bluuebunny/crossref_metadata_2025 huggingface dataset:
Adsorption of Cu and Pd on α-Al2O3(0001) surfaces with different stoichiometries
Authors: Łodziana Z. , Nørskov J. K.
Published: July 2002

We report density functional theory calculations of the interaction of Cu and Pd with the (0001) surface of α-Al2O3. The interaction of those metals with the oxide surface varies from covalent-like for the aluminum rich surface to ionic-like for the oxygen terminated surface. Stoichiometric hydroxylation of the surface does not increase the metal–oxide interaction significantly. We suggest that defects created upon hydroxylation are the main mechanism to explain the observed metal wetting of the oxide.
Effect of strong surface pinning on the surface impedance of a superconductor in a mixed state
Authors: Luzhbin D. A.
Published: July 2002

The field and frequency dependences of the surface impedance of a type-II superconductor in a mixed state in a constant magnetic field perpendicular to the surface are calculated taking account of the nonlocality of the elastic moduli of the vortex lattice. It is shown that if there is no surface pinning, a correct analysis of the elastic properties of the vortex ensemble results in the Coffey–Clem model with small corrections. For strong surface pinning, the surface resistance is predicted to decrease substantially at low frequencies. It is shown that the results obtained when dispersion is taken into account by renormalizing the elastic moduli [E. B. Sonin, A. K. Tagantsev, and K. B. Traito, Phys. Rev. B 46, 5830 (1992)] are incorrect.
Theory of magnetic-field enhancement of surface-field terahertz emission
Authors: Johnston M. B. , Whittaker D. M. , Corchia A. , G. Davies A. , Linfield E. H.
Published: July 2002

We present a theoretical treatment of surface–field THz generation in semiconductors, which explains the power enhancement observed when a magnetic field is applied. Our model consists of two parts: a Monte Carlo simulation of the dynamics of carriers generated by a subpicosecond optical pulse, and a calculation of the resulting THz radiation emitted through the semiconductor surface. The magnetic field deflects the motion of the carriers, producing a component of the THz dipole parallel to the surface. This causes the power transmitted through the surface to be increased by more than one order of magnitude.
On the non-Heisenberg contribution to the spin–spin interaction of an antiferromagnet with S=3/2
Authors: Kalita V. M. , Lozenko A. F.
Published: July 2002

It is shown that the spin Hamiltonian of the spin–spin interactions, written with the use of effective spinons defined on the two lowest doublets of magnetic ions with a partially frozen orbital moment, has a non-Heisenberg form. The Heisenberg contribution is represented by spin–spin terms containing three powers of ion spin projections and is anisotropic and comparable to the bilinear spin–spin interaction.
A black-box self-consistent field convergence algorithm: One step closer
Authors: Kudin Konstantin N. , Scuseria Gustavo E. , Cancès Eric
Published: July 2002

A direct inversion iterative subspace version of the relaxed constrained algorithm is found to be a very powerful convergence acceleration technique for the solution of the self-consistent field equations found in the Hartree–Fock method and Kohn–Sham-based density functional theory (KS-DFT). The present algorithm, abbreviated EDIIS, is benchmarked against the direct inversion iterative subspace method based on the commutator of the density and Fock matrices developed by Pulay (DIIS). Our findings indicate that while EDIIS is able to rapidly bring the density matrix from any initial guess to a solution region, the DIIS method is faster when the density matrix is close to convergence. Consequently, we propose a combination of EDIIS and DIIS methods, which is both very robust and highly efficient. We also show how EDIIS can detect the presence and determine the value of fractional occupations in KS-DFT.
First principles determination of the bound levels of Li−(H2)
Authors: Chang Daniel T. , Surratt G. , Ristroff G. , Gellene Gregory I.
Published: July 2002

An analytical potential energy surface is developed from high quality ab initio calculations for the electrostatic region of the Li−+H2 interaction. The Li−(H2) electrostatic complex is found to have a linear minimum energy structure with a De of 64.44 cm−1. A numerical determination of the bound levels supported by this potential indicates a D0 of only about 7 cm−1 for Li−(para H2) and a considerably larger D0 of about 22 cm−1 for Li−(ortho H2). Altogether, the Li−(para H2) interaction is predicted to support 11 bound levels: ν3=0, J=0–6; and ν3=1, J=0–3, whereas the Li−(ortho H2) interaction is predicted to support 28 bound levels: ν3=0, J=0–10; ν3=1, J=0–8; ν3=2, J=0–5; and ν2=1, J=1–2. Analogous results for the D2 and HD isotopolouges are reported.
Theory of the ternary layered system Ti–Al–N
Authors: Holm Bengt , Ahuja Rajeev , Li Sa , Johansson Börje
Published: July 2002

We present calculations on the physical properties of Ti4AlN3 obtained from first principles, as well as some general observations concerning this group of materials. We further report mechanical data obtained through numerical simulations for some representative phases, and elaborate on the issue of high pressure stability. By examining the density of states of Ti4AlN3, we conclude that it is a semimetal, and we predict an opening up of the band gap around the Fermi level with increasing pressure.
Computer modeling of disordered molecular solids: Six-state model of glassy crystal cyanoadamantane
Authors: Fabiański Robert , Firlej Lucyna , Kuchta Bogdan
Published: July 2002

