bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunneling microscopy of NbSe2 in air
Authors: Dahn D. C. , Watanabe M. O. , Blackford B. L. , Jericho M. H.
Published: July 2002

We have obtained atomic-resolution images of NbSe2 single crystals in air. Constant-height images clearly show the expected atomic structure, and can distinguish the two inequivalent halves of the unit cell. Constant-current images show an anomalously high atomic corrugation, associated with elastic deformation of the sample. Surface contamination probably plays an important role in transmitting the tip-sample forces. A larger-scale apparent buckling of the surface with a period of several times the atomic spacing is sometimes observed.
Application of the Avrami rate equation to electromigration damage in Al–1%Si interconnections
Authors: Patrinos A. J. , Vankar V. D. , Schwarz J. A.
Published: July 2002

Al–1%Si test stripes were subjected to life testing, and the resistance-versus-time behavior of individual conductors were collected for subsequent analysis. The failure times were log-normally distributed and the median time to failure decreased with increasing temperature. Analysis of the resistance-versus-time data was performed using the Avrami rate expression, R(t)=R0 exp[(Bt)n] proposed earlier by Rodbell, Rodriguez, and Ficalora [J. Appl. Phys. 61, 2844 (1987)]. It is found that values of the parameter n vary for identical conductors stressed under the same conditions. Thus the physical significance of this parameter with respect to identifying mechanisms of the rate-determining step is questionable. On the other hand, dramatic changes in the value of this parameter are observed over the temperature range of this study; this reflects changes in the growth geometry. It is suggested that such effects can be used to establish appropriate stressing conditions during life testing to avoid possible complications in the interpretation of the reliability data.
Electronic structure of ultrathin (GaAs)n(AlAs)n [001] superlattices and the Ga0.5Al0.5As alloy
Authors: Wei S.-H. , Zunger Alex
Published: July 2002

Using self-consistent electronic structure calculations we contrast the energy levels of the ultrathin (GaAs)n(AlAs)n [001] superlattices (n=1,2) with those of the disordered Ga0.5Al0.5As alloy and a long period (n→∞) superlattice. Conventional Kronig–Penney and effective mass models suggest that, because of the relatively light electron effective masses and small barrier heights, only delocalized superlattice conduction states would exist in the n=1 limit. We find a number of such conventional ‘‘averaging states’’ (delocalized on both sublattices). In addition, we also find states localized on a single sublattice. For small n’s, the latter are divided into two classes: (i) ‘‘repelling states’’ (distinct alloy states which fold in the superlattice into states of identical symmetry, which, in turn, repel each other and tend to localize), and (ii) ‘‘segregating states’’ (a pair of localized states Ψα and Ψβ, where symmetry compels Ψα to have a vanishing angular momentum component l on a subset α of unit cell atoms, whereas the complementary state Ψβ is localized on the other atoms β. These states are split by the potential difference Vβl −Vαl). We analyze new luminescence, reflectance, and Raman data in light of our theoretical model. Studies of the II-VI superlattices (CdTe)1(HgTe)1 shows similar behavior.
Acoustic loss phenomena in alpha quartz-crystal resonators over the 1.4–77 K temperature range
Authors: Suter J. J.
Published: July 2002

The acoustic loss phenomena of conventional and electrodeless quartz-crystal resonators over the temperature range from 1.4 to 77 K were investigated. The AT electrodeless resonator of the BVA type exhibited smaller acoustic losses than the conventional FC crystals at the ionic sodium impurity relaxation temperature (∼50 K) and acoustic wave interaction with thermal phonon temperature (∼20 K). A dramatic decrease in the acoustic losses in the electrodeless resonator was observed at temperatures in the range from 1.4 to 5 K. This decrease is attributed to a decrease in two-level energy excitations in this type of resonator. Furthermore, acoustic attenuation was shown to be a function of microwave power absorbed by the crystal plates.
Wave aspects of balance equations for nonlinear space-charge dynamics in GaAs
Authors: Vassell M. O. , Lee Johnson
Published: July 2002

