bluuebunny/crossref_metadata_2025 huggingface dataset:
The role of dielectric relaxation in transient electrorheological response
Authors: Tang Xinlu , Conrad Hans
Published: July 2002

The influence of dielectric relaxation on the complex relative polarizability β*(ω) of particles with and without a thin surface film is calculated as a function of the electric field frequency ω. Further, a simple model is employed to calculate the transient shear stress response of electrorhological (ER) fluids under unipolar and bipolar periodic-step, electric field excitation. Good accord exists between the calculated stress-time responses and measurements on four types of ER suspensions. The non-ohmic conductivity of the host oil was found to be important in transient ER response.
Sensitivity and spatial resolution for electron-spin-resonance detection by magnetic resonance force microscopy
Authors: Zhang Z. , Roukes M. L. , Hammel P. C.
Published: July 2002

The signal intensity of electron spin resonance in magnetic resonance force microscopy (MRFM) experiments employing periodic saturation of the electron spin magnetization is determined by four parameters: the rf field H1, the modulation level of the bias field Hm, the spin relaxation time τ1, and the magnetic size R(∂H/∂z) of the sample. Calculations of the MRFM spectra obtained from a 2,2-diphenyl-1-picrylhydrazyl particle have been performed for various conditions. The results are compared with experimental data and excellent agreement is found. The systematic variation of the signal intensity as a function of H1 and Hm provides a powerful tool to characterize the MRFM apparatus.
Design of high temperature permanent magnets
Authors: Kim Andrew S.
Published: July 2002

There has been a demand for permanent magnet materials which can be used at up to 450–500 °C. Normal Sm2TM17 magnets have a coercivity Hci of ∼10 kOe at room temperature. At 150 °C, the coercivity is decreased by about 50%. The maximum operating temperature (MOT) of these magnets is 150–200 °C. By increasing the Hci to 30 kOe, the MOT is increased to about 300 °C. Further increases in MOT are anticipated with further increases in coercivity. However, the degree of increase in a MOT will be small as long as the same temperature coefficient of coercivity (β) is maintained, resulting in MOT that is far below the required operating temperature. Therefore we studied the effect of β when they have the same Hci at room temperature. This study indicates that the MOT is more effectively increased by reducing the β than increasing the Hci. For example, a 1:7 magnet having a β of −0.13%/°C and a Hci of 18.6 kOe starts an irreversible loss at 550 °C, while a 2:17 magnet having a β of −0.29%/°C and a Hci of 28.5 kOe starts an irreversible loss at 350 °C. Therefore, high temperature permanent magnets (MOT>500 °C) are possible when the β is lower than −0.15%/°C with moderately high coercivity (∼20 kOe).
Re-entrant flux reversal in 80Ni–4.2Mo–Fe permalloy wires
Authors: Li Lin
Published: July 2002

The re-entrant flux reversal effect was observed in commercially produced wire (44 and 117 μm in diameter) that had been made with Carpenter HyMu “80”® alloy (Ni80Mo4.2Febal permalloy). The critical field (Hj) for the flux jump decreased (to 0.2 Oe) with increased annealing temperature (e.g., 700 °C). However, after annealing at higher temperatures (e.g., 900 °C) the re-entrant effect disappears. The factors that affect re-entrant flux reversal in permalloy wire, such as wire diameter, length, heat treatment, and magnetic field, were investigated. The signals generated from amorphous and permalloy wires were also compared.
The effect of an auxiliary discharge on anode sheath potentials in a transverse discharge
Authors: Foster J. E. , Gallimore A. D.
Published: July 2002

A novel scheme that employs the use of an auxiliary discharge has been shown to reduce markedly anode sheath potentials in a transverse discharge. An 8.8 A low-pressure argon discharge in the presence of a transverse magnetic field was used as the plasma source in this study. In such discharges, the transverse flux that is collected by the anode is severely limited due to marked reductions in the transverse diffusion coefficient. Findings of this study indicate that the local electron number density and the transverse flux increase when the auxiliary discharge is operated. Changes in these parameters are reflected in the measured anode sheath voltage. Anode sheath potentials, estimated by using Langmuir probes, were shown to be reduced by over 33% when the auxiliary discharge is operated. These reductions in anode sheath potentials translated into significant reductions in anode power flux as measured using water calorimeter techniques. The reductions in anode power flux also correlate well with changes in the electron transverse flux. Finally, techniques implementing these positive effects in real plasma accelerators are discussed.
Synthesis and characterization of cluster-assembled carbon thin films
Authors: Milani P. , Ferretti M. , Piseri P. , Bottani C. E. , Ferrari A. , Li Bassi A. , Guizzetti G. , Patrini M.
Published: July 2002

Nanostructured carbon thin films have been produced by deposition of supersonic cluster beams. The clusters are generated by a pulsed arc cluster ion source modified in order to achieve high fluxes and stability. Scanning electron microscopy, Raman, and optical spectroscopy show that the films are a low density network of nanometer-size particles. The nature of the films is essentially graphite-like with a large number of distorted bonds. The formation of structures based on sp3 bondings is not observed. The use of cluster beam deposition for the synthesis of nanocrystalline thin films is discussed.
Effects of MeV ion irradiation of thin cubic boron nitride films
Authors: Ullmann J. , Baglin J. E. E. , Kellock A. J.
Published: July 2002

