bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogen-plasma etching of ion beam deposited c-BN films: An in situ investigation of the surface with electron spectroscopy
Authors: Reinke P. , Oelhafen P. , Feldermann H. , Ronning C. , Hofsäss H.
Published: July 2002

In the present study nanocrystalline c-BN films deposited with a mass selected ion beam were subjected to a hydrogen plasma or atomic hydrogen produced by the hot filament method. Film composition and electronic properties of the surface were subsequently analyzed in situ by photoelectron spectroscopy in the x-ray and ultraviolet regime, and by electron energy loss spectroscopy. The sp2-bonded surface layer, which is native to ion beam deposited c-BN films, is rapidly removed by hydrogen plasma etching and the almost phase pure c-BN layer uncovered. Continuation of hydrogen plasma exposure leads to a removal of the c-BN layer at an etch rate of about 0.65 nm/min until finally the sp2-BN nucleation layer is detected. No preferential etching of either B or N is observed and an equal concentration of the constituents is maintained throughout the experiment. The large atomic hydrogen flux from the plasma is held responsible for the efficient etching, which could not be achieved with the hot filament method. The valence band spectra of nanocrystalline c-BN can be understood in the framework of published density of states calculations and allows to identify characteristic spectral features for both phases. The nanocrystalline c-BN surface exhibits a negative electron affinity, which might be connected to a hydrogen termination of the surface. The hydrogen plasma etching offers a comparatively easy route for future investigations of surface reactivity and the exploitation of the NEA property of the material.
Solvent effects in emission spectroscopy: A Monte Carlo quantum mechanics study of the n←π* shift of formaldehyde in water
Authors: Coutinho Kaline , Canuto Sylvio
Published: July 2002

Supermolecular calculations that treat both the solute and the solvent quantum-mechanically are performed to analyze the solvatochromism of the first emission transition of formaldehyde in water. The liquid structures are generated by NVT Metropolis Monte Carlo simulation assuming a fully relaxed excited state. The autocorrelation function is calculated to obtain an efficient ensemble average. A detailed analysis of the hydrogen bonds and their contribution to the solvation shift is presented. On average, 0.7 hydrogen bonds are formed in the excited state, about three times less than in the ground state. Quantum-mechanical calculations using the intermediate neglect of differential overlap with singly excited configuration interaction (INDO/CIS) are then performed in the supermolecular clusters corresponding to the hydrogen bond shell and the first, second, and third solvation shells. The third solvation shell extends up to 10 Å from the center of mass of formaldehyde, showing the very long-range effects on the solvation shift of this polar molecule. The largest cluster includes one formaldehyde and 142 water molecules. INDO/CIS calculations are performed on this cluster with a properly antisymmetric reference ground state wave function involving all valence electrons. The estimated limit value for the solvatochromic shift of the n-π* emission transition of fully relaxed formaldehyde in water, compared to the gas phase, is ≈1650 cm−1. The total Stokes shift of formaldehyde in water is calculated as ≈550 cm−1.
A direct method to calculate tip–sample forces from frequency shifts in frequency-modulation atomic force microscopy
Authors: Giessibl F. J.
Published: July 2002

Frequency-modulation atomic force microscopy (FMAFM) has proven to be a powerful method for imaging surfaces with true atomic resolution. However, the tip–sample forces are not directly accessible by FMAFM. Here, an algorithm to recover the tip–sample forces from the frequency shift curve is introduced and demonstrated with experimental data. Also, an intuititive connection between frequency shift Δf and tip–sample force gradient kts that simplifies the calculation of FMAFM images is established: Δf is a convolution of kts with a semispherical weight function.
The role of high-temperature island coalescence in the development of stresses in GaN films
Authors: Böttcher T. , Einfeldt S. , Figge S. , Chierchia R. , Heinke H. , Hommel D. , Speck J. S.
Published: July 2002

The formation of dislocations and stress in GaN layers grown by metalorganic vapor phase epitaxy on sapphire is investigated with regard to the average grain diameter. The grain diameter was determined by monitoring the high-temperature GaN island coalescence process during growth using reflectometry. It is found that the density of edge threading dislocations decreases and the compressive stress measured after cooling to room temperature increases when the coalescence thickness and the grain diameter increase. The data are consistent with models of development of tensile stress due to island coalescence during growth.
Modeling of the gas-phase chemistry in C–H–O gas mixtures for diamond chemical vapor deposition
Authors: Petherbridge James R. , May Paul W. , Ashfold Michael N. R.
Published: July 2002

The boundaries of the diamond deposition region in the C–H–O (Bachmann) atomic phase composition diagram have been reproduced successfully for 38 different C, H, and O containing gas mixtures using the CHEMKIN computer package, together with just two criteria—a minimum mole fraction of methyl radicals [CH3] and a limiting value of the [H]/[C2H2] ratio. The diamond growth/no-growth boundary coincides with the line along which the input mole fractions of C and O are equal. For every gas mixture studied, no-growth regions are found to coincide with a negligible (<10−10) mole fraction of CH3 radicals, while for gas mixtures lying within the diamond growth region the CH3 mole fraction is ∼10−7. Each no-growth→diamond growth boundary is seen to be accompanied by a 2–3 order of magnitude step in CH3 mole fraction. The boundary between diamond and nondiamond growth is less clearly defined, but can be reproduced by assuming a critical, temperature dependent [H]/[C2H2] ratio (0.2, in the case that Tgas=2000 K) that reflects the crucial role of H atoms in the etching of nondiamond phases. The analysis allows prediction of the composition process window for good quality diamond growth for all stable input gas mixtures considered in this study.
Measurement of ballistic phonon conduction near hotspots in silicon
Authors: Sverdrup P. G. , Sinha S. , Asheghi M. , Uma S. , Goodson K. E.
Published: July 2002

