bluuebunny/crossref_metadata_2025 huggingface dataset:
Crystallization of sputtered BaTiO3-based thin films: A differential scanning calorimetry study
Authors: Chu J. P. , Wang S. F. , Lee S. J. , Chang C. W.
Published: July 2002

Crystallization behavior of sputtered BaTiO3 and Nb-doped BaTiO3 thin films has been characterized by means of the differential scanning calorimetry (DSC). The crystallization of as-deposited amorphous structure to the equilibrium crystalline structure is confirmed as an irreversible, exothermic, first-order transition. Our in situ DSC result confirming the metastable nature of sputtered films is in agreement with that of a conventional, ex situ x-ray diffraction result. At a heating rate of 20 °C/min, the crystallization peak temperature of BaTiO3 film is found to be 467.6 °C and to increase with the addition of Nb. Activation energies for crystallization are determined by Kissinger's method to be 139.2, 99.3, and 81.0 kJ/mol for pure BaTiO3, BaTiO3–2 at. % Nb, and BaTiO3–3 at. % Nb films, respectively. The decrease in activation energy suggests the addition of Nb tends to assist the thermal-activated crystallization process in sputtered films.
Improved strong magnetic field performance of low angle grain boundaries of calcium and oxygen overdoped YBa2Cu3Ox
Authors: Daniels George A. , Gurevich Alex , Larbalestier David C.
Published: July 2002

Critical currents and extended voltage–current characteristics of [001] tilt, low angle boundaries in thin film bicrystals of pure YBa2Cu3Ox and Y0.7Ca0.3Ba2Cu3Ox in strong magnetic fields (H<10 T) have been measured. For 5°[001] tilt boundaries, there are clear benefits to Ca additions, particularly when the sample is oxygen treated to further overdope the sample after growth. Ca-doping decreases excess grain boundary resistance and diminishes the gap between inter- and intragrain current density over a wide field and temperature range. In spite of the lower Tc of the 0.3 Ca-doped samples, Ca doping increases the intergrain critical current density at 44 K as high as 30% over 0–3 T and threefold at 5 T.
Modeling of dispersive nonphotochemical hole growth kinetics data: Al-phthalocyanine tetrasulphonate in hyperquenched glassy water
Authors: Reinot T. , Small G. J.
Published: July 2002

Zero-phonon hole (ZPH) growth kinetics data that span six decades of burn fluence are reported for Al-phthalocyanine tetrasulphonate (APT) in hyperquenched glassy water (HGW) at 5.0 K. The kinetics are highly dispersive. The hole growth equation used for analysis of the dispersion incorporates three distributions (λ, α, and ω) where λ is the tunnel parameter associated with nonphotochemical hole burning (NPHB), α is the angle between the transition dipole and the laser polarization and the ω-distribution stems from off-resonant absorption of the zero-phonon line (ZPL). The single site absorption profile used includes the phonon sideband as well as the ZPL. The homogeneous width of the ZPL and shape of the phonon sideband were determined from experiment. Eight models, which include the possible combinations of the above distributions, were used to fit the data. As in previous works the λ-distribution was taken to be a Gaussian peaked at λ=λ0 with a standard deviation of σλ. The results show that the contribution to the dispersive kinetics from the λ-distribution is of primary importance. It provides a good fit to the data over the first three decades of burn fluence (∼80% of the saturated ZPH depth). The intrinsic contributions from the α- and ω-distributions become important for the last ∼20% of the burn. These two distributions by themselves or in combination yielded poor fits to the data. The three distributions in combination (λαω-model) provided a good fit over the first five decades of burn fluence. Importantly, the λ0 and σλ values of 8.3 and 0.95 from the λ-distribution alone are nearly the same as those from the λαω-distribution. The above findings for APT/HGW should be widely applicable since previous studies of other NPHB systems led to σλ values ≳1. It is emphasized that APT/HGW is an ideal system for hole growth studies because of its very narrow ZPL and weak electron-phonon coupling (S∼0.2) and because it satisfies the homogeneity condition, i.e., all sites are burnable.
Braided coaddition on the quantized braided matrices
Authors: Gao Ya-Jun , Gui Yuan-Xing
Published: July 2002

An additive braided coproduct (also be called braided coaddition) is introduced on the quantized braided matrices (QBMs). It gives QBMs another braided-Hopf algebra structure. The coaddition is also shown to be compatible with the existing coproduct such that they together form a so-called quantized-braided ring. And some quantized braided differential operator bialgebras (Hopf algebras) relating to this braided coaddition are constructed. These give a unification and generalization of the known results about braided and quantum matrices. Moreover, the coaddition construction and the related differential calculi on the QBMs are further extended to a kind of more general quantum matrix algebraic systems. Some examples are also given.
Microstructure and wet oxidation of low-temperature-grown amorphous (Al/Ga,As)
Authors: Chang K. L. , Pickrell G. W. , Wohlert D. E. , Epple J. H. , Lin H. C. , Cheng K. Y. , Hsieh K. C.
Published: July 2002

