bluuebunny/crossref_metadata_2025 huggingface dataset:
On optimal contact cooling of high-heat-load x-ray mirrors
Authors: Khounsary A. M. , Yun W.
Published: July 2002

Under appropriate conditions, optimal contact cooling provides an attractive cooling strategy for the design of high-heat-load mirrors. This approach avoids a number of problems and uncertainties inherent in the fabrication, assembly, and operation of internally cooled high-heat-load optics. In this paper, the optimal contact-cooling concept is described, its advantages, disadvantages, and limitations are pointed out, and various design aspects and options are discussed. Simple heuristic guidelines for the design of such substrates are provided. The mirror assembly consists of the polished substrate and two cooling blocks in contact with it. The mirror and cooling block dimensions as well as the location of the cooling blocks on the mirror, are optimized to provide a thermo-mechanically balanced system so that there are negligible tangential slope errors in the mirror without any bending mechanism. As an example, the design of a high-heat-load first mirror for the Advanced Photon Source (APS) is described. This mirror, which is 1.2 m long, is exposed to a total power of 2 kW with a uniform heat flux of 0.38W/mm2. This mirror is currently being fabricated. The concept of a thermo-mechanically balanced system can also be applicable to other high-heat-load optical components, such as monochromators.
The de-agglomeration and dispersion of small dust particles—Principles and applications
Authors: Blum Jürgen , Schnaiter Martin , Wurm Gerhard , Rott Martin
Published: July 2002

A mechanism for the generation of dust dispersions consisting of single micrometer-sized particles in a low pressure gas is discussed. The particles are de-agglomerated and dispersed by injecting a dust sample (powder) together with a small amount of an arbitrary type of gas into a modified turbomolecular pump. The discussed sample release mechanism/dust disperser offers a wide range of possible gas and particle parameters, suitable for different kinds of experiments. We present two applications of our de-agglomeration mechanism. In the first experiment, homogeneous dust clouds were produced under microgravity conditions, and the Brownian motion of the single dust grains was studied. The second application is a matrix isolation technique for the spectroscopical investigation of single dust particles.
An ultrahigh vacuum high speed scanning tunneling microscope
Authors: Curtis Robert , Mitsui Toshiyuki , Ganz Eric
Published: July 2002

We have developed an ultrahigh vacuum high speed scanning tunneling microscope (STM) which is based on a prototype STM that operates in air. The prototype operates with a tip scan speed up to 105 nm/s. At this high tip speed, we have made real time videos of nickel and gold at rates up to 15 Hz. The ultrahigh vacuum version operates at speeds up to 8000 nm/s with atomic resolution on the Si(111) surface and incorporates sample transfer and sample heating.
A counter-rotating Couette apparatus to study deformation of a sub-millimeter sized particle in shear flow
Authors: de Haas K. H. , van den Ende D. , Blom C. , Altena E. G. , Beukema G. J. , Mellema J.
Published: July 2002

We describe a new counter-rotating Couette apparatus that has been developed for deformation studies of single sub-millimeter sized particles in shear flow. New features are the adaption to the low viscosities of water-based systems and temperature control of the device. The inner to outer radius ratio of the cylinders used is 0.9785 and the height to width aspect ratio of the gap is 4.0, while the inner radius is 100 mm. Because of the limited particle size a high mechanical accuracy of the Couette geometry is necessary. The swing of the inner cylinder is less than 2 μm and that of the outer cylinder less than 4 μm. We achieved this by carefully choosing the design parameters of the aerostatic bearing and the coupling between cylinder and motor unit. Furthermore, special drive units give a shear rate resolution of 0.018 s−1, while the maximum shear rate is 100 s−1. For a liquid viscosity on the order of 1 mPas the effective maximum shear rate is 30 s−1. We have shown that deformations as small as (L−B)/(L+B)≈0.01 of giant bilayer vesicles (typical radius 10 μm) with length L and width B can be observed with our device.
New high temperature furnace for structure refinement by powder diffraction in controlled atmospheres using synchrotron radiation
Authors: Margulies L. , Kramer M. J. , McCallum R. W. , Kycia S. , Haeffner D. R. , Lang J. C. , Goldman A. I.
Published: July 2002

A low thermal gradient furnace design is described which utilizes Debye–Scherrer geometry for performing high temperature x-ray powder diffraction with synchrotron radiation at medium and high energies (35–100 keV). The furnace has a maximum operating temperature of 1800 K with a variety of atmospheres including oxidizing, inert, and reducing. The capability for sample rotation, to ensure powder averaging, has been built into the design without compromising thermal stability or atmosphere control. The ability to perform high-resolution Rietveld refinement on data obtained at high temperatures has been demonstrated, and data collected on standard Al2O3 powder is presented. Time-resolved data on the orthorhombic to rhombohedral solid state phase transformation of SrCO3 is demonstrated using image plates. Rietveld refinable spectra, collected in as little as 8 s, opens the possibility of performing time-resolved structural refinements of phase transformations.
A one-dimensional axial electron cyclotron resonance source model
Authors: Edgell D. H. , Kim J. S. , Wong S. K. , Pardo R. C. , Vondrasek R.
Published: July 2002

