bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of exchange energy and magnetic anisotropy on the nanocrystalline alloy
Authors: Kim K. S. , Yu S. C. , Rao K. V.
Published: July 2002

We have studied the influence of exchange energy and magnetic anisotropy on the magnetic properties of nanocrystalline materials. Amorphous ribbons of composition Fe88Zr7B4Cu1 have been annealed between about 500 and 700 °C. Temperature dependence of magnetization was carried out and nanocrystallized magnetic entities were investigated using the mean field theory and random anisotropy model obtained ferromagnetic correlation length and magneto-crystalline anisotropy constant. The best soft magnetic properties were observed annealed at 650 °C for the smallest magneto-crystalline anisotropy constant (K1) of about 1.40×104 erg/cm3. We obtained average magnetic anisotropy of 0.81 erg/cm3 which is almost negligible compared with K1 at an annealing temperature of 650 °C. The local magneto-crystalline anisotropy is randomly averaged out by strong exchange interaction so that this mechanism provides the basis for the good soft magnetic properties.
On the relationships between the temperature dependence of the magnetization and the average grain size in nanostructured samples
Authors: González J. M. , de Julián C. , Cebollada F. , Giri A. K. , González J.
Published: July 2002

We report on the low temperature thermal demagnetization behavior observed in samples of the Vitrovac 7600TM commercial amorphous alloy which were submitted to different crystallization treatments, all of them resulting in the induction of nanostructures. Our experimental data were analyzed in terms of the predictions of different models of ferromagnetism allowing us to conclude that our samples behaved as weak ferromagnets with nanostructure dependent spin wave spectra and a significant contribution to the demagnetization processes from single particle excitations.
Structure and perpendicular magnetization of Fe/Ni(111) bilayers on W(110)
Authors: Sander D. , Enders A. , Schmidthals C. , Kirschner J. , Johnston H. L. , Arnold C. S. , Venus D.
Published: July 2002

Scanning tunneling microscopy and low energy electron diffraction show that high quality fcc Ni(111) films can be prepared on W(110). The subsequent coverage of this Ni template by monolayers of Fe leads to a Fe/Ni bilayer with striking magnetic properties. The Fe cap layer induces a spin reorientation of the easy axis of magnetization from in-plane to perpendicular to the film, as checked with the magneto-optic Kerr effect. At higher Fe coverages, an in-plane magnetization of the bilayer is found, which is proposed to be caused by the fcc to bcc transition in the Fe layer.
Extended energy loss fine structure analysis of hard and elastic carbon nitride thin films
Authors: Csillag S. , Räty R. , Zou W. Y. , Sjöström H. , Sundgren J.-E. , Colliex C.
Published: July 2002

In this article an electron energy loss spectroscopy investigation of CNx thin films is reported. The bonding, composition, and structure are discussed and a more thorough extended energy loss spectroscopy investigation is carried out to determine the interatomic distances. The extended energy loss fine structure analysis reveals a component with an unusually high frequency in the data corresponding to an interatomic distance of approximately 7.3 Å. This is suggested to originate from backscattering from distant curved atomic layers.
Assessment of thin-film hardness through elastic/plastic stress analysis in a microindentation test
Authors: Ahn Jeong-Hoon , Kwon Dongil
Published: July 2002

The true hardness of thin film is assessed through micromechanical analysis of the composite hardness measured by a microindentation tester. In partitioning the respective contributions of the film and the substrate to the composite hardness, we apply the modified plastic-zone volume-law-of-mixtures theory by analyzing the indentation stress field of a film/substrate system. At this time, while the substrate is assumed to undergo radial deformation, the film is assumed to be deformed mainly parallel to the surface. On the basis of these stress analyses, the influence of interface is incorporated; the deformation in the softer material is constrained by interface bonding and the plastic-zone radii are modified by considering the respective virtual pressures that the film and the substrate actually support. When an interface has perfect bonding, the hardness value for a film can be calculated analytically from the condition of strain matching at the interface—approximately 2200 kgf/mm2 for diamondlike carbon (DLC) films deposited on WC–Co substrates. On the other hand, when the interface is weak, an interface parameter κ is introduced to describe the partial release of film constraint due to interface delamination. For DLC films on Si substrates, the expansion rate of the extent of this partial release is constant regardless of both the film thickness and indentation load, so that it can be used as a measure of adhesion loss.
Quantitative analysis of piezoelectricity in simultaneously stretched and corona poled polyvinyl chloride films
Authors: Bharti Vivek , Nath R.
Published: July 2002

Piezoelectric activity in cyclohexane-treated polyvinyl chloride films has been found to increase when processed under simultaneously stretched and corona poled (SSCP) conditions. The piezoelectric coefficient d31=7.2±1.1 pC/N was obtained in these films under optimum poling conditions. The main contributor to the improved piezoelectricity has been the increased dipolar polarization. Crystallinity and compliance have contributed marginally to the former. Thermally stimulated current (TSC) experiments show that maximum oriented dipole density has been achieved during the SSCP. Dipolar polarization ≈10−2 C/m2 has been obtained. The TSC and the shrinkage effect involving molecular rotation and/or chain motion are closely related. An application of the extended Mopsik and Broadhurst theory shows that 56% of the measured d31 is due to the dimensional effect and 44% due to the local field effect.
Configurational transformation of an Er center in GaAs:Er,O under hydrostatic pressure
Authors: Hogg R. A. , Takahei K. , Taguchi A. , Takarabe K. , Mizushima T. , Minomura S.
Published: July 2002

