bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic correlations in the bilayer manganite La1.2Sr1.8Mn2O7
Authors: Rosenkranz S. , Osborn R. , Mitchell J. F. , Vasiliu-Doloc L. , Lynn J. W. , Sinha S. K. , Argyriou D. N.
Published: July 2002

Neutron scattering has been used to study the magnetic correlations in the double-layer manganite La1.2Sr1.8Mn2O7, which exhibits colossal magnetoresistance in the vicinity of the ferromagnetic transition at Tc=112 K. Over a large temperature range above Tc, there is evidence for two-dimensional magnetic correlations which peak in intensity at the transition. Although the in-plane correlations are predominantly ferromagnetic, a strong canting of spins in neighboring planes within the bilayers, at an angle that is dependent on both the temperature and magnetic field, is observed. There are indications of a crossover to three-dimensional critical scattering very close to Tc, but the nature of the phase transition has yet to be determined.
Microstructure-related anelastic and magnetoelastic behavior of nanocrystalline nickel
Authors: Bonetti E. , Campari E. G. , Pasquini L. , Sampaolesi E.
Published: July 2002

Nanocrystalline nickel was prepared by a planetary ball milling apparatus working in a vacuum of 10−4 Pa in the 150–300 K temperature range. The kinetic of the milling process and the microstructure evolution upon annealing were followed by x-ray diffraction and mechanical spectroscopy measurements. It was observed that thermal annealing up to 600 K induces a strong reduction of the internal strains without significant grain growth. Measurements of elastic energy dissipation and dynamic elastic modulus as a function of temperature showed that in the nanocrystalline samples, anelastic relaxation processes occur, with the activation energy of grain boundary diffusion. A systematic study of the magnetic field dependence of the dynamic modulus (ΔE effect) revealed a correlation between the ΔE magnitude and the strain values obtained by x-ray diffraction analysis.
Spatial characterization of doped SiC wafers by Raman spectroscopy
Authors: Burton J. C. , Sun L. , Pophristic M. , Lukacs S. J. , Long F. H. , Feng Z. C. , Ferguson I. T.
Published: July 2002

Raman spectroscopy has been used to investigate wafers of both 4H–SiC and 6H–SiC. The wafers studied were semi-insulating and n-type (nitrogen) doped with concentrations between 2.1×1018 and 1.2×1019 cm−3. Significant coupling of the A1 longitudinal optical (LO) phonon to the plasmon mode was observed. The position of this peak shows a direct correlation with the carrier concentration. Examination of the Raman spectra from different positions on the wafer yielded a rudimentary spatial map of the carrier concentration. These data are compared with a resistivity map of the wafer. These results suggest that Raman spectroscopy of the LO phonon–plasmon mode can be used as a noninvasive, in situ diagnostic for SiC wafer production and substrate evaluation.
On the thermoluminescence of LiF from 83 to 320 K
Authors: da Rosa L. A. R. , Caldas L. V. E.
Published: July 2002

Thermoluminescence was studied in LiF (TLD-100) single crystals from 83 to 320 K. In this temperature range, the glow curve presents six peaks at 138, 153, 193, 260, 283, and 300 K and a seventh peak at approximately 240 K. Based on annealing response, linearity, phototransference, and optical bleaching experiments, the glow peak at 138 K was related to VK hole centers, while the remaining glow peaks were associated with electron traps, although a clear relation between the highest temperature glow peak, at 300 K, and electron centers could not be established with certainty. The seven glow peaks emit at three wavelengths, namely, 270, 300, and 420 nm. The emission band at 270 nm is more intense for the glow peaks at 138 and 153 K. For the remaining glow peaks, the emission bands at 300 and 420 nm are the most important ones. The emission band at 270 nm was related to the recombination of VK holes at recombination centers other than the ones normally associated with the emission band at 420 nm. The emission band at 300 nm was related to the recombination of electrons and VK centers. It was verified that the glow peaks at 138, 153, 193, and 260 K obey first order kinetics.
Phase selection governed by different growth velocities in the early stages of the Ti/Al phase reaction
Authors: Wöhlert S. , Bormann R.
Published: July 2002

The solid-state reaction between Ti and Al is characterized by a distinct phase selection in favor of TiAl3. The absence of phases other than TiAl3 was investigated in order to understand the selection mechanism using synthetically prepared trilayers with a layer of α2-Ti3Al or γ-TiAl sandwiched between Ti and Al layers. The growth of TiAl3 in competition with α2 or γ was observed by transmission electron microscopy. Upon annealing, the α2 and the γ phases are completely consumed by growth of TiAl3. The results indicate that the absence of phases other than TiAl3 is not due to the existence of nucleation barriers for α2 and γ, but is caused by growth selection. A kinetic model is presented which quantitatively explains the observed phase selection on the basis of different growth velocities which result from different diffusion fluxes in the competing phases.
Photoinduced effects in the Sb2Se3–BaCl2–PbCl2 glasses
Authors: Wasylak J. , Kucharski J. , Kityk I. V. , Sahraoui B.
Published: July 2002

