bluuebunny/crossref_metadata_2025 huggingface dataset:
Amorphous ferroelectric (Ba0.67Sr0.33)Ti1.02O3 thin films with enhanced H2 induced interfacial polarization potential
Authors: Zhu W. , Tan O. K. , Yao X.
Published: July 2002

Ferroelectric (Ba0.67Sr0.33)Ti1.02O3 thin films have been prepared by the sol-gel technology and characterized using thermogravimetric analysis, differential thermal analysis, x-ray diffraction, dielectric characterizations, and gas sensing measurements. The (Ba0.67Sr0.33)Ti1.02O3 thin film capacitive devices are made on Si substrate to detect hydrogen gas and to study hydrogen induced interfacial polarization potential. Experimental results show that the Schottky I–V behavior appears in these Pd/amorphous (Ba,Sr)TiO3 thin film/metal capacitive devices, both in air and in diluted hydrogen gas environment, and that the enhanced interfacial dipole potential as large as 4.5 V at 1042 ppm hydrogen gas in air has been observed. Compared with the available data in the literature, the obtained value of hydrogen induced interfacial polarization potential in our experiment is about seven times larger than the best one reported under similar testing conditions. It has been clearly shown that the hydrogen induced interfacial polarization potential is closely correlated with the microstructure of ferroelectric thin films and the enhancement of this interfacial polarization potential is mainly attributed to the high dielectric constant of amorphous ferroelectric thin films. A model is also proposed to explain this interesting phenomenon. In this model, hydrogen H2 molecules are dissociated at the top surface of the catalytic Pd layer and ionized under the positive bias. The H+ ions then diffuse through this Pd layer, accumulate at the interface between the Pd layer and the amorphous ferroelectric passivation film. Dipoles are thus formed so that the polarization potential is built up at the interface. Moreover, the high dielectric constant of ferroelectric films enhances dipole polarization, thus greatly improves the H2 gas induced polarization potential. Though in a preliminary stage, our experimental results offer great promise in fabricating large-scale, Si based ferroelectric thin film gas sensors and related electronic devices.
First and second order velocity shifts and reflection coefficients for periodic interdigital transducers by boundary condition matching
Authors: Thoma C. , Hahn Y.
Published: July 2002

The method of boundary condition matching is utilized to determine reflection coefficients and velocity shifts for surface acoustic wave devices mechanically and electrically loaded by a periodic grating. The method has previously been limited to grooved gratings in isotropic substrates, but is here extended to allow for anisotropic and piezoelectric substrates as well as arbitrary overlay materials which allows for the modeling of metallic electrode interdigital transducers. Upper and lower stop band frequencies are determined by imposing the proper grating boundary conditions. From these frequencies is calculated both first and second order contributions in h/λ (where h is the grating finger height and λ the wavelength) to the reflection coefficient and velocity shift due to the loading of the substrate. The presence of the grating also couples the incident surface acoustic waves to evanescent and bulk modes. The attenuation of the incident Rayleigh waves due to bulk mode conversion is discussed as well. Numerical results for some common substrate and overlay materials are given and a comparison of the relative strengths of the first and second order contributions for specific values of h/λ is made. Some comparisons with experimental data are also given with good results for sufficiently small h/λ.
A study of deep centers in Zn1−xMgxSe crystals using deep-level transient spectroscopy
Authors: Beyer R. , Burghardt H. , Firszt F. , Zahn D. R. T.
Published: July 2002

Defects in Zn1−xMgxSe have been studied using deep-level transient spectroscopy. The crystalline material with low Mg concentration (x=0.09, 0.15), obtained by the high-pressure Bridgman method, has a sphalerite structure and shows n-type conductivity. A major electron trap was found with a thermal activation energy of 0.37 eV (x=0.09) and 0.38 eV (x=0.15), respectively. The trap concentration was 1.5–2.7×1014 cm−3. From capture measurements we obtained the electron capture rate cn as a function of the temperature, providing information about the thermal activation energy of the capture process. The capture barrier ΔEb splits into a high-temperature section with Eb=145 meV and a low-temperature section with a value of 66 meV for Eb. It is assumed that the predominant trap in Zn1−xMgxSe has the same origin as the well-known ∼0.32 eV trap in ZnSe.
Self-consistent analysis of persistent photoconductivity data in Si δ-doped AlxGa1−xAs (x=0.32) superlattices
Authors: Mejri H. , Ben Jazia A. , Maaref H. , Etienne B. , Priester C.
Published: July 2002

Hall measurements were performed on a series of Si δ-doped AlxGa1−xAs (x=0.32) superlattices. The DX center has been detected in these structures through the observation of persistent photoconductivity at low temperature. Electrical properties of this defect are investigated as a function of the δ-doping content. The results have been analyzed using the large lattice relaxation model. We have calculated the minibands for the δ-doping AlxGa1−xAs:Si superlattices studied as well. Evidence that the Si DX center is present in these structures and has a negative-U character will be given in this study.
Strong interface effects in graded SiO2/Si/SiO2 quantum wells
Authors: de Sousa J. S. , Farias G. A. , Freire V. N. , da Silva E. F.
Published: July 2002

