bluuebunny/crossref_metadata_2025 huggingface dataset:
The photodissociation of N,N-dimethylformamide: A complete active space self-consistent field study
Authors: Liu Dan , Fang Weihai , Lin Zhenyang , Fu Xiaoyuan
Published: November 2002

The S0, S1, and T1 equilibrium geometries of N,N-dimethylformamide (NNDMF) and transition state structures of the HC(O)–N(CH3)2 and HC(O)N(CH3)–CH3 bond cleavages in the S1 and T1 states were determined with the complete active space self-consistent field method using the 6-31G** basis set. The surface intersections among the S0, S1, and T1 states were optimized with the state-averaged complete active space self-consistent field method. The most probable mechanisms, leading to different products for the NNDMF photodissociation at 193 nm, were provided in the present paper. The obtained mechanisms are slightly different from those proposed previously on the basis of qualitative analyses of state correlation and electronic configurations between the reactants and products. This is discussed in detail.
The generalized Douglas–Kroll transformation
Authors: Wolf Alexander , Reiher Markus , Hess Bernd Artur
Published: November 2002

We derive the most general parametrization of the unitary matrices in the Douglas–Kroll (DK) transformation sequence for relativistic electronic structure calculations. It is applied for a detailed analysis of the generalized DK transformation up to fifth order in the external potential. While DKH2–DKH4 are independent of the parametrization of the unitary matrices, DKH5 turns out to be dependent on the third expansion coefficient of the innermost unitary transformation which is carried out after the initial free-particle Foldy–Wouthuysen transformation. The freedom in the choice of this expansion coefficient vanishes consistently if the optimum unitary transformation is sought for. Since the standard protocol of the DK method is the application of unitary transformations to the one-electron Dirac operator, we analyze the DKH procedure up to fifth order for hydrogenlike atoms. We find remarkable accuracy of the higher-order DK corrections as compared to the exact Dirac ground state energy. In the case of many-electron atomic systems, we investigate the order of magnitude of the higher-order corrections in the light of the neglect of the DK transformation of the two-electron terms of the many-particle Hamiltonian. A careful analysis of the silver and gold atoms demonstrates that both the fourth- and fifth-order one-electron DK transformation yield a smaller contribution to the total electronic energy than the DK transformation of the two-electron terms. In order to improve significantly on the third-order correction DKH3, it is thus mandatory to include the DK transformation of the two-electron terms as well as the spin-dependent terms before proceeding to higher orders in the transformation of the one-electron terms. However, an analysis of the ionization energies of these atoms indicates that already DKH3 yields a highly accurate treatment of the scalar-relativistic effects on properties.
Implementation of generalized valence bond-inspired coupled cluster theories
Authors: Van Voorhis Troy , Head-Gordon Martin
Published: November 2002

We present an implementation of the recently proposed imperfect pairing (IP) and generalized valence bond restricted coupled cluster (GVB-RCC) methods. Our algorithm centers on repeated construction of Coulomb and exchange matrices. These operations are the computational bottleneck, scaling with the third power of system size for large systems. Robust optimization of the valence orbitals is attained using a geometrically consistent form of direct minimization. Analytic gradients of the IP and GVB-RCC energies are also obtained by a simple modification of the energy optimization scheme. As an illustration of the potential of these new methods, we use IP to compute the equilibrium geometry and energetics of a Si9H12 cluster that is a crude model for silicon dimerization on the Si(001) surface. We thus demonstrate a valuable role for IP and GVB-RCC as a diagnostic for the accuracy of reduced active space calculations as compared to their full valence analogs.
Thermodynamic consistency in dissipative particle dynamics simulations of strongly nonideal liquids and liquid mixtures
Authors: Trofimov S. Y. , Nies E. L. F. , Michels M. A. J.
Published: November 2002

Dissipative particle dynamics (DPD) is a mesoscopic simulation method for studying hydrodynamic behavior of complex fluids. Ideally, a mesoscopic model should correctly represent the thermo- and hydrodynamic properties of a real system beyond certain length and time scales. Traditionally defined DPD quite successfully mimics hydrodynamics, but is not flexible enough to accurately describe the thermodynamics of a real system. The so-called “multibody” DPD (MDPD) is a pragmatic extension of the classical DPD that allows one to prescribe the thermodynamic behavior of a system with only a small performance impact. Here we present a practical improvement to the “multibody” DPD model and test it on a number of single-component examples. We also generalize MDPD to multicomponent systems, which are an important target of DPD studies. The improved model provides a correction for particle correlations in strongly nonideal systems that were neglected in the original MDPD model. The implications of the coarse-graining procedure on the MDPD are discussed.
Accurate method for the Brownian dynamics simulation of spherical particles with hard-body interactions
Authors: Barenbrug Theo M. A. O. M. , Peters E. A. J. F. (Frank) , Schieber Jay D.
Published: November 2002

