bluuebunny/crossref_metadata_2025 huggingface dataset:
The optical gain mechanism in solid conjugated polymers
Authors: Schweitzer B. , Wegmann G. , Giessen H. , Hertel D. , Bässler H. , Mahrt R. F. , Scherf U. , Müllen K.
Published: July 2002

We report on stimulated emission from a solid conjugated methyl-substituted ladder-type poly(paraphenylene). The threshold for stimulated emission as well as the spectral narrowing is measured. Upon exciting within the low energy tail of the S1-S0 (0–0) absorption band narrow emission lines offset from the excitation by the energies of the dominant vibrational modes are observed. From these measurements we conclude that vibronic sublevels play a major role in the optical gain mechanism. Hence, the gain mechanism can be explained in terms of molecularly excited states.
Photoluminescence and ultraviolet lasing of polycrystalline ZnO thin films prepared by the oxidation of the metallic Zn
Authors: Cho Sunglae , Ma Jing , Kim Yunki , Sun Yi , Wong George K. L. , Ketterson John B.
Published: July 2002

We report a simple method for preparing polycrystalline ZnO thin films with good luminescent properties: the oxidization of metallic Zn films. In photoluminescence (PL) studies at room temperature for wavelengths between 370 and 675 nm, we have observed a single exciton peak around 390 nm without any deep-level emission and a small PL full width at half maximum (23 meV), indicating that the concentrations of the defects responsible for the deep-level emissions are negligible. We have also observed optically pumped lasing action in these films. The threshold intensity for lasing was ∼9 MW/cm2.
Silicon dopant imaging by dissipation force microscopy
Authors: Stowe T. D. , Kenny T. W. , Thomson D. J. , Rugar D.
Published: July 2002

Noncontact damping of a cantilever vibrating near a silicon surface was used to measure localized electrical dissipation. The dependence of the damping on tip-sample distance, applied voltage, carrier mobility, and dopant density was studied for n- and p-type silicon samples with dopant densities of 1014–1018 cm−3. Dopant imaging with 150 nm spatial resolution was demonstrated.
Microwave properties of tetragonally distorted (Ba0.5Sr0.5)TiO3 thin films
Authors: Kim W. J. , Chang W. , Qadri S. B. , Pond J. M. , Kirchoefer S. W. , Chrisey D. B. , Horwitz J. S.
Published: July 2002

A strong correlation is observed between the structure and the microwave dielectric properties of epitaxial Ba0.5Sr0.5TiO3 (BST) thin films deposited onto (001) MgO by pulsed laser deposition. Films were deposited at 750 °C in an oxygen pressure that was varied from 3 to 1000 mTorr. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D=a/c) of the films depends on the oxygen deposition pressure. D varied from 0.996 at 3 mTorr to 1.003 at 800 mTorr. At microwave frequencies (1–20 GHz), BST films with low distortion have higher dielectric constants (ε∼500), and lower dielectric loss (tan δ∼0.02) compared to films with higher distortion. The correlation of the microwave properties with the film structure can be attributed to stresses and polarizability in the film. The BST film grown at the oxygen deposition pressure of 50 mTorr exhibits a large dielectric constant change and a low dielectric loss at the same time, which corresponds to the film in low stress (D=1.0004). For tunable microwave applications, BST films with low stress are desirable in order to achieve both low dielectric loss and large tunability.
Microlaser made of disordered media
Authors: Cao H. , Xu J. Y. , Seelig E. W. , Chang R. P. H.
Published: July 2002

We have fabricated microlasers with disordered semiconductor nanoparticles. The physical mechanism of optical confinement is based on Anderson localization of light in micrometer-sized random media. Pulsed lasing occurs around 380 nm at room temperature under optical pumping.
Second-order perturbation corrections to singles and doubles coupled-cluster methods: General theory and application to the valence optimized doubles model
Authors: Gwaltney Steven R. , Sherrill C. David , Head-Gordon Martin , Krylov Anna I.
Published: July 2002

We present a general perturbative method for correcting a singles and doubles coupled-cluster energy. The coupled-cluster wave function is used to define a similarity-transformed Hamiltonian, which is partitioned into a zeroth-order part that the reference problem solves exactly plus a first-order perturbation. Standard perturbation theory through second-order provides the leading correction. Applied to the valence optimized doubles (VOD) approximation to the full-valence complete active space self-consistent field method, the second-order correction, which we call (2), captures dynamical correlation effects through external single, double, and semi-internal triple and quadruple substitutions. A factorization approximation reduces the cost of the quadruple substitutions to only sixth order in the size of the molecule. A series of numerical tests are presented showing that VOD(2) is stable and well-behaved provided that the VOD reference is also stable. The second-order correction is also general to standard unwindowed coupled-cluster energies such as the coupled-cluster singles and doubles (CCSD) method itself, and the equations presented here fully define the corresponding CCSD(2) energy.
Calibration techniques and datatype extraction for time-resolved optical tomography
Authors: Hillman Elizabeth M. C. , Hebden Jeremy C. , Schmidt Florian E. W. , Arridge Simon R. , Schweiger Martin , Dehghani Hamid , Delpy David T.
Published: July 2002

