bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of organometallic vapor phase epitaxy of InAs and InAsBi at temperatures as low as 275 °C
Authors: Ma K. Y. , Fang Z. M. , Cohen R. M. , Stringfellow G. B.
Published: July 2002

InAs and InAsBi have been grown by atmospheric pressure organometallic vapor phase epitaxy (OMVPE) over a broad temperature range from 600 to as low as 275 °C. This is the lowest growth temperature ever reported for standard OMVPE. It is demonstrated that lowering the growth temperature is the most effective approach for increasing the Bi content in InAsBi alloys. For example, InAsBi samples with Bi concentrations as high as 6.1 at.% have been successfully grown at a temperature of 275 °C. Trimethylindium, arsine, and trimethylbismuth were used as precursors for most experiments. The growth efficiency is a constant for temperatures above 400 °C, indicating the growth rate is diffusion limited in this temperature regime. For lower temperatures, it decreases exponentially with decreasing temperature with an activation energy of 24 kcal/mol. Incomplete pyrolysis of TMIn limits the growth rate in this temperature regime. However, by substituting ethyldimethylindium for TMIn the diffusion controlled regime can be extended to lower temperatures. Hall effect measurements show that the n-type background concentration increases from approximately 2.3×1016 to 1019 cm−3 as the growth temperature decreases from 600 to 325 °C. Secondary-ion mass spectroscopy results show that the dominant impurity is carbon. Thus, carbon is mainly a donor in these materials.
AlGaAs diode laser blue shift resulting from fast neutron irradiation
Authors: Camparo J. C. , Delcamp S. B. , Frueholz R. P.
Published: July 2002

Six TJS (transverse junction stripe) AlGaAs diode lasers were exposed to fast neutrons with fluences ranging from 2×1012 to 3×1013 n/cm2, and their tuning, gain, and dispersion curves were measured. The tuning and gain curves of four lasers showed blue shifts of several meV at neutron fluences as low as 2×1012 n/cm2; the other two lasers never showed this blue shift. In addition, the lasers that displayed a blue shift also showed an increase in their threshold currents. None of the lasers exhibited any change in their dispersion curves. To explain the blue shift, it is hypothesized that neutron irradiation reduces the efficiency for lasing in the bandtail states of these devices, forcing lasing action to occur between states with greater energy separation. A model in which the reduced efficiency takes the form of a decrease in the transition matrix element is found to yield blue shifts of the correct order of magnitude. Though bandtail effects may also explain the difference in radiation sensitivity among the TJS lasers, at present the reason why only four of the six lasers blue shifted after neutron irradiation is not well understood.
Electro-optical effect in aluminum nitride waveguides
Authors: Gräupner P. , Pommier J. C. , Cachard A. , Coutaz J. L.
Published: July 2002

We fabricated thin optical layers in aluminum nitride by magnetron sputtering. Using the techniques of integrated optics, we characterized the layers and we determined the electro-optic coefficients of AlN: r13 = 0.67 pm/V and r33 = −0.59 pm/V.
Magnetic resonance spectroscopy in silver-doped silicon
Authors: Son N. T. , Gregorkiewicz T. , Ammerlaan C. A. J.
Published: July 2002

In silver-doped silicon, several new electron paramagnetic resonance spectra were observed. Three of these, labeled Si-NL45, Si-NL46, and Si-NL47, were detected in n-type samples. The spectra have trigonal symmetry; the effective electron spin value S equals 1/2. The spectra Si-NL45 and Si-NL46, with the perpendicular g values close to 6 and 8, can alternatively be described by electron spins S=5/2 and S=7/2, respectively, and g values close to 2. The Si-NL47 center has both principal g values significantly deviating from 2. They can be accounted for by taking crystal-field splitting and spin-orbit coupling into consideration. On the basis of the correlation of the production of these centers with the Si-NL44 center and the analysis with high values of electron spin, all these centers are tentatively identified as silver-related complexes.
Critical current density and resistivity measurements for long patterned lines in Tl2Ba2CaCu2O8 thin films
Authors: Holstein W. L. , Wilker C. , Laubacher D. B. , Face D. W. , Pang P. , Warrington M. S. , Carter C. F. , Parisi L. A.
Published: July 2002

Epitaxial Tl2Ba2CaCu2O8 films of thickness 0.65±0.05 μm and Tc of 105±1 K were prepared on (100) LaAlO3 through a two-step post-deposition thallination process and patterned by standard photolithographic techniques and ion beam milling. Using the voltage per unit length criteria Ec=1.0 μV/cm, transport critical current density Jc in zero applied field for a 1.8-m-long, 12±1-μm-wide meander line separated by 8±1 μm spaces was measured to be 1.04×107 A/cm2 at 20 K, 1.82×106 A/cm2 at 80 K, and 1.02×105 A/cm2 at 100 K. The uniformity in Jc was measured for eight line segments of about 11.7 cm length, yielding variations in Jc of 1.44–3.02×106 A/cm2 at 80 K. Jc values independent of linewidth were also measured for three 0.7-cm-long lines with widths of 7, 27, and 52 μm. For design of electronic circuits, resistivity may be a more useful design parameter than Jc, and detailed measurements of resistivity ρ as a function of current density J were carried out. At low temperatures (T/Tc<0.2), ρ increased by about two orders of magnitude for a 10% increase in J, and Jc is well defined. At high temperatures (T/Tc≳0.7), ρ is less strongly dependent on J near Jc. At 90 K, where Jc=7.6×105 A/cm2, ρ remained less than 10−10 Ω cm (3000× less than oxygen free high conductivity copper at 90 K) even for J=1.4×106 A/cm2. The results suggest the potential for the use of patterned Tl2Ba2CaCu2O8 films in high Jc electronic applications such as chip-to-chip interconnects operating at temperatures below 90 K.
Current-density relations for nonisothermal modeling of degenerate heterostructure devices
Authors: Lindefelt U.
Published: July 2002