A six-state pseudo-spin model has been proposed to understand the phase transition mechanism and kinetics of the cyanoadamantane glassy crystal. Implementation of this model has significantly improved ergodicity in computer simulations (Monte Carlo technique) of the system. In particular, an order–disorder phase transition from an orthorhombic to a cubic phase has been reproduced for the first time. It has been also shown that this transformation goes through metastable states.
Dynamical disease: Identification, temporal aspects and treatment strategies of human illness
Authors: Bélair Jacques , Glass Leon , an der Heiden Uwe , Milton John
Published: July 2002

Dynamical diseases are characterized by sudden changes in the qualitative dynamics of physiological processes, leading to abnormal dynamics and disease. Thus, there is a natural matching between the mathematical field of nonlinear dynamics and medicine. This paper summarizes advances in the study of dynamical disease with emphasis on a NATO Advanced Research Worshop held in Mont Tremblant, Québec, Canada in February 1994. We describe the international effort currently underway to identify dynamical diseases and to study these diseases from a perspective of nonlinear dynamics. Linear and nonlinear time series analysis combined with analysis of bifurcations in dynamics are being used to help understand mechanisms of pathological rhythms and offer the promise for better diagnostic and therapeutic techniques.
Dynamical phenomena in systems with structurally unstable Poincaré homoclinic orbits
Authors: Gonchenko S. V. , Shil’nikov L. P. , Turaev D. V.
Published: July 2002

Recent results describing non-trivial dynamical phenomena in systems with homoclinic tangencies are represented. Such systems cover a large variety of dynamical models known from natural applications and it is established that so-called quasiattractors of these systems may exhibit rather non-trivial features which are in a sharp distinction with that one could expect in analogy with hyperbolic or Lorenz-like attractors. For instance, the impossibility of giving a finite-parameter complete description of dynamics and bifurcations of the quasiattractors is shown. Besides, it is shown that the quasiattractors may simultaneously contain saddle periodic orbits with different numbers of positive Lyapunov exponents. If the dimension of a phase space is not too low (greater than four for flows and greater than three for maps), it is shown that such a quasiattractor may contain infinitely many coexisting strange attractors.
Radiant-energy penetration effect in the thermal-diffusivity flash technique for layered and porous polymers
Authors: Bretzlaff R. S.
Published: July 2002

Thermal insulation characterization of polymers and other materials is an important requirement for present and future aerospace missions, as well as for the home construction industry. Some dispersed composites and layered samples have been successfully characterized by the thermal-diffusivity flash technique, while the interpretation of these and other experiments for other systems remains problematical. One refinement for layered-sample data reduction is investigated here. An exponentially decaying spatial penetration of radiant energy into the sample is accounted for. Under the conditions chosen for numerical evaluation, an 18% reduction in the back-face rise time t1/2 is predicted for a case in which only 5% as much radiant energy is deposited on the midplane as on the front surface. It is suggested how the use of the entire V(t) response function might lead to experimental methods of estimating the penetration depth of the radiant energy for particular samples.
Transient nonlinear laser heating and deposition: A comparison of theory and experiment
Authors: Allen S. D. , Goldstone J. A. , Stone J. P. , Jan R. Y.
Published: July 2002

Time-dependent nonlinear laser-heating calculations have been applied to the laser chemical vapor deposition of reflective metallic coatings on absorptive low thermal conductivity substrates such as fused quartz. The surface temperature profile is modified by the changing optical properties of the substrate as the film is deposited, leading to a decrease in the surface temperature and flattening of the temperature profile with increasing irradiation time and film deposition. The results of the theoretical calculations are compared with real-time optical measurements of the deposition rate of Ni from Ni(CO)4 on SiO2 using a CO2 laser as a function of irradiation time, yielding excellent agreement.
Optical properties of hydrogenated amorphous silicon
Authors: Demichelis F. , Minetti-Mezzetti E. , Tagliaferro A. , Tresso E. , Rava P. , Ravindra N. M.
Published: July 2002

A detailed study of the optical properties of sputtered hydrogenated amorphous silicon films with varying hydrogen concentration is presented here. The energy dependence of the absorption coefficient is looked into, in detail, from a point of view of understanding the well known Tauc rule and the alternate relations being proposed in recent years. Spectroscopic and band-structural models like Wemple–Didomenico and Penn are then utilized to analyze the optical parameters near the band-gap region of the wavelength spectra. Extensive comparisons of our results are made with those of sputtered a-Si:H films of other workers, glow discharge prepared a-Si:H, chemically vapor deposited and evaporated a-Si, and also crystalline silicon. The similarities in the variation of the optical properties of a-Si:H with increasing hydrogen concentration (or decreasing measurement temperature) to that of crystalline silicon with decreasing measurement temperature lead us to interesting conclusions. Thus, it seems that decreasing disorder (topological or thermal) in a-Si:H is equivalent to decreasing thermal disorder in c-Si, at least as far as the disorder-optical property relationships are concerned.
Deposition and electrical properties of i
n s
i
t
u phosphorus-doped silicon films formed by low-pressure chemical vapor deposition
Authors: Learn Arthur J. , Foster Derrick W.
Published: July 2002