The Fourier law for lattice heat conduction fails when the source of heat is small compared to the phonon mean free path. We provide experimental evidence for this effect using heating and electrical-resistance thermometry along a doped region in a suspended silicon membrane. The data are consistent with a closed-form two-fluid phonon conduction model, which accounts for the severe departure from equilibrium at the hotspot. The temperature rise exceeds predictions based on the Fourier law by 60% when the phonon mean free path is a factor of 30 larger than the resistor thickness. This work is improving the constitutive modeling of heat flow in deep-submicron transistors.
Epitaxial growth of the first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series
Authors: Haeni J. H. , Theis C. D. , Schlom D. G. , Tian W. , Pan X. Q. , Chang H. , Takeuchi I. , Xiang X.-D.
Published: July 2002

The first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series, i.e., Sr2TiO4, Sr3Ti2O7, Sr4Ti3O10, Sr5Ti4O13, and Sr6Ti5O16, have been grown by reactive molecular beam epitaxy. A combination of atomic absorption spectroscopy and reflection high-energy electron diffraction intensity oscillations were used for the strict composition control necessary for the synthesis of these phases. X-ray diffraction and high-resolution transmission electron microscope images confirm that these films are epitaxially oriented and nearly free of intergrowths. Dielectric measurements indicate that the dielectric constant tensor coefficient ε33 increases from a minimum of 44±4 in the n=1(Sr2TiO4) film to a maximum of 263±2 in the n=∞(SrTiO3) film.
Structural and magnetic properties of η-phase manganese nitride films grown by molecular-beam epitaxy
Authors: Yang Haiqiang , Al-Brithen Hamad , Smith Arthur R. , Borchers J. A. , Cappelletti R. L. , Vaudin M. D.
Published: July 2002

Face-centered tetragonal (fct) η-phase manganese nitride films have been grown on magnesium oxide (001) substrates by molecular-beam epitaxy. For growth conditions described here, reflection high energy electron diffraction and neutron scattering show primarily two types of domains rotated by 90° to each other with their c axes in the surface plane. Scanning tunneling microscopy images reveal surface domains consisting of row structures which correspond directly to the bulk domains. Neutron diffraction data confirm that the Mn moments are aligned in a layered antiferromagnetic structure. The data are consistent with the fct model of G. Kreiner and H. Jacobs for bulk Mn3N2 [J. Alloys Compd. 183, 345 (1992)].
Crystal growth and structure, electrical, and optical characterization of the semiconductor Cu2SnSe3
Authors: Marcano G. , Rincón C. , de Chalbaud L. M. , Bracho D. B. , Pérez G. Sánchez
Published: July 2002

X-ray powder diffraction by p-type Cu2SnSe3, prepared by the vertical Bridgman–Stockbarger technique, shows that this material crystallizes in a monoclinic structure, space group Cc, with unit cell parameters a=6.5936(1) Å, b=12.1593(4) Å, c=6.6084(3) Å, and β=108.56(2)°. The temperature variation of the hole concentration p obtained from the Hall effect and electrical resistivity measurements from about 160 to 300 K, is explained as due to the thermal activation of an acceptor level with an ionization energy of 0.067 eV, whereas below 100 K, the conduction in the impurity band dominates the electrical transport process. From the analysis of the p vs T data, the density-of-states effective mass of the holes is estimated to be nearly of the same magnitude as the free electron mass. In the valence band, the temperature variation of the hole mobility is analyzed by taking into account the scattering of charge carriers by ionized and neutral impurities, and acoustic phonons. In the impurity band, the mobility is explained as due to the thermally activated hopping transport. From the analysis of the optical absorption spectra at room temperature, the fundamental energy gap was determined to be 0.843 eV. The photoconductivity spectra show the presence of a narrow band gap whose main peak is observed at 0.771 eV. This band is attributed to a free-to-bound transition from the defect acceptor level to the conduction band. The origin of this acceptor state, consistent with the chemical composition of the samples and screening effects, is tentatively attributed to selenium interstitials.
Electronic and optical properties of BaTiO3 and SrTiO3
Authors: Ahuja R. , Eriksson O. , Johansson B.
Published: July 2002

We have used the linear muffin-tin-orbital method, without geometrical approximations, to calculate the electronic structure of tetragonal BaTiO3 and cubic SrTiO3. Using our self-consistent potential, we have calculated the frequency-dependent dielectric function and the reflectivity spectrum. For tetragonal BaTiO3 the anisotropic optical properties are calculated. We compare our calculations with experimental data and find good agreement. We analyze the origin of the different calculated features in the optical spectra by means of our energy-band structure.
Molecular beam epitaxy control of the structural, optical, and electronic properties of ScN(001)
Authors: Smith Arthur R. , AL-Brithen Hamad A. H. , Ingram David C. , Gall Daniel
Published: July 2002

