bluuebunny/crossref_metadata_2025 huggingface dataset:
Relationship between optical properties and crystallinity of nanometer Y2O3:Eu phosphor
Authors: Igarashi T. , Ihara M. , Kusunoki T. , Ohno K. , Isobe T. , Senna M.
Published: July 2002

Y 2 O 3 -doped Eu3+ (Y2O3:Eu) nanometer particles (NPs) were synthesized via a chemical route. The particle size estimated by x-ray diffractometry and transmission electron microscopy was about 61 nm. Two photoluminescence peaks were observed at 582 and 587 nm, being attributed to the transition of D05→F1a7 of Eu3+ in the S6 and C2 symmetry sites of Y2O3, respectively. The intensity ratio of the photoluminescence peaks at 582 nm to at 587 nm was larger for NPs than for micrometer particles (MPs). The excitation peak of NPs due to the charge-transfer band shifted toward the high-energy side as compared with that of MPs. According to x-ray diffractometry, the lattice distortion and the lattice constant were larger for NPs than for MPs, showing the restructure at the near surface and the increase in ionicity of the Eu–O bond with decreasing particle size. This explains the blueshift in the excitation peak.
Experimental evidence for recombination-assisted leakage in thin oxides
Authors: Ielmini Daniele , Spinelli Alessandro S. , Lacaita Andrea L.
Published: July 2002

Carrier separation experiments in degraded p-channel metal–oxide–semiconductor devices reveal the occurrence of both electron and hole leakage processes. A detailed study of the excess currents as a function of time after stress, stress fluence, and polysilicon doping is presented. Evidence for a linear correlation between electron and hole leakage currents is provided, suggesting that the same defect species are responsible for both leakage phenomena. The dependence on polysilicon gate type confirms this explanation, supporting a leakage model based on trap-assisted tunneling and recombination in oxide traps.
Extended x-ray absorption fine-structure measurement of bond-length strain in epitaxial Gd2O3 on GaAs(001)
Authors: Nelson E. J. , Woicik J. C. , Hong M. , Kwo J. , Mannaerts J. P.
Published: July 2002

Extended x-ray absorption fine structure (EXAFS) has been used to measure the bond length in a 23 Å epitaxial Gd2O3 film grown on GaAs(001). The Gd–O bond length is determined to be 2.390±0.013 Å, which corresponds to a +0.063±0.013 Å increase or a +2.7%±0.6% bond-length strain relative to the bond length in a bulk Gd2O3 powder. Using a simple model for the strained film that matches the [001] and [−110] axes of Gd2O3 with the [110] and [1–10] axes of the GaAs(001) surface, the measured bond-length increase of the film determined by EXAFS agrees well with the perpendicular lattice distortion of the film determined by diffraction.
Structure and optical properties of Si/InAs/Si layers grown by molecular beam epitaxy on Si substrate
Authors: Zakharov N. D. , Werner P. , Gösele U. , Heitz R. , Bimberg D. , Ledentsov N. N. , Ustinov V. M. , Volovik B. V. , Alferov Zh. I. , Polyakov N. K. , Petrov V. N. , Egorov V. A. , Cirlin G. E.
Published: July 2002

Epitaxial Si/InAs/Si heterostructure grown on (001) Si substrate by molecular beam epitaxy and annealed at 800 °C was investigated by high resolution transmission electron microscopy. Extensive interdiffusion leads to the formation of an InAs solid solution in the Si cap layer. Additionally, InAs-enriched regions with extensions of ∼6 nm, which exhibit two kinds of ordering are observed. The ordering of InAs molecules has occurred, respectively, in (101) and (101̄) planes inclined and (110) and (11̄0) planes parallel to the [001] growth direction. It is attributed to the energy gain from the reduced number of mixed Si–As and Si–In bonds. The sample show photoluminescence in the 1.3 μm region, which is tentatively attributed to the recombination of excitons localized in the ordered regions.
Structure and morphology of polymer-stabilized cholesteric diffraction gratings
Authors: Kang S. W. , Sprunt S. , Chien L. C.
Published: July 2002

We report a study of polymer network morphology in planar diffraction gratings formed from polymer-stabilized cholesteric liquid crystals. Photopolymerizing a mesogenic monomer yields a regular sawtooth morphology, composed of polymer sheets running at an acute angle between thick, uniformly aligned bundles which reside on the opposing substrates. To explain this morphology and evidence of two grating vectors in the diffraction pattern, we propose a model for the liquid-crystal director where out-of-phase twists at the surfaces gear into an interior twist, which has half the period of the surface twist, via splay distortions running between the substrates. The morphology also changes significantly when a nonmesogenic monomer is substituted or when selectively polarized states of UV radiation are used to cure the polymer.
Nonparaxial one-dimensional spatial solitons
Authors: Blair Steve
Published: July 2002

Scalar and vector nonlinear nonparaxial evolution equations are developed for propagation in two-dimensions. Using standard soliton scalings, it is found that nonparaxial propagation is accompanied by higher-order linear and nonlinear terms and an effective quintic nonlinear index. The presence of an intrinsic quintic nonlinearity arising from χ(5) must also be considered at the order of the analysis. These terms represent corrections to the well-known nonlinear Schrödinger equation. Exact and approximate solutions to these higher-order evolution equations are obtained and are shown to exhibit quasi-soliton behavior based on propagation and collision studies.
Quantum control of chemical reaction dynamics in a classical way
Authors: Umeda Hiroaki , Fujimura Yuichi
Published: July 2002

