bluuebunny/crossref_metadata_2025 huggingface dataset:
The initial permeability of polycrystalline MnZn ferrites: The influence of domain and microstructure
Authors: van der Zaag P. J. , Ruigrok J. J. M. , Noordermeer A. , van Delden M. H. W. M. , Por P. T. , Rekveldt M. Th. , Donnet D. M. , Chapman J. N.
Published: July 2002

The validity of nonmagnetic grain-boundary (NMGB) models for the initial permeability of polycrystalline ferrites is examined. The domain size in a series of wet-chemically prepared polycrystalline MnZn ferrites, in the demagnetized state, has been determined by neutron depolarization. A transition in the intragranular domain structure from mono- to two domain is observed at grain size D≊4 μm. An expression for this transition has been derived for a dense magnetic material. The grain size dependence of the initial permeability of the ferrites studied, particularly for monodomain grains, is consistent with the NMGB model. The grain-boundary width and composition have been determined with a nanoprobe (1.5 nm resolution). Composition variations extend ≊10 nm into the grain. The findings suggest that, although the NMGB model considers ‘‘nonmagnetic’’ grain boundaries, in practice the grain boundary may be hard magnetic.
(Photo)electrochemical characterization of tin-doped indium oxide
Authors: van den Meerakker J. E. A. M. , Meulenkamp E. A. , Scholten M.
Published: July 2002

Tin-doped indium oxide (ITO) films, prepared by dc magnetron sputtering, were characterized by (photo)electrochemical measurements in aqueous H2SO4 solutions. Wavelength dependent photocurrent measurements were used to determine the optical band gap energy of these films. Electron excitation from the valence band to localized states in the band gap was observed. The presence of such energy levels resulted in an Urbach tail. Impedance measurements were used to determine the flatband potential and the charge carrier concentration of ITO. A change in the charge carrier concentration due to different deposition conditions resulted in a change of the resistivity and in a shift of the flatband potential. This shift could be explained by a Moss–Burstein shift of the optical band gap.
Vacuum ultraviolet spectroscopy study of excimer-laser-generated plasmas
Authors: Mehlman G. , Chrisey D. B. , Burkhalter P. G. , Horwitz J. S. , Newman D. A.
Published: July 2002

The dispersed emission in the vacuum ultraviolet (VUV) (1200–3000 Å) from the plasma generated by the interaction of a KrF excimer laser with an Al and a YBa2Cu3O7 target has been measured. Emission spectra were collected as a function of distance above the target surface and as a function of laser fluence. The qualitative features of the plasmas from the two different targets were similar. The character of the emission spectra changed from a pseudocontinuous emission at the target surface to discrete emission from singly, doubly ionized species as well as neutrals at distances greater than ∼1.5 mm. The spatial variation indicated two regions: a high-density sheath along the target where the core emission is close to blackbody; and beyond, a plasma with large opacity emitting a UV spectrum of intensity decreasing fast with distance. Estimates of the plasma temperature and density were between 2 and 4 eV and ∼1018/cm3, respectively.
Isotopically engineered semiconductors
Authors: Haller E. E.
Published: July 2002

Scientific interest, technological promise, and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled semiconductor crystals. This review of mostly recent activities begins with an introduction to some past classical experiments which have been performed on isotopically controlled semiconductors.

A review of the natural isotopic composition of the relevant elements follows. Some materials aspects resulting in part from the high costs of enriched isotopes are discussed next. Raman spectroscopy studies with a number of isotopically pure and deliberately mixed Ge bulk crystals show that the Brillouin-zone-center optical phonons are not localized. Their lifetime is almost independent of isotopic disorder, leading to homogeneous Raman line broadening. Studies with short period isotope superlattices consisting of alternating layers of n atomic planes of 70Ge and 74Ge reveal a host of zone-center phonons due to Brillouin-zone folding. At n≳40 one observes two phonon lines at frequencies corresponding to the bulk values of the two isotopes. In natural diamond, isotope scattering of the low-energy phonons, which are responsible for the thermal conductivity, is very strongly affected by small isotope disorder. Isotopically pure 12C diamond crystals exhibit thermal conductivities as high as 410 W cm−1 K−1 at 104 K, leading to projected values of over 2000 W cm−1 K−1 near 80 K.

The changes in phonon properties with isotopic composition also weakly affect the electronic band structures and the lattice constants. The latter isotope dependence is most relevant for future standards of length based on crystal lattice constants. Capture of thermal neutrons by isotope nuclei followed by nuclear decay produces new elements, resulting in a very large number of possibilities for isotope selective doping of semiconductors. This neutron transmutation of isotope nuclei, already used for homogeneous doping of floating zone Si with P, holds perhaps the biggest promises for isotopically controlled semiconductors and is discussed in some detail. Local vibrational modes of low-mass impurities are sensitive to the mass of the impurity as well as the masses of the host atoms neighboring the impurity. High-resolution infrared-absorption studies of O in Ge crystals of different isotopic composition demonstrate the extreme simplification in such spectra which is achieved by isotope control. Interdiffusion of GaAs and Ge isotope superlattices with 0.1–1 μm thick layers have been studied with secondary-ion-mass spectroscopy. This kind of internal diffusion avoids the problems with surface effects and can produce accurate data without the need for radioactive tracers. The review closes with an outlook on the exciting future possibilities offered through isotope control of a wide range of semiconductor materials.
Carrier lifetime in InP/InGaAs/InP by open-circuit voltage and photoluminescence decay
Authors: Zemel A. , Gallant M.
Published: July 2002

