bluuebunny/crossref_metadata_2025 huggingface dataset:
Annealing of Bragg gratings in hydrogen-loaded optical fiber
Authors: Patrick H. , Gilbert S. L. , Lidgard A. , Gallagher M. D.
Published: July 2002

We have conducted a detailed study of the thermal stability of Bragg gratings written in hydrogen-loaded and unloaded germanium-doped optical fiber. Interference of either continuous-wave or pulsed ultraviolet light was used to induce the index modulation gratings. Some gratings were kept at room temperature and others were annealed at fixed temperatures for 10–20 h. For temperatures between room temperature and 350 °C, gratings in the hydrogen-loaded fiber showed significantly greater decay than those in the unloaded counterpart. The ultraviolet-induced index modulation in hydrogen-loaded fiber was reduced by 40% after 10 h at 176 °C, whereas it was reduced by only 5% in unloaded fiber under the same conditions. The annealing behavior of gratings written using the pulsed source was identical to that of gratings written with the continuous-wave source, and the thermal stability of gratings in hydrogen-loaded fiber did not depend on the magnitude of the index modulation. We also observed that the annealing of ultraviolet-induced OH absorption in the hydrogen-loaded fiber was not correlated with the grating decay. Our annealing results show that the species responsible for the index change in the hydrogen-loaded fiber are less stable than those in the unloaded fiber.
Size effects and giant magnetoresistance in unannealed NiFe/Ag multilayer stripes
Authors: Sanders S. C. , Cross R. W. , Russek S. E. , Roshko A. , Oti J. O.
Published: July 2002

We have observed giant magnetoresistance (GMR) in unannealed NiFe/Ag multilayer thin-film stripes. Rectangular stripes having constant thickness and a constant 11:1 length-to-width aspect ratio, but varying widths down to 0.5 μm, were measured. Two types of multilayer configurations were tested, a system of five NiFe/Ag bilayers with 5.5-nm-thick Ag spacer layers, and a system of nine bilayers with 4.4-nm-thick Ag layers. In contrast to the characteristic of annealed NiFe/Ag multilayer stripes, the unnannealed stripes produced increasing GMR ratios for decreasing stripe sizes, with the 0.5-μm-wide stripe of the five-bilayer system exhibiting a ΔR/R of 2.5%. Barkhausen noise and response broadening also increased with decreasing stripe size, however. The results are discussed in terms of magnetostatic coupling of the NiFe layers within the stripes.
Magnetic and electrical properties of single-phase, single-crystal Fe16N2 films epitaxially grown by molecular beam epitaxy (invited)
Authors: Sugita Yutaka , Takahashi Hiromasa , Komuro Matahiro , Igarashi Masukazu , Imura Ryo , Kambe Takashi
Published: July 2002

The average magnetic moment per Fe atom for a single-phase, single-crystal Fe16N2(001) film epitaxially grown on a GaAs(001) substrate by molecular beam epitaxy has been confirmed to be 3.5μB at room temperature by using a vibrating sample magnetometer (VSM) and Rutherford backscattering. The value was in good agreement with that obtained by using a VSM and by measuring the film thickness (3.3μB per Fe atom). The saturation magnetization 4πMs has been found to increase with decreasing temperature, obeying T3/2 law at lower temperatures. The slope was steeper than that of a pure Fe film, suggesting a lower exchange constant for Fe16N2. The g factor for Fe16N2 has been accurately measured to be 2.17 by using ferromagnetic resonance with changing frequencies of 35.5–115 GHz, which is not unusual compared with the g factor of 2.16 for pure Fe. The resistivity for Fe16N2 has been measured to be around 30 μΩ cm at room temperature compared with 10 μΩ cm for pure Fe and decreases linearly with decreasing temperature. The behavior was that for normal metal and nothing unusual was seen. The anomalous Hall resistivity for Fe16N2 was 4×10−7 V cm/A, which is about three times as large as that for pure Fe. The relationship between the giant magnetic moment and the anomalous Hall resistivity has not been clarified yet.
Designing logic functions using an electron waveguide Y-branch switch
Authors: Palm Thomas , Thylén Lars
Published: July 2002

We propose and analyze theoretically logical circuitry based on electron waveguides. The circuit elements are branching electron waveguides (Y-branch switches) where an applied field gives a nonlinear digital response function, giving standardized signal levels. The proposed circuitry is shown to exhibit features analogous to those of complementary metal-oxide semiconductors. Circuits implementing NAND and NOR functions are shown, as well as a realization of a logical implication that only requires one Y-branch switch.
Free silicon and crystallization in silicon nitride based ceramics and in oxynitride glasses
Authors: Rouxel Tanguy , Piriou Bernard
Published: July 2002

Changes in microstructure induced by heat treatments between 1000 and 1500 °C in Si3N4 based glasses and ceramics were characterized using x-ray diffraction and Raman scattering. The presence of free silicon appears to be a common feature of these families of materials. Si precipitates were observed by transmission electron microscopy in a bulk glass and in a polycrystalline ceramic. It is shown that heat treatments affect the free silicon content and that its decrease upon annealing is closely related to the crystallization of secondary phases. The incidence of the amount of free silicon on the material color strongly suggests that silicon microcrystals are responsible for most of the color fluctuations reported in silicon nitride glasses and ceramics. The higher the free Si content, the darker they are.
Single-projection radiography for noncircular symmetries: Generalization of the Abel transform method
Authors: Gueron Shay , Deutsch Moshe
Published: July 2002