An optical spectroscopic study of Er-related luminescence in GaAs:Er,O under hydrostatic pressure is reported. The application of pressure results in new Er-related centers becoming optically active under host photoexcitation. Two new sets of I413/2→4I15/2 transitions are observed to appear in different pressure ranges. It is concluded that these two sets of crystal-field-split transitions are due to the same Er center, and that this Er center undergoes an abrupt change in local configuration at a specific pressure.
Defects in metamorphic InxAl1−xAs (x<0.4) epilayers grown on GaAs substrates
Authors: Shieh Jia-Lin , Chang Mao-Nan , Cheng Yung-Shih , Chyi Jen-Inn
Published: July 2002

Defects in Si-doped InxAl1−xAs (0<x<0.4) epilayers on GaAs substrates were systematically investigated by deep-level transient spectroscopy. Three electron traps, ranging from 0.22 to 0.89 eV, were observed in InxAl1−xAs grown by molecular beam epitaxy. Their energy levels can be extrapolated from those in the InxAl1−xAs/InP system. A low-temperature-grown buffer layer as well as thermal annealing were found to be capable of reducing the defects in the active layer effectively.
Lateral magnetoresistances of epitaxial ZnSe and CdMnTe thin films measured by the microwave contactless method
Authors: Wang Zongxin , Chu Youling , Ling Zhen , Jin Caixia , Wang Jie , Hou Xiaoyuan , Wang Xun
Published: July 2002

A microwave contactless method is developed to measure the magnetoresistances of ZnSe and CdMnTe epitaxial layers grown by molecular beam epitaxy on semi-insulating GaAs substrates. The carrier concentrations and Hall mobilities are derived from the experimental data.
Simple theory for the Levitron®
Authors: Jones T. B. , Washizu Masao , Gans Roger
Published: July 2002

The Levitron® is a popular toy that provides a unique and interesting demonstration of levitation using permanent magnets. It consists of a small, spinning magnetic top and a magnetic base plate. Stable levitation is possible because of a unique coupling of the magnetic forces and torques with the gyroscopic action of the top. A simple theory of its operation, using a general axisymmetric form for the magnetic field of the base, is based on the dipole force model. With this model, the stable behavior of the spinning, levitated top may be investigated by testing various assumptions for its orientation. Stability is not possible if the top remains rigidly parallel to the axis of the base. On the other hand, if one assumes that the top remains aligned parallel to the local magnetic field during radial excursions, then stability is possible. This simple model, combined with measurements of the magnetic field along the axis, permits fairly accurate prediction of the upper and lower limits of the locus of stable equilibria.
Crack softening damage model for ceramic impact and its application within a hydrocode
Authors: Hazell Paul J. , Iremonger Michael J.
Published: July 2002

A physically based crack softening damage model has been developed and used in a non-linear transient dynamic computer code (AUTODYN-2D). It is assumed that there is a finite number of orientated pre-existing flaws within the ceramic target. The mode I and mode II stress intensity factors are calculated in compression and tension and the strain energy release rate is then estimated and compared to a critical dynamic strain energy release rate. At initiation, a tension crack propagates at a velocity dependent on the mode I stress intensity factor and failure occurs in a computational cell when two neighbouring microcracks coalesce. The model was used to simulate two different plate impact experiments of alumina on alumina with encouraging results. The model was also used to analyze the impact of a steel sphere on alumina and shows strong correlation between experimental and predicted results.
A study of electrically active defects created in p-InP by CH4:H2 reactive ion etching
Authors: Goubert L. , Van Meirhaeghe R. L. , Clauws P. , Cardon F. , Van Daele P.
Published: July 2002

The electrical effects of reactive ion etching (RIE) of p-InP by CH4:H2 are investigated. By using optical deep-level transient spectroscopy, several deep-level defects could be detected. The main defect (E3) is found to be a donor with an energy level at Ec−0.38 eV. From depth profiles of both the net acceptor concentration and this defect, it follows that after RIE, the E3 defects are passivated by hydrogen. Simultaneous passivation of both acceptors and donors on p-InP is reported. Subsequent rapid thermal annealing at increasing temperatures shows that the E3 defects are first depassivated and then annealed out. The change of the Schottky barrier height with the anneal temperature could be explained by Fermi-level pinning due to E3 and depinning either by its passivation for low enough temperatures or by its annealing out at higher temperatures. Some possibilities concerning the physical nature of E3 are discussed.
Control of stability and current-voltage characteristics in amorphous hydrogenated silicon nitride thin film diodes
Authors: McGarvey B. , Curran J. E. , Ford R. A. , French I. D. , Gale I. G. , Hewett J. , Sandoe J. N.
Published: July 2002

Current transport and instability mechanisms in thin film diodes with nonstoichiometric silicon nitride (a-SiNx:H) semiconducting layers have been investigated. In common with amorphous silicon thin film transistors the electrical characteristics of these diodes have been found to drift during use. We found that the initial current-voltage characteristics are related to the choice of interfacial treatments. This is explained by trapping of electrons at interface defect states and by tunneling of electrons into the conduction band via these states. We have also found a relationship between the initial characteristics of the diodes and the rate of drift due to electrical stressing. A threshold exists below whose drift is independent of the current-voltage characteristics and above which there is a strong dependence. This dependence of drift on current-voltage characteristic is consistent with field enhanced defect creation in the a-SiNx:H layer.
Visible photoluminescence from porous a-Si:H and porous a-Si:C:H thin films
Authors: Estes M. J. , Hirsch L. R. , Wichart S. , Moddel G. , Williamson D. L.
Published: July 2002