The electron and hole current-density relations, valid to first order in the deviation from thermal equilibrium, are derived under general conditions. These conditions are: position-dependent band structure, Fermi-Dirac statistics, several nonequivalent conduction-band minima, spin-orbit splitted valence bands, band nonparabolicity, and nonconstant temperature. Electrical boundary conditions at ohmic contacts are also derived and the effects of nonisothermal conditions on these boundary conditions are specified.
Superlinear electron transport and noise in quantum wires
Authors: Mickevičius R. , Mitin V. , Harithsa U. K. , Jovanovic D. , Leburton J. P.
Published: July 2002

We have employed a Monte Carlo technique for the simulation of electron transport and noise (diffusion) in GaAs rectangular quasi-one-dimensional quantum wire structures at low temperatures. It is demonstrated that with the heating of electron gas the efficiency of acoustic phonon scattering decreases and the mobility increases. The increase of electron mobility appears as a superlinear region on velocity-field dependence. It is shown that electron noise increases in the superlinear region. The transition from superlinear transport to the regime close to electron streaming with a further increase of electric fields is reflected on the diffusivity-frequency dependence by the appearance of a separate peak at the streaming frequency. The electron streaming regime which takes place at higher fields causes the collapse of the diffusion coefficient (noise spectral density) to the streaming frequency.
Modulation-induced giant magnetoresistance in a spinodally decomposed Cu-Ni-Fe alloy
Authors: Jin S. , Chen L. H. , Tiefel T. H. , Eibschutz M. , Ramesh R.
Published: July 2002

Giant magnetoresistance in a spinodally decomposed, bulk 60 Cu-20 Ni-20 Fe alloy is reported. An annealed, quenched, and heat-treated sample with a compositional modulation of ≲50 Å size exhibits a ΔR/R value as high as 9% at 4.2 K. Optimization of the ferromagnetic phase particle geometry through a combination of spinodal decomposition and uniaxial deformation led to a locally multilayered, superlattice-like structure and a dramatic increase in room-temperature magnetoresistance from ∼0.6 to ∼5%. This improvement in magnetoresistance is accompanied by a decrease in coercivity from ∼620 Oe in the fully decomposed material to ∼45 Oe in the optimized structure. Interestingly, this structure no longer exhibits the commonly observed temperature-dependent behavior of ΔR/R increasing at low temperatures, but rather shows a decrease at 4.2 K.
A magnetic, neutron diffraction, and Mössbauer spectral study of Nd2Fe15Ga2 and the Tb2Fe17−xGax solid solutions
Authors: Hu Z. , Yelon W. B. , Mishra S. , Long Gary J. , Pringle O. A. , Middleton D. P. , Buschow K. H. J. , Grandjean F.
Published: July 2002

An x-ray diffraction study of the substitution of gallium in Tb2Fe17 to form the Tb2Fe17−xGax solid solutions indicates that the compounds adopt the rhombohedral Th2Zn17 structure. The unit cell volume and the a-axis lattice parameter increase linearly with increasing gallium content. The c-axis lattice parameter increases linearly from x=0 to 6 and then decreases between x=7 and 8. Magnetic studies show the Curie temperature increases by ∼150° above that of Tb2Fe17 to reach a maximum between x=3 and 4, and then decreases with further increases in x. Neutron diffraction studies of Nd2Fe15Ga2 and Tb2Fe17−xGax, with x equal to 5, 6, and 8, indicate that the gallium completely avoids the 9d site, occupies the 6c ‘‘dumbell’’ site only at high values of x and strongly prefers the 18f site at high values of x. The magnetic neutron scattering indicates both that the terbium sublattice magnetization couples antiferromagnetically with the iron sublattice and that there is a change in easy magnetization direction from planar to axial with increasing gallium concentration. This change in easy magnetization direction is explained in terms of a sign reversal of the second-order crystal field parameter, A02, the most important parameter responsible for determining the terbium sublattice anisotropy. The Mössbauer effect spectra indicate a larger room-temperature average hyperfine field at the iron site in the Tb2Fe17−xGax solid solutions than in several related R2Fe17 compounds. The large observed increase in the isomer shift with increasing gallium content results from interatomic charge transfer and intraatomic s-d charge redistribution in the presence of gallium.
Ion-beam-assisted etching of Si with fluorine at low temperatures
Authors: Mullins C. B. , Coburn J. W.
Published: July 2002

The ion-assisted etching of Si with F atoms has been studied over the temperature range from 77 K to room temperature. Separately controllable beams of F atoms and 1 keV Ar+ ions are used in an ultrahigh-vacuum environment. Neutral etch products are measured with modulated beam mass spectrometry. The ion-assisted etch rate is seen to increase as the temperature is decreased whereas the spontaneous etch rate goes to zero at low temperatures. The nature of the etch products is essentially independent of temperature over this temperature range. Evidence is presented indicating that the spontaneous etching of Si by F atoms at 77 K is blocked by the formation and condensation of Si2F6.
Study of electroless Au film deposition on ZnCdTe crystal surfaces
Authors: George M. A. , Collins W. E. , Chen K. T. , Hu Zhiyu , Egarievwe S. U. , Zheng Y. , Burger A.
Published: July 2002

