bluuebunny/crossref_metadata_2025 huggingface dataset:
Current saturation mechanisms in carbon nanotube field emitters
Authors: Dean Kenneth A. , Chalamala Babu R.
Published: July 2002

Recent studies have shown a current limiting effect in the field emission behavior of carbon nanotubes. In this letter, we demonstrate that an individual nanotube exhibits current saturation above 100 nA of emission current, and we show that this current saturation is a direct result of an adsorbate-enhanced field emission mechanism. Current saturation results from the displacement of adsorbates from configurations of tunneling enhancement as electric field and current are increased. Saturation is concurrent with rapid fluctuations in emission current and distinctive changes in the field emission patterns. At high fields, the adsorbate states are completely removed from the nanotube. A single, clean single-walled nanotube shows no evidence of current saturation for emission currents reaching 2 μA.
Thermally induced stress relaxation and densification of spin-on-glass thin films
Authors: Chiang C. K. , Wallace W. E. , Lynn G. W. , Feiler D. , Xia W.
Published: July 2002

The stress–temperature relationship of silica spin-on-glass thin films on silicon wafers was studied. Upon heating, the stress–temperature curves showed a dramatically increasing slope when the temperature of the film was greater than 340 °C. At 450 °C, a significant, irreversible change in the stress of the film was observed. This change in stress was correlated with an increase in film electron density and a decrease in film thickness. The observed thermally activated stress–relaxation behavior was interpreted in terms of reflow of the glassy hydrogen–silsesquioxane-based material.
Micromagnetics of submicron (110) Fe elements
Authors: Thomas Luc , Parkin Stuart S. P. , Yu Jun , Rüdiger Ulrich , Kent Andrew D.
Published: July 2002

The magnetization processes and the domain structure of epitaxial (110) Fe submicron-sized elements are studied experimentally and with micromagnetic simulations. Magnetic force microscopy and Kerr magnetometry results are in good agreement with the simulations, thus allowing the development of a consistent picture of the micromagnetics of these systems.
Dielectric function of aggregates of small metallic particles embedded in host insulating matrix
Authors: Grechko Leonid G. , Pustovit Vitaly N. , Whites Keith W.
Published: July 2002

The optical properties of clusters with metallic spherical particles embedded in an insulating matrix are studied. A theoretical approach is proposed for the calculation of the macroscopic dielectric response for a collection of spheres at random positions embedded in a homogeneous medium. While accounting for the dipole–dipole interaction between particles, we have considered the frequency dependence behavior of the imaginary part of the effective dielectric constant in this system with two kinds of particles of different sizes.
Oxide thinning percolation statistical model for soft breakdown in ultrathin gate oxides
Authors: Chen Ming-Jer , Kang Ting-Kuo , Liu Chuan-Hsi , Chang Yih J. , Fu Kuan-Yu
Published: July 2002

An existing cell-based percolation model with parameter correlation can find its potential applications in assessing soft-breakdown (BD) statistics as long as the oxide thinning due to the localized physical damage near the SiO2/Si interface is accounted for. The resulting model is expressed explicitly with the critical trap number per cell nBD and the remaining oxide thickness tox′ both as parameters. Reproduction of time-to-bimodal (soft- and hard-) breakdown statistical data from 3.3-nm-thick gate-oxide samples yields nBD of 3 and 4 for soft and hard breakdown, respectively. The extracted tox′ of 1.0 nm for soft breakdown, plus the transition layer thickness of 0.5 nm in the model, is fairly comparable with literature values from current–voltage fitting. The dimension and area of the localized physically damaged region or percolation path (cell) are quantified as well. Based on the work, the origins of soft and hard breakdown are clarified in the following: (i) soft breakdown behaves intrinsically as hard breakdown, that is, they share the same defect (neutral trap) generation process and follow Poisson random statistics; (ii) both are independent events corresponding to different tox′ requirements; and (iii) hard breakdown takes place in a certain path located differently from that for the first soft breakdown.
Double diffusive convection in a cubic enclosure with opposing temperature and concentration gradients
Authors: Sezai I. , Mohamad A. A.
Published: July 2002

A three-dimensional numerical study has been performed to investigate double-diffusive, natural convection in a cubic enclosure subject to opposing and horizontal gradients of heat and solute. The flow is driven by buoyancy forces due to temperature and solutal gradients. Constant temperature and concentration are imposed along the two vertical side walls of the cubic enclosure, while the remaining walls are impermeable and adiabatic. The numerical simulations presented here span a wide range of thermal Rayleigh number (10.0<Ra<2×105), buoyancy ratio (−2.0<N<0) and Lewis number (0.1<Le<150) to identify the different flow patterns and bifurcations. Nusselt and Sherwood numbers are presented as functions of the governing parameters. Most of the published results approximate the problem as two dimensional. In the present results we indicate that the double diffusive flow in enclosures with opposing buoyancy forces is strictly three dimensional for a certain range of parameters.
Light absorption enhancement in thin silicon layers
Authors: Sukmanowski J. , Paulick C. , Sohr O. , Andert K. , Royer F. X.
Published: July 2002