Photoinduced optical phenomena in amorphous Sb2Se3–BaCl2–PbCl2 glasses are studied using experimental spectroscopic and theoretical quantum chemical methods. Photoinduced two-photon absorption (TPA) (for λ=10.6 μm) and second harmonic generation (SHG) (for the output wavelengths 5.3 μm) were measured. CO laser (λ=5.5 μm) was used as a source of photoinducing light. We have found that with an increasing CO-laser exposure the SHG maximum output signal increases and achieves its maximum value at CO photon fluence 18×1013 phot./cm2 per pulse after two hours of illumination. The output SHG signal was more than one order less comparing with ZnS crystals in case of its third rank nonlinear optical susceptibility tensor components χ222 (λ=10.6 μm). With decreasing temperature the SHG signal strongly increases within the 26–32 K temperature range. Time-dependent probe-pump measurements show an existence of SHG maximum at time delay about 45 ps. Good correlation between SHG and TPA was observed. Spectral positions of the TPA maxima strongly depend on the photoinducing power. We observed at least two maxima of the TPA: at 26 and 70 ps. We explain this dependence within a framework of the quantum chemical approach taken into account electron-vibration anharmonicity. Coming out from the performed calculations and infrared (IR)-spectroscopy Fourier technique measurements in the spectral region between 100 and 300 cm−1 we have ascertained a key role of Sb–Se fragments in the observed photoinduced effects. We have also compared different existing models of the photoinduced changes. The studied glasses have relatively low nonuniformity through the sample surfaces and can be used as a promising material for picosecond IR laser devices.
Simultaneous atomic force microscopy measurement of long range forces and adhesion energy between tungsten and oxide surfaces under ambient atmosphere and ultrahigh vacuum
Authors: Sounilhac S. , Barthel E. , Creuzet F.
Published: July 2002

In the field of metal/oxide adhesion, it is contended that long range interactions may contribute to the work of adhesion. The combination of dc and ac atomic force microscopy is shown to provide a quantitative answer to this question by the simultaneous measurement of the long range forces and the adherence force. Different systems are considered: W/MgO under ambient atmosphere, where we show that adhesion is completely accounted for by the capillary force, and W/TiO2 (stoichiometric and nonstoichiometric) under ultrahigh vacuum, where the results suggest that the van der Waals contribution has to be taken into account in the adhesion energy.
Effect of modulating field on photoreflectance simulated by electroreflectance
Authors: Chiou S. J. , Sung Y. G. , Wang D. P. , Huang K. F. , Huang T. C. , Chu A. K.
Published: July 2002

Photoreflectance (PR) of surface-intrinsic-n+ (s-i-n+) type doped GaAs has been simulated by electroreflectance (ER). The simulated spectra of the s-i-n+ sample have exhibited many Franz–Keldysh oscillations, which enable the electric field (F) to be determined. It is known that F’s determined from PR are subjected to photovoltaic effect and the measured F is close to Fbi−δF/2 when the modulating field, δF≪Fbi, where Fbi is the built-in field of the sample and δF is the modulating field. In this work, we have investigated the relation between the measured F and δF not only for the region where δF≪Fbi holds, but also for a whole range of δF. In order to determine the magnitude of δF, we have used ER to simulate PR, that is, the measurements of ER under a forward bias, which is set to be equal to δF/2.
High density submicron magnetoresistive random access memory (invited)
Authors: Tehrani S. , Chen E. , Durlam M. , DeHerrera M. , Slaughter J. M. , Shi J. , Kerszykowski G.
Published: July 2002

Various giant magnetoresistance material structures were patterned and studied for their potential as memory elements. The preferred memory element, based on pseudo-spin valve structures, was designed with two magnetic stacks (NiFeCo/CoFe) of different thickness with Cu as an interlayer. The difference in thickness results in dissimilar switching fields due to the shape anisotropy at deep submicron dimensions. It was found that a lower switching current can be achieved when the bits have a word line that wraps around the bit 1.5 times. Submicron memory elements integrated with complementary metal–oxide–semiconductor (CMOS) transistors maintained their characteristics and no degradation to the CMOS devices was observed. Selectivity between memory elements in high-density arrays was demonstrated.
Giant positive magnetoresistance of Bi nanowire arrays in high magnetic fields
Authors: Hong Kimin , Yang F. Y. , Liu Kai , Reich D. H. , Searson P. C. , Chien C. L. , Balakirev F. F. , Boebinger G. S.
Published: July 2002

We have studied the magnetoresistance of electrodeposited Bi wires with diameters between 200 nm and 2 μm in magnetic fields up to B=55 T. In zero field, the resistance increases with decreasing temperature, indicating that the mean free path is strongly influenced by the nanowire geometry. The high-field magnetoresistance shows strong dependence on field orientation; typically 200% for B parallel to the wires, and 600%–800% for B perpendicular to the wires. The perpendicular magnetoresistance is well described by a modified two-current model which suggests that the high-field response of the arrays is fairly insensitive to the wire diameter, and is dominated by bulk properties of Bi.
Fractional contributions of microscopic diffusion mechanisms for common dopants and self-diffusion in silicon
Authors: Ural Ant , Griffin Peter B. , Plummer James D.
Published: July 2002

An identical set of thermal oxidation and nitridation experiments has been performed for four common dopants and self-diffusion in Si. Selectively perturbing the equilibrium point-defect concentrations by these surface reactions is a powerful tool for identifying the relative importance of the various atomic-scale diffusion mechanisms. We obtain bounds on the fractional contributions of the self-interstitial, vacancy, and concerted exchange mechanisms for arsenic, boron, phosphorus, antimony, and self-diffusion in Si at temperatures of 1100 and 1000 °C. These bounds are found by simultaneously solving a system of equations making only very conservative assumptions. The validity of common approximations found in previous work and their effects on the results are also analyzed in detail. We find that B and P diffuse by a self-interstitial mechanism, whereas Sb diffusion is almost exclusively vacancy mediated. As and self-diffusion, on the other hand, exhibit evidence for a dual vacancy-interstitial mechanism with the possibility of some concerted exchange component.
A method of measuring anisotropy field of polycrystalline thin-film media
Authors: Igarashi Masukazu , Kambe Tetsuya , Yoshida Kazuetsu , Hosoe Yuzuru , Sugita Yutaka
Published: July 2002

