bluuebunny/crossref_metadata_2025 huggingface dataset:
Time-averaged hydrodynamic roughness of a noncolloidal sphere in low Reynolds number motion down an inclined plane
Authors: Galvin K. P. , Zhao Y. , Davis R. H.
Published: July 2002

A system consisting of a spherical particle in motion down an inclined planar surface in a viscous liquid was investigated theoretically and experimentally to examine the effects of surface roughness on the interactions between the sphere and the plane. Two characteristic roughness scales were used to describe the microscopic surface roughness of the sphere. The smallest roughness elements are assumed to dominate the surface, and the largest roughness elements are more sparse. The time-averaged nominal separation between the sphere and the plane was found to increase as the planar surface was made steeper. This apparent hydrodynamic roughness is governed by the heights of the smallest roughness when the sphere resides on a horizontal plane, whereas the largest roughness elements govern the apparent hydrodynamic roughness when the plane is inclined at a steep angle. On a steep incline, the normal component of the gravitational force that drives the sphere toward the plane is relatively weak. Hence, as the sphere migrates toward the plane after contact with a large asperity ends, its rotation may result in another large asperity forcing the sphere away from the plane before contact with the smaller asperities occurs. The time-averaged separation at intermediate angles increases with increasing surface coverage by the largest roughness elements. The method of Smart and Leighton [Phys. Fluids A 1, 526 (1989)] was modified to determine the hydrodynamic separation between the sphere and the plane during its motion down the incline. The apparent hydrodynamic roughness values obtained in the experiments increase as the angle of inclination of the plane was increased, and provide a satisfactory validation of the model. The relatively large but sparse roughness elements have a disproportionate effect on the time-averaged hydrodynamic roughness, especially at high angles of inclination. These findings may be important in the interaction of pairs of spherical particles in viscous suspensions, where the effective angle of inclination varies significantly. For example, the presence of a low concentration of relatively large roughness elements should result in significantly higher levels of hydrodynamic diffusion.
Elastic properties of langasite-type crystals determined by bulk and surface acoustic waves
Authors: Chilla E. , Flannery C. M. , Fröhlich H.-J. , Straube U.
Published: July 2002

We have determined the elastic constants of langasite-type crystals (La3Ga5SiO14, La3Ga5.5Nb0.5O14, and La3Ga5.5Ta0.5O14) from measurements of the sound velocity of acoustic waves. Starting with the elastic tensor determined from bulk acoustic waves, we optimized the data set by investigating the influence of the elastic tensor components on the angular dispersion of surface guided waves. This procedure is particularly useful for accurate determination of the nondiagonal elements of the elastic tensor. Phase velocity calculations based on the new set of constants show an increased agreement with experimental data compared to the data set derived from bulk waves and previously published material data.
Identification of Si and O donors in hydride-vapor-phase epitaxial GaN
Authors: Moore W. J. , Freitas J. A. , Braga G. C. B. , Molnar R. J. , Lee S. K. , Lee K. Y. , Song I. J.
Published: July 2002

Donor impurity excitation spectra in the infrared from two high-quality, not-intentionally doped, hydride-vapor-phase epitaxial GaN wafers are reported. Two previously observed shallow donors which we designate N1 and N2 were observed in both wafers. However, spectra of one wafer are dominated by N1 and spectra of the other by N2. A comparison of infrared and secondary ion mass spectroscopic data allows identification of N1 as Si and N2 as O. Silicon is the shallowest uncompensated donor in these samples with an activation energy of 30.18±0.1 meV in the freestanding Samsung wafer. The activation energy of O is found to be 33.20±0.1 meV. An unidentified third donor with an activation energy of 31.23±0.1 meV also was observed. Integrated absorption cross sections are found to be 8.5×10−14 cm for Si and 8.6×10−14 cm for O.
Effect of average grain size on the work function of diamond films
Authors: Abbott Patrick , Sosa Edward D. , Golden David E.
Published: July 2002

The work function of hydrogen-terminated polycrystalline diamond films deposited by electrophoresis on molybdenum was studied using ultraviolet photoelectron spectroscopy with 21.2 eV photons for average grain sizes ranging from 0.32 to 108 μm. The work function has a maximum of about 5.1 eV at 0.32 μm, then decreases with increasing grain size to a minimum of about 3.2 eV at an average grain size of about 4 μm and then increases to a value of about 4.8 eV at a grain size of 108 μm. The results are consistent with a model in which the work function is controlled by the work function of single crystal diamond (111) at the larger grain sizes, graphitic carbon at the smaller grain sizes, and by a negative electron affinity that increases with decreasing grain size due to defects near diamond (111) crystallite edges for the intervening grain sizes. The large change in work function (almost a factor of 2) could be useful to make conductors with different work functions for microelectronic gate structures.
Measuring nanomechanical properties of a dynamic contact using an indenter probe and quartz crystal microbalance
Authors: Borovsky B. , Krim J. , Syed Asif S. A. , Wahl K. J.
Published: July 2002