A simplified approach to quantum control of chemical reaction dynamics based on a classical, local control theory was developed. The amplitude of the control pulse is proportional to the linear momentum of the reaction system within the dipole approximation for the system-radiation field interaction. The kinetic energy of the system is the controlling parameter. That is, the reaction is controlled by accelerating the representative point on a potential energy surface before crossing over a potential barrier and then by deaccelerating it to the target after passing over the potential barrier. The classical treatment was extended to control of wave packet dynamics by replacing the classical momentum by a quantum mechanically averaged momentum on the basis of the Ehrenfest theorem. The present method was applied to a quantum system of a simple one-dimensional, double-well potential for checking its validity. A restriction of the applicability of the simplified method was also discussed. An isomerization of HCN was treated as a model system for wave packet control of a two-dimensional reaction.
Cryogenic magnetic force microscope
Authors: Roseman M. , Grütter P.
Published: July 2002

We describe our cryogenic magnetic force microscope, operating between 4.2 and 300 K, in fields of 0–8 T. The system uses a fiber optic interferometer to measure cantilever deflections, permitting the tracking of the resonance frequency through the use of a phase locked loop. Piezoelectric positioners, capable of operation in high magnetic fields, perform in situ tip and fiber approaches. As an effective means of vibration isolation, we suspend the microscope from a soft bellows which attenuates vibrations by more than an order of magnitude. A detailed noise analysis indicates that although the microscope is thermally limited, the system frequency resolution is currently limited by the shot noise of the interferometer.
Noncontact scanning force microscopy based on a modified tuning fork sensor
Authors: Göttlich Hagen , Stark Robert W. , Pedarnig Johannes D. , Heckl Wolfgang M.
Published: July 2002

Distance control using a tuning fork setup for the detection of shear forces is a standard configuration in scanning near-field optical microscopy (SNOM). Based on this concept, a modified sensor was developed, where a standard silicon tip for atomic force microscopy (AFM) is attached to the front end of one prong of a 100 kHz quartz tuning fork oscillator. Comparison of force curves of a standard tapping-mode AFM cantilever, a conventional fiber tip SNOM sensor and the novel AFM tip shear force sensor demonstrate an enhanced stability and sensitivity of the new sensor. Due to the rigid sensor design the force curves of the AFM tip shear force sensor indicate a perfect noncontact behavior under normal conditions in air. Noncontact images show a comparable resolution to conventional force microscopy.
Thickness-shear mode quartz crystal resonators in viscoelastic fluid media
Authors: Arnau A. , Jiménez Y. , Sogorb T.
Published: July 2002

An extended Butterworth–Van Dyke (EBVD) model to characterize a thickness-shear mode quartz crystal resonator in a semi-infinite viscoelastic medium is derived by means of analysis of the lumped elements model described by Cernosek et al. [R. W. Cernosek, S. J. Martin, A. R. Hillman, and H. L. Bandey, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45, 1399 (1998)]. The EBVD model parameters are related to the viscoelastic properties of the medium. A capacitance added to the motional branch of the EBVD model has to be included when the elastic properties of the fluid are considered. From this model, an explicit expression for the frequency shift of a quartz crystal sensor in viscoelastic media is obtained. By combining the expressions for shifts in the motional series resonant frequency and in the motional resistance, a simple equation that relates only one unknown (the loss factor of the fluid) to those measurable quantities, and two simple explicit expressions for determining the viscoelastic properties of semi-infinite fluid media have been derived. The proposed expression for the parameter Δf/ΔR is compared with the corresponding ratio obtained with data computed from the complete admittance model. Relative errors below 4.5%, 3%, and 1.2% (for the ratios of the load surface mechanical impedance to the quartz shear characteristic impedance of 0.3, 0.25, and 0.1, respectively), are obtained in the range of the cases analyzed. Experimental data from the literature are used to validate the model.
Dielectric breakdown of ultrathin aluminum oxide films induced by scanning tunneling microscopy
Authors: Magtoto N. P. , Niu C. , Ekstrom B. M. , Addepalli S. , Kelber J. A.
Published: July 2002

Dielectric breakdown of 7-Å-thick Al2O3 (111) films grown on Ni3Al(111) under ultrahigh vacuum conditions is induced by increasing the bias voltage on the scanning tunneling microscopy tip under constant current feedback. Breakdown is marked by the precipitous retreat of the tip from the surface, and the formation of an elevated feature in the scanning tunneling microscopy image, typically greater than 5 nm high and ∼100 nm in diameter. Constant height measurements performed at tip/sample distances of 1 nm or less yield no tip/substrate physical interaction, indicating that such features do not result from mass transport. Consistent with this, current/voltage measurements within the affected regions indicate linear behavior, in contrast to a band gap of 1.5 eV observed at unaffected regions of the oxide surface. A threshold electric field value of 11±1 MV cm−1 is required to induce breakdown, in good agreement with extrapolated values from capacitance measurements on thicker oxides.
Development of the tracking laser vibrometer: Performance and uncertainty analysis
Authors: Castellini P. , Paone N.
Published: July 2002

