bluuebunny/crossref_metadata_2025 huggingface dataset:
ElPed: An interactive PC-based learning program for basic electrical circuits
Authors: Johansen G. A. , Nordtvedt J. E. , Vold P.
Published: July 2002

A survey among 400 upper secondary school physics students reveals serious difficulties in their understanding of the dynamics of fundamental electrical circuits. The students are unable to explain how currents and voltages vary when a dynamic change in a resistance is introduced. In order to evaluate the effect of computer-assisted learning in this field, an electrotechnical pedagogical program (ElPed) has been developed. The program is divided into three sections: circuit dynamics simulation, questionnaire, and reasoning. The structure and function of the program are discussed in detail. The major goal of the program has been to demonstrate the dynamics of fundamental electrical circuits and develop the students’ ability to perform systematic and structured reasoning in a simple and highly interactive manner. This implies that the students can work on the problems at their own pace until they master the whole reasoning process satisfactorily. A second survey, where the effect of ElPed is evaluated, is also presented. The results indicate that the students that have used ElPed perform better, and, on questions where complex reasoning is required, significantly better.
Time evolution and decay of an excited atom in a weak electric field
Authors: Wang J. B.
Published: July 2002

A Mathematica notebook for describing the time evolution and decay of the hydrogen n=2 states in the presence of a weak external electric field is presented. The work involves (1) solving a set of differential equations coupled by the Hamiltonian of the external electric field and (2) deriving a set of formulas for a complete description of the polarization state of the emitted photons. It is demonstrated how problems with such complexity can be treated with ease and in an error-free manner by using symbolic software such as Mathematica. © 1996 American Institute of Physics.
Boundary elements and surface plasmons
Authors: Goloskie R. , Thio T. , Ram-Mohan L. R.
Published: July 2002

We present an elementary introduction to the boundary element method (BEM). We use the BEM to calculate the electromagnetic field enhancements in the vicinity of metallic wires of arbitrary cross-section. The local electric field shows resonant peaks, as a function of the frequency of incident radiation, at the natural frequencies of the electronic plasma in the wires. We consider the effect of the shape of the cross-section of the wire on the resonant frequency, the proximity effect due to neighboring wires, and the influence of a substrate. The calculations are performed using enhanced continuity conditions at the boundaries. We discuss the advantages of such an approach. The field enhancements are predicted for a number of geometries. © 1996 American Institute of Physics.
Magnetomechanical properties of flash annealed Metglas 2605 SC
Authors: Kabacoff L. T. , Wun-Fogle M.
Published: July 2002

Ribbons of Metglas 2605 SC were flash annealed at various temperatures and under various tensile stresses. The resulting samples exhibited a magnetomechanical coupling K33 of 0.37, independent of the annealing temperature or the magnitude of the stress. Secondary annealing in a strong magnetic field greatly improved the value of K33. As quenched ribbons subjected to the same field annealing had significantly smaller values of K33, except for relatively long anneals (greater than two hours). Secondary annealing in a small magnetic field produced a much smaller increase in K33.
Growth and characterization of GaAs/Ge epilayers grown on Si substrates by molecular beam epitaxy
Authors: Sheldon P. , Yacobi B. G. , Jones K. M. , Dunlavy D. J.
Published: July 2002

Epitaxial GaAs/Ge films are grown by molecular beam epitaxy (MBE) on Si substrates. The effect of various MBE growth conditions on the sample morphology, the defect density, and the optical properties of GaAs and Ge epilayers is examined. Scanning electron microscopy, plan-view and cross-sectional transmission electron microscopy, reflection high-energy electron diffraction, and photoluminescence are used to characterize epitaxial layers. It is found that the defect density decreases with increasing epilayer thickness. This is due to an annihilation process that affects both threading dislocations and stacking faults. The substrate temperature during Ge growth is found to affect the properties of both the Ge and GaAs films. GaAs surface morphology degrades and the stacking fault density increases at high Ge buffer-layer substrate temperatures; however, the threading dislocation density remains unchanged. Variations in growth conditions are correlated with defect densities and luminescence efficiencies to determine material quality and optimize growth conditions.
Highly conductive and wide band gap amorphous-microcrystalline mixed-phase silicon films prepared by photochemical vapor deposition
Authors: Nishida Shoji , Tasaki Hirohisa , Konagai Makoto , Takahashi Kiyoshi
Published: July 2002

Doped hydrogenated amorphous-microcrystalline mixed-phase silicon (μc-Si:H) films were prepared by the mercury photosensitized decomposition of a disilane-hydrogen gas mixture, by adding phosphine and diborane for n and p type, respectively. The maximum dark conductivity and optical band gap of the films were, respectively, 20 S cm−1 and ∼2.0 eV for n type, and 1 S cm−1 and 2.3 eV for p type. It is most significant that the gaseous ratio of hydrogen to disilane should be enhanced up to 70 for n type, and even 300 for p type, to obtain such a highly conductive and wide gap film. The crystallinity of the photodeposited μc-Si films appeared to be improved in comparison with that of films by conventional plasma glow discharge technique.
Effect of sputtering pressure on the structure and solid-state reaction of titanium-nickel compositionally modulated film
Authors: Clemens B. M.
Published: July 2002