The low-pressure chemical vapor deposition of phosphorus-doped silicon films on oxidized silicon wafers was investigated as a function of phosphine/silane mole ratio, silane partial pressure, temperature, and wafer spacing. The deposition rate decreases, concomitant with increased phosphorus incorporation, as the mole ratio increases. The deposition rate tends to saturate or vary linearly with silane partial pressure for undoped and heavily doped films, respectively. This, together with differing deposition-rate activation energies of 1.5 and 2.0 eV for undoped and doped films, respectively, is indicative of different reaction mechanisms in the two regimes. As the mole ratio increases, the deposition rate becomes increasingly dependent on the wafer spacing and the radial position on a wafer. Because phosphorus incorporation varies inversely with deposition rate, it develops similar dependencies on wafer spacing and radial position. The majority of these observations are interpretable in terms of a model that has been defined for the growth of oxide from the silane-oxygen reaction where the phosphorus and oxygen play analogous roles. The resistivity of annealed films decreases with increased phosphorus incorporation (mole ratio) and for 0.5-μm-thick films reaches a minimum value of approximately 440 μΩ cm at about 1021 phosphorus atoms cm−3. The resistivity decreases with increasing deposition temperature which may be attributable to one or a combination of the decreased phosphorus incorporation at higher temperature or deposition rate, or decreased grain size at higher temperature. The decrease in resistivity with increased thickness is attributed to increased grain size with increasing thickness. At least for thicknesses less than 0.5 μm, lower resistivity is achieved by in situ doping than by doping of films subsequent to deposition.
Photoenhanced chemical vapor deposition of zinc phosphide
Authors: Kato Yoshimine , Kurita Shoichi , Suda Toshikazu
Published: July 2002

Thin films of zinc, phosphorus, and zinc phosphide (Zn3P2) have been deposited by photodissociation of dimethylzinc (DMZ) and phosphine (PH3) on various substrates using a low-pressure mercury lamp as a light source. The substrate temperature was varied between room temperature and 250 °C. The deposition rates of the films were significantly affected by the UV light intensity, the density of gases, the PH3/DMZ molar ratio, and the substrate temperature. Zn3P2 microcrystallites were grown on Si(111) substrates at a temperature of 250 °C. Those crystallites were studied by using a scanning electron microscope and reflection high-energy electron diffraction. A very weak photoluminescence spectrum at 808 nm (1.53 eV) was observed at room temperature.
Native defects in gallium arsenide
Authors: Bourgoin J. C. , von Bardeleben H. J. , Stiévenard D.
Published: July 2002

We describe information which has been obtained on point defects detected in various types of GaAs materials using electron paramagnetic resonance as well as electrical and optical techniques. From a comparison of their characteristics and those of simple intrinsic defects (As and Ga interstitials, vacancies and antisites) it is concluded that native defects are not simple intrinsic defects, with the exception of the antisites, but complexes formed by the interaction of such defects between themselves or with impurities. Particular emphasis is given to the As antisite complexed with an As interstitial, the so-called EL2 defect which plays a major role in the electrical properties of bulk materials. Differential thermal analysis, positron annihilation, and x-ray diffraction demonstrate that bulk materials contain a large concentration of vacancy-related defects and As precipitates located along dislocations which play the role of gettering centers. Presumably, bulk materials also contain other As clusters of various sizes although only the smallest ones (EL2) have been detected. All these As clusters are sources of As interstitials which play an important role in thermal treatments. As to semi-insulating materials, their electrical properties result mainly from the compensation between the double donor, called EL2, associated with the As antisite and the double acceptor ascribed to the Ga antisite.
The return of a hysteretic Josephson junction to the zero-voltage state:   I-V characteristic and quantum retrapping
Authors: Chen Y. C. , Fisher Matthew P. A. , Leggett A. J.
Published: July 2002

We study the behavior of a hysteretic current-biased Josephson junction in the vicinity of its return to the zero-voltage states, with primary though not exclusive emphasis on the limit of weak damping (βJ≪1), and under the assumption that the zero-point and thermal energies are both small compared to Icφ0 so that fluctuation effects are important only very close to the return point. We consider in detail the resistively shunted junction (RSJ) and quasiparticle-tunneling models, and also make predictions for more general models. Denoting the value of imposed current I at which return to the zero-voltage state would take place in the absence of fluctuations by Ir, we study in particular (a) the dc current-voltage characteristic in the running state for I−Ir≪Ir, and (b) the first-passage-time statistics of the return to the zero-voltage state induced by both classical and quantum fluctuations. With regard to (b), we express our results in the form of a prediction of the width σ of the distribution of retrapping events as a function of imposed current; this prediction extends down to zero temperature and can be compared directly with the experimentally measured widths.

Our two principal results are as follows: (a) In the running state, for I≪Ir, the current-voltage characteristic should be given quite generally by the formula (I−Ir)/Ir =[(AV0/V)+B]exp−V0/V, where A and B are constants specific to the model, and V0 is a characteristic voltage which for the simplest models is given in the weak-damping limit by V0=ωJφ0+0(β2J) , with ωJ the junction plasma resonance frequency at zero current bias. (b) The square σ2 of the width of the retrapping distribution plotted as a function of I/Ir is given to within logarithmic factors by σ2(T)=const μf(T), where μ≡ℏωJ/Icφ0, the constant is of order 1, and f(T) is a function which tends to 1 as T→0 and is proportional to T in the limit of high T; it is computed explicitly for the RSJ model. We also suggest an explanation (other than lead effects) of the ‘‘forbidden voltage regions’’ which appear to be a characteristic of many high-quality junctions. We discuss the application of our results to the determination of the parameters of Josephson junctions necessary for the investigation of quantum effects on the macroscopic level.
Estimation of the composition parameter of electrochemically colored amorphous hydrogen tungsten oxide films
Authors: Kaneko Hiroko , Miyake Kiyoshi
Published: July 2002

