bluuebunny/crossref_metadata_2025 huggingface dataset:
High-resolution neutron powder diffraction study on nitrogenated Nd2Fe17
Authors: Kajitani Tsuyoshi , Morii Yukio , Funahashi Satoru , Iriyama Takahiko , Kobayashi Kurima , Kato Hiroaki , Nakagawa Yasuaki , Hiraya Kenji
Published: July 2002

High-resolution neutron powder diffraction measurements with λ=1.8232 Å and collimation of 6′-20′-6′ on Nd2Fe17Nx with x=0, 2.85, and 2.91 were carried out at room temperature. Structural parameters and magnetic moments were determined by the Rietveld profile-fitting calculation. The magnetic moments of iron atoms increased with increasing nitrogen concentration; e.g., the magnetic moments of iron located at 6c, 9d, 18f, and 18h sites increase from less than 0.7μB in the x=0 sample to about 2.1μB in the x=2.91 sample. It is noticed that a local atomic group composed of two Fe(1) atoms at the 6c site plus six Fe(3) atoms at the 18f site keeps its shape against the nitrogen uptake.
Electrical activation of bismuth implanted into silicon by rapid thermal annealing and kinetics of defects
Authors: de Souza J. P. , Fichtner P. F. P.
Published: July 2002

The rapid thermal annealing (RTA) of Si implanted with Bi+, to a dose of 5.0×1014 cm−2 at the energy of 150 keV, was investigated using sheet resistivity, Hall measurements, and channeling analysis. Approximately 95% of the Bi dose is found substitutional and 90% is electrically active after annealing is performed at 600 °C for times longer than 1 min. The electrical activation yield of Bi after RTA at temperatures ≥700 °C is observed to decrease when increasing the temperature and time of the annealing process. The data taken from electrical measurements and angular scan across the 〈100〉 axis are evidence that the electrically inactive concentration of the Bi correlates with the concentration of Bi atoms located slightly displaced from the crystal rows.
GaAs free-standing quantum-size wires
Authors: Hiruma Kenji , Yazawa Masamitsu , Haraguchi Keiichi , Ogawa Kensuke , Katsuyama Toshio , Koguchi Masanari , Kakibayashi Hiroshi
Published: July 2002

Ultrathin GaAs wires as thin as 15–40 nm and about 2 μm long have been grown on a GaAs substrate by metal-organic vapor-phase epitaxy. The wires, which consist of whiskers, are grown between 380 and 550 °C using trimethylgallium and arsine (AsH3) as source materials. It is found that the wire growth direction is parallel to the [111] arsenic dangling-bond direction and can be perfectly controlled by the crystallographic orientation of the GaAs substrate surface. From transmission electron microscopic analysis it is revealed that the crystal structure of the wire coincides with the zinc-blende type for the growth temperature range of 460–500 °C, but it changes to the wurtzite type at 420 °C and temperatures higher than 500 °C. It is also found that the wires have a twin-type structure around the [111] growth axis for zinc blende and [0001] growth axis for wurtzite. Photoluminescence study of these wires shows that the luminescence peak energy shifts to a higher energy as the wire width decreases from 100 to about 35 nm. In terms of luminescence polarization it is confirmed that the luminescence intensity parallel to the wires is four times greater than that perpendicular to the wires. These results clearly indicate the quantum-size effect of carriers confined in the wire. As a preliminary application to devices, a p-n junction has been formed along the GaAs wire. Light emission by current injection to the p-n junction wires has been observed in continuous operation at room temperature.
Conducting thin films of pentacene doped with alkaline metals
Authors: Minakata Takashi , Ozaki Masaru , Imai Hideaki
Published: July 2002

Donor doping of thin films of pentacene (PEN) with alkaline metals such as sodium, potassium, and rubidium was carried out. Drastic changes in conductivities of PEN films from an insulator to an n-type semiconductor due to the doping was observed. Maximum conductivity of 2.8 Ω−1 cm−1 was shown in the film doped with rubidium. Alkaline metal doping of thin films of other acene compounds such as tetracene and hexacene was also performed.
Nanoindentation of Ag/Ni multilayered thin films
Authors: Ruud J. A. , Jervis T. R. , Spaepen F.
Published: July 2002

Nanoindentation was used to study the mechanical properties of Ag/Ni multilayered thin films. Both the hardness and the elastic modulus of the multilayered thin films had values between those for homogeneous Ag and Ni thin films. The trend in the hardness with layer repeat length can be explained by the effects of both the stress and the microstructure. No evidence for interfacial effects on hardness was found. A decrease in modulus at the smallest repeat lengths was compared with literature data on the elastic constants of Ag/Ni multilayers.
Amorphous phase formation by ion bombardment: Direct comparison between ion implantation and ion-beam mixing
Authors: Thomé L. , Benkoulal T. , Jagielski J.
Published: July 2002

A direct comparison between ion implantation and ion-beam-mixing experiments for the formation of an amorphous phase in metals is provided. The system investigated is Ni-Zr and the technique used to monitor amorphization is Rutherford backscattering and channeling. It is demonstrated that the crystalline-to-amorphous phase transformation results from the introduction of a threshold concentration of foreign atoms into the surface layer of the bombarded target in both cases. However the onset and spatial development of the amorphization process are shown to strongly depend on the type of experiments performed.
Structural and magnetic properties of RFe10.5V1.5Nx
Authors: Yang Jun , Dong Sheng-zhi , Yang Ying-chang , Cheng Ben-pei
Published: July 2002