A conventional zero-dimensional (uniform plasma parameters with no spatial variations) fluid model will provide a good match with an experimental electron cyclotron resonance ion source (ECRIS) charge-state distribution (CSD) if provided with a judicious set of user inputs. However, this arbitrarily chosen set of inputs is not necessarily unique. To be truly predictive, an ECRIS model should rely on experimental parameters as inputs. A multi-species model for an ECRIS plasma using experimental parameters as inputs is under development. The model eliminates electron temperature as a user input by employing a 2 V(v,θ) Fokker–Planck code with an ECR heating term to calculate the non-Maxwellian anisotropic electron distribution function. Further arbitrary user inputs are eliminated in favor of controlled parameters by bounce averaging the Fokker–Planck coefficients for a one-dimensional (1D)/2 V axial model. The neutral gas modeling has been extended to 1D using axially coupled particle balance equations. The improved model is able to reproduce experimental Faraday cup (CSDA) from the Argonne National Laboratory’s ECR-II. Further elimination of arbitrary inputs is expected when the ion model is extended to 1D.
High resolution infrared direct absorption spectroscopy of ionic complexes
Authors: Linnartz Harold , Verdes Dorinel , Speck Thomas
Published: July 2002

A sensitive and generally applicable technique for high resolution infrared spectroscopy of weakly bound ionic complexes is presented. The method is based on the direct absorption of tunable diode laser radiation in an expansion cooled planar plasma. The plasma is generated by electron impact ionization of gas that is expanded supersonically through a long and narrow slit. This technique allows a fast and effective production modulation. Online monitoring by a quadrupole mass spectrometer yields direct information on the plasma characteristics. Rotational temperatures as low as 15 K and densities of the order of 109–1010 complexes/cm3 are routinely obtained. The technique is discussed on the example of N4+ and demonstrated with results on the proton bound complexes N2–H+–N2, Ar–HN2+, and Ar–HCO+.
Heterojunctions of solid C70 and crystalline silicon: Rectifying properties and barrier heights
Authors: Chen K. M. , Wu K. , Chen Y. , Jia Y. Q. , Jin S. X. , Li C. Y. , Gu Z. N. , Zhou X. H.
Published: July 2002

Heterojunctions of solid C70 and n- or p-type crystalline Si have been made. Current–voltage measurements show that both C70/n-Si and C70/p-Si contacts are rectifying but their directions of rectification are opposite to each other. Thermal activation measurements at a fixed forward bias show an exponential dependence of current on the reciprocal of temperature, from which we determine the effective barrier height as 0.23 eV for C70/n-Si and 0.27 eV for C70/p-Si. Relative dielectric constant of solid C70 was determined to be 4.96 through the study of high-frequency capacitance–voltage characteristics for Ti/C70/p-Si structures.
An annealing study of luminescent amorphous silicon-rich silicon oxynitride thin films
Authors: Augustine B. H. , Hu Y. Z. , Irene E. A. , McNeil L. E.
Published: July 2002

We report the effect of the rapid thermal annealing (RTA) of amorphous silicon oxynitride (a-SiOxNy:H) thin films on the room-temperature photoluminescence (PL). Samples were prepared by plasma enhanced chemical vapor deposition. The PL intensity increased by one order-of-magnitude after annealing for 20 min at 800 °C compared to that of the as-deposited sample. We have followed the changes in the chemical microstructure of the materials by in situ real-time ellipsometry, and suggest that the annealing is characterized by two distinct mechanisms. The first is that hydrogen effusion from clustered hydrogen and/or =N–H bond breaking occurs in the first 5 min resulting in increasing dangling bond density and no increase in the PL intensity. The second occurs after longer annealing times due to local reconstruction of Si–O and Si–N bonds, rather than diffusional rearrangement of the atomic species. This results in a more dense material with a higher refractive index, fewer nonradiative recombination centers, and more efficient PL. A model is presented to describe both the radiative and nonradiative recombination processes.
The size dependence of the resonant Kerr nonlinearity of Cd(S,Se)-doped glasses revisited
Authors: Schanne-Klein M. C. , Piveteau L. , Ghanassi M. , Ricard D.
Published: July 2002

Recently, the mechanisms leading to the resonant Kerr nonlinearity of Cd(S,Se)-doped glasses could clearly be assessed. On this basis, we reconsider experimentally the size dependence of the fast component of this nonlinearity. We observe that the figure of merit corresponding to this fast component decreases when the particle size decreases. The various results that had been previously reported are discussed in light of this work.
A voltage-controlled tunable two-color infrared photodetector using GaAs/AlAs/GaAlAs and GaAs/GaAlAs stacked multiquantum wells
Authors: Zhang Yaohui , Jiang D. S. , Xia J. B. , Cui L. Q. , Song C. Y. , Zhou Z. Q. , Ge W. K.
Published: July 2002

A voltage-controlled tunable two-color infrared detector with photovoltaic (PV) and photoconductive (PC) dual-mode operation at 3–5 μm and 8–14 μm using GaAs/AlAs/AlGaAs double barrier quantum wells (DBQWs) and bound-to-continuum GaAs/AlGaAs quantum wells is demonstrated. The photoresponse peak of the photovoltaic GaAs/AlAs/GaAlAs DBQWs is at 5.3 μm, and that of the photoconductive GaAs/GaAlAs quantum wells is at 9.0 μm. When the two-color detector is under a zero bias, the spectral response at 5.3 μm is close to saturate and the peak detectivity at 80 K can reach 1.0×1011 cmHz1/2/W, while the spectral photoresponsivity at 9.0 μm is absolutely zero completely. When the external voltage of the two-color detector is changed to 2.0 V, the spectral photoresponsivity at 5.3 μm becomes zero while the spectral photoresponsivity at 9.0 μm increases comparable to that at 5.3 μm under zero bias, and the peak detectivity (9.0 μm) at 80 K can reach 1.5×1010 cmHz1/2/W. Strictly speaking, this is a real bias-controlled tunable two-color infrared photodetector. We have proposed a model based on the PV and PC dual-mode operation of stacked two-color QWIPs and the effects of tunneling resonance with narrow energy width of photoexcited electrons in DBQWs, which can explain qualitatively the voltage-controlled tunable behavior of the photoresponse of the two-color infrared photodetector.
Chemical vapor deposition of copper with a new metalorganic source
Authors: Choi E. S. , Park S. K. , Shin H. K. , Lee Hong H.
Published: July 2002