The structure and annealing behavior of compositionally modulated nickel-titanium films is a function of the sputter deposition pressure. Films with a composition modulation wavelength of 20 close-packed atomic planes of each constituent per layer were prepared with argon sputter deposition pressures of 2, 5, and 10.2 mTorr. The structure was studied with x-ray diffraction in symmetric reflecting, and transmission geometries, and by x-ray rocking curves. Anneals of 1 h at 300 °C were performed to access the effect of the structure on solid-state formation of an amorphous alloy. Samples prepared under low deposition pressure exhibited textured, relatively high quality crystalline layers resulting from layer nucleation and growth. These films were not transformed to an amorphous phase by an anneal. Higher deposition pressures resulted in island nucleation and growth, random orientation, and a higher degree of disorder. Amorphous phase growth was observed during the anneal of the sample prepared at 10.2 mTorr of Ar. Increase in deposition pressure will result in lower impact energy of the deposited species, which will result in lower atomic mobility. Higher mobility with the negative surface tension between nickel and titanium will result in layer nucleation and growth. The disorder in the high deposition pressure samples may be acting as a nucleus for amorphous phase growth.
The effect of a Ga prelayer on the beginning of GaAs epitaxy on Si
Authors: Bringans R. D. , Olmstead M. A. , Ponce F. A. , Biegelsen D. K. , Krusor B. S. , Yingling R. D.
Published: July 2002

The formation of GaAs islands is a major effect at the beginning of GaAs-on-Si epitaxy. The density of nucleation sites for the islands and the manner of their subsequent coalescence will influence the dislocation density of the final GaAs film. In this paper the effect on GaAs-on-Si epitaxy of a Ga-prelayer treatment is studied with photoemission core level spectroscopy and high resolution transmission microscopy (HTEM). Experiments are carried out with GaAs film thicknesses in the range from one monolayer to around 50 nm. Core level spectroscopy results for the monolayer films give information about the bonding character at the interface and suggest methods of improving the degree of two-dimensional growth. A particular Ga-prelayer technique is examined with HTEM using a wedge-shaped GaAs-on-Si sample. This allows side-by-side comparisons of areas with and without the Ga prelayer as a function of GaAs thickness. At thicknesses of around 5 nm, it is shown that the Ga prelayer yields islands with a lower wetting angle than those obtained with no prelayer. The Ga-prelayer technique also gives better surface morphology at thicknesses of around 50 nm.
Elimination of thermally induced biaxial stress in GaAs on Si layers by post-growth patterning
Authors: van der Ziel J. P. , Chand Naresh , Weiner J. S.
Published: July 2002

The biaxial tensile stress of 2.65 kbar in as-grown GaAs/Si is reduced by post-growth patterning of the GaAs and the reduction in stress is dependent on the pattern size and shape. For stripe patterns less than 15 μm wide the stress becomes largely uniaxial with stress relief normal to the stripe direction. Rectangular patterns exhibited stress relief in orthogonal directions, and have the lowest stress in the narrow direction of the rectangle. A 9×12 μm2 rectangle exhibited an average stress of 0.5 kbar.
Nondestructive evaluation of adhesive joints by guided waves
Authors: Nagy Peter B. , Adler Laszlo
Published: July 2002

Guided waves in an adhesive layer between two adherend half-spaces are shown to be uniquely sensitive to most types of bond defects. A simple experimental technique based on ultrasonic transmission mesurement is introduced to detect leaky guided modes in the interface layer. Experimental results for these dispersive guided modes are shown to be in good quantitative agreement with theoretical predictions. The suggested technique might find numerous applications in nondestructive evaluation of different bonds of layered structure such as adhesive and brazed joints.
The effects of scattering on current-voltage characteristics, transient response, and particle trajectories in the numerical simulation of resonant tunneling diodes
Authors: Jensen K. L. , Buot F. A.
Published: July 2002

The relaxation-time approximation is used in the numerical simulation of the Wigner distribution function to incorporate scattering. The effects within the constant relaxation-time approximation are (a) a decrease in the peak-to-valley ratio of the current-voltage curve; (b) a reduction in the oscillations of the Wigner distribution function, especially at resonance bias; (c) a suppression of the decay time of current oscillations after a sudden bias shift, indicating a smaller switching time than for no scattering; (d) a degradation in the resonant tunneling trajectories towards the characteristics of nonresonant trajectories; (e) a decrease in the spatial range of the quantum influences near resonance; and (f) ballistic transport sets in [i.e., the mean free path of the electrons is greater than the barrier region (110 Å)] for temperatures less than 74 K.
Surface chemical processes in metal organic molecular-beam epitaxy; Ga deposition from triethylgallium on GaAs(100)
Authors: Murrell A. J. , Wee A. T. S. , Fairbrother D. H. , Singh N. K. , Foord J. S. , Davies G. J. , Andrews D. A.
Published: July 2002

The adsorption of triethylgallium on the GaAs (100) (4×1) surface has been studied using the techniques of low energy electron diffraction, x-ray photoelectron and Auger spectroscopies, high resolution electron energy loss spectroscopy and temperature-programmed desorption. Condensed multilayers of the organometallic compound formed following adsorption at 150 K desorb from the surface at ∼170 K to leave a chemisorbed molecular monolayer of triethylgallium. Upon further heating this layer partially desorbs and partially decomposes to form diethylgallium in two competing processes. The diethylgallium so formed can also desorb or otherwise decompose ultimately to adsorbed Ga atoms in a reaction which results in the formation of hydrogen, ethene, and ethane. The temperature-programmed desorption characteristics of these latter species are found to be similar to those observed for a dissociated layer of ethyl bromide. A reaction scheme is proposed to account for the observations and kinetic parameters are obtained from computer modeling of the temperature-programmed desorption results. The reaction scheme is also used to evaluate the temperature-dependent growth rate expected in metal organic molecular beam epitaxial growth of GaAs. Comparison with experimental results is made and the work is discussed in the light of the previous model which has been proposed for the epitaxial growth of GaAs by metal organic molecular-beam epitaxy.
Raman analysis of laser-annealed amorphous carbon films
Authors: Bowden M. , Gardiner D. J. , Southall J. M.
Published: July 2002