A nonlinear wave approach is suggested for analyzing systems of balance equations used for modeling nonstationary and hot-carrier dynamics in semiconductors. The approach is applicable since balance equations in conventional use comprise quasilinear systems of partial differential equations, and so are describable naturally in terms of nonlinear hyperbolic waves. A general review of the pertinent wave properties is given for systems written in one space dimension and time for an arbitrary number of conserved quantities. Special attention is paid to the peculiar roles of flux, source, and relaxation terms in specifying signal speeds and wave types, and in controlling wave interaction. The pervasive nature of discontinuous solutions, and the possibility of nonlinear resonance phenomena are discussed. The latter are claimed to have important consequences for the numerical resolution and stability of solutions, and for device behavior. Certain restrictions on the proper posing of initial-boundary value problems are stressed. These ideas are illustrated in particular for the single-electron gas version of the semi-empirical transport equation model for GaAs. This model uses fluxes and relaxation rates taken from steady-state Monte Carlo data to describe the spatial and temporal evolution of the three conserved quantities: particle density, velocity field, and energy. Geometrical effects resulting from variation of the channel cross section have been introduced into the discussion.
The crystallography and defect chemistry of structural faults in lithium niobate
Authors: Twigg M. E. , Eaglesham D. J. , Maher D. M. , Nakahara S. , Holmes R. J.
Published: July 2002

Transmission electron microscope studies of Ti-doped, congruent lithium niobate (LiNbO3) have shown that extended structural faults and domain boundaries are only present within the Ti-diffused layer (i.e., the waveguiding region). Structural faults have also been observed in undoped Li-deficient LiNbO3, though not in undoped control crystals of congruent and stoichiometric LiNbO3. In both Ti-doped and Li-deficient LiNbO3, crystal chemistry predicts an increase in the concentration of the point defects V′′′′′Nb and Nb⋅⋅⋅⋅Li. Therefore, it appears that the coalescence of these defects is responsible for the creation of structural faults. Since the extent of these structural faults and domain boundaries is tens of microns, they are clearly potential scattering sites for photons. In this regard, a systematic understanding of their origin and thermal stability is crucial to integrated optical device technologies based on LiNbO3 and on the Ti-doped waveguide fabrication technique.
The effect of various buffer-layer structures on the material quality and dislocation density of high composition AlxGa1−xAs laser material grown by metalorganic chemical vapor deposition
Authors: Givens M. E. , Coleman J. J. , Zmudzinski C. A. , Bryan R. P. , Emanuel M. A. , Miller L. M.
Published: July 2002

The effect of various types of buffer layers on the generation and propagation of dislocations in epitaxial layers of high composition (x=0.85) AlxGa1−xAs grown by metalorganic chemical vapor deposition (MOCVD) on horizontal Bridgman (HB) and liquid-encapsulated Czochralski (LEC) substrates is examined. Bulk epilayers of high composition (x=0.85) AlxGa1−xAs and graded-barrier quantum-well laser structures with confining layers of the same composition were grown simultaneously on high-quality/low etch-pit density (EPD) HB substrates and comparatively lower quality/high EPD LEC substrates with one of four types of compositionally graded and/or superlattice buffer-layer structures. The bulk material was characterized by delineation and measurement of surface EPD and the observation of overall surface morphology. Data are also presented on the device characteristics of graded-barrier quantum-well laser diodes grown with these same buffer layers in order to determine the correlation between dislocation density and laser threshold current. The various buffer-layer structures were seen to be effective in reducing the defect density and improving the surface morphology of high composition epilayers grown on both HB and LEC substrates. The threshold-current density of the laser diodes, however, was independent of both the type of prelayer and/or substrate utilized.
Scale dependence of resistance fluctuations at metal-semiconductor junctions
Authors: Boudville W. J. , McGill T. C.
Published: July 2002

