bluuebunny/crossref_metadata_2025 huggingface dataset:
Square-loop cobalt/gold multilayers
Authors: Gambino R. J. , Ruf R. R.
Published: July 2002

Multilayers of Co and Au with perpendicular hysteresis loop squareness ratios of ∼1 have been prepared by e-beam evaporation. These films have perpendicular anisotropy in the as-deposited condition in contrast to other work in which Co/Au multilayers, prepared by ion beam sputtering, showed perpendicular anisotropy only after annealing at 300 °C. The Faraday rotation of these square-loop multilayers is about 9×105 deg/cm of Co or 1×105 deg/cm of total thickness at a wavelength of 633 nm. These values indicate an enhancement of the Faraday rotation of Co at this wavelength by about a factor of 2. This may be a plasma-edge enhancement effect similar to that reported by Katayama et al. [Phys. Rev. Lett. 60, 1426 (1988)] in the Kerr effect of Fe/Au multilayers.
Magnetic mechanisms of high-temperature superconductivity (invited)
Authors: Emery V. J.
Published: July 2002

A survey of the relationship between magnetism and superconductivity in high-temperature superconductors is presented. It is shown that strong electron correlations are responsible for magnetic behavior and for the specific electronic structure of the charge carriers. A survey of various proposals for a magnetic origin of high-temperature superconductivity is given, including the possibilities of unconventional pairing, new mechanisms, and new forms of the many-body theory. An important issue is whether superconductivity is an intrinsic property of mobile holes in an antiferromagnet. This possibility is compared and contrasted with more conventional approaches to the problem.
Crystal and magnetic structure of Er2(CoxFe1−x)17 compounds
Authors: Kumar R. , Yelon W. B.
Published: July 2002

Room-temperature neutron-diffraction spectra on compositions x=0.0, 0.20, 0.40, 0.55, 0.80, and 1.0 have been obtained and analyzed for crystal and magnetic structures. The crystal structures deviate from the ideal Th2Ni17 structures, as has been observed in the other hexagonal 2-17 materials. Structure analysis from the nuclear intensities at 350 K leads to a precise stoichiometry of Er1.91Fe18.25 in Er2Fe17. Because of the high Tc of the Co-rich compounds, the Er atoms align at relatively high temperature and the axial Er anisotropy dominates at room temperature. Only at x=0.4 composition is a spin reorientation from planar to axial observed below room temperature. The f site, which is analogous to the c site in rhombohedral Th2Zn17 and the j2 site in Re2Fe14B, does not support a moment as large as on the c site and j2 site in respective phases. The moment reduction is probably related to atomic defects on the dumbbell f site.
Magnetic-field source for bichambered electron-beam devices
Authors: Leupold H. A. , Potenziani II E. , Basarab D. J. , Tilak A.
Published: July 2002

Some electron-beam devices such as gyro amplifiers are composed of tandem coaxial chambers with different uniform axial magnetic fields in their interiors. Therefore, there is a step in the field profile in the vicinity of the juncture of the two chambers, which are sometimes of different diameter as well. An electric solenoid is usually employed to furnish such a field, but such sources are bulky, cumbersome, and consumptive of energy, and usually provide only rough approximations to the ideal field profile. This paper describes a permanent magnet field source that provides a constant axial field of 2.0 kOe in the larger of the two chambers and 0.5 kOe in the smaller. The basic field is provided by tandem permanent magnet solenoids. The rather broad transition between the field values in the two chambers is considerably narrowed (40%) by application of a radially magnetized ring to the juncture of the two solenoids. The field profile can be made to conform to that specified to within a few percent.
Spin reorientation phenomenon in Nd0.5Er1.5Fe14−xMxB (M=Al and Co), as determined by ac susceptibility measurements
Authors: Malik S. K. , Adroja D. T. , Ma B. M. , Boltich E. B. , Sohn J. G. , Sankar S. G. , Wallace W. E.
Published: July 2002

Spin reorientation transitions (SRTs) in Nd0.5Er1.5Fe14−xMxB, where M=Al or Co and x=0–3, were determined by ac susceptibility measurements. In Nd0.5Er1.5Fe14B, the SRT is observed at about 195 K. The addition of Al was found to shift the SRT to lower temperatures, presumably due to the weakening of the rare earth–transition metal exchange interaction. For M being Al and x=2, an additional SRT is observed at about 25 K, possibly due to a change of cone angle. Similar reductions in the SRT temperature were observed with Co substitution.
Small-angle neutron scattering from bacterial magnetite
Authors: Krueger S. , Olson G. J. , Rhyne J. J. , Blakemore R. P. , Gorby Y. A. , Blakemore N.
Published: July 2002

Small-angle neutron scattering was used to study the magnetic properties of intracellular magnetite crystals (magnetosomes) in the bacterium, Aquaspirillum magnetotacticum, grown in pure culture. An average of 20 single-domain magnetite (Fe3O4) particles of diameter 400–500 Å are arranged in a chain that longitudinally traverses each cell. The net magnetic dipole moment carried by each chain is sufficient to orient the bacterium in the geomagnetic field. A contrast variation series, performed on a nonmagnetic variant, established that a 30% D2O/H2O ratio in the suspension buffer resulted in the minimization of the scattering from the bacterium itself, thereby enhancing that from the magnetosomes. Measurements of A. magnetotacticum in 30% D2O and an applied field of 25 G confirmed that the scattering from the magnetite particles in bacteria grown under iron-rich conditions is of ferromagnetic character, while that from magnetosomes in bacteria grown under iron-starved conditions is characteristic of superparamagnetic particles. In both cases, the wave-vector dependence of the scattering can be represented by a sum of Lorentzian plus Lorentzian squared terms, from which longitudinal as well as transverse spin correlation lengths were extracted. The results describe dynamic spin correlation lengths on the order of 50 Å, which are superposed on the static magnetosome cluster.
Hysteretic behavior of cobalt powders
Authors: Arajs Sigurds , Nanna Louis C. , Rasmussen Don H. , Bush Robert L.
Published: July 2002