We have succeeded in preparing RFe10.5V1.5 compounds and their corresponding nitrides RFe10.5V1.5Nx, where R=Y, Ce, Nd, Sm, Gd, Td, Dy, Ho, and Er. The structural and magnetic properties of RFe10.5V1.5Nx have been investigated by x-ray diffraction techniques and magnetic measurements and compared with those of RFe10.5V1.5, NdFe10.5V1.5Nx is a new potential hard magnetic material with excellent intrinsic magnetic properties.
Enhanced electrical conductivity of zinc oxide thin films by ion implantation of gallium, aluminum, and boron atoms
Authors: Kohiki Shigemi , Nishitani Mikihiko , Wada Takahiro
Published: July 2002

Effect of ion implantation on the conductivity of zinc oxide was examined by using highly resistive zinc oxide thin films deposited by rf magnetron sputtering at room temperature to reduce the effect of oxygen vacancies. With the doping by 1×1017 atoms/cm2 gallium the conductivity is 1.0×103/Ω cm for as-implanted film and it increases up to 3.7×103/Ω cm, the highest conductivity reported for zinc oxide films, with raising the annealing temperature in either a nitrogen or oxygen atmosphere. The conductivity of aluminum-doped films is slightly lower than those of gallium-doped films. Among the elements gallium, aluminum, and boron, gallium is the most effective in enhancing the conductivity and boron is the least. The order of the effectiveness is explained by the electronegativity of the dopants.
The effect of static electric fields on ice friction
Authors: Petrenko Victor F.
Published: July 2002

The effect of an external electric dc bias applied between ice and sliders on both static and dynamic ice friction has been examined. The measurements were performed on pure polycrystalline ice in the temperature range from −5 to −30 °C and for sliding velocities from 0.5 to 8 m/s. The sliders were made of aluminum, stainless steel, and polyethylene. It was found that an application of an external dc bias to the ice/slider interface can double the force of dynamic and static friction between ice and metals and between ice and dielectrics. These new physical phenomena are discussed in terms of electrostatic pressure, effect of electric fields on ice plastic deformation, and field-sensitive ice adhesion. Both the experimental results and the discussion show that frictional self-electrification may play a significant role in forming the ice friction coefficient and that the friction can be modified intentionally.
Magnetic properties and microstructural analysis of sputter-deposited and annealed Co-Pt alloys
Authors: Park Sung-Eon , Jung Pu-Young , Kim Ki-Bum
Published: July 2002

We have produced Co1−xPtx (x=0.53 and 0.75) alloy films by dc magnetron sputtering at various substrate temperatures (RT-300 °C) and sputtering pressures (2–10 mTorr). The magnetic properties of the films were examined by using vibrating sample magnetometry and Kerr hysteresis loop tracer. A (111) texture is observed in the as-deposited film, and its degree is dependent on the substrate temperature and Ar pressure. The degree of the (111) texture does not affect the magnetic anisotropy. While the magnetic properties of the Co0.25Pt0.75 alloy films are not sensitive to heat treatment, the coercivity and the loop squareness of the Co0.47Pt0.53 alloy films are drastically increased by annealing. Structural analysis using transmission electron microscopy and x-ray diffractometry reveals that CoPt(L10) and CoPt3(L12) ordered phases, respectively, are formed, each with a strong (111) texture. By comparing the magnetic properties between the CoPt(L10) and CoPt3(L12) ordered phases in relation to the atomic arrangements in a unit cell, it is concluded that the perpendicular magnetic anisotropy in the sputter-deposited Co-Pt alloy system depends mainly on the atomic arrangements of the Co and Pt.
Recent progress in computer-aided materials design for compound semiconductors
Authors: Ito Tomonori
Published: July 2002

Recent progress in computational materials science in the area of semiconductor materials is reviewed. Reliable predictions can now be made for a wide range of problems, such as band structure and structural and thermodynamic properties of various compound semiconductors, using electronic theories such as the pseudopotential method. Further applications are examined by investigating the behavior of various atomic species in semiconductors, including the stability and band structure of heterostructures, superlattices, lattice defects, alloy systems, and surface-related properties such as surface reconstruction, surface passivation, and adatom migration during thin film growth. The empirical interatomic potentials, pseudopotential, and stochastic Monte Carlo methods are used. An overview of these issues is provided and the latest achievements are presented to illustrate the capability of the theoretical-computational approach by comparing experimental results. The constituents of the semiconductors that are most applicable to electronic and optical devices, mainly group-II, -III, -IV, -V, and -VI elements, are focused on. These successful applications of the theoretical-computational approach lead to future prospects for the computer-aided materials design for semiconductors introduced as ‘‘bond engineering.’’
MeV ion irradiation-induced creation and relaxation of mechanical stress in silica
Authors: Snoeks E. , Weber T. , Cacciato A. , Polman A.
Published: July 2002