Carrier lifetime in n-InP/n-InGaAs/n-InP and Zn-diffused p-InP/p-n InGaAs/n-InP double heterostructures (DH) with undoped n-InGaAs was measured using the small signal open-circuit voltage and photoluminescence decay techniques. The structures were grown by the metalorganic chemical-vapor-deposition growth technique. The theoretical considerations required to correctly apply both techniques to structures containing a p-n junction are discussed in detail. Carrier lifetimes between 3 and 5 μs and between 8 and 15 μs were obtained for the p-InP/p-n InGaAs/n-InP and n-InP/n-InGaAs/n-InP DH, respectively. The carrier lifetimes measured in n-InP/n-InGaAs/n-InP confirm the long lifetimes previously deduced indirectly from continuous-wave and pulsed-optical-beam-induced current measurement techniques. The much shorter lifetimes measured in the Zn-diffused DH are basically due to a joint recombination process involving processes on both sides of the p-n junction structure. The analysis of the results show the possible reduction of bulk lifetime due to Zn-induced nonradiative recombination processes in the highly doped Zn-diffused region.
Dielectric properties of rutile and its use in high temperature superconducting resonators
Authors: Klein N. , Zuccaro C. , Dähne U. , Schulz H. , Tellmann N. , Kutzner R. , Zaitsev A. G. , Wördenweber R.
Published: July 2002

The microwave properties of single crystalline TiO2 (rutile) were investigated. At a frequency of 7.5 GHz the loss tangent tan δ was found to increase from 1.4×10−7 at 4 K to 4×10−6 at 70 K for electric fields parallel to the crystallographic a,b plane. The high permittivity of 105 and the small tanδ in combination with the low microwave losses of high temperature superconductors (HTS) were utilized to construct a miniaturized X-band resonator with a high quality factor Q. An assembly of two YBa2Cu3O7 films of 8 mm in diameter separated by a rutile cylinder of 2 mm height provides a TE011 resonance at 9.7 GHz with Qs ranging from 6×105 at 10 K to 105 at 70 K. Frequency scaling of the losses in rutile and in the HTS films indicates Qs in excess of 106 at 1.8 GHz using YBa2Cu3O7 films of two inches in diameter. Such resonators are considered to be key elements for high-power filters in mobile communications.
Mechanism of charge transport in polypyrrole, poly(N-methyl pyrrole) and their copolymers
Authors: Singh Ramadhar , Narula Amarjeet K. , Tandon R. P. , Mansingh A. , Chandra Subhas
Published: July 2002

The temperature dependence of the dc conductivity of electrochemically polymerized films of polypyrrole, poly(N-methyl pyrrole) and their copolymers, poly(N-methyl pyrrole-pyrrole), having different percentage of BF−4 ions has been investigated in the temperature range 77–350 K. The observed behavior could be explained in terms of Mott’s variable range hopping model involving a single phonon process. The estimated values of polaron radius yield the realistic values of density of states at the Fermi level which are in good agreement with the values reported earlier for other polyconjugated systems.
Exchange biasing in MBE grown Fe3O4/CoO bilayers: The antiferromagnetic layer thickness dependence
Authors: van der Zaag P. J. , Ball A. R. , Feiner L. F. , Wolf R. M. , van der Heijden P. A. A.
Published: July 2002

Exchange biasing has been studied for a series of [100] and [111] oriented, epitaxial Fe3O4/CoO bilayers grown by oxidic MBE. The low-temperature exchange biasing versus CoO layer thickness is compared to theoretical models for exchange biasing. We argue that the Malozemoff random field model does not apply to this system. The exchange biasing calculated according to the Meiklejohn–Bean model, assuming nearest-neighbor exchange coupling across a flat and magnetically uncompensated interface, differs for [100] oriented bilayers by a factor of ≂8 from the experimental value.
Metastable RFe7 compounds (R=rare earths) and their nitrides with TbCu7 structure
Authors: Mao O. , Yang J. , Altounian Z. , Ström-Olsen J. O.
Published: July 2002

RFe7 compounds with the TbCu7 structure have been synthesized by annealing of mechanically alloyed powders. The lattice constants a and c decrease with increasing atomic number of the rare earths except for Ce which has a different valence state. Introduction of nitrogen interstitial atoms into the structure results in a ∼6.0% volume expansion. In addition, nitriding gives rise to a dramatic enhancement of the magnetic properties of the compounds. Curie temperatures are increased by ∼300 °C upon nitriding. RFe7 nitrides with R=Sm, Tb, Dy, and Ho have high coercivities. A substantial enhancement in the magnetic remanence for the RFe7 nitrides with R=Tb, Dy, and Ho is attributed to the magnetic exchange coupling between grains of nanocrystalline structure in the powders.
Structural comparisons of ion beam and dc magnetron sputtered spin valves by high-resolution transmission electron microscopy
Authors: Bailey William E. , Zhu Nan-Chang , Sinclair Robert , Wang Shan X.
Published: July 2002