Spherical cobalt powders with mean diameters D between 0.5 and 8 μm and standard deviations σ, scaling approximately as 0.25D, have been prepared by a nonaqueous solution technique. Characterization of these powders has involved x-ray diffraction analysis, scanning electron microscopy, and differential thermal analysis. SEM reveals additional levels of substructure with size domains ranging from 0.2 to 0.3 μm and below. ‘‘As-prepared’’ powders revealed a microcrystalline structure through XRD analysis. High-temperature annealing (923 K) in flowing N2 resulted in a crystallographic change from microcrystalline to a preferred cubic crystal structure as evident through XRD. Room-temperature magnetization studies have been made by a vibrating sample magnetometer as a function of magnetic fields up to 20 kOe. Particular attention has been given to the coercivity and reduced remanence. The dependence of these quantities on the crystallinity of the powders will be briefly discussed.
Direct overwrite capability in transition-metal-rich side rare-earth transition amorphous films (abstract)
Authors: Takayama Shinji , Shimizu Teruhisa , Tanaka Hiroshi
Published: July 2002

The feasibility of direct overwrite utilizing a demagnetization field was successfully demonstrated by employing transition-metal-rich side Tb-Fe-Co-Cr amorphous films, which is different from previously reported rare-earth-rich side films.1,2 For this purpose, 3.5-in. magneto-optical discs were made in the form SiN/TbFeCoCr/SiN on a pregrooved polycarbonate substrate. To evaluate the capability of direct overwrite, we chose the writing and erasure conditions in such a way that erasure laser power (PE) was set higher than the writing power (PW), applying a constant weak external field (Hex) directed in the direction of erasure. Thermomagnetic writing and erasure at 1.76 MHz were carried out on a disk rotating at 1800 rpm. The carrier-to-noise ratio, C/N, thus obtained was 26 dB in the conditions of PW=8 mW and Hex=−100 Oe. This signal was completely erased by the dc erase procedure (PE=10 mW and Hex=−100 Oe). Furthermore, this signal value was repeatedly obtained by the above writing and erasure sequence. However, when the writing laser power is set higher than the erasure power, the initial readout C/N decreases rapidly within a few write/erasure cycles and cannot be repeatedly reproduced. Observation of the above written bits by using a polarized optical microscope showed no irregular shape domains, but did show a relatively weak light intensity coming from the whole domain area. A more detailed analysis of domain formation will be discussed.
H atom reactions with GaAs 〈001〉
Authors: Chuang Mei-Chen , Coburn J. W.
Published: July 2002

A GaAs 〈001〉 crystal at room temperature has been exposed to a flux of hydrogen atoms with and without simultaneous 2-keV argon ion bombardment. A modulated molecular beam mass spectrometric detection system is used to monitor the volatile products evolved from the surface, and in situ Auger electron spectroscopy is used to monitor the surface conditions. With the H atom flux alone no volatile products were observed but an arsenic deficiency was seen with Auger electron spectroscopy. With simultaneous H atom exposure and Ar+ bombardment, arsenic hydrides were observed with the mass spectrometer and a larger arsenic deficiency was observed on the processed surface. No gallium hydrides were observed at any time.
Direct overwriting capability of magneto-optical disks
Authors: Hashimoto Y. , Nakashima K. , Maeda M. , Inoue H. , Ogawa S.
Published: July 2002

Erasing magneto-optical disks were studied using constant frequency pulses. The magnetic layer was Gd12Tb12Fe76. The linear velocity was 7.2 m/s and writing and rewriting were done at 1.8 MHz. The magnetic field was the same during writing and erasing. If the erasing power is too low, bits are incompletely erased, and if the erasing power is too high, erase pulses nucleate domains. Erasing performance can be improved by high frequencies and low duty cycles. Erasing performance decreases as the magnetic field in write direction becomes stronger. However, almost the same rewriting C/N as the original C/N can be obtained in a weak magnetic field. We obtained a rewriting C/N of 29 dB, while an original C/N was 31 dB at a magnetic field of +50 Oe.
Film thickness dependence of magneto-optical and magnetic properties in Co/Pt and Co/Pd multilayers
Authors: Hashimoto S. , Ochiai Y. , Aso K.
Published: July 2002

Co/Pt and Co/Pd multilayers were prepared by two source dc-magnetron sputtering. It was found that magneto-optical and magnetic properties of the multilayers were affected by total film thickness. Kerr rotation angle of the films was remarkably enhanced at the film thickness below several hundred angstroms. The theoretical calculation clarified that the increase of Kerr rotation was due to optical interference and multiple reflection. The shape of Kerr loop also depends upon the film thickness and a perfect squareness is attained only at the ultrathin region. The improvement in the squareness of Kerr loop can be explained by the change in the domain structure from a stripe domain to a single domain. These behaviors at the ultrathin region in the multilayers are suitable for new magneto-optical recording media.
Superconducting melt-textured thin films prepared by a spray pyrolysis method
Authors: Ban E. , Matsuoka Y. , Ogawa H.
Published: July 2002