In situ wafer curvature measurements were performed to study mechanical stress in amorphous SiO2 during Xe, Ne, and Er ion irradiation at energies in the 0.27–4.0 MeV range. Three phenomena are observed: network compaction, radiation-induced viscous flow, and a nonsaturating anisotropic deformation phenomenon. The radiation-induced viscosity is shown to be inversely proportional to the energy density deposited into atomic displacements. The relation between radiation-induced flow and diffusion is discussed in the context of the Stokes–Einstein relation. Viscous flow serves to relax stress, yet a continuous nonsaturating anisotropic deformation effect causes the stress in the irradiated layer to saturate at nonzero values: Xe irradiation at an energy below 3.6 MeV results in a tensile saturation stress; for higher energies a compressive stress builds up. These effects are explained in terms of competing bulk and surface deformation processes resulting from local heating of the SiO2 around the ion tracks. The macroscopic effect of deformation phenomena is illustrated by showing the surface morphology after 4.9 MeV Er irradiation of silica through a contact implantation mask. Finally, an in situ stress study of an alkali borosilicate glass is presented. In this case a fourth radiation induced effect is observed, namely, the generation and annihilation of volume occupying point defects. These defects are shown to anneal out at room temperature, following a broad spectrum of activation energies.
Correlations between CdTe/CdS/SnO2/glass solar cell performance and the interface/surface properties
Authors: Feng Z. C. , Chou H. C. , Rohatgi A. , Lim G. K. , Wee A. T. S. , Tan K. L.
Published: July 2002

Surface and interface properties of CdTe/CdS/SnO2/glass heterojunction solar cells are studied by means of x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and optical reflectance (OR) techniques. First, n-type CdS layer was grown by solution growth technique on the SnO2 coated glass substrate and then the p-type CdTe was deposited on CdS by metal-organic chemical-vapor deposition. Despite many other efficiency limiting mechanisms in CdTe solar cells, this article shows that surfaces and interfaces play an important role in determining the cell efficiency. In an attempt to correlate the surface and interface properties to the cell performance, a series of CdTe/CdS solar cells with different conversion efficiencies were fabricated and analyzed. It was found that high efficiency cells possess Te-rich CdTe surface along with smooth interfaces, as revealed by XPS, SIMS, and OR measurements, while low efficiency cells display near stoichiometric or Cd-rich CdTe surface and abrupt interfaces. The impact and role of interface/surface properties on CdTe solar cell performance are discussed.
Electronic structure and magnetism in Sm2Fe17−xAx (A=Al,Ga,Si)
Authors: Sabirianov R. F. , Jaswal S. S.
Published: July 2002

Self-consistent spin–polarized electronic structure calculations have been carried out to derive the magnetic properties of the Sm2Fe17−xAx (A=Al, Ga, Si and x=0 and 1). The influence of impurities on the Curie temperature (TC) of Sm2Fe17 has been studied using the spin–fluctuation approach of Mohn and Wohlfarth. The calculated TC generally goes up with the addition of each impurity but the relative increase in TC depends on the type of the impurity and its site. The calculated TC enhancement weighted with the impurity-site occupancy from Mössbauer data are in good agreement with the experimental data.
Effect of O2(a1Δg) on plasma structures in oxygen radio frequency discharges
Authors: Shibata Mari , Nakano Nobuhiko , Makabe Toshiaki
Published: July 2002

Oxygen rf glow discharges between parallel plates were numerically analyzed by using the relaxation continuum model. The result at a frequency of 13.56 MHz, sustaining voltage of 150–350 sin ωt V, pressure of 0.15–1.0 Torr, and stainless steel surface, shows that O2(a1Δg) has a number density that is an order of magnitude larger than that of atomic oxygen. The plasma density as a function of pressure has a maximum at about 0.2 Torr, and decreases with increasing pressure due to the increase in the net rate of associative detachment from O− by O2(a1Δg). The comparison between the discharges in two surface materials, stainless steel and copper, indicates that the number densities of O2(a1Δg) and atomic oxygen strongly depend on the surface loss probability, and that consequently the plasma density is also changed by replacing the surface material.
Amorphization induced by ion mixing in Fe–Mo multilayered films: Correlation with the number of metal layers
Authors: Zhang Z. J.
Published: July 2002

In the Fe–Mo system, amorphous alloys are thermodynamically unstable and are hard to form in the bulk state. In multilayered films, however, amorphous alloys were produced by room temperature 200 keV xenon ion mixing. It was found, when keeping the total thickness of the multilayered film constant at 40 nm, that the composition range favoring amorphization extended from the equiatomic stoichiometry towards the two metal sides with increasing number of metal layers. This alloying behavior is discussed in terms of a simple thermodynamic calculation.
Quantitative mobility spectrum analysis of multicarrier conduction in semiconductors
Authors: Meyer J. R. , Hoffman C. A. , Antoszewski J. , Faraone L.
Published: July 2002

We demonstrate an optimized quantitative mobility spectrum analysis (QMSA) technique for determining free electron and hole densities and mobilities from magnetic-field-dependent Hall and resistivity data. The procedure is applied to an In1−xGaxAs–In1−xAlxAs single quantum well, GaAs–AlAs asymmetric double quantum wells, and Hg1−xCdxTe epitaxial thin films containing multiple carrier species. The results illustrate the reliability, versatility, and sensitivity of the analysis, which is fully computer automated following input of the magnetic-field-dependent data. QMSA is found to be a suitable standard tool for the routine electrical characterization of semiconductor material and device transport properties.
Size distribution and magnetic behavior of lead inclusions in silicon single crystals
Authors: Milants K. , Verheyden J. , Barancira T. , Deweerd W. , Pattyn H. , Bukshpan S. , Williamson D. L. , Vermeiren F. , Van Tendeloo G. , Vlekken C. , Libbrecht S. , Van Haesendonck C.
Published: July 2002

