bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of irradiation temperature and ion flux on electrical isolation of GaN
Authors: Kucheyev S. O. , Boudinov H. , Williams J. S. , Jagadish C. , Li G.
Published: July 2002

We study the evolution of sheet resistance of n-type GaN epilayers irradiated with MeV 1H and 12C ions. Results show that both implantation temperature (varied from 77 up to 423 K) and ion beam flux affect the process of electrical isolation in the case of irradiation with 12C ions. This behavior is consistent with significant dynamic annealing occurring in GaN during MeV light-ion bombardment, which suggests a scenario where the centers responsible for electrical isolation are defect clusters or anti-site-related defects. Dynamic annealing causes simple ion-beam-generated Frenkel pairs to annihilate (or cluster) during irradiation at liquid nitrogen temperature and above. These beam-flux and irradiation-temperature effects are not observed during bombardment with lighter 1H ions, which produce very dilute collision cascades. A qualitative model is proposed to explain temperature and flux effects in GaN in the MeV light-ion bombardment regime used for electrical isolation.
Strongly opalescent liquid network formed in a porous silicon matrix
Authors: Kovalev D. , Gross E. , Künzner N. , Polisski G. , Koch F. , Timoshenko V. Yu. , Bel’kov V.
Published: July 2002

We report on a medium exhibiting extremely efficient light scattering properties: a liquid network formed in a porous silicon matrix. We find that the scattering efficiency depends strongly on the filling factor of the liquid in the pores, its dielectric constant, and the type of termination of the internal surface of the layer. The spectral dependence of the scattering length of photons evidences the fact that the phenomenon is governed by a Mie-type scattering mechanism. The mean free path of photons in this medium is found to be in the micrometer range.
Nanoscale characterization of hydrogenated and oxidized B-doped homoepitaxial diamond by conductive atomic force microscopy
Authors: Zhang L. , Sakai T. , Yoshida H. , Yamanaka S. , Okushi H.
Published: July 2002

A comparison of topographical and electrical properties of hydrogenated and oxidized high-quality B-doped homoepitaxial films was carried out by conductive atomic force microscopy. The topography images of both hydrogenated and oxidized films showed nanoscale flatness and uniformity. The hydrogenated film showed ohmic property indicating the existence of a high-conductivity layer on the surface. A reproducible periodical stripe pattern in the conductivity distribution was also observed indicating the imperfection of H-termination on the step edges. On the other hand, the oxidized surface showed Schottky I–V characteristics and a uniform conductivity distribution without structural dependence, although the surface morphology was modified after the current measurements.
Scanning electron filling modulation reflectance of charged InGaAs self-assembled quantum dot
Authors: Hsu T. M. , Chang W.-H. , Lai C. Y. , Yeh N. T. , Chyi J.-I.
Published: July 2002

We present the scanning electron-filling modulation reflectance (SEFR) of charged In0.5Ga0.5As self-assembled quantum dots. The SEFR is performed by applying a small constant modulation voltage and scanning the dc bias through the quantum dot energy levels. The spectral intensity of the SEFR can be related to the electron density confined in each quantum state, which shows a good agreement with the apparent carrier concentration deduced from the capacitance–voltage measurements. In this study, we find that the transition energy of quantum-dot excited state is about 10 meV smaller than the values obtained from the photoluminescence and electron-filling modulation reflectance. This phenomenon is explained by the Coulomb interactions between the optical excited electron–hole pair and the electrons occupied in the quantum dots.
Calculated and measured reflectivity of some p-type SiC polytypes
Authors: Ismail Adnan M. , Abu-Safia Hassan
Published: July 2002

In this work, we performed measurements of reflectivity in the energy range 1–6 eV at room temperature for 4H and 6H p-type SiC polytypes. The optical constants of these samples were determined. Experimental results were compared to theoretical calculations based on Kramers–Kronig relations. A good agreement between the experimental and calculated values of reflectivity was obtained.
Hydrodynamics for a granular binary mixture at low density
Authors: Garzó Vicente , Dufty James W.
Published: July 2002

Hydrodynamic equations for a binary mixture of inelastic hard spheres are derived from the Boltzmann kinetic theory. A normal solution is obtained via the Chapman–Enskog method for states near the local homogeneous cooling state. The mass, heat, and momentum fluxes are determined to first order in the spatial gradients of the hydrodynamic fields, and the associated transport coefficients are identified. In the same way as for binary mixtures with elastic collisions, these coefficients are determined from a set of coupled linear integral equations. Practical evaluation is possible using a Sonine polynomial approximation, and is illustrated here by explicit calculation of the relevant transport coefficients: the mutual diffusion, the pressure diffusion, the thermal diffusion, the shear viscosity, the Dufour coefficient, the thermal conductivity, and the pressure energy coefficient. All these coefficients are given in terms of the restitution coefficients and the ratios of mass, concentration, and particle sizes. Interesting and new effects arise from the fact that the reference states for the two components have different partial temperatures, leading to additional dependencies of the transport coefficients on the concentration. The results hold for arbitrary degree of inelasticity and are not limited to specific values of the parameters of the mixture. Applications of this theory will be discussed in subsequent papers.
Wider bandwidth with high transmission through waveguide bends in two-dimensional photonic crystal slabs
Authors: Chutinan Alongkarn , Okano Makoto , Noda Susumu
Published: July 2002