We present a new method which extends the use of the single projection radiographic Abel method, hitherto applicable only to objects of circular and elliptical cross sections, to objects having general, noncircular symmetries. This is done by developing a new integral equation that is similar in applications to Abel’s equation, and includes it as a special case. The use of the new equation is discussed for objects having a smooth and convex cross-section boundary (e.g., elliptic), a piecewise smooth convex boundary (e.g., bi-parabolic), and a boundary with regions of zero curvature (e.g., polygons). Specific examples are given for each of these three classes, and analytic inverses are calculated for these cases. Also, numerical inversion of the integral equation is given, showing satisfactory results. We show that in contrast to Abel’s equation in many cases the kernel of the integral equation is non-singular. Consequently, fairly simple inversion techniques are sufficient. Finally, the azimuthal variation of the transmitted intensity is employed to provide a convenient and fast nondestructive evaluation test of the deviation of the radiographed object from a prescribed symmetry.
A high-temperature x-ray-diffraction study of epitaxial PbTiO3 thin films on MgO(100) grown by metal-organic chemical-vapor deposition
Authors: Batzer R. S. , Yen Bi Ming , Liu Donhang , Chen Haydn , Kubo H. , Bai G. R.
Published: July 2002

Epitaxial lead titanate (PbTiO3) thin films were grown on MgO(100) single-crystal substrate by metal-organic chemical-vapor deposition at a growth temperature of 650 °C. The films were dense, stoichiometric, and epitaxial. The domain structure was studied via x-ray-diffraction ω scans and in-plane Φ scans. Existence of c and a domains in the PbTiO3 films was evident. Consistent with literature reports, the a domains were found to have four variants 90° apart from each other, tilting about 2.2° away from the surface normal. The population ratio between c and a domain of the PbTiO3 films was determined to be approximately 2.3 to 1. The domain structure was found to be stable and reproducible during repeated thermal cycling above and below Tc. Furthermore, lattice parameters and Curie temperatures of PbTiO3 thin films were measured during the thermal cycles. The lattice parameters were found to be smaller and the Curie temperatures were shifted lower than the corresponding bulk values. This is attributed to the film stress effect. A theory based on the Landau–Ginzburg–Devonshire function has been developed to explain the shift of the Curie temperatures.
Electrostatic forces acting on the tip in atomic force microscopy: Modelization and comparison with analytic expressions
Authors: Belaidi S. , Girard P. , Leveque G.
Published: July 2002

With the model of equivalent charge distribution, we calculated the exact electrostatic force acting on the real (conical) tip of an atomic force microscope. This model applies to a conductive tip in front of a conductive plane. We compared the equivalent charge model with several analytic models used to date to approximate the electrostatic forces and discussed their degree of validity. We estimated the contribution of the cantilever to the total force and showed, on the basis of theoretical calculations and experimental results, that the contribution of cantilever may constitute the essential part of the electrostatic force in the range of distances used in electrostatic force microscopy in the air.
Ab initio pseudopotential calculations of InAs/AlSb heterostructures
Authors: Ren Shang-Fen , Shen Jun
Published: July 2002

The InAs/AlSb heterojunction system is a lattice matched semiconductor heterostructure with type-II band alignment at the interfaces. This system is attractive to ultrafast electronic device applications because of its high mobility and large conduction-band offset. There are two different interfaces, i.e., AlAs-like or InSb-like, for InAs/AlSb heterostructure systems, with which very different properties, such as interface roughness, mobility, and carrier concentrations, were found to be associated. There are experimental facts indicating preferential local arrangements of the interfacial atoms. In this research, first principle pseudopotential calculations are performed on InAs/AlSb superlattices associated with different interface arrangements, and their total energy is compared with that of superlattices with ideal sharp strained interfaces. The results show that superlattices with switched layers at the interfaces are more energetically favorable than superlattices with ideal sharp strained interfaces, the switching of the Al layer with the In layer at the AlAs-like interface is the most energetically favorable, with about 0.08 eV lower energy for each switch. These calculations provide more understanding about the roughness at AlAs-like interfaces in InAs/AlSb superlattices.
Carrier relaxation and recombination in GaAs/AlGaAs quantum heterostructures and nanostructures probed with time-resolved cathodoluminescence
Authors: Lin H. T. , Rich D. H. , Konkar A. , Chen P. , Madhukar A.
Published: July 2002