Amorphous and polycrystalline compounds of (Ga,As) and (Al,As) grown at very low temperatures by molecular-beam epitaxy are characterized. The ultimate microstructure and the amount of excess arsenic incorporated in the (Ga,As) or (Al,As) layers are found to depend on the arsenic overpressure during the low-temperature growth. With lower arsenic overpressure, a polycrystalline structure prevails and less excess arsenic is observed inside the layer. In contrast, a high incorporation of excess arsenic achieved by high-arsenic overpressures leads to the formation of amorphous films. Upon wet oxidation, the lateral oxidation rate of (Al,As) is found to depend on the crystallinity of the (Al,As) layer and the amount of excess arsenic. During the same process, recrystallization proceeds in the (Ga,As) layer.
Comment on “From Rouse dynamics to local relaxation: A neutron spin echo study on polyisobutylene melts” [J. Chem. Phys. 111, 6107 (1999)]
Authors: Harnau Ludger
Published: July 2002

The dynamic structure factor of polyisobutylene chains in a melt is calculated using a semiflexible chain model. It is demonstrated that recently reported deviations between the theoretical calculations and experimental data are due to the use of an inappropriate model parameter.
Structure of alkali tellurite glasses from neutron diffraction and molecular orbital calculations
Authors: Niida Haruki , Uchino Takashi , Jin Jisun , Kim Sae-Hoon , Fukunaga Toshiharu , Yoko Toshinobu
Published: July 2002

The structure of pure TeO2 and alkali tellurite glasses has been examined by neutron diffraction and ab initio molecular orbital methods. The experimental radial distribution functions along with the calculated results have demonstrated that the basic structural units in tellurite glasses change from highly strained TeO4 trigonal bipyramids to more regular TeO3 trigonal pyramids with increasing alkali content. It has also been shown that the TeO3 trigonal pyramids do not exist in the form of isolated units in the glass network but interact with each other to form intertrigonal Te⋯O linkages. The present results suggest that nonbridging oxygen (NBO) atoms in tellurite glasses do not exist in their “pure” form; that is, all the NBO atoms in TeO3 trigonal bipyramids will interact with the first- and/or second-neighbor Te atoms, resulting in the three-dimensional continuous random network even in tellurite glasses with over 30 mol % of alkali oxides.
Three formulas for the off-diagonal density matrix of a dirac spinor, with an application
Authors: Wightman A. S.
Published: July 2002

Expressions for the off-diagonal density matrix of a Dirac spinor in space–time dimensions 2, 3, and 4 are derived. They are used to obtain formulas for the dual potential of a free Dirac current in two, three, and four dimensions of space–time.
Exact numerical derivatives of the pair-correlation function of simple liquids using the tangent linear method
Authors: Charpentier I. , Jakse N.
Published: July 2002

We present for the first time the tangent linear method applied to the algorithm for solving integral equations of the theory of simple liquids. The purpose of this method is to compute the exact derivatives of the pair-correlation function g(r) with respect to density and temperature, which appear in thermodynamic properties and in the self-consistent procedure. An automatic differentiation tool is applied to the numerical algorithm of the integral equation method that combines the Newton–Raphson and the traditional iterative techniques. It results in a tangent linear code for the integral equation proposed by Vompe and Martynov, which is applied to calculate the thermodynamic properties and the bridge functions of the Lennard–Jones fluid. The results suggest that the tangent linear method should be preferred over the different strategies of differentiation that have been used up to now. Moreover, a local formulation of the Gibbs–Duhem relation is tractable in this framework.
Absolute total and partial cross sections for the electron impact ionization of tetrafluorosilane (SiF4)
Authors: Basner R. , Schmidt M. , Denisov E. , Becker K. , Deutsch H.
Published: July 2002

We measured absolute partial cross sections for the formation of various singly charged and doubly charged positive ions produced by electron impact on SiF4 from threshold to 900 eV using a time-of-flight mass spectrometer. Dissociative ionization was found to be the dominant process, although we found evidence of the presence of the SiF4+ parent ion in our experiment. The SiF3+ fragment ion has the largest partial ionization cross section with a maximum value of 4.3×10−16 cm2 at 90 eV. All other singly charged fragment ion cross sections are about one order of magnitude smaller at this impact energy. The cross-section values of the doubly charged ions with the exception of SiF2++ are about two orders of magnitude smaller. A comparison is made with available previously measured data. Additional measurements using a sector-field mass spectrometer revealed that all fragment ions are formed with excess kinetic energy. The experimentally determined total single ionization cross section of SiF4 is compared with results of semiempirical and semiclassical calculations and reasonable agreement is found.
Quantum-mechanical effects on eikonal cross section for elastic electron–ion collisions in dense high-temperature plasmas
Authors: Jung Young-Dae
Published: July 2002

The eikonal method is applied to investigate quantum-mechanical effects on elastic electron–ion collisions in dense high temperature plasmas. An effective pseudopotential model taking into account both quantum-mechanical effects and plasma screening effects is applied to describe the electron–ion interactions in dense high-temperature plasmas. The straight-line trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal cross section as a function of the impact parameter, thermal de Broglie wavelength, Debye length, and projectile energy. In the first-order eikonal approximation, the quantum-mechanical effect reduces the elastic electron–ion scattering cross section. It is also found that the quantum mechanical effect is increased with increasing the projectile energy.
Temperature-dependent electroluminescence in poly [2-methoxy-5(2′-ethylhexyloxy)-p-phenylenevinylene] light-emitting diode
Authors: Saha S. K. , Su Y. K. , Juang F. S. , Yokoyama M.
Published: July 2002

