bluuebunny/crossref_metadata_2025 huggingface dataset:
On the interpretation of force extension curves of single protein molecules
Authors: Makarov Dmitrii E. , Wang Zhisong , Thompson James B. , Hansma Helen G.
Published: July 2002

The atomic force microscope can be used to forcibly unfold and extend single polypeptide chains. The resulting force versus distance curves have been widely interpreted to arise from the loss of entropy that the unfolded polypeptide chain experiences as it is extended. Here, we have used Monte Carlo simulations of unfolded polypeptide chains to examine the average distance between the ends of a polypeptide chain as a function of the force that pulls these ends apart. We examine two types of experiments: (a) A rigid force-sensor (bead-type) experiment: The chain is subjected to a constant stretching force f and the resulting chain extension is measured. (b) A flexible force-sensor (cantilever-type) experiment: The force is measured by the deflection of a cantilever that is attached to one end of the chain. The total length of the chain plus the displacement of the cantilever is fixed. In case (b), in the limit of a large cantilever force constant, the entropic force f is related to the free energy of the chain F(r) constrained to have the end-to-end distance r by the usual thermodynamic relationship: f=dF/dr. However in case (a) this relationship is invalid. The reason of its failure is that large fluctuations in the end-to-end distance r cannot be neglected at the single molecule level and so macroscopic thermodynamics relationships cannot be used. Thus the two types of experiments measure different force extension curves f(r). We compute the force extension curves for a model of a polypeptide chain in each case and find that they are significantly different. We further discuss implications of our findings with regard to the results of cantilever-type unfolding experiments.
Global prescribed mean curvature foliations in cosmological space–times. II
Authors: Henkel Oliver
Published: July 2002

This paper is devoted to the investigation of global properties of prescribed mean curvature (PMC) foliations in cosmological space–times with local U(1)×U(1) symmetry and matter described by the Vlasov equation. It turns out that these space–times admit a global foliation by PMC surfaces as well, but the techniques to achieve this goal are more complex than in the cases considered in Paper I [Henkel (2002)].
A random-field model for polarization reversal in Pb(Yb1/2Nb1/2)O3–PbTiO3 single crystals
Authors: Zhang Shujun , Priya Shashank , Furman Eugene , Shrout Thomas R. , Randall Clive A.
Published: July 2002

Polarization switching and dielectric temperature behavior have been investigated in 0.55Pb(Yb1/2Nb1/2)O3–0.45PbTiO3 (PYN–PT) single crystals. These compositions are of interest owing to their unusually high Curie temperature and high coercive fields. The dielectric permittivity was determined as function of temperature and the polarization was measured as a function of applied field for different drive frequencies. Analysis of the experimental results with a random-field model was found to show similar trends to recent reports for the Pb(Mg1/3Nb2/3)O3 relaxor materials. The frequency dispersion of the maximum in dielectric spectra, Tmax, was also analyzed with the Vogel–Fulcher relation yielding a high freezing temperature of 577 K. Polarization behavior was found to follow a stretched exponential time dependence and a fractal dimension of ∼2.2 was obtained. It is proposed that the polar cluster state in PYN-PT single crystals do not freeze in random orientations but rather arrange locally in preferred configurations. The fraction of the clusters decreases under increasing external field drive, resulting in the formation of the percolated network at higher fields.
Effects of barrier alloy composition and number of stacks in the optical and structural characteristics of strain compensated InxGa1−xAsyP1−y/InzGa1−zAstP1−t/InP multiquantum wells
Authors: Maltez R. L. , Ribeiro E. , Bernussi A. A. , Carvalho W. , Gobbi A. L. , Ugarte D.
Published: July 2002

We have optically and structurally investigated the effect of the barrier alloy composition and number of wells in strain compensated InGaAsP/InP multiquantum well (MQW) samples grown by low-pressure metalorganic vapor phase epitaxy that emit close to 1.55 μm. Low temperature photoluminescence (PL) spectra exhibited two additional anomalous emission lines, below the fundamental QW transition. The PL temperature evolution of such anomalous transitions deviates significantly from the bulk-like band gap behavior expected. Cross-sectional transmission electron microscopy (XTEM) shows that partial elastic relaxation, characterized by localized surface deformations (like surface channels) and/or interfacial undulations occurs for all samples in which anomalous PL lines are observed. TEM results also show that the extra PL lines are correlated to neither Cu–Pt type atomic ordering in the well/barrier layers nor to phase separation. These anomalous transitions are tentatively attributed to the presence of interfacial undulation as well as to surface deformation and to the presence of interfacial domains with an intermediary composition between those of the barrier and QW layers.
A new tool for measuring polar magneto-optical Kerr hysteresis curves at high fields and low temperatures
Authors: Geerts Wilhelmus J. , Donehew Brian , Williams Vaughan , Schmiedel Thomas
Published: July 2002

