bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarizing-field orientation and thermal treatment effects on the dielectric behavior of fluorapatite
Authors: Vassilikou-Dova Aglaia , Kalogeras Ioannis M. , Macalik Bogus’aw , Londos Charalambos A.
Published: July 2002

A thermally stimulated depolarization currents (TSDC) study in natural fluorapatite single crystals has established different relaxation mechanisms for two polarization orientations (Ep parallel and perpendicular to the crystallographic c axis), which are discussed in relation to the defect chemistry and the specific columnar structure in apatite. The intensities of the thermostimulated current signals between the two poling field orientations demonstrate a difference of at least one order of magnitude, with the higher one recorded for the electric field parallel to the c axis. The TSDC thermogram appearing with the electric field parallel to c axis, in the 10–320 K range, consists of a broad and complex band (HT), with a maximum around 300 K. The relative intensity of associated current signals is indicative of extensive dipole-like ionic motions along c axis with a distribution in their activation energies ranging between 0.14 and 0.85 eV. The microdomain structure of fluorapatite along c axis permits the formation of charge layers at the interfaces. After annealing, the induced changes of size and/or shape of the interfaces could explain the observed changes of band intensity and location. With the electric field perpendicular to c axis, the spectrum consists of at least five well-defined relaxation bands, the high temperature ones (HT1, HT2, HT3) decreasing after heating at 673–873 K. The most dramatic change was recorded for an intermediate LT2 single-relaxation band located around 185 K, with a high activation energy of 1.06 eV, which manifested a significant growth after annealing. Rietvelt analysis of the x-ray diffraction patterns of the original and annealed apatite powders, indicates change in the unit cell parameters of the hexagonal structure (i.e., a increases from 9.3921 to 9.3940 Å after annealing), which can be related to the establishment of a new equilibrium distribution of the abundant trivalent rare-earth (Ce, La, Nd, Pr,…) impurity ions. The origin of the TSDC bands is discussed and tentative correlations are suggested, in terms of substitute aliovalent ions-vacancy dipoles. The thermal response of the high temperature relaxation bands in the case of Ep⊥c axis, is characteristic of dipole clustering phenomena—although an explanation based on localized changes in the structural environment of the pertinent dipoles/ions cannot be disregarded.
Growth and characterization of beryllium-based II–VI compounds
Authors: Cho M. W. , Chang J. H. , Bagnall D. M. , Koh K. W. , Saeki S. , Park K. T. , Zhu Z. , Hiraga K. , Yao T.
Published: July 2002

We report on the growth and characterization of beryllium–chalcogenide layers prepared on GaAs (100) by molecular beam epitaxy. Be- and Te-terminated BeTe surfaces show (4×1) and (2×1) reconstructions, respectively. The stability of each surface is investigated by reflection high energy electron diffraction as a function of substrate temperature. The dependence of growth rate of BeTe on growth temperature and Be cell temperature is investigated. The best full width at half maximum (FWHM) of a (400) x-ray rocking curve of BeTe is 78 arcsec. The dependence of the ZnBeSe energy gap on Be composition is obtained by four-crystal x-ray diffraction (XRD) and low temperature photoluminescence measurements. The energy gap of Zn1−xBexSe varies as Eg=0.0107x+2.790 (eV) for small Be composition (x<0.25) at 77 K. Lattice-matched ZnBeSe (Eg=2.82 eV) and ZnMgBeSe (Eg=2.975 eV) layers show narrower XRD peaks, the FWHM values of which are 64 and 21 arcsec, respectively. The variation of FWHM of x-ray rocking curve due to lattice misfit is investigated for ZnMgBeSe quaternaries with various lattice misfits extending from compressive to tensile strain. The FWHM value under compressive strain increases more steeply with lattice misfit than that under tensile strain.
Temperature-dependent photoluminescence of In0.5Al0.5As/Al0.25Ga0.75As self-organized quantum dots
Authors: Phillips J. , Kamath K. , Bhattacharya P. , Venkateswaran U.
Published: July 2002

The photoluminescence spectra of large ensembles of self-organized In0.5Al0.5As/Al0.25Ga0.75As quantum dots were studied. The spectra reveal a number of sharp peaks with energy separation and full width at half maximum values of ∼20 meV for 150 K⩽T⩽275 K. Significant changes are observed in the luminescence characteristics with varying temperature, which are attributed to a changing distribution of photoexcited carriers between dots.
Magnetic properties and microstructural observations of oxide coated FeCo nanocrystals before and after compaction
Authors: Turgut Z. , Nuhfer N. T. , Piehler H. R. , McHenry M. E.
Published: July 2002

A radio frequency (rf) plasma torch has been used to produce FeCo nanoparticles with a thin protective oxide coating from metal powder precursors. Structural characterization by conventional and synchrotron x-ray diffraction indicated a disordered bcc α-FeCo phase. High resolution transmission electron microscopy revealed spherical particles with several monolayer thick protective oxide coatings. Thermomagnetic measurements were carried out using a superconducting quantum interference device magnetometer and a vibrating sample magnetometer at temperatures between 5 and 1050 K. Antiferromagnetic (exchange bias) coupling was observed due to the presence of the oxide layer. Relatively high coercivities were observed (280 Oe at 5 K and 250 Oe at room temperature). Néel’s surface (interface) anisotropy model was employed to explain the origin of the observed coercivities. As produced powders were hot isostatically pressed at 1023 K and 22 ksi for 2 h. Dense structures were observed and compacted particles revealed coercivities as low as 25 Oe at room temperature.
Chain-of-cubes iron nanoparticles prepared by borohydride reduction of acicular akaganeite particles
Authors: Chen Min , Nikles David E.
Published: July 2002