A study of the contact mechanics of a probe tip interacting with a quartz crystal microbalance (QCM) has been performed, involving simultaneous measurements of normal load, displacement, and contact stiffness with changes in QCM resonant frequency. For metal–metal and glass–metal contacts in air, the QCM frequency shifts were observed to be positive, and directly proportional to the contact area as inferred from the contact stiffness. Interfacial characteristics of the probe–tip contact (elasticity, contact size, and an estimate of the number of contacting asperities) were deduced by extending a prior model of single asperity contact to the case of multiple contacts. The extended model clarifies a number of seemingly disparate experimental results that have been reported in the literature.
Efficient coupling of surface plasmon polaritons to radiation using a bi-grating
Authors: Worthing P. T. , Barnes W. L.
Published: July 2002

A nanostructured surface in the form of a bi-grating is shown to efficiently couple surface plasmon polaritons to free-space radiation in the visible part of the spectrum. Coupling was achieved for all propagation directions of the surface mode and the efficiency found to be independent of the propagation direction, taking a mean value of 60% for the structure examined. The consequences of the findings for emissive devices that make use of surface plasmons are discussed.
Surface wrinkling of two mutually attracting elastic thin films due to van der Waals forces
Authors: Ru C. Q.
Published: July 2002

This article studies surface instability of two mutually attracting elastic films due to van der Waals forces under plane strain conditions. The analysis is based on an approximate method which reduces a two-dimensional problem of an elastic film to a one-dimensional surface problem. This method is much simpler than the conventional method and allows one to easily identify the critical value of the interaction coefficient and the surface instability mode. The accuracy of this method is demonstrated by the excellent agreement (with relative errors less than 5%) between the predicted results and the known exact data for a special case of an elastic film interacting with a rigid flat surface. The incompatibility of the individual fundamental modes of two elastic layers with unequal thickness results in two distinct metastable instability modes when the thickness ratio exceeds a critical value, in contrast to the uniqueness of the surface instability mode of an elastic film attracted to a rigid flat surface. This nonuniqueness of the metastable instability modes would cause complex surface instability phenomena of two interacting elastic films (such as jump between the two instability modes).
Helium nanodroplet isolation rovibrational spectroscopy: Methods and recent results
Authors: Callegari Carlo , Lehmann Kevin K. , Schmied Roman , Scoles Giacinto
Published: July 2002

In this article, recent developments in helium nanodroplet isolation (HENDI) spectroscopy are reviewed, with an emphasis on the infrared region of the spectrum. We discuss how molecular beam spectroscopy and matrix isolation spectroscopy can be usefully combined into a method that provides a unique tool to tackle physical and chemical problems which had been outside our experimental possibilities. Next, in reviewing the experimental methodology, we present design criteria for droplet beam formation and its seeding with the chromophore(s) of interest, followed by a discussion of the merits and shortcomings of radiation sources currently used in this type of spectroscopy. In a second, more conceptual part of the review, we discuss several HENDI issues which are understood by the community to a varied level of depth and precision. In this context, we show first how a superfluid helium cluster adopts the symmetry of the molecule or complex seeded in it and discuss the nature of the potential well (and its anisotropy) that acts on a solute inside a droplet, and of the energy levels that arise because of this confinement. Second, we treat the question of the homogeneous versus inhomogeneous broadening of the spectral profiles, moving after this to a discussion of the rotational dynamics of the molecules and of the surrounding superfluid medium. The change in rotational constants from their gas phase values, and their dependence on the angular velocity and vibrational quantum number are discussed. Finally, the spectral shifts generated by this very gentle matrix are analyzed and shown to be small because of a cancellation between the opposing action of the attractive and repulsive parts of the potential of interaction between molecules and their solvent. The review concludes with a discussion of three recent applications to (a) the synthesis of far-from-equilibrium molecular aggregates that could hardly be prepared in any other way, (b) the study of the influence of a simple and rather homogeneous solvent on large amplitude molecular motions, and (c) the study of mixed He3/He4 and other highly quantum clusters (e.g., H2 clusters) prepared inside helium droplets and interrogated by measuring the IR spectra of molecules embedded in them. In spite of the many open questions, we hope to convince the reader that HENDI has a great potential for the solution of several problems in modern chemistry and condensed matter physics, and that, even more interestingly, this unusual environment has the potential to generate new sets of issues which were not in our minds before its introduction.
Triplet states of rubidium dimers on helium nanodroplets
Authors: Brühl F. R. , Miron R. A. , Ernst W. E.
Published: July 2002

Rubidium dimers in their 1 3Σu+ states are formed through collisions of Rb atoms that have been deposited on the surface of helium nanodroplets. Visible absorption spectra between 550 and 690 nm were probed by laser induced fluorescence and emission spectra measured for selected excitation wavelengths. A system absorbing around 595 nm with its emission to the ground state centered at 604 nm is identified as the Rb2 2 3Πg–1 3Σu+ transition. A broad unstructured band is measured near 667 nm. Following its excitation, two fluorescence channels are detected, one representing the 1 1Πu–1 1Σg+ (Rb2 B–X) transition and the other leading to atomic Rb D1 and D2 emission. Various explanations of this observation are discussed, each of which requires the presence of a third rubidium atom on the droplet. All spectra have been modeled using energy potentials from previous theoretical work and the results are compared. Relaxation after laser excitation experiences various bottlenecks, which show up in the vibrational emission intensities and may be interpreted in terms of intramolecular interactions.
Nonlinear optical tagging diagnostic for the measurement of Fokker–Planck diffusion and electric fields
Authors: Claire N. , Dindelegan M. , Bachet G. , Skiff F.
Published: July 2002