Multilayer systems containing one layer from amorphous silicon (a-Si) are analyzed regarding the absorption of light and the distribution of the electrical field within them. The local electric field strength is derived from the intensity of the Raman lines of the a-Si. Suitable designed multilayer structures may provide more absorption of light in a layer of 55 nm and less than in bulk a-Si. Applications for thin layer solar cells are discussed.
Large improvement in quantum fluorescence yield of Er3+-doped fluorozirconate and fluoroindate glasses by Ce3+ codoping
Authors: Meng Zhicong , Yoshimura Tomoshi , Fukue Kei , Higashihata Mitsuhiro , Nakata Yoshiki , Okada Tatsuo
Published: July 2002

The fluorescence characteristics of different Ce3+:Er3+-codoped fluoride host glasses, such as fluorozirconate and fluoroindate glass, are reported. It is shown that Ce3+ codoping into Er3+ doped fluoride glasses resulted in a quenching of Er3+ ions from the I11/24 to the I13/24 state, and the branching ratio for the Er3+ I11/24→I13/24 transition increased from 0.20 to over 0.80 by codoping 2.0 mol % Ce3+. Further, the fluorescence quantum yield at 1.55 μm was also significantly improved. A Ce3+:Er3+-codoped fluorozirconate fiber laser operating at 1.55 μm band with 980 nm excitation has been realized for the first time, which clearly indicates the effectiveness of Ce3+ codoping.
Mechanics in space and counterspace
Authors: Conradt Oliver
Published: July 2002

The completely dual approach to Clifford algebra is used to enlarge the concept of the projective split and to develop a new geometric representation for the Pauli algebra (space and counterspace), for the momenta (planelike vectors), and for the phase space. The Pauli algebra appears in this context as the phase space extended by time (scalar) and energy (pseudoscalar). Lagrangian and Hamiltonian mechanics are embedded into the dual framework of space and counterspace. Several examples illustrate the new techniques. The dual approach to mechanics provides a new possibility to interpret symplectic geometry.
Stably stratified turbulent channel flows. I. Stratification regimes and turbulence suppression mechanism
Authors: Garg Rajat P. , Ferziger Joel H. , Monismith Stephen G. , Koseff Jeffrey R.
Published: July 2002

Our objective in this study is to study inhomogeneous stratified shear flows using large eddy simulation; stratified pressure-gradient-driven channel flow was selected. The flows can be separated into three regimes: buoyancy affected, buoyancy controlled, and buoyancy dominated flows. The regime boundaries are defined by Richardson and Reynolds numbers based on the friction velocity. Buoyancy affected flows remain actively turbulent and attain a statistical steady state that resembles a lower Reynolds number unstratified flow. Flows in the buoyancy controlled regime are not in equilibrium. In the cases studied, an asymmetry develops with respect to the channel centerline leading to one-sided turbulence. Eventually, the “inactive” half undergoes a transition initiated by the active half and symmetry is restored. At higher Richardson numbers, the flows are buoyancy dominated, the near-wall burst-sweep process is completely disrupted and turbulence production ceases, leading to relaminarization. In relaminarizing flows, the inner and outer regions behave nearly independently. While the inner region turbulence decays monotonically, large-scale restratification, internal waves, and potential energy-driven motions are observed in the outer region. The simulation results are used to construct a physical model of stratified wall-bounded flows. Stable stratification weakens the interaction between the inner and outer regions by decreasing the vertical transport, leading to near-decoupling of the two layers at strong enough stratification. The notion that the disappearance of the log region marks the onset of buoyancy control provides a criterion for estimating the Richardson number delineating the transition from buoyancy affected to buoyancy controlled flows. Data that should be useful for creating parametrizations for prediction of stratified flows are also presented.
Si self-interstitial injection from Sb complex formation in Si
Authors: Fage-Pedersen J. , Gaiduk P. , Lundsgaard Hansen J. , Nylandsted Larsen A.
Published: July 2002

It has recently been established that Si self-interstitials are generated during annealing of high-concentration Sb layers in Si. In the present work, we make use of samples grown with molecular-beam epitaxy. We monitor, at different times and temperatures, the diffusion enhancement or retardation of deep B or Sb marker layers next to a 1.1×1020 cm−3 Sb box, as well as the formation of Sb precipitates within the box. It is concluded that the interstitials are not associated with precipitate growth, but that they are generated from formation of Sb-vacancy complexes, primarily involving 2 Sb atoms.
Using a classical potential as an efficient importance function for sampling from an ab initio potential
Authors: Iftimie Radu , Salahub Dennis , Wei Dongqing , Schofield Jeremy
Published: July 2002

In this paper the ab initio potential of mean force for the formic acid–water system is calculated in a Monte Carlo simulation using a classical fluctuating charge molecular mechanics potential to guide Monte Carlo updates. The ab initio energies in the simulation are calculated using density-functional theory (DFT) methods recently developed by Salahub et al. [J. Chem. Phys. 107, 6770 (1997)] to describe hydrogen-bonded systems. Importance sampling methods are used to investigate structural changes and it is demonstrated that using a molecular mechanics importance function can improve the efficiency of a DFT simulation by several orders of magnitude. Monte Carlo simulation of the system in a canonical ensemble at T=300 K reveals two chemical processes at intermediate time scales: The rotation of the H2O bonded to HCOOH, which takes place on a time scale of 3 ps, and the dissociation of the complex which occurs in 24 ps. It is shown that these are the only important structural “reactions” in the formic acid–water cluster which take place on a time scale shorter than the double transfer of the proton.
Improved band alignment for hole injection by an interfacial layer in organic light emitting devices
Authors: Chkoda L. , Heske C. , Sokolowski M. , Umbach E.
Published: July 2002