Ellipsoid shaped akaganeite (β-FeOOH) particles, 70 nm long and 10 nm in diameter, were prepared by forced hydrolysis of ferric chloride. The β-FeOOH particles were suspended in a lamellar liquid crystal phase at 0 °C and reduced with dilute sodium borohydride solution to iron particles. These conditions allowed us to slow the reduction reaction and isolate partially reduced iron particles. Transmission electron microscopy images showed that the particles were being reduced from the outside and there were nanometer-sized spherical iron clusters on the surface. Fully reduced iron particles were chains of cubes, with the cubes about 20 to 25 nm in size. The cubes were connected either corner to corner, edge to edge, or face to face. The face to face chains of cubes had the desired acicular shape, with aspect ratios exceeding five. This face to face chain of cubes microstructure was also observed in commercial iron particles.
Near-field coupling effect in normal-incidence absorption of quantum-well infrared photodetectors
Authors: Fu Y. Y. , Willander M. , Lu W. , Xu W. L. , Li Ning , Li Na , Liu X. Q. , Chen Y. D. , Shen S. C.
Published: July 2002

Experiments have shown a significant photoresponsivity in quantum well (QW) infrared photodetectors (QWIP) with normal incidence without optical grating. Different explanations based on the energy band structures have been given, but no concrete conclusions can be drawn at the present stage. We develop a theory based on the optical near-field effect in the QW active region to explain the observed photoresponsivity in the normal incidence condition. Our theory indicates that the near-field effect results in an evanescent wave which induces a nonzero electrical component in the optical field along the QWIP growth direction in the quantum well region inducing the observed photoresponsivity.
Propagation of the magnetic domain wall in submicron magnetic wire investigated by using giant magnetoresistance effect
Authors: Ono T. , Miyajima H. , Shigeto K. , Mibu K. , Hosoito N. , Shinjo T.
Published: July 2002

The magnetization reversal phenomenon in a submicron magnetic wire with a trilayer structure consisting of NiFe(400 Å)/Cu(200 Å)/NiFe(50 Å) was investigated by measuring the electric resistance in external magnetic fields. It is shown that the magnetization reversal can be very sensitively investigated by utilizing the giant magnetoresistance effect. The time variation of resistance during the magnetization reversal was also measured and the velocity of the magnetic domain wall propagating in the wire was determined at 77 K.
Formation of discharge plasma on the surface of cathodes with different dielectric constants
Authors: Krasik Ya. E. , Dunaevsky A. , Felsteiner J.
Published: July 2002

We present a detailed study of the spatial and temporal evolution of visible light emission from plasma formed on the surface of ferroelectrics [Pb(Zr,Ti)O3 and BaTiO3] and a printed-circuit board under the application of a high-voltage pulse. For all samples studied, plasma formation occurs within the first 5 ns from the start of the high-voltage pulse. It is shown that the plasma appears at the edges of the front strip electrodes. Further, the plasma spreads along the dielectric surface covering it partially. It is found that the uniformity of the plasma and the distance of its propagation along the surface depend on the properties of the dielectric, the polarity and amplitude of the high-voltage pulse, and the vacuum conditions. A qualitative model which explains the observed experimental data is suggested.
Electron/ion emission from the plasma formed on the surface of ferroelectrics. I. Studies of plasma parameters without applying an extracting voltage
Authors: Dunaevsky A. , Krasik Ya. E. , Felsteiner J. , Dorfman S.
Published: July 2002

We present experimental results of plasma formation on the surface of ferroelectric samples. Different poled and unpoled ferroelectric samples having a disk or tube form and made of Pb(Zr, Ti)O3 or BaTiO3 were tested. Using fast framing photography and different electric probes it was found that the application of a high-voltage driving pulse to the ferroelectric sample causes a fast surface plasma formation. This plasma formation occurs within a few nanoseconds from the start of the driving pulse for all the tested ferroelectric samples and the methods of applying the driving pulse. It was found that reversing the polarization of a ferroelectric does not play a significant role in the process of the plasma formation. Parameters of the plasma and of the neutral flow formed during the plasma formation versus the polarity and the amplitude of the driving pulse are presented.
Structure evolution and magnetic properties of a Co/Ni/Fe multilayer film
Authors: Ding Xing-zhao , Wu Xian-ying , Chiah M. C. , Cheung W. Y. , Wong S. P. , Wilson I. H. , Zhu Xiang-rong , Shen Hong-lie , Liu Xiang-huai
Published: July 2002

In this article, a 16-multilayer Co/Ni/Fe film was prepared by using a three-source magnetically filtered metal vapor vacuum arc deposition system. The microstructure evolution and the variation of magnetic properties of the film during a vacuum annealing process were monitored by Rutherford backscattering spectroscopy, x-ray diffraction, vibrating sample magnetometer, scanning atomic force microscopy, and magnetic force microscopy experiments. It was found that the saturation magnetization of the film was enhanced gradually by the interdiffusion and alloying effect within the multilayers caused by vacuum annealing. Meanwhile, however, the magnetic domain grew to a larger scale progressively, and the film became magnetically harder and harder.
Effects of annealing on the electrical properties of Fe-doped InP
Authors: Zhao Y. W. , Fung S. , Beling C. D. , Sun N. F. , Sun T. N. , Chen X. D. , Yang G. Y.
Published: July 2002