We investigate the influence of SiOx interfaces in SiO2/Si/SiO2 single quantum wells in terms of the changes that the transition layers impose on the electron bound states. In our numerical calculations, we consider the spatial dependence of the electron effective mass through the SiOx interfacial monolayers. The electron energy levels in graded SiO2/Si/SiO2 wells are strongly blue-shifted in comparison to those calculated when the existence of interfacial SiOx transition layers is disregarded. In the case of an 11 Å SiO2/Si/SiO2 well with a single interfacial SiOx monolayer, we show that the first and the second energy levels are blueshifted by as much as 285 and 720 meV, respectively.
Starting mechanisms and dynamics of bubble formation induced by a Ho:Yttrium aluminum garnet laser in water
Authors: Frenz Martin , Könz Flurin , Pratisto Hans , Weber Heinz P. , Silenok Alexander S. , Konov Vitaly I.
Published: July 2002

The starting mechanisms and dynamics of laser-induced bubble formation at a submerged fiber tip in distilled water were experimentally investigated using pressure measurements and fast flash videography. A fiber guided Ho:YAG laser operating in the free running (τ=200 μs) and Q-switched (τ=45 ns) mode at a wavelength of λ=2.12 μm was used as a light source. It is shown that the beam profile at the distal fiber tip (multimode fiber d=300 μm) exhibits hot spots that result in an inhomogeneous temperature distribution in the heated water volume. Depending on the laser irradiance, three different bubble formation processes are distinguished: bubble formation by heating, by rarefraction (cavitation), and by a combination of these two processes. For laser irradiances of less than 0.5 MW/ cm2 bubble formation takes place at temperatures near the critical point of water (T=280 °C). A rapid decrease in the threshold temperature for bubble formation was found for laser irradiances between 0.5 and 1 MW/cm 2. At laser irradiances higher than 3 MW/cm2, microbubbles with radii of up to 20 μm were formed at the front of the laser pulse even though the average water temperature was far below 100 °C. The water temperature distribution during the laser pulse was determined by numerical simulation. Simultaneous pressure measurements revealed that each subablative laser spike induces a bipolar pressure transient. The onset of the bubble expansion was found to be correlated with a characteristic pressure increase that can be used for on-line monitoring of the ablation process. The distortion of the temporal profile of the pressure wave is shown to be an effect of diffraction. The reduction of pressure by the negative part of the bipolar pressure transients leads to a lowering of the evaporation pressure and therefore to the initiation of bubbles by cavitation. With increasing irradiance this mechanism becomes more efficient.
Luminescence decay kinetics in homogeneously and delta-doped ZnS:Mn
Authors: Park W. , Jones T. C. , Tong W. , Schön S. , Chaichimansour M. , Wagner B. K. , Summers C. J.
Published: July 2002

The luminescence decay kinetics of homogeneously and delta-doped ZnS:Mn thin film phosphors was investigated. A quantitative model based on the hopping model of energy transfer theory was developed to described the concentration quenching phenomenon in ZnS:Mn. The model predicted the dependence of the energy transfer rate on material parameters such as the Mn and defect concentrations. The luminescence decay of homogeneously doped ZnS:Mn consisted of two exponential components at 10 K. The fast component of 120 μs was attributed to exchange-coupled pair emission and the slow component of 1.6 ms to isolated Mn ions. As the temperature was increased, the exchange-coupled pair emission disappeared and the decay became strongly nonexponential. The nonexponentiality was attributed to nonradiative energy transfer processes. The concentration dependence of the effective lifetime was also found to change with temperature. The investigation on the temperature dependence revealed two regimes of concentration which showed distinct temperature dependencies. From the temperature dependence, it was concluded that the energy transfer between Mn ions was active only when the Mn concentration was greater than 2 at. %. By comparing these results with the results of Dexter’s theory, the energy transfer between Mn ions was found to be mediated by an electric dipole–dipole interaction. The delta-doped ZnS:Mn showed faster decay times due to the enhanced overlap between 3d and s-p host states caused by lattice strain. From the temperature dependence, a two-dimensional confinement of energy transfer was observed when the doping planes were far apart. However, when the doping planes were brought close together, the delta-doped samples behaved similarly to the homogeneously doped ZnS:Mn indicating that the energy transfer was no longer two-dimensionally confined.
The effect of the coupling agent on the packing density and corrosion behavior of NdFeB and SmCo bonded magnets
Authors: Chen Q. , Asuncion J. , Landi J. , Ma B. M.
Published: July 2002

The effects of the coupling agent addition on the specific density and corrosion behavior of NdFeB and SmCo bonded magnets have been investigated. The application of LICA44, a titanate coupling agent, improved the filled volume of the epoxy-based bonded magnets, especially at high compression pressures. When molded at 10.5 T/cm2, specific densities of 6.2 and 6.5 g/cm3 have been obtained for the NdFeB and SmCo bonded magnets, respectively. Incorporating a coupling agent was found to eliminate visible cracks on the SmCo bonded magnets, possibly due to the improvements in the wettability and compatibility of the system. The percentage of the filled volume was increased by 2%–7%. Incorporating LICA44 also improved corrosion resistance of the bonded magnets. For NdFeB, LICA44 reduced the steady state corrosion current Icorr from 69 to 38 μA/cm2. No significant changes in the Icorr were observed for SmCo due to the stable nature of SmCo.
Ferromagnetic/antiferromagnetic structures with ferromagnetic interlayer coupling
Authors: Edelstein A. S. , Kodama R. H. , Miller M. , Browning V. , Lubitz P. , Sieber H.
Published: July 2002