We demonstrate the use of a defect to improve the transmission property of waveguide bends in two-dimensional photonic crystal slabs. We show that high reflection in the two-dimensional photonic crystal slab previously reported is due to the fact that the waveguide is multimoded at the bend while it is single moded along the straight waveguide. By making the waveguide single moded at the bend, the transmission property can be significantly improved. An extension of more than twice of high-transmission bandwidth is achieved.
Submillimeter spectroscopic study of concentrated electrolyte solutions as high density plasma
Authors: Dodo Taro , Sugawa Masao , Nonaka Eiji , Ikawa Shun-ichi
Published: July 2002

The transport property of ions in highly concentrated electrolyte solutions, which are governed by many body Coulomb interaction, are found to be well described by a model that ions behave as high density strongly coupled plasma irrespective of ion species. The frequency of plasma oscillation, which is the long-range collective motion of plasma, is estimated to be order of 1 THz for concentrated electrolyte solutions, and evidence of the plasma oscillation can be searched by submillimeter spectroscopy. Transmission and reflection spectra of the concentrated electrolyte solutions have been measured in the 10 to 50 cm−1 range, and the real and imaginary parts of refractive indices for the solutions are obtained. A close inspection of these spectral data suggests that the plasma oscillation is not excited in the highly concentrated electrolyte solutions. This is probably caused by that the long-range collective motion of ions is disturbed by short-range solute–solvent interaction.
An excess electron bound to urea. III. The urea dimer as an electron trap
Authors: Skurski Piotr , Simons Jack
Published: July 2002

The possibility of electron binding to urea dimers in the gas-phase was studied at the coupled cluster level with single and double excitations using aug-cc-pVDZ+7s6p5d basis sets. It was found that two kinds of anions can be formed—dipole-bound anions and solvated-electron anions. The global minimum on the ground-state anionic potential energy surface corresponds to the dipole-bound anion whose adiabatic and vertical electron binding energies are 484 and 1443 cm−1, respectively. It was also found that solvated electron structures, which are locally geometrically stable yet thermodynamically unstable, are considerably higher in energy than dipole-bound anions. However, the vertical electron detachment energies of the latter are significantly larger (i.e., 1267, 4129, and 7540 cm−1, depending on whether two canonical, one canonical, and one zwitterionic, or two zwitterionic urea monomers are involved, respectively), so their presence in any experimental source should allow them to be identified.
A new correlation functional based on analysis of the Colle–Salvetti functional
Authors: Imamura Yutaka , Scuseria Gustavo E. , Martin Richard M.
Published: July 2002

The behavior of the Colle–Salvetti (CS) correlation functional based on the Hartree–Fock (HF) second-order density matrix is investigated in the case of the He atom. The analysis of the correlation hole and energy contributions show that correlation effects are not taken into account appropriately due to the missing kinetic correlation. The CS final simplified energy expression also has some problems. To address these problems, we have constructed a new correlation functional based on the HF second-order density matrix including the effects of not only electron–electron interaction but also the kinetic energy by using an adiabatic connection formula. In addition, correlation effects for opposite and parallel spins are treated independently. This functional reproduces accurate correlation energies for H–Ar atoms. Combining it with the Becke 1988 exchange functional, we get reasonable atomization energies for the G2 set.
Critical adsorption and boundary layer structure of 2-butoxyethanol+D2O mixtures at a hydrophilic silica surface
Authors: Howse Jonathan R. , Manzanares-Papayanopoulos Emilio , McLure Ian A. , Bowers James , Steitz Roland , Findenegg Gerhard H.
Published: July 2002

Neutron reflectivity experiments have been performed to investigate the adsorption behavior of aqueous solutions of n-2-butoxyethanol (n-C4E1) and i-2-butoxyethanol (i-C4E1) in D2O against a hydrophilic silica substrate. The measurements were made in the one-phase region near the lower critical solution point of the aqueous systems. At temperatures removed from the lower critical solution temperature, TLCS, evidence for the existence of a microscopically thick (∼40 Å) adsorbed D2O-rich layer is presented along with data suggesting that the amphiphile is strongly depleted from the boundary layer. Experiments using a thick “tuning” layer of silica reinforce these observations. Analysis of the data as T→TLCS reveals that the region beneath the adsorbed D2O-rich layer becomes increasingly enriched with D2O, and the nature of the composition profile normal to the interface has been considered. Analysis identifies the exponent describing the power-law decay part of the critical adsorption profile appended to the D2O-rich region and associates a temperature-dependent correlation length with the critical adsorption profile. Ellipsometric data are presented which complement the neutron reflectivity results. The adsorption behavior of the n-C4E1 and i-C4E1 systems are found to be very similar with the only discernible difference being the magnitude of the adsorption preference of water for the n-C4E1 system in the long-range profile.
Generalized boundary conditions for the Aharonov–Bohm effect combined with a homogeneous magnetic field
Authors: Exner P. , Št’ovı́ček P. , Vytřas P.
Published: July 2002