We report on the influence of doping, temperature, porosity, band gap, and oxidation on the photoluminescence (PL) properties of anodically etched porous a-Si:H and a-Si:C:H thin films. Only boron-doped, p-type a-Si:H samples exhibited visible photoluminescence. Two broad PL peaks at ∼1.6 and ∼2.2 eV are apparent in room temperature PL spectra. The intensity of the 2.2 eV peak as well as the nanovoid density in the unetched a-Si:H layers both correlate well with boron concentration. We see evidence of discrete defect or impurity levels in temperature-dependent luminescence measurements, where we observe multiple luminescence peaks. Unlike in porous crystalline silicon, the luminescence energy in porous amorphous silicon does not change with porosity. We do, though, observe a correlation of luminescence energy with band gap of the starting a-Si:C:H films. Oxidation, either native or anodic, reduces photoluminescence intensity. We discuss the implications of these observations on the nature of the luminescence mechanism.
Off-axis extra lines in powder pattern electron paramagnetic resonance of Cr3+ in YAlO3
Authors: Rakhimov R. R. , Wilkerson A. L. , Loutts G. B. , Ries H. R.
Published: July 2002

Powder sample electron paramagnetic resonance (EPR) spectrum of Cr3+ ions in YAlO3 crystals contains off-axis extra lines, which do not belong to the principal X, Y,and Z components of the fine structure. Orientation dependence of the EPR spectrum of Cr3+ doped YAlO3 single crystal confirms the existence of extra turning points corresponding to directions lying off the X, Y, and Z axes, consistent with theoretical predictions. This results in five EPR lines due to the transition between the central Kramers doublet in the powder system. The observed powder pattern EPR has been used in identification of Cr3+ as an impurity ion in undoped YAlO3 crystals.
Period expansion of Co/C and CoN/CN soft x-ray multilayers after annealing
Authors: Bai H. L. , Jiang E. Y. , Wang C. D. , Tian R. Y.
Published: July 2002

Period expansion of Co/C and CoN/CN soft x-ray multilayers has been investigated by x-ray diffraction and Raman spectroscopy. Below the anneal temperature of 400 °C, the period expansion (< 12%) of Co/C multilayers is mainly caused by the graphitization of the amorphous carbon layers. By 500 °C, the crystallization and agglomeration of Co layers induce an enormous period expansion (∼40%). The period expansion of CoN/CN multilayers is only 4% at 400 °C, which is much smaller than that of Co/C multilayers. The interface patterns of the CoN/CN multilayers still exist even if they were annealed at 700 °C. The Raman spectroscopy analyses indicate that the formation of the sp3 bonding can be suppressed effectively by doping N atoms, and thus the period expansion is decreased considerably at annealing temperatures below 600 °C. The significant suppression of grain growth above 600 °C is believed to be attributed to the coexistence of hcp and fcc Co structures induced by interstitial N atoms, which cause the high-temperature period expansion decrease. The results also imply that the structural stability of Co/C soft x-ray multilayers can be significantly improved through doping N atoms.
Comment on “A universal optimum work rate potential for continuous endoreversible Carnot heat engine cycles” [J. Appl. Phys. 80, 3619 (1996)]
Authors: Chen Jincan , Yan Zijun
Published: July 2002

It is pointed out that there is not a certain universal relation for the power outputs of continuous endoreversible and reversible Carnot heat engine cycles operating between the same temperature limits, and the relevant conclusion obtained by Blank and Bhattacharyya in a recent article of this journal is incorrect.
Reaction of Ti with WSi2
Authors: Gambino J. P. , Cunningham B. , DeHaven P. , Adams E. D.
Published: July 2002

Reactions between Ti and WSi2 have been studied between 400 and 800 °C. Reactions at the Ti–WSi2 interface begin at 600 °C, with the formation of TiSi. The TiSi is converted to TiSi2 at 800 °C. The formation temperatures for TiSi and TiSi2 are higher than those observed for Ti on Si, presumably because the Si supply is limited by relatively slow diffusion of Si through WSi2. At 700 °C, localized formation of Ti silicide is also observed below the WSi2, due to Ti diffusion through grain boundaries in the WSi2. These results suggest that post-metallization anneals of Ti on WSi2 polycide structures should be kept below 700 °C to avoid device degradation.
Effects of surface heavy ion implantation on superconducting melt-textured YBa2Cu3O7−δ
Authors: Mezzetti E. , Gerbaldo R. , Ghigo G. , Gozzelino L. , Minetti B. , Cherubini R.
Published: July 2002

The possibility of predictably enhancing the critical current density of a bulk material with a strong uncorrelated disorder by introducing linearly correlated defects into just the top and bottom surface layers has been investigated. The task was fully demonstrated for melt-textured YBa2Cu3O7−δ having irradiation modified percent thickness ranging from 13% to 5%. The characteristic signatures of vortex localization were observed in the higher range of temperature and magnetic field.
Diffraction anomalous near-edge structure in ordered GaInP
Authors: Alagna L. , Prosperi T. , Turchini S. , Ferrari C. , Francesio L. , Franzosi P.
Published: July 2002