Copper deposition based on a newly synthesized metalorganic compound shows promise for reproducible deposition which has been a major problem. The deposition can be carried out without adverse effect on the film qualities at the bubbler temperature of 65 °C. Preliminary deposition results reveal that the resistivity is 2.5 μΩ cm in the deposition temperature range between 175 and 200 °C. Unlike (hfac)Cu⋅VTMS, the resistivity increases with increasing deposition temperature due to loose packing of the grains at higher temperatures.
Growth defects in GaN films on 6H–SiC substrates
Authors: Chien F. R. , Ning X. J. , Stemmer S. , Pirouz P. , Bremser M. D. , Davis R. F.
Published: July 2002

Lattice defects in GaN epilayers grown on 6H–SiC(0001) by metalorganic vapor phase epitaxy (MOVPE) have been characterized by transmission electron microscopy (TEM). The predominant defects in the film were threading dislocations and half-loops with Burgers vectors 1/3〈112̄0〉, [0001], and 1/3〈112̄3〉. Planar faults were also observed in the film that are thought to be inversion domain boundaries (IDBs).
Feedback positioning cantilever using lead zirconate titanate thin film for force microscopy observation of micropattern
Authors: Fujii Toru , Watanabe Shunji
Published: July 2002

An atomic force microscope cantilever with PZT thin film with pyramidal stylus was used for actuation of feedback motion. The maximum stroke of the cantilever was more than 1 μm within a frequency range from direct current up to natural resonant frequency, e.g., 28.48 kHz. An image of a pit of a compact disk with a depth of 100 nm was successfully obtained using only cantilever actuation in a vertical direction.
The role of point defects and arsenic precipitates in carrier trapping and recombination in low-temperature grown GaAs
Authors: Lochtefeld A. J. , Melloch M. R. , Chang J. C. P. , Harmon E. S.
Published: July 2002

GaAs epilayers were grown with a wide range of excess arsenic concentrations and subjected to various anneals to study the role of the point defects and arsenic precipitates in carrier trapping and recombination. Prior to anneal, the point defects rapidly trap photogenerated electrons and holes—usually on subpicosecond time scales. However, full electron-hole recombination occurs on a significantly longer time scale. After anneal, the full electron-hole recombination lifetime appears to be greatly reduced, indicating that the arsenic precipitates play a significant role.
Magneto-optic observation of anomalous Meissner current flow in superconducting thin films with slits
Authors: Baziljevich M. , Johansen T. H. , Bratsberg H. , Shen Y. , Vase P.
Published: July 2002

Slits patterned into a YBa2Cu3O7−δ thin film were observed to obstruct Meissner sheet currents leading to an imbalance in the local Meissner screening properties. The new phenomenon was studied with magneto-optic imaging where twin lobes of opposite flux polarity were seen to form near the slits and inside the Meissner region. The lobe closest to the sample edge is always polarized opposite to the applied field. At weak fields, the anomalous flux generation is reversible. At higher fields, but still sufficiently small to keep the vortex penetration front away from the slits, the anomalous current starts nucleating flux lines which become trapped when the field is removed.
Piezoelectric properties of Ca-modified PbTiO3 thin films
Authors: Kholkin A. L. , Calzada M. L. , Ramos P. , Mendiola J. , Setter N.
Published: July 2002

Low-field piezoelectric coefficient and strain were investigated in Ca-modified PbTiO3 thin films by means of optical interferometry. A remanent piezoelectric coefficient of 70 pm/V was observed that agrees well with the values previously reported for the bulk ceramics of the same composition. The efficient poling of the films can be achieved at room temperature under the field of 300 kV/cm. Electrically induced strains as high as 0.8% were observed using a bipolar driving field of 1 MV/cm. Strain response under a unipolar driving field exhibited a good linearity and small hysteresis. These properties combined with the low dielectric constant and high dielectric strength are shown to be attractive for micromechanical applications.
Photoreversible optical nonlinearities of polymeric films containing spiropyran with long alkyl chains
Authors: Tamaoki Nobuyuki , Van Keuren Edward , Matsuda Hiro , Hasegawa Kan , Yamaoka Tsuguo
Published: July 2002

It was demonstrated that the third-order optical nonlinearity of polymer films could be reversibly controlled by the photochromic conversion and following J-aggregate formation of spiropyran embedded in an inert polymer matrix. Patterns consisting of areas with high and low third-order nonlinear optical activities were prepared by imagewise exposure of the photochromic films followed by thermal treatment to form thermally stable J aggregates of photoinduced merocyanine in the exposed area. This system could be useful in developing new unconventional optical devices based on optical nonlinearities, including new types of spatial light modulators.
Periodic electric field poling of KTiOPO4 using chemical patterning
Authors: Risk W. P. , Lau S. D.
Published: July 2002

We report a technique for periodic poling of KTiOPO4 (KTP). In this approach, we exploit the observation that Rb-exchanged regions of KTP resist electric field poling under conditions that produce domain inversion in unexchanged KTP. Thus, a pattern of Rb-exchanged regions can be transformed into a corresponding pattern of domain inverted regions by application of a uniform electric field. We applied this technique to in situ poling of segmented waveguides and report quasiphase matched second-harmonic generation in these devices.
Porous silicon structure studied by nuclear magnetic resonance
Authors: Petit D. , Chazalviel J.-N. , Ozanam F. , Devreux F.
Published: July 2002

