bluuebunny/crossref_metadata_2025 huggingface dataset:
Processing dependent thermal conductivity of nanoporous silica xerogel films
Authors: Jain Anurag , Rogojevic Svetlana , Ponoth Shom , Gill William N. , Plawsky Joel L. , Simonyi Eva , Chen Shyng-Tsong , Ho P. S.
Published: July 2002

Sintered xerogel films (porous SiO2) show a much higher thermal conductivity than other low dielectric constant (low-K) materials available for the same value of K. The thermal conductivity of xerogels which we have processed using different methods is compared with that of other low-K materials such as silica hybrid (silsesquioxanes) and polymeric low-K materials. The methods used were: (1) single solvent (ethanol) method, (2) binary solvent (mixture of ethanol and ethylene glycol) method, (3) sintering. For the xerogel films, we show that process history is as important as the chemistry of the solid matrix or the porosity in determining the thermal conductivity. The thermal conductivity, measured by the 3-ω method or the photothermal deflection method, is affected by phonon scattering, which in turn is effected by the size and distribution of pores and particles and the presence of imperfections such as interfaces, substituted chemical species, impurities, microcracks, and microporosity. The thermal conductivity extrapolated to zero porosity for porous sintered xerogel films approaches that of thermally grown SiO2 indicating the least phonon scattering of all processing methods. For these films, the elastic modulus is proportional to thermal conductivity squared, in agreement with theories developed for materials with few defects and a connected matrix.
C lattice site distributions in metastable Ge1−yCy alloys grown on Ge(001) by molecular-beam epitaxy
Authors: Park S. Y. , D’Arcy-Gall J. , Gall D. , Kim Y.-W. , Desjardins P. , Greene J. E.
Published: July 2002

Epitaxial metastable Ge1−yCy alloy layers with y⩽0.045 were grown on Ge(001) by solid-source molecular-beam epitaxy (MBE) at temperatures Ts=200–400 °C. Using calculated strain coefficients and measured layer strains obtained from high-resolution reciprocal lattice maps (HR-RLMs), we determine C lattice site distributions as a function of Ts and total C concentration y. HR-RLMs show that all as-deposited alloys are fully coherent with their substrates. Ge1−yCy(001) layers grown at Ts⩽350 °C are in a state of in-plane tension and contain C in substitutional sites, giving rise to tensile strain, as well as in nanocluster sites which induce negligible lattice strain. Ts=400 °C layers are strain neutral with negligible substitutional C incorporation. Increasing y and/or Ts leads to a decrease in substitutional C concentration, consistent with Raman spectroscopy results, with a corresponding increase in the C fraction incorporated in nanocluster sites. The latter suggests that nanocluster formation is kinetically limited during film deposition by the C–C adatom encounter probability at the growth surface. Overall, the results show that it is not possible by MBE to obtain fully substitutional C incorporation in Ge1−yCy(001) alloys, irrespective of y and Ts. This is consistent with ab initio density functional calculations results showing that C incorporation in nanoclusters sites is energetically favored over incorporation in substitutional Ge lattice sites. Annealing the Ge1−yCy(001) layers at Ta=550 °C leads to a significant decrease in the substitutional C fraction and, hence, lower tensile strain. Layers annealed at 650 °C are strain free as all substitutional C has migrated to lower-energy nanocluster sites.
Polymer–nanotube composites: Burying nanotubes improves their field emission properties
Authors: Alexandrou I. , Kymakis E. , Amaratunga G. A. J.
Published: July 2002

Composite structures of single walled carbon nanotubes (SWNTs) and poly(3-octylthiophene) polymer were tested as field emitters. The structures studied were formed on n+-Si by mixing the two materials after they had been dissolved in chloroform. Three different structures were tested: (a) a SWNT mat, (b) a SWNT mat on a SWNT (30 wt %)–polymer composite and (c) a SWNT mat on pure polymer. Although the conductivity of the composite is very much lower than that of the nanotube mat, experiments show that the field emission threshold is lower and the emission current higher from structure (b) compared to the pure SWNT mat on n+-Si. A comparison between the field enhancement factors and the expected aspect ratios shows that enhancement due to an additional field enhancement mechanism other than that due to the geometrical shape of the SWNTs must exist. We propose that the field due to the surface potential modification at the triple junction between SWNTs, polymer and vacuum is adequate to create additional field emission from such sites. This mechanism can also be used to explain previously reported field emission characteristics of CNx nanocomposite films.
Photoluminescence behavior of poly(quinoline)s in silica glasses via the sol–gel process
Authors: Huang W. Y. , Ho S. W. , Kwei T. K. , Okamoto Y.
Published: July 2002

A full color display with its spectra covering the entire visible color range using a single polymer is presented here. Different concentrations of poly(2,6-[4-phenylquinoline]) and poly(2,6-[p-phenylene]-4-phenylquinoline) were incorporated into silica gels via the sol–gel technique. At high concentrations, the conjugated polymers form multiple excimers in the channels within the silica network, leading to the emission of red light (∼600 nm). At low concentrations, the polymer chains are isolated and are being trapped individually in the silica domain, which results in the emission of blue light (∼400 nm). For concentrations in-between, moderate extensive chain interaction leads to the emission of green, yellow, and orange colors. Therefore, the color tunability can be achieved by simply varying the concentration of quinoline polymers in the silica glasses.
Study on bias-enhanced nucleation of diamonds by simulating the time dependence of bias current
Authors: Perng Kuoguang , Liu Kuo-Shung , Lin I-Nan
Published: July 2002