The size distribution of Pb inclusions formed by high-dose ion implantation in crystalline Si has been studied with a variety of experimental techniques. Results obtained from small angle x-ray scattering, transmission electron microscopy, and low-temperature magnetic moment measurements are compared. For samples implanted at room temperature, the results depend on which technique has been used, due to the amorphization of the silicon. The experiments on the samples implanted at an elevated temperature yield compatible results.
Accurate method for the determination of bulk minority-carrier lifetimes of mono- and multicrystalline silicon wafers
Authors: Schmidt Jan , Aberle Armin G.
Published: July 2002

An accurate method for the determination of the bulk minority-carrier recombination lifetime of crystalline silicon wafers of typical thickness (<0.5 mm) is presented. The method consists of two main steps: first, both wafer surfaces are passivated with silicon nitride films fabricated at low temperature (<400 °C) in a remote plasma-enhanced chemical vapor deposition system. Second, the effective minority-carrier lifetime of the wafer is measured by means of the contactless microwave-detected photoconductance decay technique. Due to the outstanding degree of surface passivation provided by remote-plasma silicon nitride films, the bulk minority-carrier lifetime can be very accurately determined from the measured effective minority-carrier lifetime. The method is suited for the bulk minority-carrier lifetime determination of p-type and n-type silicon wafers with doping concentrations in the 1014–1017 cm-3 range. We demonstrate the potential of the method for commercially available float-zone, Czochralski, and multicrystalline silicon wafers of standard thickness.
Wave propagation in a piezoelectric layer
Authors: Ramos R. R. , Otero J. A. , Pérez-Alvarez R.
Published: July 2002

The dispersion relations of oscillation modes in a piezoelectric slab with hexagonal symmetry are given, considering the slab infinite with respect to the axes x1 and x2 with the six-order symmetry axis perpendicular to the x1–x2 plane. Four types of modes are identified: two of them are transversal horizontal and the other two are associated to electrostatic potential waves which determine the longitudinal and flexural modes. The secular equations for these modes are given and numerical results for the piezoelectric transducer ceramic slab are obtained and compared to experimental results with a very good agreement.
Study of lasing action based on Förster energy transfer in optically pumped organic semiconductor thin films
Authors: Kozlov V. G. , Bulovic V. , Burrows P. E. , Baldo M. , Khalfin V. B. , Parthasarathy G. , Forrest S. R. , You Y. , Thompson M. E.
Published: July 2002

We present a study of optically pumped waveguide and microcavity lasers based on vacuum-deposited thin films of small molecular weight organic semiconductors. Lasing action in waveguide lasers is characterized by high output peak power (50 W), high differential quantum efficiency (70%), low lasing threshold (1 μJ/cm2), and long operational lifetime (>106 laser pulses at 100 times the threshold pump power). Microcavity laser characteristics include 3 W peak output power, 300 μJ/cm2 lasing threshold, and lifetimes of >106 pump laser pulses (operating at 6 times the threshold power). We demonstrate wavelength variability from 460 to 700 nm by changing the composition of the organic films. The confinement of excitations on the dopant molecules leads to quantum dot-like behavior such as high temperature stability of the lasing threshold, output power, and emission wavelength in the temperature range from 0 to 140 °C. The linewidth of laser emission from microcavity structures is found to be 0.2±0.1 Å and is transform limited by 40 ps long relaxation oscillations. We present a theoretical estimate for the linewidth of organic semiconductor laser emission which is found to be at least an order of magnitude smaller than for inorganic laser diodes. The prospects for realizing electrically pumped organic semiconductor lasers are considered.
Comparative study between macrotribology and nanotribology
Authors: Liu E. , Blanpain B. , Celis J.-P. , Roos J. R.
Published: July 2002

The behavior of different materials under macrotribological and nanotribological conditions has been compared. The materials, hydrogenated amorphous diamondlike carbon, highly ordered pyrolitic graphite, and mica, were submitted to a fretting test (macrotribological behavior) and a combined atomic force microscopy/lateral force microscopy (AFM/LFM) (nanotribological behavior). The coefficient of friction in the macroscopic regime under fixed experimental conditions considerably changed with the test duration, whereas the friction coefficient measured from the nanoscopic regime was relatively constant during the test. In the macroscopic regime, evident wear, elastoplastic deformation, and material transfer were noticed, while in the nanoscopic regime under the used testing conditions no wear phenomena were observed. The coefficient of friction showed some dependence on the amplitude and frequency of lateral movement of the sample relative to the counterbody in both the fretting and AFM/LFM tests as well as on the normal load in the fretting test. This work leads to the conclusion that although some similitudes could be found, the macrotribological behavior of materials cannot simply be related to their nanotribological behavior due to the different wear and friction processes.
Magneto-optical properties of FeAu alloys and Fe/Au superlattices
Authors: Gavrilenko V. I. , Wu R.
Published: July 2002

Magneto-optical Kerr effect (MOKE) of the Fe/Au superlattices and the FeAu L10 alloy has been studied by using the first principles full-potential linearized augmented plane-wave method. The calculated MOKE spectra show pronounced structures in the region from 2 to 6 eV, in agreement with experiment. The line shape of the MOKE spectra of Fen/Aun superlattices strongly depends on their lattice period (n) and interfacial interdiffusion.
Performance evaluation of high-power wide band-gap semiconductor rectifiers
Authors: Trivedi Malay , Shenai Krishna
Published: July 2002