This article describes the preprocessing and calibration methods currently applied to data acquired with the University College London multichannel time-resolved optical tomography system. We briefly outline the imaging system and describe the features of our experimentally collected data, sources of stochastic noise, and systematic errors. We examine two methods of calibrating data: “difference imaging” using two image data sets with and without the features of interest to produce an image, and “absolute imaging” using an independent calibration measurement. We describe the methods developed to apply each calibration to raw data. Although the difference imaging performed is found to produce images with fewer artifacts, analysis indicates that it will not be directly applicable for clinical applications. Also examined are the effects of using a two dimensional (2D) reconstruction scheme to produce images from measured data. For absolute imaging, artifacts are shown to dominate such images even in the case of a homogeneous third dimension. The feasibility of deriving an ad-hoc correction factor to allow the use of a 2D reconstruction for measured data is examined, and is shown to reduce artifact. Difference imaging is demonstrated to be more robust to such effects.
Adhesion of micrometer-sized polymer particles under a scanning electron microscope
Authors: Miyazaki Hideki T. , Tomizawa Yasushi , Saito Shigeki , Sato Tomomasa , Shinya Norio
Published: July 2002

Techniques for manipulating micrometer-sized objects and assembling them into a microstructure in a scanning electron microscope (SEM) are important for research related to microscale physics. It has been demonstrated that micro-objects ranging from sub-μm to several 10 μm can be freely manipulated by adhering them to the tip of a probe. However, the present micromanipulation technique in a SEM is still inefficient, because little is known about the adhesion mechanisms of micro-objects in a SEM environment. In this study, the adhesion forces of micrometer-sized polymer particles deposited on a substrate during SEM observation have been directly measured. The adhesion forces between a polyvinyltoluene sphere of 1 μm radius deposited on a Au substrate, and a glass probe with a hemispherical tip with a typical radius of 0.75 μm coated with Au, were found to show various complicated behaviors. An irreversible increase in the adhesion forces initiated by the electron-beam (EB) irradiation, and the dependence of the adhesion on the electron flux and the probe voltage were observed. On the other hand, the dependence on the pressing force and the probe diameter, predicted by a conventional theory, was not confirmed. This observed complicated phenomena were successfully explained using the model based on the formation of an electric double layer at the contact interface by the EB irradiation, and the successive progress of creep deformation.
Design of an optical cell for pulse radiolysis of supercritical water
Authors: Takahashi Kenji , Cline Jason A. , Bartels David M. , Jonah Charles D.
Published: July 2002

The design of a flow cell that is applicable to pulse radiolysis/transient absorption experiments on supercritical water is described. The cell is designed to minimize dead volume and prevent the accumulation of radiolytic products. It is also necessary to minimize emission and absorption of sapphire windows from high energy electron beam irradiation. To obtain an optical throughput of f/4, the inner diameter is 6 mm, and distance between windows is 25 mm. The effective optical path length is 20 mm for irradiation from the side through a thin Hastelloy wall. Belleville spring washers were used to keep a constant force on the 3 mm sapphire windows, which were sealed to the Hastelloy body with copper gaskets. An application of this cell to measurements of solvated electrons in supercritical water is demonstrated.
In situ monitoring of GaN metal-organic vapor phase epitaxy by spectroscopic ellipsometry
Authors: Peters S. , Schmidtling T. , Trepk T. , Pohl U. W. , Zettler J.-T. , Richter W.
Published: July 2002

Epitaxy of high-quality GaN on sapphire requires a rather sophisticated substrate preparation prior to the GaN epilayer growth, namely nitridation of the substrate’s surface, growth of a GaN nucleation layer at a relative low temperature, and reduction of the defect density of this layer by a subsequent annealing step. For studying both, the detailed mechanisms of this complex procedure and its growth parameter dependencies, we attached an in situ spectroscopic ellipsometer to a nitride metal-organic vapor phase epitaxy reactor. First, the high-temperature dielectric function of GaN was measured using samples from different suppliers. Based on these data, the effect of growth parameter variations on the crystal quality of GaN epilayers could be monitored in situ. In particular, we determined the threshold temperature and the duration of the substrate nitridation under ammonia as well as the thermal threshold and duration of the nucleation layer transformation. Additionally, based on the in situ measurements a qualitative estimate for the crystalline quality of the nucleation layer and the epilayer is provided. Finally, the surface roughness of differently prepared GaN layers was evaluated by using the high-energy spectroscopic range of our vacuum-ultraviolet ellipsometer (3.5–9.0 eV).
Role of ion diamagnetic effects in the generation of large scale flows in toroidal ion temperature gradient mode turbulence
Authors: Smolyakov A. I. , Diamond P. H. , Medvedev M. V.
Published: July 2002

The secondary instability of large scale flows, such as zonal flows and streamers, in toroidal ion temperature gradient turbulence is investigated. It is shown that diamagnetic effects (finite pressure fluctuations), such as those for toroidal ion temperature gradient modes, significantly increase the total Reynolds force. In general, pressure fluctuations have a finite phase shift relative to the electrostatic potential. It is shown that both parts (in-phase and out-of-phase) contribute to the diamagnetic Reynolds stress tensor. Also, the out-of-phase component of the pressure fluctuations is responsible for anomalous energy flux. It is shown here, that a specific contribution of the out-of-phase component to the Reynolds stress tensor allows us to decouple zonal flow dynamics from slow evolution of the ion pressure profile. General equations for the zonal flow and streamer instabilities in toroidal ion temperature gradient turbulence are derived and the growth rates are determined.
Composition, temperature, and crystal orientation dependence of the linear electro-optic properties of Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals
Authors: Barad Y. , Lu Yu , Cheng Z.-Y. , Park S.-E. , Zhang Q. M.
Published: July 2002

