bluuebunny/crossref_metadata_2025 huggingface dataset:
Pyroelectric, piezoelectric, and dielectric properties of β-BaB2O4 single crystal
Authors: Guo R. , Bhalla A. S.
Published: July 2002

Dielectric and pyroelectric properties of single-crystal β-barium metaborate (β-BaB2O4) are measured in the temperature range from 50 °C down to −190 °C. Thermal expansion and piezoelectric properties are measured at room temperature. Piezoelectric and electromechanical coupling coefficients in β-BaB2O4 are low where as pyroelectric figure of merit p/K is quite attractive for some device applications.
The effect of laser annealing of thin W(100) films on positron transmission reemission properties
Authors: Jacobsen F. M. , Charlton M. , Chevallier J. , Deutch B. I. , Laricchia G. , Poulsen M. R.
Published: July 2002

Results on e+ transmission reemission properties are presented for three single-crystal W(100) films of thicknesses 1000, 2000, and 3100 Å for incident e+ energies of 1.4–16 keV. The films were first cleaned with a 10-s laser pulse, then annealed in O2 (10−6 Torr), and finally heated in a vacuum (10−9 Torr). Thereafter the films showed good reemission properties. The maximum transmitted yields of slow e+ were 38% (1000 Å), 27% (2000 Å), and 17% (3100 Å) at incident e+ energies of 4, 5.2, and 7.2 keV, respectively. The energy distributions of the reemitted e+ were characterized by a full width half maximum of 0.15–0.2 eV located at 2.8 eV containing 60%–70% of the e+ superimposed onto a nearly uniform distribution covering the energy interval 0–2.8 eV.
Low-temperature dielectric properties of candidate substrates for high-temperature superconductors: LaAlO3 and ZrO2 : 9.5 mol % Y2O3
Authors: Samara G. A.
Published: July 2002

The effects of temperature (4–400 K), hydrostatic pressure and frequency on the dielectric constant, ε′, and dielectric loss of single crystals of LaAlO3 and cubic yttria(9.5 mol %)-stabilized zirconia (ZrO2) were investigated. Both crystals are relatively low-loss, low-dispersion dielectrics in the temperature range (T<150 K) of most interest for high-temperature superconductors. The pressure results make it possible to evaluate the various contributions to the temperature dependence of ε′. It is found that this dependence is dominated by the change of polarizability with volume. The dielectric properties of the two crystals are compared with those of other candidate substrate materials.
Phase separation in amorphous ErNi2Hx prepared by hydrogenation
Authors: Chung U-In , Kim Yong-Gyoo , Lee Jai-Young
Published: July 2002

X-ray diffraction and observations of phase separation are reported in a hydrogen-induced amorphous ErNi2 alloy. Two separate diffuse maxima in x-ray diffraction and two principal halos in electron diffraction, which are evidence of phase separation into two amorphous states, are observed at hydrogenation conditions of 50 atm H2 and elevated temperatures. The phenomenon of phase separation is manifest with increasing hydrogenation temperature. The compositions of both phases are estimated by comparing the nearest-neighbor distances calculated by using the Ehrenfest relation with all possible atomic distances in amorphous ErNi2 hydride. These distances are calculated by assuming that the configurational atomic arrangement in the amorphous phase is composed of tetrahedral units. The compositions of the two phases seem to correspond to an Er-rich phase (mixture of 4-Er and 3Er+1Ni tetrahedra) and a Ni-rich phase (mixture of 4-Ni and 1Er+3Ni tetrahedra). This phase separation may be caused by the motion of Ni atoms over short distances.
Low-temperature beam-induced deposition of thin tin films
Authors: Funsten H. O. , Boring J. W. , Johnson R. E. , Brown W. L.
Published: July 2002

Ion and electron beam-induced deposition (BID) of thin (1–4 μm), conductive films is accomplished by dissociating and removing the nonmetallic components of an adsorbed, metal-based, molecular gas [SnCl4 and (CH3)4Sn]. Previous research has focused primarily on room-temperature (monolayer adsorption) BID using electrons and slow, heavy ions. This study investigates low-temperature (120 K) BID in which the condensation rate of the precursor gas is well controlled. The residual metallic films are produced by using as incident beams either 2-keV electrons, 25-keV H2+, or 50-keV H2+, all of which provide predominantly electronic energy deposition, or 30-keV Ar+, which provides predominantly nuclear energy deposition. Residual films are analyzed ex situ by scanning electron microscopy, mechanical thickness measurements, resistivity measurements, Rutherford backscattering spectroscopy, and infrared spectrometry. A model is developed that considers bulk and surface dissociation mechanisms and sputtering to describe the BID process. The derived cross sections for the formation of a residue from condensed (CH3)4Sn are nonlinearly related to the total deposited energy approximately to the 1.4 power. The lowest electrical resistivity values of the residues (650 μΩ cm) are obtained only by significant loss of carbon, which is strongly dependent on the nuclear stopping power.
Tungsten etching mechanisms in low-pressure SF6 plasma
Authors: Petri R. , Henry D. , Sadeghi N.
Published: July 2002