The kinetics of bias-enhanced nucleation (BEN) of diamonds in microwave plasma-enhanced chemical vapor deposition were investigated. During a BEN step, the increase of bias current was proportional to the area covered by diamond nuclei. This result provided information regarding the kinetics of nucleation. Avrami’s kinetics were carefully introduced to describe the time dependence of the bias current. Our analysis shows that the decrease of activation energy (0.08–0.18 eV) of atomic diffusion increased as the negative bias voltage is increased from −80 to −120 V. The mechanism of enhancement of atomic diffusion by ion bombardment in the incubation period was also presented.
Thermoelectric properties of Bi2.2−xPbxSr2Co2Oy system
Authors: Xu Gaojie , Funahashi Ryoji , Shikano Masahiro , Matsubara Ichiro , Zhou Yuqin
Published: July 2002

Polycrystalline samples of Bi2.2−xPbxSr2Co2Oy (x=0, 0.4, and 0.8) with a layered structure have been prepared using a hot pressing technique and their thermoelectric properties methodically investigated. It is found that the substitution of Pb for Bi is a fairly effective means of improving the thermoelectric properties of the Bi2.2−xPbxSr2Co2Oy system owing to resistivity (ρ) decreasing and thermoelectric power (S) increasing simultaneously. The thermal conductivity (κ) at high temperature also decreases markedly with Pb substitution. As a result, for the x=0.8 sample the dimensionless figure of merit, ZT(=S2T/ρκ), increases with temperature and reaches 0.26 at 1000 K, which indicates that this material is a strong candidate as a high-temperature thermoelectric material.
Subgrid-scale modeling for large-eddy simulations of compressible turbulence
Authors: Kosović Branko , Pullin Dale I. , Samtaney Ravi
Published: July 2002

We present two phenomenological subgrid-scale (SGS) models for large-eddy simulations (LES) of compressible turbulent flows. A nonlinear model and a stretched-vortex model are tested in LES of compressible decaying isotropic turbulence. Results of LES at 323, 483, and 643 resolution are compared to corresponding 2563 direct numerical simulations (DNS) at a turbulent Mach number, Mt∼0.4. We use numerical schemes based on compact finite differences and study the effects of their order of accuracy on LES results. Both models give satisfactory agreement with DNS for the decay of the total turbulent kinetic energy. The probability densities (pdf) of energy transfer to subgrid scales obtained from filtered DNS and the SGS models are compared. Both models produce a narrower distribution of energy transfer than corresponding filtered DNS data, with less backscatter. The pdf of the alignment of components of the subgrid stress tensor and the eigenvectors of the rate-of-strain tensor obtained from the models reproduces some features of the DNS results. The pdfs of both energy transfer and relative eigenvector alignment are obtained from DNS and LES after about one large-eddy turnover time from the same initial condition. All tests of the present LES models are therefore a posteriori and none is a priori.
Direct calculation of cumulative reaction probabilities from Chebyshev correlation functions
Authors: Xie Daiqian , Li Shenmin , Guo Hua
Published: July 2002

The transition-state wave packet method of Zhang and Light [J. Chem. Phys. 104, 6184 (1996)] for the direct calculation of cumulative reaction probabilities is implemented in the Chebyshev order domain to take advantage of exactness and efficiency of the Chebyshev propagator. Numerical testing for three-dimensional H+H2 reactive scattering (J=0) confirms the accuracy and efficiency of the proposed algorithm. This new implementation is then used to compute the cumulative reaction probability of the Li+HF→LiF+H reaction (J=0) up to 0.65 eV. It is found that the latter reaction is dominated in low energy region by numerous narrow resonances.
Reduced dimensionality quantum calculations of acetylene↔vinylidene isomerization
Authors: Zou Shengli , Bowman Joel M.
Published: July 2002

The isomerization of acetylene to vinylidene is examined in four degrees of freedom using a full dimensional potential. Fifteen hundred vibrational wave functions and eigenvalues that extend roughly 1500 cm−1 above the isomerization threshold are calculated using C2–H2 diatom–diatom Jacobi coordinates. Eigenfunctions of the four degree-of-freedom Hamiltonian are obtained using a series of truncation/recoupling procedures that begins with the eigenfunctions of a two degree-of-freedom Hamiltonian. The double-well nature of the isomerization is clear in these calculations and is exploited to divide the eigenfunctions into symmetric and antisymmetric groups. The lowest energy doublet states of vinylidene at two levels of dimensionality reduction indicate a splitting of the order of a wave number. Franck–Condon factors between these molecular eigenstates and a model, ground state vibrational wave function for the vinylidene anion are calculated to simulate the photodetachment spectrum of the vinylidene anion.
Ab initio studies of (H2O)28 hexakaidecahedral cluster with Ne, N2, CH4, and C2H6 guest molecules in the cavity
Authors: Khan Arshad
Published: July 2002