In Brownian Dynamics simulations, the diffusive motion of the particles is simulated by adding random displacements, proportional to the square root of the chosen time step. When computing average quantities, these Brownian contributions usually average out, and the overall simulation error becomes proportional to the time step. A special situation arises if the particles undergo hard-body interactions that instantaneously change their properties, as in absorption or association processes, chemical reactions, etc. The common “naı̈ve simulation method” accounts for these interactions by checking for hard-body overlaps after every time step. Due to the simplification of the diffusive motion, a substantial part of the actual hard-body interactions is not detected by this method, resulting in an overall simulation error proportional to the square root of the time step. In this paper we take the hard-body interactions during the time step interval into account, using the relative positions of the particles at the beginning and at the end of the time step, as provided by the naı̈ve method, and the analytical solution for the diffusion of a point particle around an absorbing sphere. Öttinger used a similar approach for the one-dimensional case [Stochastic Processes in Polymeric Fluids (Springer, Berlin, 1996), p. 270]. We applied the “corrected simulation method” to the case of a simple, second-order chemical reaction. The results agree with recent theoretical predictions [K. Hyojoon and Joe S. Kook, Phys. Rev. E 61, 3426 (2000)]. The obtained simulation error is proportional to the time step, instead of its square root. The new method needs substantially less simulation time to obtain the same accuracy. Finally, we briefly discuss a straightforward way to extend the method for simulations of systems with additional (deterministic) forces.
Secondary grain growth and surface morphology of post-annealed nanocrystalline Pb0.97Nd0.02(Zr0.55Ti0.45)O3 thin films
Authors: Lappalainen Jyrki , Ivanov Sergey A. , Lantto Vilho
Published: November 2002

Nd-modified PZT [PNZT,Pb0.97Nd0.02(Zr0.55Ti0.45)O3] thin films were deposited using a pulsed XeCl-excimer laser on single-crystal MgO(100) substrate at room temperature. After deposition, the PNZT thin-film samples were post-annealed at three different temperatures of 700, 800, and 900 °C. The crystal structure and crystallite size distribution were measured using conventional x-ray diffraction methods. Atomic force microscopy was used for the surface morphology characterization of both amorphous and polycrystalline films. Detailed grain structure of annealed PNZT films was studied in perpendicular film-substrate cross sections using field-emission scanning electron microscopy (FESEM). When annealing temperature increased from 700 to 900 °C, an orientation of growing grains in the film with (001) planes parallel to the film surface took place. The mean crystallite size increased from 210 to 330 Å, respectively. FESEM micrographs revealed a subpopulation of columnar grains in the films growing downward from the upper surface of the film as a result of secondary grain growth due to minimization of the surface, strain, and ferroelectric depolarization energies. At the same time, the surface roughness was considerably decreased.
A molecular-dynamics simulation study of the switching dynamics of a nematic liquid crystal under an applied electrical field
Authors: Tian Pu , Bedrov Dmitry , Smith Grant D. , Glaser Matthew , Maclennan Joseph E.
Published: November 2002

To understand the microscopic mechanism of nematic switching in many liquid crystal devices, we have performed a molecular-dynamics simulation study of the switching dynamics of nematics with positive polarizability anisotropy under an applied electrical field. Both pretilted nematics (PNs) and nonpretilted nematics (NPNs) under different field strengths are studied to investigate the effects of pretilt and field strength on the switching dynamics. Nematic molecules were modeled as rigid rods which experienced electrical torque, in a mean field approximation, imposed by uniform electrical fields. Our measured switching dynamics agree qualitatively with experiment by exhibiting initiation, fast reorientation, and slow relaxation stages. Coherence lengths under applied fields were estimated from the elastic constants calculated from our simulations. For all systems where the coherence length was larger than the simulation cell size (weak fields), unidirectional switching was observed. For field strengths yielding a coherence length smaller than the simulation cell size (strong fields), NPNs exhibited bidirectional switching. For the PNs, the reorientation of the global nematic director in response to the applied field was well described by a simple Leslie–Ericksen equation with the rate of reorientation being closely related to the torque due to the external field. For NPNs in the strong field regime, the local director within each unidirectionally switching domain exhibited reorientational dynamics similar to that of the PNs.
Simulation of hot-electron oxide tunneling current based on a non-Maxwellian electron energy distribution function
Authors: Gehring A. , Grasser T. , Kosina H. , Selberherr S.
Published: November 2002

For the simulation of gate oxide tunneling currents in sub-quarter-micron devices, the correct modeling of the electron energy distribution function is crucial. Our approach is based on a recently presented transport model which accounts for six moments of the Boltzmann transport equation. A corresponding analytical model for the electron energy distribution function shows good agreement with Monte Carlo data. Using this model, we show that the gate current behavior of short-channel devices can be reproduced correctly. This is not the case for the heated Maxwellian approximation which leads to a massive overestimation of gate currents especially for devices with small gate lengths. We develop a formalism to distinguish between cases where the heated Maxwellian distribution delivers correct results and cases where it overestimates the tunneling current at low drain bias and find that for oxide thicknesses around 2 nm, the heated Maxwellian approximation is only valid for electron temperatures below about 1000 K.
Thermal stability of the exchanged biased CoFe/IrMn electrode for the magnetic tunnel junction as a function of CoFe thickness
Authors: Lee J. H. , Kim S. J. , Yoon C. S. , Kim C. K. , Park B. G. , Lee T. D.
Published: November 2002

