bluuebunny/crossref_metadata_2025 huggingface dataset:
Electro-optic sampling of surface fields
Authors: Eesley Gary L.
Published: July 2002

We demonstrate the use of time-resolved electro-optic sampling to externally monitor the dynamics of carrier heating and transport in semiconductors. This technique can be used to follow the time evolution of surface electric fields which result from the ultrafast optical generation of excess electron-hole pairs.
Cold neutron incoherent scattering for hydrogen detection in industrial materials
Authors: Kvardakov V. V. , Chen-Mayer H. H. , Mildner D. F. R. , Somenkov V. A.
Published: July 2002

Neutron incoherent scattering (NIS) is proposed as a method for the rapid detection of hydrogen and corrosion in industrial materials. As an example, we apply the NIS method to the detection of hydrogen in graphite and in titanium alloy, and compare the results with those obtained with the prompt gamma activation analysis (PGAA) method. The scattering cross section for hydrogen is much greater than the capture cross section, which enables the NIS method to have a detection limit and accuracy close to that for PGAA, and allows real time experiments on hydrogen detection to be performed. We report preliminary results on using the NIS method to determine hydrogen in urea [CO(NH2)2]/graphite and in titanium matrices.
The optical absorption edge of diamond-like carbon: A quantum well model
Authors: Pelton Matthew , O’Leary Stephen K. , Gaspari Franco , Zukotynski Stefan
Published: July 2002

This article presents an analysis of the optical absorption edge of diamond-like carbon, based on transitions between confined electronic states in quantum wells. This theory is proposed to replace the commonly-used Tauc and Urbach expressions. It uses the cluster model of sample structure: the wells correspond to islands of graphitic, sp2-bonded material embedded in a diamond-like, sp3-bonded skeleton. A percolation model is used to give the cluster size distribution. Near-edge optical absorption coefficients are determined approximately for macroscopic thin films that have low fractions of sp2 bonding. Experimental results provide preliminary confirmation of the approach.
Dark current in p-type quantum well structures
Authors: Petrov A. G. , Shik A.
Published: July 2002

The current from a p-type quantum well caused by the hole tunneling in applied electric field, is calculated theoretically for semiconductors with the valence band described by the Luttinger’s Hamiltonian. Although the ground state in a well is formed by heavy holes, at nonzero hole momenta, it contains the admixture of light hole states with a low effective mass and high probability of tunneling through the barrier. This fact increases the dark current by several orders of magnitude, as compared with a primitive theory assuming light and heavy holes as independent particles. The current–voltage characteristic for a dark current is calculated and its dependences on quantum well parameters and doping level are discussed.
Approach to the magnetic saturation in nanocrystalline ferromagnets in the random anisotropy model
Authors: Aranda G. R. , González J. , Kulakowski K.
Published: July 2002

Recent experimental data on the magnetization curve M(H) of magnetically soft nanocrystalline Fe73.5Cu1Nb3Si13.5B9 sample are analyzed in terms of the random anisotropy model. Numerical calculations of magnetization are supplemented by a high-field expansion. Near the saturation we get M(H)/Ms=1−4〈K〉2/(15Ms2H2), where 〈K〉 is the averaged anisotropy of crystalline grains, contained in large magnetic domains. Section I is an introduction. In Sec. II, reference data on the model parameters of the nanocrystalline system Fe73.5Cu1Nb3Si13.5B9 are collected in order to justify the model approximations and to prepare the comparison with experimental results. In the two subsequent sections, the results of the calculations are described and discussed.
Magnetic moment, thermal and electrical transport in the inverse Mott systems BaCo0.9Ni0.1S2−y and CuIr2S4
Authors: Kang H. , Mandal P. , Medvedeva I. V. , Liebe J. , Rao G. H. , Bärner K. , Poddar A. , Gmelin E.
Published: July 2002

The antiferromagnetic BaCo0.9Ni0.1S2−y and the paramagnetic CuIr2S4 Mott systems show an inverse conductivity behavior, i.e., the metallic phase occurs at low or high temperatures, respectively, which also suggests inverse behavior in other transport properties. In this contribution we present magnetization data, conductivity, thermal conductivity, and Seebeck effect in the temperature range 4.2 K<T<350 K and in particular through the metal-insulator transition at TMI. We recover the anomalies in χ(T),σ(T), which are discussed using Jahn-Teller and anti-Jahn-Teller models; also, we compare the anomalies in σ(T), S(T), and of the heat conductivity κ(T) at TMI. In particular, for BaCo0.9Ni0.1S1.95, we find a step in κ of almost one order of magnitude (times 6) and a large pressure shift, ⩽−24 K/kbar, which makes this material a potential pressure activated heat switch, while CuIr2S4 shows a small step of κ (times 1.2) and a pressure shift of +2.8 K/kbar.
A numerical study of shock-induced particle velocity dispersion in solid mixtures
Authors: Yano K. , Horie Y.
Published: July 2002

Shock-induced particle velocity dispersion in solid mixtures was numerically investigated using two approaches: discrete element simulation and continuum mixture calculation. Results show (i) a trend-wise agreement between the two models, (ii) nonequilibrium distributions of particle velocity dispersion, and (iii) particle velocity dispersions of 20–100 m/s for a 10 GPa shock wave in Ni/Al mixtures and 5–70 m/s for a 5 GPa shock wave in Ti/Teflon mixtures. Particle velocity dispersions of this magnitude are thought to be the driving mechanism for initiation of chemical reactions in reactive solid mixtures.
Anomalous Hall conductivity and magnetoresistance in epitaxial La0.67Ca0.33MnO3−δ thin films
Authors: Li Kebin , Cheng Rongsheng , Chen Zhixiang , Fang Jun , Cao Xiaowen , Zhang Yuheng
Published: July 2002