The ‘‘electroless’’ deposition method of Au thin films on n-type ZnCdTe crystal surfaces has been investigated by atomic force microscopy, x-ray photoelectron spectroscopy, and low temperature photoluminescence. The blocking contact behavior of these films was strongly dependent on post deposition annealing treatments which were also found to induce modifications in the surface morphology and surface chemical composition. Heat treatments (at 300 °C) in vacuum eliminates most of the interfacial tellurium oxide introduced during the deposition. Annealing also reduces the radiative recombination at defects in the region below the interface and increases the barrier height of the contact.
Correlation between the Cu-related luminescent center and a deep level in silicon
Authors: Erzgräber H. B. , Schmalz K.
Published: July 2002

The correlation between the copper-induced deep centers with Ev+0.1 eV and the appearance of luminescent copper centers, leading to a characteristic line spectrum with the most intense Cu00 no-phonon line at 1.014 eV, has been examined using deep level transient spectroscopy (DLTS) and photoluminescence (PL). Concentrations of the 0.1 eV copper centers ranging from 1011 to about 1014 cm−3 were obtained by a copper contamination treatment of floating zone p-type silicon samples without quenching. The dependence of the Cu00 line intensity on the excitation power in the transition region to intensity saturation was used to determine the saturation intensity ICusat, which represents the concentration of luminescent copper centers. The saturation intensities and therefore also the concentrations of luminescent copper centers, show a linear dependence on the concentrations of the 0.1 eV deep centers with good correlation, suggesting that the same Cu-induced centers are detected by DLTS and PL.
Infrared dielectric constant of cubic SiC
Authors: Moore W. J. , Holm R. T. , Yang M. J. , Freitas J. A.
Published: July 2002

The real dielectric constant for chemical vapor deposition 3C-SiC grown on silicon (Si) has been determined at 300 K and at 5 K from an analytic fit to interference fringes in transmission over the spectral range from the near infrared to the submillimeter region. This technique is capable of high accuracy being limited typically by the sample thickness and accuracy with which the thickness is measured. The resulting real dielectric constant is lower than the values usually attributed to this material. We find: at 300 K ε 0=9.52 and ε ∞=6.38; at 5 K ε0=9.28 and ε ∞=6.22. In all cases the estimated error is ±0.8%. The observed ratio ε0 /ε∞ agrees with the Lyddane–Sachs–Teller relation to 0.1% at 300 K and 0.2% at 5 K.
Thickness dependence of the poling and current–voltage characteristics of paint films made up of lead zirconate titanate ceramic powder and epoxy resin
Authors: Egusa Shigenori , Iwasawa Naozumi
Published: July 2002

A specially prepared paint made up of lead zirconate titanate (PZT) ceramic powder and epoxy resin was coated on an aluminum plate and was cured at room temperature, thus forming the paint film of 25–300 μm thickness with a PZT volume fraction of 53%. The paint film was then poled at room temperature, and the poling behavior was determined by measuring the piezoelectric activity as a function of poling field. The poling behavior shows that the piezoelectric activity obtained at a given poling field increases with an increase in the film thickness from 25 to 300 μm. The current–voltage characteristic of the paint film, on the other hand, shows that the increase in the film thickness leads not only to an increase in the magnitude of the current density at a given electric field but also to an increase in the critical electric field at which the transition from the ohmic to space-charge-limited conduction takes place. This fact indicates that the amount of the space charge of electrons injected into the paint film decreases as the film thickness increases. Furthermore, comparison of the current–voltage characteristic of the paint film with that of a pure epoxy film reveals that the space charge is accumulated largely at the interface between the PZT and epoxy phases in the paint film. On the basis of this finding, a model is developed for the poling behavior of the paint film by taking into account a possible effect of the space-charge accumulation and a broad distribution of the electric field in the PZT phase. This model is shown to give an excellent fit to the experimental data of the piezoelectric activity obtained here as a function of poling field and film thickness.
Elimination of orientation domains and antiphase domains in the epitaxial films with chalcopyrite structure
Authors: Tseng Bae-Heng , Lin Song-Bin , Gu Gin-Lern , Chen Wei
Published: July 2002

Thin films of CuInSe2 and CuGaSe2 were grown by molecular-beam epitaxy. Domain structures in these films were examined by transmission electron microscopy. We demonstrate that orientation domains may not form by the use of (001)GaAs substrate while antiphase domains can be eliminated by annealing the films in the presence of a Se beam. The annealing temperatures were 400 °C for CuInSe2 and 450 °C for CuGaSe2. Photoluminescence measurements on an annealed CuInSe2 film exhibited a decrease in intensity of optical emissions associated with antisite defects and an enhancement in near-band-edge emission intensity.
A new approach to optimum design in thermoelectric cooling systems
Authors: Yamanashi M.
Published: July 2002