We report the diffraction anomalous near-edge structure (DANES) of a nominally lattice matched GaxIn1−xP/GaAs (x=0.51) heteroepitaxial layer, grown by metal organic chemical vapor deposition, which shows long range ordering in the cationic sublattice along the 〈111〉 direction. DANES spectra, originating from the 004 reflections of the substrate and of the epi-layer and that from the “forbidden” −5/2 5/2 −5/2 reflection of the superstructure, have been recorded at the Ga K edge. A full theoretical simulation, based on the kinematic formalism, largely agrees with the experimental data.
Improvement in buried silicon nitride silicon-on-insulator structures by fluorine-ion implantation
Authors: Rauthan C. M. S. , Virdi G. S. , Pathak B. C. , Karthigeyan A.
Published: July 2002

As an effort to improve buried Si3N4–Si interfaces in silicon-on-insulator (SOI) structures, fluorine was implanted either before or after the synthesis of buried silicon nitride layers by high dose nitrogen ion implantation. In these synthesized SOI structures, the thickness of Si overlayer and the buried silicon nitride layer was found to be 2400±25 and 1725±25 Å, respectively. Auger electron spectroscopy depth profile analysis illustrates that the fluorine ion implantation in the SOI structures modifies the distribution of nitrogen that results in better stoichiometry of the buried silicon nitride layers and abrupt Si3N4–Si interfaces. Current–voltage and high frequency capacitance–voltage characteristics were measured, and electrical breakdown measurements were performed on metal-nitride-silicon (MNS) structures, fabricated after removing the Si overlayer in the SOI structures. In the fluorine-implanted SOI specimens, the buried Si3N4 layers show a higher breakdown field strength of 4.4–5.2 MV/cm as compared to 3.5–4.0 MV/cm in the nonfluorine-implanted one. The capacitance–voltage analysis of the MNS capacitors reveals that the buried Si3N4–Si substrate interface exhibits a better quality with reduced fixed insulator charge and interface state densities after the fluorine ion implantation. Midgap interface state density at the buried Si3N4–Si substrate interface was as low as 1.48×1011 cm−2 eV−1 after the fluorine implantation, which is comparable to that of silicon nitride films deposited on silicon by the conventional low pressure chemical vapor deposition technique. The role of fluorine in improving the basic electrical properties of these SOI structures has been discussed.
Evolution of Si0.982C0.018 pseudomorphic layer after excimer laser annealing
Authors: Guedj C. , Calvarin G. , Piriou B.
Published: July 2002

We have studied the effect of excimer laser annealing on a high-quality Si0.982C0.018 pseudomorphic layer, using Fourier transform infrared (FTIR) spectroscopy, Raman spectrometry, and x-ray diffraction. The substitutional carbon concentration is found to decrease as a function of fluence for 50 laser pulses performed at room temperature and vacuum. The strain profile evolution is obtained with x-ray diffraction and dynamical diffraction simulations. After melting, most of the strain is released, and a new FTIR and Raman peak appears around 830 cm−1. This feature is attributed to the formation of SiC microprecipitates and V-O asymmetrical centers. A mechanism for the substitutional carbon removal is proposed. It involves SiC precipitation and reaction between V-O and substitutional carbon to form volatile CO. In the case of pulsed laser induced epitaxy, we predict that the highest substitutional carbon content should be obtained with one laser pulse in an oxygen-free ambiant. From these results, it is inferred that pulsed-laser-induced epitaxy is suitable for the localized patterning of ultrashallow buried Si1−yCy junctions.
Energy dependence of C60–graphite surface collisions
Authors: Pan Z. Y. , Man Z. Y. , Ho Y. K. , Xie J. , Yue Y.
Published: July 2002

The resilience of C60 fullerene with graphite (0001) surfaces has been investigated by means of molecular dynamics simulations with empirical model potentials. The initial energy of C60 ranges from 30 to 300 eV. It is shown that when the impact energy is above 60 eV, the rebounding energy of C60 is nearly independent of the impact energy. The scattering is highly inelastic and the internal excitation energy of the scattered molecule increases with the incident energy. These results are consistent with experiment. Furthermore, the simulations provide insight into the microscopic aspects of the scattering. The rebounding processes at different energies are found to exhibit similar dynamic behavior and the molecular center-of-mass motion can be regarded as moving in a quadratic harmonic potential. All of these results support a schematic picture to describe nonreactive C60-surface collisions.
Determination of the structure of amorphous nitrogenated carbon films by combined Raman and x-ray photoemission spectroscopy
Authors: Bhattacharyya S. , Hong J. , Turban G.
Published: July 2002

Amorphous nitrogenated carbon (a-CN:H) films are prepared from a mixture of methane and nitrogen in an electron cyclotron resonance plasma at a pressure of 2 mTorr and applying a substrate bias of −300 V. Based on the characterization done by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy on the films containing different amounts of nitrogen, a similarity in variation between the binding energy of C 1s peak and position of Raman active G peak is noticed. XPS C 1s and N 1s are deconvoluted into four components to separate the contribution of CC, C–N, C=N, and C≡N bonds. The change of intensity ratio of Raman active D and G peaks shows a marked correlation with the intensity ratio of single and double bonds between carbon and nitrogen. Combined XPS and Raman spectroscopy throw some light on the variation in structure of carbon films as a function of nitrogen concentration.
Storage-ring free-electron lasers and self-consistent harmonic generation
Authors: Dattoli G. , Di Pace A. , Mezi L. , Sabia E.
Published: July 2002