We have examined the kinetics of carrier relaxation in GaAs/AlGaAs quantum wells (QWs), quantum wires (QWRs), and quantum boxes (QBs) with time-resolved cathodoluminescence (CL). In the cases of QWRs and QBs, the nanostructures were grown via a size-reducing growth approach on pre-patterned GaAs(001) substrates composed of stripes and mesas, respectively. The growth involved deposition of multiple GaAs/AlGaAs layers in order to establish both structural and optical markers which facilitated the identification of important features in transmission electron microscopy (TEM) and CL experiments. In TEM measurements, the lateral dimensions of the top-most GaAs layers in typical stripe and mesa structures comprising the QWRs and QBs delineate GaAs regions expected to exhibit 2D and 3D quantum confinement effects, respectively. Time-delayed CL spectra of all three structures reveal that the initial capture of carriers in the active regions occurs on a time scale less than the temporal resolution of the CL system, ∼100 ps, during the onset of luminescence. Hot carriers, as a result of re-emission out of thin QWs surrounding the QWRs and QBs, exhibit diffusive transport followed by relaxation into laterally confined regions which exhibit confined states of lower energy. This thermalization gives rise to a relatively slow onset and decay of luminescence attributed to the lowest energy optical transitions. By comparing time-resolved CL transients in these three structures, we find that the average luminescence onset and initial-decay rates both decrease as the dimensionality of the system reduces from 2D to 0D. These results demonstrate that the rate of carrier relaxation, including the re-emission and diffusive transport of carriers, will depend on details of the total surrounding structure which comprises the excitation region.
Effective dielectric constant of random composite materials
Authors: Sareni B. , Krähenbühl L. , Beroual A. , Brosseau C.
Published: July 2002

The randomness in the structure of two-component dense composite materials influences the scalar effective dielectric constant, in the quasistatic limit. A numerical analysis of this property is developed in this paper. The computer-simulation models used are based on both the finite element method and the boundary integral equation method for two- and three-dimensional structures, respectively. Owing to possible anisotropy the orientation of spatially fixed inhomogeneities of permittivity ε1, embedded in a matrix of permittivity ε2, affects the effective permittivity of the composite material sample. The primary goal of this paper is to analyze this orientation dependence. Second, the effect of the components geometry on the dielectric properties of the medium is studied. Third the effect of inhomogeneities randomly distributed within a matrix is investigated. Changing these three parameters provides a diverse array of behaviors useful to understand the dielectric properties of random composite materials. Finally, the data obtained from this numerical simulation are compared to the results of previous analytical work.
Enhancement of the critical current density in Chevrel phase superconducting wires
Authors: Cheggour N. , Decroux M. , Gupta A. , Fischer O/. , Perenboom J. A. A. J. , Bouquet V. , Sergent M. , Chevrel R.
Published: July 2002

The critical current densities Jc of Chevrel phase wires with niobium as an antidiffusion barrier were measured in magnetic fields up to 24 T. At 20 T and 1.9 K, Jc reaches 5.4×108 A/m2 and decreases slightly down to 3.1×108 A/m2 at 24 T. A wire with a 20% superconducting cross section has been successfully drawn and its overall critical current density Jcov exceeds 100 A/mm2 at 1.9 K up to a magnetic field slightly above 20 T. This demonstrates the ability of Chevrel phase wires to be used in high magnetic field applications. Moreover, some parts of the coil have certainly higher Jc, since Jc is very often limited by a thermal excursion of the entire coil. The effective upper critical field μ0Hc2*, deduced from the magnetic field dependence of Jc, is too low compared to the expected bulk value, indicating that superconducting properties at the grain boundaries are still degraded. If the bulk μ0Hc2 can be restored at the grain surfaces, Jcov should be higher than 100 A/mm2 up to at least 30 T.
Dependence of the morphology of polymer dispersed liquid crystals on the UV polymerization process
Authors: Carter S. A. , LeGrange J. D. , White W. , Boo J. , Wiltzius P.
Published: July 2002

Using confocal microscopy, we have studied the morphology of polymer dispersed liquid crystals (PDLC) as a function of polymer/liquid crystal composition, polymer cure temperature, and ultraviolet (UV) curing power and determined how this morphology affects the electro-optical properties. The PDLC morphology consists of a spongelike texture where spherically shaped liquid crystalline domains are dispersed in a polymer matrix. These domains grow as the fraction of liquid crystal increases and as the UV curing power decreases. We observe no significant changes in domain size with changes in the curing temperature. Instead, high-temperature cures result in coalescence and the formation of elliptical-shaped liquid crystal domains. The temperature at which this coalescence starts to be observed marks a threshold temperature Tth, above which the switching properties are strongly dependent on morphology. Below Tth the switching properties are largely independent of morphology.
Switching field studies of individual single domain Ni columns
Authors: O’Barr R. , Schultz S.
Published: July 2002

Quantitative measurements of the switching field properties of individual single-domain particles are both scientifically and technologically important. Such data are relevant for the engineering of a patterned magnetic storage medium where a single bit of information corresponds to individual single-domain particles with sizes consistent with storage densities >50 Gbit/in2. By electroplating into Al2O3 and track-etched polycarbonate filters, we have prepared particles over a large range of radii from R=20–500 nm, and have characterized them by transmission electron microscopy. Using a magnetic force microscope with an in situ electromagnet, we have quantified the extent to which the particles can be considered “single-domain.” We have also measured the angular dependence of the switching field Hs for these particles. At zero degrees, Hs lies above the curling curve for R/R0>4, and falls below the curling curve for R/R0<2, where R0 is the exchange length (R0=20.6 nm for Ni). At intermediate radii, the angular dependence data are close to the curling curve with a crossover of the data and the curling solution at R/R0∼3.
Crystallographic and magnetic structures of Fe7Se8 at low temperature (4–78 K)
Authors: Kim Eng Chan , Kang Sin Giu , Cho Ick Han , Hwang Yong Soon , Hwang Hyeun Gook , Kim Jung Gi
Published: July 2002