The longitudinal magnetoresistance ρxx and Hall resistivity in-plane ρxy were simultaneously measured on an epitaxial La0.67Ca0.33MnO3−δ thin film as a function of temperature at several magnetic fields. The zero-field resistivity ρxx reaches a maximum at Tp=225 K which is very close to the Curie temperature Tc=221 K determined by linearly extrapolating the reciprocal of the magnetization [M−1(T)] data to zero. The amplitude of the ρxx peak is reduced while the position of Tp moves to higher temperatures as the external magnetic field is increased. The measurements of the Hall effect show that the carrier is holelike, and the carrier density is about 0.004 per unit cell above TC, which corresponds to the contribution of 1% of holes of Mn4+ ions. ρxy depends strongly on temperature and magnetic field. An extraordinary ρxy peak in the vicinity of ρxx peak TP and Hall sign reversal in the temperature range of 210–240 K have been observed. These anomalous behaviors of Hall effect are discussed in the frame of skew-scattering theory.
Engineering the strain field for the control of quantum confinement: An analytical model for arbitrary shape nanostructures
Authors: Mazzer M. , De Giorgi M. , Cingolani R. , Porello G. , Rossi F. , Molinari E.
Published: July 2002

We describe an analytical method to calculate the strain field and the corresponding band gap modulation induced in a quantum well by a surface stressor of arbitrary shape. In this way, it is possible to engineer the confinement potential of different strained nanostructures based on patterned heterojunctions. Band gap modulations up to 130–140 meV are predicted for suitably designed II–VI/III–V and III–V/III–V heterostructures.
Photogeneration of charge carriers in anisotropic multilayer structures of phthalocyaninato-polysiloxane
Authors: Gattinger P. , Rengel H. , Neher D.
Published: July 2002

The photoinduced generation and the transport of charge carriers have been investigated in sandwich structures of Langmuir–Blogdett films of phthalocyaninato-polysiloxane between aluminum and gold electrodes. The diodelike current–voltage characteristics as well as the photovoltaic properties are directly related to the asymmetric electrode configuration. Under short-circuit conditions the photoactive region is located close to the aluminum electrode. By applying an external bias, the photoactive region is enlarged throughout the whole film thickness, and collection efficiencies of up to 0.3% are observed. Recombination effects have been studied by investigating the intensity dependence of the photocurrent. Nearly square root behavior under short circuit conditions is due to efficient charge carrier recombination whereas a linear intensity dependence is observed under reverse bias, indicating charge carrier diffusion to the electrodes. The transition between these two regimes is analyzed with respect to the charge carrier mobility yielding a value as small as 10−11 cm2/V s. The anisotrope molecular orientation in the film causes a photoconductivity clearly depending on the polarization of the incident light. The formation of a microcavity in these structures strongly influences the spectral properties and it allows, in agreement with theoretical calculations, to obtain an improved efficiency in a selected region of low film absorption. Atmospheric oxygen doping results in clear increase in the dark current, but the charge generation process remains nearly unaffected.
Influence of the real shape of a sample on the pinning induced magnetostriction
Authors: Nabiałek A. , Szymczak H. , Sirenko V. A. , D’yachenko A. I.
Published: July 2002

The pinning induced magnetostriction in an isotropic superconductor was calculated for different sample shapes. We analyzed some special shapes for which the solution can be found analytically. The magnetostriction of a finite slab was considered. In order to determine the influence of demagnetization effects, the pinning induced magnetostriction of an infinitely long and thin strip was calculated. A simple, approximate formula can be used in particular cases to analyze the magnetostriction induced by pinning forces. In this formula the magnetostriction of a sample is connected directly with its magnetization. We also present some experimental results on high temperature superconductors which are analyzed in frames of the developed theory.
Low-temperature metalorganic vapor phase epitaxial growth of ZnS using diethyldisulphide as a sulphur precursor
Authors: Prete P. , Lovergine N. , Cannoletta D. , Mancini A. M. , Mele G. , Vasapollo G.
Published: July 2002

The atmospheric pressure metalorganic vapor phase epitaxy (MOVPE) growth of ZnS using diethyldisulphide (Et2S2) as a sulphur precursor in combination with dimethylzinc:triethylammine adduct is reported. Et2S2 is used to achieve low-temperature (<400 °C) growth of ZnS by pyrolytic MOVPE. The substitution of diethylsulphide (Et2S) with Et2S2 lowers the temperature of the process by almost 150 °C, leading to growth rates in excess of 0.5 μm/h at around 400 °C. The growth study is supplemented by the H1 and C13 nuclear magnetic resonance analysis of the Et2S2 batch used and by the results of mass spectrometry. The latter measurements allowed us to study the fragmentation path of the Et2S2+ ion. The ∼150 °C reduction of the ZnS growth temperature is explained as being due to the weaker S–C bonds occurring in the Et2S2 molecule with respect to those of Et2S; this is ascribed to the occurrence of the stronger S–S bond, which destabilizes the S–C bonds.
Second-harmonic generation in nonlinear grating coupler
Authors: Yokota Mitsuhiro , Takaishi Sin’ichi , Yamane Jun’ichi
Published: July 2002