A thermoelectric cooling system usually consists of a thermoelectric cooler (TEC) and heat exchangers at the cold side and the hot side. Heat exchangers in TEC systems are designed to minimize their thermal resistance under restrictions such as the size of the system and heat transfer method, because as the thermal resistances of heat exchangers increase, the performance of TEC systems decreases. In the TEC system in which thermal resistance of heat exchangers is minimized, the optimum TEC is analyzed to maximize the coefficient of performance (COP) and the exergetic efficiency in absorbing a given heat load. The effects of the thermal resistance of heat exchangers on the performance of the TEC system and the design parameters of that system are also investigated. To discuss these matters, dimensionless entropy flow equations of the TEC system are introduced, and the COP and the exergetic efficiency are expressed as a function of dimensionless quantities by using these dimensionless equations.
Suppression of Auger recombination in arsenic-rich InAs1−xSbx strained layer superlattices
Authors: Ciesla C. M. , Murdin B. N. , Pidgeon C. R. , Stradling R. A. , Phillips C. C. , Livingstone M. , Galbraith I. , Jaroszynski D. A. , Langerak C. J. G. M. , Tang P. J. P. , Pullin M. J.
Published: July 2002

Room-temperature pump–probe transmission experiments have been performed on an arsenic-rich InAs/InAs1−xSbx strained layer superlattice (SLS) above the fundamental absorption edge near 10 μm, using a ps far-infrared free-electron laser. Measurements show complete bleaching at the excitation frequency, with recovery times which are found to be strongly dependent on the pump photon energy. At high excited carrier densities, corresponding to high photon energy and interband absorption coefficient, the recombination is dominated by Auger processes. A direct comparison with identical measurements on epilayers of InSb, of comparable room-temperature band gap, shows that the Auger processes have been substantially suppressed in the superlattice case as a result of both the quantum confinement and strain splittings in the SLS structure. In the nondegenerate regime, where the Auger lifetime scales as τ−1aug=C1N2e, a value of C1 some 100 times smaller is obtained for the SLS structure. The results have been interpreted in terms of an 8×8 k⋅p SLS energy band calculation, including the full dispersion for both k in plane and k parallel to the growth direction. This is the strongest example of room-temperature Auger suppression observed to date for these long-wavelength SLS alloy compositions and implies that these SLS materials may be attractive for applications as room-temperature mid-IR diode lasers.
Charge build-up in Si-processing plasma caused by electron shading effect
Authors: Kamata T. , Arimoto H.
Published: July 2002

We investigated the influence of electron temperature and rf bias on charge build-up caused by electron shading in inductively-coupled plasmas (ICP) at 2 to 40 mTorr in Ar. We used Si substrates covered with a 500-nm-thick SiO2 film which had a line-and-space pattern. We measured the electron and ion currents going into the Si substrate through the dielectric structure. When the pattern size decreases, the electron current through the dielectric structure is suppressed and the floating potential increases. We also measured the change in the floating potential of the sample as the electron temperature was increased. As the electron temperature is increased from 2 eV to 4 eV by controlling the gas pressures, the floating potential difference increases between samples with different pattern sizes. To investigate the influence of rf bias (13.56 MHz) on charge build-up, we measured differences in the dc self-bias voltage between samples with different pattern sizes. dc self-bias voltage differences increase with increasing rf bias voltage. This cannot be explained by the slight increase (3 eV to 5.5 eV) in the electron temperature near the rf electrode due to sheath oscillation heating. Therefore, the dc self-bias voltage difference is mainly caused by a decrease in the ion flux impinging on the sidewall of the trench because of the increase in the ion energy.
Low-frequency perturbation theory in eddy-current non-destructive evaluation
Authors: Harfield N. , Yoshida Y. , Bowler J. R.
Published: July 2002

A method is presented by which series solutions for the impedance change in an eddy-current test probe due to closed cracks in a non-magnetic, conducting half-space can be derived at low frequency. The series solution is applicable for flaws whose dimensions are much smaller than the electromagnetic skin-depth. The problem is formulated using an approach in which the flaw is represented by an equivalent distribution of current dipoles. The electric field scattered by the flaw is then written as an integral, over the flaw, of the product of the dipole density distribution and an appropriate Green’s function. Terms in the series expansion for the dipole density are calculated by solving the integral equation at each order in the chosen small parameter, using perturbation theory and a dual integral equation method. The impedance change due to the crack is then calculated from the dipole distribution using the reciprocity theorem. Example solutions are given for semi-circular surface-breaking cracks and for long, uniformly deep surface-breaking cracks. Results are compared with other analytical solutions and the predictions of an independent numerical scheme, and very good agreement is observed.
BaTiO3 thin films grown on SrTiO3 substrates by a molecular-beam-epitaxy method using oxygen radicals
Authors: Shigetani Hisashi , Kobayashi Kazuyoshi , Fujimoto Masayuki , Sugimura Wataru , Matsui Yoshio , Tanaka Junzo
Published: July 2002

BaTiO3 (BT) thin films were grown by a molecular-beam-epitaxy method using an oxygen radical source. BaO and TiO2 layers were alternately deposited on SrTiO3 (001) (ST) substrates, and the structure of the thin films obtained was evaluated by x-ray diffraction, reflection high-energy electron diffraction, transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. The BT thin films were oriented in the [001] direction and epitaxially grown without misfit dislocations. The lattice constants of the thin films varied with distance from the interface of BT and ST. Near the interface the a value was shorter than that for bulk BT while the c value was longer than that for bulk BT. The surface analysis indicated that adsorbed oxygen was enriched on the BaO-terminated surface in comparison with the TiO2-terminated surface.
Switching field variation in patterned submicron magnetic film elements
Authors: Zheng Youfeng , Zhu Jian-Gang
Published: July 2002