Ferromagnetic resonance (FMR) is used to measure the magnetic anisotropy field Hk and its dispersion of poly-crystalline recording media. This method does not use any single crystal substrate. A high-intensity field is applied vertically to a sample surface to completely saturate the magnetization and a high-frequency (over 50 GHz) magnetic field is applied in the surface. An FMR simulator based on the Landau–Lifshitz–Gilbert equation confirms that the magnetic (exchange and/or magnetostatic) interactions have very little effect on the resonant frequency in the configuration used. The linewidth of the FMR spectrum is affected by the Hk dispersion and damping constant. Methods using a torque or magnetization curve include magnetic interactions, resulting in a systematic error to the value of Hk. This FMR method is applied to CoCrPt-alloy thin-film media with a Cr-alloy underlayer grown on a glass substrate. A conventional torque method is also used for comparison. The axis of hexagonal-close-packed Co magnetic grain is distributed at random in the surface. In an FMR measurement, external magnetic field is applied vertically to the surface up to 55 kOe for two frequencies (71 and 117 GHz). The obtained Hk is 7.0±0.2 kOe, which is a little higher than the Hk of 6.4 kOe estimated with the torque loss method. Important physical parameters that affect the recording performance are also estimated using the linewidth of FMR spectra for different frequencies with a micromagnetic simulation: Hk dispersion=1.2±0.5kOe, Gilbert’s damping constant α=0.03±0.01, and a g value=2.19±0.02.
Purely thermal-wave photopyroelectric interferometry
Authors: Wang Chinhua , Mandelis Andreas
Published: July 2002

A thermal-wave interferometric technique based on pure thermal-wave interference using two-cavity photopyroelectric (PPE) detection has been developed. The salient features of this interferometry are investigated with a general theory, which describes the dependence of the PPE signal on the optical, thermal, and geometric parameters of the thermal-wave cavity configuration. Preliminary experimental results are presented. A major feature of the technique is the efficient suppression of the background PPE signal base line. The physical origins of the interferometry are also discussed.
Radial plasma flow in a hot anode vacuum arc
Authors: Beilis I. I. , Keidar M. , Boxman R. L. , Goldsmith S. , Heberlein J. , Pfender E.
Published: July 2002

The free, steady state, two-dimensional radial plasma flow initiated between a pair of disk-shaped electrodes of a hot anode vacuum arc was analyzed in the hydrodynamic approximation. Studies include the influence of the self-magnetic field on the plasma density, velocity, radial spreading of the arc current and potential distribution. The free plasma boundary was calculated by solving the equations for the normal and tangential velocity components at the free boundary. It was found that the plasma significantly expands over a radial distance of about half of the interelectrode gap counted from the electrode edge and the plasma density in the center plane decreases by factor of 2, whereas the density of the fringe current decreases by a factor of 10. The self magnetic field does not influence the plasma flow and current spreading at radial distances larger than the interelectrode gap. The potential distribution is strongly nonsymmetric with respect to the central plane due to the influence of the plasma density gradients on the current spreading.
Transfer and back transfer processes in Yb3+–Er3+ codoped fluoroindate glasses
Authors: Martı́n I. R. , Rodrı́guez V. D. , Lavı́n V. , Rodrı́guez-Mendoza U. R.
Published: July 2002

The kinetics of forward and backward energy transfer processes in fluoroindate glasses double doped with Yb3+ and Er3+ has been studied. It was possible to excite selectively Yb3+ ions and detect emission due only to these ions or combined with the emission coming from the Er3+ ions. At low concentration of Yb3+ (0.1 mol %) the emission decay of these ions is nonexponential when there is back transfer from Er3+ ions; the dynamics is well described using the fluorescence “transfer function” model. Moreover, the evolution of the emission from the I11/24 (Er3+) level is in good agreement with the behavior predicted by the model. The critical radii obtained from the fitting for forward and backward energy transfer are compared with those calculated with the Dexter formula. When the Yb3+ concentration is increased, the migration among these ions is very important and the emission decay of the Yb3+ ions is nearly exponential with the lifetime of these ions. This behavior is explained considering that, at high Yb3+ concentration and due to the migration, the transfer from Yb3+ to Er3+ ions is restricted to very closed couples of ions with high back transfer probability. The limitations of the fluorescence “transfer function” model associated with the correlation effect in the excitation of Er3+ ions by energy transfer are discussed.
Current dependence of grain boundary magnetoresistance in La0.67Ca0.33MnO3 films
Authors: Westerburg W. , Martin F. , Friedrich S. , Maier M. , Jakob G.
Published: July 2002

We prepared epitaxial ferromagnetic manganite films on 45° bicrystal substrates by pulsed laser ablation. Their low- and high-field magnetoresistance (MR) was measured as a function of magnetic field, temperature and current. At low temperatures hysteretic changes in resistivity up to 70% due to switching of magnetic domains at the coercitive field are observed. The strongly nonohmic behavior of the current–voltage leads to a complete suppression of the MR effect at high bias currents with the identical current dependence at low and high magnetic fields. We discuss the data in view of tunneling and mesoscale magnetic transport models and propose an explicit dependence of the spin polarization on the applied current in the grain boundary region.
Thermal stress in a trigonal crystal rod: Second corrected solution
Authors: Greninger Charles E. , Rodriguez Sergio E.
Published: July 2002

