bluuebunny/crossref_metadata_2025 huggingface dataset:
Space-charge-limited current diode model for amorphous silicon solar cells and their degradation
Authors: Partain Larry D.
Published: July 2002

A quantitative space-charge limited-current (SCLI) diode model for the rectifying current-voltage properties of diodes with a discrete trap distribution is extended to the case of diode solar cells with a continuous distribution of traps in the energy band gap. Analyzing hydrogenated amorphous silicon (a-Si:H) solar cells indicates that their light-exposed degradation causes two major changes in their high resistance, intrinsic regions. These are a sharp increase in the equilibrium and steady-state carrier concentrations and a change in the distribution of traps in the energy band gap. The measured trap concentration densities span the 1014–1018 cm−3eV−1 magnitude range at 0.8–0.2 eV below the conduction-band edge. The measured carrier concentrations cover the 105–1012 cm−3 range. The model indicates that higher device efficiencies are obtained when the quasi-Fermi level in the high resistance region lies just above a peak in the trap concentration density in the band gap. This peak steady-state density in a-Si:H is above 1017 cm−3 eV−1 and is located approximately 0.5–0.6 eV below the conduction-band edge. Fermi–Dirac statistics are employed in the model and quantitative explanations are given for the dark and light current-voltage curves of a-Si:H under forward and reverse bias. Conditions are defined that predict maximum a-Si:H fill factors in the 0.74–0.76 range and device efficiencies at the 11%–13% levels. The trap distributions that give higher device performance are defined, and a method for measuring these distributions is described and illustrated. The model indicates that the specific conductivity changes measured on light-exposed a-Si:H by Staebler and Wronski would tend to increase the fill factor and efficiency of a-Si:H solar cells.
Laser-induced fluorescence study of discharge plasma in He+Xe mixture gas
Authors: Uchiike Heiju , Shindo Haruo , Nishi Hiroshi , Arinaga Kenji , Fukushima Yoshifumi , Tamaru Takeshi
Published: July 2002

This paper contains experimental results on the population of the 21S atom of HeI in the discharge plasma of a He+Xe gas mixture. The population measurement was made by the laser-induced fluorescence method with the laser light tuned to the HeI line of 5016 A(21S-31P). The fluorescence intensity of HeI lines was measured as a function of the mixture ratio of Xe to He gas. As the Xe gas was mixed, the fluorescence intensity was significantly reduced at pressures ranging from 0.2 to 1 Torr. Further, an increment of the discharge current made the population of the 21S atom increase more remarkably in the Xe-mixed gas than in the pure He. These behaviors were considered using the measured temperature and density of the electrons, and it was concluded that at 1-Torr the Penning collision of the 21S atom with Xe was effective and dominated the behavior of the 21S atom.
Domain wall size model in amorphous ribbons
Authors: Magaña L. F. , Valenzuela R.
Published: July 2002

The domain wall size model of polycrystalline ferrimagnetics is extended to the case of ferromagnetic amorphous ribbons. The systematic pinning of the domain wall to grain boundaries is substituted by surface pinning. We also assume that the friction term of the model that opposes the wall displacements is a linear function of the magnetization. We compare with experimental results obtained on commercially available ribbons (‘‘Metglas’’). The agreement is good.
Electron energy loss spectra of SmCo5 and Nd2Fe14B films
Authors: Cheung T. D. , Guo Xiaoqin , Cadieu F. J.
Published: July 2002

Electron energy loss spectra (EELS) have been obtained on SmCo5 and Nd2Fe14B film samples for losses in the range from 0 to 80 eV. The SmCo5 samples investigated were sputtered samples that exhibited in the film plane static energy products of 15–20 MG Oe and coercivities in the 15–25 kOe range. The Nd2Fe14B samples were also sputtered films with coercivities of 15 kOe. The EELS spectra were obtained by using a 200-eV primary electron beam and then energy analyzing the backscattered electrons with a cylindrical mirror analyzer. Transition-metal (TM 3p-3d) and rare-earth (RE 5s-4f, RE 5p-5d, and RE 4f-5d) transitions were readily observed. It was also observed that the Sm 4f electrons in SmCo5 were more localized than the Nd 4f electrons in Nd2Fe14B from the RE 5s-4f transition which was consistent with the observed superior anisotropy field in the SmCo5 lattice. In contrast, the Fe in Nd2Fe14B was indicated to be more localized than the Co in SmCo5 as deduced from the TM 3p-3d transitions which suggested a weaker d electron polarization cloud in Nd2Fe14B and thus leading to a lower Curie temperature than SmCo5.
Magnetic structure of α-FeC2O4⋅2D2O
Authors: Simizu Satoru , Chen Jiing-Yann , Friedberg S. A. , Martinez J. , Shirane G.
Published: July 2002

Long-range spin ordering has been studied in the quasi-one-dimensional antiferromagnet α-FeC2O4⋅2D2O by neutron diffraction on a powdered sample above and below TN=11.7 K. Data at 14 K show essentially the room-temperature chemical structure. The space group is C2/c, Z=4, and unit cell dimensions are a=11.700 Å, b=5.568 Å, and c=9.847 Å with β=128.05°. Magnetic peaks appear at TN and data at 10.5 K show doubling of the unit cell along the b direction, the chain axis. The ordered structure belongs to the magnetic space group P2b2/n with spins parallel to the b axis. Cancellation of certain interchain interactions gives the structure a pronounced two-dimensional character. At T2=9.5 K a discontinuous transition to a second magnetic phase occurs with additional doubling of the unit cell in the a direction. Below T2 the symmetry of the magnetic structure is reduced to triclinic, P2s1̄ with the spins deviating slightly from the b axis.
Magnetic properties of Nd-Fe-CoB permanent magnetic alloys
Authors: Wang Yizhong , Yu Boliang , Feng Minying , Xiang Jiong , Song Lin , Huang Yongcheng , Wang Zhenxi
Published: July 2002