The design of a unique probe to measure the magneto-optical (MO) Kerr rotation and MO Kerr ellipticity of thin films and multilayers at high magnetic fields (0–25 T) and low temperatures (2–325 K) is described. The design of the probe is based on direct optics. Magnetic iron was used to screen the optical components from the stray field of the magnet. The equipment has sensitivity better than 0.25 mdegree at 632.8 nm and the residual background that results from the Faraday rotation in the optical components is smaller than 2 mdegree/T. The probe measures the polar MO hysteresis curve of samples with dimensions from 0.5–2 cm. Preliminary results on a 9 monolayer iron film and a single-crystalline FePt sample show the performance of this new characterization tool. This new MO magnetometer can be used in one of the resistive magnets of the National High Magnetic Field Laboratory in Tallahassee and is available for all internal and external users.
Band gap energy bowing and residual strain in CuAl(SxSe1−x)2 chalcopyrite semiconductor epilayers grown by low-pressure metalorganic vapor phase epitaxy
Authors: Harada Yoshiyuki , Nakanishi Hisayuki , Chichibu Shigefusa F.
Published: July 2002

A quadratic dependence of the band gap energy on the alloy composition x was quantified for CuAl(SxSe1−x)2 films grown by low-pressure metalorganic vapor phase epitaxy, by means of photoreflectance and photoluminescence excitation spectroscopies. The bowing parameter for the A-exciton energy was estimated to be 0.20 eV. Several high-quality films grown on GaAs(001) substrates exhibited excitonic photoluminescence peaks in the blue to ultraviolet spectral ranges. The flow rate of the Al precursor was found to affect the incorporation ratio of S/Se, indicating that the Al–S compound plays a key role in controlling x. All films grown on GaAs(001) showed c(001) orientation. Conversely, the epitaxitial orientation of the films on GaP(001) changed from a(100) to c(001) with an increase in x. The critical value of x was around 0.5. The preferred orientations were explained by the natural selection rule under which the lattice strain in the epilayer is minimized. The residual strain in the 0.5-μm-thick epilayers on GaAs(001) was nearly constant for all x, although the lattice mismatch between the epilayer and the GaAs substrate varied from 0.62% to 5.39% with an increase in x. Consequently the strain was attributed to thermal stress.
Coherent states and annihilation–creation operators associated with the irreducible unitary representations of 𝔰𝔲(1,1)
Authors: Sakaguchi Fuminori , Hayashi Masahito
Published: July 2002

We construct a kind of annihilation–creation operators related to the affine coherent states. Next, we re-interpret them as annihilation–creation operators associated with the irreducible unitary representation of the algebra 𝔰𝔲(1,1), by adding another generator to the two generators of the unitary representation of the one-dimensional affine group.
A simple tool to cut the protruding flange of photoresist formed in spin coating process for lithography technology
Authors: He Y. Z. , Niu H. , Qi J. J. , Zhao Y. G. , Cao B. S.
Published: July 2002

A facile tool is suggested for cutting the protruding flange of photoresist (PR) along the edge of a small substrate surface, which is caused by the spin-coating process in lithography. When the dimension of the substrate reduces to a few millimeters, the edge problem of PR cannot be ignored. Since the PR's thickness along the edge of the sample is much larger than that in the middle, a traditional double exposure method does not work well. The cutter proposed here consists of a blade and a sample holder, which are installed in a base. By adjusting the blade at a suitable dip angle and pushing it to the flange of PR along the edge of substrate fixed by the sample holder, the flange of PR of small samples will be eliminated completely with great ease.
Investigation of the interlayer characteristics of Ta2O5 thin films deposited on bare, N2O, and NH3 plasma nitridated Si substrates
Authors: Lai Yi-Sheng , Chen Kuan-Jen , Chen J. S.
Published: July 2002

Low-temperature N2O and NH3 plasma nitridations on Si surfaces are conducted to prevent the thermodynamic instability at the Ta2O5/Si interface. The surface and interface of the Ta2O5/Si systems, without or with nitridation, were examined by x-ray photoelectron spectroscopy (XPS), atomic force microscopy, transmission electron microscopy, and secondary ion mass spectrometry. Capacitance–voltage (C–V) measurements were carried out to investigate the electric/defect characteristics of the Ta2O5/Si systems. For the non-nitrided Si substrate XPS detects no surface oxide formed prior to Ta2O5 deposition. In contrast, NH3 plasma nitridation forms a nitrogen-rich SiOxNy layer, while N2O plasma nitridation produces an oxygen-rich SiOxNy layer on Si. C–V measurement reveals high densities of fixed charges, trapping sites, and interface states in the Ta2O5/non-nitrided Si sample, indicative that a defective interlayer was formed during Ta2O5 deposition. Ta2O5 on both nitrided systems exhibits a reduced amount of fixed charges and trapping sites. Nevertheless, NH3 plasma nitridation is not as effective as N2O plasma nitridation in diminishing the density of interface states. The different electric/defect characters between the NH3 and N2O nitrided systems are attributed to the further oxidation of the nitrogen-rich SiOxNy layer produced by NH3 plasma nitridation, which induces defects at the dielectric/Si interface. Effectiveness of the nitridation process, therefore, depends on the stability of the nitrided layer during deposition of Ta2O5.
Structure, collective hydrogen transfer, and formation of Si(OH)4 in SiO2–(H2O)n clusters
Authors: Cheng Hai-Ping , Barnett Robert N. , Landman Uzi
Published: July 2002