Multilayer samples and structures composed of NiO(t)/Co(2.5 nm)/NiO(t)/Py(2.5 nm)/NiO(t) with t=6.6 and 13.2 nm exhibit ferromagnetic interlayer coupling below 400 K. A crossover is observed between the behavior near 300 K and at 10 K in ferromagnetic(F)/antiferromagnetic(AF) structures. Effects observed near 300 K, but not at 10 K, are an enhancement of Hc and the M–H loop squareness in F/AF multilayers as one increases the number of layers. A possible explanation for the crossover is given based on the domain wall thickness becoming less than the AF film thickness.
Effects of Cr, CrV, and CrTi underlayers on magnetic, structural, and materials reliability properties of CoPt thin films
Authors: Singleton E. W. , Narayan P. B. , Xiong Wei , Raman Ramas , Hoo Hung-Lee
Published: July 2002

Co 80 Pt 20 thin films on Cr, Cr80V20, or Cr90Ti10 underlayers were studied to improve the magnetic properties for permanent magnetic stabilization layer application in anistropic magnetoresistance/giant magnetoresistance devices. A significant improvement of nearly 15% in coercivity was achieved for CoPt films on CrTi underlayer, when compared to either Cr or CrV underlayer. X-ray diffraction pattern analysis showed that CrTi underlayer increased the intensity of CoPt (10.0) indicating enhanced crystallographic in-plane texture. With CrTi underlayer, CoPt coercivity increased linearly as Mrt was reduced from 4.3 to as low as 0.4 memu/cm2. The squareness and S* values started to drop sharply when the Mrt of CoPt film decreased to below 1.0 memu/cm2. In addition to 15% higher coercivity, another important property of CrTi/CoPt was that for CrTi underlayer thickness between 3 and 20 nm very little change in the coercivity, squareness, and S* of CoPt layer was observed. The CoPt films showed excellent corrosion resistance towards high humidity and good thermal stability up to 300 °C. The CoPt films with the underlayer of CrTi and CrV showed slightly less corrosion resistance towards industrial pollutants than those with no underlayer and Cr underlayer.
Enlarged paramagnetism by electron–electron exchange interactions in n-type modulation doped Cd1−xMnxTe/Cd1−x−yMnxMgyTe single quantum wells
Authors: Shen J. X. , Oka Y. , Ossau W. , Fischer F. , Waag A. , Landwehr G.
Published: July 2002

Electron spin polarization is studied on n-type modulation doped Cd1−xMnxTe/Cd1−x−yMnxMgyTe single quantum wells. The influence of the electron–electron exchange interaction on the electron spin polarization is discussed. In magnetic fields, the electronic states undergo an initial giant splitting due to the s–d and p–d exchange interactions with localized magnetic moments. Such a giant Zeeman splitting creates an initial spin polarization which will be further enlarged due to the many-body effects of the electron self-energy corrections.
Magneto-optical studies in NiO/Fe2O3 multilayers
Authors: Krishnan R. , Lopušnı́k R. , Višňovský Š. , Guyot M. , Keller N. , Tessier M.
Published: July 2002

[ Fe 2 O 3 /NiO ] N multilayer films prepared by pulsed laser deposition display oscillation observed in the magnetization M as a function of N or the number of interfaces i=2N−1 indicating that they are not symmetric. In this work magneto-optic (MO) polar Kerr rotation spectra of some samples in the photon energy range 1.2–4.5 eV are reported. Faraday rotation (FR) was studied at 632.6 and 550 nm laser wavelengths. It shows the oscillations with N in accordance with M. Polar Kerr rotation spectra show a series of peaks which could be assigned to nickel ferrite NiFe2O4 formed at the interfaces between NiO and αFe2O3. The amplitudes change strongly with N as expected from N dependence of FR and M. A simple analytical expression is proposed to explain the complex phase relations among the contributions from individual ferrimagnetic interfaces to the total MO Kerr rotation in the multilayers.
Preparation of colored iron particles by multistratification with alternate thin layers of SiO2 and TiO2
Authors: Kishimoto Akira , Nakatsuka Katsuto , Atarashi Takafumi
Published: July 2002

Coloring ferromagnetic fine particles was achieved only by coating the particles with SiO2 and TiO2 alternate layers of appropriate thickness that act as light interference filters. We propose a new model called the “hemisphere model” for the calculation of the optimum thickness of layers on spherical particles to obtain any specific colored particles. The stratification of alternative thin layers of SiO2 and TiO2 layers was achieved by deposition of amorphous SiO2 and quasigranular TiO2 layers by the sol-gel method.
High field electron transport properties of bulk ZnO
Authors: Albrecht J. D. , Ruden P. P. , Limpijumnong S. , Lambrecht W. R. L. , Brennan K. F.
Published: July 2002

The Monte Carlo method is used to simulate electron transport for electric field strengths up to 350 kV/cm in bulk, wurtzite structure ZnO. The relevant parts of the conduction bands of a first-principles band structure are approximated by spherically symmetric, nonparabolic valleys located at the Γ and Umin symmetry points of the Brillouin zone. It is shown that the analytic expressions represent the band structure and the density of states well over a range of nearly 5 eV from the bottom of the conduction band. The simulated electron steady-state drift velocity versus electric field characteristics are calculated for lattice temperatures of 300, 450, and 600 K. For room temperature, drift velocities higher than 3×107 cm/s are reached at fields near 250 kV/cm. Examination of the electron energy distributions shows that the strong decrease of the differential mobility with increasing electric field in the field range studied is to be associated with the pronounced nonparabolicity of the central valley and not with transfer of electrons to satellite valleys.
Exchange coupling of NiFe/CrMnPtx bilayers prepared by a substrate bias sputtering method
Authors: Xi Haiwen , White Robert M.
Published: July 2002