A detailed study of tungsten low-pressure etching in a helicon source reactor was performed. In correlation with surface analyses (transmission electronic microscope, Rutherford backscattering, nuclear reaction analysis), a complete parametric study of the plasma and etching parameters versus the macroscopic parameters [gas pressure, radio frequency (rf) power, substrate bias voltage) has been carried out. Using a model developed by Hoffman and Heinrich for silicon etching (Proceedings of the 9th ISPC, Pugnochiuso, Italy, 1989, p. 1003), and taking into account the experimental results, it has been shown that tungsten etching mechanisms can be expressed by the formation and destruction of a low-density reactive top layer. Sputtering of this layer under argon-ion bombardment has been studied by optical emission spectroscopy. Consequently, the tungsten etch rate can be expressed as an analytical function of the macroscopic parameters such as gas pressure, rf power, and substrate bias voltage.
Waveguiding epitaxial LiNbO3 layers deposited by radio frequency sputtering
Authors: Schwyn S. , Lehmann H. W. , Widmer R.
Published: July 2002

Thin films of LiNbO3 were rf sputter deposited onto a number of different substrates and were investigated with a variety of characterization methods. The morphology of the films was investigated by the scanning electron microscope and the crystal structure was determined by x-ray diffraction and small area electron diffraction. The composition was obtained from electron spectroscopy for chemical analysis and was confirmed by Rutherford backscattering. Some optical properties of the film were also investigated, for example, the refractive index which was determined from the coupling angles or the losses of the guided modes measured by scanning a fiber probe along the mode. These analyses showed that best layers can be obtained when depositing LiNbO3 layers with a thickness of approximately 200 nm on (001)-sapphire at a temperature of 500 °C and a deposition rate of 0.2–0.5 nm/min. Using these parameters, single crystalline epitaxial films can be deposited having stoichiometric composition and minimal surface roughness. It was also possible to excite waveguide modes in these films with losses less than 5 dB/cm.
Accurate determination of minority carrier- and lattice scattering-mobility in silicon from photoconductance decay
Authors: Sproul A. B. , Green M. A. , Stephens A. W.
Published: July 2002

Accurate measurements of the minority carrier- and lattice scattering-diffusion constant and mobility in float zone silicon have been determined using photoconductance decay. For the more lightly doped specimens our results indicate slightly higher mobility than published majority carrier values. This is attributed to purer samples which allow a more accurate measurement of the lattice scattering mobility. In the dopant range 1015–1017 cm−3 the results for both electrons and holes are, within experimental error, equal to the majority carrier values. Unlike other methods this technique is a very direct measurement of the diffusion constant as only the thickness and decay time of the wafer need to be determined. The method is estimated to have a one standard deviation uncertainty of 2%–4% which is comparable to the best accuracy previously obtained for majority carrier measurements.
Ion implantation in PbTiO3 films
Authors: Wang Yuguo , Qu Baodong , Zhong Weilie , Zhang Peilin , Wang Keming
Published: July 2002

Lead titanate films prepared by the Sol-Gel process were ion implanted at various temperatures. The structural changes were examined by x-ray diffraction, and the composition distribution was studied by secondary ion mass spectra and Rutherford backscattering. If the implantation is carried out at room temperature, Pb segregates in crystalline form. For high-temperature implantation, both crystallized Pb and PbO were observed and the temperature-induced crystallization of PbTiO3 was hindered. However, the implantation-induced Pb and PbO are metastable. Annealing at high enough temperatures can convert the implanted films into a purely perovskite structure.
Self-trapped state and phonon localization in TiO2 quantum dot with a dipole layer
Authors: Zou Bingsuo , Zhang Yan , Xiao Liangzhi , Li Tiejin
Published: July 2002

The discrete phonon spectra of TiO2 quantum dot with a dipole layer is reported. The absorption intensities of different LO phonons are related to the phonon-electron coupling in accord with the prediction of the continuum polarization model. The two-phonon bound state appeared due to the localization effect. The optical absorption of the self-trapped state [reported in Appl. Phys. Lett. 59, 1826 (1991)] may be related to the strong electron-phonon interaction in this system, i.e., a bipolaronic state might form with the presence of surface donor.
Maximum work from isothermal chemical engines
Authors: Gordon J. M.
Published: July 2002

Upper bounds are derived for the work that can be extracted from simple isothermal chemical converters (chemical engines) that operate from a finite chemical potential reservoir. The incremental gains that stem from adding an arbitrary number of engines is derived. Results are analogous to those for heat engines fed by a finite heat capacity reservoir. Systems of practical interest include chemical reactions, electrochemical cells, and solid-state converters. The effectiveness of introducing more engines to utilize unexploited chemical potential is expressed in terms of the specific relation between mass transfer and chemical potential difference, often referred to as the device’s current-voltage curve.
Dielectric, pyroelectric, and thermally stimulated depolarization current investigations on lead-lanthanum zirconate-titanate-x/95/5 ceramics with La content x=0.5%–4%
Authors: Handerek J. , Ujma Z. , Carabatos-Nédelec C. , Kugel G. E. , Dmytrow D. , El-Harrad I.
Published: July 2002

It is shown that La dopants have a strong influence on the antiferroelectric- ferroelectric-paraelectric (AFE-FE-PE) phase transitions and properties of the lead-lanthanum zirconate-titanate-x/95/5 ceramics (with La contents x=0.5%–4%). The occurrence of the ferroelectric microdomains, dispersed in the AFE and PE matrices, gives rise to properties typical for ferroelectric relaxors. The space-charge concentration polarization, formed by some mobile ion defects, plays an important role in this behavior. This space-charge polarization and its interaction with the spontaneous polarization was investigated by dielectric, pyroelectric, and thermally stimulated depolarization current measurements. A qualitative model of interpretation is discussed.
Effect of ligand field on branching ratio of ultraviolet and blue upconversions of Tm3+ ions in halide and oxide glasses
Authors: Tanabe Setsuhisa , Hanada Teiichi
Published: July 2002