SiO 2 –water clusters are studied using first-principles Born–Oppenheimer molecular dynamics based on density functional theory and generalized gradient approximations. Systematic investigations of structure and energetics as functions of cluster size demonstrate the roles of water molecules in chemical reactions. The water-assisted formation of a Si(OH)4 molecule from a single SiO2 molecule is revealed at the atomic level. The dynamics of dissociation of water molecules and formation of Si–OH bonds is investigated via simulations at finite temperature. A complex process that involves double and triple hydrogen atom transfer is discovered to be the reaction path.
Surface-plasmon-resonance-induced absorption of a metal–oxide nanoparticle composite
Authors: Zhao J. P. , Lu M. , Chen Z. Y. , Rabalais J. W.
Published: July 2002

A surface-plasmon-resonance (SPR)-induced absorption band has been found for low-energy Ti+ implanted (subplanted) into single-crystalline SiO2 at specific substrate temperatures. The observed SPR absorption band is in the 650–850 nm wavelength range, i.e., in the visible (red) and near-infrared regions, indicating the formation of Ti nanoparticles in the subsurface layer of the SiO2 matrix. This was confirmed by calculations based on the Mie scattering theory. The SPR absorption band becomes distinguishable only at temperatures ⩾600 °C and reached its maximum at 800 °C (1/2Tm of elemental Ti). The intensity is significantly reduced at a temperature of 1000 °C (2/3Tm). The evolution of the SPR absorption with substrate temperature is discussed.
Modeling of elastic deformation of multilayers due to residual stresses and external bending
Authors: Hsueh Chun-Hway
Published: July 2002

A general closed-form solution for elastic deformation of multilayers due to residual stresses and external bending is derived. Based on the general solution, simplified solutions for residual stress distributions in multiple layers of thin films on a thick substrate are obtained. These simplified solutions can be expressed as functions of either mismatch strains between film layers and substrate or the curvature of the system. The simplified solution for the special case of one film layer on a substrate is also presented, and its accuracy is discussed.
Lattice polymers with structured monomers: A Monte Carlo study of thermodynamic properties of melts and solutions
Authors: Buta Dorel , Freed Karl F.
Published: July 2002

The influence of monomer structure on the thermodynamic properties of polymer melts and solutions is explored in Monte Carlo computations. The internal energy, the specific heat, and the chemical potential are determined for united atom lattice models of polyethylene, polypropylene, polyethylethylene, and polyisobutylene, all with M=60 united atom groups. Computations are performed for a range of melt volume fractions 0<φ⩽0.7 and effective temperatures 3.3⩽T*⩽∞. The results reveal fine differences between the internal energies of the four different melts (solutions) and a pronounced influence of monomer structure on the specific heat and the chemical potential. The computed thermodynamic quantities are confronted with the analytical results of the lattice cluster theory (LCT). It is shown that for most thermodynamic states the LCT correctly predicts the qualitative effect of monomer structure on the thermodynamic quantities. However, large errors observed in the LCT values for the specific heat, and a qualitative departure between simulation and theory for the difference between the internal energies of polyethylethylene and polyethylene, stress the need both for improvements in the analytical solution of the LCT and for further tests of its accuracy.
Self-consistent effective interactions in charged colloidal suspensions
Authors: Anta Juan A. , Lago Santiago
Published: July 2002

We use an integral equation scheme to obtain self-consistently the effective interaction between colloids in salt-free charged colloidal suspensions. The colloid–counterion direct correlation function (DCF) is obtained for the fixed colloid–colloid pair structure by solving the corresponding hypernetted-chain equation (HNC). This DCF is then used to formulate an effective colloid–colloid pair potential for which the one-component reference hypernetted-chain equation is solved. Both processes are iterated until self-consistency is achieved. Counterion–counterion correlations are considered linear and uncoupled from the rest of the correlations. The method is based on a similar treatment utilized in liquid metals [Phys. Rev. B 61, 11400 (2000)] and provides equivalent results to those obtained using the standard multicomponent HNC equation for mixtures of charged hard spheres. The theory proves rather accurate when compared with molecular dynamic simulations of charged hard and soft spheres for colloidal charges of up to 300. We study in detail the existence of net attractions between colloids in certain cases (especially in the presence of divalent and trivalent counterions) and how this attraction may lead to phase instability. The problem of the lack of solution of the integral equation for more realistic cases (larger charges) is also discussed.
Ab initio study of vibrational anharmonic coupling effects in oligo(para-phenylenes)
Authors: Heimel G. , Somitsch D. , Knoll P. , Zojer E.
Published: July 2002

In this work, we clarify the nature of a previously not precisely identified Fermi dyad in the frequency range around 1600 cm−1 in oligo( para-phenylenes). To this end, we deploy a novel method to calculate third order anharmonic coupling effects in molecules. This Fermi dyad is shown to yield important information on the structural properties of the investigated materials. The nature of all vibrations contributing to this quantum mechanical resonance phenomenon is explained on the basis of a detailed normal coordinate analysis. The anharmonic coupling is then closely investigated by applying our theoretical model. In particular, we discuss the intensity redistribution among the two components of the Fermi doublet as well as their energetic separation. Subsequently, we establish a relation between these features and the structural conformation of the molecules. We show, how oligomer length and planarity of these systems can be determined from experimental Raman spectra by extracting the positions and relative intensities of the two components of the Fermi doublet. Furthermore, this Fermi resonance is shown to be sensitive to chemical modification on the molecules such as deuteration or substitution. Finally, we extend our model to electronically excited states in this class of molecules, as well as to charged species.
Strain-induced tuning of metal–insulator transition in NdNiO3
Authors: Tiwari Ashutosh , Jin C. , Narayan J.
Published: July 2002