YBa2Cu3O7−x thin films were prepared on (100)yttria-stabilized zirconia (YSZ) by a spray pyrolysis method. The films were fired rapidly at 1000–1020 °C in flowing helium atmosphere and then annealed in oxygen. These melt-textured thin films had the aligned grain structure consisting of large, platelike grains over a wide area. The films thus formed showed relatively high critical current density Jc more than 4000 A/cm2, and the best Jc of the film fired at 1010 °C was 4800 A/cm2 at 77 K in zero magnetic field. The melt-textured growth processing was found to be more useful for the enhancement of the Jc value of thin films than the conventional sintering method.
Nonlinear reflection in a bandpass filter
Authors: Waltz R. E. , Rosenbluth M. N. , Ikezi H.
Published: July 2002

It is shown that a nonlinear lumped-circuit transmission line with a bandpass can transmit weak signals and reflect strong signals. Such nonlinear bandpass filters have an absolute limit on the power passed and may prove useful as circuit protectors.
Transmission electron microscopy study of CdTe(111) grown on GaAs(100) by molecular-beam epitaxy
Authors: Reno J. L. , Carr M. J. , Gourley P. L.
Published: July 2002

We have used transmission electron microscopy to investigate CdTe(111) grown on GaAs(100) by molecular-beam epitaxy. The loop structure previously observed by photoluminescence microscopy has been identified as the boundary between twinned microcrystallites that extend from the CdTe/GaAs interface to the CdTe surface. When viewed along the growth axis, these boundaries between the columnar twins appear as loops and segments. Surface roughness of the GaAs substrate contributes to the initial growth of twinned material. This leads to competitive growth between the twins and the creation of the observed columnar twins.
Stability of multijunction a-Si:H-based solar cells
Authors: Bennett M. S. , Rajan K.
Published: July 2002

It is demonstrated that multijunction a-Si:H-based solar cells are more stable than their single-junction counterparts and that the stability improves as the number of junctions increases. It is further shown that the rate of degradation of multijunction devices can be explained in terms of the rates of degradation of their individual i layers. It is thus possible to predict the amount of photodegradation of multijunction devices after prolonged light soaking, knowing only the rates of degradation of single-junction cells in terms of the i-layer thicknesses and the intensity of the illumination to which they are exposed.
An unstable resonator laser with a modified output coupling scheme
Authors: Singh B. , Dixit S. K. , Mittal J. K. , Bhatnagar R.
Published: July 2002

Unstable resonator lasers with a scraper mirror as output coupler suffers from diffraction effects associated with uniformly illuminated aperture. The near-field intensity distribution is annular in shape with minima at the center. In this communication, a modified resonator is suggested in which the resonator is folded and the scraper mirror is replaced by a partially transmitting mirror to provide the output coupling. The modified resonator is applied to a copper vapor laser. Observed near-field distribution is uniform across the beam and the intensity of the diffraction rings in the far-field profiles is considerably reduced.
Channeling study of local distortion in indium-doped semi-insulating GaAs
Authors: Satoh M. , Kuriyama K.
Published: July 2002

The local distortion in the indium-doped semi-insulating GaAs has been evaluated by means of ion channeling experiments along three directions for various indium concentrations up to 6.3×1020/cm3. The local distortion due to the indium doping gradually increases with the indium concentration, accompanied by the increase in minimum yield along the 〈100〉, 〈110〉, and 〈111〉 axes. The increase of the local distortion is independent of the crystal growth method.
Effects of oxide-trapped charges and interface trap generation in metal/oxide/semiconductor structures with ultradry oxides after Fowler–Nordheim stressing
Authors: Nishioka Yasushiro , Ohji Yuzuru , Yoshida Ikuo , Mukai Kiichiro , Sugano Takuo
Published: July 2002

The effects of charge trapping and interface trap generation in fully processed metal/oxide/semiconductor (MOS) structures with wet, dry, and ultradry oxides are investigated. This is done by measuring the electrical properties of MOS capacitors with these oxides after Fowler–Nordheim electron injection stress. The data show that ultradry oxides have more hole traps than the other two. In addition, the generation of interface traps and electron traps is smaller in the ultradry oxides than in the other two.
Gain measurement of a CuBr laser by means of modified amplified spontaneous emission
Authors: Chan W. C. , Liu H. P. , Yen S. H. , Chen W. Y. , Lin Y. H. , Chen W. C.
Published: July 2002

A modified amplified spontaneous emission technique has been introduced to measure the radial distributions of the gain and the saturation energy density of the output of a double-discharge pulsed CuBr laser. An asymmetric distribution of the gain was obtained. With the laser tube temperature at 420 °C, the peak value of the gain and the saturation energy density are 70 db/m and 85 μJ/cm3, respectively.
Fabrication and performance characteristics of buried-facet optical amplifiers
Authors: Dutta N. K. , Piccirilli A. B. , Lin M. S. , Brown R. L. , Wynn J. , Coblentz D. , Twu Y. , Chakrabarti U. K.
Published: July 2002