Permanent magnets in the Nd16.5(Fe1−xCox)76.5B7 system were prepared by powder metallurgical methods. Magnetic hysteresis loops, with the magnetic field parallel and perpendicular to the aligning direction, were measured in the temperature range from 4.2 to 450 K. The magnetic anisotropy constants K1 and K2 of Nd2(Fe1−xCox)14B with 0≤x≤0.4 were determined by using aligned sintered magnets Nd16.5(Fe1-xCox)76.5B7. The intrinsic coercive force iHc, magnetic anisotropy constants K1 and K2, and spin reorientation temperature decrease monotonically with increasing Co concentration.
Mössbauer spectroscopy analysis of plasma-driven carbide formation on graphite limiters in the axis symmetric divertor tokamak experiment
Authors: Sawicka B. D. , Sawicki J. A. , Behrisch R.
Published: July 2002

The microstructure and chemical composition of dropletlike deposits formed on the surface of graphite limiters due to plasma discharges in the axis symmetric divertor tokamak experiment were analyzed using 57Fe Mössbauer transmission and conversion electron spectroscopy. It is found that the droplets consist mostly of alloy cementite, (Fe,Cr,Ni)3C, with an admixture of austenite smaller than ≂20%. The amount of cementite and the contribution of austenite depend considerably on the location of the deposits relative to plasma-drift direction, which reflects the varying conditions of metal and carbon redeposition and agglomeration in the tokamak chamber.
Mössbauer study of iron aggregation on graphite; thermal simulation of dropletlike metals deposition in tokamaks
Authors: Sawicka B. D. , Sawicki J. A.
Published: July 2002

In order to explain the processes of metal deposition and dropletlike aggregation on graphite tiles in tokamaks, we have studied thin iron films vapor deposited on graphite substrates by means of Mössbauer spectroscopy. It was found that after heating at 900 °C most of the iron has aggregated into small (submicron) droplets, similar to those formed on the surfaces of graphite limiters in tokamaks after prolonged exposure to plasma discharges. The Mössbauer spectra showed that the droplets consist of α-Fe, austenite γ-Fe, and cementite Fe3C. The results imply that the aggregation takes place in the solid phase, at temperatures significantly lower than the eutectic temperature of the Fe-C system. It is suggested that thin iron films on graphite investigated by Mössbauer spectroscopy can be sensitive probes for transient effects and radiation damage processes at the first wall in thermonuclear research.
Examination of uncoated photoresist by the low-loss electron method in the scanning electron microscope
Authors: Wells Oliver C. , Cheng Ping-Chin
Published: July 2002

Earlier studies of photoresist by the low-loss electron method are extended in three ways. First, more work has been done on the imaging of fine details on the surface of a poorly conducting, low-density specimen. Second, the method has been applied to show the surface of a self-supported membrane. Finally, it has been found that the low-loss image (especially when used at low magnifications) can show the roughness of a flat surface.
A millimeter wave Josephson mixer employing a high-T
c GdBaCuO point contact
Authors: Olsson H. K. , McGrath W. R. , Claeson T. , Eriksson S. , Johansson L.-G.
Published: July 2002

We have investigated a Josephson effect heterodyne mixer for the millimeter wave band employing high-Tc GdBaCuO point contacts. Mixer performance was in qualitative agreement with theory. A mixing response was observed up to 55 K, the highest operating temperature achieved for such a device to date. The voltage separation of rf-induced steps gave a value of h/2e=2.08×10−15 V s which is in excellent agreement with the value expected for Cooper pairs. In addition, the temperature dependence of the I0R product was found to agree with Bardeen–Cooper–Schrieffer theory in the weak coupling limit.
Electrical resistivity and ferromagnetism in Ni-Pd alloys
Authors: Ikeda Kôki
Published: July 2002

The electrical resistivity and magnetization of Ni-Pd alloys were respectively measured between 4.2 and 1200 K and at 77 K. The temperature gradient of resistivity at higher temperatures than the Curie point is almost independent of the Pd concentration. This suggests that the carrier concentration hardly changes with alloying. The magnetic and residual resistivities are discussed in terms of the s-d interaction between the conduction electron and the localized moment.
Parametric up-conversion of a microwave off a relativistic electron beam in a waveguide
Authors: Maheshwari K. P. , Tripathi V. K.
Published: July 2002

An electron beam traveling through a waveguide Raman up-converts a microwave traveling in the opposite direction, thus generating a coherent laser radiation. The frequency of the laser is tunable by the relativistic factor γ of the beam. A theory of this process is developed for an electron beam of arbitrary size. The pump and the laser eigenmodes are determined by the size of the waveguide, whereas the beam mode is confined to exist within the beam. The coupled eigenmode equations for the daughter waves are solved by first-order perturbation technique. To simplify the problem, a nonmagnetized relativistic electron beam in which the electron density is parabolic with a maximum in the center (on the axis) is considered. An analytic expression for the growth rate of the scattered wave is obtained. It is found that the growth rate is roughly (a/b)1/2 times the growth rate for stimulated Raman scattering in an infinite plasma with a uniform pump, where 2b is the separation of conducting planes and a is the half-width of beam mode. This is because the mode extent of the sideband wave extends to a width ∼b, whereas the region of parametric instability is ∼a. Our results show that for a given value of b/a, the growth rate becomes maximum for a particular value of pump frequency. We plot our results for the growth rate of the TE1 mode of the scattered wave as a function of the pump wave frequency.
Coplanar-waveguide excitation of magnetostatic surface waves
Authors: Bhattacharya D. , Chakraborti N. B.
Published: July 2002

