bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of silicon surface gratings with high-pulse-energy ultraviolet laser
Authors: Chen Cheng-Yen , Ma Kung-Jeng , Lin Yen-Sheng , Liu Chee-Wee , Hsu Chih-Wei , Chao Chung-Yen , Gurtler Steffen , Yang C. C.
Published: July 2002

We report the morphology, composition, and interaction mechanisms of silicon surface gratings fabricated with the fourth harmonic (266 nm) of a Q-switched Nd:YAG laser. We paid particular attention to the laser fluence dependence of silicon grating formation. It was found that at low fluence levels, grating formation was mainly caused by silicon oxidation. However, at high fluence levels gratings were formed with thermal ablation. In the former case, it was found that water vapor, instead of oxygen molecules, in the air was the key species providing oxygen for silicon oxidation. In the latter case, grating morphology was controlled by laser fluence level. These conclusions were supported by the measurement results of atomic force microscopy, energy-dispersive x-ray spectroscopy, Fourier-transform infrared spectroscopy, and chemical etching. The results of real-time monitoring of grating growth are also reported.
Trans-projected-range gettering of copper in high-energy ion-implanted silicon
Authors: Gueorguiev Y. M. , Kögler R. , Peeva A. , Mücklich A. , Panknin D. , Yankov R. A. , Skorupa W.
Published: July 2002

Strong gettering of Cu atoms beyond the projected ion range RP has been found in single-crystal Si implanted with P+ and As+ ions at MeV energies. We call this phenomenon the “trans-RP effect.” The formation of a separate Cu gettering band below RP, as detected by secondary ion mass spectrometry, indicates the presence of a significant amount of defects therein. These defects have not been detected by transmission electron microscopy and we suggest that they are small interstitial clusters. The amount of Cu atoms gettered beyond RP is, particularly for the P implants, much greater than that in the gettering layer at RP, indicating that the gettering ability of the point defects beyond RP is higher than that of the extended defects at RP. A mechanism responsible for their formation and clustering in the trans-RP region is proposed, and an explanation is given of the differences in the results for the P and As implants.
Quantum scattering calculations for the electronically nonadiabatic Br(2P1/2)+H2→HBr+H reaction
Authors: Takayanagi Toshiyuki , Kurosaki Yuzuru
Published: July 2002

Three-dimensional quantum scattering calculations have been carried out for the electronically nonadiabatic Br(2P1/2)+H2→HBr+H reaction. The calculations have been done using two methods: the time-independent hyperspherical close-coupling formalism for the total angular momentum quantum number J=0 and the generalized R-matrix propagation method with negative-imaginary potentials which absorb the reactive flux for J⩾0, but employing the coupled-states approximation for J>0. The (2×2) diabatic model, which was originally developed by Truhlar and co-workers, has been employed in the present calculations. The results calculated with the two methods agree very well with those obtained by Truhlar and co-workers, indicating that our results are numerically converged. Detailed analyses of the calculated probabilities show that the electronically nonadiabatic transitions from Br(2P1/2)+H2(ν) to Br(2P3/2)+H2(ν+1) effectively occur in the entrance region of the potential surface but that the contribution of the electronically nonadiabatic chemical reaction, Br(2P1/2)+H2(ν)→HBr+H, is small.
Experimental analysis of CO213 infrared stimulated emissions in solid argon
Authors: Chabbi H. , Dahoo P. R. , Dubost H. , Gauthier-Roy B. , Vasserot A.-M. , Abouaf-Marguin L.
Published: July 2002

Vibrational stimulated emissions of CO2 trapped in low temperature argon matrices have been found experimentally not only in the 16 μm region (v2 manifold) but also at 10 μm (v3−v1 transition) in the double trapping site of concentrated samples. A detailed experimental description of these emissions is reported, including spectral analysis, time-resolved studies, laser energy dependence, and concentration and temperature effects. The characteristics of the emissions are discussed, giving some insight into the nonradiative intramolecular V–V transfers from the laser excited level v3=1 towards the v2 manifold, and inside this manifold.
Photoabsorption spectra of argon cation clusters: Monte Carlo simulations using many-body polarization
Authors: Gascon Jose A. , Hall Randall W.
Published: July 2002

A simple, semiempirical model that includes many-body polarization is used to study the ground and excited state properties of ArN+ clusters (N=3–23) at 80 K. For purposes of comparison, a model that does not include many-body polarization is used to study clusters with N=3–27. Monte Carlo simulations are used to calculate the average properties of these clusters. The model is similar to one previously used to study argon cation clusters without many-body polarization. The photoabsorption spectrum is in good agreement with experiment; in particular, the photoabsorption spectra for cluster sizes 4–10 do not show the blueshift that is seen with models that do not include many-body polarization.
Temperature dependence of electron transmission through organized organic thin films
Authors: Haran Avner , Dimitrov Dimitre , Trakhtenberg Sofia , Naaman Ron
Published: July 2002