In this article, a micromagnetic study of magnetic switching properties on submicron scale single layer and multilayer thin film elements is presented. Even at deep submicron scale, there exist various edge domain configurations at the saturation remanent state. It is found that the switching field of these patterned film elements can strongly depend on these edge domain configurations. If the edge domains are not controlled, switching field of a patterned magnetic film element can vary significantly during repeated switching processes.
Control of the resonance frequency of soft ferromagnetic amorphous thin films by strip patterning
Authors: Perrin G. , Peuzin J. C. , Acher O.
Published: July 2002

Amorphous nearly zero-magnetostrictive soft ferromagnetic thin films with an in-plane uniaxial anisotropy exhibit a gyromagnetic resonance with frequency generally lower than 2 GHz. A convenient way to increase this frequency is to take the advantage of controlled demagnetizing effects: a periodic pattern of metal strips of width w, parallel to the easy axis, separated by a gap of width e is etched in the amorphous layer of thickness h. By playing with the geometrical parameters w, e, h, it is possible to adjust the resonance frequency. In this article, we demonstrate this principle for amorphous CoZrNb and CoFeMoMnSiB films.
Transfer matrix method for interface optical-phonon modes in multiple-interface heterostructure systems
Authors: Yu SeGi , Kim K. W. , Stroscio Michael A. , Iafrate G. J. , Sun J.-P. , Haddad G. I.
Published: July 2002

Interactions of carriers with interface optical phonons dominate over other carrier–phonon scatterings in narrow quantum-well structures. Herein, a transfer matrix method is used to establish a formalism for determining the dispersion relations, electrostatic potentials, and Fröhlich interaction Hamiltonians of the interface optical phonons for multiple-interface heterostructure systems within the framework of the macroscopic dielectric continuum model. This method facilitates systematic calculations for complex structures where the conventional method is very difficult to implement. Several specific cases are treated to illustrate the advantages of the general formalism.
Scattered refractive index profiles of thin silicon oxide film on silicon substrate
Authors: Wang Y.-B. , Han P. , Chen Q. , Willander M.
Published: July 2002

Randomly scattered depth profiles of refractive index are experimentally observed on thin silicon dioxide films on silicon substrates by means of conventional ellipsometry and gradually chemical etching. Computer simulations with an abrupt model and random uncertainties in ellipsometric angles Ψ and Δ show very similar behavior as experiments, and the distribution of the measured profile in the near interface region is in good agreement with 0.02° standard deviation in Ψ. Simulations also indicate that even very small errors in Ψ, as small as 0.001°, can result in divergent depth profile of the refractive index in the near interface region.
Study of the laser-driven spallation process by the velocity interferometer system for any reflector interferometry technique. I. Laser-shock characterization
Authors: Tollier L. , Fabbro R. , Bartnicki E.
Published: July 2002

This article presents the technique for measuring an amplitude and a profile of a laser-generated stress pulse using a Doppler velocity interferometer velocity interferometer system for any reflector (VISAR). The compressive stress, produced by a laser pulse, moves target’s rear free surface. The velocity of this displacement recorded by VISAR can then be related to the laser-driven load applied at the front face of the sample. In the first part of our study, the effect of the laser power density on the shape of the pressure pulse was experimentally and numerically studied for aluminum samples. Subsequently, this method was applied to other materials (copper, molybdenum, and tantalum) in order to find an influence of the material kind on the amplitude and the shape of the loading profile.
Study of the laser-driven spallation process by the VISAR interferometry technique. II. Experiment and simulation of the spallation process
Authors: Tollier L. , Fabbro R.
Published: July 2002

This article presents the experimental and numerical study of laser-driven spallation using a Doppler velocity interferometer VISAR (velocity interferometer system for any reflector). Experiments were carried out on two ductile metals: aluminum and copper. A kinetic damage model was developed in order to predict the damage effects on the mechanical properties of the material and on the waves propagation through the sample. Laser-driven loadings were simulated by a temporal pressure profile, applied at the front surface of the sample. The amplitude and shape of this profile is discussed in Part I [J. Appl. Phys. 83, 1224 (1998)]. The data input to the model was determined for both materials thanks to the comparison of experimental and numerical free-surface velocity profiles. Good agreement was obtained between experimental and predicted results with the model for the spallation threshold and for the damaged zone location.
Theory of electrical noise induced in a wire loop by the thermal motions of ions in solution
Authors: Gillespie Daniel T.
Published: July 2002

Continuous Markov process theory is used to model the electrical noise induced in a passive wire loop by the thermal motions of ions in a nearby solution. The ions, being charged particles in Brownian motion, generate a fluctuating magnetic field, and that in turn induces a fluctuating electromotive force (emf) that augments the loop’s Johnson emf. It is shown that the spectral density function of the equilibrium current in the wire loop is thereby increased, for moderate cycle frequencies ν, by approximately a factor (1+αν2), where α is determined by the geometry of the system, the resistance of the loop, and the charges, diffusion coefficients, and concentrations of the solution ions. It is also shown that the temporal trajectory of the loop current becomes “thickened,” in a randomly fuzzy way, by an approximate factor of the form (1+β)1/2, where β depends not only on the aforementioned parameters that determine α, but also on the hydrated masses of the ions. These findings may be useful for estimating the intrinsic background noise in the detector coil of a medical magnetic resonance imaging machine, or any other sensitive electronic circuit that is required to operate in an immediate “salt water” environment.
Area scaling of planar ferromagnetic tunnel junctions: From shadow evaporation to lithographic microfabrication
Authors: Boeve H. , van de Veerdonk R. J. M. , Dutta B. , De Boeck J. , Moodera J. S. , Borghs G.
Published: July 2002