We demonstrate that a thin organic interfacial layer of 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) can be utilized to improve the band alignment of N,N′-di-(3-methylphenyl)N,N′diphenyl-4,4′diaminobiphenyl (TPD) films on [indium–tin–oxide (ITO)] (InSnO) substrates in, e.g., organic electroluminescent devices. A photoemission study of the highest occupied molecular orbital (HOMO) and vacuum level position as a function of the organic overlayer thickness reveals that due to chemisorptive bonding a thin PTCDA interlayer results in a reduced barrier between the Fermi level of ITO and the HOMO of TPD. Furthermore we detect a new molecular state 0.6 eV below the Fermi level at the PTCDA/ITO interface. Both effects are expected to improve the hole injection from the ITO anode into the TPD hole transport layer, e.g., in organic light emitting devices.
Isotropization of two-component fluids
Authors: Goliath Martin , Nilsson Ulf S.
Published: July 2002

We consider the problem of late-time isotropization in spatially homogeneous but anisotropic cosmological models when the source of the gravitational field consists of two noninteracting perfect fluids—one tilted and one nontilted. In particular, we study irrotational Bianchi type V models. By introducing appropriate dimensionless variables, a full global understanding of the state space of the gravitational field equations becomes possible. The issue of isotropization can then be addressed in a simple fashion. We also discuss implications for the cosmic “no-hair” theorem for Bianchi models when part of the source is a tilted fluid.
Self-consistent field theory of twist grain boundaries in block copolymers
Authors: Duque Daniel , Schick M.
Published: July 2002

We apply self-consistent field theory (SCFT) to twist grain boundaries of block copolymer melts. The distribution of monomers throughout the grain boundary is obtained as well as the grain boundary free energy per unit area as a function of twist angle. We define an intermaterial dividing surface in order to compare it with minimal surfaces which have been proposed. Our calculation shows that the dividing surface is not a minimal one, but the linear stack of dislocations seems to be a better representation of it for most angles than is Scherk’s first surface.
A high pressure, high temperature, scanning tunneling microscope for in situ studies of catalysts
Authors: Weeks Brandon L. , Durkan Colm , Kuramochi Hiromi , Welland Mark E. , Rayment Trevor
Published: July 2002

We describe the construction and operation of a novel scanning tunneling microscope, which is capable of operating throughout a wide range of pressures (atmospheric—60 bar) and temperatures (300–650 K). The instrument has cylindrical symmetry and is constructed out of materials with low thermal conductivity and low coefficients of thermal expansion that allows imaging the same area over a wide temperature range. With this microscope we present images of the oxidation of polycrystalline nickel.
The quantum vibrational dynamics of Cl−(H2O)n clusters
Authors: Schenter Gregory K. , Garrett Bruce C. , Voth Gregory A.
Published: July 2002

The centroid molecular dynamics technique is applied to the case of chloride–water clusters to estimate their finite temperature quantum vibrational structure. We employ the flexible RWK2 water potential [J. R. Reimers, R. O. Watts, and M. L. Klein, Chem. Phys. 64, 95 (1982)] and the parametrization of a chloride–water interaction potential of Dorsett, Watts and Xantheas [J. Phys. Chem. A 103, 3351 (1999)]. We then investigate the temperature-dependent vibrational structure (infrared spectra). We find that the centroid molecular dynamics technique is capable of recovering a majority of the red shift associated with hydrogen bonding.
Applications of harmonic morphisms to gravity
Authors: Mustafa M. T.
Published: July 2002

We introduce the notion of gravity coupled to a horizontally conformal submersion as a modification of the well-known concept of gravity coupled to a harmonic map, thus obtaining a coupled gravity system with a more geometric flavor. By using integral techniques we determine the necessary conditions for coupling and cosmological constants. Finally, in the context of higher dimensional gravitation theory, we show that harmonic morphisms provide a natural ansatz to trigger spontaneous splitting and reduction of the gravity system coupled to a harmonic map on (4+D) (D⩾1) dimensional space–times.
Crystallographic transformations of rapidly quenched Nd10Fe90−xTix
Authors: Yu M. H. , Zhang Z. D. , Xiao Q. F. , Geng D. Y. , Liu W. , Zhao X. G.
Published: July 2002

The structural transformations from amorphous, metastable-to-equilibrium phases during annealing treatment of rapidly quenched Nd10Fe90−xTix (x=0, 2, 4, 6, 8, and 10) alloys have been studied systematically. By means of ac initial susceptibility and x-ray diffraction, a CaCu5-type metastable phase was observed for 0⩽x⩽10 as the crystallization of the amorphous alloys occurs below 700 °C. The CaCu5-type structure transforms into a TbCu7-type one with increasing the annealing temperature (above 700 °C). These melt-spun alloys have different structures including Th2Zn17 type for 0⩽x⩽4 and TbCu7 and ThMn12 types for 6⩽x⩽10, when annealed at high temperature (above 1000 °C). Nitrogenation was carried out at 400 °C for 10 h. Both Nd(Fe,Ti)5 and its nitride are soft-magnetic phases, like Nd(Fe,Ti)7 and Nd2(Fe,Ti)17.
Hydrogen bonding at the diatomics-in-molecules level: Water clusters
Authors: Grigorenko Bella L. , Nemukhin Alexander V. , Topol Igor A. , Burt Stanley K.
Published: July 2002