The most general admissible boundary conditions are derived for an idealized Aharonov–Bohm flux intersecting the plane at the origin on the background of a homogeneous magnetic field. A standard technique based on self-adjoint extensions yields a four-parameter family of boundary conditions; the other two parameters of the model are the Aharonov–Bohm flux and the homogeneous magnetic field. The generalized boundary conditions may be regarded as a combination of the Aharonov–Bohm effect with a point interaction. Spectral properties of the derived Hamtonians are studied in detail.
Droplet spreading: A Monte Carlo test of Tanner’s law
Authors: Milchev A. , Binder K.
Published: July 2002

The spreading of polymer droplets under conditions of complete wetting on an ideally flat and structureless solid substrate has been studied by computer simulation, using a coarse-grained bead–spring model of flexible macromolecules. Evidence is obtained that a power law close to Tanner’s law for the growth of the lateral droplet radius {r(t)∝t0.14} and contact angle {θ∝t−0.31} holds on nanoscopic scales. We observe the formation of a precursor film around the spreading droplet and find that the film attains diffusive dynamics at late times.
Lowering of operational voltage of organic electroluminescent devices by coating indium-tin-oxide electrodes with a thin CuOx layer
Authors: Hu Wenping , Manabe Kaoru , Furukawa Takumi , Matsumura Michio
Published: July 2002

We devised a method of modifying indium-tin-oxide (ITO) electrodes for organic electroluminescent devices. It consists of deposition of a nanometer-thick Cu layer on the ITO electrode and an oxygen plasma treatment. By this modification, the surface of the ITO substrate is covered with a partly oxidized Cu layer (CuOx). The CuOx-coated ITO electrode possesses strong hole-injection ability, which leads to lowered operational voltage and high luminance from the devices consisting of tris(8-quinolinato)aluminum and diamine hole-transport layers. The hole-injection ability of the CuOx-coated ITO electrode is better than that of the ITO electrode modified by conventional methods, such as insertion of a Cu-phthalocyanine buffer layer. Moreover, the CuOx layer is effective to improve the durability of the devices.
Universal behavior in irradiated high-electron-mobility transistors
Authors: Weaver B. D. , Jackson E. M.
Published: July 2002

Radiation-induced decreases in the drain current Id in a wide range of GaAs/AlGaAs high-electron-mobility transistors (HEMTs) are shown to be directly proportional to the induced defect concentration. The constant of proportionality depends only weakly on gate length, doping profile and concentration, layer thickness, device geometry, and operating voltages, and is hence nearly device independent. The same proportionality holds when the channel layer material is changed from GaAs to InGaAs. However, when the donor layer material is switched to InGaP (in InGaAs/InGaP HEMTs), the constant of proportionality changes, and the drain current becomes 16–17 times more radiation tolerant. The drain current in InGaAs/InAlAs HEMTs is about 30 times more radiation tolerant than in InGaAs/AlGaAs. We propose that the linear dependence of Id on defect concentration arises from high-efficiency scattering of carriers out of the two-dimensional electron gas in the channel layer, and that the slope of the linear relationship is determined by the efficiency with which the donor layer reinjects scattered carriers.
Constant voltage stress induced degradation in HfO2/SiO2 gate dielectric stacks
Authors: Xu Zhen , Houssa Michel , Carter Richard , Naili Mohamed , De Gendt Stefan , Heyns Marc
Published: July 2002

Defect generation in HfO2/SiO2 gate dielectric stacks under constant voltage stress is investigated. It is found that the stress induced electrical degradation in HfO2/SiO2 stacks is different than in the SiO2 layer. The variation of the gate leakage current with different polarities shows different degradation characteristics after stress. Positive charge generation is also observed under both negative and positive gate voltage polarities. These degradation phenomena are explained by the composite effect of three components: neutral trap generation, electron trapping, and positive charge generation in the gate stacks.
Band alignment between GaAs and partially ordered GaInP
Authors: Zhang Yong , Mascarenhas Angelo , Wang Lin-Wang
Published: July 2002

An empirical pseudopotential method is used for calculating the band structure of partially CuPt ordered GaxIn1−xP alloy with order parameter η varying from 0 to 1. Because the relative band alignments between the binaries (GaAs, GaP, and InP) are taken into account in the pseudopotential fitting, such a calculation naturally yields the conduction and valence band alignment between the GaInP alloy and GaAs, as well as shows how the alignments change with the order parameter. The band alignment is found to change from type I to type II at η=0.46 (0.54) for x=0.50 (0.52), which is in good agreement with experimental data.
Mechanism for Ohmic contact formation of Ti on n-type GaN investigated using synchrotron radiation photoemission spectroscopy
Authors: Kim Jong Kyu , Jang Ho Won , Lee Jong-Lam
Published: July 2002

Changes of the work function of contact metal and the atomic composition at the vicinity of metal/GaN interface were studied in in situ annealed Ti on n-type GaN using synchrotron radiation photoemission spectroscopy. Ti layer transformed to TiN and its work function increased as annealing temperature increased. Meanwhile, the atomic composition ratio of Ga-to-N below the TiN contact increased indicating the creation of N vacancies. This provides evidence that N vacancies produced below the contact, acting as donors for electrons, play a main role in forming the Ohmic contact.
Bruhat Poisson structure on CPn and integrable systems
Authors: Foth Philip
Published: July 2002