Applicability of GaN in unipolar and bipolar devices for high-power electronic applications is evaluated with respect to similar devices based on other materials. Specific resistance is used as a measure of unipolar performance. In order to evaluate bipolar performance, 700 and 6000 V p-i-n diodes based on Si, 6H-SiC, and GaN are compared with respect to forward conduction and reverse recovery performance at room temperature and high-temperature conditions. It is shown that GaN is advantageous not only for high voltage unipolar applications, but also for bipolar applications. An empirical closed-form expression is presented to predict the avalanche breakdown voltage of wide band-gap semiconductors. Formulation of the expression is based on an approximation of the impact ionization coefficient in terms of seventh power of the electric field.
Structure and magnetic properties of PrFe10VxMo2−x compounds and their nitrides
Authors: Tang S. L. , Yin J. H. , Jin Z. Q. , Zhang J. R. , Zhang S. Y. , Du Y. W.
Published: July 2002

We have succeeded in synthesizing PrFe10VxMo2−x compounds (x=0–1.5) and their nitrides with ThMn12-type structure. The phase formation and magnetic properties have been investigated by x-ray diffraction, differential thermal analysis, and magnetic measurements. It is found that PrFe10V2 compound with ThMn12-type structure does not form and that PrFe10VxMo2−x compounds with ThMn12-type structure can be formed in the as-cast sample (x=0) or by annealing the as-cast samples in a lower temperature (x=0.5–1.5). The substitution of Mo for V in the compounds results in a decrease of the magnetization and Curie temperatures. The PrFe10VxMo2−xNy with x=0–1.5 has a Tc about 416–265 °C. The hard magnetic properties of PrFe10VMoNy magnets prepared by hydrogenation-disproportionation-desorption-recombination technique have been assessed.
Band gap narrowing in n-type and p-type 3C-, 2H-, 4H-, 6H-SiC, and Si
Authors: Persson C. , Lindefelt U. , Sernelius B. E.
Published: July 2002

Doping-induced energy shifts of the conduction band minimum and the valence band maximum have been calculated for n-type and p-type 3C-, 2H-, 4H-, 6H-SiC, and Si. The narrowing of the fundamental band gap and of the optical band gap are presented as functions of ionized impurity concentration. The calculations go beyond the common parabolic treatments of the ground state energy dispersion by using energy dispersion and overlap integrals from band structure calculations. The nonparabolic valence band curvatures influence strongly the energy shifts especially in p-type materials. The utilized method is based on a zero-temperature Green’s function formalism within the random phase approximation with local field correction according to Hubbard. We have parametrized the shifts of the conduction and the valence bands and made comparisons with recently published results from a semi-empirical model.
Magnetic and magneto-optic properties of orthoferrite thin films grown by pulsed-laser deposition
Authors: Schmool D. S. , Keller N. , Guyot M. , Krishnan R. , Tessier M.
Published: July 2002

Thin magnetic layers have been prepared by pulsed-laser deposition from targets of the orthoferrites: DyFeO3, GdFeO3, SmFeO3, and YFeO3. All layers were deposited onto quartz substrates at a temperature of 450 °C under high-vacuum conditions, i.e., in the absence of additional oxygen. The as-grown films show evidence of the following phases: RFeO3, R3Fe5O12, Fe3O4, and R2O3, where R=Dy, Gd, Sm, and Y. The magnetic properties of these films have been investigated using superconducting quantum interference device magnetometry and magneto-optic (Faraday rotation) measurements taken before and after annealing. These studies reveal that the orthoferrite phase is almost completely (>90%) recovered after an annealing treatment at 860 °C for 1 h. We also report on the temperature dependence of the Faraday rotation for these orthoferrites in the temperature range 25–250 °C.
Solvent dependent nonlinear refraction in organic dye solution
Authors: Sinha Sucharita , Ray Alok , Dasgupta K.
Published: July 2002

Nonlinear refraction in rhodamine-B (Rh-B) organic dye solution at 510.6 nm was studied using the Z-scan technique with 40 ns laser pulses from a copper vapor laser. The observed nonlinear refraction is of thermal origin resulting from nonradiative energy transfer from the dye molecules to the solvent molecules and is therefore strongly dependent on the thermal properties of the solvent. Using this scheme, the thermal properties of an aqueous dye solution were compared, with that using water-surfactant mixture as the solvent.
Mössbauer spectra of ferrofluids characterized using a many state relaxation model for superparamagnets
Authors: van Lierop J. , Ryan D. H.
Published: July 2002

Using a many-state relaxation formalism and combining it with an accurate expression for the relaxation time of single-domain ferromagnetic particles and a log-normal size distribution we are able to fit Mössbauer spectra which span temperatures from where a pure hyperfine field distribution is present to where a single broad line occurs due to high relaxation rates. With this model, we have fitted the Mössbauer spectra of two Fe3O4 ferrofluids, from 12 to 180 K. Fits to the ferrofluid with a mean particle size of 4.5 nm yield an anisotropy energy (K) of 3.0±0.1×104 J/m3 with the onset of magnetic relaxation at 30±5 K while data for a mean particle size of 6.0 nm give K=2.4±0.2×104 J/m3 with magnetic relaxation starting at 55±5 K. These blocking temperatures agree with those extrapolated from frequency dependent χac data.
Order–disorder effects in nitrided Sm–Fe permanent magnets
Authors: Meacham B. E. , Shield J. E. , Branagan D. J.
Published: July 2002