Further developments of the intermolecular diatomics-in-molecules (DIM) theory towards construction of potential energy surfaces of hydrogen-bonded molecular aggregates are presented. Compared to the previously studied hydrogen fluoride clusters (HF)n [J. Chem. Phys. 111, 4442 (1999)], considerably more complicated and challenging systems, namely, water clusters (H2O)n (n=2–6) have been analyzed in this work. The present DIM, or more precisely, diatomics-in-ionic-systems, scheme is based on the balanced treatment of neutral and ionic contributions to the electronic properties of polyatomic species, and in this case takes into account the mixing of the OH and O−H+ electronic states within the valence bond description of water molecules. The potential curves of diatomic molecules required for the present application, including ionic species O−H, OH+, O2−, have been computed by ab initio quantum chemistry tools. The results of DIM calculations of equilibrium geometry configurations, binding energies, and relative energies for the low-lying isomers of (H2O)n (n=2–6) are compared to the reference data showing a good predictive power of this method.
Direct iterative solution of the generalized Bloch equation. II. A general formalism for many-electron systems
Authors: Meißner Holger , Paldus Josef
Published: July 2002

A general nonperturbative formulation of the recently proposed [H. Meißner and E. O. Steinborn, Int. J. Quantum Chem. 61, 777 (1997); Part I] quadratic iterative scheme for the wave function expansion coefficients (WECs), enabling a direct solution of the generalized Bloch equation, is given for the ab initio electronic Hamiltonians, thus enabling the computation of the molecular electronic structure. The method exploits the concepts of a multidimensional reference or model space, a (non-Hermitian) effective Hamiltonian, and the generalized Bloch equation. The formulation in terms of WECs provides a considerable freedom in the design of various approximation schemes by combining direct iterations on WECs with their approximation by disconnected cluster components based on the exponential cluster ansatz for the wave operator. The resulting formalism is capable of handling a rather large class of both ground and excited states. While the general formulation represents a multiconfigurational, multireference scheme, a special attention is paid to its two-dimensional state selective or state specific version.
Transport barrier dynamics
Authors: Horton W. , Zhu P.
Published: July 2002

The properties of the internal transport barriers are developed using theory and radial transport simulations that evolve local turbulent energy density with the temperature profiles. Standard ion temperature gradient models for the nonlinear radial fluxes driven by drift wave turbulence and stabilized by flow shear are implemented in a new high resolution multiple space–time transport code. A dimensionless parameterization of the input power is introduced and shown to characterize the bifurcation to an internal transport barrier. Examples of the interaction and feedback loops of the turbulence with the transport profiles are given for transport barriers as in the Tokamak Fusion Test Reactor [D. J. Grove et al. Nucl. Fusion 25, 1167 (1985)] and the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) [Ninomiya et al., Phys. Fluids B 4, 2070 (1992)]. For the JT-60U the high performance discharge E 27969, which reached an equivalent QDT of unity, is modeled with an appropriate set of turbulent thermal, angular momentum and particle diffusivities. The bifurcation analysis suggests a scaling law for the critical power for the onset of internal transport barriers.
Three novel high-resolution nonlinear methods for fast signal processing
Authors: Belkić Dž. , Dando P. A. , Main J. , Taylor H. S.
Published: July 2002

Three novel nonlinear parameter estimators are devised and implemented for accurate and fast processing of experimentally measured or theoretically generated time signals of arbitrary length. The new techniques can also be used as powerful tools for diagonalization of large matrices that are customarily encountered in quantum chemistry and elsewhere. The key to the success and the common denominator of the proposed methods is a considerably reduced dimensionality of the original data matrix. This is achieved in a preprocessing stage called beamspace windowing or band-limited decimation. The methods are decimated signal diagonalization (DSD), decimated linear predictor (DLP), and decimated Padé approximant (DPA). Their mutual equivalence is shown for the signals that are modeled by a linear combination of time-dependent damped exponentials with stationary amplitudes. The ability to obtain all the peak parameters first and construct the required spectra afterwards enables the present methods to phase correct the absorption mode. Additionally, a new noise reduction technique, based upon the stabilization method from resonance scattering theory, is proposed. The results obtained using both synthesized and experimental time signals show that DSD/DLP/DPA exhibit an enhanced resolution power relative to the standard fast Fourier transform. Of the three methods, DPA is found to be the most efficient computationally.
Transport diffusion of argon in AlPO4-5 from equilibrium molecular dynamics simulations
Authors: Hoogenboom J. P. , Tepper H. L. , van der Vegt N. F. A. , Briels W. J.
Published: July 2002

Transport diffusion of argon in the unidirectional channels of the molecular sieve AlPO4-5 has been studied using molecular dynamics simulations. Using the Green–Kubo formalism, this nonequilibrium property is, for the first time, extracted from just one equilibrium simulation. Apart from the computational advantages above nonequilibrium simulations, the new method also provides a way to check the validity of the assumption of linear response, which is at the basis of both methods. The transport diffusion coefficient for argon at 87 K and half the maximum loading is found to be equal to Dt=(1.4±0.1)×10−5 cm2/s, of which approximately 20% can be attributed to correlated, collective motion.
Comment on “Generation of plasma rotation by ion cyclotron resonance heating in tokamaks” [Phys. Plasmas 6, 1969 (1999)]
Authors: Shaing K. C.
Published: July 2002