The temperature dependence of electron transmission through various organized organic thin films (OOTFs) was investigated. For most systems a strong dependence of the transmission efficiency on temperature was observed, even for a relatively small temperature range. The well defined structure of the OOTFs and the monitoring of the angular dependence of both the initial and the final velocities of photoelectrons were used to reveal the mechanism behind the temperature effect and further elucidate the transmission mechanism. A simple model, which assumes that the electron transmission yield is much higher along the organic chains than in any other direction, was able to reproduce the experimental observations. We find that the photoelectron transmission yield through OOTFs is extremely sensitive to the structural order in the film.
Characterization of surface defects on MgO thin films by ultraviolet photoelectron and metastable impact electron spectroscopies
Authors: Kolmakov A. , Stultz J. , Goodman D. W.
Published: July 2002

Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy are used in this study to investigate low-defect and defective MgO(100) thin films. Unlike low-defect films, defective films exhibit a new spectroscopic feature located ∼2 eV above the top of valence band. Exposing the defective film to oxygen quenches the emission of electrons from F centers created on the surface and in the subsurface regions. Extended defects, unseen in the MIES spectra of the clean surface, are detectable using NO titration. MIES and thermal programmed desorption indicate that at ∼100 K NO adsorbs dissociatively on defects, forming N2O. Only a small fraction of the MgO surface becomes covered with N2O at ∼100 K for the low-defect MgO film indicating that N2O molecules preferentially adsorb on the extended defects. The saturation coverage of N2O increases appreciably for the defective sample.
A mesoscale strength model for silica-filled polydimethylsiloxane based on atomistic forces obtained from molecular dynamics simulations
Authors: Hanson D. E.
Published: July 2002

We present a novel mesoscale model that describes the tensile stress of silica-filled polydimethylsiloxane (PDMS) under elongation. The model is based on atomistic simulations of a single chain of PDMS, interacting with itself and/or a hydroxylated silica surface. These simulations provide estimates of the microscopic forces required to stretch or uncoil a chain of PDMS, or detach it from a silica surface. For both stretching and detachment, we find that the internal potential energy is linear with the distance the chain end is moved, albeit with differing slopes. From these calculations and recent atomic force microscopy (AFM) experiments, we conclude that the forces are constant. We apply this analysis to the case of uncrosslinked, silica-filled PDMS systems and develop a mesoscale, inter-particle strength model. The strength model includes the atomistic forces determined from the simulations, a small entropic component, and a Gaussian probability distribution to describe the distribution of chain lengths of PDMS strands connecting two silica particles and the chain lengths in the free ends. We obtain an analytic stress/strain expression whose predictions agree with experiment. This model also suggests mechanisms to explain the phenomena of hysteresis and permanent set.
A Gibbs ensemble Monte Carlo study of phase coexistence in the solvent primitive model
Authors: Kristóf Tamás , Boda Dezsö , Szalai István , Henderson Douglas
Published: July 2002

The phase coexistence behavior of the solvent primitive model (SPM) is studied by constant pressure and temperature Gibbs ensemble Monte Carlo simulations. In the SPM, the ions are modeled with charged hard spheres, while the solvent molecules are represented by neutral hard spheres. Fluid–fluid phase separation into a salt poor and salt rich phase is found. At constant pressure, the critical temperature increases with respect to the critical temperature of the primitive model (PM) where no hard spheres are present. At constant temperature and for low pressures, the phase separation of the SPM transforms into the phase separation of the PM. For high pressures, it remains an open question whether there is an upper critical immiscibility pressure or whether a solid–fluid phase transition occurs first.
Semiempirical tight binding method study of small Ge and Sn clusters
Authors: Jo Chulsu , Lee Keeyung
Published: July 2002

The ionization potentials and cohesive energies of small Gen and Snn (n=2–13) clusters have been calculated using the semiempirical tight binding total energy method. Matrix elements were obtained from the Slater–Koster terms using the universal parameters of Harrison. The structures of clusters were determined by adopting well-known shapes and adjusting the bond lengths using our total energy scheme. The calculated ionization potentials of Ge and Sn clusters are in reasonably good agreement with the recent experiment and are found to exhibit the characteristic features of experimental data. The calculated cohesive energies are in very good agreement with the experimentally observed data for both Ge and Sn clusters.
H
3
O
+
/Cl
−
ion-pair formation in high-temperature aqueous solutions
Authors: Chialvo A. A. , Cummings P. T. , Simonson J. M.
Published: July 2002

The radial profiles of the mean force and corresponding potential of mean force for the Cl−⋯H3O+ pair are determined by constraint molecular dynamics of an infinitely dilute near-critical aqueous solution, as described by the SPC/E water model and either the Gertner–Hynes or the Kusaka et al. hydronium model. These profiles are used to test the prediction of a continuum primitive model, and to predict the ion-pair association constant. The reliability of these intermolecular potential models is assessed by comparing the predicted association constants with those determined experimentally by conductance and solubility measurements. This comparison suggests that the most accurate experimental data available for the association constant of HCl fall between the predictions of the two models, and tends to support the superiority of the Gertner–Hynes over the Kusaka et al. hydronium model. Moreover, the simulation results allow a quick test of the reliability of the simple continuum dielectric model to represent the solvation behavior of the ion-pair in solution.
High level ab initio molecular orbital theory study of the structure, vibrational spectrum, stability, and low-lying excited states of HOONO
Authors: Li Yumin , Francisco Joseph S.
Published: July 2002