Adherent boron nitride thin films with a high cubic phase content (up to about 90%) were synthesized over large areas by means of ion assisted evaporation. Besides the well known superior properties of cubic boron nitride (c-BN) nearly nothing is known about the behavior of c-BN under MeV ion irradiation where the ions penetrate through the thin film and come to rest far away in the substrate. Therefore, exploratory MeV ion implantation experiments into as-deposited c-BN rich films were conducted at room temperature. Ions with different masses were chosen to study the effect of the nuclear and electronic stopping processes on the highly cubic BN films. Furthermore the influence of various ion doses (3.5×1013–2.0×1017 ions/cm2) on the cubic structure was investigated. To get an insight into the effects of the different post-deposition treatments, the as-deposited and ion implanted films were analyzed by Fourier transformed infrared spectroscopy, Rutherford backscattering, x-ray diffraction, and Auger electron spectroscopy. Depending on the ion dose, the heavy Xe ion implantation results in four different regimes for the resulting material. The implantation induced the total number of displacements per unit volume as an essential measure of the effect of the postdeposition treatment. Depending on the total number of displacements per unit volume the cubic BN structure can be transformed into a hexagonal BN dominated network.
Measurement of the magnetostriction constants of amorphous thin films on kapton substrates
Authors: Ouyang C. , Kim T. W. , Gambino R. J. , Jahnes C.
Published: July 2002

The saturation magnetostriction constants of thin films of amorphous Co39Ni31Fe8Si8B14 and CoZrTb have been measured either by the small angle magnetization rotation (SAMR) method or by the initial susceptibility method. The SAMR method is used for the soft materials. When the material is magnetically hard or has a strong perpendicular anisotropy, the initial susceptibility method is used. It is found that the amorphous Co39Ni31Fe8Si8B14 prepared by ion beam deposition from an alloy target shows very soft magnetic properties and has a very small negative saturation magnetostriction, λs, of −1×10−7. Sputtered films of CoZrTb show a strong perpendicular anisotropy when the concentration of Tb is high. We have found that the SAMR method can be applied to CoZrTb films when the Tb content is low. The saturation magnetostricition constant of a sputtered film of Co78.4Zr20.8Tb0.8 is 2×106. When the Tb content is high, however, the initial susceptibility method is used to measure magnetostriction.
Field and stress dependence of the irreversible magnetization changes in pure iron
Authors: Pearson J. , Squire P. T.
Published: July 2002

Measurements of the irreversible magnetization changes accompanying the application and removal of stress to a sample of 99.99% pure iron have been made as a function of applied field and stress magnitude for both compressive and tensile stress. The fields for peak change have been compared with those for the peak difference MAN−MINI between the anhysteretic and initial magnetization curves and the field derivative of the magnetostriction. It is shown that at low tensile stress the magnetostrictive contribution is dominant, as found by Pravdin. At higher stress the wall pinning term is dominant, and the magnetization changes are proportional to MAN−MINI, in accordance with the Jiles–Atherton model.
Coupling of reversal modes for an infinite ferromagnetic cylinder
Authors: Chang Ching-Ray , Lee Ching-Ming
Published: July 2002

We used amplitude equations to study the dynamic behavior of magnetization nucleation. Under certain conditions, the amplitudes of higher modes become dominant at large field and the reversal process moves to a new metastable state. Mode-mode coupling excites a cascade of nucleation modes and indicates that a common single-mode analysis is invalid.
High frequency properties of glass-coated microwire
Authors: Antonenko A. N. , Sorkine E. , Rubshtein A. , Larin V. S. , Manov V.
Published: July 2002

Static and high-frequency 1–30 MHz properties of the (Co100−xMnx)75B15Si10 microwires cast using the modified Taylor’s technique are reported. The hysteresis loop and the frequency dependence of permeability indicate that the longitudinal magnetization process occurs by coherent rotation of the moments. The values of static permeability are in good agreement with those predicted by the theory of ferromagnetic resonance where damping is taken into account. We observed a close correlation between the static permeability and the high-frequency limit (the frequency up to which the real part of permeability is still constant). For glass-coated CoMn microwires, this limit is considerably higher than that given by the Snoek equation.
Magnetocrystalline anisotropy in (111) CoPt3 thin film with growth-induced chemical anisotropy investigated by x-ray magnetic circular dichroism
Authors: Grange W. , Kappler J.-P. , Maret M. , Vogel J. , Fontaine A. , Pétroff F. , Krill G. , Rogalev A. , Goulon J. , Finazzi M. , Brookes N. B.
Published: July 2002

Angle-dependent x-ray magnetic circular dichroism (XMCD) experiments performed at both the Co and Pt L2,3 edges for a cubic CoPt3 thin film, which exhibits growth-induced chemical anisotropy, are used to determine the element specific magnetic anisotropy. The large decrease of the 3d orbital moment, observed when the spins are forced out the easy axis of magnetization by the applied magnetic field, is a consequence of a strong magnetocrystalline anisotropy (MCA). In addition, a weak but systematic variation of the Pt orbital moment indicates that the 5d atoms could play an important role in the MCA. These results correlate the microscopic XMCD evaluation of the MCA and the local structural anisotropy as observed with x-ray absorption fine structure (XAFS).
Application of spin-dynamics methods to a study of magnetization tunneling in many-spin systems
Authors: Dobrovitski V. V. , Harmon B. N.
Published: July 2002