Raman microline focus spectrometry (MiFS) has been used to show that hydrogen containing amorphous carbon (a-C-H) films are readily annealed by a focused argon ion (514.5 nm) laser operating at a power typical of that used for conventional Raman microscopy. The annealing process produces microcrystallites of disordered graphite localized to a 2-μm-diam region within the laser spot focus. Annealing threshold power densities vary from film to film, but are of the order 1500 W mm−2.
The decrease of ‘‘random telegraph signal’’ noise in metal-oxide-semiconductor field-effect transistors when cycled from inversion to accumulation
Authors: Dierickx B. , Simoen E.
Published: July 2002

The low-frequency (LF) noise behavior of metal-oxide-semiconductor field-effect transistors (MOSFETs) is studied when cycled between inversion and accumulation. On large-area devices the decrease of the LF noise is systematically found, and supports the observations by Bloom and Nemirovsky [Appl. Phys. Lett. 58, 1664 (1991)]. The random telegraph signal (RTS) noise observed in small (submicrometer) devices disappears when the transistor is cycled into accumulation. The drop in LF noise observed may thus be explained by the fact that most or all of the RTSs, which are caused by carrier trapping into slow oxide states, no longer contribute to the noise of the system. The method indicates a possibility to separate the contributions of different sources of 1/f noise in MOSFETs.
Preparation and properties of (Pb,La)(Zr,Ti)O3 thin films by metalorganic chemical vapor deposition
Authors: Okada Masaru , Tominaga Koji
Published: July 2002

Lanthanum-modified lead zirconate titanate (PLZT) thin films have been grown on Pt/MgO(100) and Pt/SiO2/Si substrates at 650 °C by metalorganic chemical vapor deposition using Pb(C2H5)4, La(DPM)3, Zr(DPM)4 and Ti(i − OC3H7)4 as the source materials. The properties of PLZT films were investigated as a function of the La content in a range of 0–15 at.% with a composition ratio of Zr/(Zr+Ti)<0.5. The films were of a single perovskite structure, and highly oriented to the c axis of the tetragonal phase on a Pt/MgO(100) substrate. The films were transformed from tetragonal to nearly cubic as the La content was increased. The films were colorless and transparent, and the surface morphology became smoother through an La addition. The relative dielectric constants increased as the La content was increased up to about 5 at.%. The remanent polarizations were 25–30 μC/cm2, and the coercive field decreased from 53 to 30 kV/cm as the La content was increased in the films having Zr/Ti=40/60.
Modeling the dc superconducting quantum interference device coupled to the multiturn input coil. III
Authors: Enpuku K. , Cantor R. , Koch H.
Published: July 2002

The radio frequency (rf) properties of a dc superconducting quantum interference device (SQUID) coupled to a multiturn input coil have been studied both experimentally and theoretically using an expanded model of the coupled SQUID. The input coil and washer resonances observed in the measured impedance of the expanded model are shown to be in quantitative agreement with the theory presented previously. When the input coil resonances are suppressed by resistive damping, it is shown that the impedance of the coupled SQUID at low frequencies is determined solely by an effective inductance that is much lower than the SQUID inductance. A simple equivalent circuit of the coupled SQUID for this case is proposed. The rf properties of SQUIDs with gradiometric washer configurations are also discussed. It is shown that the rf properties of these devices can be predicted from the properties of the individual washers.
Nucleation of diamond during hot filament chemical vapor deposition
Authors: Singh Jogender , Vellaikal M.
Published: July 2002

We have investigated the mechanism of nucleation of diamond during hot filament chemical vapor deposition process by high resolution transmission electron microscopy. Diamond was deposited on copper grids which were directly mounted on a high resolution transmission electron microscopy holder in order to study nucleation and growth characteristics of crystalline diamond. We have found direct evidence for the formation of a diamond-like amorphous carbon layer 8–14 nm thick in which small diamond microcrystallites about 2–5 nm across were embedded. These diamond microcrystallites were formed as a result of direct transformation of diamond-like carbon into diamond. Large diamond crystallites were observed to grow from these microcrystallites. It is envisaged that the diamond microcrystallites present in the amorphous diamond-like carbon layer provided nucleation sites for diamond growth on which the large diamond crystallites grew.
The magneto-acoustic emission (invited)
Authors: Guyot M. , Cagan V.
Published: July 2002