Ab initio molecular orbital methods have been employed to determine the molecular structure, vibrational frequencies, and stability of HOONO. These parameters were determined using quadratic configuration interaction methods with Dunning’s correlation consistent basis sets. Low-lying excited states for HOONO have been determined using complete active space self-consistent field (CASSCF) and multireference internally contracted configuration interaction (MRCI) methods. The first excited state (1 1A″) is calculated to be 4.19 eV above the ground state of HOONO. Potential energy curves for the ground and excited states are presented.
Enhancement of the giant magnetoresistance in spin valves via oxides formed from magnetic layers
Authors: Gillies M. F. , Kuiper A. E. T.
Published: July 2002

An enhancement of the giant magnetoresistance effect is investigated in spin valves where oxide layers, which are formed from magnetic layers, are incorporated in the structure. Information about Co–Fe based nanooxide layer (NOL) is obtained via x-ray photoelectron spectroscopy and Rutherford backscattering spectrometry. Cross-section transmission electron microscopy is also used to explore the effect of an NOL on the polycrystalline structure of the spin valve.
Distribution of Nb and Co in an α-Fe/Nd2Fe14B-type nanocomposite
Authors: Chen M. W. , Sakai A. , Wang X. M. , Inoue A. , Sakurai T.
Published: July 2002

Atom probe field ion microscopy was employed to investigate the distribution of the alloying elements in a nanocrystalline Fe76.5Nd8Co8B6Nb1.5 (at. %) alloy consisting of soft magnetic α-Fe and hard magnetic Nd2Fe14B. It was found that Co atoms prefer to partition into Nd2Fe14B phase. The partitioning factor of Co in the hard magnetic phase is approximately 2 with respect to the soft magnetic phase, α-Fe. Atom probe concentration and integrated depth profiles showed that the Nb atoms segregate at the interfaces between the soft and the hard magnetic phases. Based on the atom probe results, the beneficial effects of Nb and Co on the microstructure and the hard magnetic properties of the nanocomposite are discussed.
Room temperature luminescence from (Si/SiO2)n (n=1,2,3) multilayers grown in an industrial low-pressure chemical vapor deposition reactor
Authors: Pucker G. , Bellutti P. , Spinella C. , Gatterer K. , Cazzanelli M. , Pavesi L.
Published: July 2002

A simple complementary metal–oxide–semiconductor compatible process for the preparation of very thin (1–5 nm thick) poly-Si layers embedded in SiO2 is presented. The process consists of repeated cycles of poly-Si deposition, oxidation, and wet etching steps. Periodic structures with up to three Si/SiO2 layers were grown using this process. Transmission electron microscopy analyses show that the layered structure can be conserved down to a Si layer thickness of 2 nm. For thinner layers the resulting structure is more granular like. Samples with a Si-layer thickness lower than 3 nm show room temperature photoluminescence at about 1.55 eV that shifts to higher energies when the thickness is further reduced. The maximum shift obtained with respect to the c-Si band gap is 0.55 eV. Intensity of the photoluminescence as a function of temperature shows a behavior similar to the one observed for 0 and one-dimensional Si structures. On the basis of the thickness dependence, the temperature dependence and the saturation studies, this emission is attributed to recombination of electron–hole pairs in quantum confined Si.
Multiphoton dynamics of H2 with 248 nm picosecond and femtosecond pulses
Authors: Bakker Bernard L. G. , Parker David H. , Samartzis Peter C. , Kitsopoulos Theofanis N.
Published: July 2002

Nonresonant excitation of H2 at 248 nm using 5 ps and 0.5 ps laser pulses with intensities 5×1011 and 5×1012 W/cm2, respectively, is studied by measurement of the angle-speed distributions of the H+ photofragment using velocity map imaging. Three main H+ production channels are observed: (a) three-photon dissociation to H(n=1)+H(n=2) followed by photoionization of H(n=2); (b) dissociative (auto)ionization following four-photon absorption to form H(n=1)+H++e−; and (c) autoionization from a doubly excited state producing a H2+ vibrational distribution peaking at (v+=2,7,13) and subsequent photodissociation of these vibrationally excited ions. In the neighborhood of the energy sum of three 248 nm photons lie transitions to the B′ 1Σu+(3pσ) vibrational continuum and the two bound states B″ 1Σu+(4pσ)(v=2) and D 1Πu+(3pπ)(v=4). These states are believed to play an important role in three-photon absorption and at the subsequent fourth photon level. Their effect on the observed kinetic energy and angular distributions is discussed.
Transparent conducting films of ZnO–ZrO2: Structure and properties
Authors: Qadri S. B. , Kim H. , Horwitz J. S. , Chrisey D. B.
Published: July 2002

Transparent conducting films comprised of ZnO and ZrO2, with ZrO2 content varying between 0 and 10 wt %, have been deposited using pulsed-laser deposition. These films have resistivities ranging between 5.4×10−4 and 5.6×10−2 Ω cm with an optical transparency of more than 85% for the 250–900-nm-wavelength range. For films with ZrO2 content of less than 5 wt %, the hexagonal ZnO structure is observed with Zr substituting at the Zn site. The structural, optical, and electrical properties of these films make them suitable for high-temperature electronic applications.
Defect generation in ultrathin silicon dioxide films produced by anode hole injection
Authors: DiMaria D. J.
Published: July 2002