We have investigated the unidirectional exchange anisotropy between a ferromagnetic Ni81Fe19 film and a disordered antiferromagnetic CrMnPtx (x=3, 6, or 9) film prepared by substrate bias sputtering. A hysteresis loop with the shape predicted by the planar domain-wall model was found in the sample without substrate bias, suggesting that the interfacial exchange coupling energy between the Ni81Fe19 and CrMnPtx layers is comparable to the antiferromagnetic domain-wall energy. The substrate bias on the Ni81Fe19 underlayers improved the texture of the CrMnPtx layers but also increased the interface roughness. The substrate bias effect on the exchange bias can be understood in terms of the planar domain-wall model. The interface roughness reduces the interfacial exchange coupling by introducing defects and disordering the antiferromagnetic moments at the interface. This leads to a triangular hysteresis loop. The texture improvement increases the exchange bias by increasing the domain-wall energy of the antiferromagnet. The differences between these substrate bias effects and those observed in the Ni81Fe19/Fe50Mn50 bilayer system are also discussed.
Residual lattice strain in thin silicon-on-insulator bonded wafers: Thermal behavior and formation mechanisms
Authors: Iida Tsutomu , Itoh Takamasa , Noguchi Daisuke , Takano Yukio
Published: July 2002

The residual lattice strain in annealed bonded silicon-on-insulator (SOI) wafers was investigated as a function of the temperature and duration of the heat treatment. Measurements were made in a temperature range from 900 to 1050 °C and annealing times were from 6 to 30 h. In as-received SOI wafers, a tensile strain parallel to the surface of the order of 10−4 was observed. For specimens annealed at greater than 950 °C, an abrupt change from tensile to compressive strain was observed after 12–15 h annealing, and then the strain approached a saturation value with increasing annealing time. However, at 900 °C no such change to compressive strain was observed even after long annealing times. In view of this, the onset of this strain behavior is consistent with viscous flow of the SiO2. Although it might be expected that the strain in the SOI layer would be greater for thinner layers, the observed strain was found to decrease with decreasing SOI layer thickness. There was no significant variation in the radius of curvature of samples with different SOI layer thicknesses. The radius of curvature is determined mainly by the Si substrate and the buried oxide, and the thermal behavior of these two layers depends upon the heat treatment conditions, particularly the time and the temperature. Taking into account the strain formation mechanism, these results suggest the possibility of plastic deformation of the SiO2 at the SOI–SiO2 and the SiO2–Si substrate interfaces during heat treatment.
Surface-energy-driven intermixing and its effect on the measurement of interface stress
Authors: Clemens B. M. , Nix W. D. , Ramaswamy V.
Published: July 2002

We show that the technique of extracting interface stresses in multilayers using a force balance between layer stresses and substrate forces is extremely sensitive to small amounts of intermixing. Intermixing of as little as one half of a monolayer can produce apparent interface stresses of −2 N/m unless changes in the unstrained lattice parameter are accounted for. Diffusion and intermixing driven by large surface energy differences has been observed in several systems with large positive heats of mixing and limited bulk solid solubility, and is consistent with recently reported lattice parameter measurements in Ag/Ni multilayers. We also present in situ stress measurements in the Ag/Ni system that are consistent with this surface-energy-driven intermixing. We conclude that the reported imbalance between substrate and layer stress forces in Ag/Ni is probably not due to negative interface stresses but is more consistent with the effects of surface-energy-driven intermixing.
Spectral response of porous silicon based photovoltaic devices
Authors: Ünal Bayram , Bayliss Sue C. , Clarke David T.
Published: July 2002

The spectral responses of photovoltaic devices based on metal/porous silicon (PS)/silicon (Si) sandwich structures are presented. At room temperature, the photoresponse (photovoltage or photocurrent) of the devices has been measured by the variation of the optical excitation energy from a synchrotron radiation source from the near UV to the near IR. The highest photosensitivity was found to be around a wavelength of ∼500 nm, corresponding to the maximum intensity of the solar spectrum. Having a wider photoresponse full width at half maximum (FWHM) than conventional forms of silicon used for solar cells within the earlier excitation energy range, PS has promising potential for enhanced photosensitivity efficiency. Such improvements could be made by modification of the Si nanostructure, since the peak position and FWHM were found to correlate with the size and size distributions of the pores, respectively, of the Si nanocrystallites in the porous Si layers. Gains could also be made in the contact design, and in any case such devices are more stable than their light emitting diode analogues.
Optical properties of GaN quantum dots
Authors: Ramvall Peter , Riblet Philippe , Nomura Shintaro , Aoyagi Yoshinobu , Tanaka Satoru
Published: July 2002

We report on an investigation of the optical properties of GaN quantum dots (QDs) grown by means of metalorganic vapor phase epitaxy. The growth regime for GaN on AlxGa1−xN was observed to change from two- to three-dimensional, forming GaN QDs, when Si was deposited on the AlxGa1−xN surface prior to the GaN growth. These QDs showed a redshift of the photo luminescence (PL) energy from the increased Coulomb energy induced by a compression of the exciton Bohr radius. Furthermore, a diminishing temperature-dependent shift of the PL energy with decreasing QD size caused by a reduction of the longitudinal-optical phonon coupling was found. We also show that the size of the QDs is a critical parameter for the optical nonlinearities. For large dots, the dominant nonlinearity in the PL is the bandgap renormalization but when the size of the dots was reduced below the critical size of 10 nm thick and 30 nm diameter, the state-filling effect became dominant.
Mechanism for strong nonlinearities in the Faraday rotation and absorption in the ferromagnetic semiconductor CdCr2Se4
Authors: Golik L. L. , Kun’kova Z. E. , Heide C.
Published: July 2002