This work presents the development of a laser tracking Doppler vibrometer (TLDV), which is based on a laser scanning vibrometer improved for tracking the same point of a bladed disk during a complete circular rotation, so that a Lagrangian approach can be applied to vibration measurement of a rotating object. The TLDV is presented and applied to the measurement of vibration velocity map of a rotating object. Measurements have been carried out on a bladed disk rotating up to 62 rps. This article discusses the development of the measurement system, with particular attention to the problems of measurement uncertainty and laser spot positioning accuracy. Results of TLDV measurements performed under different conditions of rotation and of vibration are presented. An analytical model of the measurement system was developed in order to evaluate its behavior and analyze its accuracy on the positioning and uncertainty of the measured data. In this way it was possible to identify the most critical elements in the system and to guide its optimization. The misalignment between the optic axis of the tracking system and the rotation axis of the object is the most relevant source of uncertainty, inducing the presence of pseudovibrations at the frequencies of rotation and causing relative motion of the laser beam on the target surface, which turns out to be a positioning error and a source of speckle noise on the laser vibrometer signal. A comparison between static measurement and numerical analysis has been performed, so that capabilities of the tracking technique were highlighted.
Tandem injection of charge carriers across a metal-dielectric interface
Authors: Arkhipov V. I. , Bässler H.
Published: July 2002

A tandem mechanism of charge injection from a metal contact into an organic dielectric via an intermediate injection-facilitating layer is considered. It is shown that introducing such a layer can enhance charge injection considerably if the energy of transport states in this layer is in between the Fermi level of the contact and the energy of transport states in the bulk dielectric. The optimum energy of transport states in the intermediate layer and the minimum thickness thereof are calculated.
Site-specific dissociation dynamics of ethylene at 157 nm: Atomic and molecular hydrogen elimination
Authors: Lin Jim J. , Wang Chia C. , Lee Yuan T. , Yang Xueming
Published: July 2002

The atomic and molecular hydrogen elimination processes from ethylene have been studied using a molecular beam apparatus. Site and isotope effects on the molecular hydrogen elimination from ethylene have been clearly observed from the photodissociation of ethylene at 157 nm. Experimental results show that there are three different types of molecular elimination processes: 1,1 elimination, 1,2-cis elimination, and 1,2-trans elimination. Significant differences have been detected between 1,1 elimination and 1,2 eliminations in their kinetic energy distributions. Noticeable difference is also found between 1,2-cis elimination and 1,2-trans elimination for molecular deuterium elimination. Branching ratios for atomic and molecular hydrogen elimination processes have also been determined for ethylene and its isotopomers. Isotope and site effects on the branching ratios of different molecular elimination channels have been observed. The experimental results are also compared with recent theoretical studies.
Epitaxial growth and dielectric properties of (111) oriented BaTiO3/SrTiO3 superlattices by pulsed-laser deposition
Authors: Nakagawara Osamu , Shimuta Toru , Makino Takahiro , Arai Seiichi , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

Epitaxial BaTiO3(111)/SrTiO3(111) multilayered thin films have been investigated with various periods of stacking layer between 0.45/0.45 and 10/10 nm on Nb-doped SrTiO3(111) substrates by a pulsed-laser deposition technique. Upon decreasing the period of each layer, the spacing of (111) plane (d111) of the multilayered film increases, and the relative dielectric constant goes up to 594 which is twice as large as that of (111) oriented (Ba0.5, Sr0.5)TiO3 solid-solution film. The expansion of d111, which might be attributed to an in-plane pressure effect due to the large lattice strain in the heteroepitaxial interface, contributes to the enlargement of relative dielectric constant. Remanent polarization observed in polarization versus applied voltage hysteresis loop is no more than 2.7 μC/cm2 with 2.0/2.0 nm period of layer.
Statistical mechanics with three-dimensional particle tracking velocimetry experiments in the study of anomalous dispersion. II. Experiments
Authors: Moroni Monica , Cushman John H.
Published: July 2002

In paper I [Phys. Fluids 13, 75 (2001)] we provided a theory for simulating anomalous dispersion which relied on the self-part of the intermediate scattering function. Here we obtain Lagrangian trajectories for a conservative tracer in a porous medium and then use these trajectories to obtain the self-part of the intermediate scattering function. We then use the scattering function as data for the inverse problem and obtain the generalized wave-vector and frequency dependent dispersion tensor developed in paper I. The transverse components of this tensor are then examined as a function of wave vector to see if or when the dispersive process goes asymptotic (Fickian). The matched index (of refraction) technique has been used to obtain a transparent porous medium and three dimensional particle tracking has been used to obtain the trajectories. Over the life of the experiment the transverse dispersive process remained anomalous, though it was gradually approaching the Fickian limit.
Effects of anneals in ammonia on the interface trap density near the band edges in 4H–silicon carbide metal-oxide-semiconductor capacitors
Authors: Chung Gilyong , Tin Chin Che , Williams John R. , McDonald K. , Di Ventra M. , Chanana R. K. , Pantelides S. T. , Feldman L. C. , Weller R. A.
Published: July 2002

Results of room temperature capacitance–voltage measurements are reported for SiO2/4H–SiC (n and p type) metal-oxide-semiconductor capacitors annealed in ammonia following oxide layer growth using standard wet oxidation techniques. For n-SiC capacitors, both the interface state density near the conduction band edge and the effective oxide charge are substantially reduced by the ammonia anneals. For 2 h anneals, the oxide charge appears to have a minimum value for an anneal temperature of approximately 1100 °C. However, for p-SiC, anneals in ammonia produce no improvement in the interface state density near the valence band edge, and the effective oxide charge is not reduced compared to samples that were not annealed. Results are compared to those reported previously for anneals in nitric oxide. Ion beam analyses of the oxide layers show substantially more nitrogen incorporation with the ammonia anneals, although for n-SiC, the decrease in Dit is comparable for both nitric oxide and ammonia anneals. Results reported here for ammonia and those reported previously for nitric oxide are the first to demonstrate that significant passivation of the interface state density near the conduction band edge in SiC can be achieved with high temperature anneals using either gas. This demonstration has important implications for SiC metal-oxide-semiconductor field effect transistor technology development.
Smooth wet etching by ultraviolet-assisted photoetching and its application to the fabrication of AlGaN/GaN heterostructure field-effect transistors
Authors: Maher H. , DiSanto D. W. , Soerensen G. , Bolognesi C. R. , Tang H. , Webb J. B.
Published: July 2002