Strong optical pumping of ions can result after a few microseconds of resonance excitation in a laser beam. However, both Fokker–Planck diffusion and acceleration due to macroscopic electric fields can remove an ion from resonance by changing the ion velocity on a similar timescale. Therefore, the time dependence of laser induced fluorescence can be influenced by particle acceleration and velocity–space diffusion. This effect which has already been used to measured Fokker–Planck diffusion, is extended to include the influence of an electric field and used to measure the electric field associated with an electrostatic shock in a multipolar gas discharge.
Electrochromic writing and erasing on tungsten oxide films in air by scanning tunneling microscopy
Authors: Qiu H. , Lu Y. F. , Mai Z. H.
Published: July 2002

The scanning tunneling microscope (STM) was used for both spot coloring and bleaching on α-WO3−x thin films. By wetting the STM tip with 1 M KOH solutions or de-ionized water before the tip approached, and keeping the humidity higher than 40%, the electrochemical reaction occurred at the tip–surface gap while the electrochromic reactions took place at the film surface. With a constant negative sample bias and grounded tip, a brown color circle or a light blue disk was formed on the surface with an enlarged diameter. The circle and disk formations were attributed to the production of alkali and hydrogen tungsten bronzes. By applying a positive sample bias with a grounded tip, the color of the hydrogen tungsten bronze could be bleached. The cathodic reduction of W6+ forms a color center at the W5+ site, while the anodic reaction removes an electron from the color center W5+ state. Raman spectroscopy indicated that for the colored state, the frequency for the W–O bond stretching mode increased, while for the W=O bonds it decreased.
Photodissociation and intramolecular dynamics of vibrationally excited CHF2Cl
Authors: Li Lianbin , Dorfman Gabriela , Melchior Aviva , Rosenwaks Salman , Bar Ilana
Published: July 2002

Photodissociation of jet-cooled, vibrationally excited CHF2Cl molecules was studied in a time-of-flight mass spectrometer to elucidate bond rupture and intramolecular dynamics. The molecules were first excited with infrared photons to the N=3, N=7/2, and N=4 C–H stretch-bend polyad components, representing stretch-bend mixed states. They were then dissociated via promotion to excited electronic states by ∼235 or 243.135 nm photons, which also tagged Cl35(2Pj) and Cl37(2Pj) or H photofragments, respectively, by (2+1) resonantly enhanced multiphoton ionization. Comparison of the photofragment yield spectra to the simultaneously measured room-temperature infrared absorption spectra revealed significant narrowing of the former due to the reduction of rotational inhomogeneous structure. These spectra, and particularly the band contraction, afforded observation of resonance splitting in the vicinity of the 31, 34, 41, and 44 components, reflecting redistribution times in the range of 1–18 ps. These times manifest the vibrational redistribution of the mixed states to other states of the molecule and are longer than those for the coupling of the stretch-bend. The initial vibrational excitation enhanced C–Cl and C–H bond cleavage with the former producing both ground-, Cl P3/22[Cl], and excited-, Cl P1/22[Cl*], spin-orbit states. The branching ratio of Cl*/Cl was ∼0.5 and of H/[Cl*+Cl]∼0.1, independent on the initially prepared state, signifying preferential production of Cl photofragments and energy flow from the initially excited bond.
Photoemission and conduction currents in vacuum ultraviolet irradiated aluminum oxide
Authors: Lauer J. L. , Shohet J. L. , Cismaru C. , Hansen R. W. , Foo M. Y. , Henn T. J.
Published: July 2002

A temporary increase in the conductivity of aluminum oxide sputter deposited on the surface of aluminum wafers was made by exposure to vacuum ultraviolet (VUV) radiation produced by a synchrotron light source. The oxide was charged, either positively or negatively, by exposure to a nonreactive inductively coupled plasma, under typical plasma processing conditions. We show the dependence of the conductivity on the energy of the incoming radiation, and conclude that only those photons whose energy is above the band gap of the oxide are capable of producing a temporary increase in the conductivity. Two processes, photoemission and enhanced conductivity, create currents flowing across the oxide layer. A circuit model was developed to show the contributions from both processes to the total current. We conclude that VUV radiation may be used to significantly decrease plasma-induced surface charging of dielectrics.
Rutile-type oxide-diluted magnetic semiconductor: Mn-doped SnO2
Authors: Kimura H. , Fukumura T. , Kawasaki M. , Inaba K. , Hasegawa T. , Koinuma H.
Published: July 2002

Epitaxial films of an oxide-diluted magnetic semiconductor with rutile structure, Mn-doped SnO2, have been fabricated by pulsed-laser deposition. As the Mn content increases, systematic changes in lattice constants and in-gap absorption are observed. Magnetization measurements show almost paramagnetic behavior. The injection of n-type carrier over 1020 cm−3 is achieved by Sb doping. A Sn0.95Mn0.05O2:Sb film shows giant positive magnetoresistance as large as 60% at 5 K.
Phase transitions in a nematic binary mixture
Authors: Rauch A. Christine , Garg Shila , Jacobs D. T.
Published: July 2002