Fe-doped liquid encapsulated Czochralski InP has been annealed between 500 and 900 °C for different durations. The electrical property of annealed InP has been studied by temperature-dependent Hall measurement. Defects in annealed Fe-doped InP have been detected by room-temperature transient photocurrent spectroscopy. Upon annealing, the change of electrical property in this material is indicative of the formation of a high concentration of defects. The formation process of these thermally induced defects is discussed.
Octahedral void defects and related circular stains on thermal oxides
Authors: Itsumi Manabu , Ueki Takemi , Yabumoto Norikuni , Suzuki Mineharu
Published: July 2002

We have found that thermal oxidation of Czochralski-grown-silicon substrates produces local circular stains around octahedral void defects. A plan view revealed that an octahedral void defect is situated near the center of a circular stain. Oxidation at 1150 °C for 400 min in a pure oxygen ambient (oxide thickness 430 nm) produced stains with a diameter of about 3 μm, which was clearly larger than the diameter (1 μm or less) of the oxide surface pit grown on the octahedral void detect. The signal intensity of the circular stain decreased with repeated scanning observations, and, in many cases, the circular stain vanished after two or three observations. From the etching rate caused by the scanning observation, we estimated that the thickness of the stain was about 0.5–1.0 nm. Micro Auger analysis revealed that the concentration of carbon was higher in the circular stain than in the surrounding region. A model is presented which shows how octahedral void defects on the silicon surface lead to the formation of circular stains during oxidation.
Thickness and temperature dependence of electrical properties of Bi2(Te0.1Se0.9)3 thin films
Authors: Damodara Das V. , Selvaraj S.
Published: July 2002

Thin films of different thicknesses have been vacuum deposited onto clean glass plates held at room temperature using the flash evaporation technique in a vacuum of 2×10−5 Torr. The structural characterization of the bulk and the thin films was carried out using x-ray diffraction, transmission electron microscopy, and selected area electron diffraction techniques. Electrical resistance and thermoelectric power of the films were measured in the same vacuum of 2×10−5 Torr in the temperature range 300–450 K. The conduction activation energy of the films was calculated using the electrical resistivity and thermoelectric power data of the films. The thickness dependence of the activation energy observed is attributed to the polycrystalline nature of the films. Grain growth and reorientation of the grains take place during the annealing process. The thickness dependence of electrical resistivity and thermoelectric power of the films are explained by the effective mean free path model [C. R. Tellier, Thin Solid Films 51, 311 (1978)]. The important physical parameters like mean free path, Fermi energy, power index of the energy dependant expression for the mean free path, the hypothetical bulk resistivity and the thermoelectric power have been calculated by the combined analysis of electrical resistivity and thermoelectric power. The electron-phonon scattering mechanism is found to be dominant in the material.
A near-infrared photoluminescence study of GaAs nanocrystals in SiO2 films formed by sequential ion implantation
Authors: Kanemitsu Yoshihiko , Tanaka Hiroshi , Kushida Takashi , Min Kyu Sung , Atwater Harry A.
Published: July 2002

We have studied photoluminescence (PL) properties of Ga+ and As+ implanted SiO2 films on Si substrate. After thermal annealing, zinc blende GaAs nanocrystals are formed in SiO2 films and several PL bands appear in the red and near-infrared spectral region. Defects and impurities in GaAs nanocrystals and SiO2 cause weak luminescence in the near-infrared spectral region at low temperatures. After low-energy deuterium implantation, the defect PL intensity decreases and the red PL from GaAs nanocrystals is clearly observed. It is demonstrated that GaAs/SiO2 nanocompostites with low defect density are fabricated by sequential ion implantation followed by thermal annealing and hydrogen passivation.
Analytical treatment of wave packet tunneling through a resonant double barrier heterostructure
Authors: Stoica F. H. , Dragoman D.
Published: July 2002

An analytical expression is found for the transmitted wave packet through a general, asymmetric double barrier structure, as well as for its width. This analytical expression is shown to be in agreement with previous treatments of wave packet tunneling and, as such, is in agreement with previous numerical calculations.
Thermoelectric characterization of Si thin films in silicon-on-insulator wafers
Authors: Liao C. N. , Chen C. , Tu K. N.
Published: July 2002

Heat dissipation becomes an important issue for highly compacted integrated circuit chips because of device performance and reliability concerns. Thermoelectric cooling has been considered as one of the possible solutions for cooling high density, locally heated integrated circuits. It excels in scalability and reliability. In this study, the thermoelectric behavior of silicon thin films in silicon-on-insulator wafers has been investigated. We have designed a testing structure to measure Seebeck coefficient which is one of the most important thermoelectric parameters. In addition, a testing structure with platinum temperature sensors was built to measure Peltier cooling effects for the powered Si channels. We found that the implanted dopant concentration and postimplantation annealing play important roles on thermoelectric properties of the Si thin films. Also, an asymmetrical thermal behavior due to Peltier effect was observed for the implanted Si thin films, especially for samples annealed at lower temperatures. The thermoelectric power of the Si thin films could be improved by appropriate ion implantation and annealing processes.
Vertical electric dipole radiation over a sphere: Character of the waves that propagate through the sphere
Authors: Houdzoumis Vassilios A.
Published: July 2002