We characterize a KOH-based ultraviolet (UV) photoassisted wet etching technique using K2S2O8 as the oxidizing agent. The solution provides a well-controlled etch rate and produces smooth high-quality etched surfaces with a minimal degradation in surface roughness as measured by atomic force microscopy. The evolution of the solution pH upon exposure to UV radiation is identified as key to obtaining smooth etched surfaces and a controlled etch rate: Unless steps are taken to maintain the pH above 12.0, the etch rate displays a sharp drop that coincides with a gross roughening of the etched surface. The applicability of the present technique is demonstrated by the fabrication of high-quality mesa-isolated AlGaN/GaN hetrostructure field-effect transistors. In addition, the etch presented here features a high selectivity to C-doped layers which should prove useful in the fabrication of AlGaN/GaN hetrostructure bipolar transistors. The method is well adapted to device processing applications because it does not require connection to the sample to an external electrochemical cell.
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enhancement of YBa2Cu3O7 films on polycrystalline silver substrates by metalorganic chemical vapor deposition in high magnetic fields
Authors: Ma Yanwei , Watanabe Kazuo , Awaji Satoshi , Motokawa Mitsuhiro
Published: July 2002

High-quality YBa2Cu3O7 (YBCO) films were grown on polycrystalline silver substrates by a metalorganic chemical vapor deposition process in high magnetic fields. Property characterizations revealed that the YBCO films deposited in high magnetic fields have excellent structural and superconducting properties. At 77 K, the transport critical current densities of such films were over 10 000 A/cm2 in the field of 1 T (H∥c axis). The improvement of the critical currents in YBCO films deposited in high magnetic fields is correlated with the better intergrain connections, as observed by scanning electron microscopy.
Highly efficient 2% Nd:yttrium aluminum garnet ceramic laser
Authors: Lu Jianren , Prabhu Mahendra , Xu Jianqiu , Ueda Ken-ichi , Yagi Hideki , Yanagitani Takakimi , Kaminskii Alexander A.
Published: July 2002

A highly efficient diode-laser end-pumped polycrystalline 2% Nd:yttrium aluminum garnet (YAG) ceramics laser that is comparable in efficiency with Nd:YAG single crystal laser has been developed. With 883 mW pumping, 465-mW-cw laser output at 1064 nm has been obtained corresponding to an optical-to-optical efficiency of 52.7%. The laser threshold is only 30 mW with R=97% output coupler.
A new experimental facility for investigating the formation and properties of gas hydrates under simulated seafloor conditions
Authors: Phelps Tommy J. , Peters David J. , Marshall Simon L. , West Olivia R. , Liang Liyuan , Blencoe James G. , Alexiades Vasilios , Jacobs Gary K. , Naney Michael T. , Heck Jack L.
Published: July 2002

A seafloor process simulator (SPS) has been developed for experimental investigations of the physical, geochemical, and microbiological processes affecting the formation and stability of methane and carbon dioxide hydrates at temperatures and pressures corresponding to ocean depths of 2 km. The SPS is a corrosion-resistant pressure vessel whose salient characteristics are: (i) an operating range suitable for study of methane and carbon dioxide hydrates; (ii) numerous access and observation ports, and (iii) a large (0.0722 m3) internal volume. Initial experiments have shown that the SPS can be used to produce large amounts of high-purity methane hydrate over a wide range of experimental conditions.
Lateral variations in threshold voltage of an AlxGa1−xN/GaN heterostructure field-effect transistor measured by scanning capacitance spectroscopy
Authors: Schaadt Daniel M. , Miller Eric J. , Yu Edward T. , Redwing Joan M.
Published: July 2002

Local dC/dV spectroscopy performed in a scanning capacitance microscope (SCM) was used to map, quantitatively and with high spatial resolution (∼50 nm), lateral variations in the threshold voltage of an AlxGa1−xN/GaN heterostructure field-effect transistor epitaxial layer structure. Scanning capacitance and the associated threshold voltage images show small round features less than 150 nm in diameter with a corresponding shift in threshold voltage of about 1.5–2 V, and larger features several microns in size with a corresponding shift in threshold voltage of approximately 1 V. The small features in the SCM and threshold voltage images are consistent with the presence of charged threading dislocations, while the variations in threshold voltage over large areas could be a result of thickness and/or composition variations in the AlxGa1−xN layer.
Zinc oxide doping effects in polarization switching of lithium niobate
Authors: Peng L.-H. , Zhang Y.-C. , Lin Y.-C.
Published: July 2002