Mössbauer spectra of ferrimagnetic Fe7Se8 prepared with 57enriched iron have been taken at 298 K and below 78 K on a “3c” absorber. Three sets of six-line hyperfine patterns were obtained and assigned to three magnetically nonequivalent sites of a superstructure of the crystal. It is shown that the results obtained at the low-temperature region are well consistent with Okazaki’s [A. Okazaki and K. Hirakawa, J. Phys. Soc. Jpn. 11, 930 (1956).] “3c” superstructure model.
Resolution and contrast in near-field photocurrent imaging of defects on semiconductors
Authors: Xu Qin , Gray Matthew H. , Hsu J. W. P.
Published: July 2002

To understand the factors that determine the resolution and contrast in near-field photocurrent (NPC) imaging of electrically active defects on semiconductor surfaces, a two-dimensional carrier diffusion model with a localized generation term is employed. Numerically simulated images of photo-generated carriers are compared with experimental NPC images acquired using a near-field scanning optical microscope (NSOM). Good agreement in image lateral resolution, contrast, and feature sharpness is obtained. It is found that the NPC lateral resolution of dislocation defects is limited not by the minority carrier diffusion length in the defect-free region as previously believed, but rather by either the defect size or the NSOM tip aperture size, whichever is larger. The NPC contrast increases with larger defect size, smaller tip aperture size, longer diffusion length in the defect-free region, and shorter diffusion length inside the defect. By comparing the theoretical results to experimental NPC data, an estimate for the carrier recombination lifetime inside a single defect can also be obtained.
A model for ion-irradiation induced hydrogen loss from organic materials
Authors: de Jong M. P. , Maas A. J. H. , van Ijzendoorn L. J. , Klein S. S. , de Voigt M. J. A.
Published: July 2002

In the study of interfacial diffusion processes in polymer light-emitting diodes, the use of high-energy ion-scattering techniques can be of great value due to the possibility of quantitative elemental depth profiling. However, ion irradiation of polymers is known to cause various degradation effects, including the loss of hydrogen. Since the hydrogen loss determines the accuracy of depth profiling, it is an interesting subject for study in order to define experimental conditions in which the degradation is suppressed. The loss of hydrogen from porphyrins (organic solar cells) has been measured by means of elastic recoil detection analysis with 2, 4, and 7.6 MeV He+ beams. A theoretical model is proposed in which the hydrogen loss is described through the formation and recombination of free hydrogen radicals. A distinct difference is introduced between direct recombination processes and the diffusion of radicals out of the ion track.
A novel diffraction phenomenon with the first order Fredericksz transition in a nematic liquid crystal film
Authors: Chen Shu-Hsia , Fan J. Y. , Wu Jin-Jei
Published: July 2002

We present the experimental results of the Ar+ laser- and electric-field-induced diffraction effect in a planar-parallel-aligned nematic liquid crystal film. It is found that only one set of diffraction rings occurs in the second order Fredericksz transition; however, two sets of diffraction rings can occur in the first order Fredericksz transition. The appearance of two sets of rings distinguishes the first order Fredericksz transition from the second order. We propose a theoretical explanation for the phenomenon.
Anisotropic magnetotransport properties of epitaxial thin films of conductive ferromagnetic oxide SrRuO3
Authors: Rao R. A. , Kacedon D. B. , Eom C. B.
Published: July 2002

We have studied the crystallographic domain structure dependent anisotropy in the magnetotransport properties of epitaxial thin films of the conductive ferromagnetic oxide SrRuO3. Single domain SrRuO3 thin films on miscut (001) SrTiO3 substrates exhibit a strong anisotropy in magnetization and magnetoresistance that reflects the inherent magnetocrystalline anisotropy of the material. In contrast, the SrRuO3 thin film on (001) LaAlO3 substrate shows identical magnetization and magnetotransport properties in two orthogonal directions on the film due to the presence of 90° domains in the plane. For both films, large negative magnetoresistance effects (∼10%) are observed when the applied magnetic field is parallel to the current due to the suppression of spin fluctuations near Tc and the anisotropic magnetoresistance effect at low temperatures.
Magnetic properties of Fe films deposited by Ar, Kr, and Xe ion beam sputtering
Authors: Iwatsubo S. , Takahashi T. , Naoe M.
Published: July 2002

Fe films have been deposited by ion beam sputtering using Ar, Kr, and Xe gases. The sputtering voltage VS was varied in the range of 900–1800 V. The magnetic properties of the films had the significant relationship with the element of the sputtering gas. The saturation magnetization 4πMS was 21 kG for Ar and 20 kG for Kr or Xe. The coercivity HC took the minimum value of 5 Oe at VS of 1200 V for Ar. The energy/the number of the energetic particles, such as the sputtered atoms and the recoiled ions bombarding to the substrate was calculated by Monte-Carlo simulation in the sputtering system with an amorphous Fe target. The average energy of the sputtered atoms ES was 40–60 eV for Ar, 30–34 eV for Kr and 24–34 eV for Xe. The average energy of the recoiled ions ER was 200–370 eV for Ar, 60–100 eV for Kr, and 25–50 eV for Xe. The energy was remarkably different among their sputtering gases with different atomic mass. The bombardment by the recoiled ions at the high energy reduced the coercivity HC of the Fe films.
Photoemission spectroscopy investigation of magnesium–Alq3 interfaces
Authors: Rajagopal A. , Kahn A.
Published: July 2002

