bluuebunny/crossref_metadata_2025 huggingface dataset:
Synchronized swinging of electroluminescence intensity and peak wavelength with Si layer thickness in Au/SiO2/nanometer Si/SiO2/p-Si structures
Authors: Qin G. G. , Wang Y. Q. , Qiao Y. P. , Zhang B. R. , Ma Z. C. , Zong W. H.
Published: July 2002

SiO 2 /nanometer amorphous Si/SiO2 structures with Si layers of twelve different thicknesses in a range of 0–3.0 nm have been deposited with the two-target alternative magnetron sputtering technique. Electroluminescence (EL) from the Au/SiO2/nanometer amorphous Si/SiO2/p-Si structures has been observed. It is found that the EL peak intensity and peak wavelength synchronously swing with increasing Si layer thickness. The experimental results strongly indicate that the EL originates from luminescence centers in SiO2 layers rather than from the Si layers in the structures. The tunneling of electrons and holes and the quantum confinement effect for them in the nanometer Si layers play important roles in the EL.
Stability, geometry, and electronic structure of the boron nitride B36N36 fullerene
Authors: Alexandre Simone S. , Mazzoni Mário S. C. , Chacham Hélio
Published: July 2002

We apply first-principles calculations to study the structural and electronic properties of a boron nitride fullerene-like cluster, B36N36. This cluster has shape and dimensions comparable to those of a single-shelled BN fullerene recently produced in an electron-beam irradiation experiment. The calculations show that B36N36 is energetically less favorable than C60, when both are compared to nanotube structures. This is consistent with the experimental difficulty to obtain BN fullerenes. On the other hand, B36N36 presents a large energy gap, larger in fact than that of a BN nanotube of the same diameter. This is an indication that the molecule is a stable one, once it is formed.
Efficient channel-waveguide laser in Nd:GGG at 1.062 μm wavelength
Authors: Gerhardt R. , Kleine-Börger J. , Beilschmidt L. , Frommeyer M. , Dötsch H. , Gather B.
Published: July 2002

Channel waveguide lasers in crystals of neodymium-doped gadolinium-gallium-garnet are realized. They are based on single-mode rib waveguides prepared by liquid phase epitaxy. By this growth technique the incorporation of certain impurities, which may cause severe quenching, is inevitable. The dominant quenching process could be identified and eliminated. Channel waveguides with extremely low losses, down to 0.25 dB/cm for both TE and TM modes, are fabricated by ion-beam etching. As a result, low thresholds of 5 mW and high slope efficiencies of 48% at the laser wavelength of 1.062 μm could be achieved when pumping at a wavelength of 807 nm.
Effects of irradiation temperature and dose on exfoliation of H+-implanted silicon carbide
Authors: Gregory R. B. , Wetteroth T. A. , Wilson S. R. , Holland O. W. , Thomas D. K.
Published: July 2002

H + implantation of SiC is the basis for a thin-film transfer process, which when combined with oxidation and hydrophilic wafer bonding, can be exploited to produce silicon carbide-on-insulator material useful as a wide-band-gap semiconductor. This thin-film transfer process has been successfully applied to Si to produce a commercial silicon-on-insulator material. The efficacy of hydrogen to produce thin-film separation was studied by investigation of H+-induced exfoliation in implanted SiC. Results showed that the onset and degree of exfoliation of SiC depends initially upon the concentration of implanted H+. However, the dose dependence of exfoliation exhibits a rather marked retrograde behavior. The degree of exfoliation eventually starts to decrease with increasing ion dose until exfoliation is completely suppressed. This behavior is attributed to a competition between the positive effects of hydrogen on exfoliation and the negative effects of ion-induced damage. Experiments were done to isolate the effects of the hydrogen–silicon chemistry from that of implant damage. Damage is reduced independently of H+ dosage by elevating the temperature of the SiC during implant in order to promote dynamic annealing. This will be shown to have a dramatic effect upon exfoliation. The “hot” implant lowers the H+ fluence required to affect thin-film separation, making the process more efficient, and producing SiC material with fewer defects.
Ultrafast hot-carrier dynamics in semiconductor saturable absorber mirrors
Authors: Joschko M. , Langlois P. , Thoen E. R. , Koontz E. M. , Ippen E. P. , Kolodziejski L. A.
Published: July 2002

Femtosecond pump-probe experiments have been used to study the ultrafast nonlinear dynamics of InGaAs/InP semiconductor saturable absorber mirrors. The relative contributions of absorption bleaching and induced absorption are investigated by varying the excitation fluence over more than four orders of magnitude, well beyond complete absorption saturation. Enhanced free carrier absorption due to highly excited carriers with an extended relaxation time of 2.8 ps dominates the differential reflectivity at ultrahigh fluences and has been studied via a two-color pump-probe measurement.
Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties
Authors: Li Y. , Meng G. W. , Zhang L. D. , Phillipp F.
Published: July 2002

Ordered semiconductor ZnO nanowire arrays embedded in anodic alumina membranes (AAM) were fabricated by generating alumina templates with nanochannels, electrodepositing Zn in them, and then oxidizing the Zn nanowire arrays. The polycrystalline ZnO nanowires with the diameters ranging from 15 to 90 nm were uniformly assembled into the hexagonally ordered nanochannels of the AAM. Photoluminescence (PL) measurements show a blue PL band in the wavelength range of 450–650 nm caused by the singly ionized oxygen vacancy in ZnO nanowires.
Silver ion implantation in epitaxial La2/3Ca1/3MnO3 thin films: Large temperature coefficient of resistance for bolometric applications
Authors: Bathe Ravi , Adhi K. P. , Patil S. I. , Marest G. , Hannoyer B. , Ogale S. B.
Published: July 2002