The theory of excitation of magnetostatic surface waves with a coplanar-waveguide (CPW) transmission line has been developed. An expression for an equivalent radiation admittance relating the energy carried away by magnetostatic surface waves to electromagnetic energy propagating along the CPW line has been obtained. The input admittance of an open-circuited yttrium iron garnet (YIG)-loaded CPW transmission line has been computed with gap width of CPW and YIG film thickness as parameters.
Wavelength- and thickness-independent optical coatings for integrated circuit metallization layers
Authors: Draper Bruce L. , Mahoney A. R. , Bailey G. A.
Published: July 2002

Detailed measurements have been made of the optical properties of sputtered tantalum silicide films on aluminum layers used in integrated circuit fabrication. This new multicomponent conductor (TaSix on aluminum), which is currently in use because of its exceptional electrical, physical, and chemical properties, was also found to have superior optical properties compared to aluminum alone. The addition of the thin silicide layers reduces both the total hemispherical and diffuse reflectance properties by up to 45% over the 265–800-nm wavelength range with almost no dependence on film thickness. Unlike other optical coatings used on metal layers in integrated circuit manufacturing, the silicide films do not need to be removed after photolithography and pattern transfer processes are completed: aluminum wire bonding from the completed circuit (with silicide coating) to the package is highly reliable and reproducible.
Dependence of the polar phonon limited mobility in the two-dimensional electron gas in GaInAs/InP heterostructures on hydrostatic pressure and carrier concentration
Authors: Fisher M. A. , Adams A. R. , Lancefield D. , Kane M. J. , Taylor L. L. , Bass S. J.
Published: July 2002

We report measurements of the pressure dependence of the mobility of a two-dimensional electron gas in GaInAs/InP single and multiple quantum well systems and at a single heterojunction. The mobility dependence on both carrier density and effective mass is derived and shown to be the same in each system. Current theories of polar optic phonon scattering do not explain the mobility variation with carrier density but can describe the effective mass dependence.
Simultaneous study of interferograms and the emission of shock-heated plasmas behind strong shock waves in argon
Authors: Aslam S. , Ryazin A. P.
Published: July 2002

A technique of simultaneous study of optical interferograms and emission of radiations of shock-heated plasmas behind strong shock waves (M>14) was used. This enabled the recording of both the interferogram and emission characteristics of the plasma on the same photofilm by using streak photographic techniques.
Short-range order and stability in Gd-Ni and Y-Ni systems
Authors: Colinet C. , Pasturel A. , Buschow K. H. J.
Published: July 2002

We present experimental determinations of the enthalpies of formation of various amorphous and crystalline alloys in the Gd-Ni and Y-Ni systems. For the amorphous alloys, crystallization enthalpy is studied by means of differential scanning calorimetry. The enthalpy effects observed in amorphous alloys point to the occurrence of chemical short-range order. This manifestation is analyzed using microscopic calculations of chemical short-range order based on a tight-binding Hamiltonian.
Bulk properties of composite media. I. Simplification of bounds on the shear modulus of suspensions of impenetrable spheres
Authors: Sen Asok K. , Lado F. , Torquato S.
Published: July 2002

We study third-order upper and lower bounds on the shear modulus of a model composite made up of equisized, impenetrable spherical inclusions randomly distributed throughout a matrix phase. We determine greatly simplified expressions for the two key multidimensional cluster integrals (involving the three-point distribution function for one of the phases) arising in these bounds. These expressions are obtained by expanding the orientation-dependent terms in the integrand in spherical harmonics and employing the orthogonality property of this basis set. The resulting simplified integrals are in a form that makes them much easier to compute. The approach described here is quite general in the sense that it has application in cases where the spheres are permeable to one another (models of consolidated media such as sandstones and sintered materials) and to the determination of other bulk properties, such as the bulk modulus, thermal/electrical conductivity, and fluid permeability.
Proton exchanged waveguides for surface acoustic waves on LiNbO3
Authors: Hinkov V.
Published: July 2002

For the first time waveguides for surface acoustic waves (SAW) have been fabricated by exchanging Li+ ions with protons on Y-cut LiNbO3 substrates. Different waveguiding structures were investigated. The influence of the proton source dilution, the annealing or the Ti prediffusion on the SAW velocity, and the parameters of the SAW waveguides have been studied.
Low-energy broad area electron beam for etching microelectronic materials
Authors: Verhey T. R. , Rocca J. J. , Boyer P. K.
Published: July 2002

We have generated beams with electron current densities of more than 10 mA/cm2 at energies between 150 and 900 eV using a plasma source. This electron source can be operated at background pressures between 0.01 and 0.1 Torr in helium and with the addition of various gases, producing electron beams up to 3.8 cm in diameter. The broad area beam can be used to induce gas phase and surface reactions and has been successfully used to achieve anisotropic etching of SiO2 films in the reactant gas CF4.
Reflection of magnetostatic forward volume waves by ion-implanted periodic arrays
Authors: Hartemann P.
Published: July 2002