Stability of hexakaidecahedral clusters in the presence of guest molecules like Ne, N2, CH4, and C2H6 molecules are examined by first optimizing geometry at the Hartree–Fock (HF) level with the 6-31G* basis set followed by a single point energy calculation at the MP2 level (second-order Møller–Plesset perturbation) and using the same basis set. For N2 and CH4 molecules, both the single and double occupancy of the cage cavity have been examined. In each case the cluster with single occupancy is more stable than that with the double occupancy, and contradicts an earlier simulation result. Even when the guest molecules are small enough to fit into the cage cavity, there seems to be an optimum size of guest molecules at which maximum amount of stabilization is achieved.
New formula for the bulk viscosity constructed from the interatomic potential and the pair distribution function
Authors: Okumura Hisashi , Yonezawa Fumiko
Published: July 2002

We provide a physical insight into the mechanism of the bulk viscosity ζ and construct a new formula for ζ directly from microscopic information such as the interatomic potential and the pair distribution function. This is an outstanding advantage when compared to the previously proposed formulas in all of which ζ is expressed only in terms of a macroscopic quantity, i.e., pressure. In other words, our new formula makes it possible for the first time to discuss the relation between the macroscopic quantity ζ and the microscopic properties such as the interatomic potential and the pair distribution function. This new formula is derived by solving the time development equation of the pair distribution function. Among some of the previously proposed formulas, we prove that the Green–Kubo formula and the Heyes formula are both equivalent to our new formula when the macroscopic quantity—pressure—in their expressions is described by means of interatomic potentials and pair distribution functions under appropriate conditions. This fact confirms the validity of our formula, which reinforces the aforementioned advantage. We calculate ζ near the triple point of the Lennard-Jones fluid using our formula, the Green–Kubo formula, and the Heyes formula. The obtained values of ζ are in good agreement with each other.
Controllability of quantum mechanical systems by root space decomposition of su(N)
Authors: Altafini Claudio
Published: July 2002

The controllability property of the unitary propagator of an N-level quantum mechanical system subject to a single control field is described using the structure theory of semisimple Lie algebras. Sufficient conditions are provided for the vector fields in a generic configuration as well as in a few degenerate cases.
The development and study of a fiber delivery system for beam shaping
Authors: Greenaway M. W. , Proud W. G. , Field J. E. , Goveas S. G.
Published: July 2002

A high power fiber delivery system has been developed for a Q-switched Nd:YAG laser. Multimode fibers with a core diameter of 400 μm were tested with a view to transmitting the maximum possible amount of optical energy. The importance of surface finish was investigated by employing a number of different polishing procedures. Atomic force microscopy and laser-induced damage threshold measurements were used to identify a clear correlation between surface finish and transmission capability. Surface roughness measurements as low as 3 nm were made and the transmission of up to 30 J/cm2 achieved. The front face of the fibers would be improved during laser testing due to plasma formation which acts to anneal the surface. The various damage mechanisms that limit the performance of the fiber have been studied and attributed to different optical and physical effects. The nature of the light spot emerging from the fiber was analyzed and quantified by beam profilometry. A number of different methods for altering the beam profile were tried but changing the length of the fiber was found to be the most effective.
Short-lived two-soliton bound states in weakly perturbed nonlinear Schrödinger equation
Authors: Dmitriev Sergey V. , Shigenari Takeshi
Published: July 2002

Resonant soliton collisions in the weakly discrete nonlinear Schrödinger equation are studied numerically. The fractal nature of the soliton scattering, described in our previous works, is investigated in detail. We demonstrate that the fractal scattering pattern is related to the existence of the short-lived two-soliton bound states. The bound state can be regarded as a two-soliton quasiparticle of a new type, different from the breather. We establish that the probability P of a bound state with the lifetime L follows the law P∼L−3. In the frame of a simple two-particle model, we derive the nonlinear map, which generates the fractal pattern similar to that observed in the numerical study of soliton collisions.
Polarization switching in optical microsphere resonator
Authors: Smirnov A. Yu. , Rashkeev S. N. , Zagoskin A. M.
Published: July 2002

A general electrodynamical theory of a high-Q optical microsphere resonator in an external alternating magnetic field is reported. We show that such a system can change a polarization state of the whispering-gallery-mode photons confined in the sphere due to the Faraday effect. This property can be used in all-optical switches and logical devices. The contribution of the Kerr nonlinearities is also discussed.
Large nonlinear dielectric properties of artificial BaTiO3/SrTiO3 superlattices
Authors: Kim Juho , Kim Youngnam , Kim Young Sung , Lee Jaichan , Kim Leejun , Jung Donggeun
Published: July 2002

BaTiO 3 ( BTO )/ SrTiO 3 ( STO ) artificial superlattices have been made on MgO (100) substrates. The periodicity of the BTO/STO layers in the superlattice was varied from one-unit cell to 125-unit cell thickness. The dielectric constant and its nonlinearity (or voltage tunability) showed similar behavior as the periodicity was varied. The voltage tunability of the superlattice increased with decreasing stacking periodicity of the BTO/STO within the critical thickness. Similarly, the lattice distortion, i.e., the ratio of the lattice parameter along surface normal to parallel, of the BTO and STO layers increased with decreasing the periodicity. Remarkable enhancement of the voltage tunability has been achieved. The superlattice exhibited large voltage tunability (94%, the highest value to date) at the periodicity of BTO2-unit cell/STO2-unit cell at which the maximum lattice distortion of each layer was obtained. This suggests that the nonlinear dielectric property of the superlattice is closely related with the lattice distortion of the individual layers.
Decrease in the leakage current density of Si-based metal–oxide–semiconductor diodes by cyanide treatment
Authors: Asano Akira , Asuha  , Maida Osamu , Todokoro Yoshihiro , Kobayashi Hikaru
Published: July 2002