Rapidly solidified Sm11Fe89 was observed to form partially ordered Sm2Fe17. The ordering increased systematically with annealing temperature, reaching an order parameter of 0.8 after annealing at 950 °C for 15 min. The coercivity of the corresponding interstitial compound Sm2Fe17Nx varied with annealing temperature and thus order parameter. The coercivity reached a maximum for an order parameter of 0.45–5, then dramatically decreased as the order increased. This behavior is characteristic of demagnetization controlled by domain wall pinning by antiphase boundaries.
On the generalized Langevin equation: Classical and quantum mechanicala)
Authors: Cortés Emilio , West Bruce J. , Lindenberg Katja
Published: July 2002

The generalized Langevin equation and its attendant fluctuation–dissipation relation (FDR) for both classical and quantum systems is explictly derived for a large class of system-bath interaction potentials. We demonstrate for this class of potentials that the classical FDR involving only the temperature of the bath is satisfied, and that in general the decay times of the dissipative processes and of the system are temperature dependent. We also demonstrate that the quantum FDR depends in detail on the nature of the bath and on the specific system-bath interaction. Thus we conclude that while the classical Langevin equation is phenomenologically useful, its quantum counterpart is much more limited.
Coverage dependent effects on metal surfaces: O, S, F, and Cl on Ni
Authors: Bauschlicher Charles W.
Published: July 2002

The c(2×2) and p(2×2) vibrational frequencies for a rigid Ni lattice are computed for O, S, F, and Cl on Ni(100). It is found that only O/Ni(100) shows a large difference in ωre. A large shift in ωre is also found for O/Ni(111) between 0.25 and 0.5 monolayer coverages. Also associated with the change in coverage, is a change in the height above the surface R⊥. This varies from a 0.11 Å reduction for O/Ni(100), to an expansion of 0.06 Å for F/Ni(100).
Theoretical study of the H+O3↔OH+O2↔O+HO2 system
Authors: Dupuis M. , Fitzgerald G. , Hammond B. , Lester W. A. , Schaefer H. F.
Published: July 2002

The key features of the H+O3 potential energy surface have been determined using ab initio quantum mechanical methods. The electronic wave function used is a multiconfiguration Hartree–Fock wave function which provides a qualitatively correct description of various reactive channels. It is found that the H+O3→HO+O2 reaction proceeds along a nonplanar pathway in which the H atom descends vertically to the plane containing the ozone molecule to form an HO3 intermediate which then undergoes fragmentation. No planar transition state for a direct O-atom abstraction could be located. The radical–radical O+HO2 reaction was found to have no energy barrier to formation of HO3 which was determined to subsequently decompose to HO+O2. The H-atom abstraction reaction O+HO2→OH+O2 was found to have a small activation energy. The dynamical implications of these findings are discussed. The results are consistent with the observed vibrational excitation of the OH product in the H+O3 reaction. The key features of the H+O3 potential energy surface are expected to be transferable to the X+O3 systems where X=Cl, OH, NO, and NH2.
Embedded-cluster model for the effect of phonons on hydrogen surface diffusion on copper
Authors: Lauderdale Jack G. , Truhlar Donald G.
Published: July 2002

We treat surface diffusion of H on a (100) plane of copper by a model involving 21 degrees of freedom, three for the H and three each for six surface atoms. The six movable surface atoms are embedded on the surface of a bulk crystal. The interaction potential consists of pairwise H–Cu and Cu–Cu interactions, and the dynamics are treated by variational transition state theory with a small-curvature-approximation semiclassical adiabatic ground-state transmission coefficient. The classical barrier height for surface diffusion on the assumed potential energy surface is 11.7 kcal/mol, and we find an Arrhenius activation energy that increases from about 6 kcal/mol, below 160 K, to about 11 kcal/mol, above 400 K. The rate is dominated by tunneling at and below about 200 K. As compared to a treatment with a rigid surface the rate is increased by factors of 16, 3.1, 2.4, 1.6, and 1.3 at 110, 160, 200, 400, and 1000 K, respectively.
Sediment formation by Brownian dynamics simulation: Effect of colloidal and hydrodynamic interactions on the sediment structure
Authors: Ansell Geoffrey C. , Dickinson Eric
Published: July 2002

The formation of a gel-like sediment by a process of irreversible single-particle accretion has been simulated in three dimensions by the computational technique of Brownian dynamics. The model incorporates hydrodynamic interactions between spherical particles at the level of the Rotne–Prager approximation, and colloidal interactions are described in terms of the Derjaguin–Landau–Verwey–Overbeek (DLVO) pair potential of mean force for electrostatically stabilized particles. The density and structure of simulated sediments have been determined as a function of ionic strength and sedimenting field strength. Depending on the conditions, sediment volume fractions range from 0.09 to 0.33. Colloidal forces can significantly affect the short-range structure of low-density diffusion-controlled sediments. Sediment density increases strongly with increasing field strength, and sediment structures as measured by the particle pair distribution function become more liquid-like as the volume fraction increases. Hydrodynamic interactions have little effect on aggregate structure at low field strengths where Brownian aggregation is dominant, but the presence of hydrodynamic interactions is essential for the formation of high-density sediments at high field strengths. This work suggests that, in general, colloidal forces and hydrodynamic interactions cannot be neglected in simulations of colloidal aggregration and gelation.
Anomalous behavior of liquid K–Pb alloys: Excess stability, entropy, and heat capacity
Authors: Saboungi Marie-Louise , Leonard Susan R. , Ellefson Julie
Published: July 2002

