for proteins. Analytical expressions were derived for the multibody terms pertaining to the correlation between local and electrostatic interactions within the polypeptide backbone; these expressions correspond to up to sixth-order terms in the cumulant expansion of the RFE. These expressions were subsequently parametrized by fitting to the RFEs of selected peptide fragments, calculated with the empirical conformational energy program for peptides force field. The new multibody terms enable not only the heretofore predictable α-helical segments, but also regular β-sheets, to form as the lowest-energy structures, as assessed by test calculations on a model helical protein A, as well as a model 20-residue polypeptide (betanova); the latter was not possible without introducing these new terms.
Design of a three-component capacitive force transducer
Authors: Cannella M. , Marinozzi F.
Published: July 2002

A three-component capacitive force/moment transducer was designed and developed. The moments Mx, My, and the normal force Fz are sensed. The capacitor electrode conformation and disposition was designed to allow a greatly simplified circuit topology and limit the influence of stray capacitances. The capacitive displacement sensor exhibits an intrinsic sensitivity of about 20 (V/V)/rad for the tangential components and 6(V/V)/mm for the normal one. With proper amplifying circuitry a minimum detectable signal of 0.3×10−4 rad/Hz−1/2 and 0.1 μm/Hz−1/2 was achieved. Different elastic elements were tested in order to vary the nominal load of the force transducer and to achieve different sensitivities. These values are roughly 102 times the values of traditional strain gages units. The proposed device is mainly intended for the mechanical characterization of nonstandard specimens, and is presented for the purpose of performing microtraction tests on biological samples while monitoring off axis components.
Statistical analysis of semiconductor devices
Authors: Mayergoyz I. D. , Andrei P.
Published: July 2002

A technique for the analysis of random dopant-induced effects in semiconductor devices is presented. It is based on the “small signal analysis” (perturbation) technique. It is computationally much more efficient than the existing purely “statistical” techniques, and it yields the information that can be directly used for the design of dopant fluctuation-resistant structures of semiconductor devices. This technique requires only the knowledge of variances of fluctuating doping concentrations and in this sense, it is a “second-moment characterization” technique. This technique can be naturally extended to take into account random fluctuations of oxide thickness and oxide charges in metal–oxide–semiconductor filed-effect transistor. The numerical implementation of this technique is discussed and numerous computational results are presented and compared with those previously published in the literature.
Etching by atomic hydrogen of Ge overlayers on Si(100)
Authors: Zheng Y.-J. , Ma P. F. , Engstrom J. R.
Published: July 2002

X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering spectrometry (LEISS) have been employed to study the kinetics of etching by atomic hydrogen of Ge overlayers on Si(100). The coverage of Ge as deduced by both XPS and LEISS decreases dramatically after exposure to atomic hydrogen, and the decrease is exponential with exposure time. The etch rate of Ge derived from these experiments is found to be a linear function of the flux of atomic hydrogen at sufficiently low temperatures (<180 °C). The rate of etching also depends upon the substrate temperature and a maximum in the etch rate is observed at about 180 °C. A kinetic model is proposed to describe the experimental data quantitatively. The model successfully describes the variation in the rate of etching as functions of the flux of atomic hydrogen and substrate temperature. In particular, the peak observed in the etch rate as a function of substrate temperature is interpreted as due to a similar variation in the coverage of an important reaction intermediate, namely a GeH2(a) surface species.
The complex of HF2− and H2O: A theoretical study
Authors: Li G. P. , Reinhart B. , Hamilton I. P.
Published: July 2002

The complex of HF2− and H2O is studied using B3LYP, MP2, and QCISD methods. Energetics, geometries, and vibrational frequencies of the equilibrium structure and two transition states are calculated using 6-311++G(d,p), 6-311++G(2d,2p), and 6-311++G(2df,2pd) basis sets. For the equilibrium structure there is a hydrogen bond between one of the F atoms of HF2− and one of the H atoms of H2O. The two transition states are only about 0.5 kcal/mol higher. The HF2−–H2O equilibrium structure is planar and, at the B3LYP/6-311++G(2df,2pd) level, the F–H–O bond angle is nearly linear at 174.4° and the F–O distance is 2.59 Å. With zero point energy and counterpoise correction, the binding energy is 14.9 kcal/mol and the strong hydrogen bond of HF2− is weakened by 11.3 kcal/mol (25%). In HF2− the experimental F–F distance is 2.28 Å and the F–H–F bond angle is 180°. The most intense IR vibration is the F–H–F asymmetric stretch at 1331 cm−1. In HF2− the calculated F–F distance is 2.30 Å and in the HF2−–H2O equilibrium structure the F–H distance for the hydrogen bonded F atom is longer by 0.13 Å but the F–H distance for the free F atom is shorter by 0.10 Å and the F–F distance is only 0.03 Å longer. The F–H–F bond angle is very close to linear at 179.4°. The most intense IR vibration remains the F–H–F asymmetric stretch, blueshifted by 648 cm−1. The F–H–O asymmetric stretch is also an intense IR vibration, redshifted by 729 cm−1 from the O–H local mode stretch for H2O.
Electrical properties of Bi3.25La0.75Ti3O12 thin films on Si for a metal–ferroelectric–insulator–semiconductor structure
Authors: Choi Taekjib , Kim Young Sung , Yang Cheol Woong , Lee Jaichan
Published: July 2002