Our objective was to study mixtures of nematic liquid crystals with dissimilar dielectric anisotropies but similar phase properties. Using light scattering and microscopy, we have established the phase boundaries and transition widths of mixtures of 4′-n-pentyl-4-cyanobiphenyl and 4′-methoxybenzylidene-4-butylaniline. In addition to the isotropic-nematic transition, there is a second induced phase for certain concentrations, which we conclude is an induced smectic B phase. Recent theoretical works provide a model for nematic to induced smectic A transition by combining Flory–Huggins and Maier–Saupe–McMillan theories. From our phase transition data and the application of the above theoretical framework, we conclude that there is a possibility of strong interaction between the two mesogens that produces the smectic B phase.
A complete quantum description of an ultrafast pump-probe charge transfer event in condensed phase
Authors: Koch Christiane P. , Klüner Thorsten , Kosloff Ronnie
Published: July 2002

An ultrafast photoinduced charge transfer event in condensed phase is simulated. The interaction with the field is treated explicitly within a time-dependent framework. The description of the interaction of the system with its environment is based on the surrogate Hamiltonian method where the infinite number of degrees of freedom of the environment is approximated by a finite set of two-level modes for a limited time. This method is well suited to ultrafast events, since it is not limited by weak coupling between system and environment. Moreover, the influence of the external field on the system-bath coupling is included naturally. The surrogate Hamiltonian method is generalized to incorporate two electronic states including all possible system-bath interactions. The method is applied to a description of a pump-probe experiment where every step of the cycle is treated consistently. Dynamical variables are considered which go beyond rates of charge transfer such as the transient absorption spectrum. The parameters of the model are chosen to mimic the mixed valence system (NH3)5RuNCRu(CN)5−.
Magnetic properties of the low-temperature phase of MnBi
Authors: Saha S. , Obermyer R. T. , Zande B. J. , Chandhok V. K. , Simizu S. , Sankar S. G. , Horton J. A.
Published: July 2002

MnBi forms peritectically at ∼450 °C. Preparation of MnBi employing conventional techniques such as arc melting and induction melting results in the segregation of manganese. In order to avoid this segregation, we followed the procedure recommended by Guo et al. [X. Guo, A. Zaluska, Z. Altounian, and J. O. Strom-Olsen, J. Mater. Res. 5, 2646 (1990)] and prepared a low-temperature phase of MnBi by melt spinning, followed by heat treatment. Fine powder of MnBi was prepared by ball milling the melt-spun ribbons for various lengths of time. Magnetic properties of these powders were determined. In particular, the temperature dependent coercivity was studied from room temperature to 360 °C for the powders ball milled for 2 and 10 h. The coercivity is found to increase with the increase in temperature reaching a maximum of 25.8 kOe at 280 °C and then decrease as the temperature is increased further. We also found that a peak in coercivity is observed for the samples milled for 10 h. MnBi shows a first-order transition to a paramagnetic phase at 360 °C. In an attempt to increase this transition temperature, an alloy of composition Mn0.75Ni0.25Bi0.5Sb0.5 was made by induction melting. The transition temperature increases from 360 °C for MnBi to 400 °C for Mn0.75Ni0.25Bi0.5Sb0.5.
Electronic, magnetic, and structural properties of L10FePtxPd1−x alloys
Authors: Willoughby S. D. , MacLaren J. M. , Ohkubo T. , Jeong Sangki , McHenry Michael , Laughlin David E. , Choi Sang-Jun , Kwon Soon-Ju
Published: July 2002

We present theoretical and experimental results on the electronic, magnetic, and structural properties of L10 FePd, FePt, and FePtxPd1−x alloys. These alloys have large magnetizations and magnetocrystalline anisotropies and as such are potentially technologically important for applications as permanent magnets or high-density storage media. Results of first principles electronic structure calculations show that the system is a strong ferromagnet with an almost full majority Fe band, and that magnetization and magnetocrystalline anisotropy remain large over the range of composition. Total energy calculations predict phase separation on the disordered Pd/Pt layer at low temperatures. Experimental studies using energy dispersion x-ray line scans confirm regions that are Pt rich and Pd rich forming at low temperatures.
Photorefractive polymer composites fabricated by injection molding
Authors: Herlocker J. A. , Fuentes-Hernandez C. , Wang J. F. , Peyghambarian N. , Kippelen B. , Zhang Q. , Marder S. R.
Published: July 2002

We report on the fabrication of bulk samples of photorefractive polymers using the injection molding technique. The photorefractive properties of these materials are evaluated by four-wave mixing and two-beam coupling experiments. Samples with good optical quality, high diffraction efficiency, and net optical gain are obtained.
Correlation between magnetic properties and carrier concentration in Ge1−xMnxTe
Authors: Fukuma Y. , Arifuku M. , Asada H. , Koyanagi T.
Published: July 2002

The dependence of magnetic properties on the carrier concentration for Ge1−xMnxTe films prepared by ionized-cluster beam technique has been studied to clarify the mechanism of carrier-induced ferromagnetism. The experimental results suggest that the cluster of spins aligned by short-range ferromagnetic interaction tends to be formed for samples with the lower carrier concentration than the critical value for the occurrence of homogeneous ferromagnetic order. With increasing carrier concentration, the long-range ferromagnetic interaction grows. The magnetotransport properties of Ge1−xMnxTe films are found closely related to its magnetic properties, indicating carrier-enhanced ferromagnetic effects.
Vibrational spectroscopy and matrix-site geometries of HArF, HKrF, HXeCl, and HXeI in rare-gas solids
Authors: Bihary Z. , Chaban G. M. , Gerber R. B.
Published: July 2002