Scandium nitride (001) oriented layers have been grown on magnesium oxide (001) substrates by molecular beam epitaxy using a rf-plasma source and a scandium effusion cell. The Sc/N flux ratio is found to be critical in determining the structural, optical, and electronic properties of the grown epitaxial layers. A distinct transition occurs at the point where the Sc/N flux ratio equals 1, which defines the line between N-rich and Sc-rich growth. Under N-rich conditions, the growth is epitaxial, and the surface morphology is characterized by a densely packed array of square-shaped plateaus and four-faced pyramids with the terraces between steps being atomically smooth. The films are stoichiometric and transparent with a direct optical transition at 2.15 eV. Under Sc-rich conditions, the growth is also epitaxial, but the morphology is dominated by spiral growth mounds. The morphology change is consistent with increased surface diffusion due to a Sc-rich surface. Excess Sc leads to understoichiometric layers with N vacancies which act as donors. The increased carrier density results in an optical reflection edge at 1 eV, absorption below the 2.15 eV band gap, and a drop in electrical resistivity.
Determination of pair-potential parameters from experimental structure factors
Authors: Tóth Gergely
Published: July 2002

A method is developed to obtain pair-potential parameters for liquids from structure factors measured by diffraction experiments. The approach is a generalization of the method of Lyubartsev and Laaksonen [Phys. Rev. E 52, 3730 (1995)]. Each step of the iterative process consists of two parts: a Monte Carlo or molecular dynamics simulation with an assumed pair-potential and the modification of the potential due the experimental data and different quantities calculated in the simulations. The choice of the structure factor as input function makes it possible to avoid the difficulties of the Fourier transformation from the inverse to the real space, to replace the solution of the set of linear equations with simple linear combination in the case of multicomponent or molecular systems, and to use least-square fit for the modification of the pair-potential. The latter is less sensitive to the errors of the data than the unique solution of linear equations applied in the original recipe. The method is tested for one- and two-component Lennard-Jones systems, and Lennard-Jones parameters for liquid argon are determined from a diffraction experiment.
Thermal radiation from two-dimensionally confined modes in microcavities
Authors: Maruyama S. , Kashiwa T. , Yugami H. , Esashi M.
Published: July 2002

A two-dimensional array of a microcavity with a high aspect ratio is made on a Cr-coated Si surface using the micromachining technology. The thermal emission spectra whose wavelength is close to the dimension of cavity aperture (5 μm) are measured on samples with a different aspect ratio. The clear selective emission bands corresponding to the two-dimensionally confined electromagnetic modes are demonstrated experimentally. It is found that the low emissivity of the base material is essential to obtain the high spectral selectivity of thermal radiation. The direction and polarization properties are also examined. The dominant peaks of the emission spectra can be explained by a simple cavity resonator model.
Molecular dynamics simulation of gaseous–liquid phase transitions of soluble and insoluble surfactants at a fluid interface
Authors: Tomassone M. S. , Couzis A. , Maldarelli C. M. , Banavar J. R. , Koplik J.
Published: July 2002

Molecular dynamics simulations are used to study the formation of gaseous and liquid expanded phases of surfactants on a liquid/vapor interface. Both insoluble and soluble surfactants are considered, modeled as freely jointed chains in a monatomic solvent with appropriate Lennard–Jones interactions. For both insoluble and soluble cases our results indicate that the surface tension as a function of coverage shows a plateau close to the clean interface value until a critical surface concentration, beyond which the surface tension lowers steeply. For the soluble case, we also detail a complete Gibbs construction for the surface excess and report an adsorption isotherm. Snapshots of the positions of the molecules, together with analysis of correlation functions, show that these model surfactants assemble from solution and exhibit coexistence in the plateau of isolated surfactants and liquid aggregates. We have identified this coexistence as a gas–liquid phase transition, in agreement with recent experimental evidence in the soluble case.
Comparison of ab initio and density functional calculations of electric field gradients: The Fe57 nuclear quadrupole moment from Mössbauer data
Authors: Schwerdtfeger Peter , Söhnel Tilo , Pernpointner Markus , Laerdahl Jon K. , Wagner Friedrich E.
Published: July 2002

The difficulty in accurate determination of the nuclear quadrupole moment of the first I=3/2 excited nuclear state of Fe57 from electronic structure calculations of the iron electric field gradient combined with Mössbauer measurements of the nuclear quadrupole splitting in the isomer shift is addressed by comparing ab initio with density functional calculations for iron pentacarbonyl, Fe(CO)5, ferrocene, Fe(C5H5)2, and the Δg5 electronic ground states of FeCl2 and FeBr2. While the ligand field gradient tensor components change relatively little with the method applied, the iron electric field gradient is sensitive to the specific density functional used. Single reference many-body perturbation theory for electron correlation also performs poorly for the iron electric field gradient and shows extreme oscillatory behavior with a change in the order of the perturbation series. Even with larger basis sets and coupled cluster techniques a precise value for the iron electric field gradient could not be determined from electronic structure calculations due to limitations in the theoretical procedures. In order to avoid uncertainties in the measured isomer shift which enters into the nuclear quadrupole coupling constant we determined the Mössbauer spectrum of Fe(C5H5)2 between temperatures of 4.2 and 295 K. In this range two phase transitions are observed, but the quadrupole splitting is not very dependent on the solid state structure in each phase. Solid state effects for the Fe(CO)5 are determined by comparing the iron electric field gradient of the isolated molecule with the value obtained from first principle solid state calculations at various levels of theory. These calculations show that the influence of near neighboring effects to the iron electric field gradient is small. Fully relativistic Dirac–Hartree–Fock calculations for Fe(CO)5 reveal that relativistic effects for the iron electric field gradient are small as well. Fe(CO)5 is therefore an ideal test molecule for the determination of an accurate nuclear quadrupole moment from electronic structure calculations if combined with an experimental nuclear quadrupole coupling constant. Our best estimate for the Fe57 nuclear quadropole moment is 0.14(2) barn in reasonable agreement with recent nuclear structure calculations.
Effects of annealing time on defect-controlled ferromagnetism in Ga1−xMnxAs
Authors: Potashnik S. J. , Ku K. C. , Chun S. H. , Berry J. J. , Samarth N. , Schiffer P.
Published: July 2002