Epitaxial films of La2/3Ca1/3MnO3 were successively implanted with 100 and 200 keV silver ions at fluences of 4.5×1015 and 1×1016 ions/cm2, respectively, to achieve a fairly uniform implant distribution. The as-implanted films are insulating and do not show a metal–insulator transition. Postimplantation annealing at 950 °C shows a recovery of the high structural quality of the films, along with an increase in the metal–insulator transition temperature (Tp), magnetoresistance, and the peak temperature coefficient of resistance (TCR) at the transition. The peak TCR of 23% for manganite films is clearly significant for bolometric applications.
Dielectric properties of layered perovskite Sr1−xAxBi2Nb2O9 ferroelectrics (A=La, Ca and x=0,0.1)
Authors: Forbess M. J. , Seraji S. , Wu Y. , Nguyen C. P. , Cao G. Z.
Published: July 2002

In this letter, we report an experimental study on the influences of 10 at. % Ca2+ and La3+ doping on dielectric properties and dc conductivity of SrBi2Nb2O9 ferroelectric ceramics. All the samples were made by two-step solid-state reaction sintering at temperatures up to 1150 °C for 0.5–1 h in air. X-ray diffraction analysis indicated that single-phase layered perovskite ferroelectrics were obtained and no appreciable secondary phase was found. The Curie point was found to increase from 418 °C without doping to 475 °C with Ca2+ doping and to 480 °C with La3+ doping. Dielectric constants, loss tangent, and dc conductivity of SrBi2Nb2O9 ferroelectrics doped with Ca2+ and La3+ were studied and the relationships among doping, crystal structure, and dielectric properties were discussed.
Real-time precision concentration measurement for flowing liquid solutions
Authors: Krishna V. , Fan C. H. , Longtin J. P.
Published: July 2002

The precise, real-time measurement of liquid concentration is important in fundamental research, chemical analysis, mixing processes, and manufacturing, e.g., in the food and semiconductor industries. This work presents a laser-based, noninvasive technique to measure concentration changes of flowing liquids in real time. The essential components in the system include a 5 mW laser diode coupled to a single-mode optical fiber, a triangular optical cell, and a high-resolution beam position sensor. The instrument provides a large range of concentration measurement, typically 0%–100% for binary liquid mixtures, while providing a resolution on the order of 0.05% concentration or better. The experimental configuration is small, reliable, and inexpensive. Results are presented for NaCl and MgCl2 aqueous solutions with concentrations ranging from 0% to 25%, with very good agreement found between measured and true concentrations.
Superconformal symmetry in three dimensions
Authors: Park Jeong-Hyuck
Published: July 2002

Three-dimensional 𝒩-extended superconformal symmetry is studied within the superspace formalism. A superconformal Killing equation is derived and its solutions are classified in terms of supertranslations, dilations, Lorentz transformations, R-symmetry transformations and special superconformal transformations. A superconformal group is then identified with a supermatrix group, OSp(N|2,R), as expected from the analysis on simple Lie superalgebras. In general, due to the invariance under supertranslations and special superconformal transformations, superconformally invariant n-point functions reduce to one unspecified (n−2)-point function which must transform homogeneously under the remaining rigid transformations, i.e., dilations, Lorentz transformations, and R-symmetry transformations. After constructing building blocks for superconformal correlators, we are able to identify all the superconformal invariants and obtain the general form of n-point functions. Superconformally covariant differential operators are also discussed.
Comparative study of energy transfers from Er3+ to Ce3+ in tellurite and sulfide glasses under 980 nm excitation
Authors: Choi Yong Gyu , Kim Kyong Hon , Park Se Ho , Heo Jong
Published: July 2002

We have demonstrated that the population feeding from the I11/24 level to the 1.5 μm fluorescence emitting I13/24 level of Er3+ ions in low phonon energy glass hosts can be enhanced by codoping with Ce3+ under optical pumping at 980 nm. The nonradiative energy transfer Er3+: 4I11/2; Ce3+: 2F5/2→Er3+: 4I13/2; Ce3+: 2F7/2, occurs in the form of phonon-assisted energy transfer, and therefore the feeding rates are faster in the tellurite glasses, which have a comparatively higher phonon energy than in the sulfide glasses. The cross-relaxation process for I13/24: 4I13/2→4I15/2: 4I9/2, which lowers the population density of the I13/24 manifold and causes a deleterious effect in the 1.5 μm fluorescence intensity, is more severe in the sulfide glasses. Population feeding rate from the I11/24 to the I13/24 level is significantly enhanced by way of cerium codoping into tellurite glasses, which promises an efficient 980 nm pumped broadband Er3+-doped fiber amplifier.
The higher-order Douglas–Kroll transformation
Authors: Nakajima Takahito , Hirao Kimihiko
Published: July 2002