The linear electro-optic (EO) coefficients of Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals over a broad composition range were investigated at temperatures from −20 to 80 °C. The orientation effect on the EO coefficients was also examined. For crystals poled in the 〈001〉 direction, a large r33 was observed near the morphotropic phase boundary (MPB). More importantly, r33 was found to be independent of temperature for the crystals on the tetragonal side of the MPB. In contrast, r13 was nearly zero for all compositions examined at all the temperatures. The large r33 near the MPB and the observed crystal orientation effect of the EO coefficients were also analyzed.
Control of GaN surface morphologies using plasma-assisted molecular beam epitaxy
Authors: Heying B. , Averbeck R. , Chen L. F. , Haus E. , Riechert H. , Speck J. S.
Published: July 2002

The characteristic surface morphologies of GaN grown by plasma-assisted molecular beam epitaxy under various growth conditions have been investigated. Three growth regimes (one N stable and two Ga stable) are identified on a surface structure diagram (Ga/N ratio versus substrate temperature). The boundary between the N-stable regime (low Ga/N ratios) and the two Ga-stable regimes (high Ga/N ratios) is determined by the growth rate of the films and is constant over the range of substrate temperatures investigated. The boundary between the two Ga-stable regimes (the Ga-droplet regime and the intermediate regime) is determined by the formation of Ga droplets and has an Arrhenius dependence with substrate temperature. The characteristic morphologies of films grown within each of these regimes are investigated using atomic force microscopy and transmission electron microscopy. N-stable films have rough, heavily pitted morphologies. Films grown within the intermediate phase have areas of flat surface between large, irregularly shaped pits. The pits observed for films grown within both regimes are found to initiate from threading dislocations and to decrease in density with increasing Ga/N ratio at constant temperature. Ga-stable films, grown within the Ga-droplet regime, exhibit atomically flat surfaces with no pit features. The morphology transitions are associated with changes in the growth kinetics caused by variations in the coverage of the GaN surface by excess Ga.
Hydrodynamic interactions in long chain polymers: Application of the Chebyshev polynomial approximation in stochastic simulations
Authors: Jendrejack Richard M. , Graham Michael D. , de Pablo Juan J.
Published: July 2002

We have simulated Brownian bead-spring chains of up to 125 units with fluctuating hydrodynamic and excluded volume interactions using the Chebyshev polynomial approximation proposed by Fixman [Macromolecules 19, 1204 (1986)] for the square root of the diffusion tensor. We have developed a fast method to continuously determine the validity of the eigenvalue range used in the polynomial approximation, and demonstrated how this range may be quickly updated when necessary. We have also developed a weak first order semiimplicit time integration scheme which offers increased stability in the presence of steep excluded volume potentials. The full algorithm scales roughly as O(N2.25) and offers substantial computational savings over the standard Cholesky decomposition. The above algorithm was used to obtain scaling exponents for various static and zero shear rate dynamical properties, which are found to be consistent with theoretical and/or experimental predictions.
Excited states theory for optimized orbitals and valence optimized orbitals coupled-cluster doubles models
Authors: Krylov Anna I. , Sherrill C. David , Head-Gordon Martin
Published: July 2002

We introduce an excited state theory for the optimized orbital coupled cluster doubles (OO-CCD) and valence optimized orbital coupled cluster doubles (VOO-CCD) models. The equations for transition energies are derived using a similarity transformed Hamiltonian. The effects of orbital relaxation are discussed. We present results for several single-reference molecules (H2O, CH2O, C2H4O, C2H4, BeO), as well as for molecules with significant nondynamical correlation in the ground state (CH+, BH, Ã 1A1 CH2), and for rectangular O4+. We find that: (i) OO-CCD excitation energies are very close to CCSD excitation energies; (ii) similarly to the complete active space SCF (CASSCF) model, the effects of orbital relaxation are very important for VOO-CCD excited states such that the excitation energies calculated by VOO-CCD and CASSCF with orbitals optimized for the ground state are very close to each other and unsatisfactory; (iii) the VOO-CCD model with an approximate treatment of orbital relaxation describes singly (valence and Rydberg) and doubly (valence) excited states within errors of 0.2–1.0 eV at equilibrium geometries and along bond-breaking coordinates; (iv) the above accuracy of the VOO-CCD model does not degrade as molecules or basis sets grow in size; (v) the shapes of potential energy surfaces around excited states minima are reproduced well by VOO-CCD model suggesting the use of this method for excited states geometry optimization.
Memorized polarization-dependent light scattering in rare-earth-ion-doped glass
Authors: Qiu Jianrong , Kazanski P. G. , Si Jinhai , Miura K. , Mitsuyu T. , Hirao K. , Gaeta Alexander L.
Published: July 2002