It is shown by considering the general properties of solution of the kinetic equation that rf wave heated tokamak plasmas cannot be modeled correctly by a radial current.
The structure of planar and curved detonation waves with reversible reactions
Authors: Sharpe Gary J.
Published: July 2002

The structure of both steady planar and slowly varying weakly curved detonations with one reversible reaction are investigated. For reactive systems with reversible reactions there are two distinguished sound speeds in the equilibrium fluid; the frozen sound speed and the equilibrium sound speed. According to the Chapman–Jouguet condition, self-sustaining, steady and planar detonation waves in such systems are equilibrium sonic at the end of the reaction zone. In this paper, it is shown that for any small, but nonzero, curvature of the front, the solution passes through a frozen sonic point where the thermicity simultaneously vanishes, the so-called generalized Chapman–Jouguet condition. Hence, the structure for the steady, planar wave, which is frozen subsonic throughout and is equilibrium sonic at the end of the reaction zone, is a singular limit of the structure of curved detonation waves as the curvature tends to zero. Since in any real detonation there will always be some curvature of the front, however small, the classical equilibrium sonic Chapman–Jouguet point is unobtainable and instead the frozen sonic point of the generalized Chapman–Jouguet condition must be considered. Detonation speed-curvature relations are determined for the system. Very long-time, high resolution, one-dimensional numerical simulations, starting from a time-dependent ignition problem, in both planar and cylindrical geometry are also performed. The results are compared with the steady planar or quasisteady weakly curved solutions.
State-to-state and total rotational energy transfer rate constants for CN(B 2Σ+,v=0,N)+He and C2N2
Authors: Guo Jingzhong , Sadowski C. M. , Gao Q. , Morgan F. J.
Published: July 2002

Total rotational energy transfer (RET) rate constants, state-to-state RET rate constants, and radiative lifetimes have been measured for CN(B 2Σ+, v=0, Ni=0, 4, 6, 7, 8, 11 and 13). All experiments were conducted under single collision conditions in a static cell filled with previously prepared C2N2/He mixtures at various total pressures up to a maximum of 800 mTorr. The above initial states were prepared by pumping CN(X) with a pulsed dye laser tuned to appropriate frequencies in the P-branch of the CN-violet (0,0) band following the production of CN(X) by the excimer laser photolysis of C2N2 at 157 nm. Total RET rate constants and radiative lifetimes were determined from decay constants obtained with a transient digitizer over the above range of cell pressures. State-to-state RET rate constants were obtained from time-averaged fluorescence spectra in the R branch of the (0,0) band of the CN violet system at various cell pressures within the above range. A weak propensity for odd jumps in N was discernable in state-to-state RET rate constants in CN(B)/He collisions for N>4. Total RET rate constants varied only slightly with Ni for 0⩽Ni⩽19 with He as collision partner, but decreased monotonically with increasing Ni when C2N2 served as collision partner. No firm evidence for collisionally induced CN(B)↔CN(A) was apparent from our data.
Photoluminescence properties of GaNP/GaP multiple quantum wells grown by gas source molecular beam epitaxy
Authors: Xin H. P. , Tu C. W.
Published: July 2002

GaN x P 1−x / GaP multiple quantum wells (MQWs) with various N concentrations and well thicknesses were grown on (100) GaP substrates by gas source molecular beam epitaxy with a radio frequency nitrogen radical beam source. The N concentration and GaNP well thickness were determined by high-resolution x-ray rocking curve measurements and theoretical dynamical simulations. Photoluminescence (PL) measurements show that the PL wavelength of GaNP redshifts and the band edge emission integrated intensity increases with increasing N concentration, up to 2.5%. By using an infinite barrier model and various well thicknesses of GaN0.025P0.975/GaP MQWs, a large conduction-band effective mass mc*∼0.9 me is obtained for the GaN0.025P0.975 alloy, indicating a mixing of Γ and X wave functions in the conduction band.
Analytical modeling of the resolution of photorefractive multiple quantum well spatial light modulators
Authors: Abeeluck A. K. , Garmire E. , Canoglu E.
Published: July 2002

An analytical model that includes lateral drift of photocarriers is developed to explain the experimental resolution of photorefractive multiple quantum well (MQW) devices. The model is in excellent agreement with a phenomenological fit proposed earlier and it follows a small intensity modulation model closely. Charge distributions with and without lateral drift of carriers at the MQW interfaces are assumed in order to calculate the peak diffraction efficiency as a function of grating period. An effective mobility-lifetime product is used to account for the effect of lateral drift in the MQW region. It is shown that good agreement with experiment is obtained when lateral drift is taken into account. Moreover, the model is applied to the study of design tradeoff between resolution, sensitivity, and diffraction efficiency which are all of crucial importance in the performance of practical devices.
Very small temperature-dependent band-gap energy in TlInGaAs/InP double heterostructures grown by gas-source molecular-beam epitaxy
Authors: Ayabe A. , Asahi H. , Lee H. J. , Maeda O. , Konishi K. , Asami K. , Gonda S.
Published: July 2002