The higher-order Douglas–Kroll (DK) Hamiltonians in an external potential are explicitly derived. Application of an exponential-type unitary operator with the 2n+1 rule significantly simplifies the formulations of the high-order DK Hamiltonians. The third-order DK method has been implemented practically. Numerical results for one- and many-electron systems show that good accuracy can be obtained even with the DK Hamiltonian correct to third order in the external potential.
Longitudinal magnetic recording media with thermal stabilization layers
Authors: Abarra E. N. , Inomata A. , Sato H. , Okamoto I. , Mizoshita Y.
Published: July 2002

We introduce promising longitudinal recording media with enhanced thermal stability compared to conventional media. The media consist of one or more magnetic layers disposed between the recording layer and the underlayer with the magnetization direction of the layers being antiparallel to the adjacent layer or layers. Antiferromagnetic coupling is induced through a thin Ru layer between the Co-based magnetic layers. Comparison with conventional media with the same remanence magnetization and thickness product reveals improved thermal stability and read-write properties for the proposed media.
A simple submicron confocal microscope with a fiberoptic output
Authors: Ilev Ilko K. , Waynant Ronald W.
Published: July 2002

We present a simple design of a reflection-type confocal microscope with a multimode graded-index fiber output. Due to the use of an apertureless confocal arrangement, a high-aperture focusing objective and a large-core graded-index multimode fiber for signal detection, a regime of high-output power is achieved that provides high resolving power and submicron spatial resolution. The graded-index multimode fiber ensures an efficient launching and maintenance of high laser powers, a high sensitivity to spatial displacements of the focusing backreflectance emission, and a low mode noise and signal fluctuations. At axial scanning of the test object, a submicron resolution of 0.4 μm is obtained.
A dissipative particle dynamics method for modeling the geometrical packing of filler particles in polymer composites
Authors: Elliott J. A. , Windle A. H.
Published: July 2002

A method is presented for modeling the geometrical packing of polydisperse mixtures of both spherical and nonspherical particles, which are models for filler particles used in the manufacture of polymer composites. The technique is based on the calculation of the dissipative dynamics of an ensemble of fused soft spheres at constant temperature and pressure. After validation of the method at low pressures by comparison with analytical equations of state for monodisperse and binary mixtures of hard spheres, the random packing of fused soft sphere cubes was studied. The effect of packing cubes together with spheres of varying size was then examined, with the aim of developing an understanding of how to minimize the amount of void space in composites containing angular particles. In one case, entropically driven demixing was observed in a cube-sphere mixture.
Granular jets
Authors: Thoroddsen S. T. , Shen Amy Q.
Published: July 2002

When a solid sphere impacts on a deep layer of granular medium, it generates an ejecta sheet and a transient axisymmetric crater. The gravity-driven radial collapse of this crater generates a pressure spike, as the cavity closes up. This pressure spike drives up a narrow granular jet along the axis of symmetry. The maximum height of the jet is found to depend on the impact velocity, gravity as well as the effective viscosity of the granular medium, through a simple product of the Reynolds and Froude numbers. The presence of such granular jets, where surface tension is absent, may help pinpoint the role of surface tension for similar liquid jets.
The role of dispersion in ultrafast optics
Authors: Walmsley Ian , Waxer Leon , Dorrer Christophe
Published: July 2002

In this article, we review the phenomenon of dispersion, paying particular attention to its impact in the optics of ultrashort pulses, as well as its measurement and management. At present, lasers generating coherent bandwidths of several hundred nanometers have been demonstrated and correspondingly short pulses of 10 fs or so are quite usual. The limits to the breadth of optical spectra and brevity of pulse durations that may be achieved are often set by the dispersive properties of the linear optical elements of which the source is constructed. Progress in ultrafast optics to date has therefore relied extensively on the development of ways to characterize and manipulate dispersion. The means by which this can be accomplished are significantly different for laser oscillators and laser amplifiers, as well as for nonlinear interactions that are used to extend the range of frequencies at which short optical pulses are available, but in all cases it is this phenomenon that determines the output of current optical sources.
Strain-induced magnetic anisotropy in epitaxial Fe3O4/Co3O4 multilayers
Authors: Sohma M. , Kawaguchi K. , Manago T.
Published: July 2002

Epitaxial multilayers of Fe3O4/Co3O4 were successfully grown on MgO and SrTiO3(001) single crystal substrates using a molecular beam epitaxy apparatus. The multilayers showed a strong in-plane magnetic anisotropy and an increase of coercive field for thin Fe3O4 samples. Multilayered Co3O4 exhibited a significant lattice expansion along the growth direction in order to compensate lattice mismatch at the interface. A small lattice distortion in the Fe3O4 layers might be the origin of the strong anisotropy.
Effect of polarization fields on transport properties in AlGaN/GaN heterostructures
Authors: Hsu L. , Walukiewicz W.
Published: July 2002

We have calculated transport properties of unintentionally doped n-type AlGaN/GaN heterostructures. Using a thermodynamic model of defect formation, we have modeled the charge transfer process in such heterostructures, obtaining good agreement with experiment. The large polarization fields in the heterostructure dramatically lower the formation energy of the surface defects, leading to the observed extremely large two-dimensional electron gas concentrations. Calculations of the low temperature mobilities were also performed, showing that alloy disorder and, in some cases, interface roughness, are the dominant low-temperature carrier scattering mechanisms. At low temperatures a maximum intrinsic mobility of about 105 cm2/V s is predicted for these heterostructures.
Optically driven micromachine elements
Authors: Friese M. E. J. , Rubinsztein-Dunlop H. , Gold J. , Hagberg P. , Hanstorp D.
Published: July 2002