At high laser radiation intensities a strong nonlinear Faraday rotation (FR) may be observed in a relatively narrow energy band near the absorption edge of CdCr2Se4. To study the nature of this effect, we investigated the influence of the incident radiation intensity on the spectral properties of the FR and the circularly polarized light absorption, and studied the dynamics of the observed nonlinearities. As the laser radiation intensity increases, we find a strong growth of the resonance structure at the absorption edge and an enhancement of the FR in the band associated with the resonance. The nonlinear FR is a non-monotonic function of the radiation intensity and reaches a local maximum, which is about twice the value of the rotation angle for the linear effect. The dynamics of the observed nonlinearities show that the nonlinearities are caused by photo-excited charge carriers. These carriers screen the internal electrical fields and lead to a narrowing and growth of the excitonic resonance in the absorption spectra and FR.
Columnar defect induced phase transformation in epitaxial La0.7Ca0.3MnO3 films
Authors: Ogale S. B. , Li Y. H. , Rajeswari M. , Riba L. Salamanca , Ramesh R. , Venkatesan T. , Millis A. J. , Kumar Ravi , Mehta G. K. , Bathe Ravi , Patil S. I.
Published: July 2002

Epitaxial La0.7Ca0.3MnO3 thin films on the SrTiO3(100) surface have been irradiated with 250 MeV Ag17+ ions at different nominal fluence values in the range of 5×1010–4×1011 ions/cm2, resulting in columnar defects. At low fluences these defects cause changes in material properties that are small and scale linearly with dosage. Above a threshold fluence value ∼3×1011 ions/cm2 dramatic changes are observed, including an order of magnitude increase in the resistivity and 50 K drop in the Curie temperature. Transmission electron microscopy measurements show that the changes are associated with a phase transformation of the undamaged region between the columnar defects. The transformed phase has a diffraction pattern very similar to that seen in charge-ordered La0.5Ca0.5MnO3. We propose that above a critical level of ion damage, strains caused by the presence of the columnar defects induce a charge-ordering phase transition that causes the observed dramatic changes in physical properties. We speculate that a conceptually similar surface-induced charge ordering may be responsible for the “dead layer” observed in very thin strained films, and the dramatic changes in optical properties induced by polishing, and that an impurity-induced charge ordering causes the extreme sensitivity of properties to lattice substitution.
Twentyfold blue upconversion emission enhancement through thermal effects in Pr3+/Yb3+-codoped fluoroindate glasses excited at 1.064 μm
Authors: Oliveira A. S. , Gouveia E. A. , de Araujo M. T. , Gouveia-Neto A. S. , de Araújo Cid B. , Messaddeq Y.
Published: July 2002

Infrared-to-visible upconversion emission enhancement through thermal effects in Yb3+-sensitized Pr3+-doped fluoroindate glasses excited at 1.064 μm is investigated. A twentyfold increase in the 485 nm blue emission intensity as the sample temperature was varied from 20 to 260 °C was observed. The visible upconversion fluorescence enhancement is ascribed to the temperature dependent multiphonon-assisted anti-Stokes excitation of the ytterbium sensitizer and excited-state absorption of the praseodymium acceptor. A model based upon conventional rate equations considering a temperature dependent effective absorption cross section for the 2F7/2→2F5/2 transition of the Yb3+ and 1G4→3P0 excited-state absorption of the Pr3+, agrees very well with the experimental results.
Very large magnetoresistance in electrodeposited single-crystal Bi thin films (invited)
Authors: Chien C. L. , Yang F. Y. , Liu Kai , Reich D. H. , Searson P. C.
Published: July 2002

Single-crystal bismuth thin films, fabricated by electrodeposition and suitable annealing, exhibit very large magnetoresistance of 400 000% at 5 K and 300% at 300 K, as well as pronounced Shubnikov–de Haas oscillations. A hybrid structure demonstrates the potential for field sensing with sensitivities of 35%/Oe at 5 K and 0.2%/Oe at 300 K.
A photo-oxidation mechanism for patterning and hologram formation in conjugated polymer/glass composites
Authors: Levi Ofer , Perepelitsa Galina , Davidov Dan , Shalom Shoshy , Benjamin Iris , Neumann Ronny , Agranat Aharon J. , Avny Yair
Published: July 2002

Improved diffraction efficiency was observed in holograms stored in disordered conjugated polymer/glass composites. The conjugated polymers used were alkoxy substituted poly(phenylenevinylne) analogs and the glass matrices were zirconia-organosilica xerogels. Investigation of the mechanism of hologram formation revealed evidence of a photochromic process consisting of light induced photo-oxidation (bleaching) of the embedded conjugated polymer resulting in the formation of an absorption grating and a phase grating. Investigation of the hologram formation revealed that the process was oxygen dependent. Oxygen removal increases hologram formation time by more than an order of magnitude and halves the total hologram efficiency. The oxygen dependence was also highly correlated with photobleaching of the samples and beam interaction of the writing beams. The chemical transformations upon photobleaching were shown by infrared and Raman spectroscopy to involve chain scission and oxidation of the polymer at the vinylic position of the conjugated polymer. Film preparation of the composites was optimized showing a tenfold improvement in the holographic properties compared to our previous results. The optimized treatment method allows for a high, >20%, diffraction efficiency, η, to be obtained for the 2.5-μm-thick polymer/glass films. Light sensitivity was compared for several polymer/glass composites and was correlated to the absorption curves and holographic diffraction efficiency showing that the new composites and film preparation techniques are promising for holographic materials sensitive in the blue and ultraviolet spectral regions. A method of information fixing by preventing oxygen entry to the composite film resulted in a fourfold increase of the erasure time. These findings suggest that holograms can be fixed for a long term by nonoxygen permeable coating, applied after hologram formation.
X-ray emission from 30 J Blumlein operated compact diode
Authors: Zakaullah M. , Worley J.
Published: July 2002