A direct demonstration of defect generation in ultrathin silicon dioxide films due to the transport of holes through this layer is reported. These defects are observed only when the hole current to the cathode of the device exceeds the electron current to the anode. This condition is produced on p-channel field-effect transistors under negative gate-voltage-bias conditions with ultrathin gate oxide layers. These results are related to current reliability models which use anode hole injection and the defects produced to explain destructive breakdown of the oxide layer.
The superconducting quantum interference device microstrip amplifier: Computer models
Authors: Mück Michael , Clarke John
Published: July 2002

Computer models are presented for a microstrip amplifier based on a dc superconducting quantum interference device (SQUID). In this device, the signal is applied between one end of the spiral input coil and the square washer on which it is deposited. The amplifier exhibits substantial power gain when the signal frequency is such that a half wavelength is approximately equal to the length of the microstrip formed by the coil and the groundplane. The resonant frequency is lowered significantly by the inductance of the square washer transformed into the input coil; this reduction is consistent with predictions of a simple model and with analog simulations. With the washer grounded, the gain of the amplifier peaks at a frequency that is lowered from the unloaded resonant frequency by the damping of the resistance associated with the source. The position and magnitude of the peak are in good agreement with both a lumped circuit model and with a model representing the microstrip as a transmission line. When the counter electrode of the SQUID is grounded and the washer floats, feedback from the output of the SQUID to the input via the capacitance of the microstrip plays a major role and is well described by simulations using the transmission line model. Measurements of the input impedance of the microstrip amplifier show that the return loss can be positive or negative, depending on the sign of the feedback and whether the frequency is above or below the resonant frequency. This behavior is in good accord with simulations.
Townsend ionization coefficients of some argon-based mixtures in strong nonuniform electric fields
Authors: Krajcar Bronić Ines , Grosswendt Bernd
Published: July 2002

The reduced Townsend ionization coefficient α/P is derived from the measurement of the mean gas amplification factor in a low-pressure (P ⩽ 50 kPa) proportional counter filled with argon–propane and argon–(dimethyl-ether) mixtures of various concentrations. The results are compared with our previous α/P values for argon–isobutane mixtures [I. Krajcar Bronić and B. Grosswendt, Nucl. Instrum. Methods Phys. Res. B 142, 219 (1998)]. The range of the reduced electric field strengths was 7×104 V m−1 kPa−1⩽Sa⩽4×106 V m−1 kPa−1. In such strong electric fields electrons do not reach the equilibrium with the electric field and the reduced gas gain depends on the gas pressure. For the analysis of the experimental gas gain data we used the field gradient model of Townsend–Williams–Sara–Segur. We determined the pressure dependent coefficients A* and B* of the Townsend ionization coefficient α/P=A*×exp(−B*/Sa). We found that the coefficients increase (i) if the admixture concentration increases at constant pressure, and (ii) if the pressure decreases at constant admixture concentration. In both cases the coefficients increase with the increase of Sa. The effective ionization potential of the mixtures, Vi*=B*/A*, at low admixture concentrations reaches the ionization potential I of the admixture as a consequence of the transfer of energy from the excited metastable states of argon to ionization of the admixture through the Penning effect. When α/P vs Sa curves for mixtures of different concentrations at constant total pressure are compared, crossings of the α/P curves at a certain range of Sa values are observed in all kinds of mixtures and for all pressures. We show the necessity of the crossings by taking into account the mathematical properties of the function describing the ionization coefficient.
The unimolecular dissociation of the propionyl radical: A classical dynamics study
Authors: Martı́nez-Núñez Emilio , Peña-Gallego Angeles , Vázquez Saulo A.
Published: July 2002

The unimolecular dissociation of the propionyl radical to form CO and CH2CH3 was investigated by classical trajectory calculations. Various types of initial sampling conditions were employed: Microcanonical for energies ranging from 27.8 to 72.8 kcal/mol above the zero-point energy (ZPE), and selective excitations at 67.8 kcal/mol. A quasiclassical barrier sampling technique, which circumvents the problem of ZPE leakage, was also used for the calculation of product energy distributions. For energies above 43 kcal/mol, the computations showed that the intramolecular vibrational relaxation is not rapid as compared with the rate of reaction. On the other hand, it is found that vibrational modes associated to the CCO moiety are significantly coupled to the reaction coordinate, in agreement with the suggestion reported by Zewail and co-workers [J. Phys. Chem. 100, 9202 (1996)]. However, the calculations cannot predict the significant decrease of the dissociation rate observed upon deuterium substitution on the α-carbon. Product energy distributions and CO vibrational populations computed for the different excitation schemes are compared with those determined experimentally. For many ensembles, the fraction of the available internal energy resulting in CO vibration agrees with that estimated experimentally.
Coulomb blockade effects in anodically oxidized titanium wires
Authors: Schöllmann V. , Johansson J. , Andersson K. , Haviland D. B.
Published: July 2002