The electrical and optical steady state observed in electrochemical coloration has been studied using asymmetric cells consisting of evaporated amorphous tungsten oxide films with 350–6000 Å thickness. The counter electrode used is indium wire, steel wire, or antimony-tin oxide film, and the electrolyte is a 1-N H2SO4 aqueous solution containing 10 vol % glycerol. The current and optical transmittance of the cells decrease with increasing time during coloration, and simultaneously reach a steady state. The optical density (λ=0.5 μm) in the steady state is proportional to the thickness of the tungsten oxide film, and the absorption coefficient at λ=0.5 μm of the colored oxide film in the state is approximately 9.0×104 cm−1. The effective charges which contribute to the coloration of films calculated from the charge injected until the electro-optical steady state were found to be 1.03–1.20×103 C/cm3. Assuming that the evaporated tungsten oxide films used have a distorted ReO3 structure, and that a hydrogen tungsten bronze HxWO3 is formed by coloration, the composition parameter x calculated from the average value of the effective charge, is 0.36, which is comparable with that of hydrogen tungsten bronze H0.33WO3 obtained for the colored crystalline WO3 films.
Improved laser-beam uniformity using the angular dispersion of frequency-modulated light
Authors: Skupsky S. , Short R. W. , Kessler T. , Craxton R. S. , Letzring S. , Soures J. M.
Published: July 2002

A new technique is presented for obtaining highly smooth focused laser beams. This approach is consistent with the constraints on frequency tripling the light, and it will not produce any significant high-intensity spikes within the laser chain, making the technique attractive for the high-power glass lasers used in current fusion experiments. Smoothing is obtained by imposing a frequency-modulated bandwidth on the laser beam using an electro-optic crystal. A pair of gratings is used to disperse the frequencies across the beam, without distorting the temporal pulse shape. The beam is broken up into beamlets, using a phase plate, such that the beamlet diffraction-limited focal spot is the size of the target. The time-averaged interference between beamlets is greatly reduced because of the frequency differences between the beamlets, and the result is a relatively smooth diffraction-limited intensity pattern on target.
Thermal and mechanical induced stresses in superconducting YBa2Cu3Ox coatings on fibers
Authors: Hsueh C. H. , Becher P. F. , Lackey W. J.
Published: July 2002

Thermal stresses induced during cooling from the fabrication temperature of a fiber with a noncubic superconducting ceramic coating are analyzed theoretically. The coating considered has a preferred crystallographic orientation such that the c axis of the coating material is perpendicular to the fiber surface which, in turn, results in different thermal expansion coefficients in the radial and the tangential directions in the coating. The thermal stresses are shown to result both from the mismatch between the fiber and the coating, and from the thermal expansion anisotropy of the coating. Stresses due to bending of the fiber/coating to form a superconducting component are also addressed. A critical radius of curvature of bending is obtained below which segmentation of the coating is predicted.
The elastic and electromechanical properties of tetragonal BaTiO3 single crystals
Authors: Li Z. , Chan S.-K. , Grimsditch M. H. , Zouboulis E. S.
Published: July 2002

From velocity of sound measurements, obtained using Brillouin scattering and ultrasound techniques, the elastic and piezoelectric constants at room temperature have been determined on high-quality monodomain tetragonal BaTiO3 single crystals. The elastic constants are in fair agreement with those measured previously by the low-frequency equivalent circuit method. However, the electromechanical properties are significantly different. It is suggested that high frequency relaxation may have an influence on the measurements of those parameters.
Thermoelectric properties of a composite medium
Authors: Bergman David J. , Levy Ohad
Published: July 2002

We study the thermoelectric properties of a composite medium. Various approximations are developed for calculating the matrix Q̂e of the bulk effective transport coefficients of the medium, including exact upper and lower bounds for Q̂e under various conditions. Results are especially detailed for two-component composites, where a field decoupling transformation is used to reduce the thermoelectric problem to two uncoupled quasi-conductivity problems. Exact bounds are then obtained for the absolute thermopower αe and the thermoelectric figure of merit Ze in two-component composites. We prove that Ze of the composite can never exceed the largest value of Z in any component. Some of these results are extended to certain classes of multicomponent composites.
Plasma polymerization of 1-benzothiophene
Authors: Tanaka Kazuyoshi , Yamabe Tokio , Takeuchi Tomonari , Yoshizawa Kazunari , Nishio Satoru
Published: July 2002

Semiconductive thin organic polymer films have been prepared by the plasma polymerization of 1-benzothiophene. Characteristics of the polymeric products such as their electrical conductivities, optical properties, and electron spin properties have been examined in detail. Doping of these films with iodine has resulted in an increase in electrical conductivity by eight to nine orders of magnitude. The molecular structure of the products has also been discussed.
Acoustic emission from plastic deformation of a pure single crystal
Authors: Chung Jin-Bok , Kannatey-Asibu Elijah
Published: July 2002