TlInGaAs/InP double heterostructures (DHs) were grown on (100) InP substrates by gas-source molecular-beam epitaxy. Almost no occurrence of Tl interdiffusion at the InP/TlInGaAs heterointerface was confirmed. The photoluminescence (PL) intensity for the DH was approximately ten times stronger than that of the single heterostructure. The PL peak energy and its variation with temperature for the TlInGaAs/InP DH decreased with increasing Tl composition. For the DH with a Tl composition of 13%, the PL peak energy varied only slightly with temperature (0.03 meV/K). This value corresponds to a wavelength variation of 0.04 nm/K and is much smaller than that of the lasing wavelength of InGaAsP/InP distributed feedback laser diodes (0.1 nm/K).
Hartree–Fock and Kohn–Sham atomic-orbital based time-dependent response theory
Authors: Larsen Helena , Joørgensen Poul , Olsen Jeppe , Helgaker Trygve
Published: July 2002

A reformulation of general time-dependent Hartree–Fock and Kohn–Sham response theories that refers strictly to the atomic-orbital basis is presented. It is based on a recently proposed exponential parametrization of the one-electron atomic-orbital density matrix. In the presented formulation, only matrix multiplications and additions of sparse matrices are needed to compute the response functions and linear scaling with system size may, therefore, be obtained. Thus, this formalism is well suited to the computation of dynamic and static properties for large molecules at the Hartree–Fock and Kohn–Sham density-functional levels of theory.
Metal-assisted chemical etching in HF/H2O2 produces porous silicon
Authors: Li X. , Bohn P. W.
Published: July 2002

A simple and effective method is presented for producing light-emitting porous silicon (PSi). A thin (d<10 nm) layer of Au, Pt, or Au/Pd is deposited on the (100) Si surface prior to immersion in a solution of HF and H2O2. Depending on the type of metal deposited and Si doping type and doping level, PSi with different morphologies and light-emitting properties is produced. PSi production occurs on the time scale of seconds, without electrical current, in the dark, on both p- and n-type Si. Thin metal coatings facilitate the etching in HF and H2O2, and of the metals investigated, Pt yields the fastest etch rates and produces PSi with the most intense luminescence. A reaction scheme involving local coupling of redox reactions with the metal is proposed to explain the metal-assisted etching process. The observation that some metal remains on the PSi surface after etching raises the possibility of fabricating in situ PSi contacts.
Refractive index and elastic properties of z-cut quartz shocked to 60 kbar
Authors: Jones S. C. , Gupta Y. M.
Published: July 2002

Shock wave experiments were performed to characterize z-cut, α-quartz as an optical window for velocity interferometry measurements. Refractive index changes and shock velocities were determined to 60 kbar peak stress. Results indicate that the window correction to velocity measurements is a constant fraction of the actual particle velocity, Δu/u=0.081 07. Shock velocity measurements provide the Hugoniot curve to 60 kbar, and yield the fourth-order elastic constant, C3333=174 810 kbar. By combining the velocity correction factor and the shock velocity measurements, the refractive index is determined to be a linear function of density. This finding is used to show that the velocity correction appropriate for a step jump may be applied to monotonically time-dependent wave forms with negligible error. The present work has demonstrated the use of z-cut quartz as an optical window to 60 kbar.
Simplified calculation of the stability matrix for semiclassical propagation
Authors: Garashchuk Sophya , Light John C.
Published: July 2002

We present a simple method of calculation of the stability (monodromy) matrix that enters the widely used semiclassical propagator of Herman and Kluk and almost all other semiclassical propagators. The method is based on the unitarity of classical propagation and does not involve any approximations. The number of auxiliary differential equations per trajectory scales linearly rather than quadratically with the system size. Just the first derivatives of the potential surface are needed. The method is illustrated on the collinear H3 system.
Structure factors for the simplest solvable model of polydisperse colloidal fluids with surface adhesion
Authors: Gazzillo Domenico , Giacometti Achille
Published: July 2002

Closed analytical expressions for scattering intensity and other global structure factors are derived for a new solvable model of polydisperse sticky hard spheres. The starting point is the exact solution of the “mean spherical approximation” for hard core plus Yukawa potentials, in the limit of infinite amplitude and vanishing range of the attractive tail, with their product remaining constant. The choice of factorizable coupling (stickiness) parameters in the Yukawa term yields a simpler “dyadic structure” in the Fourier transform of the Baxter factor correlation function qij(r), with a remarkable simplification in all structure functions with respect to previous works. The effect of size and stickiness polydispersity is analyzed and numerical results are presented for two particular versions of the model: (i) when all polydisperse particles have a single, size-independent, stickiness parameter, and (ii) when the stickiness parameters are proportional to the diameters. The existence of two different regimes for the average structure factor, respectively above and below a generalized Boyle temperature which depends on size polydispersity, is recognized and discussed. Because of its analytic nature and simplicity, the model may be useful in the interpretation of small-angle scattering experimental data for polydisperse colloidal fluids of neutral particles with surface adhesion.
Experimental confirmation of the insensitivity of mass diffusion measurements to blockages and voids along the diffusion path
Authors: Banish R. Michael , Alexander J. Iwan D. , Jalbert Lyle B.
Published: July 2002