We report on a proof of principle demonstration of an optically driven micromachine element. Optical angular momentum is transferred from a circularly polarized laser beam to a birefringent particle confined in an optical tweezers trap. The optical torque causes the particle to spin at up to 350 Hz, and this torque is harnessed to drive an optically trapped microfabricated structure. We describe a photolithographic method for producing the microstructures and show how a light driven motor could be used in a micromachine system.
Dielectric function spectra of GaN, AlGaN, and GaN/AlGaN heterostructures
Authors: Wagner J. , Obloh H. , Kunzer M. , Maier M. , Köhler K. , Johs B.
Published: July 2002

The pseudodielectric function spectra 〈ε〉 of wurtzite GaN, AlGaN, and GaN/AlGaN heterostructures were determined for photon energies ranging from 2 to 5 eV, using variable angle spectroscopic ellipsometry (SE). Samples were grown by low-pressure metalorganic chemical vapor deposition on c-plane sapphire substrates. The experimental 〈ε〉 spectra were analyzed using a multilayer approach, describing the dielectric functions of the individual layers by a parametric oscillator model. In this way, parametric dielectric function spectra of GaN and AlxGa1−xN (x⩽0.16) were derived, as well as the composition dependence of the AlxGa1−xN band gap energy. The SE band gap data were found to be consistent with a bowing parameter close to 1 eV. Finally, the GaN and AlxGa1−xN parametric dielectric functions were used to quantitatively analyze the pseudodielectric function spectrum of GaN/AlGaN modulation doped field effect transistor structures, demonstrating the potential of SE in combination with a multilayer parametric dielectric function model for nondestructive ex situ control of GaN/AlGaN device structures.
In situ temperature measurements via ruby R lines of sapphire substrate based InGaN light emitting diodes during operation
Authors: Winnewisser C. , Schneider J. , Börsch M. , Rotter H. W.
Published: July 2002

The temperature of encapsulated green and ultraviolet light emitting diodes (LEDs) in operation has been measured optically via the ruby R lines emitted by the residual Cr3+ contaminations in the sapphire substrate. These two photoluminescent R lines, which are excited by the electroluminescence of the LED itself, show a well-characterized line shift as a function of temperature and pressure. The temperature is found to rise linearly with the applied forward current of the LED at a rate of ≈1 K/mA. This optical temperature measurement based on monitoring the two ruby R lines could qualify as a sensitive method for an in situ temperature sensor for other sapphire substrate based semiconductor devices such as laser diodes and field effect transistors.
Modeling of poling behavior of ferroelectric 1-3 composites
Authors: Hauke T. , Steinhausen R. , Seifert W. , Beige H. , Kamlah M.
Published: July 2002

Piezoelectric 1-3 composites are typically prepared from a polymer matrix and an unpoled Pb(ZrxTi1−x)O3 (PZT) based ceramic. During the subsequent poling process the PZT cannot strain freely due to the clamping by the surrounding polymer, which after poling results in the occurrence of residual mechanical stress in the composite. The poling process is investigated theoretically by a nonlinear finite element method modeling, which takes into account the ferroelectric and ferroelastic properties of PZT ceramics. Furthermore, an analytical model is developed, which predicts the remanent strain of the composite and the residual mechanical stress. The compressive residual mechanical stress acting on the PZT in the poling direction increases continuously with decreasing volume content va of the PZT ceramic. For values of va higher than about 20% it is lower than half of the coercive stress necessary to cause a mechanical depolarization. For va lower than 20% the residual stress rises rapidly. It may cause a mechanical depolarization of the PZT ceramic for va<11%. With increasing volume content the longitudinal and the transversal components of the remanent strain increase monotonously in similar way like the corresponding linear effective piezoelectric coefficients.
Reconstruction of the charge collection probability in a solar cell from internal quantum efficiency measurements
Authors: Donolato C.
Published: July 2002

A method is proposed to analyze the internal quantum efficiency spectra that are commonly obtained on solar cells by measuring their current response in monochromatic light as a function of wavelength. The efficiency values η are regarded as a function of the reciprocal y=1/α of the light absorption coefficient. The integral relation between η(y) and the charge collection probability distribution in the cell φ(z), with a simple change of variables, can be put in the form of a convolution. A transformed version of this convolution is approximately inverted to yield an explicit expression of φ(z) in terms of η(y) and its derivatives. The method is preliminarily tested on simulated data and is used to analyze published measurements obtained on a Si solar cell and a proton-irradiated InP solar cell.
A detector of small harmonic displacements based on two coupled microwave cavities
Authors: Bernard Ph. , Gemme G. , Parodi R. , Picasso E.
Published: July 2002