The surface thermal stresses from an earlier article for a rectangular trigonal crystal rod with zero average stress on each lateral surface are developed into Fourier series and ordinary (T=0) stress solutions are added to attempt to zero the remaining surface stress. To first zero the surface tensile stress, these stresses are developed as sine series so the shear stresses must be developed as cosine series. This leads to stress “swapping” between surfaces as a residual shear stress remains. Another computer code like the one developed for the earlier article has been generated to add these correcting stresses to the original stresses. For a chosen symmetric temperature distribution, iteration of a process to correct the stresses has converged to nearly zero remaining stress on the lateral surfaces.
Current transport and the role of barrier inhomogeneities at the high barrier n-InP | poly(pyrrole) interface
Authors: Jones Frank E. , Wood Ben P. , Myers James A. , Daniels-Hafer Carrie , Lonergan Mark C.
Published: July 2002

A detailed study of current transport at the Schottky-type n-InP | poly(pyrrole) interface is presented. At room temperature, this interface exhibits an average quality factor of n=1.02±0.02, a C–V barrier height of qφbCV=0.78±0.01 eV, and a surface recombination velocity over two orders-of-magnitude slower than at ideal n-InP metal interfaces. These latter two parameters imply an effective barrier height of 0.9 eV, which is among the highest values ever reported for an n-InP Schottky-type diode. The quality factor increases monotonically with decreasing temperature reaching a value of 1.23 at 98 K. Substantial curvature is also observed in a Richardson plot at reduced temperature. These temperature dependencies can be quantitatively modeled using thermionic emission theory in the presence of barrier inhomogeneities. Standard models, including thermionic emission with image force effects, interfacial layer models with and without surface states, and tunneling, do not adequately explain the temperature dependence of the quality factor and the curvature in the Richardson plot.
Phonon-assisted tunneling in reverse biased Schottky diodes
Authors: Pipinys P. , Pipiniene A. , Rimeika A.
Published: July 2002

Reverse current–temperature (IΓ/T) characteristics of Al–GaAs Schottky diodes with oxidized interfaces are measured in the temperature region of 250–400 K at various reverse bias voltage in order to elucidate a mechanism of carrier transport through the barrier. The IΓ/T dependencies are explained on the basis of phonon-assisted electrons tunneling from interface states to the semiconductor model. The barrier height, evaluated from the IΓ/T characteristics (activation energy) which depends on the applied voltage and temperature, is discussed. It is shown that these dependencies are due to the above-mentioned carrier transport through the Schottky barrier mechanism. The important of the phonon-assisted tunneling process on the current transport in the diodes is emphasized.
Optical properties of AlxGa1−xP (0⩽x⩽0.52) alloys
Authors: Choi S. G. , Kim Y. D. , Yoo S. D. , Aspnes D. E. , Woo D. H. , Kim S. H.
Published: July 2002

We report optical properties of AlxGa1−xP (0⩽x⩽0.52) alloys grown by gas source molecular-beam epitaxy on S-doped GaP(001) substrates. Room-temperature pseudodielectric function spectra from 1.5 to 6.0 eV were obtained by spectroscopic ellipsometry. By applying the parabolic-band critical point model to numerically calculated second energy derivatives of these spectra, we obtained accurate room-temperature values of the E1, E0′, E2, and E2′ critical point energies and their dependence on composition x.
Isotopic labeling studies of interactions of nitric oxide and nitrous oxide with ultrathin oxynitride layers on silicon
Authors: Lu H. C. , Gusev E. P. , Garfunkel E. , Busch B. W. , Gustafsson T. , Sorsch T. W. , Green M. L.
Published: July 2002

The interaction of nitric (NO) and nitrous (N2O) oxide with ultrathin (∼1.5–3.5 nm) oxide and oxynitride films on silicon has been studied by performing high resolution depth profiling using medium energy ion scattering and isotopic labeling methods. We observe that, after NO annealing at 850 °C, both O and N incorporate near the SiO2/Si interface. There is no nitrogen and little newly incorporated oxygen observed at the surface, implying that NO diffuses through the oxide film and dissociates and reacts at the interface. For N2O annealing, atomic oxygen resulting from decomposition of the gas can replace oxygen atoms in both oxide and oxynitride films. This replacement is most important at the surface, but also, to a smaller extent, occurs in the middle of the film. For ultrathin oxynitride films, oxide growth during reoxidation is faster in N2O than in pure O2. Atomic oxygen also influences the nitrogen distribution, which moves further into the film and accumulate at the new interface. We discuss the roles of atomic oxygen and peroxyl bridging oxygen species in explaining the observed phenomena.
Atomistic simulations of low energy ion assisted vapor deposition of metal multilayers
Authors: Zhou X. W. , Wadley H. N. G.
Published: July 2002