Temperature-dependent quantum efficiency in the device structure indium-tinoxide/[(2-methoxy-5(2′-ethylhexyloxy-)-p-phenylenevinylene)]/Ca/Al has been investigated over the temperature range from 20 to 296 K. A blueshift is observed in the electroluminescence spectra with an increasing temperature or voltage. From Fourier transform infrared spectra and UV-visible analyses have been observed that the conjugated length of the polymer chain decreases due to photo oxidation under UV component of sun light. The change in band gap resulting from the change in the conjugated length causes the observed blueshift. Quantum efficiency increases with decreasing temperature partly because of the enhancement of photoluminescence efficiency and partly because of the improved balance of holes and electrons. At a fixed temperature, an optimum voltage is required to obtain the maximum efficiency, and with increasing temperature, this voltage increases.
Semiempirical models for image electrostatics. I. Bare external charge
Authors: Rassolov Vitaly A. , Ratner Mark A. , Pople John A.
Published: July 2002

We develop a simple semiempirical Pariser–Parr–Pople-type model of a metal. It can be used for studies of adsorption and of the interactions between the metal surface and external charges, and is compatible with ab initio description of the molecules outside the metal. We investigate the dependence of the image-type field on the few parameters that control the model behavior, and develop a simple direct minimization self-consistent field technique that is useful for convergence in nearly degenerate cases.
Instantaneous normal modes analysis of amorphous and supercooled silica
Authors: Bembenek Scott D. , Laird Brian B.
Published: July 2002

The dynamics of a model for amorphous and supercooled silica (SiO2), a strong glass former, is studied using instantaneous normal mode (INM) analysis. The INM spectra at a variety of temperatures are calculated via molecular dynamics simulation. At temperatures below the glass transition temperature, the dominant contribution to the soft highly anharmonic modes comprising the imaginary frequency region of the INM spectrum are found to correspond to coupled rotations of SiO4 tetrahedral units, consistent with interpretations of neutron scattering experiments [B. B. Buchenau, H. M. Zhou, and N. Nucker, Phys. Rev. Lett. 60, 1318 (1988)] and with previous normal mode analysis of simulation results at T=0 K [S. N. Taraskin and S. R. Elliot, Phys. Rev. B 56, 8623 (1997)].
Electronic structure of the NaN2 and NaC2H2 collision complexes: Experiment and theory
Authors: Goldstein R. , Grosser J. , Hoffmann O. , Schumann V. , Wößner D. , Jungen M. , Lehner M.
Published: July 2002

The electronic structure of the NaN2 and NaC2H2 collision pairs is probed by optical excitation. Based on polarization experiments, we measure the alignment tensor of the electronic transition dipole moments. We compare the experimental data with the results of a quantum chemical calculation. The different geometric properties of the electronic wave functions of the two collision pairs are directly visible in the experimental data.
Investigation of intermittent magnetic flux in the auroral zones with kilometer radiation (AKR)
Authors: Liu S. Q. , Li X. Q.
Published: July 2002

On the basis of the nonlinear equations for self-generated magnetic fields, it is numerically shown that the magnetic fields self-generated are instable and may collapse, resulting in spatially highly intermittent flux fragment. Numerical results show that the enhanced magnetic flux has a strength about up to 10−2 Gauss in range about around 250–350 km in auroral zones with kilometric radiation (AKR), which correspond to estimated values in both the strength and characteristic scale by Mckean et al. [J. Geophys. Res. [Oceans] 96, 21055 (1991)].
Microwave reflectometry for turbulence studies on ASDEX Upgrade
Authors: Ribeiro T. T. , Serra F. , Conway G. D. , Manso M. E. , Ryter F. , Cupido L. , Kurzan B. , Silva A. , Suttrop W. , Vergamota S. ,   ASDEX Upgrade Team
Published: July 2002

We study the evolution of turbulence at several radial plasma locations in the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak, using microwave reflectometry. We analyze two types of scenarios: H-mode plasmas with improved performance and plasmas with electron cyclotron resonance heating modulation. Profound modifications of the turbulence spectra, occurring at specific plasma regions, have been found in both cases, indicating the relation between turbulence changes and confinement. The results demonstrate the ability of the reflectometry system on ASDEX Upgrade to make localized measurements. Mapping the radial distribution of turbulence can help us to understand the links between plasma rotation, shear, E×B flows, and the plasma turbulence behavior.
Quantum-confined Stark effect modulator based on multiple triple-quantum wells
Authors: Tobin Mary S. , Bruno John D.
Published: July 2002

A GaAs/AlGaAs triple-quantum-well structure is designed to have its lowest energy excitonic transition located significantly above the GaAs band gap, while maintaining large Stark shifts. We present photocurrent measurements of absorption features found in two different Al0.3Ga0.7As p-i-n diode structures: one contains multiple GaAs coupled-triple-quantum wells, and the other, multiple Al0.6Ga0.94As quantum wells. Both systems are designed to have their absorption edges at the same energy. The photocurrent spectra are compared, and preliminary 810 nm modulator results are presented based on the multiple triple-quantum-well approach.
Improved electroluminescence of ZnS:Mn thin films by codoping with potassium chloride
Authors: Waldrip K. E. , Lewis J. S. , Zhai Q. , Puga-Lambers M. , Davidson M. R. , Holloway P. H. , Sun S.-S.
Published: July 2002