We employ a semianalytical model to study the self-consistent higher harmonic generation in free-electron lasers operating with storage rings. The model combines the rate equations, governing standard storage-ring free-electron laser dynamics, with the equations describing the evolution of the bunching coefficients. The developed technique is amenable to a straightforward numerical implementation and provided reliable results, whose correctness is proved by means of a more complete numerical procedure.
Gigaelectron-volt heavy ion irradiation of gallium arsenide
Authors: Carlone Cosmo , Parenteau Martin , Khanna Shyam M.
Published: July 2002

Gallium arsenide grown by metalorganic chemical vapor deposition and n doped with silicon to nominal concentrations of 2×1015, 8×1015, and 2×1016 cm−3, was irradiated with 1.04 GeV bromine ions at a fluence of 5×109 cm−2, 1.7 GeV iodine ions at a fluence of 2.7×109 cm−2, and 1.5 GeV gold ions in the fluence range of 1.0×106–2.2×109 cm−2. The effects were analyzed by photoluminescence (PL) spectroscopy. The donor-to-gallium vacancy (D-VGa) and the donor-to-silicon-acceptor (D-SiAs) transitions are observed in the PL spectra of the irradiated samples. The former occurs at 1.476 eV, and the latter at 1.483 eV when the recording temperature is 6.5 K. The relative introduction rates of the VGa and SiAs defects for these ions are compared to those obtained in previous studies where electrons, protons, alpha particles, lithium ions, and oxygen ions were the irradiating particles. The measured values correlate with relativistic (Darwin–Rutherford) or nonrelativistic (Rutherford) scattering theory, depending on the projectile energy. The relevance of the GeV heavy ion irradiation with cosmic rays is discussed.
Critical-state model for intermodulation distortion in a superconducting microwave resonator
Authors: McDonald J. , Clem J. R. , Oates D. E.
Published: July 2002

A model is presented for the treatment of intermodulation distortion in a superconducting transmission line caused by vortex penetration and hysteresis. An analytical framework is developed, and numerical results are presented for center conductors of both circular and rectangular thin-film cross section.
On the nature of thermally stimulated discharge current spectra in polyethylene terephthalate
Authors: Gorokhovatsky Yu. , Temnov D. , Marat-Mendes J. N. , Dias C. J. M. , Das-Gupta D. K.
Published: July 2002

Thermally stimulated depolarization currents (TSDC) in polyethylene terephthalate (Hostaphan) films of different thickness (12, 25, and 50 μm) have been investigated under different environmental conditions in the temperature range of 20–150 °C. Two peaks of TSDC spectra observed at approximately 80 and 125 °C originate from a dipolar process (α peak) and a space-charge relaxation (ρ peak), as evidenced from the variation of parameters such as electrode geometry and polarizing voltage. A numerical method (Tikhonov regularization procedure) has been employed to determine the most probable values of effective frequency factors of dipolar and space-charge relaxation in polyethylene terephthalate. An apparent influence of environmental pressure on the location of the α peak has been observed whose origin is yet to be determined.
The effect of grain size on magnetic properties in Sm2Fe17Nx
Authors: Shield J. E. , Branagan D. J. , Li C. P. , McCallum R. W.
Published: July 2002

The microstructures of melt spun Sm11Fe89 and (Sm11Fe89)94Ti3C3 have been investigated under different solidification conditions. The magnetic properties of the nitrided materials have been related to the observed microstructures. Melt spinning of the Sm11Fe89 alloys resulted in grain sizes at or above the single domain limit. The addition of Ti and C resulted in an order of magnitude refinement in the microstructural scale. The magnetic properties of the samples with grain sizes below the single domain limit decreased with decreasing grain size due to increased intergranular exchange coupling.
Polarization anisotropy of sub-band gap oscillatory features in contactless electroreflectance spectrum of InxGa1−xP layers grown on GaAs (001) substrates
Authors: Ghosh Sandip , Arora B. M. , Sharma T. K. , Gokhale M. R.
Published: July 2002

We report the observation of strongly polarization sensitive sub-band gap oscillatory features in the contactless electroreflectance spectrum of InxGa1−xP layers grown on GaAs (001) substrates. At a given energy in the sub-band gap region, the peak strength of these oscillatory features decreases from a positive maximum to a negative minimum passing through zero as the polarization of the incident probe beam is rotated by 90° from [11̄0] direction to [110] direction in the (001) plane. The origin of this phenomenon is explained on the basis of optical interference coupled with linear electro-optic effect induced changes in the sub-band gap refractive index of the InxGa1−xP layers. Numerical simulations based on the above mechanism are shown to reproduce the polarization dependent observations quite well.
Relaxation of electron polarization for optically pumped rubidium atoms
Authors: Nagengast W. , Nass A. , Grosshauser C. , Rith K. , Schmidt F.
Published: July 2002