We report a pulsed-field analysis on the 180° domain reversal process in Z-cut congruent grown lithium niobate (LiNbO3) doped with zinc oxide at concentration CZnO>5 mol %. The polarization switching field is found to decrease with the ZnO doping with a threshold (Eth) and internal (Eint) field as low as 2.5 and 0.5 kV/mm, respectively, resultant on 8 mol % ZnO doped LiNbO3. The substantial decrease of Eth and Eint is ascribed to the suppression of nonstoichiometric point defects by the substitution of Zn2+ ions in the lattice site.
Gate tunneling currents in ultrathin oxide metal–oxide–silicon transistors
Authors: Cai Jin , Sah Chih-Tang
Published: July 2002

Carrier tunneling through ultrathin (1–3 nm) SiO2 layers in MOS (metal–oxide–silicon) structures is investigated using the Bardeen–Harrison transition probability method. Quantum mechanical wave function matching at the two abrupt potential boundaries of a trapezoidal Si/SiO2/Si barrier gives an electric-field dependent preexponential factor in the Wentzel–Kramers–Brillouin tunneling probability, which significantly affects the current–voltage characteristic at low fields. An analytical theory is employed to predict the relative importance of three elastic tunneling pathways (electrons, valence electrons, and holes) and two geometrical tunneling locations (channel region and source or drain overlap regions) in MOS transistors (MOSTs), showing (1) hole tunneling dominant in p+gate pMOST (p-channel MOST) at low gate voltages, and (2) overlap regions dominant prior to base-region inversion in both p+gate pMOST and n+gate nMOST (n-channel MOST). The analytic theory is used to analyze the experimental tunneling currents measured at the gate, source, well, and substrate terminals of sourced MOS capacitors to give the oxide thickness and impurity doping concentrations in the base and source regions.
Unipolar spin diodes and transistors
Authors: Flatté M. E. , Vignale G.
Published: July 2002

Unipolar devices constructed from ferromagnetic semiconducting materials with variable magnetization direction are shown theoretically to behave very similarly to nonmagnetic bipolar devices such as the p-n diode and the bipolar (junction) transistor. Such devices may be applicable for magnetic sensing, nonvolatile memory, and reprogrammable logic.
Magnetic and electronic properties of transition-metal-substituted perovskite manganites—La0.7Ca0.3Mn0.95X0.05O3 (X=Fe,Co,Ni)
Authors: Song Hajung , Kim Woojin , Kwon Soon-Ju , Kang Jeongsoo
Published: July 2002

The magnetic and electronic properties of Fe-, Co-, Ni-substituted La0.7Ca0.3MnO3 were studied. Ferromagnetic–paramagnetic and metal–insulator transitions were significantly affected by Mn-site substitution although no observable differences were found in their crystal structures from x-ray diffraction analysis. Valence-band photoemission studies also showed that there were no appreciable changes in the electronic structures of each sample. Peak temperatures (Tp) at maximum resistivity decreased from 265 K of virgin LaCaMnO to 217, 203, and 171K for Ni-,Co-, and Fe-substituted LaCaMnO samples, respectively. As the Tp decreased, the electrical resistivity increased and the saturation magnetization decreased. The overall experimental results were well explained by considering superexchange interactions induced by substitution with magnetic ions having different magnetic moments. From our study, it is suggested that the magnetic properties of substituting elements should also be considered to deal with the physical properties of these LaCaMnXO systems.
Critical interaction of a shock wave with an acoustic wave
Authors: Robinet Jean-Christophe , Casalis Grégoire
Published: July 2002

The interaction of linear waves with normal or oblique shock waves has been studied by Moore [NACA Report No. 1165 (1954)], Ribner [AIAA J. 23 (1984)], and McKenzie and Westphal [Phys. Fluids 11, 2350 (1968)] by writing the fluctuating quantities as normal modes. These different studies have shown that downstream of the shock exists a critical angle of the incident acoustic waves where the reflection coefficient is equal to 1. The purposes of this paper are first to point out that this critical angle may appear as a singularity in the linearized Euler equations, and second to show that this problem can actually be removed if the perturbation is no longer assumed to be a normal mode but has a mathematical form which is strictly deduced from the mathematical nature of the linearized Euler equations. This analysis is then applied to the shock wave oscillations occurring on a wing profile and in a nozzle. Despite many simplifications, good results are obtained in comparison with the available experimental data.
Theoretical study of the two-dimensional electron mobility in strained III-nitride heterostructures
Authors: Yu Tsung-Hsing , Brennan Kevin F.
Published: July 2002

We present calculations of the two-dimensional (2D) electron mobility in III-nitride heterojunction structures in the presence of spontaneous and piezoelectrically induced polarization effects. The calculations are made using a self-consistent solution of the Schrödinger, Poisson, charge and potential balance equations. It is found that the polarization fields act to significantly increase the 2D sheet charge concentration while reducing the mobility. The mobility reduction results from the enhanced band bending and subsequent attraction of the electrons to the heterointerface where they experience increased surface roughness scattering. Good agreement is obtained between the theoretical calculations and experimental measurements over the full temperature range examined. Comparison of the mobility in InGaN/GaN to AlGaN/GaN heterostructures is made. It is found that the mobility is significantly higher in the InGaN/GaN structure than in the AlGaN/GaN structure.
Manipulation and controlled amplification of Brownian motion of microcantilever sensors
Authors: Mehta Adosh , Cherian Suman , Hedden David , Thundat Thomas
Published: July 2002