The fabrication, performance characteristics, and design rules of buried-facet optical amplifiers are described. Chip gain of 25 dB, gain ripple of <1 dB, and gain difference of ≤1 dB for TE- and TM-polarized light are observed. The gain ripple and polarization dependence of gain correlate well with the ripple and polarization dependence of the amplified spontaneous emission spectrum. The performance of buried-facet amplifiers is comparable to that of cleaved-facet amplifiers with very good antireflection (R<10−4) coatings. The buried-facet design reduces the requirement on antireflection coatings and makes the fabrication process more reproducible.
Spontaneous emission in the uniform magnetic-field free-electron laser described by the ordinary and scalarized photons
Authors: Šoln Josip
Published: July 2002

As is the case with the usual free-electron laser with a helical magnetic field, the uniform magnetic-field free-electron laser has both the fundamental and the higher harmonic-frequency (spontaneous and stimulated) emission, where, however, emission with higher harmonic frequencies occurs only off axis. (The z component of the initial electron velocity defines the axis direction, which is also the direction of the uniform magnetic field B.) Here, within the multiphoton formalism, using both the ordinary and scalarized photons, we study only the spontaneous emission, into the fundamental and higher harmonic frequencies. It is proven that the use of the scalarized photons, whose advantage is that they simplify many calculations, is generally equivalent to the use of the ordinary photons. Specifically we find that the radiated energy, either total or partial (for a particular harmonic), scales linearly with L, the interaction length, and quadratically with B. The numerical analysis of the angular distribution of the spontaneously emitted energy indicates that, in addition to dominant on-axis fundamental frequency spontaneous emission, we can have rather significant higher harmonic-frequency spontaneous emission occurring just slightly off axis. So it appears that, at least in principle, not only on axis but also slightly off axis (higher harmonic) spontaneous emission could be used to stimulate emission from electrons further along for sufficiently wide electron beams.
Dynamic electrical response of Y1Ba2Cu3O7−x
Authors: Frenkel A. , Venkatesan T. , Lin Chinlon , Wu X. D. , Inam A.
Published: July 2002

We studied the dynamic electrical properties of epitaxial Y1Ba2Cu3O7−x superconducting thin films on strontium titanate and MgO substrates. A fast nonlinear switching of a high-Tc film between the dissipative and superconducting states has been demonstrated. These measurements show that the rise and the fall times of these transitions are faster than 250 ps (which are presumably limited by the speed of the measurement electronics). This fast nonlinear switching may lead to opportunities for new high-frequency electronic devices including, for example, fast logic gates or threshold devices. We have proposed and demonstrated the application of such a nonlinear switching in a novel noise discrimination scheme for speeds of several hundred MHz. The proposed method is simple and effective in the enhancement of signal-to-noise ratio in digital electronic circuits.
Power absorption during deposition of polycrystalline-silicon in a lamp-heated chemical-vapor-deposition reactor
Authors: Liao J. C. , Kamins T. I.
Published: July 2002

A thermocouple-instrumented wafer was used to monitor the wafer temperature during chemical vapor deposition of a polycrystalline-silicon layer in a lamp-heated rapid thermal processor. The temperature of the oxidized silicon substrate varies by more than 100 °C as the polycrystalline-silicon layer is deposited because the reflectivity of the sample changes with increasing thickness of the deposited layer. Cross-section transmission electron microscopy shows that when the temperature decreases below the polycrystalline-to-amorphous transition temperature during deposition, a phase change occurs in the structure of the deposited film. The change in temperature qualitatively corresponds to the change in reflectivity as a function of silicon film thickness calculated from a simple model.
Growth and characterization of GaAs/Al/GaAs heterostructures
Authors: Bhattacharya P. , Oh J. E. , Singh J. , Biswas D. , Clarke R. , Passos W. Dos , Merlin R. , Mestres N. , Chang K. H. , Gibala R.
Published: July 2002

Theoretical and experimental aspects of the growth of GaAs/Al/GaAs heterostructures have been investigated. In these heterostructures the GaAs on top of the buried metal layer is grown by migration-enhanced epitaxy (MEE) at low temperatures (200 and 400 °C) to provide a kinetic barrier to the outdiffusion of Al during superlayer growth. The crystallinity and orientation of the Al film itself deposited on (100) GaAs at ∼0 °C was studied by transmission electron diffraction, dark-field imaging, and x-ray diffraction measurements. It is found that the Al growth is polycrystalline with a grain size ∼60 Å and the preferred growth orientation is (111), which may be textured in plane but oriented out of plane. The quality of the GaAs superlayer grown on top of Al by MEE is very sensitive to the growth temperature. The layer grown at 400 °C has good structural and optical quality, but is accompanied by considerable outdiffusion of Al at the Al-GaAs heterointerface. At 200 °C, where the interface has good structural integrity, the superlayer exhibits twinning and no luminescence is observed.
The current leakage mechanism in InSb p+n diodes
Authors: Sun Tai-Ping , Lee Si-Chen , Yang Sheng-Jenn
Published: July 2002