The design and test of a detector of small harmonic displacements is presented. The detector is based on the principle of the parametric conversion of power between the resonant modes of two superconducting coupled microwave cavities. The work is based on the original ideas of Bernard, Pegoraro, Picasso, and Radicati, who, in 1978, suggested that superconducting coupled cavities could be used as sensitive detectors of gravitational waves, and on the work of Reece, Reiner, and Melissinos, who, in 1984, built a detector of this kind. They showed that an harmonic modulation of the cavity length l produced an energy transfer between two modes of the cavity, provided that the frequency of the modulation was equal to the frequency difference of the two modes. They achieved a sensitivity to fractional deformations of δl/l≈10−17 Hz−1/2. We repeated the Reece, Reiner, and Melissinos experiment, and with an improved experimental configuration and better cavity quality, increased the sensitivity to δl/l≈10−20 Hz−1/2. In this article the basic principles of the device are discussed and the experimental technique is explained in detail. Possible future developments, aiming at gravitational waves detection, are also outlined.
Gas phase dihydrogen bonded phenol–borane–trimethylamine complex
Authors: Naresh Patwari G. , Ebata Takayuki , Mikami Naohiko
Published: July 2002

Spectroscopic investigation of a dihydrogen bonded complex between phenol and borane–trimethylamine is reported here. Laser-induced fluorescence excitation, fluorescence detected infrared, and infrared–ultraviolet hole-burning spectroscopic studies were carried out in supersonic jets to investigate the complex formation between phenol and borane-trimethylamine. The redshift in the electronic transition and the low-frequency shift of the O–H stretching vibration established the proton donating ability of phenol in the complex. The experimental results together with the ab initio quantum mechanical calculations affirmed the formation of the dihydrogen bond in the complex.
Electrochemical tuning of electronic states in single-wall carbon nanotubes studied by in situ absorption spectroscopy and ac resistance
Authors: Kazaoui S. , Minami N. , Matsuda N. , Kataura H. , Achiba Y.
Published: July 2002

Electrochemical doping of single-wall carbon nanotube (SWNT) films and concomitant changes in their electronic states were investigated by in situ measurements of optical absorption spectra as well as of ac resistance using a nonaqueous electrolyte solution. A systematic, consistent, and reversible variation of these properties induced by the shift in the electrode potential demonstrated the practicability of fine and continuous tuning of their electronic states. Analysis of the potential dependence of the absorbance at 0.68 eV enabled the estimation of average values of the electron affinity (4.8 eV) and the first ionization potential (5.4 eV) of semiconducting SWNTs.
Physical and electrical characterization of ultrathin yttrium silicate insulators on silicon
Authors: Chambers J. J. , Parsons G. N.
Published: July 2002

This article describes the oxidation of yttrium on silicon to form yttrium silicate films for application as high dielectric constant insulators. The high reactivity of yttrium metal with silicon and oxygen is utilized to form amorphous yttrium silicate films with a minimal interfacial silicon dioxide layer. Yttrium silicate films (∼40 Å) with an equivalent silicon dioxide thickness of ∼11 Å and k∼14 are formed by oxidizing yttrium on silicon. The physical properties of yttrium silicate films on silicon are investigated using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The oxidation of yttrium silicide results in films nearly identical, although with a higher silicon fraction, to films formed by oxidation of yttrium on silicon. The oxidation of yttrium on silicon results in a competition for yttrium between silicide formation and oxidation. This competition yields yttrium silicate films for thin (<40 Å) initial metal thickness and a Y2O3/silicate bilayer for thick (>80 Å) initial metal thickness. Annealing yttrium films on silicon in vacuum to form yttrium silicide and then oxidizing the silicide is used to eliminate the competition and control the yttrium/silicon reaction. Analysis of the oxidation of yttrium on silicon reveals fast oxidation during silicate formation and a slow rate during oxidation of the silicon substrate to form SiO2. Oxidation of other metals, such as Hf, Zr, and La, on silicon is expected to result in metal silicate films through a similar simultaneous (or controlled sequential) silicide/oxidation reactions.
Electrical transport in n-type 4H silicon carbide
Authors: Pernot J. , Zawadzki W. , Contreras S. , Robert J. L. , Neyret E. , Di Cioccio L.
Published: July 2002

Free electron density and low field electron mobility of 4H–SiC in the temperature range of 35–900 K are examined experimentally and theoretically. Five samples produced by cold-wall atmospheric pressure chemical vapor deposition and doped with nitrogen from 3.5×1015 cm−3 to 7.5×1017 cm−3 are investigated using the electric conductivity and Hall measurements. A complete description of the electron density and mobility is presented taking into account inequivalent positions of cubic and hexagonal donor sites as well as valley-orbit splittings of the donor levels. A good agreement between experiment and theory is achieved for all samples and it is demonstrated that the scattering of electrons by neutral donors is a dominant mode in 4H–SiC at low temperatures. The deformation potential for the intravalley scattering by acoustic phonons and coupling constants for the intervalley scattering by acoustic and optic phonons are determined. The dependence of electron mobility on doping at constant temperatures 77 K, 292 K, and 600 K is experimentally established and successfully described. Parallel conductivity at low temperatures by an impurity band in the sample with 7.5×1017 cm−3 donors is evidenced.
Polaron absorption in amorphous tungsten oxide films
Authors: Berggren Lars , Azens Andris , Niklasson Gunnar A.
Published: July 2002

Amorphous thin films of tungsten oxide were deposited by sputtering onto glass substrates covered by conductive indium–tin oxide. The density and stoichiometry were determined by Rutherford backscattering spectrometry. Lithium ions were intercalated electrochemically into the films. The optical reflectance and transmittance were measured in the wavelength range from 0.3 to 2.5 μm, at a number of intercalation levels. The polaron absorption peak becomes more symmetric and shifts to higher energies until an intercalation level of 0.25 to 0.3 Li+/W, where a saturation occurs. The shape of the polaron peak is in very good agreement with the theory of Bryksin [Fiz. Tverd. Tela 24, 1110 (1982)]. Within this model, the shift of the absorption peak is interpreted as an increase in the Fermi level of the material as more Li ions are inserted.
Strain effect in large silicon nanocrystal quantum dots
Authors: Thean A. , Leburton J. P.
Published: July 2002