Alternating current thin film electroluminescent devices have been fabricated using sputter-deposited ZnS:Mn with and without codoped potassium chloride via both in situ and ex situ methods. In situ codoping proved to be difficult due to a memory effect in the deposition chamber. Samples codoped with potassium chloride via an ex situ diffusion method exhibited improvements in brightness of up to 70% (572 vs 337 cd/m2) and efficiency of up to 60% (1.95 vs 1.25 lm/W) over noncodoped samples. The threshold voltage increased by ≈5% (160 vs 168 V), and the brightness-versus-voltage curve stabilized more rapidly for the devices. Several possible mechanisms to explain these effects are discussed. While modest microstructural changes contribute to the improvements, changes in point defects which lead to modification of the space charge in the devices appears to be the dominant mechanism.
A new Bragg reflection detection instrument for ultrasonic measurement based on self-mixing effects for laser diode
Authors: Yarai Atsushi , Nakanishi Takuji
Published: July 2002

A new Bragg reflection detection instrument for an ultrasonic measurement that is based on self-mixing effects due to an optical feedback for a laser diode is proposed. This instrument allows measurement of the velocity and/or absorption of ultrasound in an ultrahigh frequency (UHF) range, in spite of its extremely simple configuration. The velocity of an ultrasonic wave in water was measured in an UHF range. The velocity was within 5% of the velocity in the previous results.
Heat capacity of the liquid–liquid mixture perfluoroheptane and 2,2,4-trimethylpentane near the critical point
Authors: Oby E. R. , Jacobs D. T.
Published: July 2002

The heat capacity of the liquid–liquid mixture perfluoroheptane and 2,2,4-trimethylpentane (also known as iso-octane) has been measured for the first time near its upper critical consolute point using an adiabatic calorimeter. The theoretical expression for the heat capacity near the critical point was applied to our combined data runs. The critical exponent α was determined to be 0.106±0.026, which agreed with theoretical predictions. When α was fixed at its theoretical value of 0.11, our value for the amplitude ratio A+/A−=0.59±0.05 was consistent with experimental determinations and theoretical predictions. However, the two-scale-factor universality ratio X, now consistent among experiments and theories with a value between 0.019 and 0.020, was violated in this system when using the published value for the correlation length.
Nonlinear terahertz oscillations in a semiconductor superlattice
Authors: Romanov Yuriy A. , Romanova Julia Yu. , Mourokh Lev G. , Horing Norman J. M.
Published: July 2002

The nonlinear oscillations of field and current in semiconductor superlattices excited by terahertz laser radiation are studied within a self-consistent multifrequency internal field approach. We show that the oscillatory character of the nonlinear susceptibilities and dissipative and parametric instabilities in superlattices leads to multivaluedness and hysteresis of their spectral harmonics as functions of the external field amplitude. The key mechanisms of such spectral behavior are the spontaneous generation of a static field and parametric creation and amplification of the external field harmonics and subharmonics. It is shown that the field inside the superlattice cannot be understood in terms of a single frequency, especially for superlattices having high electron concentration.
Laser micromachining using liquid optics
Authors: Ramanathan D. , Molian P. A.
Published: July 2002

Lasers are well suited for micromanufacturing by ablating nearly all the material within the focal spot before any significant heat conduction or mass flow takes place, thus enabling precise machining with little thermal damage. In this letter, the results of an idea of using nonlinear liquids as lenses to fabricate micron-sized features are reported. Our research showed that transmitting the laser pulse through a transparent condensed medium such as carbon disulfide that has very high intensity-dependent nonlinear refractive index creates a lens and focuses the beam. Results on 316 stainless steel showed a two-fold decrease in the hole size and reduced taper when liquid optics were used over traditional solid optics. Polarization effects were also substantially reduced. The beneficial effects of liquid optics are explored.
On the origin of the “coffee-bean” contrast in transmission electron microscopy images of CdSe/ZnSe quantum dot structures
Authors: Litvinov D. , Rosenauer A. , Gerthsen D. , Preis H. , Bauer S. , Kurtz E.
Published: July 2002

The origin of the “coffee-bean” strain contrast is studied, that is observed in the plan-view transmission electron microscopy (TEM) images of CdSe/ZnSe quantum dot structures. The samples were grown by two different methods: standard molecular-beam epitaxy at 350 °C and atomic layer epitaxy at 230 °C with annealing at 340 °C after the CdSe deposition. The nominal CdSe thickness was above 3 ML. In situ reflection high energy electron diffraction during the growth or during the annealing shows the transition from the two- (2D) into the three-dimensional (3D) surface morphology for both samples. The coffee-bean contrast is usually assigned to three-dimensional islands which are generated after the morphological 2D/3D transition. It is found that the coffee-bean contrast in plan-view TEM images is alternatively associated with pairs of stacking faults on {111} lattice planes which are inclined against each other. The stacking faults, which are bound by Shockley partial dislocations, are preferably generated in the vicinity of the Cd-rich regions (large islands) of the CdZnSe layer where Cd concentrations of more than 40% are found.
Scanning Hall probe microscopy of flux penetration into a superconducting YBa2Cu3O7−δ thin film strip
Authors: Grigorenko A. N. , Bending S. J. , Gregory J. K. , Humphreys R. G.
Published: July 2002

A high-resolution scanning Hall probe microscope has been used to measure flux profiles across one “wire” of a long YBa2Cu3O7−δ thin-film meander line as a function of both transport current density and applied magnetic field. Flux bundle penetration due to an applied current or magnetic field is demonstrated to occur at the same regions at the edge of the strip. A correlation between the surface topography of the meander line edges and the regions of penetration has been established. Penetrating flux profiles at low temperatures are in qualitative agreement with theories of dynamical instability of the order parameter.
n
- and p-type dopants for cubic silicon nitride
Authors: Oba Fumiyasu , Tatsumi Kazuyoshi , Adachi Hirohiko , Tanaka Isao
Published: July 2002