Spin-dynamics methods were applied for the numerical study of magnetic tunneling in many-spin systems. A numerical technique based on the instanton approach was developed and the zero-temperature case as well as finite-temperature effects were considered. We show that even for a two-spin system results can differ considerably from those of a corresponding reduced one-spin (or collective spin) system. Moreover, it is argued that such a difference can take place for real materials such as magnetic molecules (e.g., Mn12). Thus, consideration of the many-spin nature of some systems is shown to be important.
Atomic structure and phase transitions in disordered Ti1−xGaxN thin films grown by pulsed laser deposition
Authors: Ren Jian Z. , Rose Garry A. , Williams R. Stanley , Booth Corwin H. , Shuh David K.
Published: July 2002

We report here the growth by pulsed laser deposition and characterization of metastable disordered Ti1−xGaxN alloy thin films on Al2O3(0001) substrates. X-ray diffraction and x-ray-absorption fine-structure analyses showed that the films contained a single rocksaltlike atomic structure for 0⩽ x<0.45, a single wurtzite-like structure for 0.75⩽x⩽1, and a mixture of both structures for 0.45⩽x<0.75. Over most of the composition range, the alloy films were predominantly amorphous with some fraction of nanocrystalline material present. Electrical conductivity measurements showed that the structural transition near x≈0.5 is accompanied by a metal–insulator transition. This study provides an increased understanding of the TiN–GaN pseudobinary phase field, which has potential technological implications for metallic contacts to GaN devices.
Spontaneous emission from InAs/GaSb quantum wells grown by molecular beam epitaxy
Authors: Bertru N. , Baranov A. N. , Cuminal Y. , Boissier G. , Alibert C. , Joullie A. , Lambert B.
Published: July 2002

We have investigated the spontaneous emission from InAs/GaSb quantum wells grown by molecular beam epitaxy. A strong photoluminescence signal was observed only from samples grown in a narrow substrate temperature range around 400 °C with InSb-like interfaces. From an analysis of the photoluminescence energy with the excitation density, we demonstrated the formation of triangular quantum wells for holes in the GaSb barriers close to the interfaces. Finally, the temperature dependence of the photoluminescence peak energy showed a deviation from the InAs band gap evolution at low temperature, which could be related to the type II broken gap band alignment.
Nanoscale metal transistor control of Fowler–Nordheim tunneling currents through 16 nm insulating channel
Authors: Fujimaru Kouji , Sasajima Ryouta , Matsumura Hideki
Published: July 2002

A nanoscale metal/insulator tunnel transistor (MITT) with a channel length of only 16 nm is fabricated by conventional photolithography, and its operation is experimentally studied. The MITT consists of two metal electrodes, an insulating channel inserted laterally between these two electrodes, and a third metal gate electrode formed upon the gate insulator above the insulating channel. The Fowler–Nordheim tunneling currents flowing from one metal electrode to the other through the insulating channel are controlled by applying a voltage to the gate electrode. It is found that the MITT can be operated similarly to the semiconductor transistor, and the feasibility of the nanoscale metal transistor is demonstrated.
Reflection of infrared radiation from lamellar gratings on a silicon wafer
Authors: Hava S. , Ivri J. , Auslender M.
Published: July 2002

Infrared reflectance spectra in the spectral range from 1.5 to 14 μm of a smooth-surface silicon sample cut from a Si wafer and grating structures fabricated on the same wafer have been measured and analyzed theoretically. The structural dimensions of the gratings are in the range of the radiation wavelengths. Two types of experiments have been conducted. In the first experiment we investigated specular reflectance. The results show that the reflectance spectra depend on the mounting type: planar- or conical-diffraction mounting, and on the side of the sample which is irradiated. The differences become very pronounced for wavelengths larger than the grating period. For the reflection spectra in the case of conical-diffraction mounting, reasonably good agreement between experimental spectra and theoretical zero-order efficiency spectra is obtained. In the case of planar-diffraction mounting an agreement between theory and experiment is achieved only if we assume an admixture of the higher diffraction orders. In the second experiment we studied the angular diffraction pattern from the different gratings in planar-diffraction mounting. We found that it depends strongly on the radiation incidence angle, the radiation wavelength and grating parameters. It is observed and confirmed by simulation that the reflection dip at about 9 μm due to vibrational transition in complex Si–interstitial O2 is enhanced by the grating structure.
Strain profiles in overcritical (001) ZnSe/GaAs heteroepitaxial layers
Authors: Kontos A. G. , Anastassakis E. , Chrysanthakopoulos N. , Calamiotou M. , Pohl U. W.
Published: July 2002