Acoustic emission (AE) during plastic deformation is analyzed for a pure single crystal neglecting the effects of grain boundaries, impurities, and second-phase particles. Acceleration of a moving dislocation is considered to be the principal AE source. There are two major mechanisms of dislocation motion related to acceleration, initial, and continuous oscillatory motion. Initial motion induced by the creation of mobile dislocations is modeled as a step function of velocity. Continuous oscillatory motion produced by interactions with neighboring dislocations is modeled as a harmonic function. These mechanisms vary with strain and strain rate due to dislocation multiplication. AE can thus be described in terms of strain and strain rate. Annihilation at a free surface is also regarded as an AE source in addition to the initial and oscillatory motions. The kinetic and strain energies stored around a moving dislocation are dissipated during annihilation, and can be related to AE. The frequency spectrum of AE is also determined. A shift of the spectrum to higher frequencies with increasing strain is explained by an increase in the interaction force between dislocations.
YBa2Cu3Ox/PrBa2Cu3Ox/YBa2Cu3Ox Josephson ramp junctions
Authors: Gao J. , Boguslavskij Yu. M. , Klopman B. B. G. , Terpstra D. , Wijbrans R. , Gerritsma G. J. , Rogalla H.
Published: July 2002

A detailed study of the fabrication process, current voltage (I-V) characteristics, and Josephson and normal-state properties of the YBa2Cu3Ox(YBCO)/PrBa2Cu3Ox(PBCO)/YBCO ramp junctions is presented. The I-V characteristics can be well described by the resistively shunted junction model. It was found that the critical current Ic and the normal-state conductance 1/Rn scale linearly with the junction area, whereas Ic, the excess current Iex, and IcRn products decrease with increasing barrier thickness. These junctions with cross-sectional area A have a good controllability, low capacitance, and high values of IcRn and RnA products. The coherence length ξn of the PBCO barrier is estimated to be between 5 and 8 nm. As unambiguous evidence of the Josephson behavior, the microwave response as a function of the microwave power as well as the modulations of critical current Ic(H) with applied magnetic field are shown. A modulation depth of more than 95% has been observed. Small proximity effect parameters and junction capacitance (C/A∼10−7 F/cm2) show an advantage of these junctions for many applications.
The effect of microstructural evolution on the coercivity development of Pt-Co-B alloys
Authors: Qiu Ning , Wittig James E.
Published: July 2002

The undercool-rapid-quench (URQ) processing of Pt42Co45B13 (at. %) (Pt-Co-B) alloys with subsequent annealing produced an intrinsic coercivity (Hic) as high as 15.7 kOe. The evolution of Hic during heat treatment is related to the initial URQ-processed microstructure and its influence on the annealing phase transformations. Higher solidification rates refine the dendrites, decrease the interdendritic volume fraction, and enrich the interdendritic boron concentration. Annealing the materials processed with a higher quenching rate leads to more rapid interdendritic decomposition with more refined transformation products and a concomitant larger increase in Hic. The high Hic in Pt-Co-B alloys originates mainly from difficult magnetic domain nucleation owing to limited nucleation sites in magnetic single-domain size Co3B precipitates and an ordered magnetic anisotropic matrix with few defects such as antiphase domain boundaries and twins.
Frequency dependence of the ferromagnetic resonance linewidth and effective linewidth in maganese substituted single crystal barium ferrite
Authors: Karim R. , Ball S. D. , Truedson J. R. , Patton C. E.
Published: July 2002

Ferromagnetic resonance (FMR) and high field effective (HFE) linewidth measurements have been made at 10–90 GHz for low conductivity Mn substituted single crystal barium ferrite, BaFe12−xMnxO19, with x=0.1. The manganese compensates excess Fe2+ and reduces conductivity. Measurements were made on c-plane thin disks magnetized to saturation along the disk axis. A shorted waveguide technique was used to measure the FMR profiles and linewidths in the 55–90 GHz frequency range. High Q cavity techniques and an analysis procedure for the FMR high field tail response were used for the HFE linewidth determinations in the 10–60 GHz frequency range. The FMR linewidths increase linearly with frequency at 0.3 Oe/GHz and have a zero frequency intercept of 20 Oe. The HFE linewidths increase linearly with frequency at 0.4 Oe/GHz and have a zero frequency extrapolation of 4 Oe. This lower zero frequency extrapolation for the HFE linewidth supports an inhomogeneity origin for the 20 Oe FMR linewidth extrapolation. These data indicate intrinsic linewidths for barium ferrite of 0.3–0.4 Oe/GHz, a factor of 6–8 larger than intrinsic linewidths in single crystal yttrium iron garnet.
Materials constants of KNbO3 relevant for electro- and acousto-optics
Authors: Zgonik M. , Schlesser R. , Biaggio I. , Voit E. , Tscherry J. , Günter P.
Published: July 2002

The elastic, piezoelectric, dielectric, elasto-optic, and electro-optic tensors have been determined by numerically evaluating the measurements published until now and using the additional measurements presented in this work. The dependence of the speed of two elastic waves on the applied electric field is measured yielding one electroelastic constant g13133=(95±10)NV−1 m−1. The complete set of parameters consisting of the low-frequency clamped dielectric constants εSij, refractive indices, elastic stiffness constants at constant electric field CEijkl, piezoelectric coefficients eijk, elasto-optic tensor at constant electric field pEijkl, and clamped electro-optic coefficients rSijk is used to calculate effective electro-optic coefficients and effective dielectric constants that have to be used in photorefractive experiments where the elastic deformations associated with a periodic space-charge field have to be considered.
Relaxational self-enhancement of holographic gratings in amorphous As2S3 films
Authors: Ozols A. , Salminen O. , Reinfelde M.
Published: July 2002