The formation and ionization energies of impurities in cubic silicon nitride are investigated through first-principles calculations. Among the elements in the groups III to VI, P and O are preferable for n-type doping, while Al is favorable for p-type doping in terms of the formation and ionization energies. The compensation of doped carriers associated with the incorporation of these impurities into anti and interstitial sites can be suppressed if appropriate growth conditions are chosen.
Monte Carlo simulation of polymer brushes attached to a spherical surface
Authors: Lindberg Eva , Elvingson Christer
Published: July 2002

We present results of Monte Carlo simulations of polymer brushes attached to a spherical surface. We have varied the chain length, surface radius, and the grafting density, and the analysis has included the size, shape, and orientation as well as segment density profiles. We also compared different methods of generating the starting configurations to establish the results being independent of the initial structure. For flexible molecules, the segment density profiles agree with earlier results from self-consistent field calculations, and there is no depletion zone for the chain ends close to the surface, except for the largest molecules. Increasing the persistence length, the orientation of the molecules increases and the chain density close to the surface decreases, but there is still a finite probability of finding a chain end close to the particle surface. We have also looked at the effect of restricting the available space for the chains by an outer bounding surface and the change in molecular shape and size is then seen much earlier compared to the effect on the subunit density profiles.
Electrostatic control of microstructure thermal conductivity
Authors: Supino Ryan N. , Talghader Joseph J.
Published: July 2002

A technology for controlling the thermal conductivity of etch-released microstructures is proposed and demonstrated by placing test structures in and out of contact with their underlying substrate. By adjusting the duty cycle of a periodic actuation, the thermal conductivity can be adjusted linearly across a wide range. Experimental work with microfilaments in air has shown a continuous tuning range from approximately 1.7×10−4 W/K to 3.3×10−4 W/K. These numbers are limited by thermal conduction through air and thermal contact conductance, respectively. The fundamental tuning range is orders of magnitude wider, limited by radiation heat transfer and the thermal contact conductance of coated structures.
CH-stretching overtone spectra of a fast rotating methyl group. II. Toluenes C6D5CH2D and C6D5CHD2
Authors: Cavagnat D. , Lespade L.
Published: July 2002

The CH-stretching overtone spectra of the methyl group in gaseous toluene C6D5CH2D and C6D5CHD2 are recorded with conventional Fourier transform near infrared spectroscopy in the Δv=1–4 regions and by intracavity laser photoacoustic spectroscopy in the Δv=5 and 6 regions. The spectra are analyzed with a theoretical model that takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving methyl bending and rocking modes. A simultaneous successful reconstruction of the CH stretching overtone spectra of three isotopic derivatives of the methyl group of toluene is obtained with a single set of parameters from Δv=1–6 (18 spectra). The modifications caused by the partial deuteration of the rotating methyl group on its IVR mechanisms are analyzed. In both methyl deuterated compounds, the CH/CD interbond coupling induces a shift of the overtone spectra towards high frequencies and the appearance of additional features in the high energy overtone spectra corresponding to [(n−1)νCH+1νCD] resonant combination states. In toluene C6D5CHD2, this shift is progressively amplified until Δv=4 by Fermi resonance phenomena involving mainly HCD bending modes. From Δv=3, the effect of these anharmonic interactions appears as an extra structure in the low energy side of the spectra. The “tuning” of these interacting states into resonance is reached at Δv=5 and causes a strong intramolecular vibrational energy redistribution. In toluene C6D5CH2D, at Δv=1 and 2, the CH stretching spectra are perturbed by Fermi resonance with HCH bending mode “doorway” states. A transitional regime between normal and local mode is detected at Δv=2, which requires a more elaborate model. At higher energy, the HCH bending mode combinations move farther out of resonance. From Δv=4–6, the perturbation of the spectra is then increasingly due to Fermi resonance phenomena involving HCD bending modes.
Growth, structure, and magnetic properties of Fe monolayers on Cu84Al16(100)
Authors: Martins M. D. , Andrade L. H. F. , Gastelois P. L. , Macedo W. A. A.
Published: July 2002

We present experimental results on the growth and structure of Fe overlayers deposited on Cu84Al16(100) and discuss the correlation between the structural and magnetic properties of this system. Fe films 1–6 monolayers (ML) thick were grown under molecular beam epitaxy conditions onto the clean substrate at 160 K. Electron diffraction was applied to investigate the structure of the Fe films. The magnetic properties were investigated in situ by surface magneto-optical Kerr effect in the longitudinal geometry. Our results show that the onset of in-plane ferromagnetism around 3.5 ML of Fe coincides with structural changes that suggest a transformation within the Fe films from a fcc-like (100) to a bcc-like (110) structure with increasing Fe thickness, starting from 2.5 ML.
Magnetization process in FePd thin films
Authors: Klein O. , Samson Y. , Marty A. , Guillous S. , Viret M. , Fermon C. , Alloul H.
Published: July 2002

A custom made magnetic force microscope is used to study the magnetization process in thin films of FePd throughout the entire hysteresis loop. The 40 nm thick sample has a strong perpendicular anisotropy, which leads to a maze of 80 nm wide stripes of opposite polarity in the remanent state. The growth of M, when H increases, happens through an unwinding of the reversed domain along their axis. Together with the length recession, the reversed domain width also contracts with increasing field. The later effect is estimated by comparison of our images with magneto-optical Kerr measurements. A large disorder in the propagation process of the domain walls is observed. It is also found that the bubble configuration near the saturation field is unstable.
Two-dimensional to three-dimensional crossover and magnetic penetration depth study in NbTi/Co multilayers
Authors: Lee S. F. , Chuang T. M. , Huang S. Y. , Chang W. L. , Yao Y. D.
Published: July 2002