In this note we present a simple formula for the Bruhat Poisson structure on complex projective spaces in terms of the momentum coordinates. We also give a simple description of a family of functions in involution on compact Hermitian symmetric spaces obtained via the bi-Hamiltonian approach using the Bruhat Poisson structure and an invariant symplectic structure. We compute these functions explicitly on CPn and relate them to the Gelfand–Tsetlin coordinates. We also show how the Lenard scheme can be applied.
First principles molecular dynamics study of amorphous AlxGa1−xN alloys
Authors: Chen Kuiying , Drabold David A.
Published: July 2002

We have systematically investigated atomic structures, electronic and dynamical properties of amorphous aluminum–gallium–nitride alloys (a-AlGaN) by performing first principles local basis molecular dynamics simulations. The network topology and defects of the amorphous 216-atom model system have been analyzed with the radial distribution function, the angular distribution function, the ring statistics, and the local coordination. It was found that the models have mixed threefold and fourfold coordinations, and the number of threefold (fourfold) coordinated atoms in alloys decreased (increased) with increasing Al composition. No odd rings are found, indicating that no wrong bonds (homonuclear bonds) appear in the a-AlxGa1−xN alloys. The Ga–N and Al–N bond lengths show a small variation with the Al composition, which is in agreement with recent extended x-ray absorption fine structure experimental measurements. The electronic properties examined by the electronic density of states and local bonding character demonstrate that no mid-band-gap states exist. The band-gap dependence on Al fraction x in a-AlxGa1−xN alloys shows a nearly linear variation with Al composition, and exhibits a small downward bowing behavior. It was also shown that valence band tail states are mostly localized on the threefold coordinated N sites, while the conduction band tail states are mostly localized on the threefold coordinated Ga and Al sites, and the electronic localization tends to become weaker with the addition of Al. We find a mixture of sp3 and sp2 bonds present in the network and their interaction plays a key role in the dynamical properties of a-AlxGa1−xN alloys.
Kinetics and mechanisms of the thermal degradation of amorphous carbon films
Authors: Zhang Li Hong , Gong Hao , Wang Jian Ping
Published: July 2002

The thermal stability of amorphous carbon thin films is significantly improved by increasing the carbon-plasma energy in dc magnetron sputtering. The on-set decomposition temperature in air increased from 335 °C to 525 °C, while the apparent activation energy ΔE in the early decomposition stage increased from approx. 90 to 180 kJ/mol when the pressure in the sputtering chamber changed from 16×10−3 Torr to 1×10−3 Torr. Thermogravimetry (TG) analysis clearly showed that with the increase of carbon-plasma energy, the amount of unbounded and loosely bounded molecules in the structure reduced significantly. A study of decomposition mechanisms based on isothermal TG analysis showed that the decomposition of the films could best be expressed in an apparent nth order reaction, or −[ln(1−α)]n∼t. The apparent reaction order n reduced with the increase of temperature, and also with the carbon-plasma energy during film deposition. The temperature effect on n is explained from the diffusion resistance of decomposed products. The film structure deposited at higher energy carbon plasma showed higher bonding ratio and structure density, which yielded the film of higher thermal stability, ΔE, and lower n values.
Domain nucleation in arrays of perpendicularly magnetized dots
Authors: Li S. P. , Lebib A. , Chen Y. , Fu Y. , Welland M. E.
Published: July 2002

Magnetization reversal processes in Co55Ni45/Pt multilayer dots was studied with the magneto-optical Kerr effect and magneto-optical Kerr microscope. With decreasing dot diameter, a dramatic increase of coercive field distribution and macroscopic coercivity in the dot arrays was observed. Direct triggering with submicron wire channels indicates that the coercive field distributions and the enhancement of macroscopic coercivity are caused by nucleation isolation due to high density patterning. We show that this phenomenon is analogous to the solidification process of an undercooled liquid.
Numerical analysis on the dispersion process of carbon clusters synthesized by gas evaporation using dc arc
Authors: Usuba Shu , Yokoi Hiroyuki , Kakudate Yozo
Published: July 2002

The dispersion process of typical carbon cluster synthesized by gas evaporation using (dc) arc was numerically investigated under normal gravity (1 G) and nongravity (0 G) conditions to clarify the effect of natural convection. Calculated pressure dependence of residence time of C60 in the temperature state between 1000 and 2000 K under 1 G took a maximum value of about 260 ms at the pressure around 0.06 atm in helium, while under 0 G, it increased almost linearly with pressure. Such features in the pressure dependence of residence time of C60 under both 1 and 0 G were related to its yield by a simple model based on an annealing of imperfect C60 to perfect C60 structure. According to this model, experimentally observed secondary increase of C60 yield was consistently explained in terms of the effect of natural convection.
On-chain defect emission in electroluminescent polyfluorenes
Authors: Lupton J. M. , Craig M. R. , Meijer E. W.
Published: July 2002