We have used the lattice-mismatch epitaxial strain, induced by the constraint of epitaxy, to tune the metal–insulator (M–I) transition temperature of NdNiO3 films grown on Si(100) substrate. Films were integrated with the Si(100) substrate using several combinations of thin buffer layers. A systematic variation in the electrical transport properties has been observed with the change in the lattice mismatch between NdNiO3 and the buffer layer just below it. It was shown that by the proper selection of the substrate and thickness of film, it is possible to control and precisely tune the M–I transition temperature of NdNiO3 to any desired value between 12 and 300 K (temperature range of this study). Fine control over the M–I transition temperature of these films is likely to boost the potential of these films for their applications in bolometers, actuators, and thermal/optical switches in next-generation perovskite-based microelectronic devices.
Three-dimensional phase field microelasticity theory of a complex elastically inhomogeneous solid
Authors: Wang Yu U. , Jin Yongmei M. , Khachaturyan Armen G.
Published: July 2002

A phase field microelasticity theory of a three-dimensional anisotropic solid with arbitrary elastic and structural inhomogeneities is proposed. The theory is based on the strain energy for the elastically and structurally inhomogeneous system as a functional of the phase field, which is the effective misfit strain of the “equivalent” homogeneous modulus system. It is demonstrated that the misfit strain minimizing this functional fully determines the exact elastic equilibrium in the elastically and structurally inhomogeneous solid. The misfit strain minimizer is obtained as a steady-state solution of the time-dependent Ginzburg–Landau equation. Examples of the elastic equilibrium of an elastically inhomogeneous solid under applied stress are considered.
Defect-related density of states in low-band gap InxGa1−xAs/InAsyP1−y double heterostructures grown on InP substrates
Authors: Gfroerer T. H. , Priestley L. P. , Weindruch F. E. , Wanlass M. W.
Published: July 2002

We have measured the excitation-dependent radiative efficiency in a set of lattice-matched InxGa1−xAs/InAsyP1−y double heterostructures incrementally lattice mismatched to InP substrates. We find that the overall rate of defect-related recombination shows little change from the lattice-matched case. However, the excitation-dependent transition between defect-related and radiative recombination changes dramatically with mismatch. While a simple defect recombination model assuming defect levels concentrated near the middle of the band gap fits well for the lattice-matched material, the model does not fit the shape of the efficiency curve for the mismatched structures. We show that the addition of band edge exponential tails to the defect-related density of states gives a much better theoretical fit.
Pattern formation in an N+Q component reaction–diffusion system
Authors: Pearson John E. , Bruno William J.
Published: July 2002

A general N+Q component reaction–diffusion system is analyzed with regard to pattern forming instabilities (Turing bifurcations). The system consists of N mobile species and Q immobile species. The Q immobile species form in response to reactions between the N mobile species and an immobile substrate and allow the Turing instability to occur. These results are valid both for bifurcations from a spatially uniform state and for systems with an externally imposed gradient as in the experimental systems in which Turing patterns have been observed. It is shown that the critical wave number and the location of the instability in parameter space are independent of the substrate concentration. It is also found that the system necessarily undergoes a Hopf bifurcation as the total substrate concentration is decreased. Further, in the case that all the mobile species diffuse at identical rates we show that if the full system is at a point of Turing bifurcation then the N component mobile subsystem is at transition from an unstable focus to an unstable node, and the critical wave number is simply related to the degenerate positive eigenvalue of the mobile subsystem. A sequence of bifurcations that occur in the eigenspectra as the total substrate concentration is decreased to zero is also discussed.
Correlations in quantum time delay
Authors: Eckhardt Bruno
Published: July 2002

The semiclassical periodic orbit content of the form factor K(λ ), the Fourier transform of the autocorrelation function, for the quantum time delay is analyzed. In analogy to the case of bounded systems, three regimes can be identified. For small λ isolated periodic orbits can be identified. For intermediate λ, there is a λ exp(−Γλ) regime, where Γ is the classical escape rate. For large λ, this changes into an exp( − γqmλ) law, where now γqm is related to an inverse lifetime of the resonances. The transition between the latter two regimes is determined by the density of resonances. The theoretical analysis is supported by numerical data for the three disk scattering system.
Modelling complex transient oscillations for the BZ reaction in a batch reactor
Authors: Johnson Barry R. , Scott Stephen K. , Thompson Barnaby W.
Published: July 2002