Microcantilevers, such as those used in atomic force microscopy, undergo Brownian motion due to mechanical thermal noise. The root mean square amplitude of the Brownian motion of a cantilever typically ranges from 0.01–0.1 nm, which limits its use in practical applications. Here we describe a technique by which the Brownian amplitude and the Q factor in air and water can be amplified by three and two orders of magnitude, respectively. This technique is similar to a positive feedback oscillator, wherein the Brownian motion of the vibrating cantilever controls the frequency output of the oscillator. This technique can be exploited to improve sensitivity of microcantilever-based chemical and biological sensors, especially for sensors in liquid environments.
Bifurcation analysis of Landau–Lifshitz–Gilbert dynamics under circularly polarized field
Authors: Bertotti G. , Magni A. , Mayergoyz I. D. , Serpico C.
Published: July 2002

Uniform solutions of Landau–Lifshitz–Gilbert equation coupled with magnetostatic Maxwell equations are discussed in the case where the problem is rotationally invariant around a certain axis and the external field is circularly polarized in the perpendicular plane. It is shown that a remarkably rich variety of phase portraits is present in the dynamics, with two or four time-harmonic modes rigidly rotating with the field (P modes) and zero, one, or two quasiperiodic modes (Q modes). Different portraits are separated by bifurcation lines of saddle node, Andronov–Hopf, homoclinic-saddle connection, and semistable-limit-cycle type. The complete phase portrait and bifurcation diagram of thin films with negligible crystal anisotropy is presented and discussed.
Ultrasonic velocity measurements during phase transformations in steels using laser ultrasonics
Authors: Dubois Marc , Moreau André , Bussière Jean F.
Published: July 2002

This article presents accurate laser-ultrasonic measurements of longitudinal velocity in the 500–1000 °C temperature range in carbon steel samples for different conditions known to affect phase transformations such as cooling rate, carbon concentration, and rolling. Measurements were performed during continuous heating and cooling at rates varying between 0.1 and 20 °C/s. Carbon concentrations ranged from 0.0% to 0.72%. Hot-rolled and cold-rolled samples were measured. For the hot-rolled samples, a reproducible hysteresis was observed in the dependence of the ultrasonic velocity versus temperature of samples having a significant carbon concentration. This hysteresis is attributed to the combined effects of the phase transformation and of the ferromagnetic–paramagnetic transition. In particular, the rate of change of velocity with temperature during heating suddenly diminishes at the Curie temperature, and the velocity behavior during cooling shows clearly the start and end of phase transformations, even allowing discrimination between ferrite and pearlite nucleations. For the cold-rolled samples, significant drops in ultrasonic velocity were observed at the transformation temperatures when the samples were heated for the first time. However, the magnitude of these drops decreased for subsequent heating. These drops are attributed to irreversible crystallographic orientation changes caused by phase transformations. This effect was modeled using the Kurdjumov–Sach transformation relationship. The model calculation agrees well with the experimental data.
Diffuse phase transitions, electrical conduction, and low temperature dielectric properties of sol–gel derived ferroelectric barium titanate thin films
Authors: Thomas Reji , Varadan V. K. , Komarneni S. , Dube D. C.
Published: July 2002

Ferroelectric thin films of barium titanate were fabricated by sol–gel technique on platinum substrates. The processing temperature was 700 °C. The films obtained with a thickness of 1.5 μm were dense, transparent, and showed ferroelectricity. Scanning electron microscopy and x-ray diffraction were used for studying the surface morphology and crystallographic structure of the film. Films in the metal–ferroelectric–metal configuration (MFM) were used for the electrical measurements. Dielectric constant and loss tangent were found to be 430 and 0.015, respectively, at 10 kHz under ambient conditions. The ε′(T) curve shows broad peak centered around 120 °C as in the case of diffuse phase transition. The ac conductivity is proportional to ω0.9 in the low frequency region and ω1.8 in the high frequency region. The dc conductivity versus temperature curve showed a change in the slope around 125 °C, corresponding to the phase transition. To study the low temperature phase transitions, dielectric parameters on the films were measured to a temperature down to about 10 K. Remanent polarization (Pr) and coercive field (Ec) obtained from the hysteresis loop at room temperature are ∼2.0 μC/cm2 and ∼27 kV/cm, respectively. Capacitance–voltage studies performed on the MFM structures showed butterfly loop at 135 °C.
Reactions of methylamines at the Si(100)-2×1 surface
Authors: Mui Collin , Wang George T. , Bent Stacey F. , Musgrave Charles B.
Published: July 2002

We have investigated the room temperature adsorption of methylamine, dimethylamine and trimethylamine using density functional theory (DFT) and multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy. It was found that the reaction pathways of the amines resemble the precursor-mediated dissociative chemisorption of ammonia. Our calculations showed that although dissociation involving N–C bond cleavage is thermodynamically more favorable than the N–H dissociation pathway, the activation barrier for N–CH3 dissociation is significantly higher than that for N–H dissociation. This leads to selective cleavage of N–H bonds in the surface reactions of methylamine and dimethylamine, while trapping trimethylamine in its molecularly chemisorbed state through the formation of a Si–N dative bond. We also identified the products of the reactions of the amines on the Si(100)-2×1 surface by surface IR studies, confirming the theoretical predictions. The selectivity observed in the surface chemistry of simple model amines is briefly discussed in the context of organic chemistry at semiconductor surfaces.
The importance of an accurate CH4 vibrational partition function in full dimensionality calculations of the H+CH4→H2+CH3 reaction
Authors: Bowman Joel M. , Wang Dunyou , Huang Xinchuan , Huarte-Larrañaga Fermin , Manthe U.
Published: July 2002