The effect of different passivation processes, i.e., anodic oxidation and/or photochemical deposition of SiO2 on the current-voltage characteristics of the InSb p+n diodes was studied. By applying different voltages on the gate electrode over the p+n junction periphery, various kinds of current-voltage characteristics can be induced, including multiple negative differential resistance in forward bias. This strongly indicates that the major part of the current, especially the reverse leakage current, flows through the surface of the p+n junction. Tunneling due to field emission from the valence band of the p+ layer into a field-induced inversion layer at the InSb/oxide interface in which the energy levels are quantized is responsible for the multiple negative differential resistance. Reverse leakage current as low as 20 μA/cm2 at −1.1 V for a p+n type diode with n-type doping concentration of 2×1015cm−3 could be easily achieved by applying a gate voltage of −9 V. It is also found that diodes with similar performance can be fabricated by properly adjusting the photochemical vapor deposition passivation process.
Evidence of hydrodynamic processes in multimegampere copper fuses
Authors: Lindemuth I. R. , Caird R. S. , Goforth J. H. , Oona H. , Reinovsky R. E.
Published: July 2002

Experiments have been performed in which an explosively formed fuse transferred a fraction of the 10 MA output current from an explosively powered magnetic flux compression generator to a conventional exploding metallic fuse load. System malfunctions significantly reduced the current delivered to the load and changed the fuse performance. The resultant slowing of the fuse material’s trajectory through density-temperature space allowed a correlation between optical and electrical diagnostics which indicated a significant delay between the onset of hydrodynamic motion and the onset of rapid resistance increase. The experiments have confirmed a computational model of the fuse load hydrodynamic behavior.
Melt-front velocity in laser-induced melting
Authors: Cohen S. S. , Wyatt P. W. , Chapman G. H. , Canter J. M.
Published: July 2002

Laser melting processes have been studied extensively, particularly for semiconductor substrates. Values for the velocity of the melt front have been determined by several experimental methods, and also calculated in numerical simulations of the melting processes. The velocity of the melt both during melting and recrystallization is of direct consequences for the material properties of the laser treated zone. Hence, a clear understanding of the physical parameters involved is essential. For laser pulses of a Gaussian shape, and whose duration is longer than a few tens of nanoseconds, expressions are derived for the melt-front velocity for the general case and for the limiting case of a point source. In either case the velocity turns out to be nonconstant. Hence, experimentally reported values may only be regarded as indicative of the maximum velocity achievable. The simple closed-form analytical expressions obtained in the present study are amenable for a direct analysis of relevant experimental results. Comparison made with some available data reveals a general agreement between theory and experiment. Ultra high-speed photography is one possible technique that may enable observation of the varying velocity of the lateral melt front.
Pressure-induced resonance Raman scattering in Ga1−xInxAs/Ga1−yAlyAs strained quantum-well structures
Authors: Kourouklis G. A. , Jayaraman A. , People R. , Sputz S. K. , Maines R. G. , Sivco D. L. , Cho A. Y.
Published: July 2002

Pressure-tuned resonance Raman scattering (RRS) experiments in Ga1−xInxAs/Ga1−yAlyAs strained-layer quantum-well samples with x=0.15 and y=0.15, on a GaAs substrate, are reported. The photoluminescence (PL), 2LO-, and LO-phonon Raman-scattered intensities were followed as a function of pressure using the diamond cell. In backscattering geometry and with (100) samples, three sharply defined 2LO-phonon resonances are observed, at 2.1, 3.8, and 5.2 GPa, with 647.1-nm excitation (ℏωL=1.916 eV), and these are identified to be from the AlGaAs, GaAs, and InGaAs layers, respectively. At the above pressures, the 2LO peak of the layer falls right on top of the PL peak of the corresponding layer, revealing that the resonance occurs when E0=ℏωS (2LO). For the LO-phonon intensity, maxima are observed at 2.8 and 4.7 GPa, respectively, from the AlGaAs layer and GaAs substrate. While the LO-resonance profile (pressure versus phonon intensity) for the AlGaAs layer is narrow, the profile of the GaAs substrate layer is highly asymmetric. The InGaAs LO-resonance is totally masked by the strong GaAs LO phonon. The pressure dependence of the E0 gap, determined from PL data, reveals that the resonance condition for 2LO is E0=ℏωS (2LO) and, for LO, E0=ℏωL. Results with samples of different Al content and ℏωL are consistent with the above conditions. A brief discussion of the theory of the RRS cross section is given, and it is suggested that the observed 2LO-resonance scattering is dominated by Frölich interaction, and that it could be a double resonance. The asymmetry in the LO resonance of the GaAs substrate, we believe, is due to the pressure-induced transparency, and this is a problem when dealing with pressure-tuned RRS, in multiple-quantum-well structure with GaAs or AlGaAs substrates. The usefulness of the 2LO resonance in locating the E0 gap is indicated, and the indistinguishability between luminescence and Raman scattering at the center of the 2LO resonance is pointed out. The need to study photoluminescence and Raman scattering in pressure-tuned RRS experiments is emphasized.
Excimer laser photolysis study of NH3 in the presence of NO at 193 nm
Authors: Ikeda Tetsuya , Danno Minoru , Makihara Hiroshi
Published: July 2002

Excimer laser photolysis has been studied for NH3 and NO mixtures at the wavelength of 193 nm. The amount of decomposition of NO was compared with the incident photon energy. It was found that NO decomposition occurs with a quantum yield of 0.94.
The influence of the pregroove on the shape of thermomagnetically written domains
Authors: Ichihara K.
Published: July 2002

