bluuebunny/crossref_metadata_2025 huggingface dataset:
The interactions between ionic surfactants and phosphatidylcholine vesicles: Conductometry
Authors: Tsao Heng-Kwong , Tseng Wen Liang
Published: July 2002

The interaction between ionic surfactants and phosphatidylcholine vesicles, which are prepared without addition of buffer and salt, is investigated by conductivity measurements. On the basis of the vesicle acting as a trap of charge carriers, the bilayer/aqueous phase partition coefficient K and the surfactant/lipid molar ratio Re of nine surfactants are determined. The thermodynamic consistency is satisfied by the measured parameters. The effects of the alkyl chain length (C10–C16) and ionic head group are then studied. The inverse partition coefficient K−1 is linearly related to the critical micelle concentration. The solubilizing ability Reb is a consequence of the competition between the surfactant incorporation into the bilayer and the formation of micelles. Consequently, the K parameter rises whereas the Reb parameter declines as the chain length is increased. The influence due to addition of salt is also discussed.
Profile consistency of an elongated field-reversed configuration. I. Asymptotic theory
Authors: Barnes D. C.
Published: July 2002

An asymptotic theory of field-reversed configuration (FRC) equilibrium is developed, where the small expansion parameter is the square of the inverse elongation of the separatrix. It is shown that equilibrium alone completely determines the closed-field pressure profile of an elongated FRC in terms of the open-field profile. Examples show that the closed profile is insensitive to details of the open profile. A surprising result is that the open outflow plasma (axially beyond closed region) is always totally diamagnetic on the axis (β=1, where β is measured in the θ-pinch sense). The separatrix shape (axial variation) depends uniquely on the first-order pressure profile, and any separatrix shape may be realized within the limitations of the asymptotic theory. This sensitive dependence of shape on pressure profile explains extreme stiffness of the FRC equilibrium problem which was reported earlier. These results are compared favorably with experimental observations.
Application of power laws to low Reynolds number boundary layers on smooth and rough surfaces
Authors: Bergstrom D. J. , Tachie M. F. , Balachandar R.
Published: July 2002

Scaling laws for the overlap region of near-wall turbulent flows are of particular interest to turbulence researchers and engineers. For the mean flow at sufficiently high Reynolds numbers, the classical boundary layer theory proposes a logarithmic law for the overlap region. On the other hand, at low Reynolds numbers, refined measurements and direct numerical simulation results indicate that the log law region becomes negligibly small. Instead, power laws have received increasing attention as an alternative formulation for the overlap region at low Reynolds numbers. In the present study, we use open channel flow measurements to assess the ability of the power laws proposed by Barenblatt [J. Fluid Mech. 248, 513 (1993)] and George and Castillo [Appl. Mech. Rev. 50, 689 (1997)] to describe the overlap region in low Reynolds number boundary layers on smooth and rough surfaces. The skin friction laws derived from the power laws are also used to estimate the friction velocity, which values are then compared to measurements obtained by other reliable techniques. The results indicate that at low Reynolds numbers the power law formulations can model a wider extent of the flow than the classical logarithmic profile. Both Barenblatt’s and George and Castillo’s power laws give an excellent prediction of the friction velocities for flows over a smooth surface, but only the skin friction law proposed by George and Castillo gives good prediction for the rough wall data.
I
−
H
2
O
and its neutral precursors: Similarities and differences
Authors: Kowal Marek , Gora Robert W. , Roszak Szczepan , Leszczynski Jerzy
Published: July 2002

The results of a theoretical study of the I−H2O anion and its neutral precursors are presented. The hydrogen-bonded structures were predicted for both the ionic and neutral complexes. The energetically preferred isomer for IH2O however is a species with the direct O–I bond. The relation between the potential energy surfaces for ionic and neutral moieties is evaluated based on their electron affinity properties. Thermodynamic and spectroscopic (IR) properties of complexes are discussed. The interaction energy decomposition is applied to explore the differences between the nature of bonding within the studied complexes.
Stimulated scattering of a light wave in a dusty plasma
Authors: Lee Hee J.
Published: July 2002

Nonlinear coupling of an electromagnetic wave and a slow density modulation associated with a dust acoustic wave in a dusty plasma is considered. A special case is considered where the electron temperature (Te) and the electron Debye length (λe) are much greater than the corresponding ion quantities Ti and λi, respectively. It is shown that in this case the coupling equations are extremely simple. The threshold and growth rate of the stimulated scattering instability are obtained.
Crystallization and melting in the Lennard-Jones system: Equilibration, relaxation, and long-time dynamics of the moving interface
Authors: Tepper H. L. , Briels W. J.
Published: July 2002

Nonequilibrium molecular dynamics simulations have been carried out on the growth and melting of the Lennard-Jones (100) interface at small undercoolings and superheatings. Two regimes of linear growth rate were discovered: a short-time regime associated with interface relaxation and a long-time regime associated with the macroscopic limit of growth and melting. It was shown that, if system sizes or equilibration times are taken too small, one will find only the initial regime. On the basis of our very accurate results on the macroscopic growth rates close to equilibrium, the possibility of a discontinuity in the temperature dependence of growth and melting rates at the melting point was ruled out.
Electronic spectroscopy in He droplets
Authors: Stienkemeier Frank , Vilesov Andrey F.
Published: July 2002