Pinned electrodes of the magnetic tunnel junction (MTJ) consisting of Ta/AlOx/CoFe(x)/IrMn/NiFe/Ta, where 35 Å<x<87.5 Å, were thermally annealed and analyzed using Auger electron spectroscopy and transmission electron microscopy in order to study the effects of the CoFe thickness on the interdiffusion of elements in the pinned electrode. Increasing CoFe thickness reduced the Mn migration out of the IrMn layer towards the tunnel barrier. An 87.5−Å−thick CoFe layer completely blocked the Mn diffusion up to 350 °C with minimal reduction of the tunneling magnetoresistance (TMR) ratio when full junction was fabricated. Although other mechanisms could be responsible for the thermal degradation of the MTJ, the Mn diffusion appears to be related to the reduction of the TMR at 300 °C. Since the presence of Mn in the tunnel barrier as an impurity is detrimental to the junction performance, reduction of Mn migration towards the tunnel barrier by increasing the CoFe electrode should improve the postannealed performance of the junction.
Zero energy kinetic electron and mass analyzed threshold ionization spectroscopy of Na⋅(NH3)n (n=1, 2, and 4) complexes
Authors: Peng Xianzhao , Kong Wei
Published: November 2002

We report spectroscopic measurements of Na⋅(NH3)n (n=1, 2, and 4) complexes using zero energy kinetic electron spectroscopy (ZEKE) and mass analyzed threshold ionization spectroscopy (MATI). Our MATI spectrum of Na⋅NH3 agrees with the ZEKE spectrum from a previous report, and the relative vibrational intensity distribution of the ν3 mode qualitatively agrees with the corresponding Franck–Condon factors. The ZEKE spectrum of Na⋅(NH3)2 shows a long vibrational progression superimposed on a continuous feature. Based on an intensity calculation, where the vibrational angular momentum of the cation and Franck–Condon factors are taken into account, we are able to reproduce the experimental spectrum and obtain new spectroscopic information. The ionization threshold is determined to be 28 727±30 cm−1, and the vibrational frequencies for the v16 intermolecular bending mode of the cation and the neutral ground state are 28.35 cm−1 and 25.1 cm−1, respectively. The ZEKE spectrum of Na⋅(NH3)4 is structureless, and we believe it is a superposition of two processes: a real ZEKE signal and a false ZEKE signal generated by trapped energetic electrons in the excitation region. We attribute the lack of structure in the ZEKE spectrum to the high internal temperature of the cluster and the high density of states at the ionization threshold.
Propagation properties of a light wave in a film quasiwaveguide structure
Authors: Zhang Xi-Jing , Fan Xi-Zhi , Liao Jun , Wang Hui-Tian , Ming N. B. , Qiu Ling , Shen Yu-Quan
Published: November 2002

We suggest a three-dimensional theoretical model to explain the origin of light wave propagation in a quasiwaveguide structure formed by a thin film of refractive index smaller than the substrate. The physical principles involved are illustrated by the use of a combination of multiple-wave interference and coherent multiple scattering. Not only the theoretical details and the important analytic formulas describing the properties of wave propagation in the quasiwaveguide were given, but also the positions and linewidths of the quasiwaveguide modes, the intensity distribution in the film, the intensity of the reflected spot, and the intensity distribution of the scattered light (m lines) are studied in detail. We divide the modes in the quasiwaveguide into three kinds: cavity modes, leaky modes, and degenerate modes. Some interesting problems are discussed. We investigate the properties of wave propagation in a quasiwaveguide made with a doped polymer film and discuss the origin of scattering.
Chemical imaging by a computer screen aided scanning light pulse technique
Authors: Filippini Daniel , Lundström Ingemar
Published: November 2002

A scanning light pulse technique (SLPT) for the generation of selective chemical images is demonstrated, using a broadly available programmable light source provided by a standard cathode-rays tube computer screen, which supplies two-dimensional micropositioning and intensity modulation. The concept is tested with a metal–oxide–semiconductor capacitor with catalytic gates of platinum and palladium spatially distributed in transversal directions, which produce distinctive response patterns upon hydrogen or ammonia exposures. The nonlaterally resolved images formed by the weighted contribution of the chemically sensitive areas of the gate, subsequently highlighted by the exciting light along its scanning path, retains the ability of SLPT to generate distinctive chemical images.
Thickness dependent phase separation in La0.7Ca0.3MnO3 films
Authors: Rauer R. , Bäckström J. , Budelmann D. , Kurfiß M. , Schilling M. , Rübhausen M. , Walter T. , Dörr K. , Cooper S. L.
Published: November 2002

The complex dielectric function of La0.7Ca0.3MnO3 films with various thicknesses and on different substrates was determined by the analysis of measured ellipsometric parameters. The temperature-dependent redistribution of low-energy spectral weight shows that the metallicity is developed gradually below an onset temperature that is found to depend strongly on global and local strain properties, not on film thickness alone. Importantly, the 2 nm film shows no metallic conduction but an optical charge-carrier response below 240 K. This suggests that ultrathin films are characterized by phase separation and a percolation-type transition.
Ionization-assisted deposition of SrOx for electron injection layer of organic light emitting diode
Authors: Tanaka Kuniaki , Maeda Kenji , Usui Hiroaki
Published: November 2002

Thin SrOx films were prepared as the electron injection layer for organic light emitting diodes. The film deposition process gave a significant influence on the device characteristics, and the ionization-assisted deposition (IAD) gave considerable improvement over the conventional vacuum deposition. X-ray photoelectron spectroscopy and atomic force microscope observation showed that the IAD has the possibility of controlling two factors; the oxidation state and the growth morphology of SrOx film. The IAD–SrOx films had smaller oxygen content than the vacuum-deposited ones. In addition, the IAD gave uniform surface coverage at a smaller thickness, reflecting the initial film growth morphology that consisted of uniformly dispersed small grains instead of coagulated islands by the vacuum deposition.
Coefficient determination related to optical gain in erbium-doped silicon-rich silicon oxide waveguide amplifier
Authors: Han Hak-Seung , Seo Se-Young , Shin Jung H. , Park Namkyoo
Published: November 2002