The full dimensional rate constant reported by Huarte-Larrañaga and Manthe for the H+CH4→H2+CH3 reaction [Huarte-Larrañaga and Manthe, J. Chem. Phys. 113, 5115 (2000)] is corrected by using an accurate vibrational partition function for CH4 instead of the harmonic normal-mode one used by them. The correction is shown to be substantial over the temperature range considered by Huarte-Larrañaga and Manthe.
Structural, electrical, and optical properties of SnO2 nanocrystalline thin films grown on p-InSb (111) substrates
Authors: Kim T. W. , Lee D. U. , Lee J. H. , Choo D. C. , Jung M. , Yoon Y. S.
Published: July 2002

SnO 2 thin films were grown on p-InSb (111) substrates by radio-frequency magnetron sputtering at low temperature. Atomic force microscopy images showed that the root mean square of the average surface roughness of the SnO2 films grown on the InSb (111) substrates with an Ar/O2 flow rate of 0.667 and at a temperature of 200 °C had a minimum value of 2.71 nm, and x-ray diffraction and transmission electron microscopy (TEM) measurements showed that these SnO2 thin films were polycrystalline. Auger electron spectroscopy and bright-field TEM measurements showed that the SnO2/p-InSb(111) heterointerface was relatively abrupt. High-resolution TEM measurements revealed that the SnO2 films were nanocrystalline and that the grain sizes of the nanocystalline films were below 6.8 nm. The capacitance–voltage measurements at room temperature showed that the type and the carrier concentration of the nominally undoped SnO2 film were n type and approximately 1.67×1016 cm−3, respectively, and the current–voltage curve indicated that the Au/n-SnO2/p-InSb diode showed tunneling breakdown. Photoluminescence spectra showed that peaks corresponding to the donor acceptor pair transitions were dominant and that the peak positions did not change significantly as a function of the measured temperature. These results indicate that the SnO2 nanocrystalline thin films grown on p-InSb (111) substrates at low temperature hold promise for new kinds of potential optoelectronic devices based on InSb substrates, such as superior gas sensors and high-efficiency solar cells.
Tunnel magnetoresistance in magnetic tunnel junctions with a ZnS barrier
Authors: Guth M. , Dinia A. , Schmerber G. , van den Berg H. A. M.
Published: July 2002

We report on the junction magnetoresistance in magnetic tunnel junctions of the hard–soft type with magnetic layers separated by a ZnS barrier. The hard magnetic bottom electrode consists of an artificial antiferromagnetic structure in which the rigidity is ensured by the antiferromagnetic exchange coupling between two FeCo layers through a Ru spacer layer. The samples were grown by sputtering on Si (111) wafers at room temperature and have the following structure: Fe6 nmCu30 nm(CoFe)1.8 nmRu0.8 nm(CoFe)3 nmZnSx(CoFe)1 nmFe4 nmCu10 nmRu3 nm. The square tunnel elements, with lateral sizes of 10, 20, 50, and 100μm, exhibit typical tunnel resistance of 2–3 kΩ μm2 and nonlinear zero field current–voltage (J–V) variation. The most interesting result is the observation of junction magnetoresistance of about 5% at room temperature with a 2 nm thick ZnS barrier.
Sum rules for exchange and correlation potentials
Authors: Ayers Paul W. , Levy Mel
Published: July 2002

Two recent papers [S. Liu, P. W. Ayers, and R. G. Parr, J. Chem. Phys. 111, 6927 (1999); A. Görling, Phys. Rev. Lett. 83, 5459 (1999)] have stated that ∫∇2νxc(r)dr=4π, where νxc(r) is the exchange-correlation potential of density functional theory. Here, we derive this sum rule and related rules such as ∫∇2νx(r)dr=4π and ∫∇2νc(r)dr=0, where νx(r) and νc(r) are the exchange and correlation components of νxc(r). Using similar methods, we derive the sum rule for the “screening” portion of the exchange-correlation potential and also “generalized” sum rules for νc(r) and the “response” portion of the exchange-correlation potential, vxcresponse(r). From the sum rule for vxcresponse(r), we deduce the asymptotic decay of the density response of the hole-correlation function. We conclude by discussing the probable utility of these results for the development of new exchange-correlation functionals.
A two magnetron sputter deposition chamber for in situ observation of thin film growth by synchrotron radiation scattering
Authors: Matz W. , Schell N. , Neumann W. , Bøttiger J. , Chevallier J.
Published: July 2002

The design of a sputter deposition chamber for the in situ study of film growth by synchrotron x-ray diffraction and reflectivity is reported. Four x-ray windows, sealed with low cost, nonhazardous Kapton, enable scattering both in the horizontal as well as in the vertical scattering planes. The chamber fits into a standard six-circle goniometer from Huber which is relatively widespread in synchrotron laboratories. Two miniature magnetron and additional gas inlets allow for the deposition of compound films or multilayers. Substrate heating up to 650 °C and different substrate bias voltage are possible. The performance of the chamber was tested with the deposition of high quality TiN films of different thicknesses.
Configurational properties of a single semiflexible polyelectrolyte
Authors: Ghosh K. , Carri Gustavo A. , Muthukumar M.
Published: July 2002