Tm3+ doped zirconium barium lanthanum chloro-fluoride, germanate, and tellurite glasses were prepared and their upconversion luminescence and excitation spectra were measured to investigate the effect of the host matrix glass on the branching ratio of 0.36 and 0.45 μm luminescences. Since the radiative emission probabilities of electric-dipole transitions are functions of Judd–Ofelt intensity parameters, Ωt(t=2,4,6), absorption spectra were also measured to obtain Ωt parameters. The branching ratio β of 0.45 μm luminescence in zirconium barium lanthanum chloro-fluoride glasses increases slightly with increasing Cl− doping, but the ratios were much smaller than those in tellurite and germanate glasses. The result could be explained by the smaller Ω2/Ω4 ratio in zirconium barium lanthanum chloro-fluoride glasses than those in oxide glasses. The correlation between the luminescence intensity ratio and the Ωt parameters suggests that the branching ratio can be controlled by the ligand field for Tm3+ in glass matrix.
Optical constants of GaxIn1−xP lattice matched to GaAs
Authors: Schubert Mathias , Gottschalch V. , Herzinger Craig M. , Yao Huade , Snyder Paul G. , Woollam John A.
Published: July 2002

The optical constants of Ga0.51In0.49P have been determined from 0.8 to 5.0 eV using variable-angle spectroscopic ellipsometry measurements at room temperature. The metal-organic vapor-phase-epitaxy-grown samples were x-ray analyzed to confirm lattice matching to the GaAs substrate. The effects of the native oxide were numerically removed from the data to determine the intrinsic optical constants. This is important because the optical constants reported become generally useful for modeling multiple-layer structures. A Kramers–Kronig analysis was used to reduce interference-related fluctuations in the below-gap refractive index. Near the band edge a mathematical form for excitonic absorption was included. Critical point energies were extracted using a numerical second-derivative fitting algorithm.
Optoelectronic characterization by photothermal deflection: Silicon solar cells
Authors: Grunow P. , Schieck R.
Published: July 2002

Photothermal deflection is used to investigate the electronic transport in a pn silicon solar cell. The dependence of the deflection signal on an external bias voltage in the closed circuit and on additional bias illumination in the open-circuit case is simulated successfully with a semimicroscopic approach using the expressions for the current voltage characteristics obtained from the diode theory. The experimental results on various crystalline silicon solar cells demonstrate the ability of the method for contactless all-optical characterization of photovoltaic interfaces.
Structure and magnetic properties of MnBiDy(Sm) permanent magnetic films
Authors: Fang Ruiyi , Fang Qingqing , Zhang Sheng , Peng Chubing , Dai Daosheng
Published: July 2002

We have studied crystal texturing and magnetic properties of MnBiDy(Sm) permanent magnetic films, with thicknesses varying from 60 to 1200 nm, fabricated by the thermal coevaporation method and subsequently annealed in a vacuum oven at temperatures ranging from 350–425 °C. X-ray diffraction analysis has shown that the samples exhibit low-temperature phase MnBi with a NiAs-type structure when the thicknesses of the samples are less than 250 nm. When the thicknesses are larger than 250 nm, however, the Bi3Mn2 phase also appears. Magnetic measurements have revealed that saturation magnetization σs=46–78 emu/g, residual magnetic induction Br=3.2–7.3 kGs, remnant magnetization ratio r=0.64–0.98, intrinsic coercive force Hc=2.2–7.3 kOe, and magnetic energy product (BH)m=3.33–14.3 MGOe. We have also found that Hc depends on the thickness of the sample, and reaches a maximum when the thickness d=400 nm. The origin of the coercive force may be understood by the nucleation and growth of reverse magnetic domains when d<150 nm but by domain rotation when d≳600 nm.
Carrier capture processes in semiconductor superlattices due to emission of confined phonons
Authors: de Paula Ana M. , Weber Gerald
Published: July 2002

Carrier capture times due to the emission of confined longitudinal optical phonons via electron–phonon (Fröhlich) interaction are calculated for GaAs-AlxGa1−xAs and In0.47Ga0.53As-InP superlattices. A dielectric continuum model is used to describe the confined phonon modes and we use a Kronig–Penney type calculation for the electron envelope functions. We compare our results with capture times measured by several optical techniques and we discuss the importance of a knowledge of the carrier excitation details in order to obtain an appropriate interpretation of the experimental results. We show that electrons excited into confined states with a large kinetic energy strongly influence the overall capture times.
Theory of microstructural depth profiling by photothermal measurements
Authors: Lan T. T. N. , Seidel U. , Walther H. G.
Published: July 2002

An analytical solution for the photothermally measurable surface temperature of a sample with piecewise linearly inhomogeneous depth profile of the thermal conductivity k is presented. Based on this solution an inversion algorithm using a sequence of one-parameter fits is suggested in order to estimate the k profile. Numerical simulations demonstrate the performance of the approach and its insensitivity to random errors.
High-frequency, transient magnetic susceptibility of ferroelectrics
Authors: Grimes Craig A.
Published: July 2002