Electromotive force measurements of the potassium activity in liquid K–Pb alloys are reported as a function of composition for T=640, 723, and 879 K and also as a function of temperature for compositions ranging from 2–70 at.% potassium. Thermodynamic mixing properties derived from these measurements indicate anomalous behavior around the equiatomic composition. The Darken excess stability function (or its equivalent the Bhatia–Thornton concentration fluctuations) shows only one well-defined extremum, which occurs at XK=0.52. Contrary to the case of both the Li–Pb and Na–Pb systems, no peak is detected in the excess stability function of K–Pb at the composition corresponding to A4Pb, where A refers to the alkali metal. The variations of the total entropy of mixing with composition exhibit features characteristic of ordered solutions with a sharp negative minimum at about 52 at. % potassium. These results are in consonance with published measurements of electrical conductivity and its temperature coefficient which indicate ordering in the liquid phase around the potassium–lead equiatomic composition. At this composition, in a temperature interval close to the melting point of the corresponding compound K⋅Pb, the composition dependence of the changes in the average heat capacity upon mixing goes through a maximum with a magnitude of about 62 J mol−1 K−1, which is atypical of metallic systems. This unexpected result provides a surprising contrast to prior work on other alkali–lead alloys.
Bistability in chemical reaction networks: Theory and application to the peroxidase–oxidase reaction
Authors: Aguda Baltazar D. , Clarke Bruce L.
Published: July 2002

Starting with a comprehensive list of elementary steps in the mechanism of the peroxidase–oxidase (PO) reaction, we extract the part of the mechanism essential to the experimentally observed bistability. A general systematic method is used to sort out the mechanism. First the extreme currents are found and the structure of the current polytope is determined. Then conditions for the existence of multiple steady states are used to identify the dominant extreme currents that are essential for bistability. A clue to the cause of bistability came from applying stoichiometric network analysis to the much simpler classical substrate-inhibition enzyme mechanism. Three extreme currents are essential for bistability. These correspond to (i) a reversible flux of the inhibiting substrate, (ii) the catalytic cycle, and (iii) an inhibition pathway coupled to the catalytic cycle. The same three elements are found in the PO mechanism. Analysis of the model containing these elements shows that bistability requires less ferriperoxidase than compounds I and II taken together. The PO model can exhibit damped oscillations simultaneously with bistability, simulate experimentally observed phases in the kinetics of the closed system, and demonstrate that enzyme inhibition by the substrate oxygen exists.
New measurements of microwave transitions in the water dimer
Authors: Coudert L. H. , Lovas F. J. , Suenram R. D. , Hougen J. T.
Published: July 2002

New measurements of ten K=1 lines, including six Q type and four R type, were made on the completely protonated species of the water dimer. For some of these lines, as well as for some K=0 transitions known from the literature, Stark coefficients were determined, and these Stark coefficients provide a confirmation of the assignments. The new K=1 measurements show that the splitting associated with the (HF)2-like tunneling motion decreases from about 19.5 GHz for K=0 to about 16.2 GHz for K=1. To understand the fact that K=1 lines are populated in our 1 K beam, we must assume, in accordance with the results of beam studies on other molecules, that levels of different nuclear spin modification relax separately. In an attempt to gain information on tunneling splittings other than that caused by the (HF)2-like motion, we have made new measurements on 1–0 and 2–1 transitions with K=0 for several partially deuterated species, in which the (HF)2-like motion cannot occur. Small splittings ranging from 4 to 145 MHz were observed. Because of the nature of the tunneling motions involved, these new data yield only the difference of the tunneling splitting in the upper and lower states of the transition.
Fluids in micropores. II. Self-diffusion in a simple classical fluid in a slit pore
Authors: Schoen M. , Cushman J. H. , Diestler D. J. , Rhykerd C. L.
Published: July 2002

Self-diffusion coefficients D are computed for a model slit pore consisting of a rare-gas fluid confined between two parallel face-centered cubic (100) planes (walls) of rigidly fixed rare-gas atoms. By means of an optimally vectorized molecular-dynamics program for the CYBER 205, the dependence of D on the thermodynamic state (specified by the chemical potential μ, temperature T, and the pore width h) of the pore fluid has been explored. Diffusion is governed by Fick’s law, even in pores as narrow as 2 or 3 atomic diameters. The diffusion coefficient oscillates as a function of h with fixed μ and T, vanishing at critical values of h, where fluid–solid phase transitions occur. A shift of the pore walls relative to one another in directions parallel with the walls can radically alter the structure of the pore fluid and consequently the magnitude of D. Since the pore fluid forms distinct layers parallel to the walls, a local diffusion coefficient D(i)∥ associated with a given layer i can be defined. D(i)∥ is least for the contact layer, even for pores as wide as 30 atomic diameters (∼100 Å). Moreover, D(i)∥ increases with increasing distance of the fluid layer from the wall and, for pore widths between 16 and 30 atomic diameters, D(i)∥ is larger in the center of the pore than in the bulk fluid that is in equilibrium with the pore fluid. The opposite behavior is observed in corresponding smooth-wall pores, in which the discrete fluid–wall interactions have been averaged by smearing the wall atoms over the plane of the wall. The temperature dependence of D for fixed h is determined and the nature of melting of a pore solid is examined. It is found that the solid tends to melt first in the middle of the pore. All of the various results are related to the structural properties of the pore fluid, as manifested by the local density and pair correlation functions.
Water dimer tunneling states with K=0
Authors: Odutola J. A. , Hu T. A. , Prinslow D. , O’dell S. E. , Dyke T. R.
Published: July 2002