Recent experiments on the electronic spectroscopy of atoms, clusters, and organic molecules embedded in helium nanodroplets are reviewed. Electronic transitions imply a larger degree of distortion of the helium environment as compared to vibrational and rotational excitations. Thus new phenomena arise such as the appearance of side bands in the spectra, which are due to the excitation of helium collective vibrations, large changes of the effective molecular rotational constants and even the expulsion of an atom (or molecule) from the cluster upon excitation. These features make it possible to probe the helium environment and its interactions with molecular chromophores on the atomic scale. Real-time studies of the manifestations of superfluidity and of chemical processes in the droplets via femtosecond excitation techniques, provide a new perspective to this field. The considerable amount of data available so far shows the large potential of helium droplets for isolation and spectroscopy of large molecules and clusters. The low temperature and the high spectral resolution achievable because of the relative homogeneity of this medium, are instrumental for separating solvation effects that are obscured by the presence of much larger fluctuations in more classical environments. Hence, electronic and geometrical structures of even large entities become accessible.
Thermal stability of stacked high-k dielectrics on silicon
Authors: Chang J. P. , Lin Y.-S.
Published: July 2002

The thermal stability of ZrO2/ZrSixOy and ZrO2/ZrSixOy/SiNx thin films on silicon was examined by synchrotron radiation ultraviolet photoemission spectroscopy. The ZrO2/ZrSixOy layer deposited by atomic-layer-controlled deposition is stoichiometric, uniform, amorphous, and has an equivalent oxide thickness of ∼1 nm and a dielectric constant of ∼18 with low leakage current. These ZrO2/ZrSixOy samples are thermally stable in vacuum up to 880 °C at which the film decomposed to form ZrSi2, the most thermodynamically stable metal silicide at a per zirconium atom basis, and the desorption of SiO(g) and ZrO(g) accounted for the greatly reduced oxygen and zirconium photoemission intensities. The thermal stability of ZrO2/ZrSixOy is improved to 950 °C when deposited on a 0.5–0.7 nm SiNx film.
Optical dephasing effects in broadband four-wave mixing in C2: Implications for broadband FWM thermometry
Authors: Lloyd G. M. , Ewart P.
Published: July 2002

The effects have been studied of increasing laser bandwidth and intensity on temperatures derived from broadband four-wave mixing (FWM) spectra. A simple model of optical dephasing arising from pumping of coupled transitions is presented and validated by experimental measurements. The model is found to predict qualitatively the effect on relative spectral intensity in broadband FWM of increasing bandwidth and input intensity. The implications for thermometry are discussed to indicate the circumstances in which accurate and precise temperatures may be derived from broadband FWM spectra.
Sub-femtosecond dynamics and dissociation of C–H bonds in solid polystyrene and liquid benzene
Authors: Chatzidimitriou-Dreismann C. A. , Abdul-Redah T. , Streffer R. M. F. , Mayers J.
Published: July 2002

In previous work [C. A. Chatzidimitriou-Dreismann et al., J. Chem. Phys. 113, 2784 (2000)] we provided first results of the subfemtosecond dynamics of C–H bond dissociation of solid fully protonated polystyrene at T≈295 K, applying neutron Compton scattering (NCS). A striking anomalous shortfall of the proton scattering intensity with respect to that of the carbon nucleus has been observed. A theoretical explanation based on short-lived protonic quantum entanglement (QE) has also been given. Here we present a twofold extension of that work: Investigation of the found anomaly (i) in solid D-polystyrene with various degrees of aromatic protonation [(-CD2CDC6HxDy-)n with x+y=5] and (ii) in liquid C6H6 and C6H6–C6D6 mixtures. A striking decrease of the neutron cross-section density of H by 20% (on the average) was observed, which appeared to be weakly dependent on the H:D atomic ratio. This result indicates that the considered effect is mainly of intramolecular origin. In contrast to previous NCS experiments on metallic hydrides which revealed a strong scattering time (τscatt) dependence of the decrease of the neutron cross-section density of H, this effect is only slightly apparent in polystyrene and benzene, thus indicating different dynamics of the decoherence process due to different electronic environments. All presented results imply that short-lived QE may be highly relevant for chemical and biological systems.
Analysis of a dinitro-based molecular device
Authors: Seminario Jorge M. , Zacarias Angelica G. , Derosa Pedro A.
Published: July 2002

A proposed dinitro device, Au-(2′-nitro-4-ethynylphenyl-4′-ethynylphenyl-5′-nitro-1-benzene thiolate)-Au is analyzed using a combination of density functional and Green function theories complemented with information from theoretical and experimental studies of a similar nitroamino device, Au-(2′-amino-4-ethynylphenyl-4′-ethynylphenyl-5′-nitro-1-benzenethiolate)-Au. The dinitro compound might also perform as a molecular memory but with different characteristics than those of the nitroamino, showing well-defined charge states; however, the neutral charge state of the nitroamino presents well-defined resonant tunneling characteristics and a larger intrinsic dipole moment. Density of states, transmission functions, and current–voltage characteristics for the neutral, anion, and dianion of the two molecules are compared. The effect of the bias potential is explicitly considered in the calculations as well as the effect of the contacts and the spin states of the open shell systems. The theoretical results for the training molecule are in good agreement with experiment. It is concluded that observed negative differential resistance is due mainly to charge effects combined in less degree with resonant tunneling intrinsic to single molecules.
Caging of Ni clusters by benzene molecules and its effect on the magnetism of Ni clusters
Authors: Rao B. K. , Jena P.
Published: July 2002