In order to clarify the influence of pregrooved substrates on the shape of thermomagnetically written domains, the difference between the shape of the domains written on a pregrooved area and that written on a mirror area have been examined. Trilayered magneto-optical media, which had rare-earth- (RE-) rich TbFeCo films, transition-metal-rich TbFeCo films, and RE-rich GdTbFeCo films as a recording layer, were sputtered on disk substrates. The substrates had both a pregrooved area and a mirror area in a recording track. The domains were written in each medium by varying the recording power and the external field, and were observed by an Ar+ -laser scanning polarized microscope. In the case of TbFeCo media which were written with lower recording power condition, the shape of the domains on a pregrooved area were almost the same as those written on a mirror area. On the other hand, the widths of the domains written on a mirror area became larger than those of domains written on a pregrooved area when the recording power was increased. In the case of a GdTbFeCo medium, the widths of the domains written on a mirror area were much larger than those of domains written on a pregrooved area independent of the recording conditions. The lengths of the domains written on both areas were almost the same for all cases. It is believed that the reason for the experimental results is that thermal diffusion in the film plane is suppressed at the step of a pregroove. The different result between TbFeCo and GdTbFeCo films is believed to come from the differences in the contracting forces on the domain walls during the writing process.
Comment on ‘‘Fluid flow in a random porous medium: A network model effective medium approximation’’ [J. Appl. Phys. 6
2, 1616 (1987)]
Authors: Garboczi E. J.
Published: July 2002

In a recent article, a simple lattice-based crack percolation model was analyzed via an effective medium approximation [R. Blumenfeld and D. J. Bergman, J. Appl. Phys. 62, 1616 (1987)]. The effective-medium theory developed for this model gave a prediction for pc , the critical fraction of cracks at which a connected pathway of cracks first appeared in the lattice. In this comment, pc is computed numerically and is found to be equal to 0.194±0.01, which is much less than the effective medium prediction of 0.5. Reasons for this disagreement are discussed.
Sub-band-gap superconductor absorption in Bardeen–Cooper–Schrieffer framework using the Mattis–Bardeen approach
Authors: Goossen K. W. , Nuss M. C.
Published: July 2002

Closed-form expressions for losses in superconductors at photon energies well below the gap are found in the BCS framework using the Mattis–Bardeen approach, which has been found to agree well with high-Tc materials. This formulation supercedes an earlier one that is shown to be incorrect and that overestimates the loss at low temperatures and underestimates the loss at high temperatures.
Rapid thermal processing of YBa2Cu3O7 spun wires
Authors: Ginley D. S. , Venturini E. L. , Kwak J. F. , Mitchell M. A. , Morosin B. , Baughman R. J. , Halloran J. W. , Neal M. J.
Published: July 2002

Rapid thermal processing has been utilized to sinter and anneal phase-pure and CuO-rich YBa2Cu3O7 fibers and woven fibers formed by melt spinning a powder/polymer composite. The optimum temperature for sintering the powders varies for the stoichiometric (1025 °C) and 5% CuO-rich fibers (1000 °C), but in both cases the temperature window is extremely narrow (±25 °C). Typical processing consists of a rapid heating (250 °C/s) of the fibers to the sintering temperature in 1 atm of pure oxygen, holding at temperature for 1 s and cooling over a period of 2–3 min. The resulting fibers are orthorhombic YBa2Cu3O7 with large Meissner and shielding fractions, have zero resistance up to 92 K, critical-current densities in zero applied field to 1100 A/cm2 at 76 K, and show clean grain boundaries by transmission electron microscopy. Despite the short processing times, microstructural analysis shows considerable grain growth and evidence of metastable congruent melting of oxygen-rich YBa2Cu3O7.
Theoretical model of gravitational perturbation of current collector axisymmetric flow field
Authors: Walker John S. , Brown Samuel H. , Sondergaard Neal A.
Published: July 2002

Some designs of liquid-metal current collectors in homopolar motors and generators are essentially rotating liquid-metal fluids in cylindrical channels with free surfaces and will, at critical rotational speeds, become unstable. An investigation at David Taylor Research Center is being performed to understand the role of gravity in modifying this ejection instability. Some gravitational effects can be theoretically treated by perturbation techniques on the axisymmetric base flow of the liquid metal. This leads to a modification of previously calculated critical-current-collector ejection values neglecting gravity effects. The purpose of this paper is to document the derivation of the mathematical model which determines the perturbation of the liquid-metal base flow due to gravitational effects. Since gravity is a small force compared with the centrifugal effects, the base flow solutions can be expanded in inverse powers of the Froude number and modified liquid-flow profiles can be determined as a function of the azimuthal angle. This model will be used in later work to theoretically study the effects of gravity on the ejection point of the current collector.
X-ray topographic investigation of characteristic dislocation structure in sapphire single crystals
Authors: Qiang Zhang , Peizhen Deng , Fuxi Gan
Published: July 2002

The characteristic triangular cross-grid dislocation structure in sapphire single crystals has been investigated by means of x-ray transmission topography. Dislocations lay on (0001) basal planes and primarily appeared as three parallel groups of straight dislocation lines. At high temperature these dislocations can move easily on their slip planes and form different configurations by interacting and pinning. The analysis of Burgers vector and image width of dislocation lines confirmed that three parallel groups of straight dislocation lines are pure edge type having 〈112̄0〉-type Burgers vector, and a few dislocation reactions are the type of [112̄0]+[12̄10]+[2̄110]=0. The characteristic dislocation structure is closely related to the growth orientation.
On optimal heating and cooling strategies for melting and freezing
Authors: Gordon J. M. , Rubinstein I. , Zarmi Y.
Published: July 2002