We present a nuclear-magnetic-resonance (NMR) study of the structure of porous silicon. Cross-polarization–magic-angle-spinning Si29 NMR shows that the silicon crystalline structure is preserved in porous silicon and that there are three protonated silicon species at the surface, which are tentatively identified as belonging to different faces. The occurrence of different environments for the surface protons is supported by the heterogeneous H1 NMR line. HF and SiF62− species were detected by F19 NMR in the electrolyte left in the pores.
Temperature dependent, non-ohmic magnetoresistance in doped perovskite manganate trilayer junctions
Authors: Sun J. Z. , Krusin-Elbaum L. , Duncombe P. R. , Gupta A. , Laibowitz R. B.
Published: July 2002

We report spin-dependent perpendicular transport in the magnetic trilayer junction structure La0.67Sr0.33MnO3/SrTiO3/La0.67Sr0.33MnO3. Large (factor of 5) changes of magnetoresistance induced by a field of ∼200 Oe are observed at 4.2 K. Junction I–V characteristics at low temperatures are consistent with a metal–insulator–metal tunneling process with a large spin-polarization factor of 0.81 for the conduction electrons. Above 100 K, a variable range-hopping conduction shunts out the magnetoresistance contribution. This second conduction channel comes from the impurity states within SrTiO3 barrier and therefore is not an intrinsic limit to the magnetoresistance performance of the device at high temperatures.
High resolution transmission electron microscopy study on the microstructures of aluminum nitride and hydrogenated aluminum nitride films prepared by radio frequency reactive sputtering
Authors: Yong Yoon-Joong , Lee Jai-Young , Kim Hong Seung , Lee Jeong Yong
Published: July 2002

Aluminum nitride (AlN) and hydrogenated aluminum nitride (AlN:H) films on Si substrate have been deposited by the radio frequency reactive magnetron sputtering. The microstructures of the two films have been examined and compared by transmission electron microscopy (TEM) and high-resolution TEM. In the growth of the AlN:H film, it has been observed that the amorphous phase is formed at the initial stage of deposition, and c-axis oriented crystallite nucleates at the amorphous layer. The lattice mismatch between the film and substrate and the stress in the film are reduced, and the film surface is smooth due to this amorphous phase. A schematic model explaining the growth of AlN and AlN:H films is proposed, and the reason for the easy formation of the amorphous phase in AlN:H film is discussed.
Dielectric relaxation of Ba0.7Sr0.3TiO3 thin films from 1 mHz to 20 GHz
Authors: Baniecki J. D. , Laibowitz R. B. , Shaw T. M. , Duncombe P. R. , Neumayer D. A. , Kotecki D. E. , Shen H. , Ma Q. Y.
Published: July 2002

The dielectric relaxation of Ba0.7Sr0.3TiO3 thin films was investigated up to K band (20 GHz) using time domain and frequency domain measurements. Our results show that from 1 mHz to 20 GHz, the dielectric relaxation of the complex capacitance of Ba0.7Sr0.3TiO3 thin films can be understood in terms of a power law dependence known as the Curie–von Schweidler law. The small dispersion (less than 7% decrease in capacitance from 1 mHz to 20 GHz) and low loss (loss angle less than 0.006 at 20 GHz) measured in Ba0.7Sr0.3TiO3 thin films indicate that these films are applicable to device application up to at least K band.
Near-field optical study of InGaN/GaN epitaxial layers and quantum wells
Authors: Vertikov A. , Kuball M. , Nurmikko A. V. , Chen Y. , Wang S.-Y.
Published: July 2002

We have employed near-field scanning optical microscopy to investigate the influence of specific microstructural defects on the optical properties of thin InGaN/GaN epilayers and quantum wells. These defects are empty “pinholes” with a hexahedron cone morphology that are nucleated by threading dislocations from the GaN buffer layer. By correlating atomic force microscopy with spatially and spectrally resolved photoluminescence (PL) on a 100 nm spatial scale, we find that the pinholes have no clearly observable effect on the PL efficiency, at least partly due to the strong carrier localization in the InGaN nonrandom alloy.
Photorefractive Bragg diffraction in high- and low-molar-mass liquid crystal mixtures
Authors: Ono Hiroshi , Saito Isao , Kawatsuki Nobuhiro
Published: July 2002

Orientational photorefractive Bragg diffraction is observed in high- and low-molar-mass liquid crystal mixtures doped with fullerene (C60). These novel materials are in a nematic phase without phase separating. In the functionalized materials, we observe a high two-beam coupling gain coefficient (Γ=75 cm−1) with a low applied field of 4 V/μm, low total losses (32 cm−1 including scattering, reflection, and absorption), a high diffraction efficiency (9%), and response time of 200 ms.
Frequency control in laser ultrasound with computer generated holography
Authors: Clark M. , Linnane F. , Sharples S. D. , Somekh M. G.
Published: July 2002

In laser ultrasonics a laser is used to excite ultrasonic waves. The intensity profile of the laser on the sample can be used to control the frequency of the ultrasound generated. In this letter we show how the frequency content of Rayleigh (surface acoustic) waves generated with an 82 MHz mode-locked laser can be controlled using computer generated holograms (CGHs). To demonstrate the effectiveness of the frequency control the CGHs used were defocused to generate new illumination profiles. The agreement between the actual and predicted amplitudes for these profiles is striking. Using this technique, the intensity output from the CGHs may be considered as a tunable Rayleigh wave source.
Carrier relaxation and recombination in an InGaN/GaN quantum well probed with time-resolved cathodoluminescence
Authors: Zhang X. , Rich D. H. , Kobayashi J. T. , Kobayashi N. P. , Dapkus P. D.
Published: July 2002