A significant high-frequency magnetic susceptibility was measured both in weakly polarized and nonpolarized samples of barium titanate, lead zirconate titanate, and carnauba wax. Magnetic susceptibility measurements were made from 10 to 500 MHz using a thin film permeameter at room temperature; initial susceptibilities ranged from 0.1 to 2.5. These values are larger than expected for paramagnets and smaller than expected for ferromagnets. It was found that the magnetic susceptibility decreases rapidly with exposure to the exciting field. The origin of the magnetic susceptibility is thought to originate with the applied time varying electric field associated with the susceptibility measurements. An electric field acts to rotate an electric dipole, creating a magnetic quadrupole if the two moments are balanced, and a net magnetic dipole moment if imbalanced. It is thought that local electrostatic fields created at ferroelectric domain discontinuities associated with grain boundaries create an imbalance in the anion rotation that results in a net, measurable, magnetic moment. The origin of the magnetic aftereffect may be due to the local heating of the material through the moving charges associated with the magnetic moment.
New design of small-angle magnetization rotation device: Evaluation of saturation magnetostriction in wide thin ribbons
Authors: Alves F. , Houée P. , Lécrivain M. , Mazaleyrat F.
Published: July 2002

We present here a newly designed device allowing λS measurements on ferromagnetic ribbons (width up to 30 mm, thickness ≈20 μm). It is based on the “small-angle magnetization rotation” method. Estimation of the ac drive field and its homogeneity into the material have been looked into. Calculations by finite elements method (MAXWELL 2D) are compared to H-coil measurements. Influence of dc bias field on transverse demagnetizing field has been taken into account. The aim of this work is to analyze, by λS measurements, the evolution of composite structure (amorphous+crystalline) in accordance with annealing treatments between 450 and 600 °C of nanocrystalline Finemet Fe73.5Si15.5B7Cu1Nb3 ribbons. Results show that the value and the sign of the saturation magnetostriction λS are important because they are related to the magnetoelastic properties. Furthermore, the observed magnetoelastic behavior (via variation of magnetization energy in accordance with external applied stress) is highly dependent of the sign of λS even with a small value (≈10−7).
Elastic constants of α-GeO2
Authors: Grimsditch M. , Polian A. , Brazhkin V. , Balitskii D.
Published: July 2002

The elastic constants of α-GeO2, the quartzlike polymorph of germanium dioxide, have been determined using Brillouin scattering. The data were obtained using a high quality single crystal of α-GeO2. When compared with existing data on α-SiO2, our results show that the shear constants are considerably softened in GeO2. This could be an indication that the observed pressure induced phase change is related to a shear instability.
Space charge effects on dopant diffusion coefficient measurements in semiconductors
Authors: Lubomirsky Igor , Gartsman Konstantin , Cahen David
Published: July 2002

Systematic errors are likely to affect the results of indirect methods used for measuring dopant diffusion in semiconductors, which, for this purpose should be considered as mixed electronic-ionic conductors. The highest contribution to these errors is introduced by the presence of an internal electric field, i.e., by space charge effects. The electric field can be the result either of a dopant concentration gradient or of external bias, applied during the measurement. We consider here three methods in detail, viz. measurement of p-n junction motion, of current or potential decay, and of the time dependence of capacitance (transient ion drift). We show that space charge effects can lead to overestimating diffusion coefficients by a few orders of magnitude. We use the results of our analyses to review and compare the experimental data obtained by different direct and indirect methods, for Cu diffusion in CuInSe2, an issue of considerable current interest for solar cells.
Measurement of the critical current distribution for the resistively shunted junctions
Authors: Tarutani Y. , Hasegawa H. , Fukazawa T. , Takagi K.
Published: July 2002

A measurement method for the critical-current (Ic) distribution of the resistively shunted junction (RSJ) is discussed. According to numerical calculations, the Ic steps in the current–voltage (I–V) curve can be detected for the series array of the RSJ-type junctions when the I–V curve is changed from nonhysteretic to hysteretic. Bicrystal Josephson junctions in which YBa2Cu3O7−x film was used as electrodes were shunted by planar capacitors and the ratio of the Imin to Ic was decreased to less than 0.8. The Ic distribution of junctions numbering more than 400 could be measured by connecting them in series. A typical value of the standard deviation 1σ in Ic was 26%.
Exchange-spring behavior in epitaxial hard/soft magnetic bilayer films
Authors: Jiang J. S. , Fullerton Eric E. , Grimsditch M. , Sowers C. H. , Bader S. D.
Published: July 2002

We present results on the magnetic reversal process in epitaxial Sm–Co(11̄00)/TM (TM=Fe, Co) bilayer films prepared via magnetron sputtering onto Cr-buffered single-crystal MgO substrates. The magnetically hard Sm–Co films have 20 T uniaxial anisotropy and coercivities >3 T at room temperature. The magnetization of the soft layer is pinned at the interface to the hard-magnet layer and switches reversibly as expected for an exchange-spring magnet. With increasing soft layer thickness, the coercive field of the hard layer becomes significantly less than that of a single layer. We also present numerical solutions to a one-dimensional model that provide the spin configuration for each atomic layer. Comparison of the experimental results with the model simulations indicates that the exchange-spring behavior of our bilayer films can be understood from the intrinsic parameters of the hard and soft layers.
Ultrafast optical response of the Au–BaO thin film stimulated by femtosecond pulse laser
Authors: Wu J. L. , Wang C. M. , Zhang G. M.
Published: July 2002