We report the observation of memorized polarization-dependent light scattering in a Eu2+-doped fluoroaluminate glass sample. Anisotropic light scattering along the plane of the light polarization was observed in the glass sample after the excitation of a focused 800 nm, 150 fs laser beam at a repetition rate of 200 kHz. When we changed the direction of the light polarization and irradiated the same location, we observed an anisotropic light-scattering pattern identical to the original one at the beginning, but then observed a new pattern along the new direction of the light polarization while the original light-scattering pattern disappeared gradually with the passage of time. This observed phenomenon was considered to be due to the light scattering of the polarization-dependent permanent microstructure induced by the polarized ultrashort pulsed laser itself.
The active-space equation-of-motion coupled-cluster methods for excited electronic states: The EOMCCSDt approach
Authors: Kowalski Karol , Piecuch Piotr
Published: July 2002

The idea of selecting the most important higher-than-doubly excited configurations in single-reference coupled-cluster (CC) calculations for quasidegenerate ground states of molecular systems through the use of active orbitals is extended to excited electronic states via the equation-of-motion (EOM) CC formalism. The resulting EOMCCSDt method, in which triexcited clusters T3 and the corresponding three-body components of the EOMCC excitation operator R are restricted to internal and semiinternal components defined through active orbitals, is capable of significantly improving the vertical excitation energies obtained with the conventional EOMCCSD (EOMCC singles and doubles) approach at a fraction of the computer cost associated with the full EOMCCSDT (EOMCC singles, doubles, and triples) calculations. The results of pilot calculations for the H8, CH2, and CH+ molecules indicate that the EOMCCSDt method using small active spaces is as accurate as the EOMCCSDT approach. In particular, the EOMCCSDt method is capable of accurately describing states that are doubly excited relative to the reference state.
Efficient photovoltaic cells from semiconducting polymer heterojunctions
Authors: Jenekhe Samson A. , Yi Shujian
Published: July 2002

Solar cells made from spin-coated bilayer thin-film heterojunctions of poly(p-phenylene vinylene) and poly(benzimidazobenzophenanthroline ladder) were found to have photovoltaic charge collection efficiency as high as 49%. The power conversion efficiency varied from 1.4% under sunlight illumination to 2.0% at the peak wavelength. A space-charge region around the polymer/polymer interface, Ohmic contacts at the electrodes, and complementary absorption bands of the semiconducting polymers, play important roles in the efficient charge collection in the photocells.
Liquid–vapor coexistence curves of several interatomic model potentials
Authors: Okumura Hisashi , Yonezawa Fumiko
Published: July 2002

We clarify the relation between an interatomic potentials and liquid–vapor critical points. For this purpose, we calculate the liquid–vapor coexistence curves of several interatomic model potentials such as the Lennard-Jones n−6 (n=7–32), the Morse, and the modified Stillinger–Weber potentials by the NpT plus test particle method. From these results, we find several universal properties irrespective of the potential type: (1) The law of rectilinear diameter is fulfilled in the density–temperature plane. (2) The coexistence curve scaled by the critical temperature and density almost coincides with one another. On the other hand, we also find some properties which are definitely potential dependent. In order to demonstrate this point, we introduce a new parameter a1=2π/3∫xmin∞x du(x)/dx x2 dx [x: reduced distance, xmin: the minimum position of a potential, u(x): reduced potential] which expresses the effect of the attractive force. By making use of this parameter, we find that the critical temperature Tc and pressure pc change linearly to a1. This means that the wider the attractive part of the potential is, the higher Tc and pc are. In this way, we have discovered a method to estimate the critical point from microscopic information concerning interatomic potentials.
Photophysical properties of ZnS nanoclusters
Authors: Kumbhojkar Neelesh , Nikesh V. V. , Kshirsagar Anjali , Mahamuni Shailaja
Published: July 2002

Optical measurements on ZnS nanoclusters have been carried out to investigate surface effects along with quantum size effects. ZnS nanocrystals have been synthesized in the range of 1.5–2.5 nm, using different chemical methods as well as electronic passivating procedures. The size of nanoparticles has been estimated from empirical pseudopotential calculations. We have obtained a significantly narrower size distribution of ZnS nanocrystals than reported in earlier published results. We observed band gap luminescence in mercaptoethanol capped ZnS nanocrystals. Effects of various defect levels on the luminescent behavior of ZnS nanoparticles have been examined
Electrical conductivity studies in nylon 11
Authors: Neagu R. M. , Neagu E. , Bonanos N. , Pissis P.
Published: July 2002

The results of electrical and dielectric studies in semicrystalline nylon 11 (degree of crystallinity 62%) by means of dielectric relaxation spectroscopy (DRS) in the frequency range 10−2–106 Hz at temperatures between the glass transition temperature of about 40 °C and the melting temperature of about 190 °C are reported. Emphasis is put on the investigation of electrical conductivity and conductivity effects. Additional information on dc conductivity is obtained from triangular voltage measurements. The results of DRS were analyzed within the formalism of permittivity (ac conductivity), impedance, and electric modulus. At temperatures higher than about 100 °C the spectra are dominated by dc conductivity and conductivity current relaxation. Despite high values of dc conductivity (σdc=10−6–10−5 S/m at 170 °C), electrode effects and space charge polarization, giving rise to high values of ε′ at low frequencies, are negligible in the whole temperature range of measurements. This feature, as well as the rather small temperature dependence of static dielectric permittivity, are important with respect to technological applications of nylon 11 as a high-temperature electroactive material.
Mechanism of liquid crystal alignment on submicron patterned surfaces
Authors: Rastegar A. , Škarabot M. , Blij B. , Rasing Th.
Published: July 2002