Crown-ether cyanide treatment, which includes the immersion of Si in KCN solutions containing 18-crown-6 molecules, is found to greatly decrease the leakage current density of Si-based metal–oxide–semiconductor (MOS) diodes. The decrease by one order of magnitude for the single crystalline Si-based MOS diodes is attributable to the elimination of Si/SiO2 interface states by reaction with cyanide ions and formation of Si–CN bonds. The reduction in the leakage current density by two orders of magnitude is caused for polycrystalline Si-based MOS diodes, and this decrease is attributed to the passivation of trap states in poly-Si as well as the interface states.
Phase coexistence in Pb(Fe2/3W1/3)O3–PbTiO3 solid solutions
Authors: Mitoseriu Liliana , Vilarinho Paula M. , Baptista João L.
Published: July 2002

The crystalline symmetry of the solid solution formed by a relaxor and a ferroelectric material with formula: (1−x)Pb(Fe2/3W1/3)O3–xPbTiO3, with various x was investigated, at T=300 K. A continuous change from pseudocubic to tetragonal symmetry, with cell parameters dependent on the PbTiO3 amount, was found. In a large range of compositions, from x=0.20 to x=0.37, both phases coexist, with different relative amounts.
Phase-locking regions in a forced model of slow insulin and glucose oscillations
Authors: Sturis Jeppe , Knudsen Carsten , O’Meara Niall M. , Thomsen Jesper S. , Mosekilde Erik , Van Cauter Eve , Polonsky Kenneth S.
Published: July 2002

We present a detailed numerical investigation of the phase-locking regions in a forced model of slow oscillations in human insulin secretion and blood glucose concentration. The bifurcation structures of period 2π and 4π tongues are mapped out and found to be qualitatively identical to those of several other periodically forced self-oscillating systems operating across a Hopf-bifurcation point. The numerical analyses are supplemented by clinical experiments.
Thermostats: Analysis and application
Authors: Morriss Gary P. , Dettmann Carl P.
Published: July 2002

Gaussian isokinetic and isoenergetic deterministic thermostats are reviewed in the correct historical context with their later justification using Gauss’ principle of least constraint. The Nosé–Hoover thermostat for simulating the canonical ensemble is also developed. For some model systems the Lyapunov exponents satisfy the conjugate pairing rule and a Hamiltonian formulation is obtained. We prove the conjugate pairing rule for nonequilibrium systems where the force is derivable from a potential. The generalized symplectic structure and Hamiltonian formulation is discussed. The application of such thermostats to the Lorentz gas is considered in some detail. The periodic orbit expansion methods are used to calculate averages and to categorize the generic transitions in the structure of the attractor. We prove that the conductivity in the nonequilibrium Lorentz gas is non-negative.
Order and chaos in the planar isosceles three-body problem
Authors: Zare K. , Chesley S.
Published: July 2002

The planar isosceles three-body problem has been reduced to a two-dimensional area preserving Poincaré map f. Using certain symmetry properties of the underlying differential equations and numerical integration, we offer a global description of f in the case of three equal masses. This description, which is based on the mapping of areas, immediately leads to the existence of various types of motion such as capture-escape, permanent capture, ejection-collision, etc., and their corresponding measures in the map domain. Moreover, this technique readily allows one to distinguish between so-called “fast” and “chaotic” scattering. Although capture-escape is the subset with the highest measure, there exist two important distinct invariant subsets under f where the solutions neither are captured nor lead to escape. The first set is a Cantor set which has zero measure and it is the outcome of the fact that f acts similar to the Smale horseshoe map in part of the domain. On this subset the action of f is chaotic. The second subset is an invariant region with positive measure surrounding an elliptic fixed point. In this region f acts essentially as a perturbed twist mapping where the iterates of f for the points in a large subset move on invariant curves in an orderly manner. In an appendix we cast our results in the framework of the widely studied isosceles triple collision manifold.
New approach for simulating chain conformations in dense polymers using fully populated lattice models
Authors: Kulkarni Jaydeep A. , Beris Antony N.
Published: July 2002

A two-dimensional implementation of a new computational approach for the simulation of the microscopic chain conformations in dense linear polymers is presented. The macromolecular chains are represented as self-avoiding and mutually avoiding random walks on a fully populated lattice corresponding to the amorphous regions of a lamellar semicrystalline morphology. In this approach, information is generated based on a transfer matrix approach in terms of the permutations of the vertical and the horizontal bonds in the lattice rows. The data are then subsequently corrected to eliminate contributions from unwanted microscopic states containing closed loop (ring) chain structures. It is shown that the linear chain conformational entropy can be estimated from first principles by an efficient accounting of all the feasible microstates. In addition, statistical information on the chain conformations can also be obtained. The chain statistics presented here are compared with the predictions of ideal or nearly ideal random walks (Gambler’s ruin models) from the literature where little or no excluded volume effects are taken into account. It is shown that the chain connectivity influences the chain statistics significantly. © 1998 American Institute of Physics.
Crystallization of amorphous Zr2Pd and Zr3Rh alloys
Authors: Cantrell J. S. , Wagner J. E. , Bowman R. C.
Published: July 2002