Tunneling–rotational transitions of water dimer with K=0 have been observed and assigned in the radio frequency and microwave region of the spectrum. Rotational constants and electric dipole moments were obtained from these spectra. The rotational constants show surprisingly large variations with tunneling state for (H2O)2, but not for (D2O)2, indicating that the former species may be following behavior characteristic of a low-barrier tunneling case. A tunneling splitting of 19 526.73 MHz has been observed for water dimer and 1172.23 MHz for the completely deuterated species. The nuclear hyperfine structure of (H2O)2 radio frequency transitions has been assigned and was quite useful for determining the symmetries of the observed states. The nuclear spin–spin coupling constants have been interpreted in terms of the tunneling state of observation and of the water dimer structure.
A theoretical investigation of the astrophysically important molecules C3O and HC3O+
Authors: Botschwina Peter
Published: July 2002

Large-scale CEPA calculations have been carried out for the astrophysically important molecules C3O and HC3O+. C3O has a linear equilibrium geometry with short terminal CC and CO bond lengths of 1.273 and 1.149 Å and a central CC equilibrium bond length of 1.300 Å. Upon protonation the terminal CC and CO bond lengths are shortened by 0.061 and 0.026 Å, respectively, while the central CC bond length experiences an elongation by 0.052 Å. C3O has a noticeably large equilibrium dipole moment of 2.535 D with the positive end at the oxygen site. The IR intensity of the ν1 band at 2229 cm−1 is extremely large (72 756 cm2 mol−1). Although the intensity of the corresponding ν2 band of HC3O+ at 2318 cm−1 is only about one half as large this band appears to be most promising for future IR diode laser investigation. The proton affinity of C3O at 298 K is 885±5 kJ mol−1.
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o study of boron, nitrogen, and boron–nitrogen clusters. I. Isomers and thermochemistry of B3, B2N, BN2, and N3
Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

For a number of different structures and states of B3, B2N, BN2, and N3, optimum geometries and harmonic spectra were obtained at the HF/6-31G* level. The relative stability of the isomers was determined using full fourth-order Mo/ller–Plesset theory, both with and without spin projection, as well as coupled cluster methods. Estimates for the dissociation energies are based on scaled CCD+ST(CCD) binding energies. Koopmans’ vertical ionization potentials and Mulliken charge distributions, both at the UHF/6-31G* level, are quoted for the most stable isomers. B3 is found to be an equilateral triangle in its 2A′1 ground state. B2N has a symmetric linear arrangement in its 2Σ+u ground state with an extremely low bending frequency (73 cm−1), and an unusually low vertical ionization potential (6.75 eV). Its asymmetric stretching (2021 cm−1) is found to be extremely intense (8782 km mol−1). BN2 has four rather closely spaced states, of which an isosceles triangle is the absolute minimum (2A1 state). However, at high temperatures, an asymmetric linear arrangement (2Π state) is found to have equal importance, whereas a 4Σ− state plays a role there too. The same theoretical methods correctly predict for N3 a symmetric linear arrangement in the 2Πg ground state; the spectroscopic constants are found to be in reasonable agreement with experiment. Estimated dissociation energies (expected accuracy ±4 kcal mol−1) are: B3 197.9, B2N 265.0, BN2 224.9, N3 210.1 kcal mol−1. From a statistical thermodynamical analysis, B3 is stable against dissociation to B2 and B up to very high temperatures, B2N is extraordinarily stable, whereas BN2 and N3 dissociate spontaneously to B+N2 and N+N2 at all temperatures. From these results, the presence of B2N+ and B+3, the high abundance of B2N+, as well as the absence of BN+2 and N+3 in laser mass spectra of boron nitride is explained.
Solution of the Percus–Yevick approximation of the multicomponent adhesive spheres system applied to the small angle x-ray scattering from microemulsions
Authors: Robertus C. , Philipse W. H. , Joosten J. G. H. , Levine Y. K.
Published: July 2002

This paper describes the application of the numerical solution of the Percus–Yevick approximation for a multicomponent system of sticky hard spheres to small angle x-ray scattering experiments. The effect of polydispersity on the pair correlation function is quantitatively described, thus enabling the analysis of experimental data. In this way quantitative estimates of the size distribution width, interaction radii and the measure of attraction between particles can be obtained. The attractive interaction in a system of sticky spheres is known to give rise to a transition analogous to a gas–liquid phase transition. The effect of polydispersity on this transition is discussed. Finally it is shown how the model can be used to describe small angle x-ray scattering from an AOT microemulsion system.
Neon matrix electron spin resonance and theoretical investigations of 10,11BH2, 12,13CH−2, and 12,13CH+2
Authors: Knight Lon B. , Winiski M. , Miller P. , Arrington C. A. , Feller David
Published: July 2002

The BH2 and CH−2 radicals have been generated and trapped in rare gas matrices for ESR (electron spin resonance) investigation. Theoretical calculations were conducted on these radicals and the CH+2 cation for detailed comparison with the experimental results. The observed magnetic parameters (MHz) for 13 CH−2 in neon matrices at 4 K were: giso =2.0027(3); 13 C; ‖Aiso‖=59(1) and H; ‖Aiso‖=45(1). For 11 BH2 in argon at 4 K the results were: giso =2.0016(3); 11 B; Aiso=358(2) and H; ‖Aiso‖=38(1). The electronic structure information obtained for these small radicals from theoretical and experimental results is discussed and compared with other related molecules.