We present a computer simulation of strain effects on the electronic spectrum of 100 Å diam Si nanocrystal (nc-Si) quantum dots embedded in SiO2, based on the continuum strain model and deformation potential theory. We show that the coupling between the nc-Si geometry and the symmetry generated by the strain potential can enhance confinement in the quantum dot and can lift the degeneracy of the conduction band valleys for nonspherically symmetric nanocrystals.
Cubic boron nitride thin film heteroepitaxy
Authors: Feldermann H. , Ronning C. , Hofsäss H. , Huang Y. L. , Seibt M.
Published: July 2002

In this study we investigate the possibility of nucleating nanocrystalline cubic boron nitride (c-BN) thin films directly onto suitable substrates without the soft turbostratic BN (t-BN) interlayer that is usually observed. This would open a path to the epitaxial growth of c-BN films which is essential particularly for practicable applications in electronic devices. Appropriate substrates are required to exhibit a lattice that matches the c-BN crystallite structure, survives the ion bombardment imperative for c-BN film formation, and is not disturbed by the development of a heterogeneous interface layer. In accordance with these criteria, monocrystalline AlN is selected and employed as a potential substrate for direct c-BN film growth using mass selected ion beam deposition. A detailed examination of the BN/AlN interface microstructure by cross-sectional high-resolution transmission electron microscopy reveals that the AlN crystallinity is indeed retained, with no amorphous layer next to the BN film as commonly observed on Si substrates. Nanocrystalline BN grains with the cubic, and, more frequently, with the wurtzitic structure are found in direct contact with certain regions of the rugged AlN substrate, covering about one-third of its entire surface with no mediating t-BN or other interface layer. The c-BN and w-BN growth areas are textured and exhibit definite preferential orientation relationships with the faceted AlN substrate surface. The consequences of these findings for the understanding of the role of the t-BN interlayer in c-BN film nucleation are discussed.
Femtosecond photolysis of aqueous HOCl
Authors: Thomsen C. L. , Madsen D. , Poulsen J. Aa. , Thøgersen J. , Jensen S. J. Knak , Keiding S. R.
Published: July 2002

This paper reports an experimental study of the photolysis of aqueous HOCl using femtosecond pulses at 266 nm. The formation of photoproducts is monitored by transient absorption spectroscopy from 230 to 400 nm. The HOCl molecules dissociate with unity quantum yield to form OH+Cl faster than 1 ps, and as a result of the potential along the HO–Cl reaction coordinate, all excess energy is given to the fragments as translational energy. After dissociation, and solvent cage escape, the majority of the Cl and OH fragments recombine after diffusion on a time scale of 50 ps. The diffusion dynamics is studied using a simple model for diffusive recombination and a more extensive molecular dynamics simulation. A minor fraction of the Cl atoms (∼10%) reacts with HOCl in a diffusion limited reaction to form Cl2+OH.
Mushrooms and other billiards with divided phase space
Authors: Bunimovich Leonid A.
Published: July 2002

We present the first natural and visible examples of Hamiltonian systems with divided phase space allowing a rigorous mathematical analysis. The simplest such family (mushrooms) demonstrates a continuous transition from a completely chaotic system (stadium) to a completely integrable one (circle). In the course of this transition, an integrable island appears, grows and finally occupies the entire phase space. We also give the first examples of billiards with a “chaotic sea” (one ergodic component) and an arbitrary (finite or infinite) number of KAM islands and the examples with arbitrary (finite or infinite) number of chaotic (ergodic) components with positive measure coexisting with an arbitrary number of islands. Among other results is the first example of completely understood (rigorously studied) billiards in domains with a fractal boundary.
Determination of curvature corrections to the surface tension of a liquid–vapor interface through molecular dynamics simulations
Authors: van Giessen A. E. , Blokhuis E. M.
Published: July 2002

We use molecular dynamics simulations of particles interacting through a truncated Lennard-Jones potential to study the surface properties of the curved liquid–vapor interface. We determine the Tolman length δ, investigate its critical behavior, and provide first results for the rigidity constants of bending, k, and of Gaussian curvature, k̄. The rigidity constant of bending, determined at three different temperatures, is found to be positive and of the order of one-half kBT. The rigidity constant of Gaussian curvature, determined at a single temperature, is of the same order of magnitude.
Model of accumulation, multi-ionization, and extraction of ions in ECR ion sources
Authors: Cavenago M. , Kester O. , Lamy T. , Sortais P.
Published: July 2002

Charge breeding inside electron cyclotron resonance (ECR) ion sources is a promising way to produce radioactive ion beams with a high charge state, based on deceleration and capture of the primary ion beam by the ECR plasma, trapping, multi-ionization, diffusion, and extraction. A detailed analysis of these processes could help to improve the efficiency (reducing ion loss to walls) and the charge breeding time. An estimate of the deceleration length is given. Collisions, external magnetic field, and ambipolar potential are the fundamental parameters for the ion motion inside the trap, which is simulated by a program, including statistical averaging over many (400–3200) particle histories. Characteristics of trapped and extracted ions are shown with simulations taking in account the design of the PHOENIX ion source. Since particle losses are found to increase rapidly with the radius of injection, the injection system should form an image at the beam stopping plane. Analysis of additional source parameters is discussed.
Infinite-order two-component theory for relativistic quantum chemistry
Authors: Barysz Maria , Sadlej Andrzej J.
Published: July 2002