Using the real-time diffusion methodology first developed by Codastefano et al. [Rev. Sci. Instrum. 48, 1650 (1977)] we show that deviations from strictly one-dimensional transport (i.e., blockages and voids) widely cited in the literature as leading to erroneous results, have little effect on the measured diffusivity. In this methodology, radiotracer, initially located at one end of a cylindrical diffusion sample, is used as the diffusant. The sample is positioned in a concentric isothermal radiation shield with collimation bores located at defined positions along its axis. The intensity of the radiation emitted through the collimators is measured as a function of time using solid state detectors. Diffusivities are calculated from the signal difference between the detectors. These results were obtained using 114mIn radiotracer in benign indium. A 60% blockage was simulated by using a 2 mm source disk diffusing into 3 mm diameter host section. A void/bubble was simulated by inserting a 1 mm diameter by 1 mm long side plug into the sample 7 mm from the top (radiotracer section). The resulting self-diffusivities obtained were, for both modifications, comparable with other ground experiments. These results agree with numerical simulations. An analytical model describing the minimal effects of blockages and voids on the output diffusivity is presented.
Three-dimensional ab initio potential energy surface for He–O2
Authors: Groenenboom Gerrit C. , Struniewicz Izabela M.
Published: July 2002

An interaction potential with vibrational coordinate dependence is presented for the He–O2(X 3Σg−) van der Waals complex. The interaction is calculated with the partially spin-restricted open-shell single and double excitation coupled cluster method with perturbative triples [RCCSD(T)] in the supermolecule approach, with correction for the basis set superposition error. The augmented correlation consistent polarized triple zeta basis set is used with an additional set of bond orbitals. The single reference configuration method breaks down at an O–O separation of r≈3.5 a0. No points with r>3.1 a0 are used in the construction of the potential surface. A high quality 120 parameter fit to 754 ab initio points is presented. Special attention is given to the extraction of the long range coefficients.
H loss mechanism during anneal of silicon nitride: Chemical dissociation
Authors: Boehme Christoph , Lucovsky Gerald
Published: July 2002

Remote plasma enhanced chemical vapor deposited silicon nitride (SixNyHz), produced at high ammonia to silane flow rates (ammonia rich) shows a reduction of hydrogen during rapid thermal anneal at temperatures that exceed the deposition temperature. This H release could be either due to a “slow” atomic diffusion of the covalent bonded atoms between bonding sites, or to a “fast” molecular diffusion of hydrogen containing molecules (e.g., H2, NH3, SiH4), which dissociate from the network before they diffuse. In order to determine the dominant mechanism, layers of deuterated and hydrogenated silicon nitride on top of a crystalline Si substrate were annealed and the development of the NH- and ND-area densities were measured with Fourier transform infrared spectroscopy. Comparison of theoretical models with the measurements showed that chemical dissociation and subsequent rapid diffusion are the dominant processes. These results were confirmed by secondary ion mass spectroscopy. The experiments indicate that the H reduction in silicon nitride antireflection coatings of solar cells is mostly due to H migration out of the system and not into the Si area and make the hypothesis that postdeposition annealing of solar cell antireflection coatings can cause H-related bulk passivation of the underlying c-Si therefore questionable.
Composition-dependent dynamical structures of 1-propanol–water mixtures determined by dynamical dielectric properties
Authors: Sato Takaaki , Chiba Akio , Nozaki Ryusuke
Published: July 2002

Using time domain reflectometry, we carried out dielectric relaxation measurements on 1-propanol–water mixtures for the entire concentration range in the frequency range 100 MHz–25 GHz at 20, 25, and 30 °C. We have calculated the excess partial molar activation free energy, enthalpy, and entropy for 1-propanol, ΔG1PAE, ΔH1PAE, and ΔS1PAE, and those for water, ΔGWE, ΔHWE, and ΔSWE from the relaxation times. In the region of X (molar fraction of 1-propanol) ⩾0.14, ΔH1PAE and ΔS1PAE take nearly zero. This means that 1-propanol molecules in the mixtures find themselves in not a very different environment from that in pure liquid. In the water-rich region, ΔH1PAE and ΔS1PAE exhibit two maxima at X∼0.03 and X∼0.06, corresponding roughly to 0.9 and 0.78 g/cm3 of water content, respectively. This fact, together with the results of the molecular dynamics studies of Sciortino et al. suggest the formation of two kinds of saturated hydration structures: the clathrate hydration shells with tetrahedral local arrangements of water molecules around X∼0.03 and nonclathrate shells with large cavities with three-coordinated local arrangements around X∼0.06. Hydrophobic hydration seems to partly share the same mechanism with structural enhancement in pure water by lowering local density.
Melting of monolayer protected cluster superlattices
Authors: Sandhyarani N. , Antony M. P. , Selvam G. Panneer , Pradeep T.
Published: July 2002

Melting of crystalline solids (superlattices) of octadecanethiol and octanethiol protected silver clusters has been studied with x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and infrared (IR) spectroscopy. These solids have been compared with the silver thiolate layered compounds in view of their similarity in alkyl chain packing and x-ray diffraction patterns. Superlattice melting is manifested in XRD around 400 K as the complete disappearance of all the low angle reflections; only bulk silver reflections due to the cluster cores are seen at 423 K. The superlattice structure is regained upon cooling from a temperature close to its melting point. However, cooling from a higher temperature of 473 K does not regain the superlattice order, whereas thiolate melting is repeatedly reversible even at these temperatures. Transmission electron microscopy suggests aggregation of clusters during heating/cooling cycles. DSC shows two distinct transitions, first corresponding to alkyl chain melting and the second corresponding to superlattice melting. Only alkyl chain melting is observed in variable temperature IR and increased order is manifested upon repeated heating/cooling cycles. Alkyl chain assembly shows strong interchain coupling leading to factor group splitting in cluster superlattices upon annealing. In thiolates only one melting feature is seen in DSC and it produces gauche defects, whereas significant increase in defect structures is not seen in superlattices. Repeated heating/cooling cycles increase interchain interactions within a cluster and the superlattice order collapses.
Computer simulation study of the local pressure in a spherical liquid–vapor interface
Authors: El Bardouni H. , Mareschal M. , Lovett R. , Baus M.
Published: July 2002