A vertical electric dipole over an electrically homogeneous sphere generates waves that propagate through the sphere, in addition to those that propagate around the sphere. Results of a quantitative analysis of the former are reported for the case where both the dipole and the point of observation lie on the spherical interface. The results are valid for a sphere whose radius is much larger than the wavelengths inside and outside the sphere. It is verified that the waves propagating through the sphere can be visualized in terms of ray optics.
Reversible optical control of transmittance in polymer/liquid crystal composite films by photoinduced phase transition
Authors: Lee Hyoung-Kwan , Kanazawa Akihiko , Shiono Takeshi , Ikeda Tomiki , Fujisawa Toru , Aizawa Masao , Lee Bong
Published: July 2002

We have demonstrated reversible change in transmittance of composite films, which are composed of an isotropic (I) polymer matrix and a host nematic (N) liquid crystal (LC) containing donor–acceptor type azobenzene LCs (AzoLCs) as photoresponsive molecules, by means of reversible phase transition in the LC domain. The composite films were prepared by in situ thermal polymerization-induced phase separation (PIPS) method from their homogeneous mixtures. Photoisomerization of AzoLCs was induced within the polymer network, and the resultant effect brought about isothermal change in transmittance. The photoinduced reversible and repeatable changes between opaque light scattering and clear transparent states were achieved effectively by irradiation with a monochromatic light at 366 nm. These changes are attributed to photochemical N–I phase transition and rapid thermal I–N phase transition in the host LC domain based on reversible change in molecular sharp of the guest AzoLCs. The optical effects were influenced by such factors as temperature of the composite films in the initial state and photoisomerization ability of the guest AzoLCs. The composite films acted as an optical switching material without polarizers.
On the origin of low frequency noise in GaAs metal–semiconductor field-effect transistors
Authors: Dobrzański Lech , Wolosiak Zdzislaw
Published: July 2002

The origin of low frequency (LF) noise of GaAs metal–semiconductor field-effect transistors (MESFETs) was examined. A few generation-recombination components dominate the flicker noise present in the measured spectral noise intensity. It follows that the noise scales proportionally to L−3 (L stands for channel length). The earlier is obeyed perfectly when the noise originated in the channel access areas is negligible compared to the LF noise of the region underneath the gate. Next, it was observed that the corner frequency of the Lorentzian noise components depends on the gate-to-source bias and does not depend on the device geometry. The third important observation is that the LF noise power does not depend on the source-to-gate bias at frequencies higher than 100 Hz. It was shown that both fluctuations of the mobility and fluctuations in the carrier density inside the channel could not explain the observed noise. We claim that charge fluctuations in the Shockley–Read–Hall centers found inside the depleted layer below the gate electrode are likely to be the origin of low frequency noise in GaAs MESFETs.
Fully leaky guided wave determination of the polar anchoring energy of a homogeneously aligned nematic liquid crystal
Authors: Yang Fuzi , Sambles J. R. , Dong Youmei , Gao Hongjin
Published: July 2002

Optical excitation of a series of fully leaky guided modes has been used to explore the director distortion of a homogeneously aligned nematic liquid crystal (Merck-BDH E7) under application of an ac electric field (1 kHz) normal to the cell wall. Homogeneous alignment is realized by rubbed polyimide films on glass plates with indium–tin–oxide coatings. By using continuum theory to model the director profile through the cell to fit the fully leaky guided mode reflectivity and transmissivity data, both with and without applied fields, the changes of surface tilt angle and its gradient can be obtained from which the polar anchoring coefficient, Wθ, is derived. At 26.8 °C we find that Wθ=1.2×10−4 Jm−2.
Characterization of ferroelectricity in metal/ferroelectric/insulator/semiconductor structure by pulsed C–V measurement; Ferroelectricity in YMnO3/Y2O3/Si structure
Authors: Yoshimura Takeshi , Fujimura Norifumi , Ito Daisuke , Ito Taichiro
Published: July 2002

Ferroelectric properties of the metal/ferroelectric/insulator/semiconductor (MFIS) structure were investigated using the Pt/YMnO3/Y2O3/Si structure. The ferroelectric C–V hysteresis was observed for not only the Pt/YMnO3(0001)/Y2O3/Si capacitor but also the Pt/amorphousYMnO3/Y2O3/Si capacitor. The polarization evaluated by conventional C–V measurement should include interfacial polarization and rearrangement of the space charge together with the spontaneous polarization by ferroelectricity. To eliminate the generation of the interfacial polarization and the rearrangement of the space charge, the shorter charging time should be used to evaluate the ferroelectricity of the MFIS capacitor. Therefore, we propose the pulsed C–V measurement as a new method for evaluating the MF(I)S capacitor.
Micromagnetic analysis of recording processes in periodic antidot arrays: Interaction between adjacent bits
Authors: Torres L. , Lopez-Diaz L. , Alejos O.
Published: July 2002

Lithographically defined nonmagnetic structures in thin magnetic films have been proposed as future ultrahigh density storage systems. Previous investigations by our group have shown that these periodic antidot arrays present adequate remanent states for storage. The optimization of material parameters and geometrical dimensions led to maximum areal densities in the order of 750 Gbits/in.2 for materials with exchange lengths in the range of 3 nm. In this work recording processes are computed. Single bit switching is analyzed by micromagnetic simulations. It is observed how the dipolar and exchange interactions between adjacent bits play a key role. If the applied field is not carefully controlled or the magnetocrystalline anisotropy interaction is not strong enough, the switching of one bit can produce undesired switching of neighboring bits.
Rotation of exchange anisotropy in biased Co/CoO bilayers
Authors: Gredig T. , Krivorotov I. N. , Merton C. , Goldman A. M. , Dahlberg E. D.
Published: July 2002