We present time-resolved photoluminescence measurements on a range of poly- and oligofluorenes with different molecular weights in both dilute solution and thin films. The commonly observed parasitic broad green emission band, which has previously been attributed to an excimer, is identified in all solution and film samples and assigned to an on-chain emissive defect. By comparison of the luminescence decay in the solid state at different temperatures it is shown that, at room temperature, intramolecular relaxation is faster in these polyphenylenes than intermolecular exciton diffusion.
Oscillations and chaos in CO+O2 combustion
Authors: Johnson B. R. , Griffiths J. F. , Scott S. K.
Published: July 2002

The gas-phase reaction between carbon monoxide and oxygen (in the presence of small amounts of hydrogen) shows bistability and oscillatory behavior. Typically, the oscillatory ignition has a period-1 relaxation waveform. The limit cycle is born at a saddle-node loop and terminates via a supercritical Hopf bifurcation. For a mean residence time of 8 s there is a period-doubling to a period-2 solution followed by period-halving to quasisinusoidal period-1 oscillations. At longer residence times, more period-doublings forming a full cascade to chaos with subsequent periodic windows are observed. The chaotic attractor has an underlying single-humped next maximum map.
Variational principles for thermostatted systems
Authors: Choquard Philippe
Published: July 2002

A generalized Lagrangian formalism is proposed for dealing with interacting many body systems subject to time reversible friction forces. The corresponding generalized Hamiltonian and Poissonian formalisms are presented. Explicit connection is made between the new Poisson brackets obtained and the 2-forms of conformally symplectic systems recently investigated. Applications to Nosé–Hoover and isokinetic dynamics are treated in detail.
Segregation induced instabilities of granular fronts
Authors: Pouliquen O. , Vallance J. W.
Published: July 2002

Experimental investigation of granular flows containing particles of several sizes and moving down slopes shows that segregation of coarse-grained, irregularly shaped particles induces a fingering instability at the propagating front. The size-segregation mechanism involves percolation of small particles downward and a corresponding migration of large ones toward the flow surface. Large particles at the flow surface experience velocities that are greater than average so that they migrate forward and begin to collect at the flow front. In the case of dry cohesionless flows, the instability depends upon these large particles at the flow perimeter being more angular and thus more resistant to flow than the smaller rounder ones in the interior. A simple analytical model predicts the fingering instability when friction of the flow front is greater than that of the following flow. The presence of viscous liquid inhibits both size-segregation and the development of the instability. Fluidization of dry flows permits segregation of large particles to flow perimeters, thus increasing permeability and permitting a similar instability that owes its development to the dry frictional perimeter that surrounds a partly fluidized interior.
Symbolic dynamics of jejunal motility in the irritable bowel
Authors: Wackerbauer Renate , Schmidt Thomas
Published: July 2002

Different studies of the irritable bowel syndrome (IBS) by conventional analysis of jejunal motility report conflicting results. Therefore, our aim is to quantify the jejunal contraction activity by symbolic dynamics in order to discriminate between IBS and control subjects. Contraction amplitudes during fasting motility (phase II) are analyzed for 30 IBS and 30 healthy subjects. On the basis of a particular scale-independent discretization of the contraction amplitudes with respect to the median, IBS patients are characterized by increased block entropy as well as increased mean contraction amplitude. In a further more elementary level of analysis these differences can be reduced to specific contraction patterns within the time series, namely the fact that successive large contraction amplitudes are less ordered in IBS than in controls. These significant differences in jejunal motility may point to an altered control of the gut in IBS, although further studies on a representative number of patients have to be done for a validation of these findings.
Electrical activation and local vibrational mode from Si-implanted GaAs
Authors: Nakamura T. , Katoda T.
Published: July 2002

Local vibrational modes of lattice site Si implanted in GaAs were studied by laser Raman spectroscopy. From the dependence of the intensity of the local vibrational modes due to Si at As sites and Si pairs at Ga and As sites (SiGa-SiAs) on annealing temperature, implanted Si are considered to occupy the lattice sites with the structural recovery by annealing at lower than 550 °C, and form SiGa-SiAs pairs at higher than 550 °C besides the electrically active Si probably in Ga sites. The depth profiles of the intensity of the local vibrational mode and carrier concentration obtained after annealing at 850 °C indicate that Si at As sites prevent the generation of carriers, and Si at Ga sites have no contribution to the generation of carriers if they form the pairs with the nearest-neighbor Si at As sites.
Magnetic properties and anisotropy of (Fe1−xCox)7Se8
Authors: Sato Masaki , Kamimura Takashi , Iwata Takao
Published: July 2002

(Fe1−xCox)7Se8 samples have NiAs-type structures with or without ordered arrangements (3c or 4c) of cation vacancies. With increasing x, the lattice parameter a remains unchanged but c exhibits a fairly large reduction (∼10% by variation of x from 0 to 1). The Curie temperature decreases with increasing x and the ferrimagnetism disappears at x≊0.6. The effective magnetic moments (∼5.5 μB for x≤0.3) evaluated from the high-temperature portion (range of vacancy disorder) of the 1/χ versus temperature curves are close to the value expected on the ionic model. Results show that the magnetic states up to at least x=0.3 are those attained by a simple replacement of magnetic moments of Fe ions by those of Co ions. Samples with x>0.6 are weakly magnetic. Nearly temperature-independent ac susceptibility at x≊0.7 indicates the absence of localized moments. In this connection, a correlation between lattice parameters and magnetic character is pointed out for 7 : 8 isomorphs of transition element selenides. The easy-axis rotation from in the c plane towards the c axis with lowering temperature occurs in a different way in two superstructures: The easy axis in 4c structures rotates gradually according to temperature change. In 3c structures with x≤0.1, the rotation takes place at some definite temperature (e.g., 165 K for x=0.1). Corresponding torque curves clearly indicate a first order transition. Details of this anisotropy transition are interpreted as a combined effect of the spin-orbit interaction and other additional terms.
Laser excitation of acoustic resonances in a spherical resonator
Authors: Karbach A. , Hess P.
Published: July 2002