Gain-determining coefficients in Er-doped, nanocrystal-Si (nc-Si) sensitized silica waveguide amplifiers are investigated. Single-mode, Er-doped silica waveguides with nc-Si embedded in them were prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition of Er-doped a-Si:Ox (x<2) followed by a high-temperature anneal to precipitate nc-Si. Exciting the Er ions via nc-Si by pumping the waveguide from the top with the 477 nm line of an Ar laser resulted in an enhancement of the transmitted 1535 nm signal of up to 14 dB/cm, indicating a possible net gain of up to 7 dB/cm. From the dependence of the signal enhancement upon the pump power, an emission cross section of 2×10−19 cm2 at 1535 nm and an effective excitation cross section of ⩾10−17 cm2 at 477 nm is obtained.
Multiple ion-focusing effects in plasma immersion ion implantation
Authors: Tian X. B. , Chu Paul. K.
Published: November 2002

In plasma immersion ion implantation, the sample is negatively biased and a plasma sheath forms. Ions are accelerated to the sample surface through this sheath. The electric field contours dictate the shape of the plasma sheath that wraps around corners and tends to be smoother and rounder than the surface topography, for instance, at a sharp corner. Our theoretical and experimental studies reveal ion flux focusing effects leading to lateral nonuniformity of the incident ion dose. Ion focusing occurs not only at the sample edge but also in the central region even for a planar sample (wafer). In this work, we numerically and experimentally investigate this ion focusing effect and ion dose nonuniformity. A simple geometric model is also presented in this letter to understand the mechanism. The results demonstrate that ion focusing originates from plasma sheath convergence that is time and space dependent. Consequently, multiple ion focusing may occur at different local sites when the target shape and processing parameters vary, and a small plasma sheath relative to the target is of paramount importance for uniform implantation.
1212 nm pumping of 2 μm Tm–Ho-codoped silica fiber laser
Authors: Taniguchi Atsushi , Kuwayama Tetsuro , Shirakawa Akira , Musha Mitsuru , Ueda Ken-ichi , Prabhu Mahendra
Published: November 2002

A Tm–Ho-codoped silica fiber laser pumped by a third-stokes Raman Fiber Laser at 1212 nm is demonstrated. For the 30-cm-long Tm–Ho-codoped fiber, a 400 mW output power is obtained at a wavelength of 1790 nm due to the laser emission from the Tm3+ ions. For the 270-cm-long Tm–Ho-codoped fiber, a 450 mW output power is obtained at 1970 nm due to the laser emission from Ho3+ ions.
Scaling of nano-Schottky-diodes
Authors: Smit G. D. J. , Rogge S. , Klapwijk T. M.
Published: November 2002

A generally applicable model is presented to describe the potential barrier shape in ultrasmall Schottky diodes. It is shown that for diodes smaller than a characteristic length lc (associated with the semiconductor doping level) the conventional description no longer holds. For such small diodes the Schottky barrier thickness decreases with decreasing diode size. As a consequence, the resistance of the diode is strongly reduced, due to enhanced tunneling. Without the necessity of assuming a reduced (non-bulk) Schottky barrier height, this effect provides an explanation for several experimental observations of enhanced conduction in small Schottky diodes.
Atomic layer deposition of ZnSe/CdSe superlattice nanowires
Authors: Solanki R. , Huo J. , Freeouf J. L. , Miner B.
Published: November 2002

Atomic layer deposition has been employed to grow nanowires composed of ZnSe/CdSe superlattices. Growth of the nanowires was initiated using gold nanoparticles and the vapor-liquid-solid mechanism. High-resolution transmission electron microscopy shows that these structures are single crystals and the phase of alternating layers of ZnSe and CdSe is zinc blende. The (111) planes of ZnSe and CdSe are oriented at 60°.
Ultrafast dynamics of interfacial electric fields in semiconductor heterostructures monitored by pump-probe second-harmonic generation
Authors: Glinka Yu. D. , Shahbazyan T. V. , Perakis I. E. , Tolk N. H. , Liu X. , Sasaki Y. , Furdyna J. K.
Published: November 2002

We report measurements of the ultrafast dynamics of interfacial electric fields in semiconductor multilayers using pump-probe second-harmonic generation (SHG). A pump beam was tuned to excite carriers in all the layers in GaAs/GaSb and GaAs/GaSb/InAs heterostructures. The resulting carrier dynamics manifests itself via electric fields created by charge separation due to carrier redistribution at the interfaces. The evolution of interfacial fields is monitored by a probe beam through an eletric-field-induced SHG signal. We distinguish between several stages of dynamics originating from redistribution of carriers between the layers. We also find a strong enhancement of the induced electric field caused by hybridization of the conduction and valence bands at the GaSb/InAs interface.
Maximal properties of the normalized Cauchy transform
Authors: Poltoratski Alexei
Published: November 2002

We study the normalized Cauchy transform in the unit disk. Our goal is to find an analog of the classical theorem by M. Riesz for the case of arbitrary weights. Let

μ
\mu

be a positive finite measure on the unit circle of the complex plane and

f
∈

L

1

(
μ
)

f\in L^{1}(\mu )