We have used high-resolution transmission electron microscopy to compare the nanostructures of ion-beam and dc magnetron sputter-deposited giant magnetoresistive (GMR) spin valves and to correlate nanostructure with magnetic properties. Very low coercivities and strong exchange bias (<8 Oe, 125 Oe) were achieved in ion-beam-deposited spin valves of the form NiFe(50)/Co(20)/Cu(≳25)/Co(20)/NiFe(50)/FeMn(150)/Ta(30 Å); these were compared with typical dc magnetron deposited structures of the same kind, both with and without a Ta seed layer, which exhibited similar and poorer exchange biasing but superior GMR ratios (to 8%.) Cross-sectional and plane-view samples were prepared of all three structures and examined by high-resolution electron microscopy. Near-perfect (111)-textured fcc metal and c-axis hcp Co columnar grains were revealed in the ion beam deposited sample, while some (10°) dispersion of this texture and random grain orientations were observed in the Ta-seeded and unseeded dc magnetron sputter-deposited samples, respectively. No amount of the α-FeMn (A12) phase was observed in any of the films. Exchange bias strengths and coercivity of the top Co/NiFe/FeMn layers thus correlate strongly with the degree of (111) texture.
Relation for the nonequilibrium population of the interface states: Effects on the bias dependence of the ideality factor
Authors: Gomila G. , Rubí J. M.
Published: July 2002

By an analysis of the exchange of carriers through a semiconductor junction, a general relationship for the nonequilibrium population of the interface states in Schottky barrier diodes has been derived. Based on this relationship, an analytical expression for the ideality factor valid in the whole range of applied bias has been given. This quantity exhibits two different behaviours depending on the value of the applied bias with respect to a critical voltage. This voltage, which depends on the properties of the interfacial layer, constitutes a new parameter to complete the characterization of these junctions. A simple interpretation of the different behaviours of the ideality factor has been given in terms of the nonequilibrium charging properties of interface states, which in turn explains why apparently different approaches have given rise to similar results. Finally, the relevance of our results has been considered on the determination of the density of interface states from nonideal current-voltage characteristics and in the evaluation of the effects of the interfacial layer thickness in metal-insulator-semiconductor tunnelling diodes.
Fe-Cr-N soft magnetic thin films
Authors: Jin S. , Zhu W. , Tiefel T. H. , Korenivski V. , van Dover R. B. , Chen L. H.
Published: July 2002

Soft magnetic properties of new Fe-Cr-N and Fe-Cr-Ta-N alloy films have been investigated. Thin films with compositions in the range of Fe-2∼8% Cr-0∼1% Ta-5∼15% N (in at. %) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post heat treatment, e.g., Hc∼1–2 Oe (79.4–158.8 A/m) and 4πMs∼15–20 kG. The easy-axis M-H loop is square. The hard-axis loop is linear and closed, with the anisotropy field Ha=20–60 Oe (1.59–4.77 kA/m). The combination of high 4πMs and relatively high Ha in these films is conducive to the suppression of the undesirable ferromagnetic resonance (FMR) interference up to the GHz frequency range.
Cl
2
plasma etching of Si(100): Nature of the chlorinated surface layer studied by angle-resolved x-ray photoelectron spectroscopy
Authors: Layadi N. , Donnelly V. M. , Lee J. T. C.
Published: July 2002

The interaction of a Cl2 plasma with a Si(100) surface has been investigated by angle resolved x-ray photoelectron spectroscopy (XPS). It was found that the amount of chlorine incorporated into the near-surface region of Si increases with ion energy, and does not change with long exposure to the plasma. Chlorine is present as SiClx (x=1–3) with average relative coverages (integrated over depth) of [SiCl]:[SiCl2]:[SiCl3]≅1:0.33:0.13 at −240 V dc bias (mean ion energy ≈280 eV) and 1:0.34:0.087 at 0 V dc bias (mean ion energy ≈40 eV), at x-ray photoelectron spectroscopy (XPS) binding energies of 100.2, 101.2 and 102.3 eV, respectively. Moreover, there is a substantial amount of disordered Si within the chlorinated layer at high ion energy, reflected in a broadening of the 99.4 eV Si peak and the appearance of a shoulder at 98.8 eV, ascribed to Si with a dangling bond. In addition, bulk Si plasmon losses associated with the Cl(2p) and Cl(2s) core levels indicate that roughly one-third of the Cl in the near-surface region is surrounded by bulklike Si at the high ion energy. Modeling of the dependence of the relative concentration of Cl on the take-off angle was used to estimate the Cl content and thickness of the surface layer. From an inversion of the observed take-off angle dependence of the relative Cl and Si XPS signals, depth profiles were derived for the near-surface region. Cl content falls off in a graded fashion, over a depth of about 25 and 13 Å for a mean ion energies of 280 and 40 eV, respectively. The Cl areal density (coverage integrated throughout the layer) increases with increasing mean ion energy from 1.8×1015 Cl/cm2 at 40 eV to 3.5×1015 Cl/cm2 at 280 eV. From a similar inversion of the take-off angle dependence of the SiClx signals, SiCl2 and SiCl3 are found to be largely confined at the top ∼5 Å, while below the surface, disordered Si and SiCl are present.
Hydrogen migration in diamond-like carbon films
Authors: Vainonen E. , Likonen J. , Ahlgren T. , Haussalo P. , Keinonen J. , Wu C. H.
Published: July 2002