ZnSe layers of various thicknesses have been grown epitaxially on (001)-oriented GaAs substrates by metalorganic vapor phase epitaxy and studied by x-ray diffraction and Raman scattering. Consistent results have been found for the in-plane strains of both, ZnSe layers below and above the critical value of plastic relaxation. The experimental results are well described by strain profiles which are evaluated by an energy model and a geometrical model including the effects of strain and work hardening. The thickness-dependent full widths at half maximum of the x-ray reflections and the Raman resonances are accounted for by assuming uncorrelated misfit dislocations in the layers.
X-ray photoemission and absorption spectroscopy of supported nanoscale iron clusters
Authors: Edmonds K. W. , Baker S. H. , Thornton S. C. , Maher M. J. , Keen A. M. , Binns C.
Published: July 2002

Fe particles with sizes in the range 1–5 nm, formed by a gas-aggregation method and deposited onto graphite and C60 supports, were studied by x-ray photoemission spectroscopy, x-ray absorption spectroscopy, and magnetic linear dichroism. Clusters deposited onto a C60 coated graphite substrate become embedded within the fullerene film, and have an increased resistance to oxidation compared to exposed clusters supported on a graphite surface. No evidence for hybridization between the electronic states of Fe and C60 is seen. The magnetic dichroism signal of the exposed clusters increases sharply with the film thickness because of the increased cluster interactions.
Linear electrooptic effect in ordered (Al0.5Ga0.5)0.5In0.5P
Authors: Yamashita Kenichi , Kita Takashi , Nishino Taneo
Published: July 2002

We have studied linear electrooptic effects in long-range ordered (Al0.5Ga0.5)0.5In0.5P on GaAs (115)A and GaAs (001) by polarized electroreflectance spectroscopy. The electroreflectance spectra show anisotropic interference oscillations below the band gap. The amplitude of the oscillation signal linearly depends on the applied voltage modulated from the flat band. In strongly ordered (Al0.5Ga0.5)0.5In0.5P on the GaAs(001), an anisotropy of the oscillating signal caused by a difference of refractive indexes of ordinary and extraordinary direction was found. The amplitude of the [11̄0]-polarization spectrum is larger than that of the [110] polarization. Furthermore, the anisotropy becomes large with decreasing the detection energy. The dispersion of the anisotropic refractive index below the band gap depends on order parameter.
Time-resolved four-wave mixing signal in thick bulk GaAs
Authors: Yu Sungkyu , Lee Joo In , Viswanath Annamraju Kasi
Published: July 2002

We experimentally demonstrate that the temporal shape of the four-wave mixing (FWM) signal in thick semiconductors is significantly influenced by the absorption of the generated FWM signal during its propagation through the sample as well as the pulse broadening effect, by performing the time-integrated (TI) and time-resolved (TR) FWM experiments, and by measuring the pulse shape of the transmitted probe beam in the presence of a pump beam for a 500- and a 100-μm-thick undoped GaAs. We find that the temporal shape of the TR-FWM signal in the vicinity of the exciton resonance depends on the sample thickness and time delay between two incident pulses. As the excitation laser is tuned far below the exciton resonance, however, the propagation effect of the FWM signal itself through the sample becomes dramatically weaker and the temporal shape of the FWM signal resembles the distorted probe pulse shape. Furthermore, the peak intensities of the TI- FWM signals near the exciton resonance are smaller in the thicker sample than in the thinner sample, whereas in the thick sample are larger well below the exciton resonance, due to the absorption of the FWM signal.
Giant magnetoresistive spin valves with a strong exchange bias field and a weak interlayer coupling field
Authors: Lu Z. Q. , Lai W. Y. , Chai C. L.
Published: July 2002

Spin valve multilayers exhibiting enhanced giant magnetoresistive (GMR) effect and weak interlayer coupling as well as strong exchange biasing were fabricated by a two-step deposition procedure. The down sublayers (i.e., buffer layer Ta/free layer NiFe/interlayer Cu) were deposited at a lower argon pressure, then the upper sublayers (i.e., pinned layer NiFe/pinning layer FeMn/cover layer Ta) were deposited at a higher argon pressure. The former promoted formation of the strong (111) textures, smooth interfaces, and dense Cu film, resulting in a weak interlayer coupling. The latter promoted small domains and less diffusive interfaces, resulting in a strong exchange biasing and an enhanced GMR ratio. This shows that independent control of magnetoresistance, interlayer coupling, and exchange biasing is possible.
Photoluminescence study of carrier dynamics and recombination in a strained InGaAsP/InP multiple-quantum-well structure
Authors: Güçlü A. D. , Rejeb C. , Maciejko R. , Morris D. , Champagne A.
Published: July 2002

Carrier transport and recombination in a strained InGaAsP/InP multiple-quantum-well structure emitting at 1.55 μm are investigated experimentally and theoretically using both time-resolved photoluminescence and Monte Carlo simulations. A method for including carrier recombination in a Monte Carlo simulation is described. The calculated spectra are in good agreement with the experimental results. They show that nonradiative recombination is the principal recombination process in the first 100 ps and that the screened electron–phonon interactions are responsible for the high luminescence from the quantum-well higher-energy levels.
The influence of boron ion implantation on hydrogen blister formation in n-type silicon
Authors: Höchbauer T. , Walter K. C. , Schwarz R. B. , Nastasi M. , Bower R. W. , Ensinger W.
Published: July 2002