When cycling a hysteresis loop, bursts of high frequency ultrasonic waves are emitted. This general phenomenon, which must not be confused with the classical magnetostrictive effect at twice the cycling frequency, is called the magnetoacoustic emission (MAE). By using literature and authors’ results, qualitative and quantitative, the authors review the past and present knowledge of the MAE phenomenon to stress its fundamental origin: the domain wall (DW) creation/annihilation processes. On a qualitative point of view, the shape demagnetizing effect is shown to drastically change the MAE profiles, which allow us to clearly distinguish the MAE from the classical magnetic Barkhausen noise. Quantitatively and in agreement with their interpretation, the authors have established that the total MAE activity along an hysteresis loop is proportional to the hysteresis losses during the same loop; this law has been confirmed on several series of polycrystalline ferrimagnets and is valid whatever the composition, the grain size, or the measuring temperature is. The noncorrelation between the MAE and the magnetostriction is also demonstrated, contrary to some previously reported results and interpretations. Related to the DW creation/annihilation processes, the MAE might be the missing link in the process of the conversion of the magnetic energy into heat during a hysteresis loop.
Facet heating of quantum well lasers
Authors: Chen G. , Tien C. L.
Published: July 2002

This work investigates the temperature rise and heating mechanisms at the facets of quantum well lasers. An analytical solution of the heat conduction equation yields the temperature distribution in the laser and the temperature rise at the laser facets. The heat generation mechanisms are discussed and modeled through a one-dimensional carrier diffusion equation. The normalized results from the models agree well with available experimental data but the absolute value of the maximum temperature rise is about 5 times lower than that of the measurement. This discrepancy is explained by the reduction of thermal conductivity caused by phonon reflection and transmission at the GaAs/AlGaAs interfaces. Averaging the calculated results over a probe diameter around 1.5 μm, as is often used in the microprobe Raman spectroscopy measurement of the facet-temperature rise, reveals that the actual peak temperature at the facet is only 2–5 times higher than the measured value. This is a surprising result considering that the probe diameter is about two orders of magnitude larger than the active region thickness. A detailed examination of the calculation and the existing experimental data suggests a new explanation for the thermal runaway process in quantum well lasers. It is the onset of the absorption by the cladding media, rather than by the active region itself as is commonly believed, that provides the driving force for the thermal runaway process.
Tunneling behavior of extremely low resistance nonalloyed Ti/Pt/Au contacts to n(p)-InGaAs and n-InAs/InGaAs
Authors: Stareev G. , Künzel H.
Published: July 2002

Extremely low resistance nonalloyed Ti/Pt/Au contacts have been formed to n-InGaAs, p-InGaAs, and n-InAs/InGaAs layers with doping concentrations ranging from 1 to 5×1019 cm−3 for n-type and from 2 to 1×1020 cm−3 for p-type material. The comparative studies reveal specific contact resistances as low as 1.7×10−8 Ω cm2 for the n-InAs/InGaAs system, while the best values obtained for n-InGaAs and p-InGaAs are 4.3 and 4.8×10−8 Ω cm2, respectively. The electrical behavior of the contacts is discussed in relation to the effect of ion beam cleaning and post-deposition annealing. The use of low energy (60 eV) Ar+ bombardment provides atomically clean, contamination free surfaces. Very rapid thermal processing at 400 °C for 1 s was successfully employed in order to restore the original properties of the subsurface layer disordered during ion bombardment. This fabrication sequence ensures formation of intimate contacts without interfacial films and carrier compensation effects. The variation of the specific contact resistance with the reciprocal square root of the carrier concentration indicates that tunneling is the dominant mechanism of current flow through the metal-semiconductor junction. The tunneling origin of the contact characteristics has also been elucidated from the temperature dependence of the contact resistance.
Effective ionization and dissociation rate coefficients of molecular hydrogen in plasma
Authors: Sawada Keiji , Fujimoto Takashi
Published: July 2002

A simplified collisional-radiative model has been constructed for the system of the ground state, electronically excited stable states, and the ionic state of molecular hydrogen in plasma. Effective rate coefficients have been calculated for production of electrons, molecular ions, protons, and hydrogen atoms from molecular hydrogen. The ratio of the effective ionization rate of molecular hydrogen to the Balmer α photon emission rate and the effective rate coefficients for radiation and energy losses are also presented.
Comparative analysis of optically pumped intersubband lasers and intersubband Raman oscillators
Authors: Khurgin Jacob B. , Sun Greg , Friedman Lionel R. , Soref R. A.
Published: July 2002

A new tunable source of the far-infrared radiation based on intersubband electronic Raman scattering in semiconductor quantum wells is proposed. The gain and threshold of the proposed Raman oscillator are estimated and compared with the intersubband laser.
Room temperature oxidation enhancement of porous Si(001) using ultraviolet–ozone exposure
Authors: Thompson W. Howard , Yamani Zain , Abu Hassan Laila H. , Greene J. E. , Nayfeh Munir , Hasan M.-A.
Published: July 2002

The interaction of porous silicon (001) with ozone and ultraviolet (uv) was investigated at room temperature and ambient pressure. The O/Si atomic ratio in oxidized samples, as measured by Auger depth profiling, was similar to those measured from thermally oxidized samples. For long exposure times (≳15 min), the O/Si ratio stayed constant throughout the porous layer, indicating full oxidation of the porous structure. Additionally, the carbon concentration decreased significantly in the uv–ozone oxidized layers as compared to the untreated porous layers. Oxide layers up to 400 nm were measured. Fourier transform ir absorption measurements show both an increase in the oxygen concentration and a decrease in hydrogen concentration after oxidation. Photoluminescence data show that this method of oxidation quenches light emission from the porous layer. We believe that uv induced excitation of valence electrons results in higher reactivity and diffusion of oxygen and ozone as well as bond softening. The large volume of voids in the porous structure assist penetration of ozone and atomic oxygen throughout the porous structure while the narrow lateral dimensions of the Si skeleton requires a short diffusion length for full oxidation.
Microstructure and magnetism in FeTaN films deposited in the nanocrystalline state
Authors: Viala B. , Minor M. K. , Barnard J. A.
Published: July 2002