The formation of interfaces between magnesium (Mg) and the organic molecular semiconductor tris(8-hydroxy-quinoline) aluminum (Alq3) is studied via ultraviolet photoemission spectroscopy. The morphology and electronic structure of Mg-on-Alq3 and Alq3-on-Mg interfaces are compared. The energy of the Alq3 molecular levels is determined in both cases with respect to a reference Fermi level as a function of interface formation. The difference in injection characteristics between the two interfaces is examined in terms of differences in interface morphology and of electronic gap states induced by the diffusion of Mg at the Mg/Alq3 interface.
Space-charge characteristics in polyethylene
Authors: Wang X. , Yoshimura N. , Murata K. , Tanaka Y. , Takada T.
Published: July 2002

Space-charge distributions in plate and coaxial cable samples of various polyethylene (PE) materials such as high density polyethylene, linear low density polyethylene, low density polyethylene, and cross-linking polyethylene under dc voltage are systematically investigated by the pulsed electro-acoustic method. The chemical structure and morphology of the sample materials are studied. The formation mechanism of space charge and the relations among space charge, chemical structure, and morphology are discussed. Our research results show that the distribution characteristics of space charge in various PE materials can be attributed to their chemical structure and morphology.
Pressure measurement at high temperature using ten Sm:YAG fluorescence peaks
Authors: Zhao Yuechao , Barvosa-Carter William , Theiss Steven D. , Mitha Salman , Aziz Michael J. , Schiferl David
Published: July 2002

A high-temperature pressure calibration technique using Sm-doped Y3Al5O12 (Sm:YAG) crystal as the pressure calibrant has been developed by studying its Y1 through Y10 fluorescence peaks (frequencies from 15 600 to 17 200 cm−1) at pressures (p) from 1 bar to 19 GPa and temperatures (T) from 20 to 850 °C in externally heated diamond anvil cells. The entire spectrum was fit to a sum of ten Lorentzians plus a linear background. The positions, relative intensities and widths were represented by empirical functions of p and T. Several fitting routines for p determination were created based on these dependences, and were tested on various high-p and high-T experimental Sm:YAG fluorescence spectra. The p values obtained from the fitting routines are compared with those obtained from the ruby and the nitrogen (N2) vibron pressure scales. A fitting routine is proposed that can determine p from 20 to 850 °C within an estimated uncertainty of 0.4 GPa.
Low temperature sapphire nitridation: A clue to optimize GaN layers grown by molecular beam epitaxy
Authors: Widmann F. , Feuillet G. , Daudin B. , Rouvière J. L.
Published: July 2002

The sapphire nitridation temperature is investigated as a possible parameter to improve the properties of GaN epilayers grown by molecular beam epitaxy using a radio frequency plasma source. It is found out that lowering the nitridation temperature to values as low as 200 °C allows us to drastically improve the GaN structural and optical properties. Careful examination of the interface by transmission electron microscopy reveals that, in this case, the interface between the nitridated sapphire and the AlN buffer consists of an ordered array of pure edge dislocations. In contrast, high nitridation temperatures result in a perturbed interface with the occurrence of cubic crystallites in the AlN buffer. These results, complemented by a thorough reflection high-energy electron diffraction analysis of the nitridation procedure and a secondary ion mass spectrometry investigation, are interpreted in the framework of a model whereby a higher oxygen concentration is extracted from the substrate at high nitridation temperature, leading to the formation of cubic grains with a smaller lattice parameter than the surrounding matrix and to the concomitant occurrence of defects within the buffer.
Characterization of lead zirconate titanate piezoceramic using high frequency ultrasonic spectroscopy
Authors: Wang Haifeng , Jiang Wenhua , Cao Wenwu
Published: July 2002

Doped piezoceramic lead zircanate titanate has been characterized in the frequency range of 20–60 MHz using ultrasonic spectroscopy. Theoretical analyses were performed for the reflection and refraction of acoustic waves at the interface of water-piezoelectric ceramic. The incident directions of the wave were chosen to be appropriate for ultrasonic spectroscopy measurements. Shear wave spectrum was obtained through mode conversion using a pair of longitudinal transducers submerged in water. The phase velocity shows linear dependence on frequency while the attenuation may be described by a second order polynomial of frequency in the frequency range investigated. The Kramers–Kronig relation between ultrasonic phase velocity and attenuation was compared to measured results.
Low-frequency noise and linearity of a YBa2Cu3O7 dc superconducting quantum interference device magnetometer in static magnetic fields
Authors: Krey S. , Barthelmess H.-J. , Schilling M.
Published: July 2002