Spatially, spectrally, and temporally resolved cathodoluminescence (CL) techniques have been employed to examine the optical properties and kinetics of carrier relaxation for metalorganic chemical vapor deposition grown InGaN/GaN single quantum wells (QWs). Cathodoluminescence wavelength imaging of the QW sample revealed local band gap variations, indicating the presence of local In composition fluctuations and segregation during growth. A detailed time-resolved CL study shows that carriers generated in the boundary regions will diffuse toward and recombine at InN-rich centers, resulting in a strong lateral excitonic localization prior to radiative recombination.
Optical spectroscopy of Si-related donor and acceptor levels in Si-doped GaN grown by hydride vapor phase epitaxy
Authors: Jayapalan J. , Skromme B. J. , Vaudo R. P. , Phanse V. M.
Published: July 2002

The optical properties of n-type GaN grown by hydride vapor phase epitaxy, with intentional Si doping levels ranging from nominally undoped to ND−NA=4×1017 cm−3, are investigated using low temperature photoluminescence. We identify free and neutral donor-bound exciton transitions and two-electron satellites (TES) at 1.7 K. The energy difference between the principal neutral donor-bound exciton peak and its TES yields a Si donor binding energy of 22 meV. The intensity of the Si-related TES increases with increasing Si concentration. The Si donor is much shallower than the two residual donors, which have binding energies of 28 and 34 meV. This result suggests that the main residual donors in this material (and possibly in many layers grown by metal organic chemical vapor deposition and metal organic molecular beam epitaxy as well) are not Si. Silicon doping also introduces an acceptor level with a binding energy of about 224 meV.
Evidence of electron confinement in the single-domain (4×1)-In superstructure on vicinal Si(111)
Authors: Pedreschi F. , O’Mahony J. D. , Weightman P. , Power J. R.
Published: July 2002

Reflectance anisotropy spectroscopy is applied to submonolayer growth of In on the vicinal silicon (111) surface. Deposition in the region of 1 monolayer onto a clean stepped Si(111) surface at elevated temperature produces a single-domain In-induced (4×1) superstructure consisting of quasi-one-dimensional chains aligned parallel to the vicinal surface step edges. A significant optical anisotropy (1.65%), uncharacteristic of semiconductor systems, develops in the region of 1.9 eV which saturates upon completion of the (4×1) superstructure. We relate this feature to an optical transition involving a flat, highly populated filled surface state observed previously. We argue that the intensity and direction of this peak are indicative of electronic confinement within this system perpendicular to the In-induced chain length.
Site occupancy of Zr in Nd(FeZr)B magnet during hydrogenation disproportionation desorption recombination process
Authors: Ashfaq A. , Matsuura M. , Sakurai M.
Published: July 2002

In order to elucidate the role of Zr addition for the evolution of magnetic anisotropy in hydrogenation disproportionation desorption recombination treated NdFeB magnets, Zr site occupancy in Nd16Fe75.5Zr0.5B8 alloy has been investigated by means of x-ray absorption fine structure. Zr is found to occupy preferably the Fe(j2) sites in the Nd2Fe14B structure for the as-cast state and keeps this position in the disproportionated stage. Zr(j2) sites are situated at the position connecting two Fe-hexagonal nets where strong Fe–Zr bondings stabilize Nd2Fe14B structure against disproportionation.
Influence of GaAs (001) surface termination on the in-plane magnetic anisotropies of MnSb epitaxial films
Authors: Akinaga H. , Miyanishi S. , Van Roy W. , De Boeck J. , Borghs G.
Published: July 2002

We have studied the in-plane magnetic anisotropy of epitaxial MnSb (11̄01) films grown on GaAs (001) by molecular beam epitaxy. The MnSb films were grown on (2×4) and (4×6) reconstructed GaAs surfaces at 250 and 50 °C. At 250 °C, the films showed a strong twofold in-plane magnetic anisotropy independent of the GaAs surface reconstruction. In contrast, at 50 °C, the in-plane anisotropy appeared only on the (2×4) reconstructed surface. The anisotropic crystallographic domain structure of the MnSb films is thought to cause the magnetic anisotropy. The anisotropic domain formation is explained by the different chemisorption of the Mn adatom on the GaAs surface as a function of the termination.
Phase boundary structure in Nd–Fe–B sintered magnets
Authors: Makita Ken , Yamashita Osamu
Published: July 2002

The interface between tetragonal Nd2Fe14B phase and Nd-rich grain boundary phase in Nd–Fe–B sintered magnets was investigated with high resolution transmission electron microscope. Selected area diffraction patterns of the Nd-rich phase can be indexed by face-centered-cubic (fcc) structure with the lattice constant of a=0.548 nm. The orientation relationships between the two phases contacted directly with each other approximately agree with either of the following formulas: (110)tetra.//(110)fcc, [001]tetra.//[22̄3]fcc, or (110)tetra.//(010)fcc, [001]tetra.//[1̄02]fcc. These preferred orientations would be the results which minimize the phase boundary energy during postsintering heat treatment. In addition, it may change the local crystal field as well as the strain at the outermost Nd3+ ion site of the tetragonal grain both of which affect the coercivity of the magnet.
Near-field scanning optical microscopy of indium gallium nitride multiple-quantum-well laser diodes
Authors: Young D. K. , Mack M. P. , Abare A. C. , Hansen M. , Coldren L. A. , Denbaars S. P. , Hu E. L. , Awschalom D. D.
Published: July 2002