As-deposited, magnetically soft nanocrystalline FeTaN films are successfully grown by dc-magnetron reactive sputtering. Growth conditions are instrumental in extending the solubility limit of Ta in the bcc FeTa alloy. Nitrogen incorporation in FeTa films is found to be much higher than in Fe films and can be explained in terms of thermochemistry using a large Ta-N interaction coefficient. The influence of different alloying elements is discussed theoretically, with regard to the metal-nitrogen affinity. A ‘‘typical columnar microstructure’’ associated with the sputtering process is identified and its evolution versus the extent of nitrogenation is described in detail. Stress, magnetostriction, resistivity, and magnetic properties are respectively described as a function of both Ta and N contents of the films. The magnetic behavior of as-deposited nanocrystalline FeTaN is found to be very sensitive to both the dimension of the grains, their morphology and the nature of the grain boundary material which represents a non-negligible volume fraction in nanocrystalline films. It is proposed that the columnar structure plays the key role in promoting a large perpendicular anisotropy component (K⊥) and controls a ‘‘Stripe Domain’’-like behavior observed at high N contents, which cannot be explained in terms of film stress in this material. The contribution of the magnetoelastic anisotropy is also described. In summary, by breaking the columnar structure, the incorporation of nitrogen first decreases K⊥ below the critical limit for formation of stripe domains. In these conditions, N acts as a ‘‘grain refiner’’ and excellent soft magnetic behavior is reported and explained in terms of ‘‘vanishing magnetocrystalline anisotropy.’’ The good thermal stability of such soft films is confirmed. By contrast, higher nitrogen incorporation increases K⊥ above the critical limit, leading to a stable stripe domain-like structure which does not allow for soft magnetic properties. This phenomenon has been found to be reversible at low temperature where a complete restoration of the soft magnetic behavior has been observed. This anomalous result is explained by the transformation of the grain boundary material into a ‘‘low Curie temperature phase’’ for large N contents.
Dielectric properties of nematic liquid crystals in the ultralow frequency regime
Authors: Murakami Shuichi , Iga Hironori , Naito Hiroyoshi
Published: July 2002

The dielectric properties of nematic liquid crystal (4-cyano-4′-n-pentylbiphenyl: 5CB) cells in the ultralow frequency regime was investigated. A dielectric relaxation, whose relaxation time is 160 s, is observed at 303 K, and the dielectric relaxation is found to be independent of applied electric field. It is shown that the dielectric relaxation is caused by the Helmholtz double layer formed by the adsorption of impurity ions in 5CB onto the electrode surfaces of the cells, and hence the thickness of the double layer is comparable to the radius of impurity ions in 5CB. The dielectric relaxation obeys the empirical Cole-Cole circular arc law, indicating that dielectric relaxation times are distributed. The distribution of dielectric relaxation times can be explained in terms of distributed thicknesses of the Helmholtz double layer.
Long period two-dimensional gratings for 8–12 μm quantum well infrared photodetectors
Authors: Pan Dong , Li Jinmin , Zeng Yiping , Kong Meiying
Published: July 2002

By considering all possible high order diffracted waves, the authors investigate the spectral response of two-dimensional gratings for quantum well infrared photodetectors (QWIPs). A new method is proposed that using long period gratings may improve grating quality and reduce the resulting cross talk in grating-coupled QWIPs. A sensitivity analysis indicates that the influence of variation of the grating constant on the coupling efficiency is less sensitive for the long period gratings than for the short ones. A large coupling efficiency has been demonstrated for long period gratings. The calculated wide grating response spectra are in good agreement with the experiment result.
Site-selective excitation and polarized absorption and emission spectra of trivalent thulium and erbium in strontium fluorapatite
Authors: Gruber John B. , Wright Andrew O. , Seltzer Michael D. , Zandi Bahram , Merkle Larry D. , Hutchinson J. Andrew , Morrison Clyde A. , Allik Toomas H. , Chai Bruce H. T.
Published: July 2002

Polarized fluorescence spectra produced by site-selective excitation, and conventional polarized absorption spectra were obtained for Tm3+ and Er3+ ions individually incorporated into single crystals of strontium fluorapatite, Sr5(PO4)3F, also known as SFAP. Substitution of the trivalent rare earth ion for divalent strontium was achieved by passive charge compensation during Czochralski growth of the fluorapatite crystals. Spectra were obtained between 1780 and 345 nm at temperatures from 4 K to room temperature on crystals having the hexagonal structure [P63/m(C6h2)]. The polarized fluorescence spectra due to transitions from multiplet manifolds of Tm3+(4f12), including D21, G41, and H43 to manifolds H63 (the ground-state manifold), F43, H53, H43, and F33 were analyzed for the details of the crystal-field splitting of the manifolds. Fluorescence lifetimes were measured for Tm3+ transitions from D21, G41, and H43 at room temperature and from G41 at 16 K. Results of the analysis indicate that the majority of Tm3+ ions occupy sites having Cs symmetry. A point-charge lattice-sum calculation was made in which the crystal-field components, Anm, were determined assuming that trivalent thulium replaces divalent strontium in the metal site having Cs symmetry. Results support the conclusion that the nearest-neighbor fluoride (F−) is replaced by divalent oxygen (O2−), thus preserving overall charge neutrality and local symmetry. Crystal-field splitting calculations predict energy levels in agreement with results obtained from an analysis of the experimental data. By varying the crystal-field parameters, Bnm, we obtained a rms difference of 7 cm−1 between 43 calculated and experimental Stark levels for Tm3+(4f12) in Tm:SFAP. Absorption and fluorescence spectra are also reported for Er3+ ions in Er:SFAP. Measurement of the temporal decay of the room temperature fluorescence from the I11/24 and I13/24 manifolds yielded fluorescence lifetimes of 230±20 μs and 8.9±0.1 ms, respectively. The experimental Stark levels obtained from an analysis of the spectroscopic data were compared with a crystal-field splitting calculation. The initial set of Bnm parameters for Er3+(4f11) was established from the three-parameter theory and the final set of Bnm parameters obtained for Tm3+(4f12) in Tm:SFAP. The best overall agreement between calculated and experimental Stark levels is 8 cm−1 for 48 Stark levels, representing 12 observed multiplet manifolds of Er3+(4f11) in Er:SFAP.
Micromagnetic model for magnetic force microscopy tips
Authors: Tomlinson S. L. , Farley A. N.
Published: July 2002