Properties of physical vapor deposited diamondlike carbon (DLC) films and the migration of hydrogen in H+ and He+4 ion implanted and hydrogen co-deposited DLC films have been studied. Measurements utilizing Rutherford backscattering spectrometry showed that the films studied have an average mass density of 2.6±0.1 g/cm3. The bonding ratio sp3/sp2 is typically 70% measured with the electron spectroscopy for chemical analysis technique. Impurities and their depth distributions were deduced from the particle induced x-ray emission and secondary ion mass spectrometry (SIMS) measurements. Distributions of implanted and co-deposited hydrogen were measured by the nuclear resonance reaction H1(15N,αγ)12C and SIMS. It was found that annealing behavior of implanted H in DLC has a diffusion like character. The obtained diffusion coefficients resulted in the activation energy of 2.0±0.1 eV. It was observed that in H co-deposited DLC films the temperature of H release varied between 950 and 1070 °C depending on the H concentration.
Electron spin resonance of pulsed plasma-enhanced chemical vapor deposited fluorocarbon films
Authors: Labelle Catherine B. , Limb Scott J. , Gleason Karen K.
Published: July 2002

Pulsed-rf excitation of hexafluoropropylene oxide has been used to deposit poly(tetrafluoroethylene)-like thin films. Films were deposited at pulse rates of 10/20, 10/50, 10/200, and 10/400 ms on/ms off and analyzed using electron spin resonance spectroscopy (ESR). All four films produced similar broad ESR spectra, with an average width at maximum slope of ∼60 G and a g value of 2.0045. The number of free electrons in a sample decreased with increasing pulse off time. This behavior can be modeled by the reaction of a free radical with a gas species, assuming that free radicals are generated only during the pulse on time.
Spatial density distributions of C2, C3, and CH radicals by laser-induced fluorescence in a diamond depositing dc-arcjet
Authors: Luque J. , Juchmann W. , Jeffries J. B.
Published: July 2002

Quantitative measurements of C2(a3Πu), C3(X,1Π), and CH(X,2Π) have been made by calibrated linear laser-induced fluorescence in the plume of a dc-arcjet (Ar/H2/CH4 1.1:1:0.005) reactor during the chemical vapor deposition of diamond. The peak number density in the arcjet plume for CH is (3.7±0.8)×1012 molecules/cm3, for C2(3a)(3.1±1.3)×1010 cm−3, and for C3∼3×1012 cm−3 with 25 Torr reactor pressure. The radial spatial distributions for C2 and CH have a maximum in the center of the plume; however, C3 is distributed as a hollow cylinder with a pronounced minimum at the center. The variation in number density and in spatial distribution is investigated for changes in chamber pressure, methane flow, and distance from the nozzle.
Scaling relation for the inductance of a coil on a ferromagnetic half-space
Authors: Rose James H. , Tai Cheng-Chi , Moulder John C.
Published: July 2002

The calculated change in inductance when an air-core coil is placed next to a uniform ferromagnetic half-space is found to be well described by a simple two parameter scaling relation for a broad class of coils. The scaling relation is ΔL(ω)=ΔLoL*(ω/ωo). Here, ΔLo is a constant that determines the overall strength of the inductance change, while the frequency scale is set by ω0, the frequency for which Re ΔL(ω) is zero. We test the hypothesis that the inductance change of a coil next to a uniform ferromagnetic plate can be described by the scaling relation with measurements made on a suite of medium carbon steels.
Zone-center optical phonons in wurtzite GaN and AlN
Authors: Wei Guanghong , Zi Jian , Zhang Kaiming , Xie Xide
Published: July 2002

Phonons at the zone-center of wurtzite semiconductors GaN and AlN are studied theoretically within a rigid-ion model. The anisotropies of the zone-center optical phonons induced by the macroscopic electric field are investigated.
Finite-temperature behavior of anisotropic two-sublattice magnets
Authors: Skomski Ralph
Published: July 2002

The finite-temperature magnetism of rare-earth transition-metal intermetallics is investigated by extending the n-component vector spin model to two-sublattice magnets. Mean-field analysis shows that the influence of the rare-earth anisotropy on the mean-field Curie temperature is much smaller than expected from the low-temperature rare-earth anisotropy. The use of ultraspherical polynomials yields a generalization of the famous m(m+1)/2 power-law exponent to m(m+n−2)/(n−1).
Paratellurite: Propagation of the slow shear wave in the (001) plane. New formulations for the acoustic velocity and divergence
Authors: Kastelik J. C. , Gharbi M. , Gazalet M. G.
Published: July 2002

The propagation of the slow shear wave in paratellurite with an angular tilt θ from the [110] axis in the (001) plane is considered. Original formulations are given for the acoustic velocity, the power flow angle, and the acoustic divergence. For a small angular deviation, the acoustic spreading is greatly reduced. This configuration can be used for multichannel Bragg cells. In order to demonstrate the feasibility of such a device, a three-channel Bragg cell (θ=5°) has been designed and successfully tested.
Investigation of the electrical properties of SnO2 varistor system using impedance spectroscopy
Authors: Bueno P. R. , Pianaro S. A. , Pereira E. C. , Bulhões L. O. S. , Longo E. , Varela J. A.
Published: July 2002

The electrical properties of tin oxide varistors doped with CoO, Nb2O5, and Cr2O3, were investigated using the impedance spectroscopy technique with the temperature ranging from 25 to 400 °C. The impedance data, represented by means of Nyquist diagrams, show two time constants with different activation energies, one at low frequencies and the other at high frequencies. These activation energies were associated with the adsorption and reaction of O2 species at the grain boundary interface. The Arrhenius plots show two slopes with a turnover at 200 °C for both the higher and lower frequency time constants. This behavior can be related with the decrease of minor charge carrier density. The barrier formation mechanism was associated with the presence of CrSn⋅ at the surface, which promotes the adsorption of the O′ and O″ species which are in turn proposed as being responsible for the barrier formation.
Determination of surface energy of polymers by force microscopy
Authors: El Ghzaoui A.
Published: July 2002