We have investigated the low-frequency noise and effective flux sensing area of a field cooled dc superconducting quantum interference device magnetometer from YBa2Cu3O7 in magnetic fields up to 200 μT, both before and after it was patterned with holes to reduce the maximum structural width of the pickup loop. We find that even in low fields the noise of the unpatterned magnetometer steadily increases with the applied field. However, after the patterning for the holes, the noise remains at the zero field level up to a threshold field of 35 μT and is always less than in the unpatterned case. This threshold field is also found in field dependent measurements of the total harmonic distortion. The effective area of the magnetometer depends on the cooling field, and the dependence is different in the unpatterned and the patterned case.
Channel electron mobility dependence on lateral electric field in field-effect transistors
Authors: Hoyniak Dennis , Nowak Edward , Anderson Richard L.
Published: July 2002

A method is described to measure the channel charge density-mobility product in field-effect transistors as a function of lateral channel field. From knowledge of the channel charge density, the channel carrier mobility-lateral field relation is determined. This method was applied to n-channel Si metal–oxide–semiconductor field–effect transistors. The channel charge density was determined at low lateral field as a function of gate-to-channel voltage from capacitance measurements and this value was corrected to account for electron saturation velocity effects at higher fields. The electron mobility-field results were obtained for lateral fields up to 2×105 V/cm, a factor of 5–10 larger than previously obtained, and were fit to the conventional empirical relation for mobility, μ=μlf[1+(μlfξL/vsat)β]−1/β, where μlf is mobility at small lateral fields, vsat is electron saturation velocity, ξL is the magnitude of the lateral channel field, and β is an empirical fitting parameter; a best fit to this relation was obtained for unity β and vsat of 4×106 cm/s.
Resonant Raman scattering from buried AlxGa1−xN (x⩽0.17) layers in (Al, Ga, In)N heterostructures
Authors: Yoshikawa M. , Wagner J. , Obloh H. , Kunzer M. , Maier M.
Published: July 2002

Resonant Raman scattering from AlxGa1−xN (x⩽0.17) layers in (Al, Ga, In)N heterostructures has been studied using optical excitation at a photon energy of hνL=3.72 eV (333.61 nm). Tuning of the AlGaN band-gap energy, and thus of the resonance condition, was achieved by variation of the sample temperature. A pronounced outgoing resonance behavior was observed for first- and second-order Raman scattering by the AlGaN A1(LO) phonon, which allows a separation of the AlGaN LO phonon signal from the corresponding GaN phonon line even in the presence of much thicker surrounding GaN layers. The composition dependence of the AlxGa1−xN A1(LO) phonon mode was determined to ωLO(x)=734+356.8x−814.7x2 (cm−1) for the present excitation conditions (hνL=3.72 eV) and composition range (x⩽0.17). The use of resonantly enhanced Raman scattering by the AlGaN A1(LO) phonon allowed us to assess nondestructively the composition of the AlGaN cladding layers in an (Al, Ga, In)N laser structure, even though the Raman spectrum recorded for nonresonant subband-gap excitation was completely dominated by scattering from the much thicker GaN layers in the structure.
Structure and magnetic properties of SmCo7−xTix with TbCu7-type structure
Authors: Zhou J. , Al-Omari I. A. , Liu J. P. , Sellmyer D. J.
Published: July 2002

The SmCo7−xTix, x=0–0.56 bulk samples are prepared by arc melting. X-ray diffraction indicates that samples with 0.2<x<0.4 form a single disordered TbCu7-type structure phase and other minor phases appear for other values of x, which indicates that Ti helps stabilize the 1-7 phase. The lattice parameters ratio (c/a) increases with increasing Ti concentration. Room temperature saturation magnetization and Curie temperature decrease with increasing x. X-ray diffraction and magnetization measurements on aligned samples show that all samples studied have uniaxial anisotropy. The anisotropy field is found to increase with increasing x reaching a maximum of 175 kOe at x=0.28 and then decreases for higher values of x. This anisotropy field is 20% higher than that of the same compound with Th2Zn17-type structure.
Magnetic materials and structures for thin-film recording media (invited)
Authors: Johnson Kenneth E.
Published: July 2002

There have been three distinct stages in the development of magnetic materials and processes for thin-film media. The first era focused on designs for use with inductive heads. Vacuum sputter deposition techniques were developed to achieve high Hc, with high Ms materials epitaxially grown on Cr underlayers. The second phase of thin-film media development demanded media optimized for the use with MR heads. Noise reduction through the decoupling of grains by chemical segregation of Cr at the grain boundaries proved effective. The introduction of giant magnetoresistance heads required low noise and high Hc but added the necessity of the reduction in the average grain size and grain size distribution width. The next and perhaps final phase of thin-film media development will be focused on the minimization of the magnetization thermal decay by the use of materials and designs that produce high KuV/kBT ratios.
Device performance and polymer morphology in polymer light emitting diodes: The control of device electrical properties and metal/polymer contact
Authors: Liu Jie , Shi Yijian , Ma Liping , Yang Yang
Published: July 2002