The alignment mechanism of liquid crystals on polymeric surfaces that were patterned using an atomic force microscope (AFM) tip was studied by polarizing optical microscopy. Depending on the thickness of the polymer, polymer chain alignment or grooves appear to be responsible for the liquid crystal alignment. In thick polymer films (above 100 nm) the polymer chains will align in the direction of the scan due to the large lateral force exerted by the tip. In thin polymer films (5–20 nm) the polymer chains are strongly fixed to the surface and will not realign by the tip; instead grooves will be formed. The azimuthal surface anchoring the energy of thick polymer films does not depend on the scan line separation and scan force, is of the same order of magnitude as for conventionally rubbed polyimide surfaces, but increases by the number of scans. AFM patterned pixels do not show a pretilt angle.
Construction of laser-heated diamond anvil cell system for in situ x-ray diffraction study at SPring-8
Authors: Watanuki Tetsu , Shimomura Osamu , Yagi Takehiko , Kondo Tadashi , Isshiki Maiko
Published: July 2002

A double-sided laser heated diamond anvil cell (DAC) system was constructed at a high brilliance, undulator beamline (BL10XU) at SPring-8 a third generation synchrotron radiation facility, for performing in situ angle-dispersive x-ray diffraction experiments under high temperature and high pressure. The design of this system puts emphasis on reliable data collection for the structural analysis. With this system, the adjustment of the optical systems for both x-ray and laser beams can be done easily, and high quality diffraction data can be obtained typically within several minutes. A system for temperature measurement of ten points in a sample area at the same time was also developed. The performance of the laser heated DAC system was tested by observing phase transitions of natural olivine.
Critical thickness of crystallization and discontinuous change in ferroelectric behavior with thickness in ferroelectric polymer thin films
Authors: Zhang Q. M. , Xu Haisheng , Fang Fei , Cheng Z.-Y. , Xia Feng , You H.
Published: July 2002

We report on the observation of the critical thickness of crystallization of ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer thin films, which were solution spun cast on platinum coated silicon wafer. The effect occurs at about 100 nm thickness, which is significantly above any currently known spatial dimensions of the polymer, so that for films at thickness below about 100 nm, the crystallization process is strongly hindered, resulting in a low crystallinity in these films. This low crystallinity leads to a large and discontinuous change of the dielectric constant and ferroelectric polarization in the films below the critical thickness.
Mathematical model of the cell division cycle of fission yeast
Authors: Novak Bela , Pataki Zsuzsa , Ciliberto Andrea , Tyson John J.
Published: July 2002

Much is known about the genes and proteins controlling the cell cycle of fission yeast. Can these molecular components be spun together into a consistent mechanism that accounts for the observed behavior of growth and division in fission yeast cells? To answer this question, we propose a mechanism for the control system, convert it into a set of 14 differential and algebraic equations, study these equations by numerical simulation and bifurcation theory, and compare our results to the physiology of wild-type and mutant cells. In wild-type cells, progress through the cell cycle (G1→S→G2→M) is related to cyclic progression around a hysteresis loop, driven by cell growth and chromosome alignment on the metaphase plate. However, the control system operates much differently in double-mutant cells, wee1− cdc25Δ, which are defective in progress through the latter half of the cell cycle (G2 and M phases). These cells exhibit “quantized” cycles (interdivision times clustering around 90, 160, and 230 min). We show that these quantized cycles are associated with a supercritical Hopf bifurcation in the mechanism, when the wee1 and cdc25 genes are disabled.
Broadband semiconductor superlattice detector for THz radiation
Authors: Klappenberger F. , Ignatov A. A. , Winnerl S. , Schomburg E. , Wegscheider W. , Renk K. F. , Bichler M.
Published: July 2002

We report on a broadband GaAs/AlAs superlattice detector for THz radiation; a THz field reduces the current through a superlattice, which is carried by miniband electrons, due to modulation of the Bloch oscillations of the miniband electrons. We studied the detector response, by use of a free electron laser, in a large frequency range (5–12 THz). The responsivity showed strong minima at frequencies of infrared active phonons of the superlattice. A theoretical analysis of the detector delivers an understanding of the role of phonons and gives a characterization of the responsivity.
Three-dimensional analysis of shape transitions in strained-heteroepitaxial islands
Authors: Zhang Y. W. , Bower A. F.
Published: July 2002

A continuum model of shape transitions in strained heteroepitaxial islands predicts that small islands are hut shaped, while larger islands are domed. The volume of a stable hut can exceed that of a stable dome, however, indicating that the stability of hut-shaped islands plays an important role in shape transition, in addition to energy changes associated with the transition. A hut-shaped island loses stability upon reaching a critical volume, which depends on the anisotropy in surface energy. Impingement between islands reduces the critical volume by nucleating new facets at the base of a hut, which subsequently trigger a shape transition.
The effect of slight deformation on droplet coalescence in linear flows
Authors: Rother Michael A. , Davis Robert H.
Published: July 2002