We investigate the electrical transport properties of narrow titanium (Ti) wires which are anodically oxidized through a resist mask. At temperatures below 1.2 K the current–voltage characteristics of the resulting Ti/TiOx nanostructures exhibit a zero-current state, characteristic of the Coulomb blockade. The blockade region can be modulated by a capacitively coupled gate electrode. The observed experimental features can be explained in terms of a single electron tunneling model of a multi-island device.
Deep levels in GaAs due to Si δ doping
Authors: Hubı́k P. , Krištofik J. , Mareš J. J. , Malý J. , Hulicius E. , Pangrác J.
Published: July 2002

δ(Si)-doped GaAs samples grown by metalorganic vapor phase epitaxy are studied by capacitance–voltage and deep level transient spectroscopy (DLTS) techniques. A detailed analysis of the DLTS signal (including spatial profiles) is performed. DLTS spectra exhibit a clear development depending on the sheet dopant concentration ranging from 5×1014 to 2×1016 m−2. Two observed peaks do not change its activation energy with the doping level while their amplitude increases rapidly when the doping rises. We assign them to defects generated by high silicon concentration, probably related to gallium vacancy. Another peak in the most densely doped sample seems to correspond to the DX level which is occupied near the δ layer. Peculiar features of the EL2 level are observed in δ-doped GaAs and explained by the band bending due to the dopant sheet. No indication of the emission from the quantum confinement states is found in DLTS spectra taken at temperatures 80–400 K.
Formation of a defect-free denuded zone in GaP substrate by thermal annealing
Authors: Okada Hiroshi , Ohmoto Sei-ichirou , Kawanaka Takao
Published: July 2002

The vacancy-type Frank dislocation loops in GaP, which are the origin of shallow etch pit defects, dissolve during heat treatment via outdiffusion of the originating point defects. This forms a defect-free denuded zone under the surface of a GaP wafer. The activation enthalpy of the diffusion process of the gallium vacancy (which corresponds to the formation of the denuded zone) is estimated to be 4.0 eV.
Magnetization switching of a magnetic wire with trilayer structure using giant magnetoresistance effect
Authors: Shigeto K. , Okuno T. , Shinjo T. , Suzuki Y. , Ono T.
Published: July 2002

The switching fields of magnetic wires with trilayer structure consisting of NiFe/Cu/Co were investigated using giant magnetic resistance effect. The switching fields of both magnetic layers were observed to be inversely proportional to wire width (150–520 nm). We found that the magnetization of the NiFe layer switches under much lower applied field than in the case of single layer structure by the assistance of the stray field from the magnetic charge of Co at the edge of the wire. Attaching a pad at one end of the wire causes drastic decrease of the switching field. We investigated pad shape dependence of the switching field of the Co layer. For the sample with a square pad we measured the temperature dependence of the switching field between 5 and 300 K. The dependence at low temperatures between 5 and 50 K can be described by the model on thermally assisted magnetization reversal over a simple potential barrier.
Four-band fiber-optic radiometry for determining the “true” temperature of gray bodies
Authors: Scharf V. , Katzir A.
Published: July 2002

Standard radiometric thermometry is based on measuring the infrared radiation emitted from a body in one broad spectral band. However, the “true” temperature, Tbody, can not be determined accurately if the emissivity, ε, and the ambient temperature, Troom, are not known or change during the measurement. We constructed a “four band” fiber-optic radiometer for measurement of Tbody near room temperature, where measurements of the emitted radiation were carried out in four spectral bands. The radiometer was analyzed theoretically, and was then calibrated and tested on black and gray bodies. Good correlation between theory and experiment was achieved.
Generalized rheology of active materials
Authors: Soukhojak Andrey N. , Chiang Yet-Ming
Published: July 2002

High-strain active materials often exhibit relaxation in their response to an electric or magnetic field. This phenomenon has been previously described by a loss factor (tangent of the phase shift). However, the loss factor cannot express frequency-dependent, nonperiodic time-dependent, or nonlinear responses, therefore more fundamental material constants are needed. We present a phenomenological model that describes the time- and frequency-dependent behavior of electromechanically (piezoelectric and electrostrictive) and magnetomechanically (magnetic shape memory and magnetostrictive) active materials. Expanding rheology, we introduce electrorheological and magnetorheological models incorporating time constants corresponding not only to the viscoelastic response, i.e., strain versus stress, but also: (1) polarization or magnetization versus field, (2) strain versus polarization or magnetization, (3) ferro- and antiferroelectric, ferro- and antiferromagnetic, or ferroelastic domain switching under applied field and/or stress. A single set of constitutive equations is obtained that can describe pure and mixed cases of ferroic, antiferroic, and nonferroic response. The simulated behavior agrees well with experimental data for both polycrystalline piezoelectrics and high electromechanical strain single crystals.
Generating microstructures with specified correlation functions
Authors: Sheehan N. , Torquato S.
Published: July 2002