Relaxation time constants of electron polarization, 〈Sz〉, for optically pumped rubidium atoms in a drifilm coated glass cell have been investigated with the Faraday rotation method. The dependence of the 〈Sz〉 relaxation time on pump cell temperature, rubidium density and applied magnetic field has been determined with high precision. The measurements allow the determination of the local magnetic field on the surface, alkali-wall binding energy and wall-dwell time. Furthermore, the change of relaxation time caused by admixtures of molecular hydrogen, nitrogen and oxygen has been investigated. No influence due to the addition of hydrogen could be detected up to a density of 3.3×1014 cm−3 at typical rubidium densities of several 1011 cm−3. The cross section for depolarizing collisons with molecular oxygen has been determined to be 7.50×10−15 cm2. The corresponding cross section for nitrogen was found to be two orders of magnitude lower. The experimental results are discussed in respect to the performance of spin exchange sources for nuclear polarized hydrogen and deuterium.
Photorefractive properties of tungsten bronze ferroelectric lead barium niobate (Pb1−xBaxNb2O6) crystals
Authors: Lee Myeongkyu , Feigelson Robert S. , Liu Alice , Hesselink Lambertus
Published: July 2002

Lead barium niobate (Pb1−xBaxNb2O6) is a very promising material for photorefractive applications in that it has a high ferroelectric-to-paraelectric transition temperature (300–500 °C depending on composition) and simultaneously can have large electro-optic coefficients, because of the nearly vertical morphotropic phase boundary (MPB) near 1−x=0.63. Pb1−xBaxNb2O6 crystals have been grown by the vertical Bridgman method for near-MPB compositions (0.5<1−x<0.6), and their photorefractive properties were investigated by the two wave mixing experiments. Material properties such as composition and absorption coefficient varied along the growth direction because of the incongruent charge composition and the nonuniform distribution of impurities. A maximum diffraction efficiency of 50% at λ=514.5 nm was observed in a 3.2 mm thick nominally undoped as-grown crystal. The photorefractive sensitivity increased as the wavelength decreased. The gain coefficient also increased with decreasing wavelength, changing from 0.5 cm−1 at λ=633 nm to above 8 cm−1 at λ=465 nm.
Electroelastic equations describing slow hysteresis in polarized ferroelectric ceramic plates
Authors: Huang L. , Tiersten H. F.
Published: July 2002

Rotationally invariant electroelastic equations are extended to account for slow hysteretic effects in polarized ferroelectric ceramic plates by employing an internal variable in a thermodynamic state function. All material irreversibility is taken to be a consequence of the ferroelectric polarization-electric-field irreversibility. Since we are concerned with the slowest possible hysteresis, we ignore the evolution equation and take the known irreversible thickness polarization-electric-field saturation curve as the starting curve. After the general nonlinear three-dimensional equations are obtained, they are reduced to linear strain and then plane stress because all experimental data available to us are from strain measurements on a thin wide beam forced by voltages applied to very thin polarized ferroelectric ceramic actuators on the surfaces. The new irreversible material coefficients are obtained from data on one major hysteresis loop along with the coefficients in the saturation polarization-electric-field relation. These now known coefficients are then used to calculate other hysteresis loops, which are shown to agree quite well with measurement.
Rectangular normal domains in current-carrying superconductors
Authors: Kovner V. S. , Mints R. G.
Published: July 2002

We find a type of normal domain in composite superconductors—a traveling rectangular domain. Using the effective circuit model we study in detail the dynamics of this domain. We derive an analytical solution for the propagation velocity and the length of a traveling rectangular domain. Current–voltage characteristics are calculated for a superconductor with a rectangular domain.
Strain induced alteration of the gadolinium surface state
Authors: Waldfried C. , McIlroy D. N. , McAvoy T. , Welipitiya D. , Dowben P. A. , Vescovo E.
Published: July 2002

The electronic structure of strained and unstrained Gd(0001) has been studied with photoemission, inverse photoemission, and spin-polarized photoemission. A shift of the occupied majority and unoccupied minority surface states has been observed as a result of the strain, consistent with the phase accumulation model. There is a strain induced shift of the minority spin surface state across the Fermi level.
Magnetic behavior of the new low-Tc phase of SrRuO3
Authors: Joy P. A. , Date S. K. , Anil Kumar P. S.
Published: July 2002

Magnetic properties of the new phase of SrRuO3 having a magnetic ordering temperature of 140 K (low-Tc phase) is compared against that of the normal ferromagnetic phase of Tc=160 K (high-Tc phase). The symmetric ac susceptibility curve and an S-shaped M vs H curve of the low-Tc phase suggests a spin-glasslike behavior. However, the value of M at 80 K is comparable for both the phases and the low-Tc phase shows no magnetic aging effect expected for a spin glass. The low-Tc phase is thus a ferromagnetic phase, the difference in the magnetic behavior of the low-Tc phase from that of the high-Tc phase probably originates from the domain structure.
Infrared studies of magnetic surface modes on antiferromagnets (invited)
Authors: Camley R. E. , Jensen M. R. F. , Feiven S. A. , Parker T. J.
Published: July 2002

In contrast to ferromagnets, where low frequency surface excitations typically have frequencies in the 10 GHz range, surface excitations in antiferromagnets are often in the several hundred GHz to few THz range. Theoretical predictions for surface spin waves on antiferromagnets indicate that they should be highly nonreciprocal, i.e., the properties of a wave with wave vector +k would be very different from those with a reversed wave vector of −k. Surface spin waves on antiferromagnets have recently been measured using a high resolution Fourier transform infrared spectrometer. The results show evidence of both true surface modes and surface resonances. The nonreciprocal features of the surface modes are seen in a dramatic nonreciprocal reflection. For example, the reflectivity can be 80% for one orientation, but when the incident and reflected waves are reversed the reflectivity drops to near zero. While the initial measurements were done on a bulk antiferromagnet, we also present calculations showing the results for thin films.
Effects of heat treatment on the magnetic properties of polymer-bound iron particle cores
Authors: Namkung M. , Wincheski B. , Bryant R. G. , Buchman A.
Published: July 2002