The vibrational spectroscopy and the matrix-site geometries of several novel rare-gas compounds in the matrix environment were computed theoretically, and compared with experiment. Ab initio calculations are used in the fitting of analytical potential surfaces for the HRgY molecules and for the interactions between HRgY and the matrix atoms Rg. With these potentials, matrix-site geometries for the molecule in the solid are computed. Finally, the vibrational spectroscopy of HRgY in the Rg matrix is computed using the vibrational self-consistent field (VSCF) method. The VSCF includes anharmonic effects, that are essential in this case. The version of VSCF used here includes coupling between HRgY and the vibrations of the solid atoms. The vibrations of 72 matrix atoms are treated. The main results are: (1) The matrix shifts are considerably greater than typically found for neutral, strongly bond molecules, but are much smaller than discrepancies between theory and experiment. This can be attributed to the insufficient accuracy of the potentials used for the HRgY molecules. This calls for better future description of the electronic structure of HRgY. (2) The matrix shifts and splitting effects are interpreted by the calculations in terms of the site geometries involved. These effects are very different for HArF, HKrF than for HXeCl, HXeI. (3) The computed matrix-site splittings are in semiquantitative accord with experiment. This supports the interaction potentials used between HRgY and the matrix. The results provide insights on the effects of the matrix on the rare-gas molecules.
Local current density detection of individual single-wall carbon nanotubes in a bundle
Authors: Fujiwara Akihiko , Iijima Ryosuke , Ishii Kenji , Suematsu Hiroyoshi , Kataura Hiromichi , Maniwa Yutaka , Suzuki Shinzo , Achiba Yohji
Published: July 2002

We have measured the local current density on individual single-wall carbon nanotubes (SWNTs) with the conducting tip of an atomic force microscope; the SWNTs make up a nanometer-scale electronic circuit on an insulating substrate. Scanning tunneling spectroscopy measurements at certain positions on a SWNT bundle show that both metallic and semiconducting nanotubes can coexist in a bundle. The approach applied in this experiment appears as a powerful technique for the investigation of the spatial variation of current density and electronic states of nanometer-scale electronic devices.
Stability of miscible displacements of shear thinning fluids in a Hele-Shaw cell
Authors: Azaiez J. , Singh B.
Published: July 2002

A linear stability analysis of the viscous fingering of miscible non-Newtonian flow displacements in a rectilinear Hele-Shaw cell is presented. The shear-thinning character of the non-Newtonian fluid is described using the Carreau model which involves two rheological parameters De and n. Flows where either the displacing or displaced phase has a shear-thinning behavior are examined and compared with those of Newtonian flows. It is found that the shear-thinning character of the non-Newtonian fluid has an important effect on the flow instability. In particular, a flow where the driving fluid is shear-thinning is always more unstable than its Newtonian counterpart. For this flow, the maximum growth rate and the spectrum of unstable wave numbers are larger than in the Newtonian case which suggests that more ramified structures will develop as the finger instability grows. On the other hand, when the displaced fluid is non-Newtonian, a stronger shear-thinning rheological behavior leads in general to a less unstable flow. The mechanisms responsible for the changes in the flow instability are explained in terms of the different sources contributing to the generation of the vorticity disturbance.
Multiparticle interactions and stimulated emission in chemically synthesized quantum dots
Authors: Mikhailovsky A. A. , Malko A. V. , Hollingsworth J. A. , Bawendi M. G. , Klimov V. I.
Published: July 2002

We study the effect of multiparticle interactions on optical gain and stimulated emission in close-packed solids of chemically synthesized CdSe nanocrystals (nanocrystal quantum dots). An analysis of pump-dependent nonlinear absorption signals indicates that the band-edge optical gain is due to multiparticle states with a dominant contribution from doubly excited nanocrystals (quantum-confined biexcitons). We observe that optical gain dynamics are due to the competition between ultrafast hole surface trapping and multiparticle Auger decay. We analyze the effect of intrinsic Auger recombination on optical gain lifetimes and gain pump intensity thresholds.
Results from a scaled final focus experiment for heavy ion fusion
Authors: MacLaren S. A. , Faltens A. , Seidl P. A. , Rose D. V.
Published: July 2002

A one-tenth-scale version of a final focus subsystem for a heavy-ion-fusion driver has been built and used for experimental tests of concept. By properly scaling the parameters that relate ion energy and mass, current, emittance, and focusing fields, the transverse dynamics of a representative driver final focus have been replicated in a small laboratory beam. Whereas the driver beam parameters considered are 10 GeV Bi+ at 1.25 kA, the scaled experiment used a 95 μA beam of 160 keV Cs+ ions brought to a ballistic focus through a series of six quadrupole magnets. The measured focal spot size was consistent with calculations in the report of the design on which the experiment is based. In a second experimental program, a 400 μA beam was propagated through the focal system and partially neutralized after the last magnet using electrons released from a hot tungsten filament to test the predictions of the benefits of neutralization. The increase in beam current resulted in a corresponding increase in spot radius in the absence of electrons. A reduction of the spot radius and modification of its shape were observed in the presence of neutralizing electrons.
Surface and bulk phase behavior of dry and hydrated tetradecanol:octadecanol alcohol mixtures
Authors: Sloutskin E. , Sirota E. B. , Kraack H. , Gang O. , Doerr A. , Ocko B. M. , Deutsch M.
Published: July 2002