The recent observations of transient complex oscillations in the Belousov–Zhabotinsky (BZ) reaction in a batch reactor are confirmed and an attempt is made to model this behavior using the Gyorgyi–Field mechanism (an extension of the classic Field–Körös–Noyes model). It is seen that the concentration of bromomalonic acid plays an important role, acting somewhat like a “slowly-varying parameter,” causing the system to sweep through a region of oscillatory response even in the absence of reactant consumption. Complex behavior is not observed in the model if the usual “pre-equilibrium” assumption in made for the intermediate species BrO2 but does arise if a full steady-state approximation is applied to this radical. These results may bring into question some assumptions made in the usual treatment of the cerium-catalyzed BZ system in closed reactors.
Overview: The constructive role of noise in fluctuation driven transport and stochastic resonance
Authors: Astumian R. Dean , Moss Frank
Published: July 2002

Random noise is typically thought of as the enemy of order rather than as a constructive influence. Recent work has shown however that under certain circumstances, noise and Brownian motion can facilitate transmission of information via a mechanism know as stochastic resonance, and help systems use chemical energy and nonequilibrium fluctuations to drive directed motion via fluctuation driven transport. In this focus issue we have collected several articles that capture the flavor of these developing fields and point the way to new directions for research.
Cluster-growth in freely cooling granular media
Authors: Luding S. , Herrmann H. J.
Published: July 2002

When dissipative particles are left alone, their fluctuation energy decays due to collisional interactions, clusters build up and grow with time until the system size is reached. When the effective dissipation is strong enough, this may lead to the “inelastic collapse,” i.e., the divergence of the collision frequency of some particles. The cluster growth is an interesting physical phenomenon, whereas the inelastic collapse is an intrinsic effect of the inelastic hard sphere (IHS) model used to study the cluster growth—involving only a negligible number of particles in the system. Here, we extend the IHS model by introducing an elastic contact energy and the related contact duration tc. This avoids the inelastic collapse and allows to examine the long-time behavior of the system. For a quantitative description of the cluster growth, we propose a burning-like algorithm in continuous space, that readily identifies all particles that belong to the same cluster. The criterion for this is here chosen to be only the particle distance. With this method we identify three regimes of behavior. First, for short times a homogeneous cooling state (HCS) exists, where a mean-field theory works nicely, and the clusters are tiny and grow very slowly. Second, at a certain time which depends on the system’s properties, cluster growth starts and the clusters increase in size and mass until, in the third regime, the system size is reached and most of the particles are collected in one huge cluster.
Heteroclinic connections between periodic orbits and resonance transitions in celestial mechanics
Authors: Koon Wang Sang , Lo Martin W. , Marsden Jerrold E. , Ross Shane D.
Published: July 2002

In this paper we apply dynamical systems techniques to the problem of heteroclinic connections and resonance transitions in the planar circular restricted three-body problem. These related phenomena have been of concern for some time in topics such as the capture of comets and asteroids and with the design of trajectories for space missions such as the Genesis Discovery Mission. The main new technical result in this paper is the numerical demonstration of the existence of a heteroclinic connection between pairs of periodic orbits: one around the libration point L1 and the other around L2, with the two periodic orbits having the same energy. This result is applied to the resonance transition problem and to the explicit numerical construction of interesting orbits with prescribed itineraries. The point of view developed in this paper is that the invariant manifold structures associated to L1 and L2 as well as the aforementioned heteroclinic connection are fundamental tools that can aid in understanding dynamical channels throughout the solar system as well as transport between the “interior” and “exterior” Hill’s regions and other resonant phenomena.
Possible sources of coercivity in thin films of amorphous rare earth-transition metal alloys
Authors: Giles Roscoe , Mansuripur Masud
Published: July 2002

Computer simulations of a two-dimensional lattice of magnetic dipoles are performed on the Connection Machine. The lattice is a discrete model for thin films of amorphous rare earth-transition metal alloys, which have application as the storage media in erasable optical data storage systems. In these simulations the dipoles follow the dynamic equation of Landau–Lifshitz–Gilbert under the influence of an effective field arising from local anisotropy, near-neighbor exchange, classical dipole–dipole interactions, and an externally applied field. The effect of random axis anisotropy on the coercive field is studied and it is found that the fields required for the nucleation of reverse-magnetized domains are generally higher than those observed in the experiments. Various ‘‘defects’’ are then introduced in the magnetic state of the lattice and the values of coercivity corresponding to different types, sizes, and strengths of these ‘‘defects’’ are computed. It was found, for instance, that voids have insignificant effects on the value of the coercive field, but that reverse-magnetized seeds of nucleation, formed and stabilized in areas with large local anisotropy, can substantially reduce the coercivity.
Laplace flow near an ellipsoidal conductor
Authors: Trefry M. G. , Townley L. R.
Published: July 2002

An existing algebraic solution to Laplace’s equation for the steady three-dimensional temperature distribution around an ellipsoid embedded in a uniform, isotropic medium is used to study the characteristics of porous/permeable media flow near a variety of objects. Symbolic-computation techniques are used to derive new closed-form algebraic expressions for flow potentials near general ellipsoids. These expressions are used to derive simple algebraic results for various degenerate and limiting cases. Flow paths are calculated using numerical particle-tracking techniques, defining capture and release zones for the embedded objects. The application and relevance of these results to the flow of groundwater near shallow lakes are discussed, and possibilities for future work are noted. © 1998 American Institute of Physics.
Birefringence of weakly interacting fine particles
Authors: Chantrell R. W. , Bradbury A. , Menear S.
Published: July 2002