We recently demonstrated that the morphology of a spin-cast polymer film can be controlled by the proper choice of solvent, polymer solution concentration, and spin speed [Y. Shi, J. Liu, and Y. Yang, J. Appl. Phys. 87, 4254 (2000)]. In this article, we report the correlation between the morphology and the electrical properties of poly(2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene based light emitting diodes. The performance of these devices, such as device turn-on voltage, current injection efficiency, and quantum efficiency, shows a strong dependence on thin film processing conditions. It is found that aromatic solvents and nonaromatic solvents have very different effects on device performance, which suggests that the solvation induced morphology effect is one of the controlling factors for polymer-based optoelectronic devices.
Fluorescence quenching in molecules near rough metal surfaces
Authors: Pineda Andrew C. , Ronis David
Published: July 2002

A classical electrodynamic near field calculation for the interaction of a Drude oscillator molecular dipole with a rough metal surface is presented. Effects of electromagnetic coupling between surface bumps (assumed to be prolate hemispheroids) and the bulk metal are included. Fluorescence lifetimes are found to be two to four orders of magnitude smaller than those predicted for a flat plane in calculations on silver, and for sufficiently short distances, an avoided crossing splitting is predicted.
Dynamical effects in electron transfer reactions
Authors: Sumi H. , Marcus R. A.
Published: July 2002

A theoretical treatment is given for the effect of intramolecular vibrational and diffusive solvent orientational motions on the rate of electron transfer reactions. Four limiting cases are considered for the two-electronic state problem: slow reaction, wide and narrow reaction window, and nondiffusing limits. With the aid of a decoupling approximation, an expression is derived for the reaction rate which reduces to the appropriate expression for each limiting case when the latter is approached. Under certain conditions the time dependence of the survival probability is multiexponential rather than single exponential. Because of this behavior two average survival times are defined and expressions for each are obtained. Experimental data are considered with the present treatment in mind. One feature of the present work is a more general analysis for the case that both vibrational and solvent diffusive motion contribute to the activation process. The relation to previous works in the literature is described.
Indole’s solvatochromism revisited
Authors: Lami Hans , Glasser Nicole
Published: July 2002

The absorption and emission solvatochromism of five indole derivatives was studied by referring the shifts to the corresponding vapor-phase wave numbers. The emission results clearly demonstrate, in three cases, the change in the nature of the emitting state when the solvent polarity is increased. In absorption, a large red shift in the 1La band is observed in all polar solvents, and this cannot be explained by classical solvent–solute interaction forces. The temperature dependence of the emission shift in a viscous solvent (glycerol) confirms that most of the emission shift is due to solvent relaxation. The 1La–1Lb level inversion model applied to ground-state polar solvent–solute complex, can account for most of the solvatochromism features observed.
On standardization and algebraic symmetry of the ligand field Hamiltonian for rare earth ions at monoclinic symmetry sites
Authors: Rudowicz Czesl/aw
Published: July 2002

The standardization idea previously applied for an orthorhombic Hamiltonian is now extended to the monoclinic crystal-field Hamiltonian (HCF). Relations are given which allow for transformations of the three admissible forms of monoclinic HCF between themselves to achieve the ratio of B22/B02 confined to the standard range (0,±1). Thereby correlations between the apparently different crystal-field parameter (CFP) sets of various authors can be established. Algebraic symmetry of the characteristic equation of HCF for monoclinic C2, C1h(Cs), and C2h symmetry is considered by the method proposed earlier using altran. Magnitudes, signs, and/or relative signs of the CF parameters which can be determined from fitting the optical spectra of fn ions at sites of the above symmetry are established for the reduced approach (with one [B−qk]=0). Some misinterpretations on the complete approach, which yields ambiguous and unreliable results in general, as well as on the approximated approach are clarified. An extensive survey has revealed several cases of usage of the inappropriate complete approach. Based on the theoretical considerations a multipole correlated fitting technique has been proposed. The results of this paper are expected to greatly facilitate fitting of the optical spectra of fn ions as well as dn ions at sites of low symmetry.
Surface potential of the water liquid–vapor interface
Authors: Wilson Michael A. , Pohorille Andrew , Pratt Lawrence R.
Published: July 2002

The surface potential of the water liquid–vapor interface is studied by molecular dynamics using the TIP4P model. The surface potential predicted by this empirical model is −(130±50)mV. This value for the surface potential is of reasonable magnitude but of opposite sign to the expectations derived from laboratory experiments. The electrostatic potential displays a nonmonotonic variation with depth into the liquid. This nonmonotonic variation is explained on the basis of the nondipolar charge distribution of the H2O molecule and the observation that the more probable molecular orientations in the interfacial region place the molecular symmetry axis near the plane of the interface. It is shown that minor changes in the assumed molecular charge distribution can bring the computed surface potential into agreement with experimental expectations without qualitatively altering the nonmonotonic variation of the electrostatic potential through the interfacial region. Computed quantum mechanical descriptions of the electron distribution of the isolated H2O molecule are not compatible with the surface structure predicted by the TIP4P model and the experimental expectation that the surface potential of the water liquid–vapor interface is small, roughly of the order of 10–100 mV. The surface potential is sensitive to details in the large distance wings of the molecular electron distribution. It is hypothesized that the surface environment qualitatively alters the wings of the distribution from the result obtained by a superposition of the isolated molecule electron densities.
Formation of small negative and positive cluster ions of gold, silver, and copper by direct laser vaporization
Authors: Moini Mehdi , Eyler John R.
Published: July 2002