Reflection of magnetostatic forward volume waves impinging on 45° oblique incidence ion-implanted periodic arrays has been investigated. Waves propagating in pure yttrium iron garnet (YIG) epilayers are reflected by bars implanted with three kinds of ions (helium, boron, neon) at doses up to 1.5×1017 ions/cm2 and energies spread from 60 to 340 keV. For high doses, the YIG thickness damaged by incident ions was measured versus maximum ion energy. It reaches 1 μm for helium ions at 300 keV. The relative reflection response of 9 bar gratings was experimentally determined versus dose at different energies for a fundamental wavelength of 350 μm at 4 GHz and space harmonics. A saturation effect is observed for every ion type. The reflection coefficient per wave impedance discontinuity at saturation was deduced from measurement results for various implantation conditions. It can be larger than 10% for helium or boron ions at 150 keV. The origins of this reflection effect are discussed.
Time-resolved population measurements of excited Kr atoms in a KrCl excimer laser
Authors: Nakamura K.
Published: July 2002

The temporal behavior of excited state (3P2, 3P1) of Kr atoms in a discharge-pumped KrCl excimer laser has been studied using the hook method and the sidelight method with nanosecond time resolution. On the basis of these experimental results, the mechanism of the formation of KrCl* excimer is also discussed.
Field-induced magnetic anisotropy in a dilute dispersion of γ Fe2O3 particles
Authors: Potter D. K. , Stephenson A.
Published: July 2002

The application of an alternating field (AF) of 60-mT peak to an isotropic sample containing dilutely dispersed particles of γFe2O3 from magnetic tape causes it to become magnetically anisotropic. The anisotropy of initial susceptibility so produced can be described by an oblate ellipsoid of revolution with the unique axis aligned with the AF direction. A consequence of this effect is that an isothermal remanent magnetization (IRM) is then more easily acquired if the direct field is applied perpendicular to the previously applied AF direction than if it is applied parallel to this direction. The anisotropy of IRM acquisition at direct fields between 20 and 40 mT is an order of magnitude greater than the anisotropy of initial susceptibility and is 40% of the maximum possible value.
Characterization of InGaAsP/InP double-heterostructure wafers grown by metalorganic vapor phase epitaxy for semiconductor lasers by photoluminescence investigation with high-power YAG-laser excitation
Authors: Nakao Masashi , Sato Kenji , Oishi Mamoru , Itaya Yoshio , Imamura Yoshihiro
Published: July 2002

The photoluminescence (PL) spectrum distribution of a low-pressure metalorganic vapor-phase epitaxy grown wafer with an InGaAsP/InP double heterostructure (DH) was measured under the irradiation of a high-power Nd-YAG laser. The two-dimensional distribution profiles of the PL intensity and peak wavelength in the wafer are discussed in terms of the temperature profile on the substrate surface during growth. A change in the decomposition ratio of the reaction gas due to a temperature change on the substrate surface was found to cause the compositional (PL wavelength) distribution on the wafer and the subsequent lattice mismatch, which is considered to be the main factor influencing PL characteristics. Laser diodes (LDs) were fabricated from the DH wafer, and both PL and LD characteristics were compared. The PL intensity was found to be in reciprocal proportion to the threshold current of the LD, and the PL peak wavelength had a parallel red shift of 10–30 nm to the lasing wavelength. As a result, the LD characteristics, the threshold current, and the lasing wavelength could be predicted from the high-power PL measurements of the as-grown wafer before the LD fabrication processing.
The influence of electrical fields on diffusion of donors and acceptors in semiconductor junctions
Authors: Meijer Paul H. E. , Keskin Mustafa , Napiorkowski M.
Published: July 2002

Diffusion in semiconductor junctions (hetero, homo, and graded) is influenced by the fields present in the junction. The potential, present in the junction is determined by the Poisson equation, in which the charge density depends on the internal potential. This potential is determined by Fermi-Dirac statistics applied to the valence band, the conduction band, and the impurity levels in, and near, the junction. We utilize the complete set of equations, rather than the standard simplified model based on the use of impurity levels only. Solution of the problem is accomplished in two ways: first, a number of possible approximations are evaluated and discussed; second, the equations are solved numerically and the results are compared with the previously made approximations. Special attention is paid to the case of a graded junction.
A methodology for the calibration of stress and motion instrumentation for dry soils
Authors: Renick Joseph D. , Anderson Charles E. , O’Donoghue Padraic E. , O’Kelley Donna K. , Felice Conrad
Published: July 2002

A technique has been developed for calibrating stress and motion instrumentation in the stress region of 1–10 kbars based on the planar impact of a large metal flyer plate with a uniform dry soil testbed. By measuring the impact velocity of the plate and the propagation velocity of the resulting ground shock and assuming uniaxial soil strain with no strain recovery, the stress and motion fields in the testbed can be described completely. An analytical formulation based on these simple assumptions is developed. Hydrodynamic computer calculations are used to investigate the assumptions of the model. The results of numerical simulations of a flyer plate impacting a dry soil testbed are compared against the predictions of the theoretical model and experimental data.
Characterization of multilayered tungsten/carbon thin films by various processes
Authors: Rénucci P. , Pétrakian J. P. , Gaudart L. , Roux D.
Published: July 2002