Second-harmonic (SH) wave in the grating coupler is investigated by using the singular perturbation technique with multiple space scales. In order to apply the perturbation technique, we first introduce the perturbation parameter concerned with the nonlinear coefficient and the depth of the grating, and the multiple space scales in the propagation direction. Substituting the expansion of the field and the multiple space scales into the nonlinear wave equation, we can get the governing equations to each order of the perturbation. From the solvability conditions to have nontrivial solutions for each perturbation solution, that coupled equations for the guided wave, incident wave, and radiated wave can be shown. Also, the radiated angle of fundamental and SH waves and the radiated SH wave for input/output couplers are examined numerically.
Near interface oxide trap capture kinetics in metal-oxide-semiconductor transistors: Modeling and measurements
Authors: Bauza D.
Published: July 2002

The traps situated in the oxide in the vicinity of the Si–SiO2 interface in metal-oxide-semiconductor (MOS) transistors, are studied. This is achieved using a new technique, based on the measurement of drain current transients and called T-CDLTS (tunnel-current deep level transient spectroscopy). For this, the traps are repeatedly filled with majority carriers using gate pulses which bias the device in accumulation. Each time the device returns in inversion, the drain current transient induced by the filling of the traps with minority carriers is monitored. A model for extracting the interface trap depth concentration profiles from the current transients is derived. It is based on Shockley–Read–Hall’s statistics [R. N. Hall, Phys. Rev. 87, 387 (1952), W. Shockley and W. T. Read, Phys. Rev. 87, 835 (1952)] and on the Heiman and Warfield tunneling model [F. P. Heiman and G. Warfield, IEEE Trans. Electron Devices ED-12, 167 (1965)]. The slow trap densities measured in a virgin device agree with those obtained in state-of-the-art MOS transistors using noise measurements. In virgin and stressed devices they also compare favorably with the trap densities obtained using a recently proposed charge pumping technique. The evolution, with the experimental conditions, of the trap concentrations measured is discussed with respect to that expected from the model. In some experimental conditions, a very good agreement is obtained while in some others, discrepancies are observed. These discrepancies are discussed as regard to the hypothesis introduced in the model.
Influence of defects on electron emission from diamond films
Authors: Show Yoshiyuki , Matsuoka Fumihiko , Hayashi Masaharu , Ito Hirokazu , Iwase Mitsuo , Izumi Tomio
Published: July 2002

The correlation between paramagnetic defects and the electron emission in diamond films, which were deposited by the chemical vapor deposition method, has been studied using electron-spin-resonance (ESR) and field-emission measurements. The paramagnetic defects, which are a carbon dangling bond in the diamond layer (Pdia-center: g=2.003, ΔHPP=3 Oe) and a carbon dangling bond in the nondiamond phase carbon region (Pac-center: g=2.003, ΔHPP=8 Oe), exist in the diamond films. Electron emission with high current density was observed for the diamond film, which contains high spin densities for both ESR centers, because electrons are efficiently transported to the diamond surface through the defect-induced energy band(s) by hopping conduction.
Intensity-dependent photobleaching through bulk oxide glass containing silver particles
Authors: Watanabe Yuichi , Inoue Motohiro , Tsuchiya Toshio
Published: July 2002

We report on intensity-dependent photobleaching observed fully along the laser-pulse-propagating path as long as a few mm in bulk oxide glass containing colloidal silver particles. A likely mechanism for the photosensitivity is proposed in terms of two-photon ionization of silver particles in the glass, accompanied with spontaneous formation of defect species. We demonstrate that this effect can be utilized for introducing three-dimensionally integrated optical structures within bulk glasses.
Interference effects in differential reflectance spectra of the GaAs epilayers grown on Si substrate
Authors: Zhao Mingshan , Dai Zuoxiao , Li Guohua , Wang Ruozhen , Jiang Desheng
Published: July 2002

We report the observation of oscillating features in differential reflectance spectra from the GaAs epilayer grown on Si substrate in the energy range both below and above the fundamental band gap. It is demonstrated that the oscillating features are due to the difference in the interference between two neighboring areas of the sample. The interference arises from two light beams reflected from different interfaces of the sample. The calculated spectra in the nonabsorption region are in good agreement with measured data. It is shown that the interference effect can be used as a sensitive method to characterize the inhomogeneity of the semiconductor heterostructures.
Effect of cylindrical geometry on the wet thermal oxidation of AlAs
Authors: Alonzo A. C. , Cheng X. -C. , McGill T. C.
Published: July 2002

We have investigated the wet thermal oxidation of AlAs in cylindrical geometry, a typical configuration for vertical-cavity surface-emitting lasers. Through both experiment and theoretical calculations, we demonstrate a significantly different time dependence for circular mesas from what has been reported in the literature both in studies of stripes and in a study of circular mesas. We attribute this different time dependence to the effect of geometry on the oxidation.
Evolution of a two-dimensional quantum cellular neural network driven by an external field
Authors: Qiao Bi , Ruda Harry E.
Published: July 2002