. Denote by

K
μ

K\mu

and

K
f
μ

Kf\mu

the Cauchy integrals of the measures

μ
\mu

and

f
μ

f\mu

, respectively. The normalized Cauchy transform is defined as

C

μ

:
f
↦

K
f
μ

K
μ

C_{\mu }: f\mapsto \frac {Kf\mu }{K\mu }

. We prove that

C

μ

C_{\mu }

is bounded as an operator in

L

p

(
μ
)

L^{p}(\mu )

for

1
>
p
≤
2

1>p\leq 2

but is unbounded (in general) for

p
>
2

p>2

. The associated maximal non-tangential operator is bounded for

1
>
p
>
2

1>p>2

and has weak type

(
2
,
2
)

(2,2)

but is unbounded for

p
>
2

p>2

.
An international analysis of real estate journals
Authors: Newell Graeme , Acheampong Peter , Juchau Roger , Kwong Wing Chau , Webb James R.
Published: November 2002

The leading real estate journals in the USA, UK, Asia and Australia are analysed over 1991‐2000 to assess the impact of international real estate research in these journals. It is found that the focus on international real estate has expanded considerably in recent years, with this focus more evident in the leading UK real estate journals rather than the leading USA real estate journals. Reasons for this difference between the USA and UK real estate journals are identified. Issues relating to international authorship are assessed, with the leading international authors and universities identified.
REITs in the decentralized investment industry
Authors: Chiang Kevin C.H. , Lee Ming‐Long
Published: November 2002

Existing studies provide conflicting results regarding whether real estate investment trusts (REITs) effectively optimize and diversify institutional portfolios. Based on the style analysis of Sharpe, we extend Liang and McIntosh’s study with a more complete set of asset classes over a longer sample period. We provide additional evidence suggesting that practicing analysts should include REITs as an asset class to optimize their portfolios. Specifically, our results show that the price behavior of REITs is unique and cannot be satisfactorily duplicated by combining equity, fixed‐income securities, and unsecuritized real estate. The time series of the styles on REITs indicates that it is difficult to ex ante produce returns on REITs without diversifying into REITs.
A Possible Limiting Case Behavior for Brittle Material Fracture
Authors: Christensen R. M.
Published: November 2002

The self-consistent scheme is used to model the state of an elastic material with a very high density of nearly connected cracks. Then fracture mechanics is used to pose the problem of the complete and final failure of the material under uniaxial and eqibiaxial tension. These failure states are taken to be those of the extreme case of brittle fracture. A specific form for the resulting extreme brittle failure criterion is given.
On the Singularity Induced by Boundary Conditions in a Third-Order Thick Plate Theory
Authors: Huang C. S.
Published: November 2002

This paper thoroughly examines the singularity of stress resultants of the form r−ξFθ for 0<ξ⩽1 as r→0 (Williams-type singularity) at the vertex of an isotropic thick plate; the singularity is caused by homogeneous boundary conditions around the vertex. An eigenfunction expansion is applied to derive the first known asymptotic solution for displacement components, from the equilibrium equations of Reddy’s third-order shear deformation plate theory. The characteristic equations for determining the singularities of stress resultants are presented for ten sets of boundary conditions. These characteristic equations are independent of the thickness of the plate, Young’s modulus, and shear modulus, but some do depend on Poisson’s ratio. The singularity orders of stress resultants for various boundary conditions are expressed in graphic form as a function of the vertex angle. The characteristic equations obtained herein are compared with those from classic plate theory and first-order shear deformation plate theory. Comparison results indicate that different plate theories yield different singular behavior for stress resultants. Only the vertex with simply supported radial edges (S(I)_S(I) boundary condition) exhibits the same singular behavior according to all these three plate theories.
Axisymmetric Instability of Fluid Saturated Pervious Cylinders
Authors: Bardet J. P. , Iai S.
Published: November 2002

The emergence of two-phase instability is investigated analytically for the axisymmetric cylinders made of a pervious solid matrix with pores filled with an interstitial fluid. General analytical solutions are derived for a broad range of constitutive models, and are illustrated for a few specific types of solids. For particular combinations of stresses and material moduli, saturated hypoelastic and elastoplastic solids are found to undergo two-phase instability, whereas their dry solid matrices remain stable. Two-phase instability can emerge within stable single-phase solids due to the interaction between solid matrix and fluid flow. The present analysis provides general analytical solutions useful for investigating the instabilities of axisymmetric soil samples subjected to the undrained triaxial tests of geomechanics.
Thermal stability of Pr2O3 films grown on Si(100) substrate
Authors: Goryachko A. , Liu J. P. , Krüger D. , Osten H. J. , Bugiel E. , Kurps R. , Melnik V.
Published: November 2002

We have investigated the effect of thermal annealing on uncapped and Si-capped Pr2O3 films deposited on Si(100) substrate by Auger electron spectroscopy, x-ray photoelectron spectroscopy, and transmission electron microscopy. A rapid diffusion of Si out of the substrate was found for uncapped films at temperatures starting from 700 °C. The diffused Si is oxidized, forming a Pr2O3–Prx–Oy–Siz mixture. The excess Si diffuses through the film, forming a SiO2 rich layer on the surface. Annealing of uncapped films in vacuum has qualitatively similar effects as annealing in N2, which is not the case for Si-capped films. The latter were transformed into a Pr2O3–Prx–Oy–Siz mixture when annealed in N2 starting from 900 °C and into PrSix when annealed in vacuum starting from 700 °C. Two distinct PrSix phases were found, with Si-richer phases corresponding to higher annealing temperature.
Investigation of thermal flux to the substrate during sputter deposition of aluminum
Authors: Ekpe Samuel D. , Dew Steven K.
Published: November 2002