The pump-probe method was applied to study the dependence of the transient optical transmissivity upon time delay for the Au–BaO composite thin film stimulated by a femtosecond pulsed laser. It was observed that the light absorption of the thin film increased rapidly and then resumed its initial value in several picoseconds. Optical relaxation is a process in which nonequilibrium electrons, excited by laser pulses and originating from Au ultrafine particles, return to the equilibrium state. In this article, the value of the electron-phonon coupling constant g of gold ultrafine particles in the composite thin film was calculated theoretically.
Correlation between titania film structure and near ultraviolet optical absorption
Authors: DeLoach J. D. , Scarel G. , Aita C. R.
Published: July 2002

Titania is a material with structural flexibility, and as a result, readily forms both crystalline polymorphs and an amorphous structure in thin films grown near room temperature. The goal of this study is to correlate fundamental optical absorption edge characteristics with the phase constituency of titania films. To that end, films with coexistent rutile, anatase, and amorphous constituents were sputter deposited onto fused silica and 〈111〉-Si substrates. The films were then subjected to cyclic annealing in air at moderate temperature (700 and 1000 °C) to affect phase changes. Bragg–Brentano x-ray diffraction was used for phase identification and near ultraviolet-visible transmission and reflection spectrophotometry was used to determine the optical absorption coefficient at the onset of interband transitions. The optical absorption coefficient was modeled within the framework of the coherent potential approximation (CPA), with Gaussian site disorder introduced into the valence and conduction bands of a perfect virtual crystal. Two parameters of the disordered crystal were defined: the optical band gap, Ex, and the slope of absorption edge, Eo. The results are discussed in terms of two extreme cases: (1) film states containing a large rutile volume fraction (0.70–1) share a rutile virtual crystal, with Eg=3.22 eV. Data for these states was combined with single crystal data to develop an expression interrelating Eg, Ex, and Eo. This expression is applicable to any structure with a rutile virtual crystal. The relationship between structural disorder (i.e., the volume fraction of amorphous material) and electronic disorder (i.e., Eo), is quantitatively consistent with the CPA model. (2) Film states containing a small rutile volume fraction (0.02–0.17), and hence a large anatase+amorphous component, share a nonrutile virtual crystal, with Eg=3.41 eV. The effect of increasing the structural disorder (i.e., the rutile volume fraction), in these states is to shift Ex to lower values, which is qualitatively consistent with the CPA model. Furthermore, anatase and amorphous components can be modeled using the same nonrutile virtual crystal, indicating these structures have a common short-range order in the sputter deposited films of this study.
Effect of uniaxial anisotropy on anisotropic magnetoresistance: Thickness dependence in bilayer NiO(30 nm)/NiFe(t)
Authors: Kim D. Y. , Kim C. G. , Park B. S. , Hwang D. G. , Lee S. S.
Published: July 2002

The uniaxial anisotropy field HK and angle γ from exchange coupling field in anisotropic magnetoresistance sample has been analyzed on the basis of the rotational magnetization of single domain. The anisotropy field HK in bilayer NiO(30 nm)/NiFe(t) obtained from the measured magnetoresistance profiles at magnetizing angle θ=0° and 90° from the exchange coupling field changes from positive to negative as the thickness of NiFe increases, transition at about 20 nm. The anisotropy field |HK|, and angle γ show a minimum values at the transition thickness, which is the optimum thickness in anisotropic magnetoresistance sample design in order to increase the field sensitivity and reduces hysteresis loss.
Effect of growth temperature on point defect density of unintentionally doped GaN grown by metalorganic chemical vapor deposition and hydride vapor phase epitaxy
Authors: Joshkin V. A. , Parker C. A. , Bedair S. M. , Muth J. F. , Shmagin I. K. , Kolbas R. M. , Piner E. L. , Molnar R. J.
Published: July 2002

We report on the investigation of the effect of growth temperature on point defect density of unintentionally doped GaN grown by atmospheric pressure metalorganic chemical vapor deposition and hydride vapor phase epitaxy. A correlation between photoluminescence (PL) spectra and the concentration of donors and acceptors in unintentionally doped GaN is presented. The effects of oxygen and native acceptors on the electrical and optical properties of GaN epitaxial layers are discussed and a classification of PL data is presented. On this basis we show that oxygen creates a shallow donor in GaN with an activation energy of about 23.5±1 meV. We determine that the concentration of native acceptors in GaN increases with an increase in growth temperature. These native acceptors, probably gallium antisites (GaN) and/or gallium vacancies (VGa), are nonradiative defects. We show that a second donor level in GaN has an activation energy of about 52.5±2.5 meV and produces a PL peak with an energy of about 3.45 eV at low temperatures. From Hall investigations we show that a third donor in GaN has an activation energy of 110±10 meV.
Material gain of bulk 1.55 μm InGaAsP/InP semiconductor optical amplifiers approximated by a polynomial model
Authors: Leuthold J. , Mayer M. , Eckner J. , Guekos G. , Melchior H. , Zellweger Ch.
Published: July 2002

We report material gain measurements of bulk 1.55 μm InGaAsP/InP performed at constant temperature and carried out over a large range of carrier densities and a large spectral region. In addition, a polynomial model for the material gain is proposed that not only fits the experimental data well over the whole measured range but also shows stable convergence in simulation tools when carrier densities exceed the usual range. To get reliable parameters for the model we eliminated temperature effects arising from different current biases and performed material-gain measurements over the largest possible carrier density range. The material-gain model realized is used in semiconductor optical amplifier simulation tools.
Influence of Fe and Co additions on magnetization and magnetoresistivity of MnAu2 melt-spun alloys
Authors: Handstein A. , Nenkov K. , Rössler U. K. , Müller K.-H.
Published: July 2002