Global optimization of the geometry of small Ni clusters interacting with benzene molecules yields equilibrium structures where the Ni atoms find themselves caged between the benzene molecules. This leads to quenching of the Ni magnetic moment in most of the complexes even though the structure of the caged Ni clusters remain relatively unchanged from their otherwise free gas phase geometry. The accuracy of these predictions, based on density functional theory with generalized gradient approximation for exchange and correlation, is established by the good agreement achieved between the calculated and available experimental dissociation energies and ionization potentials.
Intramolecular energy flow and nonadiabaticity in vibrationally mediated chemistry: Wave packet studies of Cl+H2O
Authors: Fair Joanna R. , Schaefer Doug , Kosloff Ronnie , Nesbitt David J.
Published: July 2002

Time-dependent and time-independent quantum scattering methods are used to investigate state-to-state inelastic and reactive collision dynamics for a three-dimensional (3D) atom+triatom model of Cl+H2O→HCl+OH. The results elucidate the role of (i) intramolecular vibrational energy transfer and (ii) vibrational nonadiabaticity on the time scale of a reactive encounter in systems with nearly degenerate stretching “local modes.” Adiabatic two-dimensional (2D) vibrational eigenfunctions [ψn(r1,r2,R)] and eigenvalues [En(R)] are first obtained in OH bond coordinates (r1,r2) as a function of Cl–H2O center-of-mass separation (R), which then provides the requisite adiabatic potential energy curves and nonadiabatic coupling matrix elements for full 3D quantum wave packet propagation. Inspection of these 2D vibrational eigenfunctions indicates that near degeneracy between H2O symmetric |01+〉 and antisymmetric |01−〉 states is systematically lifted as R decreases, causing vibrational energy to flow into local-mode OH excitations pointing either toward (“proximal”) or away from (“distal”) the approaching Cl atom, respectively. This suggests a simple yet powerful physical model for mode-specific reactive scattering dynamics, the predictions of which are confirmed by full 3D quantum wave packet calculations over a range of collision velocities.
Electronic structure and chemical boning in nonstoichiometric molecules: Al3X2−(X=C,Si,Ge). A photoelectron spectroscopy and ab initio study
Authors: Li Xi , Wang Lai-Sheng , Cannon Nathan A. , Boldyrev Alexander I.
Published: July 2002

The electronic and molecular structure of a series of nonstoichiometric pentaatomic species, Al3X2−(X=C,Si,Ge) and their corresponding neutrals, have been studied by photoelectron spectroscopy and ab initio calculations. Well-resolved photodetachment transitions were observed for all three species and were compared with the calculations. Al3C2− was found to have a C2v(1A1) global minimum with an Al− ion coordinated to the CC triple bond in a quasi-linear AlCCAl molecule. Al3Si2− and Al3Ge2− were found to be different from the isoelectronic Al3C2− anion. They each have an almost planar trapezium global minimum structure with several low-lying isomers. The quasi-planarity of Al3Si2− and Al3Ge2− was understood by comparison with the previously known SiAl4− planar molecule.
Focusing and selecting the linear type HBr–N2O by using a 2 m long electrostatic hexapole field
Authors: Okano A. , Ohoyama H. , Kasai T.
Published: July 2002

Focusing and selecting the HBr–N2O cluster beam was performed by using a 2 m long electrostatic hexapole field. The observed focusing curve shows a clear evidence that the linear type HBr–N2O isomer which has symmetry of symmetric top was preferentially detected in the cluster beam formed under the experimental condition used here, even though our previous ab initio calculations predict that the bent type isomer is theoretically possible. The best fit simulation for the experimental focusing curve was achieved only if we assume vibrational excitation in the van der Waals mode of the linear type HBr–N2O. The permanent dipole moment is determined to be 0.50±0.05 D.
Optimization of wave packet coefficients in Li2 using an evolutionary algorithm: The role of resonant and nonresonant wavelengths
Authors: Ballard Joshua B. , Stauffer Hans U. , Amitay Zohar , Leone Stephen R.
Published: July 2002

Using feedback and an evolutionary algorithm (EA), the weak field pump–probe photoionization signal at a single time delay is optimized in Li2. A single launch state is prepared via excitation with a cw laser, from which a pump pulse excites a superposition of two rotational states on an excited electronic potential energy curve: E 1Σg+(vE=9, JE=27 and 29). The EA modifies the phase pattern versus wavelength of the ultrafast pump pulses using a pulse shaper with a 128 pixel liquid crystal spatial light modulator. Limitations of frequency resolution for the pulse shaper create an effective temporal window in which pulses can be shaped. Optimization of the photoionization signal at pump–probe time delays outside of this temporal pulse shaping window involves phase shifts of only the two frequencies resonant with the transition of the wave packet states, effectively introducing a phase shift in the wave packet recurrences. For pump–probe time delays inside the pulse shaping window, optimization of the photoionization shows the influence of not only resonant but also nonresonant wavelengths. In this regime, the phase shift of wave packet recurrences as well as the time-dependent wave packet amplitude coefficients are optimized. First order time dependent perturbation theory is used to explain the mechanism by which the photoionization is maximized. These experiments are the first to use an optimization routine as a tool to identify a variety of simultaneous mechanisms that underlie the maximization of a process.
The ground state of germylidene (H2C=Ge)
Authors: Hostutler David A. , Clouthier Dennis J. , Pauls Steven W.
Published: July 2002