We have studied the evolution of the magnetic, electronic, and structural properties of annealed epilayers of Ga1−xMnxAs grown by low temperature molecular beam epitaxy. Annealing at the optimal temperature of 250 °C for less than 2 h significantly enhances the conductivity and ferromagnetism, but continuing the annealing for longer times suppresses both. These data indicate that such annealing induces the defects in Ga1−xMnxAs to evolve through at least two different processes, and they point to a complex interplay between the different defects and ferromagnetism in this material.
Boundary layer flow on a long thin cylinder
Authors: Tutty O. R. , Price W. G. , Parsons A. T.
Published: July 2002

The development of the boundary layer along a long thin cylinder aligned with the flow is considered. Numerical solutions are presented and compared with previous asymptotic results. Very near the leading edge the flow is given by the Blasius solution for a flat plate. However, there is soon a significant deviation from Blasius flow, with a thinner boundary layer and higher wall shear stress. Linear normal mode stability of the flow is investigated. It is found that for Reynolds numbers less than a critical value of 1060 the flow is unconditionally stable. Also, axisymmetric modes are only the fourth least stable modes for this problem, with the first three three-dimensional modes all having a lower critical Reynolds number. Further, for Reynolds numbers above the critical value, the flow is unstable only for a finite distance, and returns to stability sufficiently far downstream.
High-temperature post-oxidation annealing on the low-temperature oxide/4H-SiC(0001)
Authors: Kosugi Ryoji , Cho Won-ju , Fukuda Kenji , Arai Kazuo , Suzuki Seiji
Published: July 2002

A silicon–oxide layer was deposited by a low-pressure chemical vapor deposition technique at temperatures as low as 400 °C for gate dielectric material of the 4H-SiC metal–oxide–semiconductor (MOS) capacitors. Interfacial properties such as the amount of effective charge (Neff), interfacial oxide trapped density (Dit) and hot-carrier tolerance of the 4H-SiC(0001) MOS capacitors were evaluated by simultaneous C–V measurement as a dependence of a post-oxidation annealing (POA) at temperatures ranging from 1000 to 1200 °C. It was found that the POA temperature dependence of the Dit was different from that of the Neff. On the other hand, the POA dependence of hot-carrier tolerance was similar to that of the Dit. In consequence, those interfacial properties were sufficiently improved and became stable above the POA temperature of 1100 °C, which corresponds to the softening temperature of SiO2 bulk.
X-ray four-wave mixing in molecules
Authors: Tanaka Satoshi , Mukamel Shaul
Published: July 2002

An effective core-exciton Hamiltonian is constructed for nitroanilines which includes 1s core hole transitions of both nitrogen atoms. The wavevector and frequency dependent third order susceptibility χ(3) is calculated and used to predict the frequency-domain pump–probe spectra which show both photobleaching and excited state absorption components. Signatures of electron delocalization and differences among the para-, meta-, and ortho-isomers are discussed.
Exchange bias in (Pt/Co0.9Fe0.1)n/FeMn multilayers with perpendicular magnetic anisotropy
Authors: Garcia F. , Casali G. , Auffret S. , Rodmacq B. , Dieny B.
Published: July 2002

We report the observation of exchange bias in (Pt/Co0.90Fe0.10)n/FeMn multilayers, with perpendicular magnetic anisotropy. We analyze the behavior of the exchange-bias field Heb as a function of temperature, FeMn thickness, number of (Pt/Co0.90Fe0.10) bilayers, and annealing conditions. Measurements carried out with field applied perpendicular to the plane of the samples indicate that the hysteresis loops present 100% of remanent magnetization, with Heb reaching 200 Oe and a tunable coercive field, depending on Co and Pt thickness ratio and on Pt buffer thickness. Furthermore, spin-valves of the form (Pt/Co0.90Fe0.10)n/NM/(Co0.90Fe0.10/Pt)m/Co0.90Fe0.10/FeMn with NM=Cu or Pt have been prepared. They exhibit two well separated hysteresis loops when the field is applied perpendicular to the plane.
Optimal control of ground-state dynamics in polymers
Authors: Zeidler D. , Frey S. , Wohlleben W. , Motzkus M. , Busch F. , Chen T. , Kiefer W. , Materny A.
Published: July 2002

Coherent control of the vibrational dynamics in crystalline polydiacetylene is demonstrated by tailoring the Stokes pulse of a coherent anti-Stokes Raman scattering (CARS) setup in a feedback-controlled self-learning loop. The feedback signal is derived from the spectral distribution of the CARS signal. Controlled excitation of one mode and simultaneous extinction of all other modes with high efficiency is demonstrated. In addition, the relative phases of the three normal modes have been controlled allowing excitations of local modes and suggesting the possibility of ground state reaction control.
Multicolor infrared detection realized with two distinct superlattices separated by a blocking barrier
Authors: Chen C. C. , Chen H. C. , Kuan C. H. , Lin S. D. , Lee C. P.
Published: July 2002