Magnetic field induced irreversible changes of the exchange anisotropy in Co/CoO bilayers were investigated. Cobalt films were grown by dc magnetron sputtering and then partially oxidized. They were then field cooled in 7000 Oe to 4.2 K to induce the exchange bias. A variable magnitude magnetic field was applied in the film plane at various angles with respect to the exchange bias direction. The effects of this variable magnetic field on the exchange coupling were studied by the reversible anisotropic magnetoresistance technique. Three qualitatively different behaviors of the exchange anisotropy direction were observed. The particular behavior was determined by the magnitude of the applied magnetic field and the rotation angle. A simple phenomenological model of the exchange anisotropy was developed to explain the experimental results, which allows for explicit quantitative separation of the unidirectional and the rotatable parts of the exchange anisotropy.
Ion beam deposition processes for improved hard bias magnetic and device properties in the abutted junction configuration
Authors: Devasahayam Adrian J. , Wang Jinsong , Hedge Hari
Published: July 2002

Permanent magnet films of Cr/CoCrPt for use in an abutted junction hard bias scheme were deposited by an ion beam deposition (IBD) system. The deposition angle control of IBD systems was employed to yield some excellent material and device related improvements. For films with the structure Cr-50 Å/CoCrPt-250 Å, increasing the deposition angle θ (as measured from the substrate normal) of the Cr layer from 20° to 60°, resulted in an increase in coercivity from 1860 to 1905 Oe. X-ray diffraction measurements showed that this improvement was related to an increase in CoCrPt in-plane texture and a decrease in c-axis perpendicular texture. The reason for this increase in in-plane texture is that there is better epitaxial matching between the CoCrPt and the Cr underlayer brought about by a change in the lattice dimension of Cr as a result of changing stress levels. Another positive effect of depositing the Cr at a larger angle is that the Cr would be thicker farther into the abutted junction and thus delay the onset of poor magnetic properties due to a thinning underlayer. The angle of deposition parameter can be used to further advantage by depositing the lead layer at a larger angle than the permanent magnet layers. This type of scheme would allow the leads to encroach over the permanent magnets and make direct contact with the sensor, yielding low contact resistance.
Micromagnetism in mesoscopic epitaxial Fe dot arrays
Authors: Xu Y. B. , Hirohata A. , Lopez-Diaz L. , Leung H. T. , Tselepi M. , Gardiner S. M. , Lee W. Y. , Bland J. A. C. , Rousseaux F. , Cambril E. , Launois H.
Published: July 2002

The domain structures of epitaxial Fe (20 nm)/GaAs(100) circular dot arrays (diameters from 50 to 1 μm) were studied with magnetic force microscopy. A transition from a single domain to a multidomain remanent state was observed upon reducing the dot diameter beneath 10 μm in dot arrays with the separation twice the dot diameter. When the separation is reduced to half the dot diameter, the single domain states were found to “collapse” into stripe-like multidomain states due to local dipole coupling between dots. Micromagnetic simulations further suggest that for ultrathin Fe dots of less than about 2 nm thickness the diameter does not have a significant influence on the domain structures due to a dramatic reduction of the dipole energy.
Optical constants of tetrahedral amorphous carbon films in the infrared region and at a wavelength of 633 nm
Authors: Chen Z. Y. , Zhao J. P.
Published: July 2002

The optical constants of tetrahedral amorphous carbon (ta-C) films in the infrared region and at a wavelength of 633 nm were investigated. The ta-C films were prepared by the filtered arc deposition technique under different carbon ion energies of 22–320 eV. The ta-C films contained high sp3 bonding of 82%–93% and had very smooth surfaces. The optical band gap was found to be controlled essentially by the presence and arrangement of the sp2 sites in the ta-C films. The real and imaginary parts, ε1 and ε2, of the dielectric constant, refractive index, n, and extinction coefficient, k, of ta-C films were determined from measurements of infrared reflection and spectroscopic ellipsometry. The results indicated that in our region of investigation ta-C films exhibited a fine transparency at a wavelength of 633 nm, especially in the infrared region. Both n and k, and consequently, ε1 and ε2, showed considerable variation in ion energy, and had minimum values approaching those of diamond for film prepared at an ion energy around 220 eV. The dependence of the optical constants on the sp3 content of ta-C films indicated that with increased sp3 content, ta-C film possessed a dielectric constant, refractive index and extinction coefficient much closer to that of diamond.
Thermoelectric power in thin film Fe–CuNi alloy (type-J) couples
Authors: Scarioni L. , Castro E. M.
Published: July 2002