A theory is presented which explains the observed birefringence of colloidal dispersions of fine magnetic particles. The system is assumed to consist of pairs of interacting particles, interactions between pairs being negligible. A pair of particles approximates to a single particle with a high degree of optical anisotropy. Application of a magnetic field to the colloid has the effect of aligning the axes of the pairs, resulting in the measured birefringence. On the basis of this model, the birefringence of a magnetic colloid is calculated. The form of the numerical results is consistent with experimental data. In particular it is shown that in small fields the birefringence is proportional to H2.
Design of improved integrated thin-film planar dc SQUID gradiometers
Authors: Ketchen M. B.
Published: July 2002

Key issues in the design of improved first and second derivative, thin-film, planar dc SQUID gradiometers are discussed. The introduction of a planar coupling scheme to optimally couple the planar dc SQUID to the gradiometer pickup loops leads to significantly increased sensitivity as well as elimination of the sensitivity differences between series and parallel gradiometer loop configurations. Two-hole and figure-8 SQUID designs are presented which are consistent with intrinsic gradiometer balance ≲10−4 against uniform field changes. Straightforward calculations together with data from existing low-noise SQUIDs suggest improvements in gradient sensitivity on the order of 102 over existing planar gradiometers.
Application of penumbral imaging to thermonuclear neutrons
Authors: Nugent K. A. , Luther-Davies B.
Published: July 2002

This paper describes a study investigating the possible application of penumbral imaging to thermonuclear neutrons generated in experiments on inertial confinement fusion. It is shown that the approach will allow neutron yields as low as 1011 to be imaged with better than 10-μm resolution over a 50-μm field of view. This performance is such that penumbral imaging of neutrons may provide an important diagnostic in near term fusion experiments.
Luminescence of CuGaS2
Authors: Massé George
Published: July 2002

The luminescence of single crystals of CuGaS2 grown by iodine-vapor transport was studied. As-grown crystals and crystals annealed in S, Ga, or in a vacuum were used. The crystals annealed in S showed a very intensive green luminescence, which was ascribed to donor-acceptor pair transitions interacting with lattice phonons. Studies in terms of excitation intensity, temperature, and time allow us to extract two impurity levels:ED∼50 meV and EA∼130 meV. The acceptor may be due to cation vacancies. The crystals annealed in Ga (or vacuum) showed a red luminescence which was attributed to S vacancies associated with another impurity.
An intense and efficient source of vacuum ultraviolet radiation: Low- pressure Xe Townsend discharge
Authors: Mikoshiba Shigeo , Shinada Shinichi , Shirai Shoji
Published: July 2002

An intense vacuum ultraviolet spike 0.2 μs in width is observed at the initial stage of low-pressure Xe discharge, i.e., Xe Townsend discharge, in submillimeter tubes. The intensity of the 147 nm resonance line reaches 1.3 W/cm2, about 200 times stronger than that obtained from the steady-state Xe positive column. Efficiency of the vacuum ultraviolet radiation exceeds 24%, or 32 lm/ W when green phosphor is excited, four times higher than that of the positive column, or more than an order of magnitude higher than that of the negative glow. This high intensity and efficiency originate from the optimized electron energy which can be adjusted externally by changing the electric field and pressure, contrary to that in the positive column or negative glow. Another important feature of the Townsend discharge is its wide dynamic range of operation, which is advantageous when driving a panel having a large number of cells with scattered discharge characteristics. Consequently, a gas discharge display panel utilizing the Townsend discharge has more than an order of magnitude higher efficiency than the positive column or negative glow panels.
Localization of biological sources with arrays of superconducting gradiometers
Authors: Carelli P. , Leoni R.
Published: July 2002

It is shown that a planar array of superconducting gradiometers with integrated dc SQUID’s fabricated with a lithographic technique is suitable in the study of biomagnetic fields. The comparisons were made between this kind of array and one made of traditional second-order gradiometers. These comparisons were made solving the inverse problem with both arrays. The localization procedure is described in the paper, considering also the effect of noise. The magnetic field generator was simulated using the simple current dipole model. Arrays of planar integrated gradiometers should permit the construction of multichannel systems for real time localization of the sources responsible for biomagnetic signals.
The spectrum of highly ionized praseodymium and dysprosium from the Texas tokamak plasma in the 50–250-Å range
Authors: Finkenthal M. , Lippmann A. S. , Huang L. K. , Yu T. L. , Stratton B. C. , Moos H. W. , Klapisch M. , Mandelbaum P. , Bar Shalom A. , Hodge W. L. , Phillips P. E. , Price T. R. , Porter J. C. , Richards B. , Rowan W. L.
Published: July 2002

Spectra of highly ionized praseodymium and dysprosium recorded from the Texas tokamak (TEXT) plasma have been analyzed in the 50–250-Å range. The spectra contain high-intensity continuum bands in the region below 100 Å and bright individual lines above this wavelength. Lines of highly ionized praseodymium and dysprosium have been identified. The highest ionization state reached in the 1-keV central electron temperature tokamak plasma was Cu i-like Pr xxxi. The composition of the bands is explained in terms of a new theoretical approach developed to treat large unresolved transition arrays. The brightness of the continuum is compared with that of intense spectral lines emitted in the same wavelength range.
Signal processing in optothermal spectroscopy
Authors: Helander Per
Published: July 2002