Single vibronic level emission spectra have been obtained by pumping individual vibronic bands in the B̃1B2–X̃1A1 electronic transitions of jet-cooled H2CGe and D2CGe. Analysis of the spectra gave for the first time the frequencies of the following ground state fundamentals: for H2CGe, ν2=1238 cm−1, ν3=782 cm−1, ν4=673 cm−1, ν6=351 cm−1, and for D2CGe, ν1=2089 cm−1, ν2=1000 cm−1, ν3=694 cm−1, ν4=520 cm−1, and ν6=282 cm−1. The vibrational data have been combined with our previously determined molecular structures to obtain approximate harmonic force fields for the ground and excited states. Franck–Condon simulations of the emission spectra in the harmonic approximation show reasonable agreement with experiment, except for unexpected activity in the out-of-plane bending mode. This is attributed to a very anharmonic out-of-plane bending potential in the S2 excited state. Germylidene also has a very low ground state CH2 rocking frequency (ν6) which is along the normal coordinate leading to isomerization to the less stable trans-bent germyne isomer, H–C≡Ge–H.
Multiply charged ion-induced secondary electron emission from metals relevant for laser ion source beam diagnostics
Authors: Láska L. , Krása J. , Stöckli M. P. , Fehrenbach C. W.
Published: July 2002

The number of secondary electrons, γ, emitted when multiply charged ions impact on metallic probe surface was measured to make the quantitative ion diagnostics based on this process more precise. The electron yield γ(q,Ei) was measured for Taq+ and Xeq+ ions (q=6–41) in the region of ion kinetic energy per atomic mass up to Ei/A∼34 keV/amu. For highly charged Xeq+ ions (q>16), a minimum of the electron yield, γMIN, was observed in its dependence on Ei. With increasing q, the γMIN shifts to higher energies. The comparison of available data for Nq+, Neq+, Arq+, Xeq+, and Auq+ ions shows that one can create a similarity law describing the dependence of γMIN for those elements in the Ei/A representation. The value of γ/q evaluated from compiled data ranges from ≈0.3 to ≈3.5 in dependence on q and Ei.
Optimization of tin dioxide nanosticks faceting for the improvement of palladium nanocluster epitaxy
Authors: Arbiol J. , Cirera A. , Peiró F. , Cornet A. , Morante J. R. , Delgado J. J. , Calvino J. J.
Published: July 2002

Semispherical palladium nanoclusters have been epitaxed on {110} facets of tin dioxide nanosticks. The synthesis of tin dioxide nanoparticles has been optimized to obtain a crystallite shape with a maximum surface area lying on the rutile structure {110} planes, which are the most active for gas sensing. For this purpose, we describe a microwave method, which allowed us to obtain monocrystalline stick-like tin dioxide nanoparticles (so-called nanosticks) with rectangular prism shape. These nanosticks present long lateral {110} faces, squared cross-section 5–25 nm wide, and lengths of up to 0.5 μm.
A spectral theory for small-amplitude miscible fingering
Authors: Ben Yuxing , Demekhin Evgeny A. , Chang Hsueh-Chia
Published: July 2002

Using the self-similar symmetry of a diffusing front, we develop a linear spectral theory for miscible fingering at inception that accurately captures the destabilization of localized disturbances (with large transverse wavelengths compared to the front width) by the unsteady front. Our theory predicts a generic selected wavelength (4πηD/U0 for gravity fingering, where η is the transverse to longitudinal dispersion ratio, and an additional factor proportional to the logarithm of the mobility ratio for viscous fingering) at the small time of O(D/U02), where D is the dispersion coefficient or diffusivity in the flow direction and U0 is the displacement velocity. This wavelength then grows in time and approaches a universal asymptotic wavelength coarsening dynamics of (η2D5/U02)1/8(t′)3/8, where t′ is the dimensional time, for all small-amplitude miscible fingering phenomena in a slot or in porous media. The 38 exponent in time is due to a unique long-wave stabilization mechanism due to transverse convection, which escapes prior quasisteady theory. Explicit and generic scalings are then derived for gravity and viscous miscible fingering phenomena and are favorably compared to experimental and numerical results on linear coarsening dynamics.
Determination of the distribution of both exchange bias field strength and direction in ferromagnet/antiferromagnet systems
Authors: Barholz K.-U. , Mattheis R.
Published: July 2002

A new method for the characterizing of ferromagnet/antiferromagnet (F/AF) systems is proposed. By measuring the longitudinal Kerr effect during rotation of a constant field (ROTMOKE technique) we derive unknown magnetic features of the F/AF system. The distribution of the strength and direction of exchange bias field can be determined. Asymmetries of the ROTMOKE curve give clues to the different orientations of the uniaxial and unidirectional anisotropy. The large influence of uniaxial anisotropy on the reversal of F/AF systems is shown.
Magnetic exchange effects in a nanocomposite Ni/NiO film
Authors: van Lierop J. , Lewis L. H. , Williams K. E. , Gambino R. J.
Published: July 2002

The unique sample preparation technique of reactive ion beam assisted deposition has been used to make a thin film sample of interdispersed Ni and NiO nanocrystallites. The ferromagnetic and antiferromagnetic crystallites in the film provide a unique geometry that lies between that of bulk exchange biased systems and layered thin films, and provides clear evidence of exchange effects. Interphase magnetic exchange is demonstrated by suppression of the Néel and Curie temperatures of the components, as well as by the typical hysteresis loop shift that is a measure of the exchange field strength. Furthermore, a strong temperature and maximum applied field dependence for both the coercivity and exchange field strength is present, indicating competition between the exchange interaction of the Ni and NiO nanocrystallites and the external field and NiO surface magnetic coupling interactions.
Prediction of ferromagnetic cubic spinel phase of Fe3N4
Authors: Xu Yong-Nian , Rulis Paul , Ching W. Y.
Published: July 2002