In order to meet the requirements for applications in magnetoelectronics (e.g., read heads and magnetic random access memory), a processing scheme for micron-scale ferromagnetic tunnel junctions has been developed. A comparative study of junctions defined by shadow evaporation and by lithographic processing was made, where a similar resistance-area product of nearly 1 GΩ μm2 and a high tunnel magnetoresistance of up to 15% at room temperature were observed for Co/Al2O3/Ni80Fe20 junctions patterned by both methods. A bipolar sensor characteristic at zero field was realized by inducing anisotropies in the two ferromagnetic layers that are orthogonal to each other.
Electron emission characteristics of GaN pyramid arrays grown via organometallic vapor phase epitaxy
Authors: Ward B. L. , Nam O.-H. , Hartman J. D. , English S. L. , McCarson B. L. , Schlesser R. , Sitar Z. , Davis R. F. , Nemanich R. J.
Published: July 2002

Selective growth of arrays of silicon-doped GaN (Si:GaN) pyramids for field emitter applications has been achieved. The electron emission characteristics of these arrays has been measured using techniques such as field emission, field emission energy distribution analysis (FEED), photoemission electron microscopy (PEEM), and field emission electron microscopy (FEEM). The field emission current–voltage (I–V) results indicate an average threshold field as low as 7 V/μm for an emission current of 10 nA. It is suggested that the low threshold field value is a consequence of both the low work function of Si:GaN and the field enhancement of the pyramids. The results of the FEEM and FEED measurements indicate agreement with the field emission I–V characteristics. The FEED results indicate that the Si:GaN pyramids are conducting, and that no significant ohmic losses are present between the top contact to the array and the field emitting pyramids. The PEEM and FEEM images show that the emission from the arrays is uniform over a 150 μm field of view.
Fast loading of a magneto-optical trap from a pulsed thermal source
Authors: Fortagh J. , Grossmann A. , Hänsch T. W. , Zimmermann C.
Published: July 2002

We report an efficient method to load atoms into a magneto-optical trap. A resistively heated alkali metal dispenser, which is mounted close to the trapping region, serves as a compact, pulsed source for Rb atoms. The atoms are captured by the magneto-optical trap within less than 3 s at a loading rate of more than 107 atoms per second. The lifetime of the trapped atoms exceeds the filling time by about 30 s.
Scaling of hysteresis and transition parameter with grain size in longitudinal thin film media
Authors: Zhou Hong , Bertram H. Neal
Published: July 2002

Scaling relations of hysteresis loops and single transitions with respect to grain size were obtained by numerical simulation of closely packed 64×64 uniform hexagonal grains. The smallest obtainable transition parameter was found to be half the grain size (D). An approximate formula to relate the simulation data with the Williams-Comstock model was found to be a/D≈0.52+0.35(aWC/D)2 valid only for t/D⩽2.0, d+t/2⩽3.5D and Ms/Hk⩽0.1.
Magnetic hardening in SmCox–Co multilayers and nanocomposites
Authors: Liu J. P. , Liu Y. , Skomski R. , Sellmyer D. J.
Published: July 2002

SmCo x – Co  (x=3.5, 4, and 5) nanostructured multilayers have been prepared by dc and rf sputtering. Magnetic hardening has been found in the as-deposited thin films. The effects of the Co and hard-phase layer thicknesses and the effect of the Cr underlayer have been studied. Further magnetic hardening has been achieved by heat treating the samples. A remarkable increase of the coercivity has been obtained in the nanocomposite films, with coercive force up to 40 kOe. Transmission electron microscopy, atomic force microscopy, and magnetic force microscopy observations have been performed to understand the nanostructure and the domain structure.
Optically optimized transmittive and reflective bistable twisted nematic liquid crystal displays
Authors: Tang S. T. , Chiu H. W. , Kwok H. S.
Published: July 2002

A new Mueller matrix approach is developed for the design of optical modes for all nematic liquid crystal displays (LCD). In particular, for linearly polarized light going into the LC cell, conditions for linear polarization and circular polarization outputs are obtained. By considering the switching between different polarization modes, new transmittive and reflective bistable twisted nematic (BTN) LCD operating conditions with optimized contrast and brightness are discovered. A passive matrix driven single polarizer reflective BTN display was fabricated with reasonably good measured optical performance and fast selection time.
Study of the magnetization reversal in individual nickel nanowires
Authors: Pignard S. , Goglio G. , Radulescu A. , Piraux L. , Dubois S. , Declémy A. , Duvail J. L.
Published: July 2002

The magnetization reversal of Ni nanowires was studied by anisotropic magnetoresistance measurements at temperatures between 15 and 300 K. The wires, synthesized by electrodeposition in a nanoporous polycarbonate membrane, are regular cylinders 22 μm long with a diameter of 75 or 35 nm. The nucleation field was measured on individual nanowires as a function of the angle between the applied field and the wire axis. The results are quantitatively analyzed using classical magnetization reversal theories. Measurements of the nucleation field first obtained as a function of temperature evidence an extra uniaxial anisotropy induced by the contraction of the membrane at low temperature. Combining SQUID measurements and x-ray diffractometry at different temperatures, a clear picture of the large magnetoelastic effect was obtained.
Band edge and phonon-assisted deep level emissions in the ordered filled tetrahedral semiconductor LiMgP
Authors: Kuriyama K. , Kushida K.
Published: July 2002