The properties of giant magnetoresistance multilayers are a sensitive function of the vapor deposition process used for their synthesis. The highest magnetoresistance occurs when deposition results in interfaces that are flat and chemically separated. Molecular dynamics simulations have been used to explore the potential benefits of low energy xenon ion assistance during the physical vapor deposition of Ni/Cu/Ni multilayers grown in the [111] direction from thermalized metal fluxes characteristic of molecular beam epitaxy. The simulations indicated that the roughness of the interfaces was significantly reduced as the ion energy was increased from 0 to 5 eV. However, increasing the ion energy above 2 eV also resulted in significant copper–nickel intermixing at the nickel on copper interface. Interface flattening without intermixing could be achieved using a modulated low energy ion assistance strategy in which the first half of each new material layer was deposited without ion assistance, while the remainder of the layer was deposited with an optimum low ion energy assistance of 4 eV. Modulated low energy ion assistance during thermalized metal atom deposition was found to be a promising approach for creating metal multilayers with improved magnetoresistance.
Magnetic domain confinement using nonplanar substrates
Authors: Patel N. K. , Pepper M.
Published: July 2002

Studies have been performed of ferromagnetic films deposited on nonplanar substrates. It is shown that the step edges act as efficient localization sites for domain boundaries. This process enables the fabrication of continuous magnetic films whose domain boundaries can be accurately positioned by prepatterning of the substrate. Magnetoresistance data confirm the formation of separate magnetic regions and the ability to switch the magnetization of these independently. This simple method has many applications for designing magnetic structures with controlled domain walls.
The optical and vibrational properties of the quaternary chalcopyrite semiconductor alloy AgxCu1−xGaS2
Authors: Choi In-Hwan , Eom Sung-Hwan , Yu P. Y.
Published: July 2002

The absorption, emission and Raman spectra of the quaternary chalcopyrite alloys AgxCu1−xGaS2 have been measured as a function of the Ag concentration. We find strong and highly nonlinear dependence on the alloy concentration in the optical band gap (a large bowing parameter). On the other hand, most of the Raman peaks exhibit either small energy shifts or large broadening with alloying. We conclude that the effect of substitution of the Ag cations by Cu in AgGaS2 on its band structure cannot be predicted by the simple virtual crystal potential approximation as a result of the large difference in electronegativity and bond length between Ag and Cu.
Optical properties of CuGaSe2 and CuAlSe2 layers epitaxially grown on Cu(In0.04Ga0.96)Se2 substrates
Authors: Shirakata Sho , Chichibu Shigefusa , Miyake Hideto , Sugiyama Kohichi
Published: July 2002

Photoluminescence (PL) study has been performed on epitaxial layers of CuGaSe2 and CuAlSe2 grown by metalorganic chemical vapor epitaxy on CuGa0.96In0.04Se2 substrates prepared by the traveling heater method. PL properties of epilayers are compared with each other for those grown on GaAs (100), CuGa0.96In0.04Se2 (100), CuGa0.96In0.04Se2 (112), and randomly oriented CuGa0.96In0.04Se2 substrates. PL results are discussed in terms of the lattice mismatches and stress in the epilayers.
Extra current channels in longitudinally biased magnetic tunnel junctions
Authors: Redon O. , Kasahara N. , Shimazawa K. , Araki S. , Morita H. , Matsuzaki M.
Published: July 2002

Experiments on the longitudinal biasing of microsized magnetic tunnel junctions have been conducted using permanent magnets partially overlapping the junction area. The tunneling magnetoresistance ratio showed a strong dependency on the overlap length, since even a 10% overlap of the sensor length resulted in a 25% drop from its initial value without overlap. Analytical and micromagnetic analyses have demonstrated that this decrease comes from extra current channels, located in the regions below the permanent magnets, that shorted a large amount of the sense current from the central active region in the antiparallel magnetization state. The high uniaxial anisotropy field, induced by the permanent magnets in the overlapped regions, created particular magnetic configurations responsible for these low resistance paths. Several alternatives, using antiferromagnetic material in place of the permanent magnets or a modified design of the magnetic tunnel junction structure, are presented and discussed to prevent this extra current channel effect.
Shape induced in-plane magnetic anisotropy reorientation in epitaxial hexagonal close packed cobalt dots
Authors: Otani Y. , Kohda T. , Novosad V. , Fukamichi K. , Yuasa S. , Katayama T.
Published: July 2002

The in-plane magnetic easy-axis reorientation was investigated for the arrays of (10⋅0) hcp Co rectangular dots 0.2 μm in width with a variable length from 3 to 5 μm perpendicular to the c axis. In the system, the magnitude of the shape anisotropy contribution is determined by a lateral aspect ratio of the dot, while the magnetocrystalline anisotropy contribution is temperature dependent. The torque curves for the dot arrays exhibited a characteristic fourfold symmetry due to the easy-axis reorientation. The reorientation angle from the initial easy axis was found to be dependent on the value of in-plane demagnetization factors as well as the temperature variable first and second magnetocrystalline anisotropy constants. The experimental results are in good agreement with the calculated effective magnetic anisotropy.
Transverse spin freezing in a-FexZr100xstudied using muon spin relaxation
Authors: Ryan D. H. , Cadogan J. M. , van Lierop J.
Published: July 2002

Analysis of the muon spin relaxation (μSR) signal from a range of a-FexZr100−x alloys shows that the entire sample orders at Tc and that the volume fraction that could be associated with isolated nonmagnetic clusters is less than 3%. Furthermore, the coincidence between the static and dynamic μSR signatures of transverse spin freezing provides a clear confirmation of predictions from numerical models.
Mechanical properties of xerogel silica films derived from stress versus temperature and cracking experiments
Authors: Chow L. A. , Dunn B. , Tu K. N. , Chiang C.
Published: July 2002