Near-field scanning optical microscopy is used to image electroluminescence from three and ten quantum-well (QW) indium gallium nitride based laser diodes. Facet cross sections are imaged with a spatial resolution of <100 nm, below and above the lasing threshold. Observation of spatially resolved spectra near the active region reveals compositional fluctuations as well as absorption and reemission of the lasing mode. Single-mode emission is observed for narrow (3 and 5 μm wide) 3 QW laser bars. Near-field measurements show a relationship between modal emission, waveguide structure, and lateral device size.
Design considerations and analytical approximations for high continuous-wave power, broad-waveguide diode lasers
Authors: Botez D.
Published: July 2002

Accurate analytical approximations are derived for the equivalent transverse spot size, d/Γ (<5% error), and the transverse beamwidth θ1/2 (<2% error), of broad-waveguide-type diode lasers, over a wide range in waveguide width: from the first-order-mode cutoff to the third-order-mode cutoff. The analytical formulas are found to be in good agreement with experimental values. For low-series-resistance and thermal-resistance devices, it is found that the junction-temperature rise ΔTj in continuous wave (CW) operation is a strong function of both the characteristic temperature T1 for the external differential quantum efficiency ηD as well as of the heatsink thermal resistance. If the device has relatively temperature-insensitive ηD (i.e., T1≳1000 K) the maximum CW power as well as the power density at catastrophic optical mirror damage, P̄COMD, are limited, for a given active-region material, only by the heatsink heat-removal ability. For large d/Γ, 0.97 μm emitting, 100 μm stripe InGaAs/InGaAs(P)/GaAs devices with T1=1800 K, record-high CW and quasi-CW (100 μs wide pulses) output powers are obtained. The ratio of quasi-CW to CW P̄COMD values is only 1.3, in contrast to devices of poor carrier confinement and subsequent low-T1 values (∼140 K), for which the ratio is 1.9, and whose maximum CW powers are ∼40% less than those obtainable from high-T1 devices.
Channeling implantations of Al+ into 6H silicon carbide
Authors: Morvan E. , Godignon P. , Vellvehi M. , Hallén A. , Linnarsson M. , Kuznetsov A. Yu.
Published: July 2002

A strong channeling effect of Al+ ions implanted into crystalline SiC has been observed by Monte Carlo simulations and experiments especially designed to demonstrate this phenomenon have been performed. Depth distributions of implanted Al were measured for on- and controlled off-axis Al implantations using secondary ion mass spectrometry (SIMS). Much deeper and wider profiles are obtained for the on-axis implantations as compared to off-axis implants. For higher doses, the experiment also reveals the growth of an intermediate peak slightly deeper than the random peak. The origin of the intermediate peak can be understood by combining SIMS results with Monte Carlo simulations, which motivates the development of advanced simulation tools for the ion implantation process in SiC.
Placement of conjugated oligomers in an alkanethiol matrix by scanned probe microscope lithography
Authors: Chen J. , Reed M. A. , Asplund C. L. , Cassell A. M. , Myrick M. L. , Rawlett A. M. , Tour J. M. , Van Patten P. G.
Published: July 2002

We report the in situ replacement of conjugated molecules in an insulating matrix by scanned probe microscope lithography. High yield, programmable patterning of a self-assembled monolayer of dodecanethiol was performed by applying voltage pulses from a scanning tunneling microscope. Conjugated oligomers were observed to be subsequently chemisorbed onto the patterned sites.
Low-frequency dielectric relaxation of BaTiO3 thin-film capacitors
Authors: Lee Su-Jae , Kang Kwang-Yong , Han Seok-Kil
Published: July 2002

Ferroelectric BaTiO3 thin films with perovskite structure were grown by sol-gel spin-on processing onto (111)Pt/Ti/SiO2/Si substrates. In order to investigate the effects of space charge in BaTiO3 thin films, we measured the relative dielectric constant and the ac conductivity of the films as a function of frequency, ac oscillation amplitude, and temperature. Dielectric constant and dielectric loss were 147 and 0.03 at 100 kHz, respectively. Also, BaTiO3 thin films exhibited marked dielectric relaxation above the Curie temperature and in the low-frequency region below 100 Hz. This low-frequency dielectric relaxation is attributed to the ionized space-charge carriers such as oxygen vacancies and defects in BaTiO3 film and the interfacial polarization. The thermal activation energy for the relaxation process of the ionized space-charge carriers was 0.72 eV.
Electronic bistability in electrochemically self-assembled quantum dots: A potential nonvolatile random access memory
Authors: Kouklin N. , Bandyopadhyay S. , Tereshin S. , Varfolomeev A. , Zaretsky D.
Published: July 2002

An electronic bistability has been observed in a two-dimensional spatially ordered array of 10 nm quantum dots self-assembled by electrodepositing CdS in nanoporous anodic alumite film. The current–voltage characteristic of the array shows switching between two stable conductance states, which can be controlled by an external bias. The bistability is observed when current flows laterally between two contacts on the top surface of the array, and also when current flows vertically between a top contact and the bottom (conducting) substrate. If the system is left in one conductance state, it remains there for at least 180 h and possibly much longer, until switched to the other state by an external bias. Such an effect may find applications in inexpensive, ultradense nonvolatile static random access memory.
Electro-optic polymer modulators for 1.55 μm wavelength using phenyltetraene bridged chromophore in polycarbonate
Authors: Oh Min-Cheol , Zhang Hua , Szep Attila , Chuyanov Vadim , Steier William H. , Zhang Cheng , Dalton Larry R. , Erlig Hernan , Tsap Boris , Fetterman Harold R.
Published: July 2002