A model of a two-dimensional quantum cellular neural network (QCNN) is presented in this article. The eigenvalues and eigenvectors for the Hamiltonian of a cell (neuron) are obtained, and we confirm that the ground or memory states are approximately two polarization states of 16 possible states in a cell (neuron) only when electron tunneling is relatively weak compared with the Coulomb repulsion. The evolution of the QCNN driven by a local external magnetic field is studied by solving the Liouville equation of the corresponding two-dimensional Ising model. The formula for the evolution of the density operator is given by using a subdynamics approach. We show that the local external magnetic field can drive the system to a global polarization state and induce a dynamical response in the original QCNN. This dynamical response can be interpreted as a computable function and measured by the system output.
Hard magnetic properties and microstructure of melt-spun Sm2Fe15−xCuxGa2C (x=0 and 0.5) ribbons
Authors: Zhang Shao-ying , Zhang Hong-wei , Shen Bao-gen , Zhan Wen-shan , Chin Tsung-shune , de Boer F. R. , Buschow K. H. J.
Published: July 2002

The structure, magnetic properties and microstructure of Sm2Fe15−xCuxGa2C (x=0 and 0.5) ribbons prepared by melt spinning and subsequent heat treatment were studied. A significant increase in coercivity has been found for ribbons prepared with the addition of a small amount of Cu. The maximum coercivity μ0Hc is 1.4 and 2.0 T at room temperature for ribbons with x=0 and 0.5, respectively, after annealing in vacuum at 1043 K for 15 min. A small amount of Cu addition has little effect on the lattice parameters, Curie temperature TC, and magnetocrystalline anisotropy field HA at room temperature, while it decreases the saturation magnetization Ms. The magnetocrystalline anisotropy constants K1 and K2 for the x=0 and 0.5 samples were determined in the temperature range between 50 and 293 K, using magnetically aligned samples. The variations of coercivity, magnetization, and remanence as functions of applied field and temperature are consistent with the view that the magnetization of Sm2Fe15−xCuxGa2C ribbons with x=0 as well as 0.5 are mainly controlled by pinning of domain walls. High-resolution transmission electron microscopy and energy-dispersive x-ray analysis demonstrate that Cu atoms dissolve partially into the 2:17-type matrix phase in Sm2Fe14.5Cu0.5Ga2C ribbons, which is effective in pinning the domain walls and enhancing the coercivity.
Dislocation mediated surface morphology of GaN
Authors: Heying B. , Tarsa E. J. , Elsass C. R. , Fini P. , DenBaars S. P. , Speck J. S.
Published: July 2002

The surfaces of GaN films grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) were studied using atomic force microscopy (AFM). Due to the high dislocation densities in the films (108 cm−2), the typical surface morphologies of layers grown by both techniques were dominated by three dislocation mediated surface structures—pinned steps, spiral hillocks, and surface depressions. The characteristics of these surface structures were found to depend on growth technique (MOCVD vs MBE) and the group-III to group-V ratio used in the growth of MBE GaN films. Pinned steps, created by the intersections of mixed character dislocations with the free surface, were found on all GaN films. The pinned steps were observed to be predominantly straight on the MOCVD GaN and curved into spiral hillock formations on the MBE GaN. Spiral growth hillocks form when pinned steps grow outward and around the dislocation under step-flow growth conditions. The tightness of the spiral hillocks on MBE GaN surfaces was found to increases with III/V ratio. Surface depressions, caused by the high strain-energy density near dislocations, were also observed on the surfaces of the GaN films. Two characteristic depression sizes were found on all MOCVD GaN films whereas depressions were observed only on MBE GaN films grown with low III/V ratios. These observations are explained using theories developed by Burton, Cabrera, and Frank [Philos. Trans. R. Soc. London, Ser. A 243, 299 (1951)] and Frank [Acta Crystallogr. 4, 497 (1951)].
Transients computation in electrical circuits in the presence of ferromagnetic hysteresis
Authors: Machado V. Maló , Ribeiro A. Lopes
Published: July 2002

This article is a contribution to the evaluation of transients in nonlinear inductive circuits containing a soft ferromagnetic sample in the magnetic circuit where hysteresis and eddy currents are taken into account. A hybrid finite element method (FEM)/boundary method is used to evaluate the instantaneous magnetic field. The FEM formulation is adequate to describe the field inside the ferromagnetic sample and, on the other hand, the boundary method is used to impose the adequate relation between the field and its normal derivative values on the boundary. This relationship is obtained from the exterior circuit equation. The dynamic field is evaluated in the time domain for which an implicit integration approach is used based on a second order trapezoidal (Crank–Nicolson) method. A classical Preisach model is used to simulate the magnetic hysteresis.
Micromagnetic analysis of remanence and coercivity of nanocrystalline Pr–Fe–B magnets
Authors: Suess D. , Dahlgren M. , Schrefl T. , Grössinger R. , Fidler J.
Published: July 2002

Nanocrystalline exchange coupled Pr11.76Fe82.36B5.88 single-phase and Pr9Fe85B6 two-phase magnets containing 20 vol % α-Fe were produced using mechanical alloying. Micromagnetic finite element simulations were used to analyze the temperature dependence of the magnetic properties. In the single-phase exchange-coupled magnet a significant enhancement of the remanence Jr=1.1 T was achieved at room temperature. A further enhancement of the remanence was observed in two-phase α-Fe containing magnets reaching a remanence of Jr=1.23 T. The corresponding values of the coercive field are μ0Hc=0.81 T and μ0Hc=0.57 T for the single-phase magnet and the two-phase magnet, respectively. Remanence enhancement becomes more effective with increasing temperature, compensating the decrease of the saturation polarization. In the two-phase magnet the remanence increases from Jr=1.12 to Jr=1.23 T as the temperature is increased from 200 to 300 K. Micromagnetic calculations clearly show that the increase of the exchange length with increasing temperature improves the effective coupling between the Pr2Fe14B and the α-Fe phase. The decrease of the coercive field with increasing temperature has to be attributed to the temperature dependence of the anisotropy field.
Symmetry and hyperfine fields in R2Fe17−xVxCy (R=Y, Gd; x=1, 1.5, 2 and
y
=0,1) intermetallics
Authors: Sorescu Monica , Valeanu Mihaela
Published: July 2002