An x-ray emitting diode of impedance 2.3 Ω with a knife-edge cathode energized by a 90 kV, 30 J, solid dielectric Blumlein driver of pulse length 10 ns is studied. X-ray emission from titanium, copper, molybdenum, tin, tantalum, and lead anodes was investigated. The radiation yield from titanium and copper is low. Molybdenum and tin emit a significant part as Kα line radiation, whereas the emission with tantalum and lead anode is essentially continuum radiation. The ratio of line to continuum with molybdenum target is estimated 17%±10%.
Characterization of defects in (ZnMg)Se compounds by positron annihilation and photoluminescence
Authors: Plazaola F. , Saarinen K. , Dobrzynski L. , Reniewicz H. , Firszt F. , Szatkowski J. , Meczynska H. , Legowski S. , Chabik S.
Published: July 2002

Defect characterization of as-grown Zn1−xMgxSe mixed crystals (0⩽x<0.6) and the effect of Zn vapor annealing has been studied by positron lifetime and photoluminescence measurements. We obtain both experimental and theoretical evidence that the bulk lifetime of free positrons increases linearly with Mg alloying. The average positron lifetime increases with temperature indicating that both vacancies and negative ions trap positrons. The decompositions of the lifetime spectra show that the vacancy has the characteristic positron lifetime of 325 ps. The comparison with theoretical calculations indicate that the lifetime 325 ps corresponds either to divacancies relaxed inwards or to monovacancies strongly relaxed outwards. We consider the latter identification more likely and attribute the positron lifetime 325 ps to Zn vacancy or a complex involving VZn. The vacancy concentration is almost independent of Mg content above x=0.2 but decreases strongly at x=0.56. The Zn vapor annealing decreases the concentration of Zn vacancies. The intensity of the green photoluminescence correlates with the concentration of VZn both as a function of Mg alloying and Zn vapor annealing. We thus conclude that the electron levels of the Zn vacancy are involved in the optical transition leading to the green photoluminescence.
Vortex pinning in microindented YBa2Cu3O7−x single crystals
Authors: Turchinskaya Marina , Smith Douglas T. , Roytburd Alexander L. , Kaiser Debra L.
Published: July 2002

A high resolution magneto-optical technique with a magnetic indicator film was used to study the effect of a regular array of micrometer-sized mechanical indentations on magnetization dynamics at the surface of a twinned single crystal of YBa2Cu3O7−δ as a function of applied magnetic field, Ha (0 to ±68 mT) at temperatures in the range of 10–70 K. Direct visualization of the magnetic flux interaction with the individual indentations was observed for the first time. During magnetization at low Ha up to 15 mT at 60 K, the first row of indentations provides resistance to the flux front. In the indented area, flux is preferably pinned at the indentation sites. With increasing Ha, the vortex density at the indentations approaches the magnetic flux density in the surrounding material. During demagnetization down to Ha=0 mT, the outgoing flux is trapped by the indentations in areas where the density of the trapped flux is small; flux is also trapped along the (110) twin boundaries behind the indentations. For Ha=0 to ±10 mT, the average value of the magnetic induction gradient in the vicinity of an indentation is ≈0.4 mT/μm at 20–60 K. The average induction gradient and, thus, the critical current density in the indented material is estimated to be 1.5–2 times greater than in the unindented material at temperatures from 40 to 70 K and Ha from 18 to 68 mT.
Electron beam stimulated formation of CdS nanoparticles within calixarene Langmuir–Blodgett films
Authors: Nabok A. V. , Ray A. K. , Hassan A. K.
Published: July 2002

Two possible types of cadmium sulphide (CdS) nanostructures can be formed within Langmuir–Blodgett (LB) films of calixarene Cd salt as a result of exposure to H2S gas: nanoclusters with the size of about 1.5 nm or two-dimensional planes with the thickness of about 1.7 nm. Transmission electron microscopy and electron diffraction studies on calixarene/CdS LB films show that CdS particles are not present in the fresh samples at the beginning, but they are gradually formed under exposure to a high energy electron beam. A blueshift of the first exciton absorption band and an increase of the extinction coefficient, caused by exposure to H2S gas, were found with UV-visible optical spectroscopy and surface plasmon resonance, respectively.
Electro-optic properties of oxide ferroelectrics grown on GaN/sapphire
Authors: Wang F. , Fuflyigin V. , Osinsky A.
Published: July 2002

Recent advances in the processing of complex oxide materials have allowed us to monolithically grow ferroelectrics, of the lead lanthanum zirconate titanate system, on a GaN/sapphire structure. The thickness of these ferroelectric films ranged from 0.5 to 5 μm. Strong electrooptic effect with field-induced birefringence, as large as 0.02, was measured for (Pb,La)(Zr,Ti)O3 layer grown on a GaN/sapphire structure. The applicability of the lead lanthanum zirconate titanate material as a light modulating medium, integrated with GaN-based devices, was further proven by demonstrating optical modulation of a laser beam in an Al/(Pb,La)(Zr,Ti)O3/GaN/sapphire structure.
Inhomogeneous Coulomb fluids with image interactions between planar surfaces. I
Authors: Kjellander Roland , Marčelja Stjepan
Published: July 2002