We have studied the formation of surface blisters in 〈100〉 n-type silicon following co-implantation with boron and hydrogen. The silicon substrates had four different n-type dopant levels, ranging from 1014 to 1019 cm−3. These substrates were implanted with 240 keV B+ ions to a dose of 1015 cm−2, followed by a rapid thermal anneal at 900 °C for 30–60 s to force the boron atoms into substitutional lattice positions (activation). The samples were then implanted with 40 keV H+ to a dose of 5×1016 cm−2. The implanted H+ distribution peaks at a depth of about 475 nm, whereas the distribution in the implanted B+ is broader and peaks at about 705 nm. To evaluate the role of the B+ implantation, control samples were prepared by implanting with H+ only. Following the H+ implantation, all the samples were vacuum annealed at 390 °C for 10 min. Blisters resulting from subsurface cracking at depths of about 400 nm, were observed in most of the B+ implanted samples, but not in the samples implanted with H+ only. This study indicates that the blistering results from the coalescence of implanted H into bubbles. The doping with B facilitates the short-range migration of the H interstitials and the formation of bubbles. A comparison of the observed crack depth with the depth of the damage peak resulting from the H+ implantation (evaluated by the computer code TRIM) suggests that the nucleation of H bubbles occurs at the regions of maximum radiation damage, and not at the regions of maximum H concentration. For given values of B+ and H+ doping, the blister density was found to decrease with increasing n-type doping, when the boron is activated. Blister formation was also observed in B+ implanted samples which had not been activated. In this case, the blister density was found to increase with increasing value of n-type doping.
A comprehensive kinetic meniscus model for prediction of long-term static friction
Authors: Chilamakuri Sameera K. , Bhushan Bharat
Published: July 2002

In some interfaces, the static friction force increases with an increase in the duration of stationary contact. In this study, a comprehensive kinetic meniscus model is proposed to explain this phenomenon, both for a single asperity and multiple asperity contacts at a liquid mediated interface. It is found that the static friction increases up to a certain equilibrium time after which it remains constant. The equilibrium time is dependent on the liquid film thickness, the liquid viscosity and the contact geometry. The developed model is applied to a rough textured disk and a smooth disk in the head-disk interface of a computer hard disk drive.
Scanning tunneling microscopy and spectroscopy characterization of ion-beam-induced dielectric degradation in ultrathin SiO2 films and its thermal recovery process
Authors: Watanabe Heiji , Baba Toshio , Ichikawa Masakazu
Published: July 2002

We have investigated dielectric degradation in ultrathin SiO2 films induced by ion-beam irradiation and its thermal recovery by using scanning tunneling microscopy (STM) and spectroscopy. Our STM results showed that although the ion-beam-induced damage uniformly spreads on the oxide, the dielectric properties are locally recovered by thermal annealing. We found that the tunneling spectra obtained from the ultrathin SiO2 films are sensitive to the process-induced damage, and observed both a leakage current through the damaged oxide film and the thermal recovery of the insulating features. In addition, we confirmed a local charging phenomena of the damaged oxide caused by electron traps around the STM tip.
Improved algorithm for modeling collision broadening through a sequence of scattering events in semiclassical Monte Carlo
Authors: Register Leonard F. , Hess Karl
Published: July 2002

An improved, more physically accurate algorithm is presented for modeling collision broadening in semiclassical Monte Carlo. More specifically, the method of connecting a sequence of collision broadened scattering events is addressed. Sequences of collision broadened scattering events are analyzed via basic theory and illustrated via first-principles simulations using the quantum transport simulator Schrödinger Equation Monte Carlo in which, in contrast to semiclassical Monte Carlo, real scattering and collision broadening are inseparable processes. It is demonstrated that the requirement for energy conservation in the coupled carrier-phonon system prohibits the accumulation of the uncertainty in the carrier energy associated with collision broadening through a sequence of scattering events, accumulation that is possible with the conventional treatment of collision broadening in semiclassical Monte Carlo. Based on this analysis, basic changes to the treatment of collision broadening in semiclassical Monte Carlo are suggested. The potential significance of these changes is then assessed via simplified semiclassical Monte Carlo simulations. In particular, the exaggeration of the high-energy tail of the carrier distribution possible with the conventional treatment of collision broadening in semiclassical Monte Carlo is demonstrated to be an artifact of the above described nonphysical accumulation of the collision broadening contributions, not of the form of the collision broadening used (Lorentzian, etc.) as often assumed; with this more physically accurate new algorithm, well-behaved high-energy tails of the carrier distribution are obtained even with full Lorentzian broadening.
Phase transitions of MnO to 137 GPa
Authors: Kondo Tadashi , Yagi Takehiko , Syono Yasuhiko , Noguchi Yuichi , Atou Toshiyuki , Kikegawa Takumi , Shimomura Osamu
Published: July 2002

An in situ x-ray study up to 137 GPa using a diamond anvil cell combined with synchrotron x-ray radiation has revealed three new phase transitions in MnO. The rhombohedral distortion from a B1 structure starts at about 30 GPa. The volume compression curve of the distorted phase is in good agreement with recent shock compression experiments, suggesting the possibility of a paramagnetic–antiferromagnetic transition associated with the increase of the Néel temperature (TN). A drastic change in the x-ray pattern was observed at about 90 and 120 GPa. The transition pressure of 90 GPa is also consistent with shock compression data. The phases above 90 GPa were expected to be metallic based on the highly reflective nature of the sample. The crystal structure of the high pressure phase above 120 GPa was successfully explained by a B8 (NiAs) structure as expected based on recent first principles calculations.
A percolation model for lifetime variability in polymeric materials under creep conditions
Authors: Zeifman Michael I. , Ingman Dov
Published: July 2002