A micromagnetic model for simulation of magnetic force microscope (MFM) tips is described. It incorporates a mesh of general polyhedra to allow magnetic films of arbitrary overall shape to be considered. A hierarchical method is employed for rapid evaluation of demagnetizing fields. Results for permalloy coatings of pyramidal tips are presented which show that the precise configuration of the magnetic film can produce differing magnetic behavior in comparison with experiment.
Low noise high-Tc superconducting bolometers on silicon nitride membranes for far-infrared detection
Authors: de Nivelle M. J. M. E. , Bruijn M. P. , de Vries R. , Wijnbergen J. J. , de Korte P. A. J. , Sánchez S. , Elwenspoek M. , Heidenblut T. , Schwierzi B. , Michalke W. , Steinbeiss E.
Published: July 2002

High-Tc GdBa2Cu3O7−δ superconductor bolometers with operation temperatures near 89 K, large receiving areas of 0.95 mm2 and very high detectivity have been made. The bolometers are supported by 0.62 μm thick silicon nitride membranes. A specially developed silicon-on-nitride layer was used to enable the epitaxial growth of the high-Tc superconductor. Using a gold black absorption layer an absorption efficiency for wavelengths between 70 and 200 μm of about 83% has been established. The noise of the best devices is fully dominated by the intrinsic phonon noise of the thermal conductance G, and not by the 1/f noise of the superconducting film. The temperature dependence of the noise and the resulting optimum bias temperature have been investigated. In the analysis the often neglected effect of electrothermal feedback has been taken into account. The minimum electrical noise equivalent power (NEP) of a bolometer with a time constant τ of 95 ms is 2.9 pW/Hz1/2 which corresponds with an electrical detectivity D* of 3.4×1010 cm Hz1/2/W. Similar bolometers with τ=27 ms and NEP=3.8 pW/Hz1/2 were also made. No degradation of the bolometers could be observed after vibration tests, thermal cycling and half a year storage. Measurements of the noise of a Pr doped YBa2Cu3O7−δ film with Tc=40 K show that with such films the performance of air bridge type high-Tc bolometers could be improved.
Vibrational local modes of a-SiO2:H and variation of local modes in different local environments
Authors: Lin Shu-Ya
Published: July 2002

The vibrational densities of states for the SiO2 Bethe lattices of different Si–O–Si angles and various local bonding structures in a-SiO2:H are calculated. It is found that the Si–O–Si bond-stretching mode shifts to a higher frequency and the bond-bending mode shifts to a lower frequency when the Si–O–Si bond angle increases. The SiH bond-bending mode lies at 876 cm−1 and there is an additional mode, which comes from the coupled motions of SiH and Si–O–Si bonds, at a lower frequency. This mode is not revealed by experiments. The Si-dangling bond has minor effects to the vibrational spectrum of the a-SiO2. The Si–O bond-stretching mode, of the O atom with dangling bond, shifts to a much lower 950 cm−1 compared to the 1200 cm−1 of the Si–O–Si bond-stretching mode of bulk SiO2. The Si–OH bond-stretching mode has a peak at 932 cm−1 which is slightly less than that of the Si–O bond of the dangling O atom. Under thermal annealing, some H atoms will evolve and the O atom will become a dangling atom or form a Si–O–Si bond with the other Si atom. The Si–Si bond gives an additional Si–Si vibrational mode at 464 cm−1 in the spectrum.
Soft magnetic properties and structures of nanocrystalline Fe–Al–Si–B–Cu–Nb alloy ribbons
Authors: Tate B. J. , Parmar B. S. , Todd I. , Davies H. A. , Gibbs M. R. J. , Major R. V.
Published: July 2002