A stochastic optimization technique has recently been developed that can reconstruct or construct random heterogeneous materials with specified statistical correlation functions. We demonstrate how this technique can be used to reconstruct a digitized image of an interpenetrating, isotropic ceramic-metal composite. In this case, the two-point probability function displays no short-range order and the image is reconstructed by optimizing in two orthogonal directions only. However, this technique results in artificial anisotropy in the unoptimized directions when one (re)constructs an image in which the isotropic two-point probability function exhibits appreciable short-range order. We show that by optimizing in more than two directions, one can effectively eliminate the artificial anisotropic effects for a system possessing significant short-range order. This is done by optimizing in three directions on a hexagonal grid and by optimizing in four directions on a square grid. Finally, an aspect of the nonuniqueness of the resulting structures is quantitatively examined.
Melting/freezing behavior of a fluid confined in porous glasses and MCM-41: Dielectric spectroscopy and molecular simulation
Authors: Sliwinska-Bartkowiak Malgorzata , Dudziak Grazyna , Sikorski Roman , Gras Roman , Radhakrishnan Ravi , Gubbins Keith E.
Published: July 2002

We report both experimental measurements and molecular simulations of the melting and freezing behavior of fluids in nanoporous media. The experimental studies are for nitrobenzene in the silica-based pores of controlled pore glass, Vycor, and MCM-41. Dielectric relaxation spectroscopy is used to determine melting points and the orientational relaxation times of the nitrobenzene molecules in the bulk and the confined phase. Monte Carlo simulations, together with a bond orientational order parameter method, are used to determine the melting point and fluid structure inside cylindrical pores modeled on silica. Qualitative comparison between experiment and simulation are made for the shift in the freezing temperatures and the structure of confined phases. From both the experiments and the simulations, it is found that the confined fluid freezes into a single crystalline structure for average pore diameters greater than 20σ, where σ is the diameter of the fluid molecule. For average pore sizes between 20σ and 15σ, part of the confined fluid freezes into a frustrated crystal structure with the rest forming an amorphous region. For pore sizes smaller than 15σ, even the partial crystallization did not occur. Our measurements and calculations show clear evidence of a novel intermediate “contact layer” phase lying between liquid and crystal; the contact layer is the confined molecular layer adjacent to the pore wall and experiences a deeper fluid–wall potential energy compared to the inner layers. We also find evidence of a liquid to “hexatic” transition in the quasi-two-dimensional contact layer at high temperatures.
Dynamics of harpooning studied by transition state spectroscopy. II. Li⋅⋅FH
Authors: Hudson A. J. , Oh H. B. , Polanyi J. C. , Piecuch P.
Published: July 2002

The van der Waals complex Li⋅⋅FH was formed in crossed beams and the transition state of the excited-state reaction, Li*(2p 2P)+HF→LiF+H, was accessed by photoexcitation of this complex. The dynamics of the excited-state reaction were probed by varying the excitation wavelength over the range 570–970 nm while recording the photodepletion of the complex. The findings were interpreted using high-level ab initio calculations of the ground and lowest excited-state potential-energy surfaces.
Asymptotics of electromagnetic radiation off a spherical superconductor
Authors: Wakano Masami , Wheeler John A.
Published: July 2002

Asymptotics of electromagnetic radiation off a spherical superconductor is analyzed in terms of creeping waves. Temporal damping behavior of the charge distribution on the surface when the sphere is rendered superconducting is studied and is compared with that on a superconductor with an infinite plane surface. Finally, the coherent states of light waves outside a spherical superconductor and a superconductor with an infinite plane surface are constructed.
The effect of unit lens alignment and surface roughness on x-ray compound lens performance
Authors: Pantell R. H. , Feinstein J. , Beguiristain H. R. , Piestrup M. A. , Gary C. K. , Cremer J. T.
Published: July 2002

The required alignment tolerances and surface roughness for unit lens elements in a compound refractive lens (CRL) for x rays are discussed. Contrary to what one might expect and what has been stated in the patent literature, alignment tolerances are large and for typical parameter values the effect of misalignment is minor. For a parabolic lens the focusing properties of the CRL are unaltered by misalignment and there is a small increase in absorption. For a lens with spherical aberration, there is a slight change in focal length, a minor translation of the image, and a small increase in absorption. This article also shows that lens gain is not appreciably reduced if the phase shift that is introduced by the roughness is limited to ±π/4 or if the transverse period of the roughness exceeds a specified value. The CRL can benefit from a managed misalignment of the elements to reduce the phase error introduced by surface imperfections of the lens.
Compressional acoustic wave generation in microdroplets of water in contact with quartz crystal resonators
Authors: McKenna L. , Newton M. I. , McHale G. , Lucklum R. , Schroeder J.
Published: July 2002