Direct laser vaporization techniques have been employed to produce small cluster cations and anions of gold, silver, and copper in a Fourier transform ion cylotron resonance mass spectrometer using pure metal samples. Aupn (p=+1 and −1, n=1–6), Ag+n (n=1–3), Ag−, Cu+n (n=1–3), and Cu−n (n=1–2) were observed, using two different experimental configurations. In one configuration ions were produced by Nd:YAG laser irradiation directed collinear with the magnetic field in the FTICR mass spectrometer while in the second laser light directed perpendicularly to the magnetic field was used. Relative kinetic energies of the aggregates, the relative intensities of positive vs negative ions, and the nature of the neutrals in the plasma formed by direct laser vaporization are discussed.
Ionic dynamics in computer simulated molten LiNO3. I. Translational and reorientational motion
Authors: Katō Toshiko , Machida Katsunosuke , Oobatake Motohisa , Hayashi Soichi
Published: July 2002

In order to study the microdynamics of translational and orientational motion of ions in liquid electrolytes, a molecular dynamics simulation of molten LiNO3, which includes all degrees of freedom of vibrating nitrate ions, has been performed. Simple Coulomb pair potentials with Born-type repulsions have been adopted in the simulation. The calculated properties include: translational velocity and force correlation functions, rotational correlation functions, and angular momentum and torque correlation functions. The nature of the ionic motion is analyzed in terms of quasioscillation and quasilibration. Local structures surrounding nitrate ions, which can be characterized by, e.g., the coordination number, are found to persist for a long time (∼1 ps). The coupling between the environmental structure and the translational and orientational behaviors of a given nitrate ion has been examined through selectively sampled correlation functions. We found that the change of coordination number enhances the translational diffusion and the rotational diffusion is slow if the coordination number remains low. This shows that the dynamics are very sensitive to the exact shape of the potential and the dynamics itself. Results of the simulation are compared with the experimental diffusion constants and dynamical behaviors revealed by Raman spectroscopy. While the interionic interaction assumed has been found to describe quite well the static structure of molten LiNO3, simulated translational diffusion and rotational relaxation seem to be somewhat too fast, suggesting too shallow an attractive well in the potential we have assumed.
Ionic hydration in sodium chloride solutions
Authors: Onori G.
Published: July 2002

The adiabatic compressibility of aqueous sodium chloride solutions has been determined as a function of both concentration and temperature. A simple model has been employed to interpret the data in terms of size and compressibility of hydration shells. On the basis of the analysis performed it has been possible to estimate the values of the volume contraction caused by ion–water interactions, the intrinsic volume of ions, the hydration numbers, the compressibility and density of water hydration shells, and their variation with temperature and salt concentration.
Neutral and ionic clusters of antimony and bismuth: A comparison of magic numbers
Authors: Geusic M. E. , Freeman R. R. , Duncan M. A.
Published: July 2002

Neutral and singly charged anion or cation clusters of antimony and bismuth are produced by laser vaporization in a pulsed nozzle cluster source. Neutral clusters are photoionized near threshold with a fluorine excimer laser, while ionized clusters are sampled directly from the laser vaporization plasma. All species are mass analyzed with a time-of-flight spectrometer. Abundance patterns in these systems exhibit ‘‘magic numbers’’ that are strikingly charge dependent. Compared at the same charge, corresponding neutral and cation spectra of these metals are quite similar, but significant differences arise in the two anion cluster distributions. Neutral cluster growth in these systems ends abruptly at five atoms while the ionized distributions fall off gradually out to beyond 12–15 atoms. These unique patterns are examined in the context of cluster growth dynamics and suspected cluster stability based on the predictions of simple electron counting models.
Collision induced dissociative chemisorption of CH4 on Ni(111) by inert gas atoms: The mechanism for chemistry with a hammer
Authors: Beckerle J. D. , Johnson A. D. , Yang Q. Y. , Ceyer S. T.
Published: July 2002

The dissociation of CH4 physisorbed on Ni(111) at 46 K is observed to be induced by the impact of incident inert gas atoms. The dynamics and mechanism of this new process, collision induced dissociative chemisorption, are studied by molecular beam techniques coupled with ultrahigh vacuum electron spectroscopies. The absolute cross section for collision induced dissociation is measured over a wide range of kinetic energies (28–109 kcal/mol) and incident angles of Ne, Ar, and Kr atom beams. The cross section displays a complex dependence on the energy of the impinging inert gas atom characteristic of neither total nor normal energy scaling. Quantitative reproduction of the complex dependence of the cross section on the Ar and Ne incident energy by a two-step, dynamical model establishes the mechanism for collision induced dissociation. Collision induced dissociation occurs by the impulsive transfer of kinetic energy upon collision of Ar or Ne with CH4, followed by the translationally activated dissociative chemisorption of the CH4 upon its subsequent collision with the Ni surface. The dependence of the probability of activated dissociation on the resultant CH4 normal energy derived from the fit of the model to the experimental cross section is in excellent agreement with the results of a previous study of the translationally activated dissociative chemisorption of CH4 on Ni(111). Collision induced activation and translational activation are shown to be consistent mechanisms for providing energy to CH4 to surmount the barrier to dissociative chemisorption.