The resistance fluctuations at a metal-semiconductor junction have been simulated as a function of the cross-sectional area of the contact at the interface. The fluctuations are due to the random spatial configurations of the dopants in the depletion region. The semiconductor is taken to be heavily doped n-GaAs. The conductance is computed over a square area. Dopings in the range of 1018–2.5×1019 cm−3 and square contacts with linear dimensions between 200 and 2000 Å are considered. An analysis of this problem yields the result that the standard deviation of the conductance of the contacts scales as 1/ln3/4, where n is the doping and l is the device size. From simulations, the relative fluctuations are found to fall significantly as the device sizes are increased beyond 1000 Å and as the doping is increased. The results of our simulations are found to agree with our scaling arguments.
Effects of energy straggling on surface analysis with fast ion beams
Authors: Kawano Akira , Kido Yoshiaki
Published: July 2002

The energy straggling effect on depth profiling by fast ion beams has been investigated. Prior to spectrum analysis, the electronic energy straggling for fast proton beams was measured precisely using nuclear resonance reactions. The present result has revealed the validity of Chu’s prediction [Phys. Rev. A 13, 2057 (1976)] based on the Hartree–Fock–Slater model. A significant difference is seen among the depth profiles of implanted ions derived using different straggling values [N. Bohr, Philos. Mag. 30, 581 (1915); Chu, no straggling] in calculating the excitation spectrum of the nuclear resonance reaction. In depth profiling by particle-induced x-ray emission, the straggling effect is negligibly small except for probing ions with energies less than 150 keV/amu. In backscattering spectrum simulation, the energy straggling contributes to the spectrum shape only in the region where individual spectrum is convoluted for the constituent elements locating in the neighboring layers. This provides a useful method to search the surface and interface morphologies nondestructively.
Josephson ac voltmeter
Authors: Peterson Robert L. , Oldham Nile M.
Published: July 2002

We propose a new technique for accurate measurement of ac voltages with Josephson junctions. Based on the counting of pulses generated by a Josephson junction, the method may be capable of precision at the ppm level for frequencies less than 100 kHz.
Effects of backsputtered material on gallium liquid metal ion source behavior
Authors: Galovich C. S.
Published: July 2002

Backsputtered material is demonstrated to be a major factor in determining gallium liquid metal ion source (LMIS) lifetimes and I-V characteristics during operation in ultrahigh vacuum. The behavior of LMIS emission current is extremely sensitive to the type of material backsputtered onto the source tip, shaft, and reservoir. The assimilation of sputtered material is important in maintaining stable ion emission and is found to depend on the miscibility of the contaminant material in gallium. Materials which do not form solid phases when mixed with liquid gallium have the least effect on ion currents.
Approximate analysis of nonlinear operation of a distributed feedback laser with parasitic losses
Authors: Szczepański P.
Published: July 2002

An approximate method for the analysis of distributed feedback lasers with parasitic losses is presented. Symmetric index grating structures with and without end reflectors are investigated. Characteristics of laser operation are obtained with the help of expressions relating the normalized small signal coefficient α0N L with the output power PNout normalized to the saturation power Ps, normalized loss coefficient αNLL, normalized coupling coefficient ‖κ‖L, complex reflectivity of the end reflectors ℛ, and the laser length L. The results are found to be in good agreement with published computer solutions.
A NMR study of YCo12B6 and GdCo12B6 intermetallic compounds
Authors: Erdmann K. , Rosenberg M. , Buschow K. H. J.
Published: July 2002

Spin-echo nuclear magnetic resonance was used to study 59Co hyperfine fields in the intermetallic compounds RCo12B6 with R=Y and Gd. Two groups of signals were obtained and interpreted in terms of anisotropic orbital contributions to the hyperfine field in the domain walls and domains.
Spin-wave instability and ‘‘true’’ foldover in single-crystal YIG films (abstract)
Authors: Zhang Y. T. , Patton C. E. , Srinivasan G.
Published: July 2002