A multicolor infrared photodetector was realized with two superlattices separated by a blocking barrier. The photoresponse is switchable between 7.5–12 and 6–8.5 μm by the bias polarity, and is also tunable by the bias magnitude in each wavelength regime. In addition, our detector exhibits advantages including little temperature dependence of the spectral response and the same order of responsivity in the two wavelength regimes. The measured peak responsivities in the two regimes are 117 mA/W at 9.8 μm under 1 V and 129 mA/V at 7.4 μm under −0.8 V, respectively. Also, the detectivities are comparable with the conventional multistack detector. The zero background peak detectivities are 2.3×1010 cm Hz0.5/W at 50 K and 9.8 μm under 0.7 V, and 8.7×1010 cm Hz0.5/W at 70 K and 7.4 μm under −0.7 V.
On creation of Hopf bifurcations in discrete-time nonlinear systems
Authors: Wen Guilin , Xu Daolin , Han Xu
Published: July 2002

Bifurcation characteristics of a nonlinear system can be manipulated by small controls. In this paper, we present a control method to create Hopf bifurcations in discrete-time nonlinear systems. The critical conditions for the Hopf bifurcations are discussed. The center manifold method, normal form technique and the Iooss’s Hopf bifurcation theory are employed in the derivation of the control gain. Numerical demonstration is provided.
Low-frequency dielectric relaxation and ac conduction of SrBi2Ta2O9 thin film grown by pulsed laser deposition
Authors: Kim Ill Won , Ahn Chang Won , Kim Jin Soo , Song Tae Kwon , Bae Jong-Sung , Choi Byung Chun , Jeong Jung-Hyun , Lee Jae Shin
Published: July 2002

Bi-excess SrBi2Ta2O9 (SBT) thin films on Pt/Ti/SiO2/Si substrate were prepared by pulsed laser deposition technique. The SBT structure was characterized by x-ray diffraction studies. The ferroelectric properties were confirmed by P–E hysteresis loops at different applied electric fields. The dielectric constant and the ac conductivity of the Pt/SBT/Pt capacitor were investigated in the frequency range from 0.01 Hz to 100 kHz and in the temperature range from 25 to 400 °C. The thermal activation energy of 0.90 eV is observed in the frequency dependent dielectric constant. The activation energy for conduction process is calculated as 0.91 eV from the slope of ac conductivity at the lowest frequency. The low-frequency dielectric relaxation and the ac conductivity of Bi-excess SBT thin film are discussed in relation to the electrical conduction of SBT/Pt junction.
Chaotic scattering: An introduction
Authors: Ott Edward , Tél Tamás
Published: July 2002

In recent years chaotic behavior in scattering problems has been found to be important in a host of physical situations. Concurrently, a fundamental understanding of the dynamics in these situations has been developed, and such issues as symbolic dynamics, fractal dimension, entropy, and bifurcations have been studied. The quantum manifestations of classical chaotic scattering is also an extremely active field, with new analytical techniques being developed and with experiments being carried out. This issue of Chaos provides an up-to-date survey of the range of work in this important field of study.
Temperature patterns on a hollow cylindrical catalytic pellet
Authors: Annamalai J. , Liauw M. A. , Luss D.
Published: July 2002

The atmospheric oxidation of a mixture containing 6 vol % carbon monoxide was carried out on a hollow cylindrical catalytic pellet. The catalyst was held in a conical reactor which enabled simultaneous measurement of the temperature patterns on the top and side of the pellet by an IR imager. Upon a decrease in the reactor temperature the fully ignited, high temperature state of the pellet is transformed to a nonuniform one with temperature fronts separating high and low temperature regions. The transition and the resulting states are rather intricate and are strongly influenced by the nonuniformity of the catalyst and the transport to and from it, as well as the global coupling, which stabilizes temperature fronts and patterns, which would not exist in its absence. Intricate pulse splitting and extinction were observed both on the top and the side of the pellet. Highly irregular motions and conversions were obtained following a decrease in the reactor temperature.
A fast explicit algorithm for the time-dependent Schrödinger equation
Authors: Visscher P. B.
Published: July 2002

An explicit algorithm for the time-stepping solution of the Schrödinger equation is described, which is second-order accurate in time. It is a staggered-time algorithm, in which the real and imaginary parts of the wave function are defined at alternate times. The method combines the speed and simplicity of explicit methods with the accuracy and stability of second-order implicit methods. Because of this simplicity and speed, the algorithm is well suited for pedagogical applications on personal computers, as well as for computation-intensive research applications.
Synthesis of compound thin films by dual ion beam deposition. I. Experimental approach
Authors: Harper J. M. E. , Cuomo J. J. , Hentzell H. T. G.
Published: July 2002

The synthesis of compound thin films by dual ion beam deposition is described. In Part I of this two-part paper, the experimental approach is presented, together with an analysis of composition for the aluminum-nitrogen system. In the accompanying paper (Part II), the properties of Al-N films are described. The dual beam technique supplies a deposition flux of Al from Ar+ ion beam sputtering of an Al target. Simultaneously, a low-energy (100–500 eV) N+2 ion beam bombards the growing film. Using a measured ion beam gradient across the substrate holder, a wide range of arrival rate ratios (0≤N/Al≤1.6) is obtained in each deposition run, producing a composition range of (0.1≤N/Al≤1.0). The film composition varies linearly with N arrival flux and saturates at the composition of stoichiometric AlN. The incorporation probabilities of Al and N are close to unity for N/Al≊0, and decrease to 0.7 at N/Al=1.0. Preferential sputtering of Al occurs for N/Al<1, with ejection of excess N for N/Al>1. We discuss the accuracy of this technique in establishing quantitative deposition parameters for compound and composite thin films.
A calculational procedure of the Fermi–Dirac integral with an arbitrary real index by means of a numerical integration technique
Authors: Ohsugi Isao J. , Kojima Tsutomu , Nishida Isao
Published: July 2002