Band edge and phonon-assisted deep level emissions in the ordered filled tetrahedral semiconductor LiMgP (space group: F4̄3m, direct band gap: 2.43 eV at room temperature), viewed as a zincblende-like (MgP)− lattice partially filled with He-like Li+ interstitials, have been studied using a photoluminescence (PL) method. Two band edge emissions A and B, consisting of two PL peaks, were observed at around 489 nm at 15 K. Emissions A and B were associated with a free carrier recombination (2.535 eV) and a donor-to-valence band transition (2.532 eV), respectively. From the temperature dependence of the band edge emission and optical absorption data, the temperature variation of the band gap was approximated by the empirical formula Eg(eV)=2.536–1.43×10−3T2/(T+912) (T in K). A broad emission involving at least three phonon lines was observed at around 625 nm with full width at half maximum of ∼150 meV, showing a large Franck–Condon shift. The main phonon lines in the broad PL emission were associated with two combinations of longitudinal–optical phonons relating to Li–P and Mg–P pairs.
IrMn based spin-filter spin-valves
Authors: Huai Yiming , Anderson Geoff , Pakala Mahendra
Published: July 2002

As recording densities increase, the sense layer (free layer) thickness in a recording head (whether spin-valve or otherwise) must also decrease so that the sense layer can be effectively saturated by the transitions on the media. However, in a traditional spin-valve structure, reducing the sense layer (free layer) thickness below ∼50 Å results in a rapid reduction in ΔR/R. One of the solutions for this is the spin-filter spin-valve, having a free layer composed of a very thin magnetic layer and an adjacent thin enhancing conduction layer. The advantage of the spin-filter spin-valve is high ΔR/R and easy control of the bias point while keeping the free layer very thin (10–30 Å). The present work reports the results on sputtered IrMn top and bottom spin-filter spin-valves. The data clearly show that ΔR/R has been significantly improved when reducing the free layer CoFe down to 10 Å. ΔR/R>8.0% remains for free layer thickness between 10 and 20 Å using a Cu enhancing layer, while ΔR/R decreased rapidly to 4.0% with no Cu enhancing layer for the same free layer thickness range. An optimized ΔR/R of ∼10% was obtained for an enhancing layer of tCu∼13 Å, as a result of the balance between the increase in electron mean free path difference and current shunting through the very conducting enhancement layer. It was also found that the softness of the CoFe free layer was improved when sandwiched by Cu layers, showing similar soft properties and magnetostriction to conventional free layers. Such a soft, thin CoFe free layer is particularly attractive for high density (30–50 Gb/in2) read sensor applications.
The effect of intermolecular interactions on the 2H and 17O quadrupole coupling constants in ice and liquid water
Authors: Cummins Peter L. , Bacskay George B. , Hush Noel S. , Halle Bertil , Engström Sven
Published: July 2002

Using ab initio SCF molecular orbital techniques, the electric field gradients (efg’s) at the oxygen and hydrogen nuclei were calculated for water clusters ranging from dimer to pentamer in an attempt to reproduce the shift in 17O and 2H nuclear quadrupole coupling constants (qcc’s) that is observed on going from ice to vapor. For 2H, where the qcc shift is due mostly to the change in O–H bond length, excellent agreement with the experimental vapor → ice shift was obtained. For 17O, the change in the qcc is found to be mainly electronic in origin, effectively due to the polarization of the charge associated with the oxygen atom, and approximately 75% of the observed qcc shift was reproduced. On the basis of the calculations, estimates of the 17O and 2H qcc’s in liquid water were made that are consistent with the values obtained from an analysis of the available NMR relaxation data, provided that librational motions are properly taken into account. We also present results of SCF calculations on water interacting with a Li+, Na+, or Cl− ion, indicating that the effect of a nearby ion on the 2H and 17O qcc’s is similar to that produced by H bonding.
Deuterium quadrupole coupling constants. A theoretical investigation
Authors: Huber Hanspeter
Published: July 2002

Deuterium quadrupole coupling constants were obtained for 39 sites from ab initio SCF calculations. An accuracy comparable to the experimental one is reached with a moderately large basis set of very high local quality. Comparison with experiment shows that electron correlation does not contribute substantially to this property. The experimental values for DNCO and DFCO should be reexamined, the value for CD3Cl in the literature was not properly transformed to the bond axes system, and the experimental value for ND3 agrees with the calculated one, thus showing that a dynamical model is not necessary. Several empirical relations are presented, and a simple classical model is proposed which relates the deuterium quadrupole coupling constant to the bond length with high accuracy. A theoretical study of the HCOOH dimer gives some insight into the importance of different mechanisms which lower the quadrupole coupling constant in the solid.
Simulations of vibrational relaxation in dense molecular fluids. I. Methods
Authors: Holian Brad Lee
Published: July 2002