The pressure profiles across a liquid–vapor interface introduced previously [J. Chem. Phys. 106, 635 (1997)] have been evaluated with the aid of molecular dynamics simulations for a system of particles interacting via a (truncated and shifted) Lennard-Jones potential. This investigation extends earlier results [J. Chem. Phys. 106, 645 (1997)] to spherical interfaces. Further evidence is found that, for the range of curvatures investigated, the surface tension is curvature independent while the investigation of larger curvatures is prevented by the considerable noise found on the liquid side of the interface.
Quantitative measurement of muscle hemoglobin oxygenation using near-infrared spectroscopy with correction for the influence of a subcutaneous fat layer
Authors: Niwayama Masatsugu , Lin Ling , Shao Jun , Kudo Nobuki , Yamamoto Katsuyuki
Published: July 2002

The inhomogeneity of tissue structure greatly affects the sensitivity of tissue oxygenation measurement by reflectance near-infrared spectroscopy. In this study, we investigated the influence of a fat layer on muscle oxygenation measurement by in vivo tests and Monte Carlo simulation, and we propose a method for correcting the influence. In the simulation, a three-dimensional model consisting of the epidermis, dermis, fat, and muscle layers was used. In in vivo tests, measurement sensitivity was examined by measuring oxygen consumption of the forearm muscle and the peak-to-peak variation of oxygenation in periodic exercise tests on the vastus lateralis using a newly developed multisensor type of tissue oximeter. Fat layer thickness was also measured by ultrasonography. The correction curve of measurement sensitivity against fat layer thickness was obtained from the results of simulation and in vivo tests. The values of corrected oxygen consumption were almost the same and had less variation between individuals (0.13±0.02 ml 100 g−1 min−1) than did the uncorrected values (0.08±0.04 ml 100 g−1 min−1).
Monte Carlo simulations of Wyoming sodium montmorillonite hydrates
Authors: Chávez-Páez M. , Van Workum K. , de Pablo L. , de Pablo J. J.
Published: July 2002

Monte Carlo simulations have been used to predict the interlayer basal separations of sodium-saturated Wyoming clays at constant stress (NPzzT ensemble) and at constant chemical potential (μVT ensemble). These simulations use the Ewald summation technique to incorporate long-range Coulombic interactions in the calculation of the total potential energy and the pressure tensor. A comparison is made between the use of one, two, and three sheets of clay. It is shown that, for small separations, at least two separate clay sheets must be used to avoid system-size effects. The stable interlamellar separations are determined by combining results from isostress–isothermal and grand canonical simulations. It is shown that, consistent with experiments, at the temperature and pressure studied here, the cations in the interlayer are hydrated, except at the smallest basal separations.
Deep-level impurities in CdTe/CdS thin-film solar cells
Authors: Balcioglu A. , Ahrenkiel R. K. , Hasoon F.
Published: July 2002

We have studied deep-level impurities in CdTe/CdS thin-film solar cells by capacitance–voltage (C–V), deep-level transient spectroscopy (DLTS), and optical DLTS (ODLTS). CdTe devices were grown by close-spaced sublimation. Using DLTS, a dominant electron trap and two hole traps were observed. These traps are designated as E1 at EC−0.28 eV, H1 at EV+0.34 eV, and H2 at EV+0.45 eV. The presence of the E1 and H1 trap levels was confirmed by ODLTS. The H1 trap level is due to Cu-induced substitutional defects. The E1 trap level is believed to be a deep donor and is attributed to the doubly ionized interstitial Cu or a Cu complex. The E1 trap is an effective recombination center and is a lifetime killer.
Parametric investigation of nonadiabatic compressible flow around airfoils
Authors: Lee Jang-Chang , Rusak Zvi
Published: July 2002

The compressible flow of a mixture of dry air (an inert carrier gas) and water vapor with nonequilibrium and homogeneous condensation around thin airfoils is examined. The investigation is based on a recent transonic small-disturbance (TSD) theory. The theory provides the governing similarity parameters of the flow and condensation process. The parameters give the relationship between the flow properties, the amount of water vapor mass in the air, and the airfoil’s chord and thickness ratio. The results of varying the similarity parameters are demonstrated by numerical simulations. It is found that heat addition as a result of condensation causes significant changes in the compressible flow behavior and affects the aerodynamic performance of airfoils. Increasing the free-stream frozen Mach number or the airfoil’s chord (with fixed free-stream temperature, pressure, and amount of water vapor) augments the condensation region and the amount of heat transfer to the flow. The heat transfer to the flow also results in the appearance of a compression wave in addition to a regular shock wave. Also, decreasing the free-stream temperature with a fixed amount of water vapor in the air or increasing the free-stream temperature with a fixed free-stream supersaturation ratio enlarges the condensation region and reduces the shock wave strength. These results demonstrate the possible use of condensation phenomena to control aerodynamic characteristics of airfoils operating in internal flow systems.