Magnetic compounds of the type R2Fe17−xVx (R=Y, Gd and x=1, 1.5, 2) were synthesized by arc melting (with or without interstitial carbon) and studied by x-ray diffraction (XRD), Mössbauer spectroscopy, and hysteresis loop measurements. Compounds with the structure Y2Fe17−xVx (x=1, 1.5, 2) were found to be hexagonal, whereas the intermetallics containing carbon introduced by melting and those of the type Gd2Fe17−xVx (x=1, 1.5, 2) were found to be rhombohedral. For each symmetry, the lattice parameters a/c were obtained from the XRD spectra and a considerable expansion of the unit cell volume was observed in the Y2Fe17−xVxC (x=1, 1.5, 2) systems as effect of carbon additions. Room-temperature transmission Mössbauer spectra were analyzed considering the various inequivalent iron sites and yielded the average hyperfine magnetic field. This field was studied as a function of vanadium content x for all classes of compounds under investigation. The maximum hyperfine field values were obtained for the carbides. The hysteresis loops recorded at 4.2 K in an applied field of 1.5 T provided complementary information on the saturation magnetization as a function of vanadium substitution for each group of intermetallics involved.
The effects of YBa2Cu3O7−δ overlayers on the magnetic and electronic properties of La0.7Sr0.3MnO3
Authors: Stadler S. , Idzerda Y. U. , Chen Z. , Ogale S. B. , Venkatesan T.
Published: July 2002

The effects of YBa2Cu3O7−δ (YBCO) cap layers on the magnetic and electronic properties of La0.7Sr0.3MnO3 (LSMO) underlayers was determined by x-ray magnetic circular dichroism and x-ray absorption spectroscopy techniques. The YBCO caps alter the LSMO in a way that effectively decreases the concentration of La at the LSMO/YBCO interface.
X-ray absorption spectroscopy study of the instability of ferromagnetism in CeFe2: Effects of Co and Al substitutions
Authors: Chaboy Jesús , Piquer Cristina , García Luis M. , Bartolomé Fernando , Wada Hirofumi , Maruyama Hiroshi , Kawamura Naomi
Published: July 2002

We present in this work an x-ray absorption spectroscopy (XAS) investigation performed at the L3-edge of Ce in the intermetallic compounds Ce(Fe1−xCox)2 and Ce(Fe1−xAlx)2. The mixed-valence behavior of Ce is found to be preserved in both series upon Fe substitution. This result rules out the development of a localized 4f magnetic moment at Ce atoms driven by the Fe–Co and Fe–Al substitution at the origin of the anomalous magnetic behavior of this series.
Carrier charge imbalance and optical properties of separate confinement heterostructure quantum well lasers
Authors: Tolstikhin Valery I.
Published: July 2002

Carrier charge imbalance and self-consistent potential effects are systematically studied regarding the optical properties of separate confinement quantum well lasers. Numerical modeling of the typical InP-based laser structure shows that violating the quasineutrality in the vicinity of quantum wells results in a dramatic change in a fraction of carriers trapped in the lasing subbands, and a corresponding modification of laser waveguide properties.
Mechanistic study of metalorganic chemical vapor deposition of (Ba,Sr)TiO3 thin films
Authors: Gao Y. , Perkins C. L. , He S. , Alluri P. , Tran T. , Thevuthasan S. , Henderson M. A.
Published: July 2002

The metalorganic precursor chemistry was studied on Pt(111) surfaces in a O162 and O182 backgrounds. Using temperature programmed desorption (TPD) and static secondary ion mass spectrometry (SIMS). The precursor chemistry of Sr(thd)2 was found to be different on oxide covered Pt(111) surface as compared to the clean Pt(111) surface. In an oxygen ambient, TPD showed at least four different reaction processes which involved the removal of carbon from the precursor ligands on oxide covered Pt(111). In two of these, gas phase oxygen was incorporated into the oxidative products. In contrast, one carbon removing reaction was observed on the clean Pt(111) surface. Isotopic labeling experiments have also been carried out to understand the film-formation reactions in the metalorganic chemical vapor deposition of (Ba,Sr)TiO3 (BST) films. Time-of-flight SIMS and nuclear reaction analysis reveal that the oxygen in the BST films originates from both the gas phase oxidants (18O) and the precursor ligands (16O). The ligand substitution by gas phase O2 plays a more prominent role in the film formation at lower temperatures. On the other hand, the reactive oxygen radicals produced by microwave plasma involved more in breaking the O–C bonds than substituting the precursor ligands for the film formation. Use of the 50% O182–50% N2 16O2 mixture results in a reduction of O18 incorporation in the BST films, indicative of the direct involvement of N2O in the film-formation reactions. The mechanistic studies are essential for understanding the new BST precursors used in this study, and provide useful information to correlate the film microstructure, step coverage, and dielectric properties with the precursor properties.
Role of cerium in lithium niobate for holographic recording
Authors: Yue Xuefeng , Adibi A. , Hudson T. , Buse K. , Psaltis D.
Published: July 2002