The biaxial modulus, coefficient of thermal expansion, and crack energy release rate of sol–gel derived silica, xerogel, films are presented. Using stress versus temperature measurements for xerogel on Si and quartz substrates, the biaxial modulus and linear thermal expansion coefficient of the film were determined. The film thickness at the onset of cracking and the corresponding stress were also found. For the critical thickness sample, the cracks initiated in the xerogel film and propagated into the Si substrate. The depth of the cracks in the substrate was roughly equal to the thickness of the film. From these data, the crack energy release rate for steady state channeling in the film was estimated; it was found to be roughly five times 2γs, where γs is the reported surface energy of the film. This is an indication of a plastic zone at the crack tip.
A quantitative conduction model for a low-resistance nonalloyed ohmic contact structure utilizing low-temperature-grown GaAs
Authors: Chen Nien-Po , Ueng H. J. , Janes D. B. , Woodall J. M. , Melloch M. R.
Published: July 2002

We present a quantitative conduction model for nonalloyed ohmic contacts to n-type GaAs (n:GaAs) which employ a surface layer of low-temperature-grown GaAs (LTG:GaAs). The energy band edge profile for the contact structure is calculated by solving Poisson’s equation and invoking Fermi statistics using deep donor band and acceptor state parameters for the LTG:GaAs which are consistent with measured bulk and surface electrical properties of this material. The specific contact resistance is then calculated using an analytic expression for tunneling conduction through an equivalent uniformly doped Schottky barrier. The model has been used to fit measured specific contact resistances versus LTG:GaAs layer thickness and versus measurement temperature. These comparisons provide insights into the contact mechanism (electron tunneling between metal states and conduction band states in n:GaAs) and indicate that low barrier heights (0.3–0.5 V) and high activated donor densities (∼1×1020 cm−3) have been achieved in these ex situ contacts.
Rotation-induced vibrational mixing in X̃ 1A1 formaldehyde: Non−negligible dynamical consequences of rotation
Authors: Dai H. L. , Korpa C. L. , Kinsey J. L. , Field R. W.
Published: July 2002

Individual rotation-vibration levels of the formaldehyde X̃ 1A1 state with 7400<Evib<8600 cm−1 have been examined by the stimulated emission pumping (SEP) technique. At low values of the rotational quantum number (J≤3), the SEP spectra were simple. The only vibrational levels which appeared in the spectra were those expected either to have large Franck–Condon overlap with the Ã 41 level or to have appreciable Fermi resonance with a nearby Franck–Condon allowed level. At higher J and Ka values, the spectra rapidly became more complex and the observed level densities at J≊10, Ka≊2 were several times larger than the known total density of vibrational levels. This increase in the density of spectrally accessible vibrational levels was a result of rotation-induced mixing of the anharmonic vibrational basis functions (Coriolis coupling) which compromised the ‘‘goodness’’ of both vibrational and Ka quantum numbers. Coriolis matrix elements computed in a harmonic normal mode basis set qualitatively confirmed the importance of rotation-vibration mixing. The failure to obtain quantitative agreement is attributed to anharmonic effects. The rotation-dependent vibrational mixing effects observed in the SEP spectra indicate the importance of rotation in intramolecular vibrational dynamics and mode-selective vibrational excitation. Rotation significantly diminishes the structural differences (manifest in Franck–Condon factors, rotational constants, electric dipole moments) between rotationless vibrational levels and promotes an averaging of the character of near degenerate vibrational levels together with a partial destruction of the Ka rotational quantum number. This means that the onset of the quasicontinuum in infrared multiphoton dissociation and the inhomogeneous widths of high overtone bands would be very different for excitation out of a single low vs high J level.
A quantum mechanical study of structure in liquid H2O and D2O
Authors: Kuharski Robert A. , Rossky Peter J.
Published: July 2002

Path integral Monte Carlo methods are applied to the ST2 model of water to examine both the importance of quantum effects on the structure of liquid water and the differences in the structures of light and heavy water. Significant differences are apparent among the results for classical water, quantum H2O, and quantum D2O. For all quantities considered, the classical liquid is the most structured while quantum H2O is the least structured. The implications of these results for neutron and x-ray diffraction experiments are examined; of the three atom–atom partial structure factors, the oxygen–oxygen function is found to be the most sensitive to quantum effects and the structural differences between light and heavy water appear to be large enough to be measured by x-ray diffraction experiments.
Absorption spectra for collinear (nonreactive) H3: Comparison between quantal and classical calculations
Authors: Engel V. , Bacic Z. , Schinke R. , Shapiro M.
Published: July 2002

Absorption spectra for the collinear (nonreactive) H+H2→H♯3 →H+H2 are calculated quantum mechanically, using the Siegbahn–Liu–Truhlar–Horowitz (SLTH) ab initio potential and a model H*3 surface as the ground and excited H3 surface, respectively. They are compared to classical spectra previously computed by Mayne, Poirier, and Polanyi using the same potential energy surfaces [J. Chem. Phys. 80, 4025 (1984)]. The spectra are calculated at several collision energies and for both H+H2 (v=0) and H+H2 (v=1). The quantal and classical spectra are shown to agree with respect to basic features and trends. Nevertheless, the two sets of spectra differ considerably in their overall appearance because of some purely quantum aspects of the H+H2 system.
The harmonic mean field model for molecular solvents: A liquid state analog of the harmonic oscillator-rigid top approximation
Authors: Adelman Steven A. , Balk Michael W.
Published: July 2002