The thermal stability behavior of the metallic glasses a-Zr2Pd and a-Zr3Rh was studied by means of x-ray diffraction, differential scanning calorimetry (DSC), and isothermal annealing. While several a-Zr2Pd alloys each exhibit two exothermic peaks during the DSC heating sequence, three different types of exothermic behavior have been observed for a-Zr3Rh alloys with nominally identical stoichiometries. These variations in DSC behavior are probably related to different conditions during the rapid quenching procedures. The three types of differential scanning calorimeter behavior shown by a-Zr3Rh alloys include (1) two peaks of about 715 K and 790 K, (2) a single large peak at about 730 K, and (3) a strong peak at about 725 K with a much weaker peak at about 850 K. The a-Zr3Rh alloys of type (1) crystallize to form a tetragonal lattice that is a newly identified Zr3Rh phase of the D0e structure type. The a-Zr3Rh alloys of types (2) and (3) crystallize first to form a face-centered cubic lattice which appears to be an E93-type structure. Both the D0e and E93 phases are metastable and ultimately anneal to the Zr2Rh phase with the tetragonal C16-type structure. The formation of the Zr2Rh phases with either the C16 or E93 structure is accompanied by α-Zr. The a-Zr2Pd alloys undergo a two-step crystallization with the initial formation of a disordered body-centered-cubic phase followed by the tetragonal Zr2Pd phase with the C11b-type structure.
Surface and bulk magnetic anisotropy in amorphous alloys
Authors: González J. M. , Vicent J. L.
Published: July 2002

Magnetic measurements of hysteresis loops and transverse susceptibilities have been done on as-cast iron based amorphous alloys on the surface and in the bulk of the ribbons. Bitter technique has been used to obtain the domain patterns. a-Fe78B13Si9 shows an anisotropy perpendicular to the ribbon plane with maze domain ‘‘island’’ structures. The surface measurements do not detect any difference in behavior compared to the observed bulk properties. a-Fe81.5B14.5Si4 shows different behavior in the surface and in the bulk of the ribbon with a perminvar-type hysteresis loop from the surface and magnetization perpendicular to the ribbon plane with well-aligned closure domains. The analysis of the different magnetic experimental data and selective changes in the internal stresses produced by different treatments support a model of the layered character of the anisotropies in this sample with tensile stresses and in-plane anisotropy in the surface and compressive stresses and anisotropy perpendicular to the ribbon’s surface in the bulk. Finally, a rough calculation shows that the thickness of the surface in-plane anisotropy region is of the order of the surface roughness.
Some ways to modify the spin-wave mode spectra of magnetic multilayers (invited)
Authors: Grünberg P.
Published: July 2002

Spin-wave modes in magnetic multilayers are of interest both for basic research and for application—for example, in microwave devices. Here I would like to discuss some ways to manipulate the mode spectra of such multilayers. One way, for example, would be to have reversed magnetization direction in adjacent layers which can be achieved by tailoring the coercive forces of the films in a proper way. The mode spectrum of such a system has been derived only recently and I would like to report a few important results. Also I would like to discuss the case where the films are coupled by exchange as well as by the dipolar interaction which is always there. Here the amount of exchange can be tailored by choosing the proper materials or thickness for the nonmagnetic intermediate layers between the magnetic films. We have studied these effects for a couple of Fe sandwiches with intermediate layers of various materials and find a short coupling length (<5 Å) for V and Cr, and a long one (≲20 Å) for Cu, Ag, Au, and Pd. Taking pinhole effects into account this could be correlated with the different crystal structures but it could also be explained by charge transfer effects considering the electronegativities of these materials.
Specific heat of GdRh
Authors: Azhar A. A. , Mitescu C. D. , Johanson W. R. , Zimm C. B. , Barclay J. A.
Published: July 2002

GdRh, a ferromagnetic intermetallic, is of interest as a working material for magnetic refrigeration. The requirements for such materials have been discussed by Barclay et al. and it is in this context that we analyze our data. Magnetic measurements of Buschow et al. have indicated TC =24 K while specific heat measurements of Olijhoek et al. have shown a maximum at 20 K and have yielded a magnetic entropy of 16.7 J/mole K. No Debye temperature has been reported previously. We have measured the specific heat from 1.8 to 77 K and our data show a peak at 19.93 K that is 17% higher than in the previous work. Fitting of the data from 40 to 77 K to C=A/T2+𝒟(T/θ{T}), where A and θ{T} are adjustable parameters and 𝒟 is the Debye function, yields a Debye temperature of 198±2 K. For the magnetic entropy we obtain 17.4 J/K, very close to R ln 8.
Temperature rise induced by a cw laser beam revisited
Authors: Liarokapis E. , Raptis Y. S.
Published: July 2002