The usefulness of optothermal spectroscopy for quantitative analysis is discussed. It is shown theoretically and experimentally that optothermal spectroscopy is comparable to ordinary transmission spectroscopy for the determination of optical absorption coefficients.
Thermal conductivity of optical coatings
Authors: Redondo Antonio , Beery Jerome G.
Published: July 2002

A model capable of explaining recent experimental results showing a considerable decrease in the thermal diffusivity of optical coatings is presented. The model assumes that the coatings contain microcracks. It predicts that such imperfections can cause reduction factors of 2 or more orders of magnitude in the thermal conductivity of the films, when compared to the corresponding bulk values. If the cracks enclose a region where light is absorbed the thermal isolation can lead to damage of the coating. A similar model consisting of a random distribution of voids only predicts a modest reduction of the net thermal conductivity.
Role of kinetics and thermodynamics in alloy clustering and surface quality in InAlAs grown by molecular-beam epitaxy: Consequences for optical and transport properties
Authors: Singh Jasprit , Dudley S. , Davies B. , Bajaj K. K.
Published: July 2002

We present results of a Monte Carlo computer simulation which was carried out to understand the molecular-beam epitaxial (MBE) growth of InAlAs. We focus on developing an understanding of the role of growth conditions on the short-range order (alloy clustering) in the system as well as the quality of the growth front during growth. We find that for common anion alloy systems for which thermodynamics favor phase separation, the conditions for high-quality alloy and high-quality surfaces (and heterointerfaces) are incompatible if the conventional MBE growth approach is used. We also examine the effect of the alloy clustering on the optical and transport properties of InAlAs.
Current neutralization of intense light ion beams in argon gas
Authors: Swegle John A. , Slutz Stephen A.
Published: July 2002

This paper describes a theoretical treatment for the current neutralization of intense ion beams propagating radially inward through the drift region of a cylindrical Applied-B diode. The drift region is bounded by conducting walls, and the beams are taken to have a current density profile that is constant across the surface transverse to the direction of propagation. The current neutralization calculation involves breaking the return current density into spatial harmonics in the transverse coordinate in such a way that the return current density vanishes at the walls. Each spatial harmonic is then shown to approximately obey a magnetic decay equation with a time constant proportional to the electrical conductivity of the gas. This conductivity is time dependent and is calculated locally from the degree of ionization and plasma heating created by the beam and the return current. Beams composed of protons and carbon ions are considered, and time-of-flight effects on the beam current and energy spectra are taken into account. The effects of drift region parameter variations on the time development of the conductivity and current neutralization are evaluated. Comparisons are made with experimental measurements taken at Sandia National Laboratories on the Proto-I accelerator, and strengths and weaknesses of the present model are discussed.
Rydberg state Stark spectroscopic measurement of electric-field profile in a glow discharge
Authors: Ganguly B. N. , Shoemaker J. R. , Preppernau B. L. , Garscadden A.
Published: July 2002

The electric-field profile in the cathode-fall region of a low-pressure helium gas discharge has been measured by high-level Rydberg state Stark spectroscopy using optogalvanic detection. The Stark manifolds of triplet atomic helium Rydberg states with principal quantum numbers n=15 up to 20 have been used to measure the electric-field vector using both Δmj=0 and ‖Δmj‖=1 laser polarizations. The absolute accuracy of the electric-field measurement was found to be 5% or better. The ‖Δmj‖=1 polarization Stark spectra exhibit pseudohydrogenic behavior in Stark splitting, as well as in the manifold intensity distribution. The Stark splitting of states corresponding to 1>4 was almost hydrogenic for all the measured principal quantum-number states and the intensity distribution became approximately hydrogenic for n≥16 at electric fields ≤700 V/cm.
Fast neutron-induced defects in undoped and iron-doped indium phosphide
Authors: Goltzené A. , Meyer B. , Schwab C.
Published: July 2002

Undoped and iron-doped InP samples have been investigated by conventional electron paramagnetic resonance measurements before and after fast neutron irradiations. Besides the expected anion antisite PIn defect, they reveal the presence of a broad spectrum that could be ascribed to an anion vacancy Vp on the basis of detailed linewidth and g-shift scalings of the resonance parameters of already identified intrinsic defects in GaP and InP. The irradiated InP:Fe sample shows both the nearest and next-nearest FeIn-Ini pairs.
Discharge instabilities initiated by nonuniform laser extraction in electron-beam sustained discharge KrF lasers
Authors: Kushner Mark J.
Published: July 2002