Force microscopy has been used to measure adhesive forces between a silica oxide tip and several polymer samples. The adhesive forces vary with the nature of the surface and seem to be correlated with the surface energy of the solids. From adhesive forces we have determined the surface energy of three polymer samples and compared these values with those found by contact angle measurements. The surface energies determined by these methods are close and agree with the literature values.
Mössbauer study of the influence of thermal treatment on giant magnetoresistance and interface structure in Fe/Cr multilayers
Authors: Kopcewicz M. , Luciński T. , Stobiecki F. , Reiss G.
Published: July 2002

The dependence of the giant magnetoresistance effect (GMR) on the interface structure in Fe/Cr multilayers was studied by magnetoresistivity and Mössbauer spectroscopy. The Fe/Cr multilayers consisting of Fe(6 nm)+[Cr(1.1 nm)/Fe(3 nm)]60+Cr(1.1 nm) were deposited by dc magnetron sputtering. Samples were annealed for 1 h at temperatures ranging from 200 to 500 °C. The interface structure was characterized by conversion electron Mössbauer spectroscopy (CEMS). Various different Fe sites: the bulk, step and perfect interface positions were identified. The evolution of the fraction of Fe atoms in different environments vs annealing temperature revealed that annealing at 300 °C induces bulk and in-plane diffusion of atoms. Higher annealing temperature causes substantial roughening of the interface related to a strong bulk diffusion of atoms. Microstructural changes observed in the CEMS spectra correspond well to the GMR measurements which reveal an enhancement of magnetoresistivity in Fe/Cr multilayers annealed at about 300 °C and its decrease due to deterioration of the interface at higher annealing temperatures.
Thermoexcited and photoexcited carrier transports in a GaAs/AlGaAs quantum well infrared photodetector
Authors: Fu Y. , Li Ning , Karlsteen M. , Willander M. , Li Na , Xu W. L. , Lu W. , Shen S. C.
Published: July 2002

A study of the carrier transport performance of our quantum well infrared photodetector (QWIP) is carried out in this work. AlGaAs/GaAs QWIP devices were fabricated and characterized. Carrier transport between adjacent quantum wells are modeled by both the tunneling and thermoexcitation effects. The strengths of the two transport effects are quite small (tunneling is small due to thick barrier and thermoexcitation is small due to the low device working temperature). However the thermoexcitation effect dominates in our QWIP devices and the thermally activated carriers transport across the optical active multiple quantum well (MQW) region in the three-dimensional drift-diffusion manner. At zero bias the energy band structure of our sample is almost at a condition of flat band. Due to the thick multiple quantum well region and the high doping levels in the contacts, the external bias largely applies linearly across the MQW region. Current increases with increasing bias and it becomes saturated when the drift velocity becomes saturated. The calculated dark current density agrees well with our experimental measurement. The photoexcited carrier density is almost independent of the external bias. The photocurrent increases with increasing bias because of the increasing drift velocity. It reaches its maximal when the drift velocity saturates and the current then decreases as the external bias further increases due to current continuity and charge conservation.
Nonlinear optical absorption and refraction by CdTe microcrystals doped in glass
Authors: Yu Baolong , Zhu Congshan , Gan Fuxi
Published: July 2002

The two-photon absorption coefficient β and the bound electronic nonlinear index γ of CdTe microcrystals doped in glass were measured by the Z-scan technique. It was found that the figure of merit γ/β of the CdTe microcrystals with nanometer size is enhanced compared to that of bulk CdTe. The optical Stark effect in the nanometer-sized CdTe is the possible origin of the enhanced nonlinearity.
Constriction energy in the presence of a solute field
Authors: Andrews Scott D. , Sehitoglu Huseyin , Karaman Ibrahim
Published: July 2002

It has been widely known that both solute concentration, i.e., frictional effects, and stacking fault energy influence the degree of cross slip and slip planarity in face-centered-cubic alloys. Cross slip is preceded by constriction of two partial dislocations. A model is proposed for the energy required to form a constriction from two parallel partial dislocations as a function of stacking fault energy, solute concentration, atomic size misfit, and modulus mismatch. The cross slip is curtailed due to interaction of solute atoms with the partials. Both the atomic size misfit and modulus mismatch influence the local solute concentration which introduce local stresses that determine the energy needed to form the constriction. The shape of partials and the energy to form the constriction was established for stacking fault energies in the range of 10–100 mJ/m2, misfit strains in the range of 0.1–0.5, modulus mismatch levels of −1.0, and nominal solute concentrations varying from 0 to 10 at. %. In extreme cases, the constriction energy has been found to increase fourfold compared to the solute-free case. The modulus mismatch effect is important in substitutional alloys with small misfit strains (<0.1) while for interstitial solute cases the misfit strain effects dominate. The results converge to the well-known solution of Stroh in the limit of zero solute concentration.
Antiferromagnetic correlations in Fe–Cu granular alloys: The role of the surface structure
Authors: Batlle Xavier , Echer Chuck J. , Nelson E. Chris , Krishnan Kannan M. , Hattink Bart Jan
Published: July 2002