A trajectory analysis is used to determine the effect of small deformations and van der Waals attractions on the collision efficiency of two non-Brownian drops freely suspended in a linear flow at small Reynolds number. Simple shear flow and uniaxial compressional and extensional flow are considered. Treating the capillary number (Ca) as a small parameter permits an approach similar to matched asymptotic expansions. For Ca≪1, the analysis shows that the deformation is mainly axisymmetric and that the tangential motion of the drops in apparent contact is unaffected to leading order by the small deformation. A comparison with full three-dimensional boundary-integral calculations confirms the accuracy of the asymptotic approach. In the dimensionless parameter space, results for the collision efficiency are mapped for four parameters: Ca, size ratio, drop-to-medium viscosity ratio, and a dimensionless Hamaker parameter. For spherical drops in uniaxial compression and extension, the collision efficiencies are identical due to the reversibility of Stokes flow. When small deformation is introduced, however, the collision efficiencies are lower for compression than for extension. For slightly deformable drops in simple shear flow, the critical capture cross section upstream is no longer a circle, in contrast to the behavior of spherical drops in the absence of van der Waals forces. For all flow types, a key result is that the collision efficiency decreases rapidly from the corresponding value for spherical drops, as the capillary number increases beyond a critical value, due to small deformations. Consequently, droplet growth by coalescence will be arrested when the drops reach a prescribed size, as shown by population dynamics simulations for a model physical system.
An effective pair potential for heavy water
Authors: Grigera J. Raul
Published: July 2002

The properties of heavy water are of interest in different disciplines. The simulation of such a substance, particularly in comparison with ordinary water, requires an appropriate interaction potential that allows the simulations of large systems. The potential presented in this work is an efective three-point charge potential type based on the well-known simple point charges/extended model and is named simple point charges/heavy water (SPC/HW). Molecular dynamics simulation done with the SPC/HW potential shows a good agreement with the experimental values for several properties.
Influence of contact electrodes on leakage characteristics in ferroelectric thin films
Authors: Nagaraj B. , Aggarwal S. , Ramesh R.
Published: July 2002

Electrodes can impact the device performance of ferroelectric capacitors in several ways. The present controlled studies on Pb (Nb, Zr, Ti)O3 with Pt, (La, Sr)CoO3 and SrRuO3 is a clear demonstration of the role of electrodes in impacting the leakage current mechanism of the ferroelectric capacitors and their reliability properties. The oxide electrode capacitors show predominantly nonblocking contact and good fatigue and imprint properties. Pt electrode capacitors show blocking contacts, long term leakage current relaxation, and poor fatigue and imprint properties. The nature of the temperature and voltage dependence of leakage current relaxation in Pt capacitors indicates trapping of charge carriers to be the cause for the observed relaxation. A good correlation between leakage current relaxation and the rate of polarization loss during fatigue and the similarity in their voltage and temperature dependence suggests trapping (of charged carriers/domains, respectively) as common to both phenomena.
The active-space equation-of-motion coupled-cluster methods for excited electronic states: Full EOMCCSDt
Authors: Kowalski Karol , Piecuch Piotr
Published: July 2002

The full version of the equation-of-motion coupled-cluster (EOMCC) method with all singles and doubles, and a selected set of triples defined through active orbitals (EOMCCSDt) has been implemented and tested using the H8, H2O, N2, C2, and CH+ systems. It is demonstrated that the full EOMCCSDt method provides the results of the full EOMCCSDT (EOMCC singles, doubles, and triples) quality at the fraction of the computer effort associated with the EOMCCSDT calculations. This includes excited states that are dominated by doubles and states that have large triexcited components. The excellent performance of the EOMCCSDt approach is observed even when the ground electronic state has a quasidegenerate character, which means that we can apply the EOMCCSDt formalism to excited states that cannot be adequately described by the perturbative triples models. The EOMCCSDt method is equivalent to the EOMCCSDT approach if all orbitals used in the EOMCCSDt calculations are active.
On the accurate prediction of the optical absorption energy of F-centers in MgO from explicitly correlated ab initio cluster model calculations
Authors: Sousa Carmen , Illas Francesc
Published: July 2002

A systematic study of the different computational requirements that affect the accuracy of the ab initio prediction of excitation energies of F and F+ centers on cluster models of MgO is reported. It is found that rather limited basis sets are enough to predict excitation energies of the F and F+ centers that are near to each other as experimentally observed. However, the absolute value of the excitation energy is in error by ∼1 eV or ∼20%. Increasing the basis set reduces the calculated excitation energy for the allowed transition, reaching a value of 5.44 eV for the F center, only 9% in error with respect to experiment. Improving the basis set does not result in a better value of the excitation energy of the charged F+ center. Attempts to improve the calculated result by geometry optimization of the region near the oxygen vacancy, enlarging the cluster model, improving the primitive Gaussian set, or enlarging the auxiliary basis set centered on the vacancy failed to further reduce the error. It is concluded that much larger basis sets are required to predict excitation energies of electrons trapped at oxygen vacancies in ionic oxides with accuracy of or better than 0.4 eV.
The Bargmann representation for the quantum mechanics on a sphere
Authors: Kowalski K. , Rembieliński J.
Published: July 2002