The authors propose a microlevel theory of damage accumulation in polymeric materials. On the basis of experimental data on submicro- and microcracks, they relate the time-dependent strength distribution in polymeric materials under applied load to the distribution of the largest cluster size in a site percolation lattice well below the percolation threshold ρc. The empty sites fraction ρ in the percolation lattice is suggested to increase with a constant rate, evaluated using the model of a trigger-type molecular reaction described in the literature. Duxbury’s initial study of the largest cluster size distribution is extended by Monte Carlo simulations. While the latter distribution is confirmed to be double exponential, the characteristic largest cluster size (the distribution mode) is found to increase exponentially with ρ within the range ρ0<ρ≪ρc, ρ0 being a constant, although for small ρ<ρ0 the conventional logarithmic dependence is preferable. For the considered two-dimensional square lattice ρ0 is estimated to be 0.05. A failure criterion is formulated and a constitutive framework for the derivation of the time-dependent strength distribution and the structural reliability is defined. The lifetime statistics for a constant enhanced load is shown to be of the Weibull type, while for low loads a triple-exponential distribution is suggested. Unlike the competitive theories, the proposed model predicts that the Weibull shape parameter depends on the applied load by a simple inverse power law with the power defined by a fractal dimension of the critical microcrack aggregate. This prediction is confirmed based on two experimental sets of lifetime statistics, reported in the literature.
H + H2 reaction barrier: A fixed-node quantum Monte Carlo study
Authors: Barnett R. N. , Reynolds P. J. , Lester W. A.
Published: July 2002

The classical barrier height for the H+H2 exchange reaction, as well as the energies at two other points along the reaction path, are calculated using fixed-node quantum Monte Carlo (FNQMC). Several single-determinant importance functions are used at the saddle point in order to relate the quality of the importance function to the accuracy and precision of the final result. The computed barrier is an upper bound since the energy of H and of H2 is obtained exactly by FNQMC. Our best upper bound (9.70±0.13 kcal/mol) has a mean within 0.1 kcal/mol of the presumed exact value. This best bound is obtained with a single determinant, double-zeta basis importance function. Contrary to experience with expansion methods, it is found that an importance function with a basis set of near Hartree–Fock quality, as well as one derived from a spin-unrestricted SCF calculation, are among the least efficient and least accurate of the importance functions used. Specifically, a nodal surface appearing in the lowest energy molecular orbital in these functions apparently increases the FNQMC energy. The FNQMC energy at the two other points along the reaction path is found to agree with the most accurate CI results of Liu to within statistical error.
Critical properties of nonionic micellar solutions
Authors: Degiorgio Vittorio , Piazza Roberto , Corti Mario , Minero Claudio
Published: July 2002

Solutions of the nonionic amphiphile n-dodecyl octaoxyethylene glycol monoether (C12E8) in D2O and in salty H2O have been investigated by static and dynamic light scattering. It is found that the micellar properties are little influenced by the choice of the solvent, whereas the critical properties (in particular, the critical exponents γ and ν) show a marked solvent effect. The interpretation of the observed critical behavior in terms of first-order correction-to-scaling contributions is discussed.
Spatial correlation functions from computer simulations
Authors: Seaton N. A. , Glandt E. D.
Published: July 2002

A method is presented for the computation of the surface–surface and surface–void spatial correlation functions from computer simulations of heterogeneous media. The method is illustrated with Monte Carlo simulations of penetrable spheres (for which exact analytical solutions are known) and of impenetrable spheres. Doi’s bound on the fluid permeability of a dispersion of impenetrable spheres is calculated.
Subpicosecond resolution studies of solvation dynamics in polar aprotic and alcohol solvents
Authors: Castner Edward W. , Maroncelli Mark , Fleming Graham R.
Published: July 2002

Subpicosecond resolution measurements of the kinetics of dipolar solvation have been made. The time resolved Stokes shift of a dye molecule, LDS-750 was measured using the fluorescence upconversion technique in the solvents acetonitrile, DMSO, nitrobenzene, methanol, and n-butanol. The solvation dynamics in both aprotic and alcohol solvents occur on a time scale roughly given by the longitudinal relaxation time as predicted by simple continuum theories. The relaxation in nitrobenzene and butanol is nonexponential and the relaxation in methanol is significantly faster than the calculated time. These deviations from simple theory are discussed in the context of (i) the significance of high frequency dispersions in the dielectric response, (ii) translational contributions to the solvent relaxation, and (iii) molecular aspects of the solvation not accounted in the continuum description.
CARS spectroscopy of O2(1Δg) from the Hartley band photodissociation of O3: Dynamics of the dissociation
Authors: Valentini James J. , Gerrity Daniel P. , Phillips David L. , Nieh Jong-Chen , Tabor Kevin D.
Published: July 2002