The preparation of thin film thermocouples by vacuum deposition is described. Combinations were made from Fe and CuNi alloy, evaporated by conventional heating resistance methods. Measurements of temperature against emf for these thermocouples gave almost bulk characteristic curves. The thickness dependence contribution to the Seebeck coefficient of Fe films is deduced from the thermal emf data on thermocouples formed with a 350 nm thick film of CuNi(58%Cu,42%Ni). It is compared with the size-effect theory based on the free-electron model, obtaining a maximum value of Seebeck coefficient of 0.058 mV °C−1, when the film thickness is greater than 250 nm. This value indicates that type J thin film couples can replace the wire without complications.
Low energy carbonaceous and graphite phases on the surfaces of thermochemically polished chemical vapor deposited diamond films
Authors: Weima J. A. , Job R. , Fahrner W. R. , Zaitsev A. M.
Published: July 2002

Micro-Raman spectroscopy was used to characterize chemical vapor deposition diamond films that were thermochemically polished at elevated temperatures. A continuous wave He–Ne laser and an argon ion laser both operated at a power of 10 mW on an area of 1 μm2 were used as excitation sources of the Raman setup. New Raman peaks were observed in a spectral range of 100–700 cm−1 in addition to the common graphite related features. These peaks are assumed to be related to carbonaceous phases formed on the surfaces of the films. A peculiarity of these new features is an increase in intensity with increasing laser exposure time. However, it was observed that the intensities of the amorphous and graphite Raman lines at 1326 and the 1577 cm−1, respectively, were independent of the laser exposure time. The temperature dependency of the intensities of the diamond and of the graphite Raman lines was investigated at different polishing temperatures. Low temperature (700 °C) polishing was also done to high temperature pre-polished samples to investigate the dissolution of nondiamond carbon in the polishing plate.
Memory effect in standard spin valve structures
Authors: Li Y. F. , Yu R. H. , Xiao John Q. , Dimitrov D. V.
Published: July 2002

Memory effect has been observed in both standard top and bottom spin valves. The change of the magnetization state in the pinned FM layer, below the blocking temperature, reverses the direction of the exchange bias and destroys the magnetoresistance properties. This reversed exchange bias is much weaker, causing severe consequences in SV applications. This behavior can be explained in terms of blocking temperature distribution in the AFM layer perhaps due to the structural randomness. By varying cooling procedures, the exchange coupling in regions with different blocking temperatures can be separated. It is found that the maximum exchange bias is very close to the sum of the exchange biases in different regions. The domain wall energy in the FM layer has to be taken into account in order to explain the behavior of the reversed bias. The insertion of a synthetic antiferromagnetic subsystem (Co/Ru/Co) stabilizes the magnetization state in the pinned layer because of the additional interlayer coupling through the Ru layer. This suppresses the memory effect.
Temperature stability of Co/Al2O3/Co junctions
Authors: Schmalhorst J. , Brückl H. , Reiss G. , Vieth M. , Gieres G. , Wecker J.
Published: July 2002

The temperature stability of magnetic tunnel junctions is an important requirement for the fabrication of magnetic memory devices and the integration in the semiconductor process technology. We have investigated the temperature evolution of the tunneling magnetoresistance (TMR) and the structural properties by isochronal annealing experiments up to 750 K. The magnetically hard electrode of the junction consists of an artificial antiferromagnet Co/Cu/Co, the soft electrode of a Co/Fe bilayer. The tunnel barriers are formed by plasma oxidized Al. The tunnel junctions have TMR signals up to 22% at room temperature. Besides a small increase of the TMR signal after annealing up to 480 K, a first decrease at 530 K and a breakdown beyond 600 K are found. This behavior can be attributed to structural changes of the junctions. Auger depth profiles show an unaltered Al2O3 barrier up to 600 K, but the beginning of interdiffusion processes within the magnetic electrodes already at 540 K and above.
Below band-gap optical absorption in semiconductor alloys
Authors: Krishnamurthy Srinivasan , Sher A. , Chen A.-B.
Published: July 2002

We have used accurate Hamiltonians and resulting wave functions to calculate the two-photon absorption coefficient and the free-carrier absorption coefficient in InAs and a HgCdTe alloy with the same band gap. Detailed results are obtained for the dependencies of the absorption on photon energy and incident intensity. Optical matrix elements are calculated from the wave number dependent wave functions. We have further solved the appropriate steady-state differential equation, with the calculated values of absorption coefficients, for depth dependence of the intensity in the material. We find that the nonlinear absorption in a HgCdTe alloy is about 100% larger than that in InAs of the same band gap and window thickness.
Using magnetoresistance to probe reversal asymmetry in exchange biased bilayers
Authors: Leighton C. , Song M. , Nogués J. , Cyrille M. C. , Schuller Ivan K.
Published: July 2002

We have measured the anisotropic magnetoresistance of Fe films exchange coupled to antiferromagnetic MnF2 layers. Exchange bias and coercivity obtained from magnetoresistance are in close agreement with superconducting quantum interference device magnetometry data. In addition the magnetoresistance reveals an asymmetry in the magnetization reversal process, despite the fact that the magnetization hysteresis loops show little shape asymmetry. These results correlate well with an earlier study of magnetization reversal asymmetry by polarized neutron reflectometry. The data imply that the magnetization reverses by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other.
Electroluminescence emission pattern of organic light-emitting diodes: Implications for device efficiency calculations
Authors: Kim Ji-Seon , Ho Peter K. H. , Greenham Neil C. , Friend Richard H.
Published: July 2002