Surface freezing was studied in dry and hydrated octadecanol:tetradecanol (C18OH:C14OH) mixtures, using surface tension and synchrotron x-ray surface diffraction techniques. Even small amounts of admixed C18OH were found to induce surface freezing in C14OH, which does not exhibit this effect when pure. The phase diagram of the bulk was measured by calorimetry and bulk x-ray diffraction. Upon increasing the bulk mole fraction of C18OH (φ) a sharp increase in the bulk supercooling occurs at φ≈0.4 in dry mixtures, while no supercooling was observed for the hydrated mixtures. A simple thermodynamical model based on the theory of s-regular mixtures is shown to account well for the dependence of the surface freezing onset temperature of both dry and hydrated mixtures, and the hydrated bulk’s freezing temperature on φ. Only a phenomenological description exists for the dry bulk’s phase diagram. This study is expected to provide a baseline for the general surface and bulk behavior of long-chain alcohol mixtures.
On gauge transformations of Bäcklund type and higher order nonlinear Schrödinger equations
Authors: Goldin Gerald A. , Shtelen Vladimir M.
Published: July 2002

We introduce a new, more general type of nonlinear gauge transformation in nonrelativistic quantum mechanics that involves derivatives of the wave function and belongs to the class of Bäcklund transformations. These transformations satisfy certain reasonable, previously proposed requirements for gauge transformations. Their application to the Schrödinger equation results in higher order partial differential equations. As an example, we derive a general family of sixth-order nonlinear Schrödinger equations, closed under our nonlinear gauge group. We also introduce a new gauge invariant current σ=ρ∇Δ ln ρ, where ρ=ψ̄ψ. We derive gauge invariant quantities, and characterize the subclass of the sixth-order equations that is gauge equivalent to the free Schrödinger equation. We relate our development to nonlinear equations studied by Doebner and Goldin, and by Puszkarz.
Coherent structures and self-consistent transport in a mean field Hamiltonian model
Authors: del-Castillo-Negrete D. , Firpo Marie-Christine
Published: July 2002

A study of coherent structures and self-consistent transport is presented in the context of a Hamiltonian mean field, single wave model. The model describes the weakly nonlinear dynamics of marginally stable plasmas and fluids, and it is related to models of systems with long-range interactions in statistical mechanics. In plasma physics the model applies to the interaction of electron “holes” and electron “clumps,” which are depletions and excesses of phase-space electron density with respect to a fixed background. In fluid dynamics the system describes the interaction of vortices with positive and negative circulation in a two-dimensional background shear flow. Numerical simulations in the finite-N and in the N→∞ kinetic limit (where N is the number of particles) show the existence of coherent, rotating dipole states. We approximate the dipole as two “macroparticles” (one hole and one clump) and consider the N=2 limit of the model. We show that this limit has a family of symmetric, rotating integrable solutions described by a one-degree-of-freedom nontwist Hamiltonian. A perturbative solution of the nontwist Hamiltonian provides an accurate description of the mean field and rotation period of the dipole. The coherence of the dipole is explained in terms of a parametric resonance between the rotation frequency of the macroparticles and the oscillation frequency of the self-consistent mean field. This resonance creates islands of integrability that shield the dipole from regions of chaotic transport. For a class of initial conditions, the mean field exhibits an elliptic–hyperbolic bifurcation that leads to the filamentation, chaotic mixing and eventual destruction of the dipole.
Optical limiting in SrBi2Ta2O9 and PbZrxTi1−xO3 ferroelectric thin films
Authors: Yang Pingxiong , Xu Jianfeng , Ballato John , Schwartz Robert W. , Carroll David L.
Published: July 2002

Optical limiting effects in SrBi2Ta2O9 (SBT) and PbZrxTi1−xO3 (PZT) ferroelectric thin films have been observed with nanosecond laser pulses at 1.064 μm. Limiting thresholds were found to be 5.84 J/cm2 for SBT and between 4.53 and 5.93 J/cm2 for PZT, depending on composition, whereas saturation thresholds for the films were about 2.92 J/cm2 and between 2.27 J/cm2 to 2.97 J/cm2, respectively. Damage thresholds around 10.0 J/cm2 and between 10.37 J/cm2 to 10.54 J/cm2, respectively for SBT and PZT, were also determined. A possible mechanism for the observed limiting, nonlinear optical scattering from the ferroelectric domains, is discussed. These results elucidate the origin of the nonlinear optical properties in perovskite-type ferroelectric thin films and show the potential role such materials can play in photonic devices based on nonlinear optical effects.
Critical behavior of the restricted primitive model revisited
Authors: Caillol J.-M. , Levesque D. , Weis J.-J.
Published: July 2002

Reassessment of the critical temperature and density of the restricted primitive model of an ionic fluid by Monte Carlo simulations performed for system sizes with linear dimension up to L/σ=34 and sampling of ∼109 trial moves leads to Tc*=0.049 17±0.000 02 and ρc*=0.080±0.005. Finite size scaling analysis based in the Bruce–Wilding procedure gives critical exponents in agreement with those of the three-dimensional Ising universality class. An analysis similar to that proposed by Orkoulas et al. [Phys. Rev. E 63, 051507 (2001)], not relying on an a priori knowledge of the universality class, leads to an inaccurate estimate of Tc* and to unexpected behavior of the specific heat and value of the critical exponent ratio γ/ν.
Orientational order parameter in α-N2 from x-ray data
Authors: Galtsov N. N. , Klenova O. A. , Strzhemechny M. A.
Published: July 2002