Rotationally and vibrationally resolved CARS spectra of the O2(1Δg) photofragment produced by the photodissociation of O3 at 17 wavelengths between 230 and 311 nm are reported. The spectra are taken under collision-free conditions, therefore, they reveal the nascent rotational and vibrational state distributions of the O2(1Δg) photofragment. At all photolysis wavelengths studied the vibrational distribution peaks very sharply at v=0, although all energetically allowed vibrational states are observed. The rotational state distributions are narrow, and peak typically at high J. The rotational distribution shifts to lower J as the photolysis wavelength increases. These observations imply vibrationally adiabatic, rotationally impulsive energy release in the dissociation. The shape and width of the rotational distributions can be completely accounted for by the spread in the O3 thermal rotation and zero-point vibration contributions to the O2(1Δg) photofragment angular momentum. The most striking observation about the O2(1Δg) photofragment quantum state distribution is an apparent propensity for even-J states. Experiments with 18O enriched ozone indicate that this propensity is observed only for 16O16O, not for 18O16O, and by implication not for 17O16O. We show that this is the consequence of a selective depletion of only odd-J rotational states of 16O16O(1Δg) by a curve crossing to O2(3Σg), but an equal depletion of both even-J and odd-J rotational states of 18O16O and 17O16O(1Δg) by the curve crossing. The odd-J selectivity for 16O16O is a consequence of the restriction of 3Σg to only odd-J states, due to the requirement of even nuclear exchange symmetry for this homonuclear species with spin-zero nuclei. As a result of the different curve crossing behavior, the quantum yield for 3Σg is twice as great for 18O16O and 17O16O as it is for 16O16O, and this imposes a mass-independent

isotopic fractionation in the photodissociation: the O2(1Δg) fragments are depleted of 17O and 18O, while the O2(3Σg) fragments are enriched in these isotopes.
Absorption and dissociative photoionization cross sections of NH3 from 80 to 1120 Å
Authors: Samson James A. R. , Haddad G. N. , Kilcoyne L. D.
Published: July 2002

The total absorption, photoionization, and dissociative photoionization cross sections of ammonia have been measured from 80 to 1120 Å. All possible fragment ions have been observed including doubly ionized ammonia. The absolute ionization efficiencies have also been measured in this spectral range. The appearance potentials of the fragment ions have been measured and are compared with the calculated appearance potentials derived from published heats of formation and ionization potentials of the fragments.
A theoretical study of the solid–electrolyte solution interface. I. Structure of a hard sphere ion–dipole mixture near an uncharged hard wall
Authors: Dong W. , Rosinberg M. L. , Perera A. , Patey G. N.
Published: July 2002

In this paper we investigate the density and orientational structure of a molecular model electrolyte solution consisting of a mixture of charged hard spheres in a dipolar hard sphere solvent, located in the vicinity of a neutral hard wall. The electrolyte is symmetrically charged and all particles have the same size. The statistical mechanical treatment is the reference hypernetted-chain (RHNC) approximation for the bulk fluid and, respectively, the linearized RHNC (LRHNC) and the full RHNC approximations for the wall–molecule and the wall–ion correlation functions. Our results for the pure solvent are in good agreement with previous theoretical work and available computer simulations. It is found that in addition to the steric effects due to the molecular solvent, image forces play an essential role in the determination of the ionic density profile near the wall. Moreover, at moderate ionic concentrations, our results indicate that beyond the first layer of solution this electrostatic effect may be well described by a continuum description of the solvent.
Direct observation of the excited-state c
i
s–t
r
a
n
s photoisomerization of bacteriorhodopsin: Multilevel line shape theory for femtosecond dynamic hole burning and its application
Authors: Pollard W. Thomas , Cruz Carlos H. Brito , Shank Charles V. , Mathies Richard A.
Published: July 2002

A dynamic hole-burning study of light-adapted bacteriorhodopsin (BR568) using 6 fs optical pulses has recently been reported [R. A. Mathies, C. H. Brito Cruz, W. T. Pollard, and C. V. Shank, Science 240, 777 (1988)]. The temporal evolution of the excited state absorption and emission spectra after excitation with 60 fs pulses provides a direct observation of the C13=C14 torsional isomerization of the retinal chromophore on the excited state potential surface. Here, we present a more detailed discussion of these spectra. The transient hole line shapes are then calculated by solving the density matrix equations for the third-order susceptibility of a multilevel system. The resulting equations are written without reference to the individual vibronic transitions by using the absorption correlation function 〈i‖i(t)〉. The calculations show that the sharp features seen at short delays arise from coherence coupling effects which occur when the pump and probe pulses overlap in time. This analysis demonstrates that the hole seen at 60 fs is consistent with the broad homogeneous absorption line shape for BR568 originally predicted from resonance Raman intensities, and points out the utility of 〈i‖i(t)〉, derived from resonance Raman intensity analysis, in understanding femtosecond dynamic hole-burning experiments.
Examination of a generalized model for radiationless energy transfer in dyes. Comparisons of theory and experiments
Authors: Ali M. A. , Ahmed S. A. , Chokhavatia A. S.
Published: July 2002