The Bargmann representation is constructed corresponding to the coherent states for a particle on a sphere introduced in Kowalski and Rembieliński, J. Phys. A: 33, 6035 (2000). The connection is discussed between the introduced formalism and the standard approach based on the Hilbert space of square integrable functions on a sphere S2.
The study of conical intersections between consecutive pairs of the five lowest A′2 states of the C2H molecule
Authors: Mebel A. M. , Yahalom A. , Englman R. , Baer M.
Published: July 2002

In this article we report findings regarding various conical intersections between consecutive pairs of the five lowest A′2 states of the C2H molecule. We found that conical intersections exist between each two consecutive A′2 states. We showed that except for small (high-energy) regions in configuration space, the two lowest adiabatic states (i.e., the 1 2A′ and the 2 2A′) form a quasi-isolated system with respect to the higher states. We also revealed the existence of degenerate parabolical intersections, those with a topological (Berry) phase zero, formed by merging two conical intersections belonging to the 3 2A′ and the 4 2A′ states, and suggested a Jahn-Teller-type model to analyze them. Finally, we examined the possibility that the “frozen” locations of the carbons can be considered as points of conical intersection. We found that the relevant two-state topological phase is not zero nor a multiple of π, but that surrounding both carbons yields a zero topological phase.
Physical model for trap-assisted inelastic tunneling in metal-oxide-semiconductor structures
Authors: Jiménez-Molinos F. , Palma A. , Gámiz F. , Banqueri J. , López-Villanueva J. A.
Published: July 2002

A physical model for trap-assisted inelastic tunnel current through potential barriers in semiconductor structures has been developed. The model is based on the theory of multiphonon transitions between detrapped and trapped states and the only fitting parameters are those of the traps (energy level and concentration) and the Huang–Rhys factor. Therefore, dependences of the trapping and detrapping processes on the bias, position, and temperature can be obtained with this model. The results of the model are compared with experimental data of stress induced leakage current in metal-oxide-semiconductor devices. The average energy loss has been obtained and an interpretation is given of the curves of average energy loss versus oxide voltage. This allows us to identify the entrance of the assisted tunnel current in the Fowler–Nordheim regime. In addition, the dependence of the tunnel current and average energy loss on the model parameters has been studied.
Mechanical properties of diamond films: A comparative study of polycrystalline and smooth fine-grained diamonds by Brillouin light scattering
Authors: Djemia P. , Dugautier C. , Chauveau T. , Dogheche E. , De Barros M. I. , Vandenbulcke L.
Published: July 2002

Brillouin light scattering, Raman light scattering and x-ray diffraction were used to investigate the elastic and microstructural properties of polycrystalline and smooth fine-grained diamond films of varying diamond quality. They were deposited on a titanium alloy by a two-step microwave plasma-assisted chemical vapor deposition process at 600 °C. Their morphology and roughness were studied by scanning electron microscopy and atomic force microscopy. Their refractive indices were determined by the M-line spectroscopy technique. The diamond purity of all these coatings in terms of the sp3 bonding fraction was deduced from visible and UV Raman spectroscopy as a function of the deposition conditions. All the samples were found to be textured with a 〈011〉 crystallographic direction normal to the film plane, leading to essentially hexagonal symmetry of the elastic tensor. By taking advantage of the detection of a number of different acoustic modes, complete elastic characterization of the films was achieved. The elastic constants C11 and C66, respectively, were selectively determined from the frequency of the longitudinal and shear horizontal bulk modes traveling parallel to the film surface. The three remaining elastic constants, namely, C44, C33 and C13, were obtained from detection of the Rayleigh surface wave a bulk shear wave and the bulk longitudinal wave propagating at different angles from the normal to the surface. The values of the elastic constants depend on the deposition conditions and on the microstructural properties of the films, especially the diamond quality and the polycrystalline or smooth fine-grained nature of the diamond. For the polycrystalline diamond film with the best quality, the elastic constants are rather close to the Voigt or Reuss average estimate values using known bulk elastic constants of diamond, whereas those of the smooth fine-grained diamond films are reduced because of the poorer diamond quality leading to lower residual stress in the films.
Size effect on the thermal conductivity of nanowires
Authors: Lü X. , Shen W. Z. , Chu J. H.
Published: July 2002

The size effect on thermal conductivity of a free-standing wire with rectangular cross-section is investigated. The electronic thermal conductivity is calculated within a Boltzmann transport equation approach. A simple expression for the reduction in conductivity due to the increase of boundary scattering is presented. The values of thermal conductivity deduced from our calculation are in good agreement with the experimental data for sodium and gold nanowires. The lattice thermal conductivities of silicon (Si) and cadmium telluride (CdTe) nanowires are examined on the basis of the equations of phonon radiative transfer. The calculated thermal conductivities of Si nanowires with square cross-section are found to be in good agreement with molecular dynamics results. The Umklapp and boundary scattering limited lattice thermal conductivities of CdTe nanowires are investigated theoretically by taking into account acoustic phonon dispersion. It is found that the thermal conductivity of nanowires decreases with decreasing side length. It is shown that this decrease of conductivity in wires is smaller than that of a film or a single layer of a superlattice with out-of-plane heat flux, but larger than in the case of in-plane heat flux.
Rapid, reversible, sensitive porous silicon gas sensor
Authors: Seals Lenward , Gole James L. , Tse Laam Angela , Hesketh Peter J.
Published: July 2002