A liquid state analog of the gas phase harmonic oscillator-rigid top model is developed. This analog, which we call the harmonic mean field model, permits one to compute the equilibrium and dynamic properties of real, i.e., vibrating, molecular solvents from the structure and dynamics of the corresponding rigid solvents. The harmonic mean field model is based on: (i) A mean field harmonic model for the solvent vibrational (V) force field. (ii) A rigid solvent model treatment of translational–rotational (TR) fluctuations. (iii) Complete neglect of explicit coupling between V and TR fluctuations. (Implicit coupling is included in the vibrational force field.) The model is developed for statics via a sequence of physically motivated approximations to the exact canonical ensemble phase space distribution function of the solvent, fCA[S]. This yields a model distribution function f(0)CA[S] =f(0)CA[pyy]fCA[pww; v0], where f(0)CA[pyy] is an effective harmonic vibrational phase space distribution function which describes mean field harmonic V fluctuations and where f(0)CA[pww; v0] is the rigid solvent canonical ensemble distribution function. The nonequilibrium version of the model is defined as the solvent dynamics generated by a model Liouville operator L(0). This is defined via the model equilibrium Liouville equation L(0)f(0)CA [S]=0. Explicit results for equilibrium averages and time correlation functions of molecular solvents are obtained. The frequency spectra of the time correlation functions contain a low frequency ‘‘acoustic’’ branch arising from solvent TR motions and high frequency ‘‘optical’’ branches arising from collective solvent V motions. A detailed analysis of the frequency spectra of autocorrelation functions of diatomic solvents is presented.
Electric dipole moments of HF–C2H2, HF–C2H4, and HF–C3H6
Authors: Nelson D. D. , Fraser G. T. , Klemperer W.
Published: July 2002

Electric dipole moments of HF–cyclopropane, HF–ethylene, and HF–acetylene have been determined using the molecular beam electric resonance technique. The dipole moments of HF–C3H6, HF–C2H4, and HF–C2H2 are 2.5084(28), 2.3839(45), and 2.3681(28) D, respectively. The induced dipole moments of these systems are discussed, along with that of HF–benzene which has been previously determined. The induced dipole moments do not scale simply with hydrocarbon polarizabilities. Moreover, they do not correlate well with the frequency shifts of the HF submolecule stretching vibration measured in matrix isolation studies.
A wave packet approach to inelastic scattering from large targets
Authors: Drolshagen Gerhard , Heller Eric J.
Published: July 2002

A theoretical approach to collision-induced energy transfer is presented that allows the direct calculation of the translational energy distribution of the separating fragments. Many features of the method are in close analogy to actual time-of-flight measurements. The energy spectrum can be calculated with high or low resolution and the target states are not explicitly used. In this study, a semiclassical implementation, using Gaussian wave packets, is applied to collinear scattering from an harmonic chain of variable length. Scattering intensities as functions of the final translational energy are calculated and analyzed for various chain lengths and different projectile masses. Results are reported for free chains and for those where an additional potential forces the atoms to a local motion around their equilibrium positions. In addition a ‘‘frozen’’ approximation to the wave packet approach is successfully tested. This ‘‘frozen Gaussian’’ approximation may have a wide field of possible applications, as it requires only slightly more computational effort than a pure classical calculation and yet describes many quantum effects correctly. Furthermore, the chain model is used to examine possible applications of the wave packet approach to inelastic gas–surface scattering. Results are reported for a chain of 40 atoms, which should allow an extrapolation to an infinite chain, and for a ten-atom damped oscillator. A target autocorrelation formula is rederived and found to be very useful in explaining many of the structures seen in the spectra.
Multiphoton ionization photoelectron spectroscopic study on NO: Autoionization pathway through dissociative superexcited valence states
Authors: Achiba Yohji , Sato Kenji , Kimura Katsumi
Published: July 2002

In the present work we have carried out measurements of total ion–current and photoelectrons to study autoionization of NO molecule through the two-photon resonant, valence-excited B 2Π state at the v′=9 level (designated as B-9). This state was selected so as to remove a possibility of direct ionization of producing the ground-state NO+ ion. It has been found that the ion–current spectrum of the B-9 state shows several anomalous-intensity rotational lines as well as normal Q-branch rotational lines. Each of the photoelectron spectra obtained at the normal rotational lines exhibits three energetically accessible vibrational bands with branching ratios of 0.65 (v+=2), 0.30 (v+=1), and 0.05 (v+=0). On the other hand, each of the photoelectron spectra obtained at the anomalous-intensity rotational lines suggests a relatively high yield of the v+=0 ion, and there seems to be no photoelectron angular dependence for the v+=0 band. From these experimental results, we have deduced the following conclusions: (1) The overall process of producing the normal rotational lines is represented by X2hν→B-9 hν→I*→NO+, where I* means the superexcited valence I 2Σ+ state and → indicates electronic autoionization forming the ground electronic state of ions. (2) The ionization scheme of producing the anomalous-intensity rotational lines is expressed by X2hν→B-9hν→N-6↔B′*→NO+, involving an accidental double resonance, where N is the Rydberg N(4dδ) 2Δ state, B′* is the superexcited valence B′ 2Δ state, and ↔ means an electronic coupling between the Rydberg and the valence states. The N-6 state has further been supported by experimental fact that the v+=6 photoelectron band appears at a higher laser power by four-photon direct ionization. In the above two schemes, the electronic coupling between the dissociative valence and the ionization continuum state is considered to play an important role in the present autoionization phenomena.
Effects of translational energy and moderators on vibrational excitation of carbon dioxide by hot hydrogen atoms
Authors: McGee T. H. , Weston Ralph E. , Flynn G. W.
Published: July 2002