Description of Coulomb particles near a surface is complicated by the effect of the surface on the ion–ion correlations, the electrostatic images (if the surface is a dielectric boundary), and the long range of the correlations in the lateral direction. We formulate the problem of the ion distribution for inhomogeneous Coulomb fluids with an arbitrary core potential and confined between two planar surfaces, with which the particles can interect via any short-ranged potential. All orders of image interactions are included into an effective position-dependent pair potential. The problem is solved by mapping the inhomogeneus three-dimensional system into a homogeneous two-dimensional one. The system is subdivided into M layers, and then shown to be isomorphic to an M-component fluid mixture in two dimensions. The mapping becomes exact as the number of layers (‘‘components’’) M→∞, and is accordingly an excellent approximation for finite, but large M. The correlations and other statistical mechanical functions can now be obtained with any conventional closure scheme. This forms the basis for a numerical method to solve the complete integral equations for the one- and two-particle distribution functions, using, e.g., the hypernetted chain (HNC) closure for the inhomogeneous pair correlations as the sole approximation. The associated numerical procedures are briefly described. The method could also be applied to study various problems for solute particles interacting via other pair potentials.
Infrared spectrum of the ν2 vibration-inversion band of H3O+
Authors: Liu Di-Jia , Haese Nathan N. , Oka Takeshi
Published: July 2002

High resolution infrared spectra of the 1−←0+ and 1+←0− ν2 vibration-inversion bands of H3O+ have been observed in the ac glow discharge by using tunable diode lasers and the velocity modulation technique. The two band origins were found to be at 954.4003(25) and 525.8237(13) cm−1 for the 1−←0+ and 1+←0− bands, respectively. The band origins, rotational constants, and observed relative band strengths are compared with predictions based on ab initio theory. The discharge chemistry of H3O+ is discussed with reference to the observed line strength-gas composition relation and known ion–molecule reactions.
An augmented coupled cluster method and its application to the first-row homonuclear diatomics
Authors: Raghavachari Krishnan
Published: July 2002

An augmented coupled cluster scheme to evaluate the higher order electron correlation effects is proposed. The method is carried out in two steps. First, a coupled cluster calculation with all double substitutions (CCD) is performed. The converged CCD wave function is then used in the evaluation of the contribution of single and triple substitutions. The method is correct to fourth order in a perturbation expansion and includes significant fifth and higher order terms. Illustrative calculations on the excitation and dissociation energies of first-row homonuclear diatomic molecules are reported. The low-lying excitation energies of B2 and C2 are accurately calculated. The dissociation energies of B2, C2, N2, O2, and F2 are all uniformly underestimated by 0.1–0.3 eV using large spdf basis sets.
Photoionization of the amidogen radical
Authors: Gibson S. T. , Greene J. P. , Berkowitz J.
Published: July 2002

The photoionization mass spectrum of NH2, prepared by the reaction H+N2H4, is presented. The adiabatic ionization potential is 11.14±0.01 eV (0.32 eV lower than reported by PES). A prominent autoionizing Rydberg series is observed, converging to the excited Ã 1A1 state at 12.445±0.002 eV. By extrapolation, NH2 should absorb strongly at ∼1150 Å. From the threshold for formation of NH+ (NH2), we obtain ΔH0f 0(NH+)=396.3±0.3 kcal/mol. With auxiliary data, we compute ΔH0f 0(NH)=85.2±0.4 kcal/mol, ΔH0f 0(NH2)=45.8±0.3 kcal/mol, D0(H2N–H)=106.7±0.3, D0 (HN–H)=91.0±0.5, and D0 (N–H)=79.0±0.4 kcal/mol. Additional photoionization measurements on N2H4 and N2H3 are also included.
The calculated ν2 (inversion) spectrum of H3O+
Authors: Liu Di-Jia , Oka Takeshi , Sears Trevor J.
Published: July 2002

We report the observation and assignment of a number of vibration–rotation transitions within 1−←1+ band of H3O+. The data are analyzed together with previously published measurements to yield a consistent set of rotational parameters that accurately reflects the vibration–rotation spectra involving the four lowest levels associated with the ν2 vibration of the ion. The analysis enables a precise estimate of the inversion splitting and the associated inversion-rotation spectrum of H3O+ which occurs in the far infrared.
UV laser excited fluorescence spectroscopy of the jet-cooled copper dimer
Authors: Rohlfing Eric A. , Valentini James J.
Published: July 2002

UV laser excited fluorescence spectra of the jet-cooled copper dimer are reported. Cu2 is produced via laser vaporization of the metal inside the throat of a pulsed, supersonic nozzle. Strong fluorescence is observed upon excitation of 63Cu2 in the range 287–324 nm, corresponding to several v″=0 bands of the II–X transition recently found in excitation using resonant two-photon ionization [Smalley et al., J. Chem. Phys. 78, 2866 (1983)]. The II–X emission spectra are characterized by extremely long progressions in the ground state vibration, enabling us to measure G(v) and ΔGv+1/2 values over the range v=0–72. This data has been fit to yield improved vibrational constants for 63Cu2 X 1Σ+g: ωe =266.43±0.59 cm−1, ωexe =1.035±0.030 cm−1, ωeye =+(1.70±0.58)×10−3 cm−1, and ωeze =−(1.78±0.37)×10−5 cm−1. The G(v) data has also been fit to a near-dissociation expansion which incorporates the correct limiting behavior of G(v), determined by the asymptotic inverse power contribution to the potential. This fit gives an improved dissociation energy for the dimer, and the vibrational index at dissociation: De =16 760±200 cm−1 and vD =128±5.
Solution of the Hartree–Fock equations for polyatomic molecules by a pseudospectral method
Authors: Friesner Richard A.
Published: July 2002