A series of multilayered thin films were obtained by triode sputtering from a W and C target in an alternating fashion. The films were analyzed by Auger spectroscopy. The resistance of the films was studied versus three parameters: aging, number n of layers, and temperature. The resistance of the stack varied linearly with n−1. This indicates the good repeatability of the geometrical characteristics of each pair. The electrical behavior of the multilayers is shown to be anisotropic. In particular it was shown that W/C multilayers have a negative TCR (temperature coefficient of resistance) in the longitudinal direction and a positive TCR in the transverse direction. A study made on transmittance in visible range showed no anomalies.
Photosensitive properties of a hydrated mordenite-AgI inclusion compound
Authors: Hirono Takushi , Kawana Akio , Yamada Tomoaki
Published: July 2002

Reversible movements of Ag and I atoms in zeolite channels due to photoirradiation and heating are observed in a hydrated mordenite-AgI inclusion compound. This hydrated mordenite-AgI inclusion compound is photosensitive to visible and near ultraviolet light with a power density of only a few μW/cm2 at room temperature, and it is bleached by heating. Even after bleaching, the inclusion compound shows photosensitivity to light due to hydration. Optical powder diffuse reflection spectroscopy and x-ray photoelectron measurements show that Ag+ and I− ions become diffusible from the mordenite channels to the surface due to hydration. Ag+ ions are reduced by photoirradiation and Ag particles are formed at the surface. Ag atoms are oxidized and Ag+ and I− ions migrate into the mordenite channels when heated. These photosensitization features are different from those of the dehydrated mordenite-AgI inclusion compound reported previously.
Comment on ‘‘Quantifying solid-fluid interfacial phenomena in porous rocks with proton nuclear magnetic resonance’’ [J. Appl. Phys. 5
9, 2788 (1986)]
Authors: Brown Robert J. S.
Published: July 2002

Far-reaching conclusions, not supported by the data presented, are drawn in this paper. An enhanced nuclear magnetization is reported by the authors and said to suggest a collective motion of the water molecules in pores of some porous rocks partially saturated with water. However, the data for these special rocks show a nuclear magnetization deficiency at partial fluid saturation and an even greater deficiency at full saturation. Multimodal distributions of pore sizes in rocks are inferred from multiexponetial relaxation data. However, these data would have to be far more accurate than experimentally possible to resolve the questions raised.
High-performance short-wavelength single quantum well pump lasers
Authors: Wagner D. K. , Waters R. G. , Tihanyi P. L. , Hill D. S. , Roza A. J. , Vollmer B. J.
Published: July 2002

Performance of graded-index separate-confinement heterostructure single quantum well lasers especially designed for cw optical-pumping applications is reported and is compared to that of conventional double-heterostructure devices. The quantum well devices have the advantage of lower threshold current, higher slope efficiency, and can be operated at longer cavity lengths, leading to lower diode-series resistance and reduced cooling requirements. Quantum wells of aluminum composition ranging from 6.7% to 12% and thicknesses of 12 and 20 nm were investigated. Useful lasing wavelengths between 780 and 820 nm were achieved.
Addendum: ‘‘Carrier concentration and activation energy in heavily donor-doped silicon’’ [J. Appl. Phys. 6
1, 591 (1987 )]
Authors: Schechter Daniel
Published: July 2002

In a previous paper [D. Schechter, J. Appl. Phys. 61, 591 (1987)] the Lee–McGill model for computing the populations and carrier concentrations in heavily doped n-type silicon was systematically explored. In this addendum, the carrier concentration calculated using the Lee–McGill model is calculated as the doping density is decreased, and is compared with that calculated using the usual dilute doping model. In particular, the extent to which the predictions of the two models agree is explored. It is found that at room temperature the two models agree very closely for doping concentrations below 1×1017 cm−3, but at reduced temperatures the coincidence is not so close.
Configuration-independent minimum voltage for the Townsend model of breakdown in gases
Authors: Miller Arthur
Published: July 2002

The Townsend model of electric breakdown in gases is considered for the case in which the field along the breakdown path may vary in an arbitrary way, corresponding to arbitrary electrode configuration. The common approximations are made that the first Townsend ionization coefficient α depends only on the field, and the second coefficient γ is a constant. It is also assumed that the dependence of α on field is such that the Stoletow constants are uniquely determined. This assumption is satisfied by a number of semiempirical approximations to α that have been suggested. It can then be shown that the breakdown voltage, regardless of electrode configuration, cannot be less than that which obtains for planar electrodes.
Reaction of atomic and molecular bromine with aluminum
Authors: Keaton A. Landauer , Hess D. W.
Published: July 2002

Bromine atom concentrations in Br2 discharges were measured by Br2 absorption spectroscopy. At 3.7 MHz, the dissociation of Br2 increased with power, reaching a maximum of ∼40%. The aluminum etch rate was proportional to the bromine atom concentration. In the discharge, atoms etched aluminum 20 times faster than molecules. The etch product molecule appears to be reversibly physisorbed on the brominated surface with an apparent binding energy of ∼0.2 eV/molecule.
Photopumped single quantum-well laser structures
Authors: Waters R. G. , Guido T. S.
Published: July 2002

Amplified spontaneous emission involving the n=1 and n=2 confined states is observed at room temperature in GaAs graded-barrier single quantum-well laser material grown by metalorganic chemical vapor deposition. These no-phonon, parity-allowed emission lines correlate well with computed transition energies.The duality of lines, their strength and sharpness, and the absence of undesirable substrate luminescence permit a specification of the quantum-well width. This validation, in conjunction with other wafer-level evaluations, provides a more complete characterization of laser material intended for fabrication.
Effective electron interactions due to virtual charge and spin excitations in an extended Hubbard model for Cu-O superconductors
Authors: Schüttler H.-B. , Fedro A. J.
Published: July 2002