We have carried out a theoretical investigation of the structure and properties of nitrogen-rich iron nitride with a cubic spinel structure (c-Fe3N4) using an ab initio method. The c-Fe3N4 structure, with a lattice constant of 7.895 Å and an N internal parameter of u=0.379, is predicted to be stable. Spin-polarized electronic structure calculation shows it to be weakly ferromagnetic with a total spin-magnetic moment of about 3.26μB per formula unit. Fe ions at the tetrahedral A site have a much smaller moment of 0.42μB/Fe compared to the moment of 2.28μB/Fe at the octahedral B site. The N ions are found to be oppositely polarized with a moment of −0.43μB per N atom, which contributes to the weak total moment. Lattice expansion (compression) tends to increase (decrease) the total magnetic moment. The bulk modulus calculated for c-Fe3N4 is 304 GPa, manifesting itself to be a very hard material. A bond order calculation shows that the low compressibility of c-Fe3N4 is due to the strong covalent bonding of Fe at the tetrahedral site.
Resistively coupled ferromagnetic single-electron transistors
Authors: Shirakashi Jun-ichi , Takemura Yasushi
Published: July 2002

We study the operation of resistively coupled ferromagnetic single-electron transistors quantitatively. Control of tunnel magnetoresistance on the drain and gate voltages is predicted. Inelastic macroscopic quantum tunneling of charge plays an important role in the enhancement of tunnel magnetoresistance. This implies that the resistively coupled ferromagnetic single-electron transistors have the possibility as magnetoresistive nanostructure devices.
Magneto-optical properties of Co|Pt multilayer systems
Authors: Vernes A. , Szunyogh L. , Weinberger P.
Published: July 2002

We are reporting, theoretical Kerr spectra of Co|Pt multilayer systems as obtained on a first principles basis including multiple reflections and interferences from all the boundaries in-between the layers.
Calculation of the resonant magneto-optical properties of transition metals and disordered alloys in the x-ray regime
Authors: Minár J. , Ebert H.
Published: July 2002

A version of the Kubo-formula is presented, that allows to calculate the resonant magneto-optical properties in the x-ray regime for arbitrary materials. This is achieved by representing the underlying electronic structure in terms of the electronic Green’s function. Calculating the Green’s function within the framework of spin-polarized relativistic multiple scattering theory (SPR-KKR) in particular gives access to spin-orbit induced resonant magneto-optical properties. Results obtained for the magnetic dichroic part of the complex reflective index n at the L2,3-edges of 3d-elements in pure metals as well as disordered alloys are found in very good agreement with recent experimental findings. This applies also for the corresponding Faraday rotation as it is demonstrated for the K-edge of Co in fcc-Co.
Proton NMR in the giant paramagnetic molecule {Mo72Fe30}
Authors: Jung J. K. , Procissi D. , Vincent R. , Suh B. J. , Borsa F. , Kögerler P. , Schröder C. , Luban M.
Published: July 2002

H 1 nuclear magnetic resonance (NMR) and relaxation measurements have been performed in the Keplerate species {Mo72Fe30}. The H1 NMR linewidth increases gradually with decreasing T and it saturates below about 4 K, as expected for a nonmagnetic ground state with ST=0. A strong enhancement of T1−1 with a peak around T=2 K is observed, a feature similar to the one observed in antiferromagnetically (AF) coupled iron rings. The peak is discussed in terms of the slowing down of magnetic fluctuations resulting from building up of AF correlations.
Enhanced coercive and remanence fields for CoFe2O4 and BaFe12O19 bilayers deposited on (111) MgO
Authors: Yoon S. D. , Oliver S. A. , Vittoria C.
Published: July 2002

High quality epitaxial bilayer films of spinel CoFe2O4 (CoF) and M-type hexagonal ferrite BaFe12O19 (BaM) were deposited onto (111) magnesium oxide (MgO) substrate by pulsed laser ablation deposition. X-ray diffraction patterns of both BaM/CoF and CoF/BaM films showed only (0001) BaM, (111) CoF, and (111) MgO peaks. The highest coercive field of Hc=1.4 kOe was obtained for a BaM/CoF film where the CoF layer was deposited at 400 °C, which was higher than typical Hc values of ∼0.4 kOe found for single layer BaM films on (111) MgO. This was less than the Hc∼3.0 kOe found for (111) CoF films deposited at 400 °C, probably due to in situ annealing effects during the growth of the overlying BaM film at 900 °C. This Hc enhancement for BaM/CoF bilayers as compared to BaM/MgO films may provide a means to combine large coercive fields with high quality hexaferrite films.
Stability and accuracy of Euler and quaternion micromagnetic algorithms
Authors: Feng Xuebing , Visscher P. B.
Published: July 2002

Micromagnetic calculations including a Langevin random-field term are increasingly important for understanding nucleation and relaxation phenomena. Although there has been recent work on development of higher order algorithms for Langevin simulation, the most straightforward algorithms remain first order. This creates problems of both accuracy and stability in micromagnetic simulations. In most practical micromagnetic contexts, the stability problem is the more serious. We have examined the linear stability properties of Euler’s method and a new quaternion-based algorithm. We find that the latter is significantly more stable.
Systematic study of magnetic tip induced magnetization reversal of e-beam patterned permalloy particles
Authors: Zhu Xiaobin , Grütter P. , Metlushko V. , Ilic B.
Published: July 2002