For a one-dimensional liquid or solid slab that undergoes freezing or melting, we solve for the cooling or heating strategy that, subject to certain constraints and approximations, minimizes entropy production and hence maximizes potentially available work. The key constraint is that the freezing or melting process must be completed in a given fixed time. This constraint can correspond, for example, to a fixed production rate in an industrial process. The validity of this optimal strategy and its practical significance are discussed.
Electrical characterization of AlAs layers and GaAs-AlAs superlattices
Authors: Feng S. L. , Zazoui M. , Bourgoin J. C. , Mollot F.
Published: July 2002

We characterize by electrical techniques uniformly Si-doped AlAs layers and short-period GaAs-AlAs superlattices grown in the same conditions by molecular-beam epitaxy. Deep level transient spectroscopy shows that both the layers and the superlattices contain the DX center. The AlAs layers contain, in addition, a distribution of electron traps emitting in the temperature range 50–200 K. Using electron irradiation to introduce defects as probes we verified that the band structures of the superlattices we deduce are consistent with theoretical calculations using a widely accepted value of the band offset (67%). Finally, we observe that the DX center remains present, with the same ionization energy, when the energy position of the first Γ miniband varies while the first L miniband remains practically at the energetical position of the L band in the AlAs barriers. From this result we conclude that the DX center is linked to the L band and thus must be ascribed to an L effective-mass state.
Quantitative examination of the thermoelectric power of n-type Si in the phonon drag regime
Authors: Behnen Erwin
Published: July 2002

A quantitative comparison between theory and experiment is presented for the thermoelectric power of n-type Si at low temperatures, where crystal boundary scattering of phonons is predominant. The expression for the Seebeck coefficient is obtained by simultaneously solving the coupled Boltzmann equations for electrons and phonons. The thermoelectric power is measured from samples of two different donor concentrations (ND=8×1014 cm−3 and 7×1015 cm−3) at temperatures ranging from 25 to 200 K, where the thickness is varied by etching the samples (370–73 μm). The phonon mean free path for crystal boundary scattering, determined from experimental thermopower, agrees well with that gained from the theory of thermal conductivity corrected for the phonon velocities of interest. An additional measurement of the thermal conductivity yields the mean free path for phonon–phonon scattering, which is about two orders of magnitude less than that obtained from thermopower. This difference confirms the influence of the different parts of the phonon spectrum involved in the calculation of the appropriate transport parameters.
The influence of heat treatment on the electrical characteristics of selenium-implanted GaAs
Authors: Tang A. C. T. , Sealy B. J. , Rezazadeh A. A.
Published: July 2002

The effects of subsequent heat treatments on the electrical properties of ion-implanted layers in GaAs have received little attention. This is of paramount importance as far as both the industrial applications and the understanding of the physics behind the incorporation of dopants in GaAs are concerned. In this work, we have observed reversible changes in the sheet carrier concentration of the rapid thermally annealed Se-implanted GaAs samples during subsequent heat treatments. The carrier concentration is observed to reach a limit for a certain temperature at sufficiently long annealing times. By noting the initial rate of change of the carrier concentration during subsequent heat treatments, an activation energy in the range of 1.6–2.5 eV has been identified for these samples. It is concluded that the rate dependent process for such reversible reactions is the diffusion of gallium atoms/vacancies. The electrical profiles of the Se-implanted samples have also been modified during the annealing processes.
Width dependence of magnetoresistance in GaAs-AlGaAs wires fabricated by mesa etching
Authors: Takagaki Y. , Gamo K. , Namba S. , Ishida S. , Takaoka S. , Murase K.
Published: July 2002

GaAs-AlGaAs quantum wires with various widths have been fabricated using electron beam lithography and Ar ion etching. Conductance was measured as a function of the etched depth and the wire width. The results indicate that defects diffuse at room temperature. Shubnikov–de Haas oscillations in GaAs-AlGaAs film and wires with different widths have been measured to investigate the effect of the surface damage on the electron transport characteristics. The analysis of the results shows that both electron density and scattering time decrease with decreasing the width of the channel. These are indicative of the side wall scattering and the damages induced during the sample preparation process.
Intrinsic thermal stability for scanning electron microscopy of thin-film superconductors
Authors: Flik M. I. , Tien C. L.
Published: July 2002

The criterion of intrinsic thermal stability (ITS) of thin-film superconductors predicts under which circumstances a thermal disturbance causes Joule heating. In the characterization method of low-temperature scanning electron microscopy (LTSEM), the absorption of the electron beam results in a well-controlled thermal disturbance. This work applies the ITS criterion to LTSEM of both high-Tc and low-Tc thin-film superconductors and predicts the conditions for which no voltage appears along the film. The analysis of the temperature field for high-Tc films differs from that for low-Tc films by the absence of the acoustic-mismatch thermal resistance between film and substrate at liquid-nitrogen temperature. The theoretical results for low-Tc films are in very good agreement with experimental data for LTSEM of lead films on sapphire substrates at 4.2 K.
Coercivity and switching behavior of amorphous magnets
Authors: Cresswell Alan , Paul David I.
Published: July 2002

Starting from a Hamiltonian based on the random magnetic anisotropy model to describe the magnetic characteristics of amorphous materials such as TbFe2, we determine the low-temperature dependence of the coercivity as well as the magnetization dynamics of these materials. The results are seen to compare well to experiments whenever data are available. Our model features clusters, each characterized by its anisotropy direction. The exchange-energy term entering the Hamiltonian is represented by an effective exchange interaction acting in the intercluster regions.
Analytical expression of the first magnetization curve in soft magnetic polycrystalline materials
Authors: Le Floc’h M.
Published: July 2002