We have studied the effect of virtual Co-O charge and spin fluctuations in the perovskite high-Tc materials by applying a strong coupling expansion to an extended Hubbard model with hybridized Cu d orbitals and O p orbitals. Results for the charge and spin fluctuation-mediated effective interactions are derived and discussed.
Analysis of the impulse response of a piezoelectric photoacoustic signal
Authors: Hangyo M. , Nakashima S. , Oohara Y. , Kimura S.
Published: July 2002

Impulse responses of the piezoelectric photoacoustic (PA) signal have been measured by using the technique of cross correlation. Experimental impulse responses of aluminum model samples are in good agreement with theoretical prediction. It is shown that thermal diffusivities of opaque materials can be determined by measured zero-crossing times of PA impulse response. Cracks in aluminum samples are detected by measuring a one-dimensional image of the impulse response of the PA signal.
Analysis and observation of current spreading and current concentrating effects in constricted double-heterojunction lasers
Authors: Du Guotong , Xiao Jianwei , Gao Dingsan
Published: July 2002

Current spreading and current concentrating effects of constricted double-heterojunction lasers (CDH) have been studied. A quantitative formula of the current profile for CDH structure has been obtained theoretically and verified experimentally. In addition, we have compared the influence of these two effects on the threshold current of CDH lasers. From this study, we proposed a scheme to reduce threshold current and fabricated lasers with typical threshold currents of 30–50 mA. The lowest threshold current is 27.5 mA (L=125 μm, s=10 μm, 21 °C, cw). Our results also indicated that the temperature coefficient T0 of CDH lasers will be improved only through increasing side leakage current.
Microphone-detected ion-acoustic signal from metals
Authors: Diószeghy T. , Bíró T. , Szőkefalvi-Nagy Z.
Published: July 2002

An experimental system for studying acoustic signals generated by a modulated MeV ion beam in metals is described. For detection, a closed cell on the rear side of a copper or aluminum sample, a half-inch condenser microphone, and a lock-in amplifier were employed. The pressure signal was found to be proportional to beam current and particle energy, and inversely proportional to cell length. Decrease of the signal magnitude and increase of the phase delay with increasing modulation frequency and sample thickness were also observed.
Boron diffusion in silicon at high concentrations
Authors: Orr Arienzo W. A. , Glang R. , Lever R. F. , Lewis R. K. , Morehead F. F
Published: July 2002

Depth profiles measured by secondary ion mass spectrometry have been used to compare boron diffusion from three different sources for temperatures ranging from 850 to 1050 °C. The sources included boron in situ doped and ion-implanted polycrystalline silicon as well as vapor using an evacuated capsule with highly doped powder. The junction depths and surface concentrations demonstrated little source dependence. Boltzmann–Matano analysis has been used to show that the concentration dependence of the diffusivity on source was minimal. We have clearly shown that conventional models of boron diffusion cannot fit the experimental data or the Boltzmann–Matano results, regardless of source. A new model has been used to describe the boron diffusion profiles more accurately.
NMR spin-echo studies in sputtered Heusler alloy films
Authors: Le Dang K. , Veillet P. , Krishnan R. , Morisako A. , Matsumoto M. , Naoe M.
Published: July 2002

We have carried out at 4-K spin-echo studies (of 55Mn, 63Cu) on sputtered Heusler alloy (Cu2MnAl) films with substrate temperature Ts in the range of 50–300 °C in order to obtain microscopic information on the magnetic structure. For Ts =60 °C a very broad spectrum is obtained, revealing incomplete (disordered) formation of Cu2MnAl. For Ts =158, 167, 170, and 210 °C, well-defined spectra are observed with Mn at 227 MHz and Cu at 241 and 259 MHz, respectively. Signal intensity studies reveal a maximum for Ts =167 °C in general agreement with the magnetization results of Morisako, Matsumoto, and Naoe. The signal frequencies are also in agreement with those obtained for the bulk alloy. For Ts =300 °C, the spectrum is broadened but the signal frequencies of Mn and Cu remain practically the same. This spectrum is also quantitatively different from that observed for the film with Ts=60 °C, where a low-frequency tail is observed. The effect of post-annealing the films is also presented.
Crystal/chemical structures and magnetic properties of naturally occurring Mn1.7Fe1.3O4
Authors: Evans B. J. , Dunham W. R. , Porter C. , Abernathy S. M. , Bluncson C.
Published: July 2002

An unusually pure Mnx Fe3−x O4 sample (x≂1.3) from a regionally metamorphosed ore body is expected to exhibit an approach to thermodynamic equilibrium unattainable in synthetic materials. This expectation is confirmed by the observation of (1) a small tetragonal distortion and (2) a complete ordering of Fe on the B sublattice. 57Fe Mössbauer spectra, obtained at 298, 175, 125, and 92 K and in an external field of 30 kG at 92 K, exhibit magnetic hyperfine splitting requiring the fitting of two subspectra, I and II. At 298 K, Heff =418 kOe, α=0.36 mm s−1 , and ε≊0 for the more intense subspectrum I, whereas for II Heff =403 kOe, δ=0.34 mm s−1 , and ε≂−0.15 mm s−1 . The two subspectra are not split by a 30 kG external field. Subspectra I and II correspond to Fe3+ ions located on a single magnetic sublattice. Subspectrum II is assigned to Fe3+ ions in regions with locally, large tetragonal distortions as a consequence of Mn3+clustering, which has often been invoked to explain the crystal chemistry and physical properties of Mnx Fe3−x O4 .
Spin dynamics of the reentrant spin-glass system amorphous (FexCr1−x)75P15C10 (abstract)
Authors: Mangin P. , Boumazouza D. , George B. , Erwin R. W. , Rhyne J. J.
Published: July 2002