Patterned nanoscale permalloy particles, with different aspect ratios and widths, were studied by magnetic force microscopy in different operating modes with various magnetic tips. The stray field from the magnetic tip itself could reverse the particle moment orientation while acquiring topography data or when the tip is very close to the sample. Model calculations show that in most cases the stray field from the tip is big enough to reverse the particle’s moment. To reduce the tip’s influence, one should be very careful in choosing the magnetic tip coating and the operating mode of the microscope. Control of the magnetic state of an individual particle is demonstrated.
Analysis of thermal stability enhancement in double layer antiferromagnetically coupled media
Authors: Guan Lijie , Zhu Jian-Gang
Published: July 2002

In this article, we present a combined analytical analysis along with micromagnetic simulations on the thermal stability of double layer longitudinal media with antiferromagnetic exchange coupling (AFC). The thermal stability enhancement in double layer AFC media is characterized quantitatively and comparisons with the performance of conventional single layer longitudinal media are made. It has been found that the thermal stability enhancement of the AFC media over a single layer media with the same medium coercivity can be achieved by optimizing the interlayer exchange coupling. The effective grain volume gain is found to be approximately V1+0.4V2, where V1 and V2 are the grain volumes of the top and bottom layers, respectively. Increasing V2 requires a larger interlayer exchange coupling constant.
Optically pumped transport in ferromagnet-semiconductor Schottky diodes (invited)
Authors: Isakovic A. F. , Carr D. M. , Strand J. , Schultz B. D. , Palmstrøm C. J. , Crowell P. A.
Published: July 2002

Optical pumping is used to generate spin-polarized carriers in epitaxial ferromagnet-GaAs Schottky diodes with InyGa1−yAs quantum wells placed in the depletion region. A strong dependence of the photocurrent on the polarization state of the incident light is observed, and a series of measurements as a function of excitation energy, bias voltage, magnetic field, and excitation geometry are used to distinguish spin-dependent transport from a variety of background effects. The spin polarization of the photocurrent for pumping energies at and above the band gap of GaAs is of order 0.5% or less. Much larger polarization dependence is observed for excitation energies near the quantum well ground state. Although background effects are very large in this regime, the field dependence of the polarization signal for several samples is suggestive of spin-dependent transport.
Ferromagnetic resonance mode interactions in periodically perturbed films
Authors: McMichael R. D. , Twisselmann D. J. , Bonevich J. E. , Chen A. P. , Egelhoff W. F. , Russek S. E.
Published: July 2002

Ferromagnetic resonance linewidth measurements are reported for thin films of Ni80Fe20 deposited on substrates with 320 nm period grooves or parallel aperiodic scratches. These films are model systems for studying the effects of inhomogeneity on ferromagnetic resonance. With the grooved substrate, the ferromagnetic resonance spectra show that interactions split the resonant frequencies of interacting modes leading to two resonances for strong interactions and broadened resonances for weak interactions. The results obtained with the scratched substrate are in agreement with the two-magnon model of linewidth, showing sharply increased linewidth when the scratches are oriented to allow coupling to degenerate spinwave modes. The results are not in agreement with a simple model of linewidth as a direct measure of the spread in local resonance fields.
Strongly capacitively coupled quantum dots
Authors: Chan I. H. , Westervelt R. M. , Maranowski K. D. , Gossard A. C.
Published: July 2002

Double quantum dots were formed in a gated GaAs/AlGaAs heterostructure with negligible interdot tunneling; strong capacitive coupling was provided by a floating interdot capacitor. The interdot capacitance was measured to be 0.28CΣ, where CΣ is the single-dot capacitance. Coulomb blockade conductance images for both dots versus side gate voltages at 70 mK show a hexagonal pattern of peaks; the double dot acts as a single-electron current switch. For weak tunneling, the conductance peaks of both dots fit thermally broadened line shapes. Charge fluctuations produced by strong tunneling on one dot are induced on the second, filling in its peak splitting.
Cathode and anode plasmas in short-pulse explosive field emission cathodes
Authors: Shiffler D. , Ruebush M. , Zagar D. , LaCour M. , Sena M. , Golby K. , Haworth M. , Umstattd R.
Published: July 2002

Explosive field emission cathodes have been a subject of research for a number of years. These cathodes offer high current densities and total current without requiring a heater for the production of electrons. Generally these cathodes consist of some structure with a series of tips or metal–dielectric regions in which a large electric field enhancement can occur. A cathode plasma is then formed from these discharge points that then supplies the electrons necessary for space charge limited emission. This article reports on a series of optical measurements in which the cathode and anode plasmas of explosive field emission cathodes are observed. Three types of cathodes are investigated. These types are a polymer velvet cathode, a metal–dielectric cathode, and a tufted carbon fiber cathode in which the fibers have been coated with a cesium iodide salt. Cesium iodide coated carbon fiber cathodes have shown a great deal of promise for various field emitter applications. From these high speed photos, the evolution of the plasmas on both cathode and anode can be qualitatively ascertained. Experimentally we find that not only does cathode plasma behavior depend on the type of cathode, but the anode plasma behavior does also. Further, we find that the best performing cathode shows the most rapid plasma formation on both anode and cathode, yet without a rapid plasma expansion across the anode–cathode gap.
Carbon deposition in Si as a consequence of H and He irradiations: A systematic study
Authors: Mörschbächer M. J. , Behar M.
Published: July 2002