A modified general model for radiationless energy transfer is examined. The model does not rely on selection criteria and is intended to apply over the range of diffusion and resonance energy transfer parameters normally encountered in quenching experiments. Experimental data is compared with theoretical predictions and found to confirm the model’s validity for a wide range of physical conditions.
The S1–S transition of indole and N-deuterated indole: Spectroscopy and picosecond dynamics in the excited state
Authors: Bickel Grant A. , Demmer David R. , Outhouse E. Allison , Wallace Stephen C.
Published: July 2002

A vibrational analysis for the excitation and fluorescence spectra of the S1–S0 transition in jet-cooled indole and N-deuterated indole is presented. This analysis yields the frequencies for eight low-lying S1 vibrational modes of both isotopic species. Single vibronic level fluorescence spectra also enable us to refine many of the S0 vibrational frequencies. Quantum interference effects in the energy-resolved fluorescence decays for one vibrational level of S1 indole are observed as a result of picosecond laser excitation. Through analysis of the fluorescence spectra and the modulated fluorescence decays, the coupling responsible for the quantum beats was found to result from a zero-order in-plane mode coupled to a level which is described by several zero-order out-of-plane modes. The effect of the deuterium substitution on the quantum beats is also investigated.
A predictive theoretical model for electron tunneling pathways in proteins
Authors: Onuchic José Nelson , Beratan David N.
Published: July 2002

A practical method is presented for calculating the dependence of electron transfer rates on details of the protein medium intervening between donor and acceptor. The method takes proper account of the relative energetics and mutual interactions of the donor, acceptor, and peptide groups. It also provides a quantitative search scheme for determining the important tunneling pathway(s) [specific sequence(s) of localized bonding and antibonding orbitals of the protein which dominate the donor–acceptor electronic coupling] in native and tailored proteins, provides a tool for designing new proteins with prescribed electron transfer rates, and provides a consistent description of observed electron transfer rates in existing redox labeled metalloproteins and small molecule model compounds.
Photochemical holes under pressure: Compressibility and volume fluctuations of a protein
Authors: Zollfrank J. , Friedrich J. , Fidy J. , Vanderkooi J. M.
Published: July 2002

From the pressure induced frequency shift of photochemical holes burnt into mesomorphyrin substituted horseradish peroxidase, we determined the compressibility of the protein and the vacuum frequency of the chromophore. From the compressibility, an estimation of the volume fluctuations of the biomolecule is possible.
Bond-orientational order in liquid Si
Authors: Wang Z. Q. , Stroud D.
Published: July 2002

We study bond-orientational order in liquid Si via Monte Carlo simulation in conjunction with empirical two- and three-body potentials of the form proposed by Stillinger and Weber. Bond-orientational order (BOO) is described in terms of combinations of spherical harmonic functions. Liquid Si is found to have pronounced short-range BOO corresponding to l=3, as expected for a structure with local tetrahedral order. No long-range BOO is found either in the equilibrium or the supercooled liquid. When the three-body potential is artificially removed, the tetrahedral bond-orientation order disappears and the liquid assumes a close-packed structure.
Fourier transform infrared-matrix isolation study of gaseous cerium dicarbide species
Authors: Feltrin A. , Guido M. , Cesaro S. Nunziante
Published: July 2002

For the first time, experimental evidence was obtained for the presence of a C=C bond in the structure of a gaseous rare-earth dicarbide species. A value of 1754.5 cm−1 could be unambiguously determined for the corresponding stretching frequency in the Ce12C2 molecule. Strong support was also obtained for the assignment of a weak broadband centered at 1720.5 cm−1 to analogous modes in 13C monosubstituted cerium dicarbide molecules. The possible assignment of a weak band at 789.1 cm−1 to stretching modes of Ce–C bonds in Ce12C2 molecules remains tentative. In order to fit the above assignments, the normal coordinate analysis was performed for each of the structures which preserve the C=C bond and the results were compared.
Rotational dynamics of nonpolar solutes in different solvents: Comparative evaluation of the hydrodynamic and quasihydrodynamic models
Authors: Roy Mita , Doraiswamy S.
Published: July 2002

The rotational reorientation times (τr) of substituted quaterphenyl and quinquephenyl molecules (neutral nonpolar solutes) of 4–5 Å radius have been measured in a series of n alkanes, n nitriles, and n alcohols, by picosecond fluorescence depolarization technique. The experimental data were compared with hydrodynamic and quasihydrodynamic models of Gierer Wirtz (GW) and Dote Kivelson Schwartz (DKS). Our studies reveal that these molecules rotated faster in alcohols compared to alkanes and nitriles. A nonlinear variation of τr vs viscosity (η) in alcohols was also observed, being more prominent for the smallest solute. The transition towards the stick hydrodynamic limit with increase in solute size was evident in the chosen set of solute probes. Both GW and DKS theories reproduce the nonlinear variation of τr vs η; however, DKS theory alone correctly predicted the faster rotation in alcohols. A test for quantitative prediction of τr of nonpolar solutes in alkanes and alcohols, using both GW and DKS models were made by compiling the existing data in literature. Our study indicates that DKS model agrees with experimental data in both solvents up to solute radius of ∼4.5 Å but fails beyond that. The GW model is seen to be adequate only for very small molecules. Thus, the ability of the quasihydrodynamic microfriction models to show a transition from microscopic to macroscopic hydrodynamic behavior with increase in solute size is lacking.