In the understanding of high-temperature and high-pressure chemistry in explosives, the first step is the study of the transfer of energy from translational degrees of freedom into internal vibrations of the molecules. We present new methods using nonequilibrium molecular dynamics (NEMD) for measuring vibrational relaxation in a diatomic fluid, where we expect a classical treatment of many-body collisions to be relevant because of the high densities (2 to 3 times compressed compared to the normal fluid) and high temperatures (2000–4000 K) involved behind detonation waves. NEMD techniques are discussed, including their limitations, and qualitative results presented.
FTIR study of D and 13C substituted C2H in solid argon
Authors: Shepherd R. A. , Graham W. R. M.
Published: July 2002

An extensive FTIR isotopic matrix study using deuterium and carbon-13 has been conducted on the midinfrared spectrum at 250 to 4800 cm−1 of the C2H ethynyl radical produced by the vacuum UV photolysis of acetylene. An absorption at 2104 cm−1 can be assigned to a combination band involving one quantum each of the ν2, bending and the ν3, C–C stretching vibrations. This result implies an unexpectedly low value of ∼260 cm−1 for ν2. Identification of the combination band in this work gives the first indication of the value of the ν2 fundamental, which has not otherwise been observed. The results of carbon-13 substitution on a band at 3610 cm−1 do not appear to be consistent with its previous assignment to the ν1, C–H stretching vibration of C2H. However, carbon-13 shifts for the C2D band at 2798 cm−1 support its assignment to ν1. Absorptions corresponding to several vibronic bands of the A 2Π←X 2Σ+ transition, which have recently been reported in gas phase studies using color center laser spectroscopy, are observed in the matrix spectra, confirming that these bands originate from the lowest vibrational level of the ground state. The effects of carbon-13 substitution on these bands are reported.
A CASSCF study of the potential curves for the X 1Σ+, A 1Π, and C 1Σ+ states of the AlH molecule
Authors: Matos J. Mauricio O. , Malmqvist Per-Åke , Roos Björn O.
Published: July 2002

The complete active space SCF (CASSCF) method was applied to compute the potential curves for the X 1Σ+, A 1Π, and C 1Σ+ states of the AlH molecule. Molecular constants were obtained from the potential curves by solving the radial Schrödinger equation. The results obtained for the ground state are in excellent agreement with experiment, and show that the intershell correlation effects only have a very small effect on the properties of AlH. Calculated values for ΔGv+1/2 differ from experiment within 4.4–5.6 cm−1for the five known band origins (v=0–4). The calculated dissociation energy (De) is 3.11 eV (experiment, 3.16 eV). The A state has a shallow minimum followed by a maximum. Two bound vibrational states have been found, the upper predissociative. The calculated value for De is 0.12 eV, which is 0.12 eV less than the experimental value. All deviations from experiment can be derived from this error. Rotational constants are in excellent agreement with experiment. Radiative lifetimes for the two bound states have been computed to be 62 (v=0) and 102 ns (v=1). The corresponding experimental values are 66±4 and 83±6 ns, respectively. The C state exhibits a double minimum in the potential curve, with the outer minimum lower in energy, in contrast to the corresponding state in BH.The dissociation energy (D0), 0.61 eV agrees well with the value 0.64 eV derived from experimental data. The second minimum is located at R=3.76 Å and has a depth of 1.16 eV. Transition moments from the six lowest levels of the ground state to any of the outer minimum levels are all very small and this minimum is not expected to be seen in absorption. The rotational levels for the second vibrational band in the inner minimum are heavily perturbed by interaction with outer minimum levels, and a normal analysis of the spectrum in terms of Bv and Dv is not possible.
State to state photodissociation of H2O in the first absorption band
Authors: Häusler D. , Andresen P. , Schinke R.
Published: July 2002

The photodissociation of H2O in the first absorption band is studied from single rotational states of vibrationally excited water. A tunable IR laser is used to prepare single rotational states in the asymmetric stretch mode. The subsequent photodissociation at 193 nm favors product formation from these single prepared states. The formation of the OH product in different rotational, Λ-doublet, and spin states is analyzed for a series of initial rotational states of H2O. This is the first direct photodissociation studied on a state to state level. The product state distributions depend sensitively upon the prepared state in the parent molecule H2O and exhibit pronounced quantum structure. The experimental results are understood almost quantitatively in terms of theory. The photodissociation of water turns out to be a limiting case of a dissociation which is governed by transfer of parent motion to products. The experiment leads to a highly improved understanding for the selective population of Λ-doublet states.
Stability and structure of cluster ions: Halide ions with CO2
Authors: Hiraoka Kenzo , Mizuse Susumu , Yamabe Shinichi
Published: July 2002

Thermodynamic data, ΔH0n−1,n and ΔS0n−1,n, for clustering reactions of halide ions with CO2 were measured with a pulsed electron-beam high-pressure mass spectrometer. The large value of −ΔH00,1 (32.3 kcal/mol) and a sudden decrease of −ΔH01,2 (7.3 kcal/mol) for reaction F−(CO2)n−1+CO2=F−(CO2)n indicate the formation of the core ion, not F−⋅CO2, but FCO−2, i.e., FCO−2 is the fluoroformate ion. The measured binding energies −ΔH0n−1,n for the clusters F−(CO2)n were reproduced satisfactorily by the ab initio MO calculations. The FCO−2 ion is attacked successively and electrostatically by five CO2 molecules to form the first solvation shell. The two oxygen atoms in the core ion FCO−2 are able to accept four CO2 molecules as ligands. The bonding of CI−, Br−, and I− to CO2 is found to be mainly electrostatic.