The effects of Al on the magnetic properties of nanocrystalline Fe73.5−XAlXSi13.5B9Cu1Nb3 alloy ribbons, where 0⩽X⩽10, are reported for the first time. The evolution of the structure and magnetic properties of the ribbons, which were initially cast into the amorphous state in an inert gas environment at subatmospheric pressure, were studied as a function of annealing temperature Tann. The minimum dc coercivity developed during annealing, Hcmin, was found to decrease significantly with increasing Al content from 0.5 A/m at X=0 to 0.3 A/m at X=2 and to remain at approximately this level over the range 2<X⩽8 before rising to 0.4 A/m at X=10. The saturation polarization, Js, was, however, found to decrease linearly over this range from Js=1.5 T at X=0 to Js=0.9 T at X=10 for samples exhibiting Hcmin. As there was little significant reduction in the mean crystallite size, dg, at Hcmin with increasing X, this decrease in coercivity was considered to result from a reduction of the magnetocrystalline anisotropy, K1, of the crystallites as a result of the incorporation of Al.
Refractive index of InGaN/GaN quantum well
Authors: Leung Mandy M. Y. , Djuriŝić Aleksandra B. , Li E. Herbert
Published: July 2002

In this article, the optical properties of the InxGa1−xN/GaN quantum well (QW) are investigated. The refractive index spectrum of a QW is essential to the design and implementation of optoelectronic devices. Yet, the refractive index of the InGaN/GaN QW system over a wide spectral range has been unavailable so far. This article presents a comprehensive model, which includes the exciton effect and most of the major critical points, to calculate the complex index of refraction of the InGaN/GaN QW at room temperature. The calculations have been performed for QW’s with various alloy compositions and well widths in the spectral range from 1 to 9 eV. The model presented here fully considers transitions near the band edge and above barrier gap contributions.
Mechanically alloyed Zr55Al10Cu30Ni5 metallic glass composites containing nanocrystalline W particles
Authors: Eckert J. , Kübler A. , Schultz L.
Published: July 2002

Composites based on the Zr55Al10Cu30Ni5 bulk metallic glass forming alloy, containing up to 17.5 vol % W particles were synthesized by mechanical alloying. Milling produces a metallic glass matrix with a homogeneous dispersion of nanoscale W particles. The composites exhibit almost the same thermal stability and no reduction of the supercooled liquid region compared to the particle-free metallic glass despite some small amount of dissolution of W into the glassy matrix. The viscosity in the supercooled liquid increases with increasing volume fraction of particles. This will be discussed with respect to the contribution of the particles as well as to changes in matrix composition and in the free volume of the material in the framework of the free volume model for viscous flow. Independent of the W content, the samples behave as moderately strong glasses. The viscous flow of the supercooled liquid is used to consolidate dense bulk samples. The Vickers hardness, HV, of the composites increases with increasing volume fraction of particles. It is suggested that both the matrix and the nanocrystalline particles contribute to the overall hardness of the composites.
Photoluminescence band near 2.9 eV in undoped GaN epitaxial layers
Authors: Reshchikov M. A. , Shahedipour F. , Korotkov R. Y. , Wessels B. W. , Ulmer M. P.
Published: July 2002

The broad photoluminescence band with a maximum at about 2.9 eV widely observed in undoped epitaxial GaN is studied as a function of temperature and excitation intensity. We attribute the band to transitions from a shallow donor to a deep localized acceptor. The zero-phonon transition for this band is at 3.098 eV as determined from the fine structure at low temperatures. A local vibrational mode in the ground state with an energy of 36 meV is found.
The GMR effect in annealed Co/Co2TiSn thin films
Authors: Kim T. W. , Gambino R. J.
Published: July 2002

The giant magnetoresistance (GMR) effect of Co/Co2TiSn thin films with two magnet phases was observed in samples annealed in vacuum and N2(10% H2) gas atmospheres. The perpendicular magnetization curve of the films have the hysteresis loop of a typical bubble domain material. Various features of electrical resistivity at room temperature of the annealed films suggest GMR behavior. The resistivity decreases with increasing field and saturates at a field of around 2 kOe which is the same as in-plane saturation field from magnetization measurements. In the film annealed in N2 gas, the magnetoresistivity, δρ(H), is 1.43×10−7 Ω cm and the GMR is 0.18%. In the film annealed in vacuum, the δρ(H) and GMR are about 3.3×10−8 Ωcm and 0.12%, respectively. The GMR effect in annealed Co/Co2TiSn films indicates the scattering of spin polarized conduction electrons because of the antiparallel exchange coupling at the phase boundary between the two magnetic phases. The scattering process of conduction electrons at the phase boundary was modeled in relation to the magnetization process.
Properties and structures of diamond-like carbon film deposited using He, Ne, Ar/methane mixture by plasma enhanced chemical vapor deposition
Authors: Sun Z. , Lin C. H. , Lee Y. L. , Shi J. R. , Tay B. K. , Shi X.
Published: July 2002

Diamond-like carbon (DLC) films have been deposited by a magnetically enhanced plasma (MEP) chemical vapor deposition (CVD) system. The properties and structures of DLC films deposited by MEP-CVD using various gases (methane, He/methane, Ne/methane, and Ar/methane) were studied. The mechanical properties in terms of hardness, Young’s modulus and stress, and optical properties in terms of optical band gap and refractive index were enhanced by adding inert gas in methane plasma. The magnitude of the effects on the properties for various inert gases was found as Ne, Ar, and He, on the surface roughness was found as Ar, Ne, and He. The Raman characteristic shows a dependence of the bias voltage and inert-gas/methane ratio, as well as the inert gases dilution. The Raman spectroscopy analysis indicates that the changes of properties of the DLC films are due to the structural changes, such as sp2 and sp3 content in the films prepared under various deposition conditions. The films deposited in Ne/methane show the lowest disordered (D) peak to graphitic (G) peak intensity ratio, the D and G peak positions; highest stress, hardness, Young’s modulus, optical band gap, and lowest reflective index. The films deposited in Ar/methane show the lowest surface roughness. This was proposed due to the optimum balance in the inert gas ionization potential and atomic mass.
Nonstatistical effects and detailed balance in quasiclassical trajectory calculations of the thermal rate coefficient for O+OH→O2+H
Authors: Miller James A.
Published: July 2002