The development of a sensitive porous silicon (PS) gas sensor which utilizes photoluminescence induced electroless metallization as a means of obtaining a highly efficient electrical contact has been demonstrated for the detection of HCl, NH3, and NO at the 10 ppm level. The problem of spreading resistance (kΩ–MΩ) is overcome as low resistance contacts ∼20–100 Ω are made to the mesoporous PS structure through electroless gold plating. The response of this device, which operates at a bias voltage of 1–10 mV, is rapid and reversible.
Normal-incidence Ge quantum-dot photodetectors at 1.5 μm based on Si substrate
Authors: Tong S. , Liu J. L. , Wan J. , Wang Kang L.
Published: July 2002

Coherent Ge quantum dots embedded in Si spacing layers were grown on Si substrate by molecular-beam epitaxy in the Stranski–Krastanov mode. Photoluminescence measurement showed a Ge-dot-related peak at 1.46 μm. p-i-n photodiodes with the intrinsic layer containing Ge dots were fabricated, and current–voltage (I–V) measurement showed a low dark current density of 3×10−5 A/cm2 at −1 V. A strong photoresponse at 1.3–1.52 μm originating from Ge dots was observed, and at normal incidence, an external quantum efficiency of 8% was achieved at −2.5 V.
Bimetallic Ag–Au nanoparticles: Extracting meaningful optical constants from the surface-plasmon extinction spectrum
Authors: Moskovits M. , Srnová-Šloufová I. , Vlčková B.
Published: July 2002

We report an approach for extracting the optical constants of bimetallic Ag–Au nanoparticles from the measured surface-plasmon (SP) extinction spectra. The dielectric function of the metal is expressed as an analytic function of the wavelength in which the interband (and all other non-Drude) contributions to the dielectric function are represented by a sum of Lorentz functions. This expression is then used to fit the experimental extinction spectra to appropriate functions based on Mie theory. Three Lorentz functions (plus a Drude term) were found to be sufficient to reproduce the dielectric functions of Ag and Au [P. B. Johnson and R. W. Christy, Phys. Rev. B 6, 4370 (1972)] over the entire 0.6–6.5 eV range reported. With functions of this type, an excellent multiparameter fit of the measured extinction spectrum of colloidal Ag was obtained. Extinction spectra of a series of (Ag)Au hydrosols, prepared by reducing a gold precursor in the presence of previously synthesized silver seed particles with Au mole fractions ranging from 0.1 to 0.8, were measured. The extinction spectra show a single band (attributed to the surface plasmon) for all of the colloids produced, suggesting alloy formation. Transmission electron microscopy (TEM) images, however, indicate clear core–shell contrast for nanoparticles with Au mole fractions 0.4 and higher. With a presumed particle structure consisting of Ag core and Ag/Au alloy shell, very good fits were obtained for all of the measured extinction spectra by using a fitting strategy that restricted the number of parameters allowed to vary freely in the aforementioned dielectric function. The values of the dielectric function of the presumed shells were extracted in this manner as a function of wavelength. For particles with Au mole fraction 0.1–0.3, the results suggest an incompletely formed shell. For particles with higher Au mole fractions, the dielectric function of the shell gradually approaches that of Au. Overall, the results are consistent with a particle structure that consists of a Ag-rich core and Ag/Au alloy shell whose composition is gradually enriched in Au with increasing overall Au mole fraction. The dielectric function of the alloy shells cannot be written merely as a weighted average of the corresponding dielectric functions of Ag and Au. An almost linear decrease of the electron scattering rate in the particle’s shell with the increasing Au mole fraction was determined through this fitting analysis. This was ascribed both to the confinement of the electrons within the shell (which increases in thickness with increasing Au mole fraction) and the gradual change of the composition, and, consequently, of the electrical conductivity of the shell.
Cylindrical and spherical dust ion–acoustic solitary waves
Authors: Mamun A. A. , Shukla P. K.
Published: July 2002

The properties of cylindrical and spherical dust ion–acoustic solitary waves (DIASWs) in an unmagnetized dusty plasma, whose constituents are inertial ions, Boltzmann electrons, and stationary dust particles, are investigated by employing the reductive perturbation method. The modified Korteweg–de Vries equation is derived and its numerical solutions are obtained. It has been found that the properties of the DIASWs in a nonplanar cylindrical or spherical geometry differ from those in a planar one-dimensional geometry.
A multipass hydrogen Raman shifter for the generation of broadband multifrequencies
Authors: Losev L. L. , Yoshimura Y. , Otsuka H. , Hirakawa Y. , Imasaka T.
Published: July 2002

A four-pass cell was employed for the multifrequency parametric Raman generation in gaseous hydrogen. More than 18 rotational-vibrational Raman lines from 450 to 830 nm were generated using an 80-mJ, 3-ns pump pulse at a wavelength of 532 nm. The output spectrum broadening was observed by increasing the number of passes of the pump pulse through the Raman cell.