In the present work we have investigated the influence of different parameters that determine the C deposition on a Si target. Among them we have studied the pressure of the irradiation chamber, the implantation fluence, the current density, the target temperature, the energy of the beam, the charge state of the ion, the ion species and the molecular state of the irradiation beam. In order to determine the quantity of C deposited on the Si substrate we have used the Rutherford backscattering/channeling technique together with the resonant C12(α,α)12C reaction. After careful analysis we arrived at the conclusion that the real independent parameters are the pressure of the irradiation chamber, the temperature of the target, the irradiation time and the electronic stopping power of the ion–target combination.
Microsolvation of small anions by aromatic molecules: An exploratory study
Authors: Le Barbu K. , Schiedt J. , Weinkauf R. , Schlag E. W. , Nilles J. M. , Xu S.-J. , Thomas O. C. , Bowen K. H.
Published: July 2002

This work was motivated by the experimental finding that the O2−/benzene interaction energy is unexpectedly large. To further explore the interactions of small anions with aromatic molecules, anion photoelectron spectroscopy was utilized to measure interaction strengths of the seed anions, O2− and NO−, complexed with several aromatic molecules, including benzene, naphthalene, pyridine, and pyrimidine. As in the case of O2−(benzene), the anion(aromatic)1 binding energies for the other complexes studied were also higher than one might have anticipated. In addition, the interaction energy of O2− complexed with a given aromatic molecule was, in every case studied, higher (by a factor of ∼1.5) than that of NO− complexed with the same aromatic. While the dependence of interaction strengths on solvent dipole moments and/or polarizabilities implied a substantial electrostatic component to the binding in these complexes, differences in the binding of O2− and NO− with these aromatic molecules showed that there is a distinct covalent aspect to the interaction as well. A significant portion of this interaction was attributed to the fact that O2− and NO− are both open-shell anions that are interacting with closed-shell aromatic molecules. In the accompanying paper [J. Chem. Phys. 116, 9672 (2002)] calculations on O2−(benzene)1 by Jalbout and Adamowicz shed additional light on the nature of small anion–aromatic molecule interactions. Last, results are also presented comparing the interaction energies of several multisolvent, anion–molecule complexes.
Strain relaxation of SiGe islands on compliant oxide
Authors: Yin H. , Huang R. , Hobart K. D. , Suo Z. , Kuan T. S. , Inoki C. K. , Shieh S. R. , Duffy T. S. , Kub F. J. , Sturm J. C.
Published: July 2002

The relaxation of patterned, compressively strained, epitaxial Si0.7Ge0.3 films transferred to borophosphorosilicate (BPSG) glass by a wafer-bonding and etch-back technique was studied as an approach for fabricating defect-free Si1−xGex relaxed films. Both the desired in-plane expansion and undesired buckling of the films concurrently contribute to the relaxation. Their relative role in the relaxation process was examined experimentally and by modeling. Using x-ray diffraction, Raman scattering and atomic force microscopy, the dynamics of in-plane expansion and buckling of Si0.7Ge0.3 islands for island sizes ranging from 10 μm×10 μm to 200 μm×200 μm for anneal temperatures between 750 and 800 °C was investigated. Lateral relaxation is favored in small and thick islands, and buckling is initially dominant in large and thin islands. Raising the temperature to lower viscosity of the oxide enhances the rate of both processes equally. For very long annealing times, however, the buckling disappeared, allowing larger, flat, and relaxed islands to be achieved. Cross-sectional transmission electron microscopy observation on a relaxed Si0.70Ge0.30 island revealed no dislocations, confirming that SiGe relaxation on BPSG is a good approach to achieve high quality relaxed SiGe.
On the evidence of deterministic chaos in ECG: Surrogate and predictability analysis
Authors: Govindan R. B. , Narayanan K. , Gopinathan M. S.
Published: July 2002

The question whether the human cardiac system is chaotic or not has been an open one. Recent results in chaos theory have shown that the usual methods, such as saturation of correlation dimension D2 or the existence of positive Lyapunov exponent, alone do not provide sufficient evidence to confirm the presence of deterministic chaos in an experimental system. The results of surrogate data analysis together with the short-term prediction analysis can be used to check whether a given time series is consistent with the hypothesis of deterministic chaos. In this work nonlinear dynamical tools such as surrogate data analysis, short-term prediction, saturation of D2 and positive Lyapunov exponent have been applied to measured ECG data for several normal and pathological cases. The pathology presently studied are PVC (Premature Ventricular Contraction), VTA (Ventricular Tachy Arrhythmia), AV (Atrio-Ventricular) block and VF (Ventricular Fibrillation). While these results do not prove that ECG time series is definitely chaotic, they are found to be consistent with the hypothesis of chaotic dynamics.
Oscillator strength of small-polaron absorption in WOx (x≤3) electrochromic thin films
Authors: Yoshimura Tetsuzo
Published: July 2002