A detailed experimental study of the relaxation of holographic gratings in disordered materials is presented. Relaxation parameters of holographic gratings in nonannealed As2S3 amorphous semiconductor films have been measured as a function of aging time, initial diffraction efficiency, recording light intensity, and grating period. The influence of the readout light intensity and sample thickness and its temperature has also been investigated. Relaxational self-enhancements of gratings were found up to 18 times with respect to the initial diffraction efficiency, with a saturation value stable over a period of more than 2 years. The relaxational self-enhancement effect is explained in terms of a phenomenological relaxation model with periodically distributed stress. Under certain conditions, a spatially periodic mechanical stress field resulting from a holographic grating causes anomalous diffusion of unfilled sites. This leads to a density modulation which increases the initial refractive index modulation. The motion of unfilled sites is enabled by the movement of kinetic particles including S atoms. The correlation length of the structural disorder of amorphous As2S3 films is estimated from the relaxational self-enhancement effect measurements to be about 0.5 μm.
Molecular mechanics modeling of shear and the crystal orientation dependence of the elastic precursor shock strength in pentaerythritol tetranitrate
Authors: Dick J. J. , Ritchie J. P.
Published: July 2002

The elastic precursor shock strengths of pentaerythritol tetranitrate explosive crystals were measured for [100], [101], [110], and [001] orientations using velocity interferometer system for any reflector instrumentation for samples 3–6 mm thick. Input shock strength was 1.14 GPa. Measured precursor amplitudes were 0.38, 0.58, 0.98, and 1.22 GPa, respectively, for the four orientations. Critical shear stress for the slip system with the maximum resolved shear stress for each shock orientation was computed. Details of the elastic and plastic wave profiles are discussed. Molecular mechanics modeling of the shear induced by the uniaxial strain of a plane shock wave in this molecular crystal was also performed using the amber code. This may be the first application of molecular mechanics computation to a shear problem. The modeling correctly predicts the dependence of the precursor amplitude on crystal orientation for the cases considered. The results confirm the importance of steric hindrance to shear in controlling the orientation-dependent strength in molecular crystals and sensitivity to shock initiation of detonation in molecular explosive crystals. Details of the molecular deformations and contributions to the energy barrier to inelastic shear for different orientations are given. The computational results also explain why the {110} 〈11̄1〉 slip system is observed in quasistatic deformation in spite of having the longest Burgers vector. The dynamics of sterically hindered, shock-induced shear is considered.
Magnetic and crystallographic order in α-manganese
Authors: Lawson A. C. , Larson Allen C. , Aronson M. C. , Johnson S. , Fisk Z. , Canfield P. C. , Thompson J. D. , Von Dreele R. B.
Published: July 2002

We have made time-of-flight neutron diffraction measurements on α-manganese metal. Powder diffraction measurements were made at 14 temperatures between 15 and 305 K, and single crystal measurements were made at 15 and 300 K. We found that the crystal structure of α-Mn is tetragonal below its Néel point of 100 K, with crystal symmetry I4̄2m and magnetic (Shubnikov) symmetry PI4̄21c. In agreement with the earlier results of Yamada et al., there are six independent magnetic atoms, and we found that their moments are weakly temperature dependent. The onset of magnetic order causes slight changes in the atomic positions and in the average atomic elastic constant.
Colossal magnetoresistance in La-Ca-Mn-O ferromagnetic thin films (invited)
Authors: Jin S. , McCormack M. , Tiefel T. H. , Ramesh R.
Published: July 2002

A colossal magnetoresistance effect with more than a thousandfold change in resistivity (ΔR/RH=127 000% at 77 K, H=6 T) has been obtained in epitaxially grown La-Ca-Mn-O thin films. The effect is negative and isotropic with respect to the field orientations. The magnetoresistance is strongly temperature dependent, and exhibits a sharp peak that can be shifted to near room temperature by adjusting processing parameters. Near-room-temperature ΔR/RH values of ∼1300% at 260 K and ∼400% at 280 K have been observed. The presence of grain boundaries appears to be detrimental to achieving very large magnetoresistance in the lanthanum manganite films. The orders of magnitude change in electrical resistivity could be useful for various magnetic and electric device applications.
Cathodoluminescent characteristics of ZnGa2O4 phosphor grown by radio frequency magnetron sputtering
Authors: Hsieh I. J. , Chu K. T. , Yu C. F. , Feng M. S.
Published: July 2002

Low voltage cathodoluminescent characteristics of ZnGa2O4 phosphor grown by rf magnetron sputtering have been investigated from 300 to 700 nm. The effects of substrate heating and annealing treatment on the luminescent characteristics are also studied. A blue cathodoluminescent emission peaked at 470 nm is observed. Better luminescent properties are achieved on the films which have crystal structure with a standard powder x-ray diffraction pattern of ZnGa2O4. The effect of the strength of the ligand field on the resultant energy levels for the ZnGa2O4 phosphor is investigated. Low-voltage phosphor films with excellent cathodoluminescent characteristics have been successfully developed in this research.
Contribution of the x-ray absorption spectroscopy to study TiO2 thin films prepared by ion beam induced chemical vapor deposition
Authors: Caballero A. , Leinen D. , Fernández A. , Justo A. , Espinós J. P. , González-Elipe A. R.
Published: July 2002