A calculational procedure of the Fermi–Dirac integral F(r,η)=∫∞0xr/(ex−η+1)dx with an arbitrary real index r by means of a numerical integration technique is proposed. Numerical values of F(r,η) can be calculated by applying an asymptotic series expansion to the integrand xr/(ex−η+1) for the domain 0≤x≤0.1 and by approximating the infinite upper limit at L=a+br+η, where a and b are constants estimated from the allowable calculational error. The total calculational error can be reduced to 10−7 at any η for − (1)/(2) ≤r≤10 by employing the constants a=17 and b=4.5 and using the first four terms of the series expansion.
Propagation of wiggler focused relativistic sheet electron beams
Authors: Booske J. H. , Destler W. W. , Segalov Z. , Radack D. J. , Rosenbury E. T. , Rodgers J. , Antonsen T. M. , Granatstein V. L. , Mayergoyz I. D.
Published: July 2002

A recent design concept for millimeter-wave free-electron lasers [J. Appl. Phys. 60, 521 (1986)] would require the stable propagation of a sheet electron beam through a narrow waveguide channel. Experimental results reported in this article support the feasibility of such a configuration by demonstrating the stable propagation of relativistic sheet electron beams through a narrow waveguide gap (3.2 mm) using focusing by a short-period electromagnet wiggler. 90% of the electron current in a 100-keV sheet electron beam was transmitted through a 5-cm-long channel with peak wiggler fields of 800 G. Almost 80% of a 400-keV beam was similarly confined with a 1600-G wiggler field. The data were consistent with single electron trajectory models, indicating that space-charge effects were minimal. No evidence of beam breakup or filamentation instabilities was observed.
Shock-induced phase transformation in cadmium sulfide dispersed in an elastomer
Authors: Tang Z. P. , Gupta Y. M.
Published: July 2002

Shock-wave response of cadmium sulfide powder has been examined by conducting impact experiments on cadmium sulfide/elastomer composites. Particle-velocity profiles at different sample depths were measured in experiments ranging from 7 to 45 kbar. Although no two-wave structure was observed in the wave profiles, the peak pressure-volume states and the wave-velocity results indicate a shock-induced phase transformation in the cadmium sulfide. Calculations based on a simple mixture model constructed from the hydrostatic data on cadmium sulfide and shock data on the elastomer are in good agreement with the experimental results. Reasons for the good agreement between the shock data and the hydrostatic results are attributed to the nearly hydrodynamic compression of the cadmium sulfide particles because of the elastomer matrix.
Nd:LNA laser optical pumping of 4He: Application to space magnetometers
Authors: Slocum R. E. , Schearer L. D. , Tin P. , Marquedant R.
Published: July 2002

We have observed Hanle signals and n=0, p=1 parametric resonances of 23S1 metastable helium atoms in a discharge cell by optically pumping the helium atoms with a tunable Nd:LNA laser. These resonances were used to construct a sensitive magnetometer for the measurement of very small magnetic fields. Since magnetometer sensitivity is proportional to the slope of the parametric resonance signal (signal amplitude divided by linewidth), the slopes for single-line laser pumping were compared with similar quantities obtained from conventional helium lamp pumping. Laser pumping yielded 45 times greater slopes with comparable power requirements, thus establishing the potential for developing ultrasensitive resonance magnetometers using single-line laser pumping.
Elastic constants of a graphite-magnesium composite
Authors: Ledbetter H. M. , Datta S. K. , Kyono T.
Published: July 2002

This study contains three components: measurement, modeling, and inverse modeling to get the fiber elastic constants. The studied composite consisted of 70 vol % continuous uniaxial graphite fibers in a magnesium matrix. By ultrasonic-velocity methods, we measured the composite’s complete orthotropic-symmetry (nine-independent-component) elastic-constant tensor: Cij in Voigt contracted notation. Approximately, the composite shows transverse-isotropic symmetry. For a model, we used a wave-scattering method in the long-wavelength limit. The model requires two inputs: the two isotropic matrix elastic constants and the five anisotropic fiber elastic constants. We measured the first and guessed the second based on graphite-fiber elastic constants reported by others. This guess gave good measurement-model agreement only for C11, C22, and C33. Especially, the shear moduli Cii (i=4,5,6) agreed poorly (a 20% difference). Using inverse modeling—calculating fiber properties from measured matrix properties and measured composite properties—we estimated the anisotropic fiber elastic constants. For the fiber we predict a Young-modulus anisotropy of 12 and a principal Poisson ratio range of 0.03–0.37.
Thermal reaction of Al/Ti bilayers with contaminated interface
Authors: Thuillard M. , Tran L. T. , Nieh C. W. , Nicolet M-A.
Published: July 2002