Laser excitation of nonradial and radial acoustic modes via transformation of absorbed optical into acoustical energy was studied in a spherical cavity. A computer-controlled technique was employed to record the resonance profiles with high precision as a function of CH4 pressure in the cavity. The conditions for efficient laser excitation of acoustic modes are discussed in terms of the overlap integral, which also explains the disappearance and reappearance of modes as a function of pressure. This effect was observed for the first radial mode in CH4. The investigation of the splitting of the degenerate first nonradial mode allowed an estimate of the deviations of the real cavity from the ideal spherical symmetry.
Measurements of thermal conductivity of materials using a transient technique. I. Theoretical background
Authors: Piorkowska E. , Galeski A.
Published: July 2002

A method of measuring the thermal conductivity of materials during transient heating or cooling is proposed based on the continuous recording of the temperature difference between sample surfaces when known heat flux is supplied to one of the sample surfaces by a heater. The analysis of the solution of the heat conduction equation for a measuring system consisting of two platelike samples separated by a heater shows that the transient heating or cooling significantly influences the temperature distribution within the samples. It follows from the theoretical consideration that this effect as well as the effects resulting from heat generation or absorption in the samples during phase transitions can be eliminated by subtracting the results of two measurements conducted with different powers supplied by the heater. The condition for validity of this procedure is the small temperature difference between sample surfaces which can be achieved by the selection of appropriate temperature change rate or sample thickness.
Electromagnetic radiation associated with the formation of an electric breakdown in air at atmospheric pressure
Authors: Bondiou A. , Labaune G. , Marque J. P.
Published: July 2002

Previous experiments on the triggering mechanisms of air discharges at atmospheric pressure (surface discharge, short gap breakdowns) showed a transient arc phase characterized by a sharp current variation with constant duration of about 10 ns. The current and potential evolution in the transient arc phase, in the case of a surface discharge, are calculated using the Rompe and Weisel’s air conductivity law and a classical propagation equation. The results of this model are consistent with experimental data and give the E field value in the transient arc zone. Furthermore, the model provides the necessary source terms for calculation of the radiated field associated with the transient arc in the dipolar approximation. This calculation shows the impulsive structure of the radiated field emitted in the VHF–UHF range during the formation of the transient arc: this pulse of a rising front of several nanoseconds is similar for amplitude and spectral content to the wide-band measurements carried out on natural lightning.
Absorption coefficient of Si in the wavelength region between 0.80–1.16 μm
Authors: Saritaş Müzeyyen , McKell Harry D.
Published: July 2002

The absorption coefficient, α as a function of wavelength in the region of 0.80–1.16 μm was determined by the transmission technique. The present α data fit an indirect transition model of L. H. Hall, J. Barden, and F. J. Blatt [Phys. Rev. 99, 559 (1954)]. The α data of W. C. Dash and R. Newman [Phys. Rev. 99, 1155 (1955)] and W. R. Runyan [Southern Methodist University Report SMU 83-13 (1967)] did not fit the indirect transition model. The reflectivity values obtained from transmission data at long wavelengths are less than those of H. R. Philipp and E. A. Taft [Phys. Rev. 120, 37 (1960)] and W. C. Dash and R. Newman. The present surface preparation technique produced a reflection loss slightly lower than would be expected from the surface characteristic of bulk material.
Variation of hardness with temperature in sintered NdFeB magnets
Authors: Luo Yang , Zhang Nin , Graham C. D.
Published: July 2002

The variation of hardness of a sintered Nd15Fe78B7 magnet has been measured in the temperature range from room temperature to 750 °C. Relatively little change in hardness is observed below 200° or above 700°. A sharp drop in hardness begins at about 300°, with hardness decreasing to half its original value at 450° and to 10% of its original value at 650°. Melting of a Nd-rich grain-boundary phase begins at about 600 °C. The observed hardness behavior is consistent with reports of compression-induced crystallographic alignment of NdFeB magnets at temperatures near 700 °C. High-temperature microscopy and differential scanning calorimeter measurements on cast magnets of similar composition show that melting of a Nd-rich grain-boundary phase begins at about 600 °C.
A novel method for depth profiling and imaging of semiconductor devices using capacitive coupling voltage contrast
Authors: Cole E. I. , Bagnell C. R. , Davies B. , Neacsu A. , Oxford W. , Roy S. , Propst R. H.
Published: July 2002