TiO2 thin films have been prepared and their structure analyzed by XRD at grazing angles and x-ray absorption spectroscopies (EXAFS/XANES). The deposition method is an ion beam induced chemical vapor deposition procedure (IBICVD) consisting of the bombardment of a substrate with accelerated O+2 ions while the vapor of a volatile metallic precursor is directed on its surface. The structure of the films was dependent on the type of substrate (glass, fused quartz, and sapphire) and on the conditions used for their preparation (temperature of substrate and beam energy of 1 or 10 keV). Thus, while on glass and silica/sapphire, an amorphous structure is formed when the substrate is at 300 K during preparation, a rutile-rich structure is obtained if the substrate is at 573 K during deposition or, even to a larger degree, when using ions of 10 keV. The amorphous structure of the films grown on glass yield 100% anatase after calcining at 773 K. On sapphire, the crystallization was less perfect and besides anatase, rutile and amorphous phases were detected after calcination at that temperature. The films prepared at 573 K or with O+2 ions of 10 keV yield after calcining a well ordered structure of rutile. A detailed analysis of the EXAFS and XANES spectra has permitted a better characterization of the amorphous and crystalline phases in the films and provided a better understanding of the crystallization processes during calcination. The contribution of the different factors controlling the crystallization of the films is discussed.
Acousto-optic modulation in diffusive semiconductors
Authors: Neogi A.
Published: July 2002

The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analyzed in a strain-dependent semiconductor crystal. The effect of the diffusion of charge carriers due to the doping of the medium has been investigated using the coupled-mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear current density of the medium. The AO modulation process has been treated as a four-wave parametric mixing process and the effective third-order acousto-optic susceptibility characterizing the instability process has been deduced. The AO modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold characteristics and the steady state growth rates are estimated from the acousto-optic polarization of the medium. Analytical estimation reveals that the modulated beam can be amplified in a dispersionless acoustic wave interaction regime in the presence of enhanced diffusion due to excess charge carriers. The relative magnitude of the pump field at various doping levels exhibits entirely different steady-state gain characteristics. A particular pump field Er exhibits maximum gain in the lightly doped regime while the same field exhibits a minimum due to reverse energy flow from electromagnetic fields to the collision dominated space-charge field in a heavily doped medium. At very high densities with the electron plasma frequency of the medium close to the pump frequency or under the influence of very high (drift) pump field the acoustoelectric domains are washed out due to domination of the drift process over diffusion instability process. This leads either to gain saturation in lightly doped regime or explosive increment of the gain constant in heavily doped regime. The magnitude of the third-order nonlinear optical susceptibility for III-V semiconductors obtained from our theoretical analyses is found to agree well with the previously reported values.
Influence of copper contamination on recombination activity of misfit dislocations in SiGe/Si epilayers: Temperature dependence of activity as a marker characterizing the contamination level
Authors: Kittler M. , Ulhaq-Bouillet C. , Higgs V.
Published: July 2002

The technique of electron-beam-induced current (EBIC) has been used to study the recombination activity of misfit dislocations in Si/SiGe epilayers. EBIC contrast measurements recorded as a function of temperature (T=80–300 K), c(T), were found to show a completely altered character following copper contamination of SiGe epilayers. In as-grown ‘‘clean’’ material the dislocations were found to exhibit a very small contrast at low temperature only. For a Cu contamination of about 1 ppb the contrast increased markedly at low temperatures but remained invisible near room temperature. This c(T) behavior can be attributed to shallow trap levels at the dislocations. For a Cu contamination around 15 ppb the majority of dislocations exhibited contrast in the whole temperature range, being a consequence of near-midgap centers at the dislocation. Hydrogen plasma treatment of these dislocations was observed to passivate the contrast near room temperature but did not show a pronounced effect on the contrast at low temperatures, so that the very small dislocation contrast found for clean material was not restored by hydrogen. A Cu contamination treatment in the ppm range resulted in a dramatic increase of the contrasts in the whole temperature range. Investigations by transmission electron microscopy (TEM) revealed in that material copper precipitates connected with the misfit dislocations. In contrast to the low-contaminated material no direct decoration of the dislocations could be observed. TEM images revealed that the EBIC dislocation line contrast corresponded to bundles of up to 15 individual dislocations.
Surface diffusion lengths of adatoms on 6H-SiC{0001} faces in chemical vapor deposition of SiC
Authors: Kimoto Tsunenobu , Matsunami Hiroyuki
Published: July 2002

Surface kinetics on 6H-SiC{0001} faces in chemical vapor deposition of SiC at 1200–1600 °C were studied. Based on a simple diffusion model, surface diffusion lengths were estimated from the lateral growth rates of macrosteps. No significant difference in surface diffusion lengths was observed on Si and C faces. The diffusion length decreased with the reduction of growth temperature. This decrease can be ascribed to the suppressed surface diffusion and to the enhanced incorporation of migrating species into growth nuclei, of which number significantly increased at low temperatures. The effects of gas flow rates on the surface diffusion is also discussed.
Perturbed angular correlations study of thin Cu–In films
Authors: Dzionk C. , Metzner H. , Lewerenz H. J. , Mahnke H.-E.
Published: July 2002