Electro-optic polymer modulators operating at 1550 nm are demonstrated based on a nonlinear optical polymer of a phenyltetraene bridged chromophore in polycarbonate. It has a large electro-optic coefficient (r33=55 pm/V at 1550 nm), good thermal stability (90 °C), and low loss (1.7 dB/cm). A thin protective layer was used in the fabrication of ridge waveguides on the nonlinear polymer. We measured Vπ of 2.4 and 3.7 V at 1300 and 1550 nm, respectively. The chip loss of the modulator at both wavelengths was 5 dB, not including fiber coupling losses.
Temperature Dependence of Physical–Chemical Properties of Selected Chemicals of Environmental Interest. II. Chlorobenzenes, Polychlorinated Biphenyls, Polychlorinated Dibenzo-p-dioxins, and Dibenzofurans
Authors: Shiu Wan-Ying , Ma Kuo-Ching
Published: July 2002

Data are compiled and reviewed of the physical–chemical properties of chlorinated benzenes, biphenyls, and dibenzo-p-dioxins and dibenzofurans, which control air–water partitioning, namely vapor pressure, aqueous solubility, and Henry’s law constant over the environmentally relevant temperature range of 5–50 °C. Recommended values at 25 °C, and equations for estimating these properties over the temperature range of 5–50 °C are provided. Corresponding enthalpies of phase transition are also reported.
Coalescence and capillary breakup of liquid volumes
Authors: Shikhmurzaev Yulii D.
Published: July 2002

The problem of the mathematical modeling of coalescence and breakup of liquid volumes surrounded by an inviscid gas is considered. As is shown, an unphysical singularity in the known self-similar solutions of the Navier–Stokes equations intended to describe the topological transition of the flow domain arises as a consequence of the assumption that the free surface becomes smooth immediately after the onset of coalescence or remains so up to the very moment of breakup. Then the standard kinematic boundary condition prescribes that fluid particles belonging to the free surface remain there at all times and thus couples the scales for lengths and velocities in a self-similar solution leading to the singularity. An alternative approach allowing one to remove the singularity at a macroscopic level is formulated. Its key idea is that the topological transition, being a particular case of an interface formation/disappearance process, is associated with a free-surface cusp either propagating away from the point of initial contact of two volumes leading to their coalescence or “severing” a liquid thread connecting them in the case of breakup. The interface becomes (or, in a reverse flow, ceases to be) smooth at a finite distance from the point where, in the standard approach, a singularity would have taken place. An earlier developed macroscopic theory of interface formation/disappearance is applied without any ad hoc changes.
Transport and quantum electron mobility in the modulation Si δ-doped pseudomorphic GaAs/In0.2Ga0.8As/Al0.2Ga0.8As quantum well grown by metalorganic vapor phase epitaxy
Authors: Babiński Adam , Siwiec-Matuszyk J. , Baranowski J. M. , Li G. , Jagadish C.
Published: July 2002

A study of transport and quantum mobility of electrons in two-dimensional electron gas (2DEG) in the modulation Si δ-doped pseudomorphic GaAs/In0.2Ga0.8As/Al0.2Ga0.8As quantum well (QW) grown by metalorganic vapor phase epitaxy is presented. Well-resolved Shubnikov–de Haas oscillations of the magnetoresistivity observed at T=4.2 K suggest that the 2DEG with high electron mobility (μt≈46 000 cm2/V s) formed in the QW with no significant parallel conduction. A persistent photoconductivity effect resulted in an increase in electron sheet density. An increase of transport and quantum mobilities up to the onset of the second subband occupancy was observed. Further illumination resulted in a decrease of both mobilities. Strong dependence of the quantum mobility on the thermal history of the investigated sample was attributed to the effect of actual distribution of ionized centers in the sample.
Estimating the overlap of an approximate with the exact wave function by quantum Monte Carlo methods
Authors: Hornik Miroslav , Snajdr Martin , Rothstein Stuart M.
Published: July 2002

This work is an extension of Langfelder, Rothstein, and Vrbik’s [J. Chem. Phys. 107, 8525 (1997), (LRV)] algorithm that samples the “exact” ground state distribution φ02. The distribution is the same as that obtained from exactly solving the Schrödinger equation, but with a small error due to the incorrect nodes imposed on the simulation by the guiding function [J. B. Anderson, J. Chem. Phys. 65, 4121 (1976)] ψ. LRV applied their algorithm to estimate the expectation values of nondifferential properties for first-row atoms and ions. Our objective is to show that at the same time, with some relatively trivial modifications, the algorithm can estimate the overlap integral of ψ with the exact wave function φ0.
Ab initio study of OH addition reaction to isoprene
Authors: Lei Wenfang , Derecskei-Kovacs Agnes , Zhang Renyi
Published: July 2002

Ab initio molecular orbital calculations have been employed to investigate the structures and energetics of the adduct isomers arising from the addition reaction of OH to isoprene. Several levels of ab initio theory were evaluated using a set of organic radical species to establish the appropriate level of approximation. The method of gradient corrected density functionals (NLDFT) in conjunction with moderate basis sets was found to yield satisfactory molecular geometries and vibrational frequencies. Single-point energy calculations were performed using various methods, including MP2, MP4, and CCSD(T). The most energetically favorable isomers are those with OH addition to the terminal carbon positions. At the CCSD(T)/6-311G** level of theory corrected with zero-point energy (ZPE), the isomers with OH additions to isoprene at C1 to C4 positions (i.e., isomers I–IV) are 34.8, 24.2, 22.4, and 32.3 kcal mol−1 more stable than the OH and isoprene, respectively. The activation energies against OH migration transforming the higher energy isomers into the lower energy ones (i.e., II to I or III to IV) are significant (25.5–26.5 kcal mol−1), indicating that thermal equilibrium of the OH–isoprene adduct isomers is unlikely to be established. In addition, we have developed and validated a computationally efficient method to calculate the energetics of the OH–isoprene reaction system.
Accelerations in isotropic and homogeneous turbulence and Taylor’s hypothesis
Authors: Pinsky M. , Khain A. , Tsinober A.
Published: July 2002