A method is suggested and validated for the deduction of orientational order parameter values η in molecular crystals consisting of diatomics directly from integrated x-ray diffraction intensities. This method is applied to pure solid nitrogen in its α phase. It is shown that to within a good accuracy the integrated intensity of a superstructure reflection is proportional to η2. The η values determined from x-ray powder diffraction measurements agree well with the values obtained by NQR and NMR.
Design of a z-axis translating laser light scattering device for particulate settling measurement in dispersed fluids
Authors: Hoffman D. L. , Brooks D. R. , Dolez P. I. , Love B. J.
Published: July 2002

A user friendly, Labview™ controlled, prototype settling device has been designed and built that incorporates a laser light source and detector fixtured to a z-axis translating stage. The Labview™ data interface drives the unit and captures data in the form of scattered intensity as a function of z-axis location. We present some examples of sample output from low- and high-density particles settling in epoxy fluids of various viscosities. This device maps all of the expected settling regimes and, more importantly, valuable scattering information about partial settling is found in instances without a discrete mud line. The effect of resin viscosity on settling has been measured and corresponds well to model predictions. Measurement of settling data in the compression regime was also achievable, validating descriptive models.
Homoclinical structures in nonautonomous systems: Nonautonomous chaos
Authors: Lerman L. M. , Shil’nikov L. P.
Published: July 2002

For general nonautonomous systems, integral sets similar to homoclinic structures of an autonomous system are introduced. A description of integral curves near such a set is given.
An expansion of system with time delayed feedback control into spatio-temporal state space
Authors: Hikihara Takashi , Ueda Yoshisuke
Published: July 2002

Time delayed feedback control is well known as an effective continuous control method for stabilizing the unstable periodic orbit embedded in chaotic attractors. As for the system with time delay, the solution is in a function space and shows characteristics governed by an infinite dimension. Therefore it is difficult to understand the system behavior analytically. In this paper, it is shown that, when the state space for the system with time delay is expanded into the spatio-temporal state space, the solution propagates in the space as a wave theoretically and numerically. The dynamic behavior experimentally obtained in the sinusoidally excited magneto-elastic beam system under time delayed feedback control is also discussed by the transformation into the expanded spatio-temporal state space.
Evidence for interfacial defects in metal-insulator-InP structures induced by the insulator deposition
Authors: Sautreuil B. , Viktorovitch P. , Blanchet R.
Published: July 2002

Photoluminescence (PL) intensity on n-type InP at room temperature is found to provide a convenient probe of the density of surface states in the upper part of the gap of InP. PL measurements are used to monitor the interaction phenomena occurring between an insulator (Al2O3, SiOx) and the InP substrate during the first stages of its deposition. Despite the use of a soft deposition technique, the insulator induces systematically interfacial defects in metal-insulator-InP structures. The nature of the insulator and, to a larger extent, the InP surface preparation and the substrate temperature (although kept below 150 °C) play a major role.
Magnetic structure of multiple bilayers of thin films of Fe and Ge
Authors: Majkrzak C. F. , Axe J. D. , Böni P.
Published: July 2002

A diffraction study of multiple bilayers consisting of thin films of Fe and Ge was performed using both x rays and polarized neutrons. A reduction in the moment of a fraction of the Fe atoms due primarily to interdiffusion at the interface is indicated.
Reflection high-energy electron diffraction studies on the molecular-beam-epitaxial growth of AISb, GaSb, InAs, InAsSb, and GaInAsSb on GaSb
Authors: Chiu T. H. , Tsang W. T.
Published: July 2002

The quality of materials grown by molecular-beam epitaxy (MBE) is closely related to the surfaces kinetics in which the surface reconstructions, growth temperatures and molecular species play important roles. We present our studies on the MBE growth conditions, surface reconstructions and surface morphologies of A1Sb, GaSb, and InAs grown on (100) GaSb, and of InAsSb and GaInAsSb lattice matched to GaSb using dimers As2 and Sb2 in a wide temperature range of ∼400–650 °C. Certain surface reconstructions for GaSb and InAs which have not been reported in the literature are identified, and the conditions for their existence are presented.
Investigation of optically allowed transitions of α-sulfur thin films
Authors: Abass A. K. , Hasen A. K. , Misho R. H.
Published: July 2002

Thin films of α-sulfur were prepared on quartz plates by a high-vacuum thermal-evaporation method. In an optical investigation of the thin films at room temperature, a direct allowed transition at 4.43 eV was observed. An indirect allowed transition was also observed with a band gap of 2.61 eV and assisting phonons of 0.11 eV. These observations were supported by reflectivity data obtained by an integrating sphere.
Deposition of refractory metal films by rare-gas halide laser photodissociation of metal carbonyls
Authors: Flynn Diane K. , Steinfeld Jeffrey I. , Sethi Dhanwant S.
Published: July 2002