Fe precipitates in a Cufcc matrix, prepared using the Bridgeman method and with an average composition of Cu97Fe3, displayed the coexistence of ferromagnetism (FM), spin glass-like (SGL) behavior and antiferromagnetic (AFM) correlations. The two former contributions may be attributed, respectively, to the segregation of FM, α-Febcc precipitates and to the few Fe spins distributed in the matrix. The annealing procedures increased the FM contribution and, as particle growth and phase segregation took place, the SGL behavior progressively disappeared. Results from high resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and electron energy-loss spectroscopy (EELS) suggest that the AFM correlations are due to the α-Fe particles that show a surface layer of a few nanometers in thickness, of either FeO and/or γ-Fefcc. XPS and EELS measurements confirm the presence of FeO; however, the latter is only tentatively suggested by the HRTEM analysis of the particle/matrix interfaces.
Formation energies and relative stability of perfect and faulted dislocation loops in silicon
Authors: Cristiano F. , Grisolia J. , Colombeau B. , Omri M. , de Mauduit B. , Claverie A. , Giles L. F. , Cowern N. E. B.
Published: July 2002

A study of the relative thermal stability of perfect and faulted dislocation loops formed during annealing of preamorphized silicon wafers has been carried out. A series of transmission electron microscopy experiments has been designed to study the influence of the ion dose, the annealing ambient and the proximity of a free surface on the evolution of both types of loops. Samples were implanted with either 150 keV Ge+ or 50 keV Si+ ions to a dose of 2×1015 cm−2 and annealed at 900 °C in N2, N2O, and O2. The calculations of formation energy of both types of dislocation loops show that, for defects of the same size, faulted dislocation loops (FDLs) are more energetically stable than perfect dislocation loops (PDLs) if their diameter is smaller than 80 nm and vice versa. The experimental results have been analyzed within the framework of the Ostwald ripening of two existing populations of interstitial defects. It is found that the defect ripening is nonconservative if the surface is close to the end of range defect layer or if the sample is oxidized during annealing. In both cases, the knowledge of the formation energy of both types of dislocation loops allows a realistic estimate of the interstitial flux towards and from the surface, respectively, during annealing, in agreement with the experimental results. During a conservative ripening process, a direct correspondence exists between the formation energy of the two defect families and the number of atoms bound to them. In this case, the relative stability of FDLs and PDLs depends on the initial supersaturation of Si interstitial atoms created during implantation.
A quantitative study of chemical waves in the Belousov–Zhabotinsky reaction
Authors: Wood Peggy Marie , Ross John
Published: July 2002

Measurements are reported of velocities, amplitudes, and profiles in space and time of chemical waves in the Belousov–Zhabotinsky reaction at various temperatures, depths of solution, and initial reactant concentrations. The measurements are made in a thin layer of a quiescent, but excitable solution by means of light absorption by ferroin, the tris(1,10-phenanthroline) ferrous sulfate complex. Propagating wave profiles are recorded on a linear photodiode array (25.6 mm length) with a spatial resolution of 50 μ. The determinations of velocity corroborate previous experimental findings. New results include experimental verification of the constancy of the concentration profile of the wave in space and time; determination of two characteristic time constants in the relaxation of the wave profile; trends in wave amplitude with variation of initial reactant concentrations and age of the reaction mixture; wave velocity as a function of temperature, and solution depth; and measurements of wave annihilation. Observations of additional structure include the onset of mosaic structure, that is the transition from a homogeneous to an inhomogeneous state due to the passage of the wave, and initiation spikes.
Depolarized scattering from p
a
r
a and normal hydrogen
Authors: Le Duff Y. , Ouillon R.
Published: July 2002

Experimental data on depolarized light scattering for the Rayleigh and vibrational Raman bands of hydrogen at 123 K are presented for densities between 20 and 130 amagat. The para-hydrogen (∼J=0) depolarized spectra are found quasiexponential contrary to the thermal equilibrium hydrogen Rayleigh spectrum. In this last spectrum the low frequencies part (Δν<40 cm−1) shows a contribution from the permanent anisotropy of the ortho molecules (J=1). The integrated depolarization ratios are measured for the Rayleigh and the vibrational Raman bands of para-hydrogen. A discussion of the results shows that the first order dipole induced dipole model fails to give an accurate description of all observed intensities.
The formation of surfaces by diffusion limited annihilation
Authors: Meakin Paul , Deutch J. M.
Published: July 2002

Computer simulations have been carried out in two and three dimensions to explore the formation of surfaces by diffusion limited annihilation of an initially smooth surface by mobile particles. From simulations carried out on two-dimensional strips of width l lattice units we find that the asymptotic variance in the surface height (ξ) scales with l according to ξ∼[ln(l)]1/2. For three-dimensional systems with a square cross section of size l×l lattice units we find that ξ quickly reaches a limiting value which is essentially independent of l(ξ∼l°). A variety of two-dimensional simulations were also carried out for other geometries. The results of these simulations indicate that diffusion limited annihilation processes cannot be used to produce sharply defined grooves by etching through an inert mask.
Preselecting paths for multiphoton dynamics using artificial intelligence
Authors: Chang Johnny , Wyatt Robert E.
Published: July 2002