The validity of Taylor’s hypothesis is analyzed by comparing the root mean square (rms) values of full (Lagrangian) and inertial accelerations in an isotropic and homogeneous turbulent flow. Full, local, and inertial accelerations in turbulence were decomposed into solenoidal and potential components, which made it possible to avoid dealing, at least directly, with the pressure-gradient term in the Navier–Stokes equation. The evaluations of the correlation functions and spectra of the accelerations are presented. These evaluations have been obtained using the Batchelor [Proc. Cambridge Philos. Soc. 47, 359 (1951)] longitudinal structure function that describes statistical properties of the turbulent velocity field. This function is equally valid for both inertial and dissipative subranges. It was shown that the ratio of the rms values of the full and inertial accelerations depends on the Reynolds number Rλ only and decreases at large Rλ as Rλ−1/2. At Rλ of about 20 this ratio is close to 0.72. At Rλ of 1000 the ratio is less than 0.1. The validity of Taylor’s hypothesis depends on the ratio of the rms values of the accelerations. The results indicate that Taylor’s hypothesis is valid for large Rλ (exceeding about 1000) and becomes questionable at Rλ below 100. At large Rλ the full acceleration in homogeneous and isotropic turbulence turned out to be independent of the Reynolds number.
Ab initio study of structure in boron nitride, aluminum nitride and mixed aluminum boron nitride amorphous alloys
Authors: McCulloch D. G. , McKenzie D. R. , Goringe C. M.
Published: July 2002

Car–Parrinello molecular dynamics is applied to the liquid quench simulation of the amorphous networks of BN, AlN and AlBN2. Structural and elastic properties were determined. It is found that AlN has a stronger tendency to chemical order than BN, driven by the greater energy penalty for “wrong bonds.” AlN, however, has a stronger tendency to form an amorphous structure as judged by the energy difference between the crystalline and amorphous states. There is some experimental evidence for an amorphous form of AlN. BN was simulated at two densities, 2.0 and 3.0 g/cm3. Even at the higher density, the fraction of tetrahedral coordination remained low, in contrast to AlN, enabling us to predict that the tetrahedral amorphous form of BN does not form under liquid quench conditions. The amorphous network with the formula AlBN2 has an intermediate tendency to form a tetrahedral structure and has a relatively high elastic modulus. This material is predicted to be of value for application as a wear resistant coating.
2 μm GaInAsSb/AlGaAsSb midinfrared laser grown digitally on GaSb by modulated-molecular beam epitaxy
Authors: Mourad C. , Gianardi D. , Malloy K. J. , Kaspi R.
Published: July 2002

Stimulated emission at 1.994 μm was demonstrated from an optically pumped, double quantum well, semiconductor laser that was digitally grown by modulated-molecular beam epitaxy. This “digital growth” consists of short period superlattices of the ternary GaInAs/GaInSb and GaAsSb/GaSb/AlGaSb/GaSb alloys grown by molecular beam epitaxy with the intent of approximating the band gaps of quaternary GaInAsSb and AlGaAsSb alloys in the active region and barriers of the laser, respectively. For a 50 μs pulse and a 200 Hz repetition rate, the threshold current density was 104 W/cm2 at 82 K. The characteristic temperature (T0) was 104 K, the maximum operating temperature was 320 K and the peak output power was 1.895 W/facet at 82 K with pumping power of 7.83 W.
Near-field fiber tip to handle high input power more than 150 mW
Authors: Jung S. T. , Shin D. J. , Lee Y. H.
Published: July 2002

A high-power near-field fiber tip is proposed and demonstrated. This high-power tip can handle an optical power of greater than 150 mW injected into the fiber core, higher than any previous tip. The tip has a unique, thick, heat-conducting metal layer deposited by an electroplating process. The subwavelength aperture of the tip is fabricated by the controlled lapping of the end face with in situ optical monitoring. We also demonstrate using this tip to record on phase change and photoresist media.
Enhanced pinning in a magnetic-superconducting bilayer
Authors: Garcı́a-Santiago A. , Sánchez F. , Varela M. , Tejada J.
Published: July 2002

Pinning of vortices in a high-temperature superconductor by the magnetic domain structure of a highly anisotropic ferromagnet is investigated by means of magnetic measurements in nanoscale period superconductor/ferromagnet (SC/FM) heterostructures. Two different samples consisting of highly epitaxial films of YBa2Cu3O7(SC) and BaFe12O19(FM) are analyzed relative to a pure superconducting YBa2Cu3O7 film. The irreversibility line obtained in the magnetic-field-reduced-temperature phase diagram for each heterostructure is found to shift upwards when compared to the line corresponding to the pure superconducting sample. This effect is interpreted as an evidence for the enhancement of pinning of vortices in the SC layer by the magnetic domain structure in the FM layer.
Crossed-beam velocity map imaging of collisional autoionization processes
Authors: Delmdahl Ralph F. , Bakker Bernard L. G. , Parker David H.
Published: July 2002

Applying the velocity map imaging technique Penning ion formation as well as generation of associative ions is observed in autoionizing collisions of metastable neon atoms (Ne* 2p5 3s 3P2,0) with ground state argon targets in a crossed molecular beam experiment. Metastable neon reactants are obtained by nozzle expansion through a dc discharge ring. The quality of the obtained results clearly demonstrates the suitability of this new, particularly straightforward experimental approach with respect to angle and kinetic energy resolved investigations of Penning processes in crossed-beam studies which are known to provide the highest level of detail.