The electroluminescence (EL) pattern emitted through the surface and edge of the glass substrate of two efficient polymer light-emitting diodes (LEDs) has been characterized. The surface emission is nearly Lambertian, while the edge emission comprises discrete substrate reflection and leaky waveguide modes. A simple “half-space” optical model that accounts for optical interference effects of the metal cathode–reflector is developed to extract the location and orientation of the emitting dipoles from these patterns. Numerical simulations for a range of polymer and metal refractive indices show that the surface out-coupling efficiency ξ of the internally generated photons can be greater than the 0.5 n−2 relation (where n is the refractive index of the emitter layer) valid for isotropic emitters that are not subjected to optical interference effects. When the emitting dipoles are optimally located for maximum rate of surface emission, the model predicts ξ to vary as 0.75 n−2 for the isotropic case, and as 1.2 n−2 for the in-plane case. For our LEDs, we found that the EL arises from in-plane dipoles that are on average almost optimally located away from the cathode. Using this result, the internal EL quantum yield is estimated to be close to 50% of the free-space photoluminescence yield of the emitter for the devices. This indicates excellent injection balance and recombination efficiency of the charge carriers. By also taking into account of optical interference effects on the radiative rate, we deduced that the lower limit for the probability of forming an emissive singlet exciton from electrical injection is 35%–45% in these conjugated polymers. This greatly exceeds the 25% probability from spin-degeneracy statistics.
Energy transfer and upconversion studies in yttrium–aluminum–garnet crystal doped with thulium and holmium under near-infrared laser excitation
Authors: Tian Wenyan , Rami Reddy B.
Published: July 2002

Upconversion emission from Tm3+ and Ho3+ ions in a yttrium–aluminum–garnet crystal was observed on resonant excitation of the H43 level of Tm3+ and the I45 level of Ho3+ with a Ti:sapphire laser at room temperature and 10 K. The upconversion and the energy transfer mechanisms between Tm3+ and Ho3+ ions are discussed. A rate equation model is developed which supported the energy transfer mechanism.
Theoretical analysis of equilibrium geometries and barriers of rotation in molecules H2X2, with X=O, S, Se, and Te
Authors: Block Ruud , Jansen Laurens
Published: July 2002

A model analysis is presented of equilibrium geometries and rotational barriers in molecules of composition H2X2, with X=O, S, Se, and Te, applying exchange perturbation theory. The basis of the model is an assumed analogy with the phenomenon of ‘‘superexchange’’ (indirect exchange) in solids with paramagnetic cations. A diamagnetic center in the superexchange model is formed by two spin-paired electrons in an ‘‘effective’’ 1s Slater orbital determined from the valence-shell diamagnetic susceptibility of the X atom. The model is conceptually related to the so-called ‘‘through-space’’ (i.e., direct) and ‘‘through-bond’’ (i.e., indirect) interactions, of recent interest in organic chemistry. A survey of the literature is given concerning alternative interpretations of equilibrium geometries and barriers (cis and trans), in particular with respect to the possible influence of lone-pair orbitals. The results of the model are found to be comparable to those of large-scale ab initio calculations.
Relaxation times in diffusion processes
Authors: Agmon Noam
Published: July 2002

Relaxation time for diffusion between reflecting boundaries is defined and the theory for its evaluation using the backward-equation approach is presented. A few simple examples are given and comparison with other results in the literature is made. We expect that relaxation time for nonreactive processes might assume the role of mean first passage times in reactive processes.
3D numerical solution of the potential of an ellipsoidal particle in an ionic solution
Authors: Lin Sching Lih , Chew Weng Cho
Published: July 2002

The distribution of the potential outside a charged ellipsoidal particle is useful in studying dielectric enhancement of a suspension of such particles. Before we study the interaction of the particle with an electric field, the equilibrium solution needs to be obtained. In this paper, we report the solution of the three-dimensional distribution of the potential outside a charged ellipsoidal particle in equilibrium in an electrolytic solution. The problem is solved numerically using Newton’s method. We also compare the inner solution with an analytic inner solution.
Reaction dynamics in the low pressure regime: The Kramers model and collisional models of molecules with many degrees of freedom
Authors: Borkovec Michal , Berne Bruce J.
Published: July 2002

The escape rate constant out of a metastable well for an impulsive collisional (BGK) model and the Fokker–Planck frictional (Kramers) model is evaluated analytically for arbitrary potentials and any number of degrees of freedom in the low collision or low friction limit. Completely statistical behavior of the collisionless dynamical system is assumed. The rate constants increase dramatically with the number of degrees of freedom. The result of the weak collision Kramers model allows us to evaluate the collision efficiency βc without adjustable parameters. It is argued that some reactions could be described by a non-Markovian Kramers model with an appropriate number of degrees of freedom.
Quantal and semiclassical studies of reactions in strong laser fields: F(2P3/2, 2P1/2)+H2+ℏω (0.117, 0.469, 1.17 eV)
Authors: Last Isidore , Baer Michael
Published: July 2002

The effects of three photons, namely the CO2 photon (hν=0.117 eV), the HF photon (hν=0.469 eV), and the Nd:glass photon (hν=1.17 eV), on the reactive [F(2P3/2), F(2P1/2)]+H2 systems were studied. Results due to exact quantal, approximate quantal (curve crossing model), and semiclassical (trajectory surface hopping model) treatments are presented for the collinear arrangement. It was found that the existence of field intensities of 0.1–2.5 TW/cm2 hardly affect the reactive process for the system F(2P3/2)+H2. In the interaction of F(2P1/2) and H2 which is, for all practical purposes, elastic in the field-free case, the field-induced electronic inelastic (the spin conversion process) and electronic reactive transition probabilities are not large. For an intensity of 0.1 TW the largest probability encountered is 20%. In comparing the quantal and the semiclassical treatments, it was found that except for one case where a quantitative fit was obtained in all (five) other cases the agreement is at most qualitative. In general, the semiclassical results are lower than the quantal ones.
The electric dipole moment of NiH X 2Δ5/2 and B 2Δ5/2
Authors: Gray Jeffrey A. , Rice Steven F. , Field R. W.
Published: July 2002