The problem of spatial distribution of the temperature rise due to a cw laser beam focused on the surface of an absorbing material is reexamined. The effect of temperature dependence of the absorption and reflection coefficients is incorporated in the calculations in a self-consistent way. A Green’s function has been developed for the heat diffusion equation in the axially symmetric case and a general steady-state solution is obtained for an arbitrary source function. Compared with previous results, our calculations predict melting at lower laser power densities and changes in the spatial temperature distribution. In the limit of large absorption constants these differences are small. However, they become increasingly significant as the absorption constant is decreased.
Lattice parameters of Zn1−xMnxSe and tetrahedral bond lengths in AII1−xMnxBVI alloys
Authors: Yoder-Short D. R. , Debska U. , Furdyna J. K.
Published: July 2002

This paper reports the results of lattice parameter measurements on the ternary semiconductor alloy Zn1−xMnxSe over the range 0≤x≤0.57. We find that the mean cation-cation distance increases linearly with manganese concentration x according to Vegard’s Law. It is also noted that this linear dependence occurs across the region in which the alloy changes crystal structure from zinc blende (x≤0.30) to wurtzite (0.33≤x). These observations are compared with the behavior of the crystal lattice as a function of composition in other AII1−xMnxBVI alloys. A fairly unifed pattern of behavior emerges, relating the lattice parameters and bond lengths for the entire family of these materials. In addition, this analysis provides an experimentally determined value of the tetrahedral radius of manganese.
Migration of two ions during electrolysis of glass waveguide
Authors: Yoshida Hiroyuki , Kataoka Takeshi
Published: July 2002

The entire fabrication process of an optical waveguide with a gradient index profile in planar soda-lime glass substrate was analyzed by using the Nernst–Planck equation for the fluxes of ion species in the glass by considering the effect of an electric current. The derived equations were solved numerically and the effect of the diffusivity ratio of ions and the electric current on the concentration profile of the impurity were discussed. A method for the fabrication of the waveguide was presented and some conditions for getting a parabolic concentration profile of the impurity were presented for Ag–soda-lime glass substrate system.
Microstructure of electrodeposited Co-W thin films
Authors: Admon U. , Dariel M. P. , Grünbaum E.
Published: July 2002

Cobalt-tungsten thin films, 300–500 Å thick, containing 0–15 at. % (0–35 wt. %) tungsten have been prepared by electrodeposition onto polycrystalline copper substrates, from acidic and basic baths of various compositions, at 22, 50, and 90 °C. The grain morphology, phase composition, texture, and defect structure were studied by transmission electron microscopy and diffraction. The deposits were either amorphous (or composed of microcrystallites) or crystalline, depending on the plating conditions. The crystalline films consisted of a dispersion of large grains (≫film thickness) within a background of small size grains (≲film thickness). The crystal structure of the latter was hcp, fcc, or mixed. The hexagonal small grains showed a varying degree of [0001] texture. The large grains consisted of alternate hcp and fcc platelets with a definite mutual orientation relationship: (0001)h∥(001)c∥ substrate surface with [101̄0]h∥[110]c or (0001)h∥(111)c∥ substrate surface with [21̄1̄0]h∥[110]c. The grains of both phases contained extensive stacking faults. The microstructure of the films is discussed and a growth mechanism is proposed in order to account, in terms of mutual epitaxy, for the observed orientation relationship between the hcp and fcc structures.
Deep electron trapping center in Si-doped InGaAlP grown by molecular-beam epitaxy
Authors: Nojima S. , Tanaka H. , Asahi H.
Published: July 2002

The properties of a deep electron trapping center found in Si-doped In0.51Ga0.49−xAlxP (x=0.24) grown on Si-doped GaAs(100) substrates using molecular-beam epitaxy are studied. The trapping parameters for this center are determined as follows: thermal activation energy 0.48 eV, photoionization energy 1.25 eV, and capture barrier 0.10 eV. The concentration of this center is about seven times as high as that of the shallow donor state and is found to increase with an increasing amount of Si atoms incorporated into the crystal. The thermally activated persistent behaviors of photocarriers are also observed at low temperatures. These results suggest that this center can be regarded as a kind of DX center originally found in AlGaAs, which should be depicted as a state with large lattice relaxation using a configuration coordinate diagram.
Magnetic properties of amorphous Tb-Fe thin films with an artificially layered structure
Authors: Sato Noboru
Published: July 2002

An alternating terbium-iron (Tb-Fe) multilayer structure artificially made in amorphous Tb-Fe thin films gives rise to excellent magnetic properties of large perpendicular uniaxial anisotropy, large saturation magnetization, and large coercivity over a wide range of Tb composition in the films. The films are superior to amorphous Tb-Fe alloy thin films, especially when they are piled up with a monatomic layer of Tb and several atomic layers of Fe in an alternating fashion. Small-angle x-ray diffraction analysis confirmed the layering of monatomic layers of Tb and Fe, where the periodicity of the layers was found to be about 5.9 Å. Direct evidence for an artificially layered structure was obtained by transmission electron microscopic and Auger electron spectroscopic observations. Together with magnetic measurements of hysteresis loops and torque curves, it has been concluded that the most important origin of the large magnetic uniaxial anisotropy can be attributed to the Tb-Fe pairs aligned perpendicular to the films.
Spontaneous radiation of an electron beam in a free-electron laser with a quadrupole wiggler
Authors: Levush B. , Antonsen T. M. , Manheimer W. M.
Published: July 2002