A method for the iterative algebraic generation of the numerically accurate two-component Hamiltonian for the use in relativistic quantum chemistry is presented. The separation of the electronic and positronic states of the Dirac Hamiltonian is accomplished by the algebraic solution for the Foldy–Wouthuysen transformation. This leads to the two-component formalism whose accuracy is primarily limited by the choice of basis functions. Its performance is tested in calculations of the most sensitive 1s1/2 energy for increasing values of the nuclear charge. These calculations show that the electronic part of the Dirac eigenspectrum can be obtained from the two-component theory to arbitrarily high accuracy. Moreover, if needed, the positronic states can be separately determined in a similar way. Thus the present method can be also used for the evaluation of quantum electrodynamic corrections in the finite basis set approximation.
Structures of small gold cluster cations (Aun+, n<14): Ion mobility measurements versus density functional calculations
Authors: Gilb Stefan , Weis Patrick , Furche Filip , Ahlrichs Reinhart , Kappes Manfred M.
Published: July 2002

We have performed ion mobility measurements on gold cluster cations Aun+ generated by pulsed laser vaporization. For clusters with n<14, experimental cross sections are compared with theoretical results from density functional calculations. This comparison allows structural assignment. We find that room temperature gold cluster cations have planar structures for n=3–7. Starting at n=8 they form three dimensional structures with (slightly distorted) fragments of the bulk phase structure being observed for n=8–10.
The ion-acoustic soliton: A gas-dynamic viewpoint
Authors: McKenzie J. F.
Published: July 2002

The properties of fully nonlinear ion-acoustic solitons are investigated by interpreting conservation of total momentum as the structure equation for the proton flow in the wave. In most studies momentum conservation is regarded as the first integral of the Poisson equation for the electric potential and is interpreted as being analogous to a particle moving in a pseudo-potential well. By adopting an essentially gas-dynamic viewpoint, which emphasizes momentum conservation and the properties of the Bernoulli-type energy equations, the crucial role played by the proton sonic point becomes apparent. The relationship (implied by energy conservation) between the electron and proton speeds in the transition yields a locus—the hodograph of the system–which shows that, in the first half of the soliton, the electrons initially lag behind the protons until the charge neutral point is reached, after which they run ahead of the protons. The system reaches an equilibrium point (the center of the soliton) before the proton flow goes sonic. It follows that the critical ion-acoustic Mach number, Mc, above which smooth, continuous solitons cannot be constructed, stems from the requirement that the two equilibrium points of the structure equation coalesce at the proton sonic point of the flow. In general the range of the ion-acoustic Mach numbers, Mep, in which solitons exist, is extended beyond the classical range 1<Mep<1.6. In the special case of cold protons and hot electrons with an adiabatic index 2, the structure equation may be integrated in closed form. This analytic solution describes the fully nonlinear counterpart to the sech2 shaped pulses characteristic of weakly nonlinear waves and shows that solitons exist only if 1<Mep<2. The corresponding maximum potential, associated with the critical ion-acoustic Mach number, can be between 1.3kTe and 10kTe depending upon the values of the adiabatic indices of the electrons and protons and the proton Mach number.
Strong electron correlations in cobalt valence tautomers
Authors: LaBute M. X. , Kulkarni R. V. , Endres R. G. , Cox D. L.
Published: July 2002

We have examined cobalt based valence tautomer molecules such as Co(SQ)2(phen) using density functional theory (DFT) and variational configuration interaction (VCI) approaches based upon a model Hamiltonian. Our DFT results extend earlier work by finding a reduced total energy gap (order 0.6 eV) between high-temperature and low-temperature states when we fully relax the coordinates (relative to experimental ones). Furthermore, we demonstrate that a charge transfer picture based upon formal valence arguments succeeds qualitatively while failing quantitatively due to strong covalency between the Co 3d orbitals and ligand p orbitals. With the VCI approach, we argue that the high-temperature, high spin phase is strongly mixed valent, with about 30% admixture of Co (III) into the predominantly Co(II) ground state. We confirm this mixed valence through a fit to the XANES spectra. Moreover, the strong electron correlations of the mixed valent phase provide an energy lowering of about 0.2–0.3 eV of the high-temperature phase relative to the low-temperature one. Finally, we use the domain model to account for the extraordinarily large entropy and enthalpy values associated with the transition.
Optical diffraction from a liquid crystal phase grating
Authors: Brown C. V. , Kriezis Em. E. , Elston S. J.
Published: July 2002

The finite-difference time-domain method has been used in the numerical analysis of the optical diffraction properties of a liquid crystal phase grating. The grating is formed using a nematic material that is switched using striped electrodes with a unity mark-space ratio. Three different surface pretilts have been investigated: 0°, 30° parallel, and 30° antiparallel. The tilt geometry determines the degree of suppression of the zero order and the asymmetry of the diffraction. Highly efficient beam-steering devices are shown to be possible using the antiparallel tilt alignment.
One-dimensional heterostructures in semiconductor nanowhiskers
Authors: Björk M. T. , Ohlsson B. J. , Sass T. , Persson A. I. , Thelander C. , Magnusson M. H. , Deppert K. , Wallenberg L. R. , Samuelson L.
Published: July 2002