La-modified bismuth titanate [Bi3.25La0.75Ti3O12 (BLT)] thin films have been grown at a low processing temperature of 620 °C by pulsed-laser deposition on a p-Si substrate with a nitrogen-doped thermal oxide SiO2 layer. This metal–ferroelectric–insulator–semiconductor structure exhibited a capacitance–voltage (C–V) hysteresis (memory window) due to ferroelectric polarization. The memory window reached a maximum of 0.8 V at a sweep voltage of 6 V. In addition, BLT films grown on Si exhibited the asymmetric behavior of C–V and current–voltage (I–V) characteristics, i.e., asymmetric shift of the threshold voltage with the sweep voltage. It is found that appreciable charge injection (indirect tunneling) occurs from Si, before the memory window does not even reach the maximum (i.e., 6 V in this structure). The trapped electrons injected from Si cause Vfb1 to shift toward the positive direction rather than the negative direction. This leads to the asymmetric behavior of the C–V curve and the decrease in the memory window.
Anharmonic effects on photo-induced electron transfer: A Redfield approach
Authors: Kalyanaraman C. , Evans D. G.
Published: July 2002

Photo-induced electron transfer experiments examine intrinsically nonequilibrium processes. A theoretical description of photoinduced processes should take into account the fact that the approximations and assumptions made for equilibrium electron transfer need not be appropriate. Under nonequilibrium conditions, anharmonic distortions in the potential energy surfaces of nuclear motion coupled to the electron transfer may effect the dynamics. This work is a study of the effects of anharmonicity on photo-induced electron transfer reactions for condensed phase systems where one vibrational mode is strongly coupled to the electron transfer and a stochastic bath. For this vibrational mode, both harmonic and anharmonic potential energy surfaces for the excited states are considered and the electron transfer dynamics is monitored in a range of system–bath coupling regimes. The study focuses on two effects due to anharmonic distortions of the intramolecular modes: changes to the system Hamiltonian, and differences in the dephasing processes caused by the anharmonic distortions. These calculations show that for small differences in the donor and acceptor state energies, the effects of vibrational anharmonicity is very small. However, when this energy difference is large, the dynamics for anharmonic and harmonic modes is significant. The relative role played by the competing physical processes is easily understood by examining the vibronic state populations obtained using a multistate Redfield approach.
Pyrolysis of sulfur tetrafluoride over boron: Excited-state rotational spectra and equilibrium structure of fluorothioborine (FBS)
Authors: Bizzocchi L. , Esposti C. Degli
Published: July 2002

The unstable FBS molecule has been produced in the gas phase by a high-temperature reaction between crystalline boron and sulfur tetrafluoride. Its rotational spectrum has been observed in the millimeter-wave region, from 75 to 460 GHz, for different isotopic species and vibrational states. All the excited states which approximately lie below 1700 cm−1, that are 1000 (F–B stretch), 0110 (FBS bend), 0001 (B=S stretch), 2000, 0200, 0220, 0310, 0330, 0400, 0420, 0440, 1110, 1200, and 1220, have been investigated for the most abundant isotopomer F11B32S. The analysis of the spectra has been performed taking simultaneously into account the Fermi interaction which couples the states ν1,ν2,ν3 with ν1−1, ν2+2,ν3, and l-type resonances between different sublevels of a given vibrational bending state. This procedure allowed us to calculate directly deperturbed parameters and, in addition, yielded reliable estimates of the vibrational energy difference between the interacting levels and of the normal coordinate cubic force constant k122. Rotational spectra in the ground and various excited states have been also recorded and analyzed for the less abundant isotopic species F10B32S, F11B34S, F10B34S, F11B33S, and F10B33S. The very weak spectrum in the 0001 state was successfully observed for the pair of isotopomers F11B32S and F10B32S, whose equilibrium rotational constants could be accurately calculated yielding the first evaluation of the equilibrium structure of fluorothioborine: re(F–B)=1.2762±0.0002 Å and re(B=S)=1.6091±0.0002 Å.
Anomalous dielectric behavior of undulated lipid membranes. Theoretical model and dielectric spectroscopy measurements of the ripple phase of phosphatidylcholine
Authors: Raudino Antonio , Castelli Francesco , Briganti Giuseppe , Cametti Cesare
Published: July 2002

We measured the static and dynamic dielectric properties of phosphatidylcholine (PC) spherical membranes (liposomes) over a wide range of temperature and for different liposome radii, pH, and liposomes concentration. Within the investigated temperature range, the physical state of the membrane goes from a gellike to a fluidlike phase, passing through a narrow (≈10 °C) intermediate state, the Ripple phase, characterized by permanent surface undulations of defined wavelength and amplitude. The dielectric properties of the ripple phase are anomalous, the mean static permittivity is higher than that of the gel and fluid phases (where no undulations are present), while the average relaxation frequencies is smaller. Furthermore, the static dielectric permittivity of the fluid phase is much higher than that of the gel phase, while the relaxation frequencies behave just in the opposite way. In order to rationalize this complex behavior we have developed a dynamic mean-field model aimed to calculate the electric polarization and the dipolar relaxation for an array of strongly interacting dipoles anchored to the membrane surface and rotating over either a planar (gel and fluid phases) or a corrugated surface (the ripple phase). The dipoles mimic the highly polar (≈20 Debyes) head groups of PC molecules. The model correctly predicts, albeit qualitatively, most of the observed dielectric anomalies essentially related to the changes of the dipole–dipole correlation induced by the membrane surface undulations.