We have studied some new aspects of thermal reactions in Al/Ti bilayers in which the interface is purposely contaminated with oxygen. After annealing at a temperature of 460 °C, an Al3Ti compound forms at the interface, moreover some Al diffuses through the Ti to form a compound at the free surface. The amount of aluminum at the free surface can be as large as at the interface. Nucleation and lateral growth of Al3Ti at the interface are locally unfavorable. This results in a competition between the lateral growth of Al3Ti at the Al/Ti interface and the diffusion of Al to the free surface. Once full coverage by Al3Ti is obtained at the Al/Ti interface, the diffusion of Al to the surface becomes negligible.
Spectroscopic ellipsometry and transmission electron microscopy study of annealed high-dose oxygen implanted silicon
Authors: Vanhellemont J. , Maes H. E. , De Veirman A.
Published: July 2002

The first results are presented of a comparative study of separation by implanted oxygen structures using spectroscopic ellipsometry (SE) and transmission electron microscopy. The strength of SE to measure the layer thicknesses of multilayer structures nondestructively is illustrated. Some limitations of the technique are also indicated.
Percolation in alloys with thermally activated diffusion
Authors: Ouyang H. , Fultz B.
Published: July 2002

For diffusion in a real alloy, some concepts of formal percolation theory may need to be reconsidered because the ‘‘immobile’’ atoms are not truly immobile. Our Monte Carlo simulations of vacancy diffusion on bcc lattices show the existence of a relationship between the activation barrier heights for the vacancy-atom exchanges and the effective percolation threshold concentration. In the language of formal percolation theory, we have modeled this problem by varying the immobile species’ barrier height from infinity to some finite value and calculating the resulting percolation threshold. When both species of atoms have a finite mobility, however, our results can be interpreted in terms of probabilities for vacancies to escape local clusters in a fixed amount of time. We find that the dynamical behavior undergoes a marked change above and below the formal percolation threshold, but the strength of the percolating cluster is much less important than in formal percolation theory.
Effect of chirality on the Doppler shift and aberration of light waves
Authors: Engheta Nader , Kowarz Marek W. , Jaggard Dwight L.
Published: July 2002

The Doppler shift and aberration effects are examined for the case of plane electromagnetic wave propagation in optically active or chiral media. These phenomena are found to separate a single monochromatic plane wave into two waves, as observed in a coordinate system in motion with respect to the source. It is shown that the Doppler effect is manifested as two shifted frequencies, one for each of the chiral eigenmodes, while the aberration results in two apparent directions of propagation, one for each eigenmode. Therefore, when motion is involved (either a moving source or a moving observer), a monochromatic single plane wave in one frame may be observed as two plane waves with unequal frequencies and differing directions of propagation in the other. The resulting double frequency shift is plotted as a function of relative velocity.
Temperature dependence of coercivity in melt-spun and die upset neodymium-iron-boron
Authors: Pinkerton F. E. , Fuerst C. D.
Published: July 2002

We compare the temperature dependence of the intrinsic coercivity Hci between 5 and 600 K in rapidly quenched Nd-Fe-B ribbons and a die upset Nd-Fe-B magnet. At room temperature, ribbons have higher coercivity than the die upset magnet, but have a smaller technical temperature coefficient of coercivity β between 25 and 125 °C: Hci=15.1 kOe and β=−0.38%/°C for the ribbons, while Hci=10.4 kOe and β=−0.64%/°C for the die upset magnet. Because the die upset magnet has a stronger temperature dependence, the Hci(T) curves converge at low temperature. The ribbons can be phenomenologically modeled using Hci(T)=cHA(T)−Neff4πMs(T), the two terms representing the strength of domain wall pinning at grain boundaries and the effective local demagnetizing field, respectively. The model parameters c=0.25 and Neff=0.26 are significantly smaller than the corresponding values obtained in sintered Nd-Fe-B magnets. This model is less successful in describing Hci(T) for the die upset magnet. Their qualitatively different behaviors indicate that different mechanisms of domain wall pinning are responsible for coercivity in ribbons and die upset magnets.
Midgap states in metalorganic vapor phase epitaxy grown AlxGa1−xAs
Authors: Hashizume Tamotsu , Hasegawa Hideki , Ohno Hideo
Published: July 2002

The electrical properties of midgap states in n-type AlxGa1−xAs grown by the metalorganic vapor phase epitaxy were investigated by the deep level transient spectroscopy and photocapacitance (PHCAP) techniques. A new PHCAP measurement procedure to avoid interference from the photoionization of the DX center was used. Two near-midgap levels, i.e., a higher lying MH level and a lower lying ML level were detected. As AlAs mole fraction, x, is increased, the energy positions of these two levels became deeper, maintaining a remarkable horizontal alignment with respect to the hybrid orbital energy level. The ML level showed a clear photoquenching for x<0.3, but the quenching disappeared for x≳0.3. No photoquenching of the MH level was observed for all x investigated. Similarity of the photoquenching behavior to GaAs 1−x Px as well as its energy position and optical cross sections led to the conclusion that the ML level is an As-related EL2-like defect. On the other hand, the MH level originates from a defect related to Al.
Spectroscopic study of plasma-enhanced organometallic chemical vapor deposition for superconducting thin film formation
Authors: Ebihara Kenji , Kanazawa Seiji , Ikegami Tomoaki , Shiga Masanobu
Published: July 2002

Plasma-enhanced organometallic chemical vapor deposition process has been developed to prepare high Tc superconducting films. The volatile organic precursors of Y(fod)3, Ba(hfa)2, and Cu(hfa)2 were decomposed with a microwave discharge plasma. Optical emission spectroscopy was used to determine the identity of the species produced during the vaporization and decomposition process. In this process, Y, Y+, Ba, Ba+, Cu, and Cu+ species were detected. After annealing in air, the films deposited on the (100) SrTiO3 substrate had preferential orientation of the crystalline c-axis perpendicular to the substrate surface. The characterization of this process is discussed comparing a hollow cathode YBaCuO plasma with the laser ablation experiments published.
Hydrodynamics of laser-produced shock waves in water: Reflection and transmission measurements
Authors: Fama’ F. , Harith M. A. , Palleschi V. , Salvetti A. , Singh D. P. , Vaselli M. , Dreiden G. V. , Ostrovsky Yu. I. , Semenova I. V.
Published: July 2002