Cerium-doped lithium niobate crystals are tested for holographic recording. A photochromic effect is observed in crystals doped with cerium and manganese. But two-center recording in the sample is not as effective as in iron and manganese doubly doped crystals. Photocurrent measurements in cerium and iron singly doped crystals indicate that the photovoltaic constant in the cerium-doped crystal is only one third of that of the iron-doped one. This is the main reason accounting for the low sensitivity of cerium-doped lithium niobate crystals. However, in the diffusion dominated case, i.e., for reflection geometry, cerium-doped lithium niobate may give a strong effect.
Observation of resonant tunneling through single self-assembled InAs quantum dots using electrophotoluminescence spectroscopy
Authors: Ohno Y. , Asaoka K. , Kishimoto S. , Maezawa K. , Mizutani T.
Published: July 2002

The resonant tunneling through single InAs quantum dots embedded in an n-GaAs/i-Al0.38Ga0.62As/n-GaAs diode has been studied by using microscopic electrophotoluminescence spectroscopy. Many sharp luminescence lines which originated from single quantum dots were observed by injecting resonant electrons from the emitter to the dots. Bias dependence of a single luminescence line was investigated. The peak intensity showed triangular dependence which was similar to the current–voltage characteristics of electron resonant tunneling from three dimension to zero dimension. When the bias voltage was increased, the peak energy slightly shifted to a lower energy indicating the existence of Stark effect, and the linewidth slightly increased. The higher the luminescence energy was, the broader the linewidth was. This result agrees with the calculated resonant level width. The lifetime of resonant states was estimated to be 2.4–27 ps for luminescence linewidth of 250–22 μeV.
Surface roughness in sputtered SnO2 films studied by atomic force microscopy and spectroscopic light scattering
Authors: Lindström Tomas , Isidorsson Jan , Niklasson Gunnar A.
Published: July 2002

In this paper we study the growth of surface roughness during sputtering of transparent SnO2 films onto rough glass substrates. Films in the thickness range 50–1200 nm were produced, and the optical characterization was made with a spectroscopic total integrating scattering instrument in the wavelength range 0.35<λ<1.0 μm. Optical constants for the different SnO2 films were determined. The observed spectral behavior of the diffuse reflectance (transmittance), as compared to the total reflectance (transmittance), could be explained by first-order vector perturbation theory in conjunction with a surface growth model incorporating both smoothing and roughening effects. Good agreement between calculated and measured reflectance and transmittance spectra was found, only by assuming partially correlated interface roughness. The scattering calculations rely on atomic force microscope measurements of the glass substrate and the front surfaces of the films, and a model of the cross correlation that describes the propagation of surface features through the new layer. The obtained replication factor gave additional information on the contribution of the substrate roughness to the film front surfaces.
Local order determination in SiCN(AlY) laser-synthesized nanopowders by x-ray photoemission spectroscopy
Authors: Ténégal F. , de la Rocque A. Gheorghiu , Dufour G. , Sénémaud C. , Mayne M. , Herlin-Boime N. , Armand X. , Cauchetier M.
Published: July 2002

Chemical bonding in SiCNAlY(O) nanometric powders was investigated by x-ray photoemission spectroscopy. The powders were prepared by laser pyrolysis of binary mixtures of liquid hexamethyldisilazane and disolved solid isopropoxydes. As-prepared powders are amorphous and the atomic proportion of aluminum and yttrium do not exceed 1 at. %. They were annealed at 1600 °C under N2 atmosphere and we followed the evolution of the bonding configuration with annealing treatment for the composition with a C/N atomic ratio equal to 0.78. Two other compositions were studied (powders annealed at 1600 °C): a carbon rich powder (C/N=5.05) and a nitrogen rich powder (C/N=0.09). The strong increase of Al 2p and Y 3d peak intensities relatively to Si 2p peak intensity with annealing underlines an increase of the Al and Y content in superficial layers after annealing. The analysis of the chemical shift of Al 2p core level after annealing underline the appearance of Al–N bonds at 1600 °C. The chemical environment of Y atoms in the nanopowders is quite the same to that found in a vitreous YSiAlON phase. The study of Si 2p, N 1s, and C 1s core levels revealed that Al and Y atoms do not change the bonding configuration in the powders. C–N bonds are detected in the powders and their stability through annealing treatment is underlined.
Patterned diamond particle films
Authors: Fox N. A. , Youh M. J. , Steeds J. W. , Wang W. N.
Published: July 2002

This article reports on a technique for patterning diamond nanogrit which utilizes commercial ink-jet printer technology. Diamond nanogrit as small as 50 nm has been successfully printed onto substrates of glass, silicon, copper, and fused quartz. The technique has been used to demonstrate a quick and simple means to seed patterned. nanocrystalline diamond films onto candidate substrates of potentially any conceivable size or shape.
Two-dimensional acousto-electro-optic effect and device
Authors: Yu Kuanxin , He Shiya , Zhao Qida
Published: July 2002

In this article, a coupled wave equation of a two-dimensional acousto-electro-optic (AEO) effect is given. A formula of diffraction efficiency of the two-dimensional AEO effect is derived. A two-dimensional AEO device with KDP crystal is made. Curves of relative diffraction efficiency versus direct current voltage are shown. The measured results agree with theoretical calculation.
Comment on “Dominant diffusing species during cobalt silicide formation” [J. Appl. Phys. 79, 153 (1996)]
Authors: d’Heurle F. M. , Zhang S.-L.
Published: July 2002