Spin-wave excitations have been studied by neutron inelastic scattering in a series of amorphous (FexCr1−x)75P15C10 alloys with x=0.75, 0.725, and 0.700 and are correlated with small-angle neutron scattering (SANS) results. The Curie temperatures of the three compositions are 212, 181, and 134 K, respectively. These alloys show reentrant spin-glass behavior characterized by a maximum in the spin stiffness parameter D below 0.5 Tc, and an increase in the spin-wave linewidth at low T. This corresponds to the development of a static spin-glass phase as observed by the onset of a Lorentzian-squared component in the SANS. The maximum in D shifts to higher values of T/Tc for smaller Fe concentrations reflecting the increased width of the exchange field distribution. The results for the T dependence of the spin stiffness parameter and the subcritical SANS are remarkably similar to those for crystalline FeCr alloys1 having the same Tc, although the Fe concentration in the analogous crystalline materials is much lower (e.g., Tc=178 K) for crystalline Fe26Cr74 and close to the expected percolation concentration.
Specific-heat measurements for the quasi-crystalline, first cubic approximant and hexagonal phase in AlMnSi (abstract)
Authors: Wang K. , Garoche P. , Calvayrac Y.
Published: July 2002

We have investigated the low-temperature specific heat of the quasi-crystalline (Q-C) Al73.2Mn21Si5.8 related first approximant and hexagonal crystalline phases. All measurements were performed on small, 2-mg, single-phase crystals. The structure of each phase was carefully analyzed by x-ray diffraction and for the Q-C phase all pics positions were correctly indexed to a ‘‘six-dimensional crystal’’ model.1 The samples where obtained by planar flow casting and for the hexagonal phase by an adequate annealing of the Q-C phase. The low-temperature specific heat is analyzed in a standard way as the sum of three electronic, magnetic, and phonon contributions. This analysis indicates unambiguously the presence in the Q-C phase of a large magnetic term (C/T=11 mJ/mole K K) that is supposed to be caused by the thermal excitations of a spin-glass-like ordering.2 Such a large magnetic contribution is observed neither in the hexagonal phase, which has a different local symmetry, nor in the cubic approximant with the same local icosanedral symmetry. Thus it is proposed that the random freezing of the Mn spins observed in the AlMnSi Q-C is not a canonical spin-glass state but a specific property of spins on a quasi-periodic lattice.
Magnetic behavior of CeFe2: Effects of ruthenium substitution
Authors: Roy S. B. , Coles B. R.
Published: July 2002

We report ac-susceptibility measurements on the Laves phase compounds Ce (Fe1−xRux)2 with x=0.02, 0.04, and 0.06. The results show that the ferromagnetic alignment of Fe moments seems to be very easily disrupted by substitution of Ru on the Fe sites. All the three compounds, as well as showing the para- to ferromagnetic transition, indicate another transition at lower temperature. In the case of the x=0.02 compound, the character of the susceptibility at the lower transition is similar to that seen in reentrant spin glasses, while in the compounds richer in Ru the suppression of the low-field response is much more drastic.
Structure and magnetic properties of quaternary Pr2−xRxCo14B (R=Dy and Er) systems and their hydrides
Authors: Pourarian F. , Jiang S. Y. , Sankar S. G. , Wallace W. E.
Published: July 2002

Single-phase materials of quaternary Pr2−xRxCo14B (R=Dy or Er), which crystallize in the Nd2Fe14B-type tetragonal structure, form for the compositions only up to about x=1.5. Measurements of lattice parameters and the bulk magnetization were carried out on these compounds and their hydrides. Saturation magnetizations Ms, magnetic ordering temperatures Tc, spin-reorientation temperatures Tsr, and anisotropy fields were determined. Pr2Co14B exhibits a transition between uniaxial and planar anisotropy, with increasing temperature. Partial replacement of Dy or Er for Pr in the Pr2Co14B lattice reduces the observed Tsr and HA values. Substitution with Dy in Pr2−xRxCo14B has a smaller effect than with Er. This behavior is believed to be a consequence of a stronger crystal field interaction in the Dy-substituted systems. All materials investigated were found to absorb about 3.5 hydrogen atoms per formula unit at an applied pressure of 10 atm. Introduction of hydrogen into the alloy expands the lattice, changes the magnetic moment of the system slightly and reduces Tsr and HA significantly. This effect is found to be stronger in the Dy-substituted compounds. The weakening of the 4f-3d exchange interactions relative to crystal field interactions of the 4f ions is considered to be responsible for lowering the anisotropy fields upon hydrogen absorption. The results obtained for the host and the hydrides are discussed in terms of their proposed magnetic phase diagram.
The antiferromagnetic spin glass with frustrations (abstract)
Authors: Blazej M. , Krzeminski S. , Rusek P.
Published: July 2002

In the antiferromagnetic spin glass (ASG) the distribution of magnetic moment is chaotic but there is an effective antiferromagnetic order in the system. The chaotic distributed positive and negative bonds are source of frustrations. We have studied dynamics of ASG using an effective Lagrangian. The presence of frustrations requires us to introduce additional parameters (gauge fields). The dynamics of frustrations were described with analogy to dynamics of dislocations in crystals. We found spin waves with linear dispersion law in ASG. Due to the frustrations those spin waves are damped and for small frequency they become purely dissipative. The present method can be applied to a description of other magnetic systems with topological solitons.
Observation of high-temperature spin fluctuations in UBe13 by nuclear-spin relaxation
Authors: Clark W. G. , Wong W. H. , Hines W. A. , Lan M. D. , MacLaughlin D. E. , Fisk Z. , Smith J. L. , Ott H. R.
Published: July 2002