A pseudospectral code for general polyatomic molecules has been developed using Gaussian basis functions. As an example, the water molecule is studied using a 6-31G** basis set. Quantitative agreement with conventional calculations is obtained for the equilibrium geometry, total energy, first ionization potential, and vibrational force constants. Timing results for a vectorized version of the code (run on a Cray X-MP) indicate that for large molecules, rate enhancements of Hartree–Fock self-consistent field calculations of order 103 can be achieved.
The iron–hydrogen system: Lattice location of hydrogen, heat of formation, and hydrogen–hydrogen binding energy
Authors: Minot Ch. , Demangeat C.
Published: July 2002

The electronic structure of the iron–hydrogen system has been performed in terms of band structure calculation based on the extended Hückel formalism. Heat of formation for a hydrogen atom at octahedral and tetrahedral positions in bcc and hcp iron lattice has been estimated. Allowing the iron lattice to relax, we minimize the energy of the system in terms of a variable lattice parameter. The heat of formation is less endothermic for hydrogen located at tetrahedral interstitial position than for hydrogen at octahedral position. Moreover, the effect of hydrogen is to stabilize the hcp phase. We have performed calculations with different geometrical arrangements, in terms of the distance between hydrogen atoms. Binding energies between hydrogen atoms at nearest and next-nearest interstitial positions are always found unstable. Numerical results obtained in the case of hcp structure are discussed in terms of recent Mössbauer experiments. The physical origin of the sign of the heat of formation in terms of charge transfer is presented. For this we use a model of a Fe4H cluster.
Universality, multiplicity, and the effect of iron impurities in the Belousov–Zhabotinskii reaction
Authors: Coffman K. G. , McCormick W. D. , Noszticzius Z. , Simoyi Reuben H. , Swinney Harry L.
Published: July 2002

In experiments on the Belousov–Zhabotinskii reaction in a flow reactor we have observed dynamical behavior that is described well by one-dimensional maps with a single maximum. A sequence of period doubling bifurcations was observed as a parameter was varied, and beyond the accumulation point for the period doubling sequence there was a sequence of periodic states that has the same symbolic dynamics as the states of the U (universal) sequence of Metropolis, Stein, and Stein (1973). However, in another experiment with malonic acid from a different vendor, we found that some states with particular symbol sequences occurred in three different parameter ranges rather than in one range as in the U sequence. Analysis of the effect of impurities in the reagents showed that some impurities (e.g., Fe3+ and esters of malonic acid) at concentrations of only a few ppm produced dramatic changes in the dynamics; such impurities are contained in commercially available malonic acid. Experiments with purified malonic acid indicate that the Fe2+/Fe3+ and Cu+/Cu2+ redox couples act as co-catalysts of the Belousov–Zhabotinskii reaction; their effect can be explained by a free radical mechanism. Other metal ions (e.g., Ni2+ and Co2+) at low concentrations have negligible effect on the dynamics. Finally, both the sequences with universal ordering and the sequences with multiplicity are described well by indented trapezoid maps of the type analyzed by Beyer, Mauldin, and Stein (1986).
Transition rates of atoms near spherical surfaces
Authors: Chew H.
Published: July 2002

We study the transition rates of atoms inside and outside dielectric spheres. The rates are calculated classically, and the results are shown to agree with those obtained using other approaches. For atoms outside, our analytic results are equivalent to those of Ruppin in the case of zero conductivity. Numerical results are presented, which show that resonances have important effects on the transition rates, and that enhancement factors of hundreds are possible under suitable conditions.
Infrared diode laser kinetic spectroscopy of the ν3 band of C3
Authors: Matsumura Keiji , Kanamori Hideto , Kawaguchi Kentarou , Hirota Eizi
Published: July 2002

Infrared diode laser kinetic spectroscopy was applied to reaction intermediates generated by the photolysis of diacetylene and allene at 193 nm. A strong band was observed around 2040 cm−1 and was assigned to the ν3 band of C3 . The observed spectrum was analyzed by using an energy level expression for a linear molecule; higher-order centrifugal distortion terms were necessary to be included in the least-squares fit. The band origin was determined to be 2040.0198 (8) cm−1. The rotational and centrifugal distortion constants obtained suggest the presence of a potential barrier at the linear configuration in both the ground and ν3 states.
A hypernetted chain structure factor for charged colloidal dispersions
Authors: Fushiki M.
Published: July 2002

The structure factor for charged colloidal dispersions is obtained by solving the multicomponent hypernetted chain (HNC) equation in the framework of the primitive model. To remove the instability of an ordinary iterative method, the iteration process is separated into two parts, and the Newton–Raphson method is applied in one of these cases. By using this method, we can obtain the solution for states close to the spinodal line. The height of the first peak of the structure factor S(kmax ) is plotted as a function of colloidal charge, and is found to have a maximum near this spinodal line. The maximum of S(kmax ) is larger than the result of the jellium approximation and agrees well with experimental results of small angle neutron scattering at moderate colloidal concentration.