The report that during the formations of the isomorphous CoSi2 and NiSi2 the dominant diffusing species are different is questioned. Not only does this contradict previous results obtained with inert gas markers, but it is based on experiments carried out with Ta markers that are not inert. The problem of using markers in determining the dominant diffusing species during thin film reactions is briefly analyzed.
A model for competitive binary and ternary ion–molecule reactions
Authors: Herbst Eric
Published: July 2002

A mechanism by which competitive binary and ternary ion–molecule reactions can occur is proposed. Calculations are undertaken for the specific system CH+3 +NH3+He which has been studied in the laboratory by Smith and Adams, and the potential surface of which has been studied theoretically by Nobes and Radom. It is shown that a potential energy barrier in the exit channel prevents the rapid dissociation of collision complexes with large amounts of angular momentum and thereby allows collisional stabilization of the complexes. The calculated ternary reaction rate coefficient is in good agreement with the experimental value but a plot of the effective two-body rate coefficient of the ternary channel vs helium density does not quite show the observed saturation.
The aperiodic crystal picture and free energy barriers in glasses
Authors: Hall Randall W. , Wolynes Peter G.
Published: July 2002

The aperiodic crystal picture associates the glass transition with freezing into a nonperiodic structure. Dynamics in the glassy state involves activated jumps between different aperiodic free energy minima. Activation barriers may be estimated through the use of freezing theory and the theory of dense solids. The results resemble, but are distinct from, free volume theory. Reasonable fits to experimental data are obtained.
Higher order interactions in molecular collisions
Authors: Kajita Masatoshi , Tachikawa Maki , Shimizu Tadao
Published: July 2002

The characteristics of three types of collisional interactions in molecular collisions (dipole–dipole, dipole–quadrupole, and dipole–induced dipole) have been studied by a novel laser spectroscopic method. The rotational relaxation rate constants of CH3CN against various kinds of collision partners are measured. It has been found that the relaxation rate constant critically depends on the energy level structure of collision partners for the long range interaction while it is less sensitive to the level structure for the short range interactions.
The overtone spectrum of acetylene in the vibron model
Authors: Hornos J. , Iachello F.
Published: July 2002

The overtone spectrum of acetylene including all bending and stretching vibrations up to 20 000 cm−1 is calculated within the framework of the vibron model. Properties of the local to normal transition are discussed. It is indicated how the method can be extended to even larger molecules.
Vibrational anharmonicity in CH3I: A joint local and normal mode study
Authors: Duncan J. L. , Ferguson A. M. , Mathews S.
Published: July 2002

The infrared spectrum of gaseous CH3I has been investigated in the range 500–16 500 cm−1, up to six quanta of excitation in CH stretching. A total of 16 perpendicular bands lying above 3700 cm−1 have been analyzed, some as Fermi resonance diads, in order to determine accurate vibrational wave numbers for the constituent levels. The local mode model has been applied to the CH stretching manifold to account for anharmonicity and the effects of Darling–Dennison resonances. Vibrations not involving CH stretching have been treated in a normal mode basis. The joint approach, which also takes into account known Fermi resonances, enables a complete set of 27 vibrational anharmonicity parameters to be determined for CH3I which fits 63 observed vibrational levels up to V=6 with a root-mean square (rms) error of 2.75 cm−1.
Femtosecond real-time probing of reactions. IV. The reactions of alkali halides
Authors: Rose Todd S. , Rosker Mark J. , Zewail Ahmed H.
Published: July 2002

The photodissociation dynamics of some alkali halides are explored via the method of femtosecond transition-state spectroscopy (FTS). The alkali halide dissociation reaction is influenced by the interaction between the covalent and the ground state ionic potential energy surfaces (PES), which cross at a certain internuclear separation. Depending upon the adiabaticity of the PES, the dissociating fragments may be trapped in a well formed by the avoided crossing of these surfaces. Here, we detail the FTS results of this class of reactions, with particular focus on the reaction of sodium iodide: NaI*→[Na---I]°* →Na+I. As in our first report [T. S. Rose, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 88, 6672 (1988)], we observe the dynamical motion of the wave packet along the reaction coordinate and the crossing between the covalent and ionic surfaces. The studies presented here characterize the effects of various experimental parameters, including pump and probe wavelengths, on the dynamics of the dissociation and its detection. Comparisons of the results with classical and quantum mechanical calculations are also presented.
Femtosecond transition-state absorption spectroscopy of Bi atoms produced by photodissociation of gaseous Bi2 molecules
Authors: Glownia J. H. , Misewich J. A. , Sorokin P. P.
Published: July 2002

Femtosecond transition-state absorption spectroscopy has been performed on Bi atoms produced by the 308 nm photodissociation of Bi2 molecules contained in bismuth vapor. The transient spectra obtained are all clearly identifiable as atomic, yet they display striking asymmetries in line shapes and enhancements in intensity that clearly demonstrate that they are spectral signatures of atoms still in the force fields of their receding partners.
Photodissociation dynamics of H2S at 121.6 nm and a determination of the potential energy function of SH(A 2Σ+)
Authors: Schnieder L. , Meier W. , Welge K. H. , Ashfold M. N. R. , Western C. M.
Published: July 2002