Resonating quartz crystals can be used for sensing liquid properties by completely immersing one side of the crystal in a bulk liquid. The in-plane shearing motion of the crystal generates shear waves which are damped by a viscous liquid. Thus only a thin layer of fluid characterized by the penetration depth of the acoustic wave is sensed by a thickness shear mode resonator. Previous studies have shown that the finite lateral extent of the crystal results in the generation of compressional waves, which may cause deviations from the theoretical behavior predicted by a one-dimensional model. In this work, we report on a simultaneous optical and acoustic wave investigation of the quartz crystal resonator response to sessile microdroplets of water, which only wet a localized portion of the surface. The relationship between initial change in frequency and distance from the center of the crystal has been measured for the compressional wave generation regions of the crystal using 2 and 5 μl droplets. For these volumes the initial heights do not represent integer multiples of a half of the acoustic wavelength and so are not expected to initially produce compressional wave resonance. A systematic study of the acoustic response to evaporating microdroplets of water has then been recorded for droplets deposited in the compressional wave generation regions of the crystals whilst simultaneously recording the top and side views by videomicroscopy. The data are compared to theoretically expected values of droplet height for constructive acoustic interference. Results are highly reproducible and there is good correlation between theory and experiment.
Stimulated-emission spectra of high-indium-content InGaN/GaN multiple-quantum-well structures
Authors: Chen Chii-Chang , Chuang Hui-Wen , Chi Gou-Chung , Chuo Chang-Cheng , Chyi Jen-Inn
Published: July 2002

High-indium-content InGaN/GaN multiple-quantum-well structures grown by low-pressure metalorganic chemical-vapor deposition have been studied by photoluminescence and optical pumping at room temperature and 25 K. The results show that the annealing process reduces the compositional fluctuation of indium content owing to diffusion and can make the quantum-well structures more identical everywhere in the active region. Several stimulated-emission peaks were observed in the spectra of optical pumping in edge-emitting geometry. These peaks result from interband transitions between quantized levels in the quantum well. By solving the time-independent Schrödinger equation, the interband transitions in the quantum well corresponding to each stimulated-emission peak in optical-pumping spectra can be identified. The band offset parameter ΔEC/ΔEg=0.38 was obtained.
Ultrafast all-optical switching in a silicon-based photonic crystal
Authors: Haché Alain , Bourgeois Martin
Published: July 2002

We study the effect of two-photon absorption and Kerr nonlinearity on the optical properties of a one-dimensional photonic crystal made with amorphous silicon and SiO2. A stop band appearing near 1.5 μm is monitored with a weak probe beam and modulated by changes in the refractive index caused by a pump pulse at 1.71 μm with 18 GW/cm2 peak intensity. Nonlinear optical characterization of the sample using Z-scan points out to two-photon absorption as the main contributor to free carrier excitation in silicon at that power level. Modulation in the transmittance near the band edge is found to be dominated by the optical Kerr effect within the pulse overlap (∼400 fs) whereas free carrier index changes are observed for 12 ps.
Photodissociation dynamics of CBrClF2 at 157.6 nm. II. A theoretical study using wave packet propagation
Authors: Yokoyama Keiichi , Yokoyama Atsushi , Takayanagi Toshiyuki
Published: July 2002

Photodissociation dynamics from the repulsive nσC−X* (X=Cl,Br) states of CBrClF2 has been studied by propagating wave packets on the two diabatic potential energy surfaces obtained by an approximate diabatization of ab initio complete active space self-consistent field (CASSCF) potential energy surfaces. The diabatization was carried out so as to eliminate the transition-dipole moment between the two excited states. The results have confirmed the occurrence of simultaneous triple dissociation CBrClF2→Br+Cl+CF2 observed in the 157.6 nm photolysis of CBrClF2. The triple dissociation has been found to occur on both the upper and lower adiabatic surfaces with oscillation in the population in between.
Relativistic effects on the optical response of InSb by time-dependent density-functional theory
Authors: Kootstra F. , de Boeij P. L. , Aissa H. , Snijders J. G.
Published: July 2002

In this paper we show how relativistic effects can be included in the time-dependent density-functional theory (DFT) for the optical response properties of nonmetallic crystals. The dominant scalar relativistic effects have been included using the zeroth-order regular approximation (ZORA) in the ground-state DFT calculations, as well as in the time-dependent response calculations. We show that this theory can also be applied to indium antimonide in the zinc-blende structure, notwithstanding the fact that it turns into a semimetal when scalar relativistic effects are included. Results are given for the band structure, the static dielectric constant ε∞ and the dielectric function ε(ω), for the various levels on which relativity can be included, i.e., nonrelativistic, only in the ground-state, or also in the response calculation. Comparisons of our calculated results are made with experiment and other theoretical investigations. With the inclusion of scalar relativistic effects, the band structure of InSb becomes semimetallic within the local density approximation and we find a deviation of 5% from experiment for the static dielectric constant. Also the dielectric function is improved and the spectra are in good agreement with experiment, although the spectral features are shifted to somewhat lower energies compared to experiment.
Phase equilibria and clustering in size-asymmetric primitive model electrolytes
Authors: Yan Qiliang , de Pablo Juan J.
Published: July 2002

The low-temperature phase coexistence of size-asymmetric primitive model electrolyte solutions has been investigated by means of Monte Carlo simulations. A multidimensional parallel tempering method is employed and results are analyzed by means of histogram reweighting. Coexistence curves and critical constants are determined as a function of size asymmetry, λ=σ+/σ−, from 0.05 to 1. It is found that the critical temperature and the critical density decrease as λ decreases. These trends appear to contradict available integral-equation theoretical predictions. For highly asymmetric systems, we report the formation of large chain-like and ring-like structures. These clusters are much larger than those observed in symmetric electrolytes, and they are shown to give rise to considerable finite-size effects.
Temperature dependence of surface photovoltage of bulk semiconductors and the effect of surface passivation
Authors: Datta Shouvik , Gokhale M. R. , Shah A. P. , Arora B. M. , Kumar Shailendra
Published: July 2002