Melt-spun MnAu2 ribbons modified by ternary magnetic additions (Fe and Co) show a large giant magnetoresistance (GMR) effect up to a magnetoresistance ratio MR={[R(H=7T)−R(0)/R(0)]} of the order of −10% at room temperature. Pure MnAu2 samples show a transition from positive magnetoresistance at low temperatures (e.g., MR=+23% at 10 K and μ0H=5 T) to a GMR behavior with values of about −8% at 295 K and 5 T. At 135 K the resistivity is nearly independent of the applied magnetic field. The strong increase of |MR| at room temperature is only found with applied fields larger than a threshold field, where a metamagnetic transition occurs. Alloying the compound with Co or Fe this threshold field can be reduced or suppressed. This effect is due to softening of the antiferromagnetic behavior of the (Mn; Co or Fe)Au2 samples. For an alloy with 5 at. % Fe as substitutes for Mn, no metamagnetic transition is observed in the temperature range between 10 and 300 K. For both 5 at. % Co or Fe additions a GMR behavior is observed in this temperature range with higher sensitivity at low fields, e.g., (Mn0.95Fe0.05)Au2 has values of MR=−12% and −1% in a field of 1 T at 10 and 295 K, respectively. At low temperatures, an anomalous increase of resistance at higher fields (above 1–2 T) is observed for these alloys.
Coulomb-enhanced Auger recombination in crystalline silicon at intermediate and high injection densities
Authors: Schmidt Jan , Kerr Mark , Altermatt Pietro P.
Published: July 2002

Band-to-band Auger recombination in crystalline silicon is investigated by means of quasi-steady-state photoconductance measurements under intermediate- and high-injection conditions. The measurements reveal that the correlation of the charge carriers via Coulombic interaction has to be taken into account to accurately model the observed injection level and doping concentration dependence of the Auger recombination lifetime. A semi-empirical Coulomb-enhanced Auger recombination model is applied in order to simulate the experimental results theoretically.
Single atom surface diffusion of sulfur on Ni(111)
Authors: Kellogg G. L.
Published: July 2002

The surface diffusion of individual sulfur atoms on the Ni(111) plane has been investigated by field ion microscopy. The sulfur atoms were segregated to the surface by annealing the tip at 700 K. Surface diffusion on the (111) plane was detected at temperatures above 107 K with an activation energy of 0.29–0.30 eV. These results indicate a very high diffusion rate for sulfur on clean Ni(111) at typical reaction temperatures (e.g., 450–900 K for CO methanation).
Temperature dependent electronic quenching of OH(A 2Σ+, v′=0) between 230 and 310 K
Authors: Copeland Richard A. , Crosley David R.
Published: July 2002

Measurements were made of the temperature dependence of the rate constant for collisional quenching of the v′=0 level of the A 2Σ+ electronically excited state of the OH radical. For all quenchers examined (CO2, H2, O2, and N2), we observed that the thermally averaged cross section σQ increased as the temperature is decreased. For N2, the slowest quencher, σQ shows a dramatic increase from 4.0±0.7 Å2 at 311±10 K to 7.0±0.6 Å2 at 232±5 K. The results of this work and other high temperature (900–1300 K) studies from this laboratory are compared with a description of the collisional encounter involving attractive forces and complex formation.
Percolation model for mixed alkali effects in solid ionic conductors
Authors: Harder H. , Bunde Armin , Dieterich W.
Published: July 2002

We investigate a random walk model for describing mixed alkali effects in ionic conductors of the β-alumina type. In our model, the observed drastic variation in the transport properties of the mixed crystals is related to critical properties near a percolation threshold, which originates from the blocking of conduction paths by complexes containing the substituted ions. The motion of a tracer ion and the conductivity are obtained by using the Monte Carlo technique and are discussed in relation to experiments. It is pointed out that anomalous diffusion, which occurs near percolation, leads to a characteristic frequency dependence of the dynamic conductivity, which should be experimentally accessible.
Band broadening of CH2 vibrations in the Raman spectra of polymethylene chains
Authors: Wunder Stephanie L. , Bell Michael I. , Zerbi Giuseppe
Published: July 2002

The isotropic and anisotropic linewidths of methylene vibrations in a homologous series of alkanes of increasing chain length have been measured in the liquid state as a function of temperature. The bandwidths of the CH2 symmetric stretching modes, which are in Fermi resonance with overtones of the CH2 bending vibrations, are temperature insensitive over a 200 K interval; this is best explained in terms of a vibrational dephasing mechanism (inhomogeneous broadening) for these modes. In contrast, for the bending and antisymmetric stretching vibrations, significant band broadening occurs over this same temperature interval. In addition, for these modes, both the absolute value of the bandwidth and the relative rate of increase of the bandwidth with increasing temperature, decrease with increasing chain length. These observations are consistent with a reorientational broadening mechanism as the principal bandwidth contribution for these vibrations. Hindered end-over-end rotation of the molecules, which contributes to the band broadening for very low molecular weight alkanes, rapidly becomes too slow to be observable on the time scale of the Raman experiment for the higher molecular weight alkanes and polyethylene. For longer chain lengths, torsional backbone motions coupled to the high frequency antisymmetric stretching modes can account for the breadth of the bands.
Photodissociation cross sections from the E,F state of H2
Authors: Zucker C. W. , Eyler E. E.
Published: July 2002