A series of NbTi single thin films, Co/NbTi/Co trilayers, and multilayers are made by dc sputtering to investigate the superconductive dimensional crossover between two-dimensions (2D) and three dimensions. From the temperature dependence of the upper critical fields measured resistively, we see a nonlinear changing to linear dependence of Hc2(T) from 60 to 80 nm of NbTi thickness for trilayers. Insertion of 0.6 nm thick Co between 30 nm NbTi films also shows 2D behavior, which suggests confinement of the superconducting wave function within individual layers. Magnetic field response is measured to study the interplay between the superconductor and ferromagnet. We inferred a magnetic penetration depth of ∼35 nm into NbTi.
Magnetic field induced irreversibility in UNiAl
Authors: Sechovský V. , Prokeš K. , Svoboda P. , Syshchenko O. , Chernyavski O. , Sato H. , Fujita T. , Suzuki T. , Doerr M. , Rotter M. , Loewenhaupt M. , Gukasov A.
Published: July 2002

We report on low-temperature measurements of magnetization, magnetoresistivity, Hall resistivity, magnetostriction, and elastic moduli on a UNiAl single crystal in magnetic fields up to 14 T applied along the c axis of the hexagonal structure. UNiAl is an itinerant 5f-electron antiferromagnet below TN=19.3 K. At temperatures below 7 K (∼0.3 TN), it exhibits a first-order metamagnetic transition (MT) at a critical field μ0Hc≈11.35 T between the low-field antiferromagnetic state and the high-field ferromagnetic one. The transition is accompanied by a sharp anomaly in transport and lattice properties. A striking observation is that the “virgin” curves expressing the field dependence of the various electronic properties clearly differ from those observed on a sample that has once appeared in the high-field state. This irreversibility in bulk properties corresponds to an irreversible change of the magnetic-structure propagation vector within the basal plane observed by neutron diffraction. The different shapes of M(H), ρ(H), and ρH(H) curves are strongly suggestive of field-induced changes in the spectrum of spin fluctuations, which play a substantial role in the physics of UNiAl. This seems to be corroborated by the effect of “high-field annealing” on the low-temperature specific-heat data. The irreversibility vanishes at temperatures T⩾7 K where the MT loses its first-order character.
Imaging and magnetotransport in superconductor/magnetic dot arrays
Authors: Silevitch D. M. , Reich D. H. , Chien C. L. , Field S. B. , Shtrikman H.
Published: July 2002

Magnetoresistance and scanning Hall probe microscopy studies of Nb-film/Ni-dot structures are reported. The dots act as pinning sites for superconducting vortices. The transport measurements focus on the effects of introducing disorder into the positions of the pinning lattice near the superconducting critical temperature Tc in structures with 250 nm diameter Ni dots randomized about an ideal square lattice with lattice constant a=560 nm. Features observable in the ordered arrays at higher multiples of the matching field H0=Φ0/a2 are washed out in the disordered arrays, but those at H0 remain. Scanning Hall probe microscope images were taken of the vortex configurations at fields up to 1.2H0 in ordered arrays of 1-μm-diameter dots on a 5.2 μm×4 μm rectangular lattice. These show that despite the relatively weak pinning of the magnetic dots, ordering commensurate with the dot lattice occurs even for fields below H0. Both transport and imaging studies point to the importance of interstitial vortices in determining the properties of superconductor/magnetic dot arrays.
High duty cycle operation of quantum cascade lasers based on graded superlattice active regions
Authors: Martini Rainer , Gmachi Claire , Tredicucci Alessandro , Capasso Federico , Hutchinson Albert L. , Sivco Deborah L. , Cho Alfred Y. , Whittaker Edward A.
Published: July 2002

High duty cycle operation of quantum cascade superlattice lasers with graded superlattice active regions is investigated with the goal of achieving high average optical power. The optical output power increases with pulse width and decreases with heat sink temperature. This behavior is explained on the basis of the laser core temperature oscillations during the pulsed, high duty cycle operation. Between 175 and 325 K heat sink temperature, optimum duty cycles vary from 10% to 1% and average power levels vary from 50 to 1 mW for various lasers used in this study.
Spin-stand imaging of transverse magnetization profiles of recorded tracks
Authors: Mayergoyz I. D. , Tse C. , Krafft C. , Adly A.
Published: July 2002

In this article the distribution of the transverse component of magnetization of recorded tracks is extensively studied for different track widths and various recording frequencies by using the spin-stand imaging technique [Mayergoyz et al., J. Appl. Phys. 87, 6824 (2000)]. It is demonstrated that for narrow tracks the transverse magnetization component can be quite substantial and it significantly protrudes toward the track center.
Spin configurations and negative coercivity in epitaxially grown DyFe2/YFe2 superlattices
Authors: Gordeev S. N. , Beaujour J.-M. L. , Bowden G. J. , de Groot P. A. J. , Rainford B. D. , Ward R. C. C. , Wells M. R.
Published: July 2002