Spherical iron particles of three different size distributions, 6–10 μ in diameter, 100 mesh and 30–80 mesh, were mixed with 2.0 wt % of soluble imide and compression molded at 300 °C under 131 MPa. Post-fabrication heat treatments were performed at 960 °C for 6 h resulting in a significant enhancement of the permeability in low field region for all the specimens except for the one made of 30–80 mesh particles. The rate of core loss of these specimens at a magnetic induction of 5 kG measured up to 1 kHz shows a noticeable increase after heat treatment which, along with the permeability enhancement, can be explained by the coalescence of particles forming a network of conductivity paths in the specimens. The scanning electron micrographs taken for the 6–10 μ particle specimens show no evidence of heat treatment-induced grain growth. The untreated specimens show a very weak f2-dependence of the core loss which clearly indicates a negligible contribution from the eddy current loss. In particular, an almost perfect linearity was found in the frequency dependence of the core loss of the untreated specimen made of 100 mesh iron particles.
Complex dynamics of resistively and inductively shunted Josephson junctions
Authors: Cawthorne A. B. , Whan C. B. , Lobb C. J.
Published: July 2002

We have measured Nb–AlOx–Nb Josephson tunnel junctions which have resistive shunts with different parasitic inductances. Numerical simulations reveal that specific features in the experimental current–voltage (I–V) characteristics of these devices are dc signatures of complex ac behavior. Depending on the inductance of the shunt loop and the capacitance of the junction, these features may either appear or disappear as the temperature of the device is increased. Examination of the simulated voltage waveforms allows us to map regions of the parameter space which exhibit complicated behavior. These regions should be avoided when a nearly sinusoidal voltage waveform is desired, as is the case for Josephson junction-based oscillators. The agreement of the experimental and simulated I–V curves also enables us to accurately determine the inductance of the shunts and the capacitance of the junctions.
Field emission properties of nitrogen-doped diamond films
Authors: Sowers A. T. , Ward B. L. , English S. L. , Nemanich R. J.
Published: July 2002

This study explores the field emission properties of nitrogen-doped diamond grown by microwave plasma chemical vapor deposition. Over 70 nitrogen-doped diamond samples were grown on silicon and molybdenum under varying process conditions. Under certain conditions, films can be grown which exhibit photoluminescence bands at 1.945 and 2.154 eV that are attributed to single substitutional nitrogen. Photoelectron emission microscopy with UV free electron laser excitation indicated a 0 or negative electron affinity. Field emission characteristics were measured in an ultrahigh vacuum with a variable distance anode technique. For samples grown with gas phase [N]/[C] ratios less than 10, damage from microarcs occurred during the field emission measurements. Samples grown at higher [N]/[C] content could be measured prior to an arcing event. Contrary to other reports on nitrogen-doped diamond, these measurements indicate relatively high threshold fields (>100 V/μm) for electron emission. We suggest that the nitrogen in these films is compensated by defects. A defect-enhanced electron emission model from these films is discussed.
Polarization-induced charge and electron mobility in AlGaN/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy
Authors: Smorchkova I. P. , Elsass C. R. , Ibbetson J. P. , Vetury R. , Heying B. , Fini P. , Haus E. , DenBaars S. P. , Speck J. S. , Mishra U. K.
Published: July 2002

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51 700 cm2/V s (T=13 K) was obtained in the Al0.09Ga0.91N/GaN structure with a two-dimensional electron gas density of 2.23×1012 cm−2.
Hard sphere colloidal dispersions: Viscosity as a function of shear rate and volume fraction
Authors: de Kruif C. G. , van Iersel E. M. F. , Vrij A. , Russel W. B.
Published: July 2002

The viscosities of suspensions of sterically stabilized (hard) silica spheres in cyclohexane are reported as a function of shear rate (γ̇) and volume fraction (6×10−4<φ<0.6). The shear thinning scales according to (ηr−η∞)/(η0−η∞) =1/(1+1.31ηγ̇a3/kT) with limiting low and high shear viscosities described up to φ∼0.35 by η0=1+5/2φ+(4±2)φ2+(42±10)φ3 , η∞=1+5/2φ+(4±2)φ2+(25±7)φ3 . At higher volume fractions the viscosity becomes more sensitive to φ and diverges at φm=0.63±0.02 (γ̇→0) , φm=0.70±0.02 (γ̇→∞) . The experimental results compare well with existing hard sphere theories and the data of Krieger (1972) for aqueous lattices. Even at the highest volume fraction neither yield stresses nor shear thickening are observed.
Resonances in the K shell excitation spectra of benzene and pyridine: Gas phase, solid, and chemisorbed states
Authors: Horsley J. A. , Stöhr J. , Hitchcock A. P. , Newbury D. C. , Johnson A. L. , Sette F.
Published: July 2002