The oscillator strength of small-polaron absorption is studied for various thin films of tungsten oxide, WOx (x≤3), prepared by rf reactive magnetron sputtering, reactive ion plating, and vacuum evaporation. The oscillator strength greatly varies depending on the procedure of film preparation and treatment. For WOx films prepared by vacuum evaporation, the effective oscillator strength decreases down to 0.04 by annealing in air at 200 °C and increases up to 0.24 by annealing in air at 400 °C. The film deposited by reactive ion plating at an rf power of 300 W exhibits a large effective oscillator strength of 0.31. Concerning the film prepared by rf reactive magnetron sputtering, the oscillator strength drastically increases with an increase in rf power. For the film deposited at an rf power of 1000 W, an effective oscillator strength of 0.51 is obtained, which is five times greater than that of the film prepared by vacuum evaporation. X-ray diffraction and XPS measurements reveal that the oscillator strength is affected by the two factors, that is, crystallinity and oxygen content of the film. It is concluded that the enhancement of oscillator strength occurs through crystallization of the film from its amorphous state and a lack of oxygen in the film. Using a configuration coordinate diagram, the mechanism of the oscillator strength enhancement is discussed in terms of a delocalization of polaron wave function.
Effect of arsenic segregation on the electrical properties of grain boundaries in polycrystalline silicon
Authors: Wong C. Y. , Grovenor C. R. M. , Batson P. E. , Smith D. A.
Published: July 2002

Equilibrium arsenic segregation to the grain boundaries of polycrystalline silicon was measured directly by x-ray microanalysis in the temperature range 700–1000 °C. A direct link was observed between arsenic segregation and resistivity. Increasing arsenic segregation at the lower annealing temperatures is consistent with an observed increase in resistivity. Fitting the enhancement levels at various temperatures with the McLean segregation isotherm, a binding energy of 0.65 eV/atom and a boundary saturation limit of 12 at. % for arsenic was obtained. A model for the effect of the segregation of arsenic to silicon grain boundaries is proposed. Segregation to boundary defects that cause trapping states can remove these interfacial traps, and segregation to other boundary sites can create a degenerately-doped interfacial layer. The electrical consequences of this segregation are considered, and by comparison of the measured resistivity changes with temperatures with those predicted from these simple models it is proposed that the major contribution to the resistivity in heavily-doped polycrystalline silicon comes from scattering of carriers by the high density of positive charge at the grain boundaries.
Spin glass in Fe-Ni Invar alloys
Authors: Miyazaki T. , Ando Y. , Takahashi M.
Published: July 2002

Initial ac susceptibility χ was examined in the temperature range from 4.2 K to Curie temperatures for polycrystalline Fe100−xNix (20≤x≤50) alloys. The reentrant spin glass is found in x=30–45 alloys. The dependence of the freezing temperature Tg on the intensity of driving magnetic field (ac: 0–0.26 Oe; dc: 0–10 Oe) and on the frequency (140–9200 Hz) was studied for the Fe65Ni35 alloy. A strong magnetic relaxation is observed below Tg. These results are discussed based on the cluster model.
Ion-beam mixing at Fe-Si interface: An interface-sensitive conversion electron Mössbauer spectroscopic study
Authors: Ogale S. B. , Joshee Rekha , Godbole V. P. , Kanetkar S. M. , Bhide V. G.
Published: July 2002

The ion-beam-induced and subsequent thermal transformations at the Fe-Si interface are studied by using the technique of interface-sensitive conversion electron Mössbauer spectroscopy (CEMS). The samples used in these experiments have been prepared by depositing a thin (∼50-Å) layer of Fe57 isotope (enriched to 95.45%) on a freshly cleaned surface of a (111) silicon crystal followed by a deposition of a 250-Å-thick overlayer of natural iron containing only 2.2% of the Fe57 isotope. A number of such composites have been bombarded with Xe+ ions having an energy of 10 keV at an ion dose of ∼1–3×1016 ions/cm2. The atomic mixing at the interface is confirmed by Rutherford backscattering (RBS) measurements and the average composition of the mixed layer is also determined. Using the CEMS technique, it is shown that the thin FeSi layer formed at the interface due to the deposition-induced reaction grows upon ion bombardment, along with the incorporation of a large number of vacancies in the structure. It has also been shown that the internal magnetic field of the ion-beam-mixed alloy shows a broad distribution over a range of field values between 100 and 330 kOe with indications of certain preferences for local atomic arrangements and compositions. The ion-beam-mixed sample shows significant changes in the values of hyperfine interaction parameters subsequent to vacuum annealing treatment at 450 °C for 1 h, while precipitation of α-Fe and Fe3Si phases upon annealing at 700 °C. The as-deposited sample exhibits distinctly different features as compared to those of the ion-beam-mixed sample, when subjected to thermal treatments. These thermal transformations in the as-deposited as well as ion-beam-mixed samples have also been monitored by resistance annealing measurements, and the result of these studies agree well with those of CEMS measurements.
An ellipsometry study of a hydrogenated amorphous silicon based n-i structure
Authors: Collins R. W. , Clark A. H. , Guha S. , Huang C.-Y.
Published: July 2002

Spectroscopic ellipsometry measurements have been used as a probe to provide information on the structure of a hydrogenated amorphous silicon based n-i two-layer system. The n-i structure is prepared under the same conditions as an a-Si:H p-i-n cell with a thin (∼150 Å) n layer deposited in a glow discharge from a mixture of SiF4, SiH4, H2, and PH3 on top of intrinsic a-Si:H. An effective medium approximation and a linear regression analysis have been used to determine the bulk microstructure of the thick (0.3 μm) n layer on quartz as a mixture of volume fractions of a-Si (0.74±0.01), c-Si (0.09±0.02), and void (0.17±0.03). When applied to the thin n layer in the solar-cell configuration, however, the corresponding analysis shows evidence of a 100-Å overlayer of very low density a-Si, not observed on the thicker n-type material, which dominates the dielectric properties of the thin layer. These results are suggestive of nucleation and growth surface microstructure on the 150-Å n layer and lead to the conclusion that, in the fabrication of high-performance a-Si:H based devices using thin layers, such structure must be minimized.