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2/YFe2 superlattice samples with a (110) growth direction. In particular, the magnetic properties of the YFe2 dominated multilayer samples [wDyFe2/4wYFe2]×N with w=20, 30, 40, 45, 50, and 55 Å are presented and discussed. In principle, the multilayer films should possess similar magnetic properties because they are all nominally Dy0.2Y0.8Fe2. However, it is shown that their magnetic properties depend strongly on the thickness of the DyFe2 layers w. Those films with w⩾45 Å possess negative coercivities, while those with w⩽40 Å are positive. It is argued that this behavior can be understood in terms of a strong increase of the intrinsic coercivity of the DyFe2/YFe2 superlattice, taken as a whole, as w is reduced. For w⩽40 Å almost none of the DyFe2 moments can be flipped over in the available field range (12 T).
Exchange bias in Fe/KNiF3 bilayers
Authors: Wee L. , Stamps R. L. , Celinski Z. , Skrzypek D.
Published: July 2002

The exchange bias effect is known to depend strongly on the condition of the ferromagnet/antiferromagnet interface. Bilayers of single-crystal Fe and polycrystalline KNiF3 were grown and exchange bias observed using superconducting quantum interference device magnetometry. Hysteresis loops after field cooling are asymmetric in the forward and reverse directions. This asymmetry disappears at 50 K, indicating a blocking temperature for the bias well below the Néel temperature of KNiF3 (250 K).
Low temperature behavior of magnetic domains observed using a magnetic force microscope
Authors: Chung S. H. , Shinde S. R. , Ogale S. B. , Venkatesan T. , Greene R. L. , Dreyer M. , Gomez R. D.
Published: July 2002

A commercial atomic force microscope/magnetic force microscope (MFM) was modified to cool magnetic samples down to around 100 K under a high vacuum while maintaining its routine imaging functionality. MFM images of a 120 nm thick La0.7Ca0.3MnO3 film on a LaAlO3 substrate at low temperature show the paramagnetic-to-ferromagnetic phase transition. Evolution of magnetic domains and magnetic ripples with decreasing temperature are also observed near the edge of a 20 nm thick patterned Co film on a Si substrate.
Anisotropic hyperfine field fluctuation in Sr2FeMoO6
Authors: Kim Sung Baek , Park Key Taeck , Kim Chul Sung
Published: July 2002

The double perovskite Sr2FeMoO6 has been studied by the Mössbauer technique, neutron, and x-ray diffraction. The structure is found to be tetragonal with lattice constants a0=5.5729 Å and c0=7.9077 Å. Mössbauer spectra and neutron diffraction measurements of Sr2FeMoO6 have been taken at various temperatures ranging from 10 to 473 K. The low-field magnetoresistance magnitude (Δρ/ρ0) at 500 Oe was 3.1% and 1.8% at 77 and 300 K, respectively. As the temperature increases toward to the Curie temperature, TC=425 K. Mössbauer spectra show the line broadening and 1, 6 and 3, 4 linewidth difference because of anisotropic hyperfine field fluctuation. The anisotropic field fluctuation of +H(P+=0.85) was great than −H(P−=0.15). We also calculated frequency factor and anisotropy energy with values of 9.8 Γ/ℏ and 149.6 erg/cm3, respectively, using the relatively accurate data for T=260 K which is associated with the large line broadening.
Seeding of InP islands on InAs quantum dot templates
Authors: Medeiros-Ribeiro G. , Maltez R. L. , Bernussi A. A. , Ugarte D. , de Carvalho W.
Published: July 2002

The ability of stacking layers of islands and their corresponding alignment have prompted a number of studies. The main focus so far has been on stacking self-assembled quantum dot (QD) layers of the same material and composition. Our goal is to create systems of coupled QDs of different electronic properties, aiming at hybridization of their different electronic levels. In this work, we investigate the early stages of the coupling of alternate InAs–InP QD layers through a GaAs spacer layer. We have found that by using an InAs layer containing QDs as seeds, we can control the size, shape and density of InP islands by varying the spacer thickness. We have observed a significant improvement of the InP island size uniformity, as well as an induced size reduction, thus providing an extra degree of tunability previously available only through growth kinetics.
Classification of R-operators
Authors: Shibukawa Youichi
Published: July 2002

We classified the R-operators which satisfy the quantum Yang–Baxter equation on a function space. In this study, we gave all the meromorphic solutions of the system of the functional equations which is a necessary and sufficient condition for the R-operator to satisfy the Yang–Baxter equation. Most of the solutions were expressed in terms of the elliptic, trigonometric and rational functions.
Comment on “Local responsivity in quantum well photodetectors” [J. Appl. Phys. 86, 7059 (1999)]
Authors: Ryzhii M. , Khmyrova I.
Published: July 2002

The response of multiple quantum well (QW) infrared photodetectors (QW) to the photoexcitation of one QW selected from many identical QWs was recently modeled [M. Ershov, J. Appl. Phys. 86, 7059 (1999)]. We point out here that the presented analysis based on the use of drift-diffusion model for a system with a few electrons is incorrect.
Electric field modulation spectroscopy by scanning tunneling microscopy with a nanometer-scale resolution
Authors: Hida Akira , Mera Yutaka , Maeda Koji
Published: July 2002

Instrumentation has been devised by coupling electric field modulation spectroscopy (EFMS) with scanning tunneling microscopy (STM) that enables the investigation of electronic band structures in semiconductors with a nanometer-scale spatial resolution. Model experiments using low-temperature-grown GaAs (LT–GaAs) epifilms show that a difference as small as 0.01 eV in electronic energy gaps between the GaAs substrate and the LT–GaAs epilayers can be distinguished in the EFMS spectra, demonstrating the high energy, as well as spatial, resolution of the STM–EFMS.
Bulk-hardened magnets based on Y2Co17
Authors: Gabay A. M. , Zhang Y. , Hadjipanayis G. C.
Published: July 2002