A detailed theoretical investigation of dynamical effects in the thermal rate coefficient for the title reaction is reported. Quasiclassical trajectory (QCT) calculations in both the forward (+1) and reverse (−1) directions are employed in the analysis. The results are compared with each other, with the variational transition state theory (VTST) calculations of Rai and Truhlar, with previous trajectory results for the reverse reaction, and with experiment. It is found that nonstatistical, or non-RRKM, ‘‘recrossing’’ effects play an important role in determining the thermal rate coefficient (the nonstatistical correction is almost a factor of 4 at 2500 K and nearly a factor of 2 even at 200 K). The results from the two trajectory methods satisfy detailed balance at high temperature, but at low temperature (250–300 K) the (−1) trajectories (through the equilibrium constant) predict a rate coefficient for the title reaction that is higher than that predicted by the (+1) method by about a factor of 2. This discrepancy is attributed to the crude handling of quantum effects near threshold in the (−1) trajectory method. The predictions of the (+1) method are in excellent agreement with experiment over the entire temperature range where experimental results are available (250–2500 K). At high temperatures (T>500 K) the VTST results of Rai and Truhlar predict accurately the rate of formation of HO*2 complexes, but at lower temperatures the VTST and trajectory results are in disagreement. The VTST calculations are unable to predict the thermal rate coefficient of the title reaction at any temperature for a combination of reasons.
Cooperative photoabsorption induced charge transfer reaction dynamics in rare gas solids. I. Photodynamics of localized xenon chloride exciplexes
Authors: Fajardo Mario E. , Apkarian V. A.
Published: July 2002

Cooperative charge transfer absorptions are demonstrated in HCl, Cl2, and Cl doped rare gas solids. The four-body, two-photon cross section for Xe+HCl+2hν→Xe+(HCl)− at 308 nm is >10−42 cm4 s. The same mechanism explains the efficient Cl atom photogeneration in Cl2 doped solids. The weak field cross sections for the three-body one-photon and four-body one-photon transitions, Xe+Cl+ hν→XeCl(B) and Xe+Xe+Cl+hν→Xe2Cl(4 2Γ), are comparable near the B←X resonance, the cross sections are estimated as 3 and 1(±0.5)×10−16 cm2. The XeCl C→A emission, B←X absorption, and C state lifetime [75 (±5) and 60 (±5) ns in Ar and Kr], can be accounted by bulk dielectric solvation of the molecular dipole. The Xe2Cl(4 2Γ) emission [573 nm in Ar, Kr, and Xe], and lifetime [250 (±10), 210 (±10) and 225(±10) ns in Ar, Kr, Xe] cannot be treated by the same model. The XeCl (B)→XeCl(C) and XeCl(C)+Xe→Xe2Cl(4 2Γ) reactions are fast, >2×1010 s−1. The XeCl(C) and Xe2Cl(4 2Γ) emissions can be modeled with one- and two-dimensional potentials using gas phase diatomic parameters: the localized excitons retain their molecular nature.
Laser-initiated half-reaction. Vibrational and rotational state distribution of NO produced from the reactant pair O(1D)⋅N2O
Authors: Honma Kenji , Fujimura Yo , Kajimoto Okitsugu , Inoue Gen
Published: July 2002

The vibrational and rotational state distribution was measured for NO produced from the reaction O(1D)+N2O→2NO via a reactant pair O(1D)⋅N2O, which, in turn, formed by the 193 nm photolysis of the N2O dimer. The dimer was generated by the supersonic expansion through a pulsed nozzle. The distribution was determined by using the laser-induced fluorescence of NO on its A–X transition. The rotational distribution was of the Boltzmann type characterized by a low temperature, 60–100 K, at each vibrational level measured. The vibrational distribution was found to be composed of the two components, one very cold and the other relatively hot. The experiment using an isotopically labeled N2O revealed that the vibrational energy was not equally distributed over two kinds of NO; the NO originally present in N2O was vibrationally cool while that formed from O(1D) and the terminal nitrogen of N2O was vibrationally hot. These results indicate that the reaction occurring is the abstraction of the terminal nitrogen by O(1D). The low rotational temperature, which sharply contrasts with the extremely high rotational excitation observed for the ordinary bimolecular reaction, can be rationalized by considering the geometrical difference in the encounter between the O(1D) atom and N2O. This fact, in turn, indicates that the product energy distribution is significantly affected by the orientation in the reactive encounter.
Rheological properties of polymer dumbbell models with configuration-dependent anisotropic friction
Authors: Biller P. , Petruccione F.
Published: July 2002

With the help of Brownian dynamics simulations we study the rheological properties of polymer Hookean dumbbell models with anisotropic friction. The friction tensor depends on the actual configuration of the dumbbell. The resulting equation of motion is a Langevin equation with multiplicative noise. The correlations of the ‘‘Langevin’’ Brownian forces appearing in this equation turn out to be anisotropic, too. They should be carefully distinguished from the ‘‘smoothed out’’ Brownian forces which appear in the standard derivations of the Fokker–Planck equation in kinetic theory. We will discuss the rheological properties in steady shear flow for two different friction tensors.