The transient and steady state thermal flux at the substrate during the deposition of aluminum film in a direct current magnetron sputter system has been determined by measuring the resistance of a complementary metal–oxide–semiconductor (CMOS) sensor. The sensor is calibrated using ohmic self-heating before the plasma is switched on. The steady state thermal flux at the substrate was measured to vary from 9.6 to 46 mW/cm2 at a substrate-target distance of 10.8 cm depending on the magnetron power (75–300 W) and gas pressure. Plasma radiation and electron bombardment are noted to be the most significant sources of the thermal flux to the substrate, each contributing about 36% and 29%, respectively, of the total thermal flux at the substrate for a magnetron power of 200 W and gas pressure of 5 mTorr. Thermal radiation is also an important factor, along with kinetic energy and condensation energy. Total energy per deposited atom is calculated to be in the range of 28–52 eV depending on the magnetron power and gas pressure, and increases with pressure but decreases with magnetron power. The trend seems to suggest that at higher magnetron powers (>300 W for a 3 in. target), a pressure independent total energy per deposited atom may be obtained.
Highly 〈100〉-oriented growth of polycrystalline silicon films on glass by pulsed magnetron sputtering
Authors: Reinig P. , Selle B. , Fenske F. , Fuhs W. , Alex V. , Birkholz M.
Published: November 2002

Nominally undoped polycrystalline silicon (poly-Si) thin films were deposited on glass at 450 °C at high deposition rate (>100 nm/min) by pulsed dc magnetron sputtering. The pulse frequency was found to have a significant influence on the preferred grain orientation. The x-ray diffraction pattern exhibits a strong enhancement of the (400) reflex with increasing pulse frequency. The quantitative evaluation reveals that over 90% of the grains are 〈100〉 oriented. The observed change in preferred grain orientation in poly-Si films at low temperatures is associated with concurrent ion bombardment of the growing film.
Growth of Be2C(100) films on Be(0001) substrate using C60 as precursor
Authors: Tzeng C.-T. , Yuh J.-Y. , Lo W.-S. , Chu R.-Y. , Tsuei K.-D.
Published: November 2002

Highly oriented crystalline beryllium carbide films were grown on Be(0001) substrate using C60 as a carbon source. The films were characterized by low energy electron diffraction, photoemission spectroscopy, and near-edge x-ray absorption fine structure. C60 begins to decompose on Be(0001) at about 250 °C, forming beryllium carbide completely after further annealing to 450 °C. The beryllium carbide film is observed as sets of (100) surfaces, arranged in three domains rotated by 120° from each other. Extra C60 deposited on Be2C(100)/Be(0001) at temperature below 200 °C and heated to 450 °C leads to an increase of the film thickness, indicating the decomposition of C60 on Be2C(100)/Be(0001) at an elevated temperature and formation of new carbide layers on the sample surface. It further implies that the Be2C/Be surface has the ability to supply Be atoms to interact with the new carbon atoms on top, and that Be atoms can diffuse through the beryllium carbide layer at the temperature of 450 °C, possibly involving a vacancy mechanism.
Column angle variations in porous chevron thin films
Authors: Harris K. D. , Vick D. , Smy T. , Brett M. J.
Published: November 2002

We describe the fabrication of porous chevron thin films deposited at glancing incidence and report some common growth peculiarities of the microstructure. In particular, the second (and additional) chevron arms of porous chevron thin films are consistently inclined at angles significantly greater than the corresponding first arms. This effect is observed in both real and simulated films and has important implications in the fabrication of optical coatings. A simple theoretical model is developed to describe this effect, and a technique for achieving consistent column angles is presented.
Atomic layer deposition of zirconium silicate films using zirconium tetrachloride and tetra-n-butyl orthosilicate
Authors: Kim Won-Kyu , Kang Sang-Woo , Rhee Shi-Woo , Lee Nae-In , Lee Jong-Ho , Kang Ho-Kyu
Published: November 2002

Atomic layer chemical vapor deposition of zirconium silicate films with a precursor combination of ZrCl4 and tetra-n-butyl orthosilicate (TBOS) was studied for high dielectric gate insulators. The effect of deposition conditions, such as deposition temperature, pulse time for purge and precursor injection on the deposition rate per cycle, and composition of the film were studied. At 400 °C, the growth rate saturated to 1.35 Å/cycle above 500 sccm of the argon purge flow rate. The growth rate, composition ratio ((Zr/Zr+Si)), and impurity contents (carbon and chlorine) saturated with the increase of the injection time of ZrCl4 and TBOS and decreased with the increased deposition temperature from 300 to 500 °C. The growth rate, composition ratio, carbon, and chlorine contents of the Zr silicate thin films deposited at 500 °C were 1.05 Å/cycle, 0.23, 1.1 at. %, and 2.1 at. %, respectively. It appeared that by using only zirconium chloride and silicon alkoxide sources, the content of carbon and chlorine impurities could not be lowered below 1%. It was also found that the incorporation rate of metal from halide source was lower than alkoxide source.
A Monte Carlo investigation of fluence profiles collimated by an electron specific MLC during beam delivery for modulated electron radiation therapy
Authors: Deng Jun , Lee Michael C. , Ma C.‐M.
Published: November 2002