Photodissociation cross sections were calculated for transitions from the v=6, J=1 level of the E,F 1∑+g state of H2 to the continua of the B 1∑+u, C 1∏u, and B′ 1∑+u states. The results include the first accurate calculations of transitions to the continua of these states for energies extremely close to threshold. The need for a small adjustment to the B′ state potential was established, and a simple ad hoc procedure for this was developed. The calculations predict a prominent resonance very near threshold, and oscillatory behavior at larger energies.
Quantum flux operators and thermal rate constant: Collinear H+H2
Authors: Park Tae Jun , Light J. C.
Published: July 2002

The exact quantum formulation of the thermal rate constant, k(T), given by Miller et al. [W. H. Miller, J. Chem. Phys. 61, 1823 (1974); W. H. Miller, S. D. Schwartz, and J. W. Tromp, ibid. 79, 4889 (1983)] is evaluated in a localized L2 basis (distributed Gaussian basis) for two model problems. In considering the accuracy, feasibility, and computational efficiency of this approach, we demonstrate novel properties of the flux operator, namely the paucity of nonzero eigenvalues. This contributes greatly to the efficiency of the L2 approach. Finally, we show that Lanczos reduction can be used effectively for determining the thermal flux projectors and their time evolution as is required for evaluation of k(T).
A theoretical study of the water dimer interaction
Authors: Szalewicz Krzysztof , Cole Samuel J. , Kol/os Wl/odzimierz , Bartlett Rodney J.
Published: July 2002

We have performed a study of the water dimer interaction using larger basis sets and higher levels of theory than have been previously applied to this system. For the minimum geometry we have used spdf basis sets containing up to 212 orbitals. Our most accurate SCF interaction energy for the minimum is −3.73±0.05 kcal/mol. We have shown that this energy can be reproduced to within 0.1 kcal/mol using much smaller basis sets containing proper (diffuse) exponents. Accounting for the basis set superposition error is shown to be essential. We computed the dispersion energy with neglect of the intramolecular correlation using basis sets of various sizes. The best value obtained in a large spdf basis set with exponents which optimize this quantity is −1.93 kcal/mol and it is expected to be accurate to 0.1 kcal/mol or better. Using some of these basis sets we have performed supermolecular many-body perturbation theory (MBPT) and coupled-cluster (CC) calculations including triple excitations. We have shown that if the correlated supermolecular interaction energies are not corrected for the basis set superposition error, the correlated part of the interaction energy varies widely with the basis set. In contrast, the corrected values converge smoothly. On the basis of the dispersion energy and the MBPT/CC results we predict the correlated part of the interaction energy to be −1.0±0.3 kcal/mol, which leads to the total interaction energy of −4.7±0.35 kcal/mol. Thus, our work favors the lower limit of the experimentally predicted interaction energy of −5.4±0.7 kcal/mol.
Proposed experimental probes of chemical reaction molecular dynamics in solution: ICN photodissociation
Authors: Benjamin I. , Wilson Kent R.
Published: July 2002

Knowledge of how translational and rotational motions are influenced by the solvent during the course of a photodissociation ‘‘half-collision’’ reaction in solution is of interest in itself and can also help our understanding of how thermally activated reactions take place in solution by means of fluctuations in translational and rotational motion. With this goal, the molecular dynamics of the photodissociation of the triatomic molecule ICN are compared in the gas phase and in Xe solution. The time evolution of the trajectories (particularly with respect to interfragment distance and CN orientation) and of the energy partitioning (particularly into fragment translational recoil and into rotation of the CN) are displayed. Two types of solution experiments are proposed and simulated, both closely related to recent gas phase studies by Dantus, Rosker, and Zewail. These experiments are designed to probe the detailed dynamics of chemical reactions in solution during the time period the reaction is in progress, in particular to reveal the dramatic effects of the solvent on translational motions and energies. Both are pump–probe experiments in which the first photon dissociates the ICN and the second induces fluorescence in the CN fragment. In the first type of experiment, which is particularly sensitive to fragment translational motion, the fluorescence intensity is measured as a function of photon energy and of time delay. In the second type of experiment, which is particularly sensitive to fragment rotation, in addition the angle between the polarizations of the pump and probe photons is varied. In the calculations presented here, the effect of the absorption of the photodissociation photon is treated using the classical Frank–Condon principle. The coupling between the assumed two upper electronic surfaces is taken into account semiclassically using a generalization to the condensed phase of the classical electron model of Miller and Meyer, which was applied to ICN photodissociation in the gas phase by Goldfield, Houston, and Ezra.
Photoelectron spectroscopy and electronic structure of heavy group IV–VI diatomics
Authors: Wang Lai-Sheng , Niu B. , Lee Y. T. , Shirley D. A. , Balasubramanian K.
Published: July 2002

Vibrationally resolved HeI (584 Å) photoelectron spectra of the heavy group IV–VI diatomics SnSe, SnTe, PbSe, and PbTe were obtained with a new high temperature molecular beam source. Ionization potentials and spectroscopic constants are reported for all the ionic states observed. Relativistic complete active space multiconfiguration self-consistent field (MCSCF) followed by multireference singles+doubles relativistic configuration interaction (CI) calculations which included up to 200 000 configurations were made on both the neutral diatomics and their positive ions. Ionization potentials and spectroscopic constants were calculated and were in good agreement with the experimentally measured values. Relativistic CI potential energy curves were calculated for all the neutral ground states and the ionic states involved. Relativistic effects were shown to play an important role in these heavy diatomics. The 2Σ+1/2 and 2Π1/2 states for all four molecular ions showed avoided curve crossings, which resulted in pronounced shoulders in the Ω=1/2 potential energy curves of PbTe+. Experimentally, autoionization transitions were also observed for the PbTe+ spectrum. The importance of the relativistic effect and chemical bonding in the heavy diatomics are discussed.
A reaction surface Hamiltonian treatment of the double proton transfer of formic acid dimer
Authors: Shida Norihiro , Barbara Paul F. , Almlöf Jan
Published: July 2002