A scanning electron microscope was used in the voltage contrast mode to examine the relationship between dynamic secondary electron emission from passivated devices and buried structure depth. A primary electron beam energy of 500 eV was used. Depth measurement was performed on floating structures with an average error of 6.59%, 4.93% standard deviation, over a depth range of 0.04–0.76 μm. Further, a method of producing capacitive coupling voltage contast (CCVC) images of unbiased devices was developed, having a spatial resolution of better than 1 μm micron through passivation. A modified version of the Gorlich model for CCVC decay is presented to explain the experimental results [S. Gorlich, K. D. Herrmann, and E. Kubalek, in Proceedings of the Microcircuit Engineering 84 Conference (Academic, London, 1985)].
On the balance between silylene and silyl radicals in rf glow discharges in silane: The effect on deposition rates of a-Si:H
Authors: Kushner Mark J.
Published: July 2002

Plasma enhanced chemical vapor deposition (PECVD) of amorphous hydrogenated silicon (a-Si:H) is typically performed in low-pressure (≤0.5 Torr) radio frequency (rf) discharges in gas mixtures containing silane (SiH4). The initiating step is electron impact dissociation of silane, whose products are primarily the silylene (SiH2) and silyl (SiH3) radicals. The ratio of SiH2 to SiH3 in the plasma is important because these radicals incorporate differently into the a-Si:H film; and therefore the characteristics of the film are a function of the ratio [SiH3]/[SiH2]. The initial silane dissociation step is followed by a series of hydrogen abstraction, silylene insertion, and silyl association reactions which subsequently alter this ratio. The branching ratios for electron impact dissociation of silane, as well as the rate constants for the subsequent reactions, have not been measured or are uncertain. Using a model for a rf discharge in silane gas mixtures, the effects of branching ratios for silane dissociation, and rate constants for key reactions are investigated. We find that in order for SiH3 to be the dominant radical, as currently thought, its branching ratio from electron impact dissociation of SiH4 must exceed 0.75, and the yield for H atoms in the branch for SiH2 must also exceed 0.75. The deposition rate of a-Si:H is then controlled by the yield of H atoms and the subsequent generation of radicals by hydrogen abstraction from SiH4. These results require that the silyl association reaction has a rate constant ≤10−11 cm3 s−1, and that the rate constant for insertion of SiH2 into silane exceeds 10−11 cm3 s−1.
Third harmonic generation in phthalocyanines
Authors: Ho Z. Z. , Ju C. Y. , Hetherington III W. M.
Published: July 2002

Third harmonic generation (THG) has been performed in reflection from thin polycrystalline films of chloro-gallium and fluoro-aluminum phthalocyanines. By comparison to THG from the 100 surface of silicon, the values of χ(3) are estimated to be 2.5×10−11 and 5×10−11 esu, respectively. The patterns of the molecular and crystal tensors are discussed for the case of multiple resonance enhancements.
Modeling of magnetic field source by distributing dipoles on a disk
Authors: Mori Keiichi
Published: July 2002

This paper treats the problem of finding a model of the magnetic field source that is long and wide from the measured field values outside of it. The modeling method by distributing the perpendicular and radial dipoles on a disk is proposed. These distributions are expressed as functions of the respective oblate spheroidal harmonic coefficients. With this method the equivalent distributions to the source can be determined uniquely based on the harmonic coefficients.
Band-edge nonlinearities in direct-gap semiconductors and their application to optical bistability and optical computing
Authors: Koch S. W. , Peyghambarian N. , Gibbs H. M.
Published: July 2002

Nonlinear optical properties of laser-excited semiconductors are discussed, and the applications to optical bistability and optical logic are reviewed. The physical origin of the nonlinearities is analyzed in terms of the many-body interactions in the system of electron-hole pairs, causing effects such as plasma screening of the Coulomb interaction, band-gap renormalization, and band filling. Theoretical results for absorption and refractive-index spectra are compared to experimental data obtained for room-temperature GaAs and other semiconductors. The experimental and theoretical results for dispersive optical bistability in semiconductors are summarized, and recent experiments on optical logic gating and pattern recognition are discussed.
Multiple coil pulsed NMR method for measuring the multipole moments of particle accelerator bending magnets
Authors: Clark W. G. , Hijmans T. W. , Wong W. H.
Published: July 2002

An important step in the development and manufacture of the bending magnets for the proposed superconducting supercollider particle accelerator is measurement of the multipole moments of the magnetic field. This paper describes a method for doing so that makes use of the simultaneous pulsed NMR signal from a large number of coils placed on the diameter of a circle. From the value of the magnetic field thereby measured on the boundary, the multipole moments are obtained. Details of the basic concepts of the method are presented, along with some results of an early test of it.
Niobium-stress influence on Nb/Al-oxide/Nb Josephson junctions
Authors: Kuroda Kenichi , Yuda Masahiro
Published: July 2002