Modulated electron radiation therapy (MERT) is able to deliver conformal dose to shallow tumors while significantly reducing dose to distal structures and surrounding tissues. An electron specific multileaf collimator (eMLC) has been proposed and constructed as an effective means of delivering electron beams for MERT. The aim of this work is to apply the Monte Carlo method to investigate the fluence profiles collimated by the eMLC in order to achieve accurate beam delivery for MERT. In this work, theEGS4/BEAMcode was used to simulate the eMLC collimated electron beams of 6–20 MeV generated from a Varian Clinac 2100C linear accelerator. An attempt was made to describe the fluence profiles with an analytic Sigmoid function. The function parameters were determined by the fittings of the Monte Carlo simulated fluence profiles. How the function parameters depend on the eMLC aperture size, the off‐axis location, and the electron beam energy has been investigated. It has been found that the eMLC collimated fluence profiles are dependent on beam energy, while almost independent of leaf location or dimension of MLC aperture. There is little difference in the fluence profiles collimated by the leaf side and the leaf end for the straight‐edged leaves. It is possible that these energy‐dependent Sigmoid functions can serve as operators to account for the energy dependence of the eMLC collimated fluence profiles. These operators can be incorporated into the inverse planning algorithm to derive desired dose distributions using a set of electron beams of variable energy and field size suitable for delivery by the eMLC.
Analytical representation of enhanced dynamic wedge factors for symmetric and asymmetric photon fields
Authors: Yu M. K.
Published: November 2002

The Enhanced Dynamic Wedge (EDW) presents many advantages over the physical wedge. However, in order to calculate monitor units (MUs) necessary to deliver a certain dose at a certain point, EDW factors (EDWFs) need to be determined. In this work, based on analysis of the golden segmented treatment table (GSTT) and the MU fraction model, an empirical analytic formula has been developed to calculate EDW factors for symmetric and asymmetric fields. This formalism is an extension of the MU fraction model. However in comparison with previous studies [J. P. Gibbons, Med. Phys. 25, 1411–1418 (1998) and M. Miften et al., Med. Dosim. 25, 81–86 (2000)], this formula is simpler, and easier to use. It is applicable to EDW fields of different sizes, wedge angles and different photon energies. For 6 and 18 MV beams from a Varian 21EX accelerator with 7 EDW angles (Varian Oncology Systems, Palo Alto, CA), more than 250 measured EDWFs for symmetric and asymmetric fields with different off‐axis distances and field sizes were compared with model calculations. Results show that 80% and 98% of calculated EDWFs match corresponding measured values to within 0.5% and 1.0%, respectively, the maximum deviation being 1.3%.
Ultrasound measurements of overlying soft tissue thickness at four skeletal sites suitable for in vivo x‐ray fluorescence
Authors: Pejović‐Milić A. , Brito J. A. , Gyorffy J. , Chettle D. R.
Published: November 2002

Due to signal attenuation in overlying soft tissue, development of x‐ray fluorescence systems to measure low atomic number elements, such as strontium, in human bone required a search for a skeletal site with thin overlying tissue. This paper reports ultrasound measurements of overlying tissue on 10 subjects, at four anatomical sites. The average tissue thickness at the finger was The average tissue thicknesses were and at forehead, tibia and heel, respectively. Additionally, both parametric and nonparametric approaches to the relationship between body mass index (BMI) and tissue thickness suggest that there is a significant linear correlation between the subject's BMI and overlying tissue at the finger and heel bone. These correlations might be used as a criterion to perform XRF measurements, however a larger data set is required to address these correlations more clearly.
Lung nodule detection on thoracic computed tomography images: Preliminary evaluation of a computer‐aided diagnosis system
Authors: Gurcan Metin N. , Sahiner Berkman , Petrick Nicholas , Chan Heang‐Ping , Kazerooni Ella A. , Cascade Philip N. , Hadjiiski Lubomir
Published: November 2002

We are developing a computer‐aided diagnosis (CAD) system for lung nodule detection on thoracic helical computed tomography (CT) images. In the first stage of this CAD system, lung regions are identified by a k‐means clustering technique. Each lung slice is classified as belonging to the upper, middle, or the lower part of the lung volume. Within each lung region, structures are segmented again using weighted k‐means clustering. These structures may include true lung nodules and normal structures consisting mainly of blood vessels. Rule‐based classifiers are designed to distinguish nodules and normal structures using 2D and 3D features. After rule‐based classification, linear discriminant analysis (LDA) is used to further reduce the number of false positive (FP) objects. We performed a preliminary study using 1454 CT slices from 34 patients with 63 lung nodules. When only LDA classification was applied to the segmented objects, the sensitivity was 84% (53/63) with 5.48 (7961/1454) FP objects per slice. When rule‐based classification was used before LDA, the free response receiver operating characteristic (FROC) curve improved over the entire sensitivity and specificity ranges of interest. In particular, the FP rate decreased to 1.74 (2530/1454) objects per slice at the same sensitivity. Thus, compared to FP reduction with LDA alone, the inclusion of rule‐based classification lead to an improvement in detection accuracy for the CAD system. These preliminary results demonstrate the feasibility of our approach to lung nodule detection and FP reduction on CT images.
Assaying line sources using a standard length well chamber
Authors: Brezovich Ivan A. , Popple Richard A. , Duan Jun , Shen Sui , Pareek Prem N.
Published: November 2002