K shell excitation spectra of the aromatic molecules benzene and pyridine in the gas phase are compared to those for the solids (ices) and for monolayers chemisorbed on Pt(111). The gas phase and solid spectra are essentially identical and even the spectra for the chemisorbed molecules exhibit the same resonances. Because of the orientation of the molecules upon chemisorption the latter spectra show a strong polarization dependence as a function of x-ray incidence. This polarization dependence in conjunction with a multiple scattering Xα calculation for the benzene molecule allows us to assign the origin of all K shell resonances. The resonances are found to arise from transitions to π* antibonding orbitals and to σ* shape resonances in the continuum. The shape resonances are characterized by potential barriers in high (l=5 and 6) angular momentum states of the excited photoelectron. The polarization dependence and energy position of the resonances allow the molecular orientation on the surface to be determined and show that the change in the carbon–carbon bond length is less than 0.02 Å.
Rotational partition functions for linear molecules
Authors: McDowell Robin S.
Published: July 2002

An accurate closed-form expression for the rotational partition function of linear polyatomic molecules in 1∑ electronic states is derived, including the effect of nuclear spin (significant at very low temperatures) and of quartic and sextic centrifugal distortion terms (significant at moderate and high temperatures). The proper first-order quantum correction to the classical rigid-rotator partition function is shown to yield Qr ≊β−1 exp(β/3), where β≡hcB/kT and B is the rotational constant in cm−1; for β≥0.2 additional power-series terms in β are necessary. Comparison between the results of this treatment and exact summations are made for HCN and C2H2 at temperatures from 2 to 5000 K, including separate evaluation of the contributions of nuclear spin and centrifugal distortion.
Integral equation study of parallel hard spherocylinders
Authors: Caillol J. M. , Weis J. J.
Published: July 2002

The hypernetted chain (HNC) and Percus–Yevick (PY) integral equations are solved numerically for parallel hard spherocylinders of length-to-width ratios L/σ ranging from 0.5 to 5.0. The PY virial pressure is in excellent agreement with Monte Carlo data. The HNC equation gives evidence for the existence of a nematic to smectic-A phase transition at densities which compare reasonably well with the Monte Carlo (MC) estimates, whereas the PY equation gives stable nematic solutions for densities beyond the MC transition densities. For a given packing fraction η, pressure and pair correlation functions (properly scaled) depend very little on the elongation L/σ in the range 3–5.
Solubility of xenon in 45 organic solvents including cycloalkanes, acids, and alkanals: Experiment and theory
Authors: Pollack Gerald L. , Kennan Richard P. , Himm Jeffrey F. , Carr Peter W.
Published: July 2002

New measurements are reported of the Ostwald solubility L(T), as a function of temperature in the approximate range 10.0–50.0 °C, for 133Xe gas in 13 liquid organic solvents, viz., three cycloalkanes, six carboxylic acids, and four normal alkanals. From our data for each solute–solvent system we determine the mole-fraction solubility x2(T), and the following thermodynamic functions of solution: chemical potential Δμ0ρ2(T)=−RT ln L, enthalpy ΔH̄0ρ2, and entropy ΔS̄0ρ2, where Δμ0ρ2=ΔH̄0ρ2 −TΔS̄0ρ2S̄0ρ2, all based on the number density scale. New results are considered together with previous measurements of xenon solubility in liquid normal alkanes, alkanols, and perfluoroalkanes; in all, data and theory are treated for xenon solubility in 45 organic solvents from six homologous series. The average observed entropy of solvation of Xe is ΔS̄0ρ2=−4.1± 0.5 cal/mol K, remarkably independent of solvent. The results are analyzed with scaled-particle theory from which are obtained hard-core diameters a1, and cavity energies gcav and enthalpies hcav for all the solvents at 25 °C. Values of a1 range from 4.08 Å (for CH3OH) to 9.18 Å (n-C20H42), and gcav ranges from 2520 cal/mol (n-C6F14) to 9430 cal/mol (HCOOH). We discuss the application to solubility in these solvents of interaction site calculations; interaction potentials for the functional groups are available but difficult to apply to these solute–solvent systems. We also discuss the role of configurational entropy, as well as molecular dynamics approaches to calculation of free energies of solubility. Finally the results are examined empirically and values are given for the contribution to chemical potential, enthalpy, and entropy of solvation, of the six functional groups: CH2 (linear molecules), CH3, OH, COOH, CHO, and CH2 (cyclomolecules).
State-selective reaction of excited potassium atom with hydrogen molecule. K*+H2→KH+H
Authors: Lin King-Chuen , Chang Hai-Chou
Published: July 2002

By using a pump-and-probe technique, we have observed for the first time the product KH formed by reaction of K*(7S) with H2 in a single collision under bulk conditions. In contrast, no detectable laser-induced fluorescence (LIF) signal of KH was detected as the K*(7S) was replaced by the K*(5D), a state having 88 cm−1 less energy. These experiments demonstrate for the first time the possibility for an alkali atom to undergo with the H2 molecule a state-selective reaction. This reaction can be satisfactorily understood in terms of the harpoon mechanism. Measurements of the temperature dependence of the rate constant confirm the proposed mechanism.
Two-photon absorption spectroscopy of ion beams: CO+2 C̃ 2Σ+g state characterization
Authors: Wyttenbach T. , Evard D. D. , Maier J. P.
Published: July 2002

Two-photon absorption spectroscopy with a mass-selected beam of CO+2 ions was used to study the predissociative C̃ 2Σ+g state of CO+2. The first photon pumped the Ã 2Πu←X̃ 2Πg transition and the second photon was used to scan through the C̃ 2Σ+g←Ã 2Πu transition. A rotational analysis of two bands in this spectrum has been made. The C̃ 2Σ+g state is linear with a C–O bond length of 1.1552(2) Å in the v=0 level.