A calculation is presented of spontaneous radiation emitted by an electron beam passing through a continuously rotating quadrupole magnetic undulator. It is shown that radiation spectrum emitted in forward direction of beam propagation has four peaks, corresponding to four betatron frequencies. Utilizing the Madey theorem, a stimulated emission is calculated and presented as gain versus frequency curves, for different values of the quadrupole magnetic field. A free-electron laser operating at two or three radiation frequencies with a quadrupole magnetic wiggler is suggested.
Spatially resolved ellipsometry
Authors: Erman M. , Theeten J. B.
Published: July 2002

Using a convergent beam approach, a spatially resolved ellipsometry (SRE) has been achieved. The modifications and the limitations due to the use of such a nonplanar wave for an ellipsometric measurement are discussed. We first established the relations between the ellipsometric data (tan ψ,cos Δ), the optical properties of the sample which are assumed to be spatially nonuniform [rp(x,y) and rs(x,y)], and the optical characteristics of the beam (i.e., electric field distribution of the light on the sample surface and its angular domain Fourier transform function). These relations are established for both a coherent and an incoherent light source. We use them to investigate the absolute accuracy achievable with SRE on an homogeneous sample. It is demonstrated that submonolayer sensitivity can be obtained using a 10×10-μm spot. In the case of a step discontinuity of rp(x,y) and rs(x,y), strong optical resonances can take place in SRE. Their quantitative analysis is performed in the (tan ψ,cos Δ) plane. In this representation, the optical resonance corresponds to a trajectory, called ‘‘spatial trajectory.’’ Another type of trajectory can be observed for a multilayer structure in the case when one (or more) thickness is slowly varying versus the position (‘‘thickness trajectory’’). The combination of both cases leads to a generalized trajectory concept which allows for a quantitative analysis of two-dimensional ellipsometric maps. This trajectory approach is illustrated on two practical situations: (i) a SiO2 layer etched through a lithographic mask on top of a GaAs wafer; (ii) metallic patterns deposited on GaAs. It is shown that the trajectory concept is capable of analyzing patterns with lateral dimensions smaller than the actual spot size (i.e., direct optical resolution of SRE).
Surface characterization of InP using photoluminescence
Authors: Chang R. R. , Iyer R. , Lile D. L.
Published: July 2002

Photoluminescence (PL) measurements have been performed on InP samples in situ during various surface treatments including chemical etching, wet anodization, and low-pressure chemical vapor deposition. It was found, in agreement with previously published results, that the magnitude of the PL signal varies markedly with surface treatment due presumably to changes in either surface-state density, and/or surface potential. In an attempt to assess the effectiveness of this noninvasive method as a tool for characterizing and monitoring the progressive development of a semiconductor surface during processing, a number of experiments on InP have been performed. The results indicate that although some uncertainty may exist in assigning a mechanism for the PL change in any given experiment, the general trend appears to be that surface degradation results in a reduced signal. As a result, process steps which enhance the PL intensity are likely to be beneficial in the preparation of a high-quality interface.
Calibration of aspirator-type ion counters and measurement of unipolar charge densities
Authors: Misakian M. , McKnight R. H. , Fenimore C.
Published: July 2002

The characterization of a parallel plate apparatus which can produce a unipolar charge density that is suitable for calibrating aspirator-type ion counters operating in the ground plane is described. The influence of a dc electric field, air motion, Coulomb repulsion, and diffusion on the transport of ions into the ion counter are examined to determine their effects on instrument calibration and measurements in the vicinity of high-voltage dc transmission lines. A charge density which is known with an uncertainty of less than ±9% is used to check the performance of an ion counter with and without a duct at its entrance.
Amorphous Ti-Si alloy formed by interdiffusion of amorphous Si and crystalline Ti multilayers
Authors: Holloway Karen , Sinclair Robert
Published: July 2002

Reactions upon rapid thermal annealing of sputtered Ti-Si multilayers have been studied by cross-section and through-foil transmission electron microscopy, glancing-angle Rutherford backscattering, and x-ray diffraction. The compositions of the samples are 40 at. % Ti, 60 at. % Si and 60 at. % Ti, 40 at. % Si, and the bilayer periodicity is about 10 nm. The silicon layers in the as-deposited films are amorphous; the titanium layers are polycrystalline hcp. After a 30-s anneal at 455 °C, significant interdiffusion occurs and we observed the formation of an amorphous Ti-Si alloy by interfacial reaction. The metastable disilicide, C49 TiSi2, nucleated along with a small amount of TiSi in the sample with higher silicon content (60%) upon annealing at 550 °C for 10 s, but the amorphous alloy remained as the only product of reaction in the 40-at. % Si sample.
Space-charge depletion studies of deep states in glassy semiconductors
Authors: Abkowitz M. , Maitra S.
Published: July 2002