The collisional thermalization of hot H atoms produced by the excimer laser photolysis of H2S (193 nm) and HI (193 and 248 nm) has been determined by measuring the decrease in vibrational excitation of CO2 (ν3) as a function of moderator pressure. Cross sections obtained for the rare gases in this way are in reasonable agreement with those calculated for energy loss in hard-sphere collisions. These experiments also indicate that the efficiency of vibrational excitation increases as the relative translational energy increases.
Symmetry assignment of vibrations in anthracene, phenazine, and acridine from infrared dichroism in stretched polyethylene
Authors: Radziszewski Juliusz G. , Michl Josef
Published: July 2002

Polarized Fourier transform infrared spectra of anthracene, phenazine, and acridine in stretched polyethylene and stretched perdeuterated polyethylene permitted complete symmetry assignment for the IR-active vibrations throughout the 100–3200 cm−1 region.
Canonical variational transition state theory for a radical combination reaction on a
b i
n
i
t
i
o potential energy surfaces: H+CH3
Authors: LeBlanc James F. , Pacey Philip D.
Published: July 2002

Canonical variational transition state theory calculations have been performed for the reaction H+CH3→CH4 on potential energy surfaces based on ab initio calculations. Most vibrations were treated as harmonic. The resulting energy levels and partition functions were compared to empirical rules. For the two rotational degrees of freedom (χ) of CH3 which become bending vibrations in CH4, changing from a harmonic oscillator treatment to a hindered rotor treatment changed the partition functions by an order of magnitude or more for C ⋅ ⋅ ⋅ H distances, R, greater than 0.3 nm. The variation of potential energy with R was taken as a standard Morse function, as a stiff Morse function with a variable parameter β or as a Lippincott function. The value of R for which the rate was minimum was found to vary between 0.25 and 0.5 nm, depending on the temperature and the assumed variation of potential energy with R and χ. Provided the χ bending modes were treated as hindered rotations for large values of R, the limiting values of the rate coefficients were similar to the results of experiments, of classical trajectory calculations, and of a modified version of simple collision theory.
Close-coupling calculations on the H+BrH→HBr+H reaction in three dimensions
Authors: Clary D. C.
Published: July 2002

Close-coupling calculations on the H+BrH(v=0, j)→HBr(v=0, j′)+H reaction in three dimensions are reported. Cross sections have been computed for an energy range sufficient to give a converged rate constant at room temperature. The results are used to test the accuracy of sudden approximations and a variational transition state theory with tunneling correction. It is found that these approximate theories slightly overestimate the reaction rate constant.
Generalized Langevin dynamics simulation of interacting particles
Authors: Guàrdia E. , Padró J. A.
Published: July 2002

We analyze the effect of including time-dependent memory functions in computer simulation of interacting Brownian particles. Numerical integration of generalized Langevin equations is carried out by an Allen-like algorithm. The inclusion of time-correlated random forces does not result in significative changes in the structure and self-diffusion coefficients, but the dynamic evolution of particles becomes closer to the real one.
The excitation of IF(B 3Π0+) by N2(A 3Σ+u)
Authors: Piper L. G. , Marinelli W. J. , Rawlins W. T. , Green B. D.
Published: July 2002

Experiments in a discharge-flow reactor show that the energy-transfer reaction between N2(A 3Σ+u ) and IF(X 1Σ+) is extremely rapid (ktotal=2.0×10−10 cm3 molecule−1 s−1) with 40% of the quenching collisions resulting in fluorescence from IF(B 3Π0+).The reaction populates IF(B) vibrational levels up through v′=9 and produces v′=0–6 with about equal probability at low pressure. The vibrational distribution relaxes rapidly in collisions with the reactor bath gas even at pressures of a few Torr. The vibrational relaxation rate coefficients for levels 3–6 of IF(B) in a mixture of 80% Ar/20% N2 are about 3×10−12 cm3 molecule−1 s−1, with levels 1 and 2 being a little slower. Differences in IF excitation between N2(A) v′=1 and v′=0 are small (kv′=1/kv′=0≤1.2). Electronic quenching of the IF(B) is sufficiently slow that even at higher reactor pressures radiative decay is the dominant loss. Rate coefficients for quenching N2(A), by CF3I, and NF3 and for vibrational relaxation of N2(A,v′=1) by CF4, CF3H, CH4, and SF6 also are reported.
Origin of the high integrated infrared intensity of the O–H stretching vibrations in ice I relative to the vapor
Authors: Whalley Edward , Klug D. D.
Published: July 2002

The integrated infrared intensity of the symmetric O–H stretching vibration of the water molecule increases by a factor of 220 when the vapor is condensed to ice I. The increase occurs for three reasons, the first of which is that the dipole moment of an isolated water molecule in its equilibrium configuration is near its maximum as a function of O–H distance. Its change with the symmetric stretch of the O–H bonds is, therefore, small, and so is the integrated absorptivity. The second is that when the molecule is put into the crystal, the dipole moment must, by symmetry, go to zero when the O–H distance is half the O--O distance, and this causes the dipole-moment derivative to become large and negative. The third is that each molecule is further polarized by the electric fields of neighboring molecules, and so its dipole moment is increased even further, which increases the magnitude of the dipole-moment derivative and hence of the integrated intensity.