The strength of intravascular sources is typically measured by the manufacturer before shipment, and treatment planning is based on that assay. However, in‐house verification of source strength is required at some institutions by state law or internal policy, is recommended by the AAPM TG 60 report on intravascular brachytherapy, and is considered a necessity by many medical physicists. To accommodate the long sources used in intravascular therapy, special well chambers with extended regions of constant response have been designed. To allow assays using a widely available standard well chamber, we have measured its position dependent sensitivity and derived from it a table of correction factors that account for the extended length of intravascular sources. An experimental verification shows that application of these correction factors yields assays with sufficient accuracy for routine quality assurance tests.
Monte Carlo evaluation of 6 MV intensity modulated radiotherapy plans for head and neck and lung treatments
Authors: Wang Lu , Yorke Ellen , Chui Chen‐Shou
Published: November 2002

Intensity modulated radiotherapy (IMRT) beams may have strong fluence variations and are advantageous at disease sites such as lung and head and neck (H&N), where neighboring tissues have very different electron densities. We use Monte Carlo (MC) dose calculations to evaluate the dosimetric effects of these inhomogeneities for 10 clinical IMRT treatment plans for five lung patients and four H&N patients. All beams are 6 MV photons. “Standard plans” were first produced on a clinical treatment planning system which optimizes beam intensity distributions to meet dose and dose‐volume constraints and calculates dose using a measurement‐based pencil‐beam algorithm with an equivalent pathlength inhomogeneity correction. Patient anatomy and electron densities were obtained from patient‐specific CT images. The dose distribution of each beam was recalculated with the MC method, using the same CT images, beam geometry, beam weighting and optimized fluence intensity distributions as the corresponding standard plan. For the lung cases, the MC calculated dose distributions are characterized by reduced penetrations and increased penumbra due to larger secondary electron range in the low‐density media, which is not accurately accounted for in the pencil beam algorithm. For the lung cases, the PTV was underdosed; except for one dose‐volume index, underdose was less than 10%. Individual H&N fields are affected to different degrees by tissue inhomogeneities, depending on specific anatomy, especially the size and location of air cavities in relation to the beam orientation and field size. For four H&N plans, PTV coverage changed by less than 2%; for the fifth, there was less than 10% difference between the standard and the MC plans. Critical normal tissue DVHs (cord, lung, brainstem) are changed by <10% at the high dose end and mean lung doses are changed by <6%.
An accelerator based system for in vivo neutron activation analysis measurements of manganese in human hand bones
Authors: Arnold M. L. , McNeill F. E. , Stronach I. M. , Pejovic‐Milic A. , Chettle D. R. , Waker A.
Published: November 2002

Manganese (Mn) is an essential nutrient for growth and development. Unfortunately, overexposure can lead to neurological damage, which is manifested as a movement disorder marked by tremors. Preclinical symptoms have been found in populations occupationally exposed to the element, and it is suggested that in late stages of the disorder, removing the Mn exposure will not prevent symptoms from progressing. Hence, it is desirable to have a means of monitoring Mn body burden. In vivo neutron activation analysis (IVNAA) is a technique which allows the concentration of some elements to be determined within sites of the body without invasive procedures. Data in the literature suggests that the Mn concentration in bone is greater than other tissues, and that it may be a long term storage site following exposure. Therefore, using the McMaster KN‐accelerator to produce neutrons through the reaction, the feasibility of IVNAA for measuring Mn levels in the human hand bone was investigated. Mn is activated through the reaction, and the 847 keV gamma rays emitted when decays are measured outside the body using NaI(Tl) detectors. An optimal incident proton energy of 2.00 MeV was determined from indium foil and microdosimetry measurements. Hand phantom data suggest a minimum detectable limit of approximately 1.8 ppm could be achieved with a reasonably low dose of 50 mSv to the hand (normal manganese levels in the human hand are approximately 1 ppm). It is recommended the technique be developed further to make human in vivo measurements.
PAIR OF HEAVY-EXOTIC-QUARKS AT LHC
Authors: CIEZA MONTALVO J. E. , DE QUEIROZ FILHO P. P.
Published: November 2002

We study the production and signatures of heavy exotic quarks pairs at LHC in the framework of the vector singlet model (VSM), vector doublet model (VDM) and fermion-mirror-fermion (FMF) model. The pair production cross-sections for the electroweak and strong sector are computed.
A NOTE ON THE CYLINDRICAL COLLAPSE OF COUNTER-ROTATING DUST
Authors: GONÇALVES SÉRGIO M. C. V. , JHINGAN SANJAY
Published: November 2002

We find analytical solutions describing the collapse of an infinitely long cylindrical shell of counter-rotating dust. We show that — for the classes of solutions discussed herein — from regular initial data a curvature singularity inevitably develops, and no apparent horizons form, thus in accord with the spirit of the hoop conjecture.
RESTRICTIONS ON DILATONIC BRANE-WORLD MODELS
Authors: ZHUK A.
Published: November 2002

We consider dilatonic brane-world models with a non-minimal coupling between dilaton and usual matter on the brane. We demonstrate that the variation of fundamental constants on the brane due to such interaction can lead to strong restrictions on model parameters. In particular, the experimental bounds on the variation of the fine structure constant rule out non-minimal dilatonic models with a Liouville-type coupling potential f(φ) = exp (bφ) where [Formula: see text].