Surface photovoltage (SPV) of n-GaAs decreases both above and below a certain transition temperature. We explain this phenomenon in terms of a Schottky contact based model and relative dominance of thermal and nonthermal parts of the dark current. This also explains the observed increase of SPV of p-GaAs below room temperature. Our analysis is further confirmed from the temperature dependence of the SPV measurements on p-InP and n-InP samples. Surface passivation is seen to lower the transition temperature of n-GaAs.
An apparatus for beam-rocking reflection high-energy electron diffraction and total reflection angle x-ray spectroscopy
Authors: Yamanaka Toshiro , Ino Shozo
Published: July 2002

Characteristic x-ray emission from surface atoms induced by irradiation of an electron beam during reflection high energy electron diffraction (RHEED) depends on the incident glancing angle θg, since distribution of electron current density near surfaces depends on θg. Depth distribution of elements, adatom height, and adatom site can be determined from θg dependence of x-ray emission. This article describes an apparatus for measuring θg dependence of x-ray emission during RHEED. X rays from the surface are sensitively detected at a grazing take-off angle θt close to the critical angle for the total reflection. A small electron gun is rotated around the sample in ultrahigh vacuum to scan θg, during which the sample and x-ray detector are fixed.
An expansion of general validity for the diffusive parameters of a charged particle in a zero-point field
Authors: Battezzati M.
Published: July 2002

The following calculations provide expressions for the drift velocity and the average diffusion coefficient, applying to diffusion in configuration space of a nonrelativistic charged particle interacting with the zero-point field of stochastic electrodynamics. The particle is assumed to evolve according to a widely popular reduced form of the Braffort–Marshall equation, which is free from runaway solutions, and supposed to be found in stationary conditions, with regard to the average values of physical quantities. The results show an interesting similarity with the equations of quantum mechanics.
Electric field induced instabilities at liquid/liquid interfaces
Authors: Lin Zhiqun , Kerle Tobias , Baker Shenda M. , Hoagland David A. , Schäffer Erik , Steiner Ullrich , Russell Thomas P.
Published: July 2002

External electric fields were used to amplify thermal fluctuations at the interface between two thin liquid films. Similar to the results shown previously for the enhancement of fluctuations at the polymer/air interface, interfacial fluctuations having a well-defined wavelength were enhanced with a characteristic growth rate. A simple theoretical framework to describe the experimental observations is presented. Both experiment and model calculation show a substantial reduction in feature size as a result of the change in surface/interfacial energy when going from the thin film to the bilayer case. Experimentally, features develop nearly 50 times faster for the bilayers in comparison to the polymer/air case. These results point to a simple route by which the nanoscopic feature can be easily and rapidly produced or replicated.
Non-Gaussian dynamics of a dilute hard-sphere gas
Authors: Yamaguchi T. , Kimura Y.
Published: July 2002

We have calculated nonlinear time correlation functions of a dilute hard-sphere gas numerically by using the Monte Carlo method, to find that the single particle dynamics in a dilute hard-sphere gas does not follow the Gaussian process. The deviation of the self-part of the dynamic structure factor from a Gaussian function is observed. This non-Gaussian character corresponds to those of Lennard-Jones liquids reported by Itagaki et al. [K. Itagaki, M. Goda, and H. Yamada, Physica A 265, 97 (1999)], if we scale the time unit by the collision frequency. Further, we trapped a particle in a harmonic well and calculated the time development of its distribution, in order to clarify the effect of collisions to the solvation dynamics. Both the Gaussian and the linear response assumptions are broken, and the deviation becomes larger as the curvature of the harmonic well gets larger.
Electrostatic surface waves in dense dusty plasmas with high fugacity
Authors: Rao N. N. , Stenflo L. , Shukla P. K.
Published: July 2002

The propagation of electrostatic surface modes on a thin dusty plasma slab has been investigated in different dust fugacity regimes. For wave frequencies much smaller than the grain charging frequency and for long wavelengths, it is shown that dusty plasmas support three different types of surface modes, namely, the dust–acoustic surface wave (DASW), the dust charge–density surface wave (DCDSW) and the dust–Coulomb surface wave (DCSW) which exist, respectively, in the low fugacity (tenuous), medium fugacity (dilute) and high fugacity (dense) regimes. In the short wavelength range, there exists a new kind of surface mode called “dusty plasma surface wave” (DPSW) which has no counterparts in the volume modes. This new mode exists over a wide range of dusty fugacity (from tenuous to dense regime). On the other hand, for frequencies much larger than the grain charging frequency, the DASWs exist also in the dilute regime, while the DCSWs are found to be absent. In the short wavelength limit, the DPSWs continue to exist even in the high-frequency range. Explicit dispersion relations as well as the damping rates due to the grain charge fluctuations are derived in each case. The results obtained for the various surface modes are compared with those for the case of volume modes.