A new and improved version of the technique of H atom photofragment translational spectroscopy has been applied to a study of H2S photodissociation at 121.6 nm. The primary fragmentation pathways leading to H+SH(A) fragments and H+H+S(1D) atoms are observed to dominate the product yield; the yield of H atoms formed in conjunction with ground state SH(X) fragments is undetectably small. The majority of the SH(A) fragments are formed in their v=0 level with a rotational state population distribution that spans all possible bound and quasibound rotational levels. The experimental determination of the energies of these hitherto unobserved high rotational states has enabled a refinement of the SH(A) potential energy function, an improved estimate of the SH(A) well depth (9280±600 cm−1), and thus of the SH(X) ground state bond dissociation energy D00 (S–H)=3.71±0.07 eV. All aspects of the observed energy disposal in the title photodissociation process may be understood, qualitatively, if it is assumed that (i) the primary fragmentations occur on the B̃ 1A1 potential energy surface and (ii) Flouquet’s ab initio calculations of portions of this surface [Chem. Phys. 13, 257 (1976)] correctly predict its gross topological features.
Triple and quadruple excitation contributions to the binding in Be clusters: Calibration calculations on Be3
Authors: Watts John D. , Cernusak Ivan , Noga Jozef , Bartlett Rodney J. , Bauschlicher Charles W. , Lee Timothy J. , Rendell Alistair P. , Taylor Peter R.
Published: July 2002

We have investigated the contribution of connected triple and quadruple excitations to the binding in Be3 by comparing various coupled-cluster (CC) and truncated configuration-interaction (CI) treatments with multireference CI (MRCI) and full CI (FCI) calculations. The CC method with single and double excitations (CCSD) produces results that differ substantially from more elaborate treatments, but most extensions to CCSD that account approximately for connected triple excitations perform very well. In contrast, good agreement with FCI for Be2 can be achieved only with the highest level CC and MRCI methods.
2+1 resonantly enhanced multiphoton ionization of CO via the E 1Π–X 1Σ+ transition: From measured ion signals to quantitative population distributions
Authors: Hines Melissa A. , Michelsen Hope A. , Zare Richard N.
Published: July 2002

The 2+1 resonantly enhanced multiphoton ionization (REMPI) spectrum of the CO E 1Π–X 1Σ+ (0,0) transition is used to determine ground state rotational populations with a detection sensitivity of approximately 3×106 molecules per quantum state per cm3. Low rotational states of CO are ionized to CO+; however, high rotational states form both C+ and CO+. This effect is shown to be both branch dependent and J dependent. In order to extract reliable ground state populations, both the C+ and CO+ channels must be measured. When the C+ channel is not accounted for, high rotational states are systematically undercounted. New rotational constants are determined for the C12O16 E 1Π state; Bf0 is 1.9526 cm−1 and Be0 is 1.9645 cm−1. The large lambda doubling (q=0.0119 cm−1 ) of the E state is attributed to a perturbation by the nearby C 1Σ+ state.
A simple model for linear polyene electronic structure
Authors: Kohler Bryan E.
Published: July 2002

A simple model based on Huckel theory for interpreting linear polyene electronic structure is presented. The essential ingredients are the use of alternating resonance integrals to obtain the proper chain-length dependence of the 1 1Bu highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) excitation energy and mixing of the HOMO to LUMO+1 and HOMO to LUMO double-excitation configurations to properly describe the 2 1Ag state. With the proper choice of parameters, this model reproduces all 25 2 1Ag and 1 1Bu 0–0 excitation energies that have thus far been measured for linear polyenes in low-temperature hydrocarbon solutions.
Comparison of rate theories for generalized Langevin dynamics
Authors: Tucker Susan C. , Tuckerman Mark E. , Berne Bruce J. , Pollak Eli
Published: July 2002

Rate constants evaluated from (1) the energy-loss turnover theory of Pollak, Grabert, and Hänggi (PGH), (2) the Grote–Hynes extension of Kramers theory (GH), and (3) the microcanonical variational transition state theory for dissipative systems of Tucker and Pollak (μVTST) are compared with rate constants determined from direct computer simulations of generalized Langevin dynamics. The comparisons are made for a cubic oscillator under the influence of a slow bath characterized by a Gaussian friction kernel. In the μVTST calculations, which are based on an effective two degree of freedom Hamiltonian, barrier crossing due to energy transfer from the bath to the effective Hamiltonian is neglected. This neglect is significant only at very strong coupling, where it causes the μVTST results to drop below the simulation results. Both GH and μVTST theories fail (as expected) in the energy diffusion regime, while PGH theory is only moderately successful. The μVTST results agree extremely well with the simulation results in the spatial diffusion regime, providing a significant improvement over the GH results at intermediate coupling strengths and over the PGH results at strong coupling strengths. This improvement is a result of nonlinear effects which are included in the μVTST approach but neglected in the PGH and GH theories.
Rotational motions of CH4 molecules in weak orientational potentials of cubic symmetry
Authors: Asmussen B. , Prager M. , Press W. , Blank H. , Carlile C. J.
Published: July 2002

Nearly free rotational motions of CH4 molecules as substitutional impurities in argon, krypton, and xenon have been observed at low temperatures with inelastic neutron scattering. Besides energy transfer, the dependence of the scattered intensities on momentum transfer Q is used for the assignment of the experimentally observed lines to the various transitions of a spherical quantumrotor in an orientational potential of cubic symmetry. The measured intensities are in good agreement with theoretical predictions based on the extended James–Keenan model. Measurements with high energy resolution on solid CH4 in its antiferrorotational phase II were devoted to the determination of the Q dependence of the tunneling lines and the nearly free rotor lines. The results give the first direct experimental evidence for the value 3 for the ratio of the orientationally ordered molecules to those which are orientationally disordered in CH4-II.