This work deals with soft magnetic polycrystalline samples (YIG) specially prepared for fundamental investigations. These investigations have previously shown, at different times, that macroscopic magnetic properties of such materials could be interpreted by a simple spherical grain model. From theoretical study of this model, a first magnetization curve formulation is proposed. Excellent agreement with measurements in a broad range from weak fields to saturation is shown. In high-quality materials with a negative magnetostriction constant, because of the very peculiar action on domain walls, parallel compression is shown to be a useful way to compare observation with theory. Quantitative study proves this specific role.
Iodine-doping effects on the vapor-phase epitaxial growth of ZnSe on GaAs substrates
Authors: Fuke Shunro , Imai Tetsuji , Irisawa Shin-ichi , Kuwahara Kazuhiro
Published: July 2002

Epitaxial growth of ZnSe on GaAs(100) and (111)B substrates is undertaken by hydrogen-transport vapor-phase epitaxial growth method. Growth behaviors under the condition of iodine doping and characteristics of grown ZnSe layers are given. By adding iodine molecules to the reactant agents, the growth rates of ZnSe on both GaAs(100) and (111)B planes are enhanced, which are similar phenomena with the iodine doping effects upon ZnS growth on GaP substrates. As the iodine transport rate increases beyond optimum value, the growth rates of ZnSe decrease in the opposite way. Furthermore, the dependence of the crystallinity of the grown layers on the iodine transport rate is also found to support the changes in the growth rate caused by iodine doping.
The measurement of boron at silicon wafer surfaces by neutron depth profiling
Authors: Downing R. G. , Lavine J. P. , Hossain T. Z. , Russell J. B. , Zenner G. P.
Published: July 2002

The thermal neutron reaction n+10B→4He+7Li is utilized to measure the boron concentration on the surface of silicon wafers. This neutron depth profiling measurement technique requires no sample preparation. Boron is determined on the as-received wafers at a level of 1012 to 1013 atoms/cm2. A boron level of about 2×1012 atoms/cm2 is found at the wafer surface after oxidation or epitaxial or polycrystalline silicon deposition. Ambient air appears to be one source of the boron. Secondary-ion mass spectroscopy performed on wafers with polysilicon provides additional support for an atmospheric source of boron.
Pressure effect on thermal relaxation behavior of the Curie temperature in amorphous alloys
Authors: Shen Zhong-Yi , Hong Jing-Xin , Yin Xiu-Jun , Zhang Yun , He Shou-An
Published: July 2002

The Curie temperature relaxation behavior in two amorphous ferromagnetic alloys (Fe10Co55Ni35)78Si8B14 and Fe40Ni40P14B6 (Metglas 2826) subjected to annealing at pressures up to 2 GPa is investigated. A comparison of the relaxed amounts of TC due to isothermal and isochronal annealings at 0, 1, and 2 GPa shows a delaying effect on the relaxation process during high-pressure annealing, while the concomitant kinetics of the processes remains unchanged. The pressure-induced percentage reductions in TC relaxation are estimated to be 8% and 12% per GPa, respectively. The reversible relaxation behavior was found still present during high-pressure annealing, but with a reduced amplitude. By annealing between high and low pressures at fixed temperature, some regenerative relaxation behavior was observed after every completion of the cyclic pressures. The obtained results may be explained by pressure-induced narrowing of regions in the amorphous structure. It is suggested that these extended regions are the main locations where the compositional short-range ordering contributed to TC relaxation.
The change of electrical conduction in the valence/conduction band to the impurity band in CdSexTe1−x thin films
Authors: Sebastian P. J. , Sivaramakrishnan V.
Published: July 2002

Reported here are the results of investigations carried out on the mechanism of electrical conduction in CdSexTe1−x thin films between 300 and 125 K in vacuum. All the films showed a transition from grain boundary limited conduction in the conduction/valence band to phonon assisted hopping via the impurity band (impurity band conduction) at around 280–290 K. The grain boundary limited conduction showed an activation energy equal to about 0.14 eV and conduction via impurity band showed an activation energy of about 0.02 eV, which is characteristic of phonon assisted hopping conduction.
Time-resolved spectra of the 1.55-eV band in Ge-doped AlxGa1−xAs
Authors: Mejri H. , Maaref H.
Published: July 2002

We report time-resolved luminescence data on the 155-eV band of Ge-doped AlxGa1−xAs with x=0.3, grown by liquid-phase epitaxy. They are interpreted in terms of donor-acceptor pair recombinations in which the donors and acceptors have a well-defined spatial separation. From the analysis of the emission line shape, the optical parameters of the pair are deduced.
Particle and power balances of hot-filament discharge plasmas in a multidipole device
Authors: Cho M-H. , Hershkowitz N. , Intrator T.
Published: July 2002

This paper considers particle and power balances to estimate the bulk plasma potential of a hot-filament discharge plasma produced in a multidipole plasma device. The bulk plasma potential dependence on positive dc bias applied to an anode is analyzed, and the predicted characteristics of the plasma potential are compared to the experiment. It is shown that the plasma potential can be more positive or more negative than the anode bias potential. When the potential is more negative, a steady-state potential dip in front of an anode is observed using emissive probes with the zero-emission inflection point method. Conditions for the potential dip formation are discussed.