A new, systematic method for selecting the most important basis functions by using an artificial intelligence tree pruning algorithm is introduced. The technique can be applied to any physical problem which gives rise to coupled equations but is applicable in general to any problem which could benefit from an orderly exploration of linked alternatives in a decision tree. The method is applied here to select the most important states for modeling the multiphoton dynamics of a general vibrating–rotating spherical top molecule. The optimal paths selected by the branch-and-bound search algorithm augmented with dynamic programming are investigated in detail with respect to what information about the dynamics they may contain.
Relativistic calculations of electronic states of PdH
Authors: Balasubramanian K. , Feng P. Y. , Liao M. Z.
Published: July 2002

Complete active space MCSCF(CASSCF)/first-order configuration interaction (FOCI) calculations followed by relativistic configuration interaction (RCI) calculations are carried out on 2Σ+, 2Π, 2Δ, and 2Σ+(II) λ–s states and 1/2, 3/2, 1/2(II), 3/2(II), and 5/2 ω–ω states of PdH. Our calculations predict that the ground state is 2Σ+1/2. The spin–orbit splitting and contamination of the excited states (2Π and 2Δ) are quite large. The two 3/2 states are quite mixed [3/2(I): 66% 2Δ3/2, 29% 2Π3/2] in character. The spectroscopic properties (Re, Te, De, and ωe) of all the above states are calculated and compared with available experimental results on the ground state of PdD. The Mulliken population analysis of the low-lying states of PdH shows that the 2Σ+ state is ionic (Pd+H−) but the excited 2Π and 2Δ states are a bit more ionic with the same polarity as the ground state. The 3/2 state exhibits an interesting avoided crossing.
Theory of enhanced energy transfer between molecules embedded in spherical dielectric particles
Authors: Druger Stephen D. , Arnold Stephen , Folan Lorcan M.
Published: July 2002

The effects of morphological resonances on radiative emission rates and on dipole–dipole energy transfer rates between molecules embedded in aerosol droplets are studied by developing and applying a theory based on electromagnetic solutions for a dielectric sphere. Expressions for the Mie-resonance contributions to the transfer and emission rates are obtained in terms of the experimentally measurable spectral shape functions for the donor and acceptor molecules in the bulk host medium. Mie resonances in droplets of about 10 μm diameter are found to cause only minor changes in the integrated radiative rate, while strongly enhancing the rate and quantum efficiency of energy transfer. Estimates of the enhancement effect and calculations of its dependence on donor/acceptor location yield results consistent with experiment, supporting our interpretation that the observed energy–transfer enhancement occurs via Mie resonances of the nearly spherical droplets, and providing further insight into the Mie-enhanced energy–transfer mechanism.
Dense fluid theory of mixtures
Authors: Hamad E. Z. , Mansoori G. A.
Published: July 2002

Previous studies have indicated that most of the existing theories of mixtures tend to fail for solutions containing species with large molecular size and intermolecular energy differences. In this work a dense fluid mixture theory, which is similar to the mixture theory of imperfect gases, is introduced. This theory is applicable for mixtures of molecules with large size and energy differences. The new theory is shown to be successful in predicting properties of Lennard-Jones fluid mixtures at, both, finite concentrations and infinite dilution.
Intramolecular dynamics of the overtone-induced isomerization of methyl isocyanide
Authors: Sumpter Bobby G. , Thompson Donald L.
Published: July 2002

Overtone-induced isomerization of methyl isocyanide to methyl cyanide is studied by using classical trajectories on several potential-energy surfaces. The several potential-energy surfaces are variations of a potential which we have developed based on available experimental and ab initio results. The trajectory results for simple potentials which neglect stretch–bend interactions show that overtone excitation of a CH stretch to the v=6 level at total energies (including overtone excitation energy) of 75, 125, and 150 kcal/mol does not enhance the rate of isomerization. However, at an initial total energy of 200 kcal/mol, the isomerization rate is enhanced by as much as a factor of 3 by selective excitation of a CH stretching overtone. However, the mode specificity is sensitive to the potential-energy surface. When a more realistic potential is used in which the bending force constants are attenuated as a function of the bond lengths or in which nondiagonal quadratic coupling terms are included, the dominant reaction is CH bond dissociation. The rate coefficient for the CH bond dissociation is an order of magnitude greater than the rate of isomerization at 200 kcal/mol. The initial energy flow out of an excited CH stretch is rapid (occurring on a times scale of less than 0.5 ps) and is primarily into the methyl bending modes. The energy that flows into the bending modes does not transfer out over the time period of 5.4 ps that the trajectories were followed. The methyl bending modes act as an energy ‘‘sink’’.
Product rotational alignment in the excitation transfer reaction Ar(3P2)+N2→Ar+N2(C 3Πu)
Authors: Tyndall George W. , de Vries Mattanjah S. , Cobb Cathy L. , Martin Richard M.
Published: July 2002

The velocity dependence of the N2(C) rotational alignment from the title excitation transfer reaction was studied over the collision energy range 42–460 meV, using a beam-gas time-of-flight method. The alignment was found to be positive, and to increase approximately linearly with energy: the extrapolated line has a large positive intercept, suggesting that the alignment persists down to zero energy. The present results, together with previous studies of this reaction, are consistent with a model in which the reaction occurs preferentially in the plane of rotation of the N2. It is suggested that this steric effect is due primarily to the orbital overlap requirements of the electron exchange mechanism. The polarization of the fluorescence from the dissociative excitation reactions of Ar*+CS2, Ar*+H2O, Kr*+CS2, and Xe*+BrCN was also measured and found to be zero at all velocities.
Self-consistent field investigation of vibrations of atomic adsorbates
Authors: Wierzbicki Andrzej , Bowman Joel M.
Published: July 2002