Bulk magnetic hardening of cast Y2Co17-based alloys was systematically studied for different compositions and heat treatments. Additions of Cu and Zr, Hf, or Ti to the Y–Co were found to be essential for developing coercivity. The performance of Y–Co–Cu–Zr magnets can be significantly improved by partial Pr and Fe substitutions for Y and Co, respectively. Anisotropic (Y0.8Pr0.2)11.5Zr2.75Co56.75Fe14Cu15 powders with intrinsic coercivity of 7.8 kOe and energy product of 14.4 MG Oe were obtained after annealing at 900 °C for 15 min and cooling to 200 °C at the rate of 4 °C/min. We also explored the effects of some other rare earths (La, Nd, Gd) and transition metals (Mn, Ni) on the magnetic properties of the Y–Co–Cu–Zr magnets. The phases present and the microstructure were analyzed with x-ray diffraction, thermomagnetic analysis, and transmission electron microscopy. The cellular/lamellar microstructure of the bulk-hardened alloys is similar to that of Sm–Co–Cu–Zr magnets. Among the most noticeable distinctions in the Y–Co–Cu–Zr alloys are a smaller average size of 2:17 cells and a variety of Zr-rich phases, like Zr2Co11 and Zr6Co23. Although the Y2Co17 phase is known to have an “easy-plane” anisotropy, the x-ray diffraction experiments with magnetically oriented powders suggest that in the bulk-hardened Y–Co–Cu–Zr and Y–Co–Cu–Fe–Zr magnets the 2:17 phase has uniaxial anisotropy.
Complex permittivity measurements and mixings laws of alumina composites
Authors: Gershon D. , Calame J. P. , Birnboim A.
Published: July 2002

The effect of microstructure on the complex permittivity of alumina composites was studied. Three different algebraic mixing laws (Maxwell–Garnett, effective medium approximation, and the Landau–Lifshitz–Looyenga formula) inaccurately predicted the measured complex permittivities of alumina/silicon carbide and alumina/copper oxide composites. The imaginary part of the complex permittivity of the four-phase composites depends significantly on the absorbed water and microstructure. Accurate calculations require that the microstructure of the composite be examined and incorporated into the electrostatic model. Incorporating the general physical characteristics of the composites enabled a three-dimensional electrostatic model to accurately predict the permittivities of four-phase composites with different microstructures.
Carrier separation measurement of leakage current under prebreakdown in ultrathin SiO2 films
Authors: Uno Shigeyasu , Ishida Akihiro , Deguchi Kazuaki , Kamakura Yoshinari , Taniguchi Kenji
Published: July 2002

The anomalously large gate leakage current observed prior to dielectric breakdown in electrically stressed n+ gate p metal–oxide–semiconductor field-effect transistors is investigated. Carrier separation measurements reveal that the leakage currents are electron tunneling current, and in some cases are accompanied by noticeable hole-related current at low gate voltages. Experimental results demonstrate the close correlation between this phenomenon and soft breakdown in terms of current–voltage characteristics.
Monte Carlo simulation study of the Fano factor, w value, and energy resolution for the absorption of soft x rays in xenon–neon gas mixtures
Authors: Santos F. P. , Dias T. H. V. T. , Rachinhas P. J. B. M. , Conde C. A. N. , Stauffer A. D.
Published: July 2002

Xenon gas proportional-scintillation counters (GPSC) have many applications in the detection of soft x rays where their energy resolution, R, is comparable to solid-state detectors when large window areas are required. However, R is known to deteriorate for energies Exr below 2–3 keV due to electron loss to the entrance window. Since the addition of a lighter noble gas increases the absorption depth, we have investigated the use of Xe–Ne gas mixtures at atmospheric pressure as detector fillings. The results of a Monte Carlo simulation study of the Fano factor, F, the w value, and the intrinsic energy resolution, R=2.36(Fw/Exr)1/2, are presented for Xe–Ne mixtures and pure Xe and Ne. The results show that the addition of Ne to Xe reduces the intrinsic energy resolution ℛ but this never compensates for the reduction in scintillation yield in GPSC applications, implying that the instrumental energy resolution R will only improve with the addition of Ne when electron loss to the window in pure Xe is significant. The simulation reproduces the photoionization process of the Xe and Ne atoms, the vacancy cascade decay of the residual ions, and the elastic and inelastic scattering of electrons by the gas atoms. The contribution of energy and charge transfer mechanisms such as Penning, associative, and transfer ionization is discussed in detail. It is shown that Penning and associative ionization are the crucial indirect ionization processes which determine the behavior of F and w at low concentrations of Xe. The importance of the nonmetastable Ne states is also assessed.
Size dependence of domain patterns in a constrained ferroelectric system
Authors: Ahluwalia Rajeev , Cao Wenwu
Published: July 2002

We have simulated the size dependence of domain patterns in a two-dimensional (square shaped) constrained ferroelectric system by means of a time-dependent Ginzburg–Landau model. The theory incorporates elastic strain in the form of an effective long-range nonlocal interaction of the polarization. A nonferroelectric layer is introduced in the free energy that enforces a decaying polarization at the surface. The results show that the number of domains decreases on decreasing the system size. We also found two critical sizes. The first one signifies a transition from a multi-domain to a single-domain state and the second indicates the disappearance of ferroelectricity.