Using different optical diagnostic methods, namely interferometric holography and shadowgraphy, and utilizing a piezoelectric transducer for precise velocity determination, the laws of reflection and transmission of laser-driven shock waves through an obstacle in water are studied. Experimental results on the velocity of transmitted and reflected shock waves from different materials are reported.
Magnetic properties of coupled layer films
Authors: Shi Longpei , Ji Z. R. , Hu L.
Published: July 2002

The magnetic properties of an alternating two-element multilayer film are studied by the Green’s function based upon the Heisenberg model with coupled interaction between the layers in the interfaces. It is proved that the order parameter changed with the position of the atomic planes in the multilayer films but the Curie temperature of the atomic planes is all the same. Applying the theory to an alternating terbium-iron multilayer structure, numerical calculation agrees with the results of magnetic measurements.
Correlation between crystalline perfection and film purity for chemically vapor deposited diamond thin films grown on fused quartz substrates
Authors: Prawer Steven , Hoffman Alon , Stuart Sue-Anne , Manory Rafael , Weiser Paul , Lim Cheryl S. , Long John M. , Ninio Fred
Published: July 2002

Chemically vapor deposited (CVD) diamond films have been deposited on quartz substrates using a configuration in which the substrate is placed parallel to the direction of the gas flow in the deposition system. Spatially resolved Raman spectroscopy and optical microscopy of the resultant films revealed that (a) as the diamond component of the films increases, the defect density (as measured by the FWHM of the Raman 1332 cm−1 line) decreases, (b) there is a decrease of the quality and perfection of the CVD diamond particles as they overgrow to form a continuous film, and (c) the best quality diamond particles (FWHM) of the 1332 cm−1 line=2.7 cm−1) are produced downstream at the bottom of the plasma ball. It is suggested that the limitations on the continuous film quality appear to be governed not so much by the details of the growth chemistry, but rather by the effects of particle overgrowth.
Magnetotransport properties of Fe/Cu multilayer films (abstract)
Authors: Florczak Jeff M. , Dahlberg E. Dan , Fullerton Eric E. , Schuller Ivan K.
Published: July 2002

In order to understand the transport properties of multilayer systems, three different series of sputtered Fe/Cu multilayer films were examined by low-frequency ac resistance measurements. In each series, the total thickness of the film was maintained at approximately 2000 Å while the bilayer thickness was varied from 20 to 300 Å. The three multilayer series differ in the ratios of Fe to Cu in a bilayer. The Fe:Cu ratios examined here are 1:3, 1:1, and 3:1. Transport and magnetotransport measurements were done on these films at temperatures of 296, 77, and 4.2 K with in-plane fields up to 10 KOe. As expected, for each Fe:Cu series the resistivities increase with decreasing bilayer thickness. Furthermore, comparing the different series, the higher Cu ratios give smaller resistivities. From the magnetotransport measurements, the anisotropic magnetoresistance also increases with decreasing bilayer thickness and appears to be linearly related to the resistivity of the multilayer, especially at the lower temperatures. For the highest Cu ratio, Fe:Cu 1:3, the AMR was unmeasureable from the thinnest periods at 296 K but becomes significant at the lower temperatures.
Internal explosion in laser ablation of superconducting targets
Authors: Lin D. L. , Li X. , Liu Z. D. , George Thomas F.
Published: July 2002

The temperature profile inside a superconducting target in laser ablation is calculated for laser pulses of various shapes. The calculation is based on the equation of heat conduction. All parameters characterizing the target material are assumed to be temperature dependent and are determined empirically by extrapolating experimental data to the melting point. The receding velocity of the vapor-solid interface is determined by the dynamical balance of energy. Our calculation shows that, in general, there exist subsurface overheating spots at different instants as long as the laser pulse intensity is strong enough. The dependence of their occurrence on the pulse shape is analyzed, and conditions to avoid them without jeopardizing the deposition process are discussed.
Anomalous Hall effect in InSb layers grown by metalorganic chemical vapor deposition on GaAs substrates
Authors: Besikci C. , Choi Y. H. , Sudharsanan R. , Razeghi M.
Published: July 2002

InSb epitaxial layers have been grown on GaAs substrates by low-pressure metalorganic chemical vapor deposition. A 3.15-μm-thick film yielded an x-ray full width at half maximum of 171 arcsec. A Hall mobility of 76 200 cm2/V s at 240 K and a full width at half maximum of 174 arcsec have been measured for a 4.85-μm-thick epilayer. Measured Hall data have shown anomalous behavior. A decrease in Hall mobility with decreasing temperature has been observed and room-temperature Hall mobility has increased with thickness. In order to explain the anomalous Hall data, and the thickness dependence of the measured parameters, the Hall coefficient and Hall mobility have been simulated using a three-layer model including a surface layer, a bulklike layer, and an interface layer with a high density of defects. Theoretical analysis has shown that anomalous behavior can be attributed to donorlike defects caused by the large lattice mismatch and to a surface layer which dominates the transport in the material at low temperatures.