High-power pulsed microwave measurements of ferromagnetic resonance (FMR) in yttrium iron garnet (YIG) films have been extended to include all three in-plane and out-of-plane microwave and static field combinations for FMR. The 5.1-μm-thick films were prepared by liquid phase epitaxy on single-crystal gadolinium gallium garnet substrates and subsequently etched to form 500-μm circles. The FMR spectra were obtained with a 9.18-GHz pulse spectrometer as described in Ref. 1. The results confirm the conclusion of Ref. 1. The pulsed microwave FMR at high-power levels yields line broadening due to spin-wave instability processes, rather than the shifts and distorted profiles which occur due to heating under cw excitation conditions. The key new result is the explicit correlation between the microwave field amplitude for the onset of the line broadening and the FMR pump configuration. In addition to these line broadening effects at high power, a ‘‘true’’ foldover effect is observed for the perpendicular-to-plane static field case: the FMR shoulder shifts down in field at high powers, due to the nonlinear response and the limitations on that response from the spin wave instability processes.
Quenching of ferrimagneticlike ordering in SrCr8Fe4O19 hexagonal ferrite
Authors: Obradors X. , Labarta A. , Tejada J. , Pernet M. , Tholence J. L. , Saint-Paul M. , Barbara B.
Published: July 2002

We report here measurements of high field (H≤150 kOe) isothermal magnetization, dc and ac susceptibility, and Mössbauer spectroscopy of the exchange frustrated insulator SrCr8Fe4O19. The isothermal magnetization and Mössbauer spectroscopy results are consistent with a ferrimagneticlike transition at TC≊250 K. Low field dc and ac susceptibility measurements reveal a reentrant spin-glass transition at low temperature but they fail to show any anomaly at the nominal Curie temperature TC. This unusual feature is interpreted as indicative of a transition where long range ferrimagnetic ordering has been replaced by a correlated spin-glass phase with finite spin correlation length.
Solutions of the magnetic Eliashberg equations for heavy fermion superconductors (abstract)
Authors: Norman M. R.
Published: July 2002

The Eliashberg equations for magnetic fluctuations are solved for heavy fermion superconductors. The susceptibility is taken from neutron data, and the Fermi surface from band calculations. For the case of UPt3, mass renormalizations of about 16 are found consistent with de Haas–van Alphen data. A superconducting transition temperature of the order of 0.12 K is calculated, due to the momentum dependence of the susceptibility, with the gap function having lines of zeros in parts of the zone. The transition temperature can be boosted up to experimental values by crude incorporation of interband matrix elements. Extensions of these calculations to other heavy fermion superconductors will be reported.
Magnetoelasticity and magnetovolume of dilute Cr-Si alloys
Authors: Alberts H. L. , Lourens J. A. J.
Published: July 2002

The recently predicted reentrant feature near the triple point on the magnetic phase diagram of the Cr-Si system was not observed in magnetoelastic and magnetovolume measurements. Known itinerant electron theories, which were previously applied successfully to other Cr systems, fail in several aspects when applied to the magnetovolume of Cr-Si alloys.
Charge transport in Li-Ni ferrospinels
Authors: Reddy P. Venugopal
Published: July 2002

The electrical conductivity (σ) and thermopower (Q) of Li-Ni mixed ferrites have been studied as a function of composition and temperature. The conduction mechanism in these ferrites has been explained with the help of carrier concentration (n) and charge carrier mobility (μ) calculated from the measured experimental values. The composition and frequency dependence of dielectric constant (ε′) and dielectric loss tangent (tan δ) have also been studied at low frequencies. A relationship between dielectric constant (ε′) and electrical resistivity (ρ) has been arrived at. A qualitative explanation for observing a maximum in the plots of dielectric loss tangent and frequency has also been given.
Magnetic-wave-like excitations and cluster modes in randomly disordered Rb2MnxCr1−xCl4 (invited)
Authors: Sieger D. , Tietze H. , Geick R. , Schweiss P. , Heger G. , Treutmann W.
Published: July 2002