The vibrational self-consistent field method is presented and applied to a study of the coupling between an atomic adsorbate and a corrugated surface. A dispersion in the adsorbate vibrational energies results from this coupling and for an infrared transition this dispersion results in a linewidth in an IR spectrum. A model calculation considering a linear corrugated surface and parameters for H–LiF (001) yields a maximum dispersion of between 1 and 6 cm−1 depending on the surface state, with the largest dispersion occurring for surface states with energies which are roughly equal to the barrier height separating adsorbate binding sites.
The use of artificial intelligence methods in studying quantum intramolecular vibrational dynamics
Authors: Lederman Steven M. , Marcus R. A.
Published: July 2002

Artificial intelligence methods are used to treat the time evolution in intramolecular quantum dynamics. Comparison is made of several AI search algorithms and evaluation functions in an application to the study of quantum intramolecular vibrational redistribution. A combination of a beam search and a best-first search is used, in conjunction with an accumulated evaluation function which encourages both searching and ultimately satisfying the uncertainty principle. The methods developed are applied to an 11-coordinate heavy central mass problem and used to treat both quantum beats and ‘‘dissipative’’ intramolecular energy transfer. Good agreement is obtained with the ‘‘exact’’ quantum dynamics.
Associative and dissociative electron attachment by SF6 and SF5Cl
Authors: Fenzlaff Marita , Gerhard Rolf , Illenberger Eugen
Published: July 2002

Electron attachment by SF6 and SF5Cl in the energy range 0–20 eV has been studied in a beam experiment at room temperature. At low energies (≊0 eV) electron attachment to SF6 yields the well known parent anion SF−*6 (associative attachment) and SF−5 (dissociative attachment), while other negative ion fragments (F−, F−2 , SF−2 , SF−3 , and SF−4) are generated with comparably low cross sections from various resonances at higher energies. In contrast to that, negative ion formation in SF5Cl is dominated by dissociative channels (F−, Cl−, FCl−, and SF−5 ) and only a weak SF5Cl− signal is observed. A time-of-flight analysis of the ionic fragments reveals that the decomposition of all resonances is characterized by a low translational excess energy release indicating effective energy randomization in the parent ion prior to dissociation. The present results are compared with negative ion formation in halogenated hydrocarbons.
Coherent propagation of intense ultrashort laser pulses in a molecular multilevel medium
Authors: Chelkowski S. , Bandrauk A. D.
Published: July 2002

The Maxwell–Schrödinger equations describing the ultrashort laser pulse propagation in multilevel molecular media are shown to be analytically solvable and to reduce to nonlinear multisine-Gordon equations for certain models of the field–molecule system. Special cases of level schemes are shown to lead to soliton propagation. A numerical study is presented in order to examine the effect of nonresonant (virtual) transitions on pulse propagation and amplification in a three-level molecular system as a function of intensity and detuning.
Bond breaking without barriers: Photofragmentation of ketene at the singlet threshold
Authors: Chen I.-Chia , Green William H. , Moore C. Bradley
Published: July 2002

Ketene (CH2CO) in a supersonic free jet was photodissociated by a tunable pulsed laser in the frequency range just above the threshold for production of singlet methylene, CH2 (ã 1A1). CH2 was detected by laser-induced fluorescence (LIF). The appearance threshold and yield curve of individual 1CH2 rotational states were obtained by scanning the photolysis laser frequency with a fixed LIF probe laser frequency. The dissociation occurs on the ground electronic state potential energy surface. The threshold for CH2CO→1CH2+CO is found to be 30 116.2±0.4 cm−1. By varying the delay between the photolysis and probe pulses, a lower bound of 7×107 s−1 was set for the dissociation rate on the triplet surface at the singlet energy threshold. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. The experimental data provide a rigorous test of theoretical models of photofragment dynamics. The data clearly show that nuclear spin is conserved through the photodissociation. PHOFEX curves calculated from phase space theory (PST) are in excellent agreement with the experiment and show that there is no barrier along the reaction coordinate. The singlet/triplet branching ratio as a function of photolysis laser frequency is inferred from PST fits to the PHOFEX data. Comparisons with the statistical adiabatic channel model (SACM) are also presented. The data show that the only dynamical constraints on product state rotational energy distributions are conservation of energy, angular momentum, and nuclear spin.
A three-dimensional quantum mechanical study of vibrationally resolved charge transfer processes in H++H2 at Ecm=20 eV
Authors: Baer Michael , Niedner-Schatteburg Gereon , Toennies J. Peter
Published: July 2002

A three-dimensional quantum mechanical study of vibrational state resolved differential cross sections for the direct inelastic and charge transfer channels of the H++H2 system has been carried out at Ecm =20 eV using the infinite order sudden approximation (IOSA). Steric factors, opacity functions, angular distributions, and integral cross sections are calculated. The integral cross sections are in very good agreement with recent experimental results, whereas the angular distributions agree only partially with the experiments. A further comparison of both the theoretical and experimental results with semi-classical calculations based on the usual trajectory surface hopping method revealed that the present quantum results provide a better description of the experimental observations. The likely shortcomings of the semiclassical method are discussed.