We report on the growth of designed heterostructures placed within semiconductor nanowhiskers, exemplified by the InAs/InP material system. Based on transmission electron microscopy, we deduce the interfaces between InAs and InP to be atomically sharp. Electrical measurements of thermionic emission across an 80-nm-wide InP heterobarrier, positioned inside InAs whiskers 40 nm in diameter, yield a barrier height of 0.6 eV. On the basis of these results, we propose new branches of physics phenomena as well as new families of device structures that will now be possible to realize and explore.
Role of reversible susceptibility in ferromagnetic hysteresis
Authors: Schneider Carl S.
Published: July 2002

An equation of state based upon saturation magnetization, Ms, coercive field, Hc, and the reversible susceptibility function of magnetization is proposed for ferromagnetic hysteresis. Reversible susceptibility divided by the initial susceptibility is the anisotropy function of magnetization, χr, ranging from one in the demagnetized state to zero at saturation, and varying with magnetic history. Its dependence on scaled magnetization, m=M/Ms on the interval (−1,1) varies with material, allowing characterization of anisotropy classes. Precise measurements have been made of reversible susceptibility, initial and saturate magnetization curves, and loops for Orthonol™, annealed 3% nickel steel and as-received 1018 steel, representing crystals, isotropic polycrystals and composite ferromagnets, respectively. Magnetization change is the product of the reversible susceptibility, change in the applied field and the cooperative function due to domain interactions. This function is 1+βm for the virgin curve with half this slope from any reversal, where β=Ms/XiHc is the hysteresis coefficient. Variation of β for 1018 steel is due to distributed coercivities, and causes sigmoid B(H) curves. In the scaled field representation, where h=H/Hc, the cooperative function is 1/(1−hχr), a hyperbolic field dependence smeared by the anisotropy function. Constant anisotropy causes closed hysteresis loops, while variable anisotropy causes creeping of cycled asymmetric loops. In ferromagnetism, 1/χ=1/χr−h, normal scaled reluctivity is reduced from its reversible value by the scaled field.
A new laboratory experiment on magnetic reconnection
Authors: Stenzel R. L. , Urrutia J. M. , Griskey M. , Strohmaier K.
Published: July 2002

In a large laboratory plasma reconnection of three-dimensional (3-D) magnetic fields is studied in the parameter regime of electron magnetohydrodynamics (EMHD). A reversed-field topology with two 3-D null points and a two-dimensional (2-D) null line is established, and its free relaxation is studied experimentally. Major new findings include the absence of tilting instabilities in an unbounded plasma, relaxation times that are fast compared to classical diffusion times, dominance of field line annihilation at the 2-D current sheet versus reconnection at 3-D null points, conversion of magnetic energy into electron thermal energy, and excitation of various microinstabilities. The experiment implies that EMHD processes near absolute magnetic null points must be considered in the multiscale physics of magnetic reconnection.
Calibration of optical tweezers with differential interference contrast signals
Authors: Capitanio M. , Romano G. , Ballerini R. , Giuntini M. , Pavone F. S. , Dunlap D. , Finzi L.
Published: July 2002

A comparison of different calibration methods for optical tweezers with the differential interference contrast (DIC) technique was performed to establish the uses and the advantages of each method. A detailed experimental and theoretical analysis of each method was performed with emphasis on the anisotropy involved in the DIC technique and the noise components in the detection. Finally, a time of flight method that permits the reconstruction of the optical potential well was demonstrated.
Simplified analysis of two-layer antiferromagnetically coupled media
Authors: Richter H. J. , Girt Er. , Zhou H.
Published: July 2002

Analytic expressions for the energy barrier at zero applied field and the switching field for a field applied along the easy axis are derived for a pair of antiferromagnetically coupled (AFC) single-domain particles at zero temperature. The energy barriers of the two single-domain particles add in zero field if the antiferromagnetic coupling exceeds a critical value |Jc|=2(KV)1(KV)2/[S((KV)1+(KV)2)]. The antiferromagnetic coupling increases both the effective energy barrier as well as the switching field, so that optimized AFC media for recording applications require less than critical coupling |Jc|.
Temperature dependent magnetic properties of highly chemically ordered Fe55−xNixPt45L1 films
Authors: Thiele J.-U. , Coffey K. R. , Toney M. F. , Hedstrom J. A. , Kellock A. J.
Published: July 2002

Magnetic media using materials with high uniaxial magneto-crystalline anisotropy, KU, combined with a thermal assist to overcome write field limitations have been proposed as one of the potential technologies to extend the areal density of magnetic disk recording beyond the limitations of current technology. Here we present an investigation on structural and temperature dependent magnetic properties of chemically ordered epitaxial Fe55−xNixPt45 thin films. Increasing Ni additions result in a steady reduction of magneto-crystalline anisotropy, saturation magnetization, and Curie temperature. The ability to control thermomagnetic properties over a wide range makes Fe55−xNixPt45 and similar FePt-based pseudo-binary alloys attractive base materials for media applications in thermally assisted magnetic recording.