Using a variational calculation, we have considered the effect of chain length, intrinsic backbone stiffness, solvent quality, and salt concentration on the behavior of a single semiflexible polyelectrolyte in dilute solution. Explicitly, we have calculated the radius of gyration (Rg) and effective persistence length for different solvent qualities and salt concentrations. For an isolated semiflexible polyelectrolyte with increasing molecular weight, there can be five regimes with effective exponent ν (defining the molecular weight dependence of Rg) being 1, 1/2, 1, 2/5, and 1/2 in the absence of nonelectrostatic excluded volume interaction. This suggests a double crossover behavior from rodlike to Gaussian and then to Gaussian again as the chain length is increased. During the second crossover, ν can be as high as 1, although the actual value of Rg is order of magnitude smaller than the rodlike value. There can be another regime in this second crossover where the apparent exponent is 2/5 due to additional self-screening arising from counterions of the polymer. This self-screening can significantly reduce Rg, although the asymptotic exponent 2/5 may not be observable due to physical constraints. A thorough analysis of the crossover behavior is presented.
Filled liquid crystal depolarizers
Authors: Diorio Nicholas J. , Fisch Michael R. , West John L.
Published: July 2002

The construction and properties of scattering type liquid crystal depolarizers are discussed. The Mueller matrices that describe these devices are measured and compared to ideal depolarizers and ideal linear materials. The present depolarizers do not perform as well as crystal pseudodepolarizers. Nevertheless, when polarized light is incident on one of these devices the polarization of the transmitted light can be uniform to within 5%. These devices have the advantage that they can be made to areas as large as 30–30 cm or larger, are inexpensive, and can be electrically switched off and on. © 2001 American Institute of Physics.
X-ray monochromator with an energy resolution of 8×10−9 at 14.41 keV
Authors: Yabashi Makina , Tamasaku Kenji , Kikuta Seishi , Ishikawa Tetsuya
Published: July 2002

An ultrahigh-resolution x-ray crystal monochromator providing a 120 μeV bandwidth at 14.41 keV is presented. The design, which uses four independent silicon crystals and has an output beam parallel to the incident beam, may be generalized to arbitrary energies. Fluxes of 1.3×106 photons/s (1.0×107 photons/s) in bandwidths of 120±15 μeV (140±15 μeV) were measured. The performance of the monochromator, including the preservation of coherence through it, was verified by measurement of a 9.6±2.0% enhancement in the coincidence rate (i.e., γ(2)−1=0.096±0.020) in an intensity correlation experiment.
Anchoring the water dimer potential energy surface with explicitly correlated computations and focal point analyses
Authors: Tschumper Gregory S. , Leininger Matthew L. , Hoffman Brian C. , Valeev Edward F. , Schaefer Henry F. , Quack Martin
Published: July 2002

Ten stationary points on the water dimer potential energy surface have been characterized with the coupled-cluster technique which includes all single and double excitations as well as a perturbative approximation of triple excitations [CCSD(T)]. Using a triple-ζ basis set with two sets of polarization functions augmented with higher angular momentum and diffuse functions [TZ2P(f,d)+dif], the fully optimized geometries and harmonic vibrational frequencies of these ten stationary points were determined at the CCSD(T) theoretical level. In agreement with other ab initio investigations, only one of these ten stationary points is a true minimum. Of the other nine structures, three are transition structures, and the remaining are higher order saddle points. These high-level ab initio results indicate that the lowest lying transition state involved in hydrogen interchange is chiral, of C1 symmetry rather than Cs as suggested by recently developed 6D potential energy surfaces. The one- and n-particle limits of the electronic energies of these ten stationary points were probed by systematic variation of the atomic orbital basis sets and the treatment of electron correlation within the framework of the focal-point analysis of Allen and co-workers. The one-particle limit was approached via extrapolation of electronic energies computed with the augmented correlation consistent basis sets (aug-cc-pVXZ, X=D−6), and, independently, by estimating the basis set incompleteness effect with the explicitly-correlated second-order Møller-Plesset method (MP2-R12). Electron correlation was evaluated at levels as high as the Brueckner coupled cluster method with double excitations and perturbatively treated triple and quadruple excitations [BD(TQ)]. Core correlation and relativistic effects were also assessed. Consideration of the aforementioned electronic effects as well as basis set superposition error leads to an estimate of 21.0 kJ mol−1 for the electronic dissociation energy of (H2O)2.
Inhomogeneous line shape theory of electronic transitions for molecules embedded in superfluid helium droplets
Authors: Dick Bernhard , Slenczka Alkwin
Published: July 2002

A model is developed for the calculation of the inhomogeneous line shapes of electronic transitions of molecules embedded in superfluid liquid helium droplets. This model takes account of the cluster size distribution, and the size dependence of the capture cross section and the resonance frequency. Fits to experimental spectra yield very good agreement and reasonable values for the model parameters.
Low-frequency noise in high-transition-temperature superconducting multilayer magnetometers in ambient magnetic fields
Authors: Cho Hsiao-Mei , Andresen Y. T. , Clarke John , DiIorio M. S. , Yang K.-Y. , Yoshizumi S.
Published: July 2002

Each magnetometer consisted of a dc superconducting quantum interference device (SQUID), with the YBa2Cu3O7−x washer patterned into 4 μm lines, coupled to a multiturn, multilayer flux transformer containing flux dams. The noise at 1 Hz did not increase when bare SQUIDs or magnetometers were cooled and operated in fields up to values well above the magnetic field of the earth. When the magnetic field was changed, the noise in a bare SQUID was constant up to a threshold field of 12 μT. The addition of a flux transformer containing flux dams increased the magnetic field sensitivity by a factor of 43, while reducing the threshold field only moderately, to 5 μT.