The double proton transfer reaction of the isolated formic acid dimer has been investigated within the reaction surface Hamiltonian framework, using a newly calculated three-dimensional ab initio potential energy surface. The symmetric (synchronous) proton movement, the asymmetric (asynchronous) proton movement and the relative motion of two formic acid molecules have been explicitly included in the calculation. The calculation gives a tunneling splitting of 0.004 cm−1, which is considerably smaller than a previous theoretical prediction (0.3 cm−1). An effective tunneling path has been calculated from the lowest vibrational eigenfunction of the reaction surface Hamiltonian, and the path deviates significantly from the minimum energy path on the potential energy surface. The new results are consistent with the conventional understanding of heavy–light–heavy mass combination reactions. The effective reaction path from this calculation reveals evidence of asymmetric proton movement. However, a synchronous double proton transfer is the major mode of reaction. Tunneling splittings for a few excited vibrational levels have also been calculated within the reaction surface Hamiltonian framework. Vibrational excitation of a large amplitude, heavy atom mode dramatically increases the tunneling splitting.
Spectroscopy of jet-cooled tyrosine derivatives
Authors: Teh Chin Khuan , Sulkes Mark
Published: July 2002

Using fluorescence-based techniques (excitation and emission spectra lifetimes), we have studied the jet-cooled tyrosine derivatives tyramine and 3-(4-hydroxyphenyl) propionic acid (HPA). For solvent addition resulting in small, mainly one adduct complexes, there is a high degree of analogy with the results for the corresponding analogs of the amino acid tryptophan. In particular, for tyramine, as had been the case with its tryptophan analog tryptamine, addition of one -OH bearing solvent molecule at the α-amine apparently results in a complex in a single conformation.
Two-photon spectroscopy of N2: Multiphoton ionization, laser-induced fluorescence, and direct absorption via the a″ 1Σ+g state
Authors: Lykke Keith R. , Kay Bruce D.
Published: July 2002

A new (2+1) resonantly enhanced multiphoton ionization (REMPI) process via the a″ 1Σ+g state of N2 with a quantum specific detection sensitivity of ∼105 molecule cm−3 is reported. The resulting spectra are easily analyzed and yield directly the relative rotational and vibrational populations in the ground electronic state of N2. Additionally, both two-photon direct absorption (TPDA) to, and laser-induced fluorescence (LIF) from two-photon excitation of, the a″ state of N2 are observed. These spectroscopies are compared with (2+1) REMPI and TPDA via the E,F state of H2.
Excitons in confined geometries: Size scaling of nonlinear susceptibilities
Authors: Spano Francis C. , Mukamel Shaul
Published: July 2002

The third-order nonlinear optical response of one-dimensional molecular aggregates of arbitrary size is calculated using equations of motion for single-exciton and two-exciton variables. These equations map the system onto the dynamics of N(N+1)/2 coupled nonlinear oscillators, where N is the number of molecules in the aggregate. The scaling of the generalized third-order hyperpolarizability γ(3) with N is analyzed in detail. The complete wave vector and frequency dependence of γ(3) is maintained and cooperative radiative decay (superradiance) is incorporated. Comparison is made with the density-matrix approach which provides a fully time-ordered expression.
Long-time collective diffusion coefficient of semidilute suspensions of spherical Brownian particles
Authors: Felderhof B. U. , Vogel J.
Published: July 2002

We study the long-time wave-number-dependent collective diffusion coefficient of a semidilute suspension of interacting spherical Brownian particles with direct and hydrodynamic interactions. Explicit results are found for the first two nonvanishing terms in an expansion in powers of the wave number for a suspension of hard spheres with hydrodynamic interactions.
Interaction-site representation of the Smoluchowski–Vlasov equation: The space–time correlation functions in a molecular liquid
Authors: Hirata Fumio
Published: July 2002

A site–site representation of the Smoluchowski–Vlasov equation is proposed based on the generalized Langevin equation, and on the site–site Ornstein–Zernike equation for the molecular liquid. The method is applied for calculating the space–time pair correlation function proposed by van Hove. The results are compared with a molecular dynamics simulation of water. The site–site Smoluchowski–Vlasov equation predicts most of the essential features of the van Hove function for water.
Infrared laser spectroscopy of jet-cooled carbon clusters: The bending dynamics of linear C9
Authors: Orden A. Van , Hwang H. J. , Kuo E. W. , Saykally R. J.
Published: July 2002

We report improved measurements for the ν6 antisymmetric stretch fundamental and observation of the (ν6+ν15)−ν15 and (ν6+2ν15)−2ν15 hot bands of the linear C9 carbon cluster by direct absorption diode laser spectroscopy of a supersonic carbon cluster beam. Analysis of these bands characterizes C9 as a semirigid molecule with a bending potential similar to that of C5 and further evidences the alternation in degree of rigidity of linear carbon clusters with the g–u symmetry of the HOMO.