Films of Cr, Mo, and W on quartz, Pyrex, Suprasil, and Al substrates were deposited by photodissociation of the respective hexacarbonyls using focused and pulsed radiation from rare-gas halide lasers. Cr was deposited by dissociation of Cr(CO)6 using XeF (308 nm), KrF (249 nm), and ArF (193 nm) lasers. Mo and W were deposited from their respective hexacarbonyls at 249 and 193 nm. Pulse energies varied between 8 and 12 mJ. Pulse rates of 10–60 Hz were used. The pulse duration was about 10 ns. Depositions with substrates both parallel and perpendicular to the excimer radiation were attempted. Only in the case of perpendicular configuration were strongly adherent films observed. The deposition rates for thicknesses up to 3000 Å appeared to be independent of the pulse rate for all three metals. The films exhibited strong adhesion to the substrate. Scanning electron microscope photographs of the films revealed the presence of continuous metal layers. Auger and x-ray analyses of the films indicated contamination from carbon and oxygen. The source of these impurities is most likely to be CO produced in the decarbonylation of the parent hexacarbonyl. Adhesion to the substrate is apparently enhanced by laser stimulated generation of strong binding sites on the surface.
High-speed InP/Ga0.47In0.53As superlattice avalanche photodiodes with very low background doping grown by continuous trichloride vapor-phase epitaxy
Authors: Mattera V. D. , Capasso F. , Allam J. , Hutchinson A. L. , Dick J. , Brown J. M. , Westphal A.
Published: July 2002

Trichloride vapor-phase epitaxy has been employed in a single chamber reactor to achieve the continuous (noninterrupted) growth of n−InP/n−In0.53Ga0.47As/InP superlattice structures for avalanche photodetector applications. Very low background doping has been achieved in both the InP (n≊1×1014 cm−3) and the superlattice layers (n=1×1015 cm−3). We report the details of the epitaxial crystal growth as well as analysis of the structures by secondary ion mass spectrometry and transmission electron microscopy. Avalanche photodiodes have been fabricated by Zn diffusion in the InP layer and mesa etching. The devices exhibit an intrinsic response time (full width at half maximum) of 66 ps at λ=1.5 μm which is the shortest so far achieved in superlattice photodetectors. Dark currents of 50 nA at unity gain, multiplication factors of 6, and breakdown voltages exceeding −120 V have been measured.
Magnetic and magneto-optical properties of Tb-Fe-Co amorphous films
Authors: Takayama S. , Niihara T. , Kaneko K. , Sugita Y. , Ojima M.
Published: July 2002

Characteristics of Tb-Fe-Co amorphous films have been systematically investigated in order to optimize compositions suitable for magneto-optical recording medium with high readout signal. In order to optimize alloy composition, Curie temperature TC and compensation temperature Tcomp are summarized as functions of both Tb and Co contents. Crystallization temperature Tx of Tb-Fe-Co amorphous films has been also measured to be about 400 °C and tends to increase with replacing Fe by either Co or Tb. To evaluate Tb-Fe-Co films as a magneto-optical recording medium, carrier C and modulation noise Nmod (defined by the difference in noise before and after writing) are measured as a function of external field Hex. It is found that written bits of large Nmod show an irregular shape, while those of small Nmod are of regular one. To reduce Nmod and obtain high readout C/N, films with lower saturation magnetization Ms (that is, low demagnetization field) at just below writing temperature are preferred.
Positive temperature coefficient resistors as high-power pulse switches: Performance limitations, temperature effects, and triggering behavior
Authors: Ford R. , Kahn M.
Published: July 2002

Positive temperature coefficient resistors are evaluated in the self-heating mode as pulse generators and as opening switches in inductive storage pulse power supplies. Results from both carbon-filled polymer devices, as well as from barium-titanate-based devices, are shown. Theoretical switched powers of close to a 1/4 MW are calculated for honeycomb-shaped BaTiO3 positive temperature coefficient resistors. Switching times in the millisecond range are achieved. Pulse duration varies with ambient temperature, but this can be counteracted by using capacitative triggering. Surface flashover was one of the critical failure mechanisms. The results point towards further improved device designs.
A kinetic model for solid-state silicide nucleation
Authors: Bené R. W.
Published: July 2002

We present a model for solid-state silicide nucleation at the interface of a thin metal film and a silicon substrate, starting with the assumption that compound nucleation in these systems is a kinetically controlled process. We consider the work being done on other systems, such as dendrite nucleation and growth, and snowflake generation, where the processes are kinetically self-limited by the structures that form. We show that the requirements for kinetically self-limited systems can be formally satisfied at the interfaces for silicide forming systems. In particular, in analogy with the selection of metastable shapes for snowflake structures, we would in general expect metastable chemical structures to form at metal-semiconductor interfaces under conditions of large supercooling. We discuss the selection criteria (maximum energy degradation rate), the probable forms of these structures, and the conditions required for their growth. We then discuss an interpretation of phenomenological rules for selection of first-phase nucleation in terms of these structures.
Prediction of optical properties of amorphous tetrahedrally bonded materials
Authors: Campi D. , Coriasso C.
Published: July 2002

A new approach is described for the prediction of the optical properties of amorphous tetrahedrally bonded materials over the entire experimental energy range, in which a model of the imaginary part of the dielectric constant is assigned and the real part is obtained by the Kramers–Kronig transform. Tight-binding theory provides direct interpretation of the model parameters. The model provides good insight into the physics of the problem and a quantitative account of experimental data as well. Results are presented for Si, Ge, Si3N4, and GaAs.