Perturbed angular correlations (PAC) of γ rays using 111In nuclear probes are applied to study the phase formation in thin Cu–In films used as precursors for the preparation of CuInS2 by reactive annealing in H2S atmosphere. The Cu–In films are prepared by the successive thermal evaporation of In and Cu in UHV. By means of PAC we are able to obtain a true image of all In-containing phases in the sample. Improved PAC parameters for the phases Cu7In3, Cu11In9, and CuIn2 are given, and the stability of the phase CuIn2 is determined. The phase compositions of copper-rich, stoichiometric, and indium-rich precursors are investigated after annealing procedures between room temperature and 150 °C. The phase formation in this temperature region is governed by two fast diffusion processes which lead to the appearance of Cu11In9 and CuIn2, respectively, as the only intermetallic compounds.
Ion kinetic-energy distributions and Balmer-alpha (Hα) excitation in Ar-H2 radio-frequency discharges
Authors: Radovanov S. B. , Olthoff J. K. , Van Brunt R. J. , Djurović S.
Published: July 2002

Excited neutrals and fast ions produced in a 13.56 MHz radio-frequency discharge in a 90% argon −10% hydrogen gas mixture were investigated, respectively, by spatially and temporally resolved optical emission spectroscopy, and by mass-resolved measurements of ion kinetic energy distributions at the grounded electrode. The electrical characteristics of the discharge were also measured and comparisons are made with results obtained for discharges in pure H2 under comparable conditions. Measurements of Balmer-alpha (Hα) emission show Doppler-broadened emission that is due to the excitation of fast atomic hydrogen neutrals formed from ion neutralization processes in the discharge. Temporally and spatially resolved emission profiles of the Hα radiation from the Ar-H2 mixture are presented for the ‘‘slow’’ component produced predominately by electron-impact dissociative excitation of H2, and for the ‘‘fast’’ component corresponding to energies much greater than can be derived from dissociative excitation. For the Ar-H2 mixture, the fast component is significantly enhanced relative to the slow component. The measured kinetic-energy distributions and fluxes of predominant ions in the Ar-H2 mixture, such as H3+, H2+, H+, and ArH+, suggest mechanisms for the formation of fast hydrogen atoms. The interpretation of results indicate that H+ and/or H3+, neutralized and backscattered by collision with the powered electrode, are the likely sources of fast hydrogen atoms that produce Doppler-shifted Hα emission in the discharge. There is also evidence at the highest pressures and voltages of ‘‘runaway’’ H+ ions formed near the powered electrode, and detected with kinetics energies exceeding 100 eV at the grounded electrode.
Tetrahedral amorphous carbon films prepared by magnetron sputtering and dc ion plating
Authors: Schwan J. , Ulrich S. , Roth H. , Ehrhardt H. , Silva S. R. P. , Robertson J. , Samlenski R. , Brenn R.
Published: July 2002

Highly tetrahedral, dense amorphous carbon (ta-C) films have been deposited using rf sputtering of graphite by an unbalanced magnetron with intense dc Ar-ion plating at low temperatures (<70 °C). The ratio of the argon ion flux to neutral carbon flux Φi/Φn is about 5. The film density and compressive stress are found to pass through a maximum of 2.7 g/cm3 and 16 GPa, respectively, at an ion plating energy of about 100 eV. Experiments with higher ion flux ratios of Φi/Φn=10 show that it is possible to deposit carbon films with densities up to 3.1 g/cm3 and sp3 contents up to 87%. Deposition of ta-C in this experiment when the energetic species is Ar appears to require a minimum stress of 14 GPa to create significant sp3 bonding, which contrasts with the continuous increase in sp3 content with stress when the energetic species is C ions themselves. These results are used to discuss possible deposition mechanisms.
Spectroscopic observation of interface states of ultrathin silicon oxide
Authors: Yamashita Y. , Namba K. , Nakato Y. , Nishioka Y. , Kobayashi H.
Published: July 2002

Interface states in the Si band gap present at oxide/Si(100) interfaces for ∼3-nm-thick Pt/2.1∼3.6-nm-thick silicon oxide/n-Si(100) metal–oxide–semiconductor devices are investigated by measurements of x-ray photoelectron spectra under biases between the Pt layer and the Si substrate, and their energy distribution is obtained by analyzing the amount of the energy shift of the substrate Si 2p3/2 peak measured as a function of the bias voltage. All the interface states observed using this new technique have discrete energy levels, showing that they are due to defect states. For the oxide layer formed in H2SO4+H2O2, the interface states have three density maxima at ∼0.3, ∼0.5, and ∼0.7 eV above the valence-band maximum (VBM). For the oxide layer produced in HNO3, two density maxima appear at ∼0.3 and ∼0.7 eV above the VBM. The energy distribution for the oxide layer grown in HCl+H2O2 has one peak at ∼0.5 eV. The 0.5 eV interface state is attributed to the isolated Si dangling bond defect. The 0.3 and 0.7 eV interface states are, respectively, due to Si dangling bonds with which Si and oxygen atoms in the silicon oxide layer interact weakly. The oxide layer formed in HCl+H2O2 has the highest-density interface states. The oxide layer produced in HNO3 has the lowest-density interface states and, thus, the final cleaning using HNO3 is recommended for the device fabrication.