The dark decay of open-circuit surface voltage on a composition series of capacitively charged a-Se-Te films has been analyzed. The bulk process controlling dark decay is demonstrated to be thermal emission of holes from discrete states deep in the mobility gap. The hole sweepout process causes the progressive development of negative deeply trapped space charge in the bulk which is experimentally manifested in the distinct shape of the associated dark discharge curves. The rate of dark discharge and the detailed shape of the dark decay function dV/dt vs t vary systematically with alloy composition. Phenomenological parameters of the dark decay can be related to an underlying Gaussian emission center distribution in the gap by numerical simulation techniques. It is demonstrated that progressive Te addition to the binary alloy broadens this distribution and shifts it toward the valence-band mobility edge.
Electron-trap generation by recombination of electrons and holes in SiO2
Authors: Chen I. C. , Holland S. , Hu C.
Published: July 2002

It is shown that after holes are injected and trapped in silicon dioxide (SiO2), subsequent electron injection will generate neutral electron traps. The density of electron traps generated is about 30% of the density of trapped holes. It is proposed that electron traps are created by the energy released through the recombination of electrons and holes, and that this is the mechanism of electron-trap generation during high-field oxide stressing. Similar oxide field and thickness dependencies of the rate of electron-trap generation and hole generation further support this model. This model can reconcile the main evidence for the electron-trapping oxide breakdown model with the hole-trapping breakdown model. It is consistent with the higher trap generation rate in irradiated SiO2. An analytical trapping model is derived and the electron capture cross sections of trapped holes and the generated neutral traps are found to be 10−14 cm2 and 5×10−16 cm2, respectively.
Synthesis of ferromagnetic τ phase of Mn-Al films by sputtering
Authors: Morisako A. , Matsumoto M. , Naoe M.
Published: July 2002

Mn-Al films have been prepared by dc magnetron sputtering and their crystallographic characteristics, magnetic properties, electrical resistivity, and internal stress have been investigated. The films deposited when substrate temperature Ts was below 100 °C and above 170 °C exhibit a high-temperature ε phase and β-Mn–like structure, respectively. Films of a ferromagnetic τ phase were synthesized with Mn content in the range of 58–62 at. % and Ts of about 150 °C. Existence of CuAu-type superstructure in the τ phase was confirmed by a change of electrical resistivity ρ of deposited films. The films deposited at a Ts of about 150 °C exhibit minimum value of 180 μΩ cm. Lattice constant a and c of the tetragonal τ-phase crystallites in the films were in the range of 3.582–3.592 Å and 3.941–3.968 Å, respectively, while those of the bulk alloy were 3.580 and 3.940 Å, respectively. The maximum value of saturation magnetization of the films was 120 emu/cc, being as small as about one fourth of that of bulk alloy. This may be caused by elongation of lattice constants and coexistence of nonmagnetic ε phase.
Magnetization reversal dynamics in the media of magneto-optical recording
Authors: Mansuripur M.
Published: July 2002

Domain wall dynamics in thin films of amorphous rare-earth–transition-metal alloys are investigated using numerical integration of the Landau–Lifshitz–Gilbert equation. A two-dimensional square lattice of dipoles with nearest-neighbor exchange interaction and random axis anisotropy is simulated and the structure of domain walls (with and without Bloch lines) is determined. The motion of domain walls under applied fields is then considered and the effects of random spatial variations of magnetic parameters upon wall mobility are discussed.
Theory of the velocity-field relation in AlGaAs
Authors: Brennan K. F. , Park D. H. , Hess K. , Littlejohn M. A.
Published: July 2002

We present ensemble Monte Carlo calculations of the steady-state electron drift velocity as a function of applied electric field in Al0.32 Ga0.68 As. The effect of various material parameters on the calculated velocity is assessed by varying each parameter independently by ±20%. It is found that both the optical phonon energy and intervalley separation energy alter the peak electron velocity greatly. Variations in the dielectric constants and central valley effective mass have little effect upon the peak drift velocity, but act to alter the threshold electric field. It is further found that the threshold electric field is greater in Al0.32Ga0.68As than in GaAs even though the central-to-satellite valley separation energy is less in AlGaAs. The combined effects of a greater central valley effective mass and a larger phonon energy in AlGaAs result in a greater threshold field. Finally, we present sets of material parameters useful in Monte Carlo models for both GaAs and AlGaAs.
Micromagnetic studies of thin metallic films (invited)
Authors: Zhu Jian-Gang , Bertram H. Neal
Published: July 2002

A computer simulation model has been developed to conduct micromagnetic studies of thin magnetic films. Thin-film media are modeled as a planar hexagonal array of hexagonally shaped grains. Each grain is a single domain particle whose magnetization reverses by coherent rotation. The computation utilizes coupled gyromagnetic dynamic equations with phenomenological Landau–Lifshitz damping. In particular, the effects of particle interactions are investigated. The effect of media microstructure on magnetic hysteresis is examined as well as the effect of intergranular exchange coupling. The difference between planar and completely random orientation of the crystalline anisotropy axes is discussed. Recorded transitions are simulated by allowing a pair of perfect transitions to relax. With no intergranular exchange coupling, the transitions show profound irregularity and zig-zag structure. Intergranular exchange coupling produces more uniform transitions with increased zig-zag structure amplitude. For a closely spaced transition pair, the equilibrium configuration yields percolated transition boundaries with stable reverse island domains. The effect of gyromagnetic precession also has been examined.