Niobium-stress influence on Nb/Al-oxide/Nb trilayer Josephson-junction characteristics is studied. Different junction fabrication techniques are examined such as conventional etching and new lift-off processes. Nb-film stress, relaxed during trilayer etching, can deteriorate junction quality in etching-processed junctions smaller than a few micrometers square. Such deterioration does not occur in lift-off processed junctions even when they are 1 μm square because stress relaxation occurs during trilayer deposition through lift-off stencils. This new all-Nb Josephson-junction fabrication technique produces high-quality junctions with Rsg/Rnn =24 (Rsg and Rnn are tunnel resistances at 2 and 4 mV, respectively) and a current uniformity of 1.5% for 100 1-μm-square junctions connected in series.
Laser flash thermal conductivity studies of porous metal fiber materials
Authors: Golombok M. , Shirvill L. C.
Published: July 2002

The laser flash method has been used to measure thermal conductivities in a porous metal fiber material at room and elevated temperatures. The anisotropy of heat conduction in this material is reported and a novel technique using a focused laser beam is described to access directly the off-diagonal components of the conductivity tensor. A simulation study shows the anisotropy to be determined by the layered structure of the material. Enhanced conductivities are reported at elevated temperatures and are shown to be due to radiation heat transfer within the porous structure.
Percolating cermet thin-film thermistors between 50 mK–300 K and 0–20 T
Authors: Gershenfeld N. A. , VanCleve J. E. , Webb W. W. , Fischer H. E. , Fortune N. A. , Brooks J. S. , Graf M. J.
Published: July 2002

Cermet thin-film resistors consisting of roughly 50-Å Pt or Mo particles in Al2O3 at compositions just below the percolation transition provide a number of useful features for thermometry. These include: continuous sensitivity from at least 50 mK to 300 K, adjustable temperature dependence, low heat capacity, small size, excellent stability, and a saturable magnetoresistance. The fabrication, microstructure, and temperature dependence of the resistance and magnetoresistance of these materials are described. We find poor agreement with existing theories of conduction in granular materials.
Evanescent wave infrared spectroscopy of liquids using silver halide optical fibers
Authors: Simhony S. , Schnitzer I. , Katzir A. , Kosower E. M.
Published: July 2002

Infrared spectroscopic measurements of liquids were performed in an attenuated total internal reflection cell, using infrared transparent silver halide fibers. As an example, we studied evanescent wave absorption in water. In particular, we analyzed absorption dependence on various physical parameters, such as the length of the absorbing medium and the geometry of the beam at the fiber entrance and exit faces. Absorption peaks were easily traced and experimental results correlated well with theoretical calculations. These studies may lead to practical uses of fiberoptic-based evanescent wave spectroscopy.
The spin-phonon interaction in FeF2 and MnF2 studied by Raman spectroscopy
Authors: Lockwood D. J. , Cottam M. G.
Published: July 2002

A temperature-dependent study of the four Raman-active phonons in MnF2 and FeF2 has shown that their Raman frequencies and intensities are affected to varying extents by the antiferromagnetic ordering. A theory for spin-dependent effects in rutile structure antiferromagnets has been developed to calculate the anomalous variation in the phonon frequency and to estimate the corresponding spin-phonon coupling coefficients.
Charged dislocation induced optical absorption in GaAs
Authors: Vignaud D. , Farvacque J. L.
Published: July 2002

We propose a model of optical absorption induced by electric fields resulting from charged dislocations. It is shown to account satisfactorily for the deformation induced absorption in GaAs, which exhibits the shift of the band-to-band transitions often observed in semiconductors. This model considers that dislocations introduce energy levels in GaAs and its implications should be rather general for most semiconductors in which dislocations introduce energy levels in the band gap.
Phase and frequency locking of a cavity vircator driven by a relativistic magnetron
Authors: Price D. , Sze H. , Fittinghoff D.
Published: July 2002

Observations of mode selection, frequency locking, and phase locking of a high-power, S-band, cavity vircator oscillator by an externally injected, relativistic magnetron are reported. The injected electric field can increase the virtual cathode oscillation growth rate and defeat chirping. The peak power extracted from the driven vircator cavity is between 100 and 500 MW. The output power from the magnetron is comparable. The virtual cathode oscillation frequency is pulled to the magnetron frequency in 3–5 ns. Locked phase measurements versus the allowable frequency detuning that yields locking agree with predictions. The output power of the driven cavity vircator is two to three times that obtained from a free-running, undriven vircator.
Surface-roughness and grain-boundary effects on the optical properties of low-pressure chemical-vapor-deposited silicon thin films by spectroscopic ellipsometry
Authors: Logothetidis S.
Published: July 2002

The optical properties of polycrystalline silicon thin films prepared by low-pressure chemical vapor deposition were studied by spectroscopic ellipsometry. The surface roughness and the microstructure of the films were investigated by combining the theory of Ohlidal and Lukeš with the Bruggeman effective-medium approximation. Furthermore, a line-shape analysis of the dielectric function, as well as a method to describe the dielectric function, was used to discuss the effect of the surface roughness and that of the crystallite size and grain boundaries in these materials. The results of this analysis show that the parameters that mainly control the growth mode in these materials, at a temperature of 630 °C, are the silane pressure and the growth rate. The different surface morphologies and the growth of a native oxide on the material surface seem to be associated with the growth mode of the film. The effect of surface roughness on the measured data must be taken into account even at energies as low as 2 eV, while the presence of a native oxide overlayer influences the measured data at higher energies. A reasonable agreement with recent electron microscopy studies on these materials was found.