The randomly disordered magnetic system Rb2MnxCr1−xCl4 with competing exchange interactions and with competing anisotropies has been studied by means of inelastic neutron scattering. We have investigated three samples of different concentrations (x=0.70, 0.25, 0.59) at various temperatures (4.2 K<T<50 K). In the samples with ferromagnetic order (x=0.25) and with antiferromagnetic order (x=0.70), a well-defined spin-wave branch has been found coexisting with a number of well-defined dispersionless excitations in the same frequency range of 0.8 THz≤ν≤2.5 THz. Of particular interest are the excitations in the sample with x=0.59 which shows no long-range order but spin-glass behavior with a freezing temperature of Tf=11 K. For T<Tf, two-dimensional ferro- and antiferromagnetic correlations are observed with the same correlation length ξ=15 Å. The excitation spectrum of this sample gives evidence for a variety of cluster modes and for two types of spin waves, namely antiferromagnetic and ferromagnetic ones, for wavelengths smaller than the correlation length. Such an excitation spectrum as that observed for our sample with x=0.59 has not yet been reported for any other spin glass.
Shift in maximum of sound attenuation with magnetic field in UPt3
Authors: Baum H.-P. , Xu M.-F. , Qian Y. J. , Schenstrom A. , Ketterson J. B. , Hinks D. H. , Levy M. , Sarma B. K.
Published: July 2002

We have measured ultrasonic attenuation and ac susceptibility simultaneously on a UPt3 single crystal, in various constant magnetic fields at 293 MHz, and in zero field at several frequencies. The temperature of the attenuation maximum Tmα appears lower than (or very close to) the susceptibility transition temperature Tcχ. A breakpoint is defined, which occurs always at a lower temperature Tbp than Tmα. The shift of this breakpoint, Tcχ −Tbp, exhibits a small frequency dependence.
Neutron scattering and magnetic susceptibilities of dilute P
tCo alloys calculated by Monte Carlo methods
Authors: Parra R. E. , López R. A.
Published: July 2002

Neutron scattering experimental data for dilute PtCo alloys show sharp forward peaks indicating the inhomogeneous distribution of magnetization in these alloys and the size and shape of magnetization clouds. These characteristics of the clouds, however, do not agree with those obtained by magnetization measurements. The disagreement suggests an important nonelastic contribution to such scattering. We, therefore, calculated the diffuse neutron cross sections for these alloys employing a magnetic environment model for the magnetization clouds, that has been used successfully for other giant moment dilute alloys such as PdFe and PtFe. The calculations were made by Monte Carlo methods for alloys of PtCo in the concentration region 1.0≤c≤6.0 at. %Co. The results show sharp peaks due to quasielastic scattering which reproduce well the experimental neutron data. We also calculated by the same methods the magnetic susceptibilities for alloys of PtCo with c<1.0 at. %Co. The results show a good agreement with experimental data. The validity of the model and of the magnetic correlations used for the calculations is supported by their agreement with experimental results. At high concentrations the model should not apply as the clouds are considered independent in the calculations.
Spin fluctuations in Ni above T
C
Authors: Rusek P. , Callaway J.
Published: July 2002

Numerical computations of the temperature dependence of the cross section for the magnetic scattering of unpolarized neutrons in nickel above TC are reported. The calculations are based on semiempirical bands assuming no exchange splitting above TC . The thermal fluctuations are taken into account through the introduction of an effective temperature-dependent interaction, which is evaluated in the mode-mode coupling approximation. Cross sections are shown for constant-q scans for three different values of q and for several different temperatures from 650 to 946 K. There is some small structure in the calculated cross section due to band-structure effects, but we do not predict propagating spin waves above TC.
Anisotropy of the upper critical field in the magnetic heavy-fermion superconductor URu2Si2
Authors: Moshchalkov V. V. , Aliev F. , Kovachik V. , Zalyaljutdinov M. , Palstra T. T. M. , Menovsky A. A. , Mydosh J. A.
Published: July 2002