The photon energy of KrF lasers (λ=248 nm) is sufficiently large to photodetach electrons from the halogen negative ion (F−) and to photoionize excited states of krypton. In electron-beam (e-beam) sustained discharge KrF lasers, the contribution of photogenerated electrons to the total rate of ionization can be large enough to significantly perturb the conductivity of the plasma. If laser oscillation is spatially nonuniform, the photoelectron contribution to the conductivity deforms the local electric field. This condition can ultimately lead to a local discharge ionization instability and an arclike termination of the discharge. In this paper, the effect of nonuniform optical extraction on the discharge stability of an e-beam sustained discharge KrF laser will be studied with results from a two-dimensional time-dependent discharge kinetics and laser extraction model. Examples of stable and unstable configurations will be presented. The time at which the arclike ionization instability terminates the discharge correlates with the time that multistep ionization becomes an appreciable fraction of the total rate of ionization . A criterion for discharge stability is proposed; gradients in conductivity caused by photogenerated electrons must be either perpendicular or parallel to the bulk electric field.
The contribution of bulk states to the ac conductance of metal-insulator-semiconductor diodes
Authors: Brunson Kevin M. , Sands David , Thomas Clive B. , Reehal Hari S.
Published: July 2002

Measurements of the density of interface states in metal-insulator-semiconductor diodes by conductance techniques have revealed two resonances in the equivalent parallel conductance. Interface states are responsible for one, but the other is due to bulk states in the silicon crossing the Fermi energy in the depletion region. This latter contribution is found to resonate at a fixed frequency independent of the bias applied to the diode, unlike the interface state response which varies in the normal way. From the maximum in the equivalent parallel conductance we have calculated the density of bulk states to be 1014 cm−3 and identified sodium as the likely impurity responsible.
New application of complex magnetic materials to the magnetic refrigerant in an Ericsson magnetic refrigerator
Authors: Hashimoto T. , Kuzuhara T. , Sahashi M. , Inomata K. , Tomokiyo A. , Yayama H.
Published: July 2002

A complex new magnetic refrigerant, suitable for the ideal Ericsson cycle, has been investigated. Above ∼15 K it is necessary to use ferromagnets as a magnetic refrigerant. However, temperature variation for the magnetic entropy change in a homogeneous ferromagnet is not suitable for the Ericsson cycle. The present paper verifies, from theoretical analysis, that a complex ferromagnetic material, for instance, (ErAl2)0.312(HoAl2)0.198 (Ho0.5Dy0.5Al2)0.490, has the most suitable characteristics for the ideal Ericsson cycle, including two kinds of isomagnetic field processes. On the basis of the above consideration, a sintered layer structural complex has been prepared, composed of three kinds of RAl2.15 layers, where R’s are rare-earth atoms. From specific heat measurements made on this complex, its entropy and entropy change have been determined. It has been concluded that the complex magnetic material is the most hopeful refrigerant for the Ericsson cycle.
Self-consistent analysis of resonant tunneling in a two-barrier–one-well microstructure
Authors: Brennan K. F.
Published: July 2002

A self-consistent solution to the resonant tunneling problem is presented based on the simultaneous solution of the time-independent Schrödinger equation with the Poisson equation. The solution is obtained from a piecewise linear matching of Airy functions. The model is used to explore the effects of the self-consistent electron charge on the transmissivity and current-voltage characteristics of a double-barrier single-well GaAs-AlGaAs device. It is found that the self-consistent potential always acts to shift the negative differential resistance onset voltage to large positive values. The self-consistent field effectively acts to screen the positive applied voltage. Therefore, the effects of the self-consistent field can essentially be modeled by a smaller applied positive bias. It is further found that the effects of the self-consistent field are most prevalent at high temperatures, ∼300 K, and at high dopings, >1.0×1018. It is necessary to include the self-consistent effects then when designing resonant tunneling structures within these constraints.
Recrystallization of amorphized polycrystalline silicon films on SiO2: Temperature dependence of the crystallization parameters
Authors: Iverson R. B. , Reif R.
Published: July 2002

This paper presents a theoretical and experimental study of the recrystallization behavior of polycrystalline silicon films amorphized by self-implantation. The crystallization behavior was found to be similar to the crystallization behavior of films deposited in the amorphous state, as reported in the literature; however, a transient time was observed, during which negligible crystallization occurs. The films were prepared by low-pressure chemical vapor deposition onto thermally oxidized silicon wafers and amorphized by implantation of silicon ions. The transient time, nucleation rate, and characteristic crystallization time were determined from the crystalline fraction and density of grains in partially recrystallized samples for anneal temperatures from 580 to 640 °C. The growth velocity was calculated from the nucleation rate and crystallization time and is lower than values in the literature for films deposited in the amorphous state. The final grain size, as calculated from the crystallization parameters, is 2.5 μm for an anneal temperature of 580 °C and 1.5 μm for an anneal temperature of 640 °C and is in agreement with transmission electron microscopy dark-field images of completely recrystallized samples that show grain sizes of approximately 1–2 μm.
Current rise-time limitation of the large volume x-ray preionized discharge-pumped XeCl laser
Authors: Champagne L. F. , Dudas A. J. , Harris N. W.
Published: July 2002

Sixty-six joules has been extracted from a 22-l active volume, 5-atm laser mixture, in a 180-ns (FWHM) optical pulse. The best efficiency obtained under these conditions was 0.8%, based on the available energy stored in the double Blumlein. For the laser chamber geometry used in this experiment it was essential that the surface track path between the positive high-voltage electrode and any ground be a minimum of 3 times the interelectrode separation. The small-signal gain under these operating conditions was 6.0% cm−1 and the measured absorption at the peak of the gain was 0.4% cm−1. We observe that for both volumetric scaling and pulse length scaling the current rise time is of critical importance.