A sputter source/supersonic jet apparatus has been developed in order to record MJ-resolved Stark spectra of refractory diatomic molecules. Dipole moments of 0.3±0.1 and 2.4±0.1 D have been measured for the NiH B 2Δ5/2(v=1) and X 2Δ5/2(v=0) levels from resolved splittings of the Q(2.5) and R(2.5) lines at 17 462.95 and 17 500.50 cm−1.
Thermodynamic derivatives of classical distribution functions
Authors: Hernando J. A.
Published: July 2002

The temperature and density derivatives of the distribution functions in the canonical ensemble and the temperature and fugacity derivatives in the grand canonical ensemble are evaluated.
Elastic constants of ammonia by Brillouin spectroscopy
Authors: Kiefte H. , Breckon S. W. , Penney R. , Clouter M. J.
Published: July 2002

The technique of high resolution Brillouin spectroscopy has been used to determine the adiabatic elastic constants of oriented single crystals of (solid I) NH3 at temperatures near the triple point. The values obtained at 194.2 K were C11=81.1, C12=43.7, and C44=48.2 (in units of kbars) with an estimated absolute uncertainty of ±2%. No evidence for acoustic attenuation near the melting curve was found.
Ionization-detected Raman studies of the 1600 cm−1 Fermi dyad of benzene
Authors: Esherick P. , Owyoung A. , Plíva J.
Published: July 2002

We present the first rotationally resolved Raman spectrum of a degenerate rovibrational transition in benzene as a demonstration of the high spectral resolving power and sensitivity attainable by ionization detected stimulated Raman spectroscopy (IDSRS). The double-resonance form of the technique allows the clean separation of the Raman spectra of the Fermi dyad composed of the ν16 fundamental and ν2+ν18 combination vibrations.
Multiphoton dissociation of GeH4: Ultraviolet emission spectrum of GeH
Authors: Osmundsen J. F. , Abele C. C. , Eden J. G.
Published: July 2002

The ultraviolet emission spectrum of GeH has been examined following the two photon-initiated dissociation of germane (GeH4) at 248 nm. The most prominent peaks lying between ∼250 and 340 nm are tentatively assigned to B 2Σ(v′=0)→X 2Π1/2,3/2 (v′) transitions. From this premise, the vibrational constants for the X 2Π states have been estimated to be ωe=(1949±5) cm−1, ωexe=16.5±2.0 cm−1, and ωeye=(0.7±0.3) cm−1. Also, the radiative lifetime of the lowest vibrational levels of the GeH A 2Δ state in transitions to ground has been measured to be 12±2 ns.
A theory of cylindrical polyelectrolyte solutions
Authors: Vlachy V. , McQuarrie D. A.
Published: July 2002

Structural and thermodynamic properties of a solution of cylindrical polyelectrolyte are calculated in the cell model using the HNC/MSA equation. The electrostatic potential and distribution functions are more sensitive to interionic correlations than are the thermodynamic properties. Satisfactory agreement with recent simulation data is found.
Structure of ice Ih. A
b i
n
i
t
i
o two- and three-body water–water potentials and geometry optimization
Authors: Yoon Byoung Jip , Morokuma Keiji , Davidson Ernest R.
Published: July 2002

The OO distance in ice (2.76 Å) is much shorter than in water dimer (2.98 Å). No first principle potential function has successfully described the observed OO shrinkage. We have calculated water–water two-body interaction potentials with an ab initio MO method by varying not only the OO distance but also the OH distance. New analytical fits of two-body potential functions have been obtained. The nearest-neighbor three-body potential has been evaluated for proton-ordered ice–Ih structures. With ab initio one, two, and nearest-neighbor three-body potentials, ∠HOH fixed at the monomer value, we have been able to obtain ROO=2.79 Å, ROH=0.977 Å with the binding energy of 15.8 kcal/mol per H2O molecule for proton-ordered antiferroelectric ice Ih and ROO=2.85 Å, ROH=0.972 Å with the binding energy of 14.3 kcal/mol per molecule for ferroelectric ice Ih. The three-body interaction, aided by the two-body interaction, contributes to the OO shrinkage. Factors that would favor larger ROH stretch and ROO shrinkage have been discussed.
Collisional energy transfer via vibrational predissociation: Relaxation of HF(v=1) by HF dimers
Authors: Rensberger K. J. , Copeland R. A. , Robinson J. M. , Crim F. F.
Published: July 2002

Overtone vibration–laser double resonance measurements determine the vibrational relaxation rate of HF(v=1) by HF dimers. Vibration-to-vibration energy transfer from the excited monomer to the dimer followed by vibrational predissociation of the dimer provides an efficient pathway for vibration-to-translation energy transfer that deexcites the monomer at 40% of the gas kinetic collision rate. Analysis of the pressure dependence of the observed decay constants using a simple kinetic model establishes a rough upper limit of 10 ns on the predissociation lifetime of the collisionally excited dimer.