We report the extension of measurements of the 9Be spin-lattice relaxation rate in UBe13 from 300 up to 1000 K. They reflect the fluctuating local magnetic field at the site of the 9Be nuclei. Above about 100 K, there appears a new contribution in addition to the rate observed at lower temperatures. The high temperature contribution is modeled in terms of the thermal excitation of low-lying states of the U3+ ion along lines used by Felton et al. to explain their measurements of the specific heat. Our results indicate a splitting of 200±20 K between the ground and excited states of the system, and a lifetime of 8×10−14 s for the excited states (width =95 K).
Magnetic field dependence of the elastic constants in Nd3Se4(abstract)
Authors: Fütterer H. , Yohannes T. , Bach H. , Pelzl J. , Nahm K.
Published: July 2002

Most of the metallic rare-earth chalcogenides of RE3X4 type, where X stands for S, Se, Te, show marked elastic anomalies which appear either in conjunction with a structural transformation or with a magnetic ordering transition. In this contribution we report on results obtained from Nd3Se4 where the elastic constants display a particular magnetic field dependence. Single crystals were grown by a Bridgman–Stockbarger method and the three cubic elastic constants were measured by ultrasonic techniques as function of temperature and in an external magnetic field. The occurrence of magnetic ordering was proved independently by susceptibility measurements. Nd3Se4 has been found to order ferromagnetically at Tc =52 K. In the absence of an external magnetic field approaching Tc from above, the shear elastic constants (C11−C12)/2 softens by about 40%. Below Tc the elastic constants harden again where the increment of C11 follows the Brillouin function of the magnetization. An external magnetic field parallel to the propagation direction of the sound waves affects the elastic anomalies in the paramagnetic as well as in the ferromagnetic phase. Above Tc the sound velocities exhibit a H2 field dependence, as is expected from a magnetoelastic coupling mechanism. At temperatures smaller Tc the field-dependent elastic anomalies are related to the hysteresis loop M(H). The change of the elastic constants as a function of the magnetic field is proportional to the field derivative dM/dH. The coupling coefficient has an opposite sign for the C44 and (C11−C12)/2 shear elastic constants.
Magnetic properties of rare-earth transition-metal borides
Authors: Aly S. H. , Singleton E. , Hadjipanayis G. C. , Sellmyer D. J. , Zhao Z. R.
Published: July 2002

The magnetic and structural properties of quaternary RCo4−xFexB alloys with R=Nd, Sm, Er have been examined with magnetometry and x-ray diffraction. The CeCo4B-type phase is found for all x in Er, for x≤3 in Sm, and for x=0, 1, and 2 in Nd. Maximum coercivities have been obtained in a crystallized SmFe2Co2B sample with Hc>17 kOe at room temperature.
Magnetochemistry of chromium (III) (abstract)
Authors: Merabet K. E. , Burriel R. , Carlin Richard L.
Published: July 2002

There are relatively few superexchange-coupled antiferromagnets of chromium (III), largely because the exchange interactions are so weak. We have begun a program to study these materials and present magnetic susceptibility measurements on several new systems. We find that a polycrystalline sample of [Cr(NH3)5Cl]Cl2 behaves as a magnetic linear chain, and orders antiferromagnetically at about 600 mK, and that single crystals of [Cr(en)3]X3⋅3H2O order at 130 mK when X is chloride. The analogous bromide orders at 112 mK. The chloride exhibits an easy axis, parallel to the longitudinal axis, while the bromide exhibits an easy plane. The data have been successfully fitted by several different procedures which result in much the same parameters. In addition to the exchange, it is necessary to include in the analysis the zero-field splitting of the chromium ions. The zero-field splitting in paramagnetic NaMg[Cr(oxalate)3]⋅9H2O has also been measured.
Anomalous electrical and magnetic properties of gadolinium hexaboride
Authors: Ali Naushad
Published: July 2002

Gadolinium hexaboride undergoes an antiferromagnetic phase transition below TN≂16 K. A second phase transition takes place below 11 K. We have studied both phases of GdB6 by experimental measurements of electrical resistivity, magnetoresistance, magnetic susceptibility, and Hall effect. The electrical resistivity data show a large thermal hysteresis associated with the lower-phase transition. Magnetoresistance data in the higher phase is isotropic with respect to sample current and applied magnetic field directions. However, magnetoresistance measurements reveal some unusual behavior in the lower phase, namely, (i) the magnetoresistance is very anisotropic with respect to current and field directions, (ii) a large field hysteresis is observed, and (iii) a field-induced transition is observed at 4.2 K in a field of ∼7 kOe. This field transition, which is also evident in magnetic susceptibility data, may be due to the formation of a single magnetic domain.
On magnetic transitions in dilute Cr-Ga and Cr-Ge alloys
Authors: Smit P. , Alberts H. L.
Published: July 2002

The behavior of the electrical resistivity at the incommensurate–commensurate spin density wave transition temperature of dilute Cr-Ga and Cr-Ge alloys was investigated in samples of different degrees of homogeneity and states of annealing. Sample homogeneity appears to play a much larger role in the behavior at the incommensurate–commensurate transition temperature than at the Néel point, to such an extent that it may suppress the resistivity anomaly at the first mentioned transition completely.