Measurements have been performed of the upper critical field Hc2 anisotropy in the magnetic heavy-fermion superconductor URu2Si2. The dHc2/dT value is constant within 5% when H is rotated in the basal plane, whereas ‖dHc2/dT‖ decreases by about 35% for H rotated by 20°–30° out of the basal plane. Contrary to CeCu2Si2 the Hc2 anisotropy in URu2Si2 is enhanced as temperature decreases below Tc. This disparity can be attributed to the different strengths of paramagnetic and spin-orbit effects responsible for the suppression of superconductivity by a magnetic field in CeCu2Si2 and URu2Si2.
Field-dependent behavior of longitudinal ultrasonic attenuation in UPt3
Authors: Xu M.-F. , Schenstrom A. , Baum H.-P. , Qian Y. J. , Ketterson J. B. , Hinks D. , Levy M. , Sarma Bimal K.
Published: July 2002

A possible phase transition is evident in the field dependent behavior of the ultrasonic attenuation in UPt3. In the low field region the behavior is linear and in the high field region close in Hc2 the behavior is quadratic in h′=(Hc2−H)/Hc2. At the crossover there is a small peak in the ultrasonic attenuation. The temperature dependence of the coefficients a(T), for the linear behavior, and b(T), for the quadratic behavior are obtained.
Overwrite temperature dependence for magnetic recording
Authors: Takeda Takayuki , Tagami Katsumichi , Watanabe Takaaki
Published: July 2002

The temperature effect on overwrite (OW) characteristics for magnetic recording, using various heads and media, has been examined. Overwrites depend linearly on the difference (T2−T1) between overwrite temperature T2 and previous recording temperature T1. Their gradients vary from −0.10 to 0.43 dB/deg, depending on head-media combinations. The gradients are proportional to the difference (αBs−αHc), between temperature coefficient for head saturation magnetic flux density Bs and that for media coercivity Hc. Therefore, temperature dependence of overwrite is reduced by selecting a mead-media combination with a small ‖αBs−αHc‖ value. This relation is explained by temperature variation in recording depth.
Recording characteristics of tapes prepared using iron oxide particles treated with organic titanate
Authors: Aoyama S. , Sumiya K. , Kishimoto M. , Amemiya M.
Published: July 2002

The adsorption behavior of organic titanate on an iron foil having an ideal iron oxide surface is examined to improve read-write characteristics of particulate magnetic recording media by the surface treatment of iron oxide magnetic particles with organic titanate. Organic titanate significantly reduces the surface hydrophilicity of iron oxide surface of the iron foil. Heat treatment is required to form the chemical bonding between the organic titanate and the iron oxide surface. The method of surface treatment is applied to Co-modified iron oxide particles. As a result, the magnetic properties and surface roughness of the tape are remarkably improved by the increase of dispersibility of particles due to the reduction of hydrophilicity with the organic titanate treatment. The carrier-to-noise ratio (C/N) at the frequency of 6.0 MHz in the tape is increased about 2 dB compared with the tape prepared using untreated particles.
Magnetic susceptibilities of actinide 3d-metal intermetallic compounds
Authors: Muniz R. Bechara , d’Albuquerque e Castro J. , Troper A. , Gomes A. A.
Published: July 2002

We have numerically calculated the magnetic susceptibilities which appear in the Hartree–Fock instability criterion for actinide 3d transition-metal intermetallic compounds. This calculation is based on a previous tight-binding description of these actinide-based compounds [A. Troper and A. A. Gomes, Phys. Rev. B 34, 6487 (1986)]. The parameters of the calculation, which starts from simple tight-binding d and  f  bands are (i) occupation numbers, (ii) ratio of d-f hybridization to d bandwidth, and (iii) electron-electron Coulomb-type interactions.
Brillouin light scattering detection of ferromagnetic resonance in thin films
Authors: Srinivasan G. , Patton C. E. , Booth J. G.
Published: July 2002

Ferromagnetic resonance (FMR) in thin films of permalloy and yttrium iron garnet (YIG) has been studied by Brillouin light scattering (BLS) techniques. The measurements were made at 9.4 GHz on 22.4 to 75-nm-thick permalloy films and on 2.1- to 12.8-μm-thick YIG films. Intensity profiles for magnon scattered light versus in-plane applied field were obtained by analyzing the forward scattered light through the films with a high-contrast Fabry–Perot interferometer. The BLS profiles show a signal-to-noise ratio of 10-100 for the permalloy and 100-1000 for the YIG films, depending on the film thickness and the microwave power level. The FMR BLS response was quantified in terms of global response function, counts/s mW versus magnon occupation number Nu. The Nu parameter relates the scattering to the uniform mode FMR response (linewidth, field, frequency, etc.), input microwave power, and active sample volume. The response for permalloy was approximately10−8 counts/s mW magnon, which translates into a limiting sample volume of 10−12 cm3.
Electroreflectance and wavelength-modulated reflectivity spectra of Zn1−xMnxSe alloys
Authors: Stankiewicz Jolanta , Fermín José R.
Published: July 2002

Low-field electroreflectance (ER) and wavelength-modulated reflectivity (WMR) spectra have been obtained on Zn1−x Mnx Se single crystals in the whole range of composition, 0≤x≤0.57. An analysis of the ER data enabled us to determine the dependence of the fundamental energy gap on temperature (8 K≤T≤300 K) and Mn molar fraction x. In samples with x≥0.35, which crystallize in a wurtzite structure, two or three temperature-dependent features are observed in WMR spectra and in some ER spectra. We identify these peaks with the ground-state excitons for different valence bands in hexagonal Zn1−x Mnx Se. The observed crystal-field splitting is ∼20 meV for x=0.39 and decreases with increasing Mn content.
Effect of heat treatment on grain-boundary microstructure in Nd-Fe-B sintered magnet
Authors: Tsubokawa Y. , Shimizu R. , Hirosawa S. , Sagawa M.
Published: July 2002

Microstructure of grain-boundary regions in sintered Nd15Fe77B8 magnets has been observed under a high-resolution TEM. Crystalline Nd-rich phase is not observed at all the grain boundaries, while amorphouslike layers are observed quite often. It has also been found that no bcc phase in as-sintered samples was observed before heating the samples but heating of the samples for annealing has caused bcc microcrystals to form in the edges of the matrix phase.
Eddy current losses in composite and/or multibodies with μr≥1
Authors: Taher Ahmed M. , Lavers J. D. , Burke P. E.
Published: July 2002

The paper deals with the computation of eddy current losses in composite and/or multiconducting cylinders due to time-harmonic transverse magnetic fields. The cylinders are linear magnetic or nonmagnetic with an arbitrary cross-section area. Two diffferent formulations in terms of the Fredholm integral equations of the second kind are used. The exact formulation comprises 2N (where N is the number of conductors) coupled one-dimensional integral equations; and the second, where the external impedance boundary condition approach is used, consists of N coupled one-dimensional integral equations. With the proper choices of the equations the highest singularity in both formulations is logarithmic (weak singularity in 2D). Numerical results are computed for the special case of a pair of circular conductors. Magnetic and nonmagnetic materials are considered and the results are compared with the corresponding results for a single circular conductor.
Structural, electrical, and magnetic properties of Fe/Pd superlattices (abstract)
Authors: Boufelfel A. , Emrick R. M. , Makous J. L. , Falco Charles M. , Majkrzak Charles
Published: July 2002

We have made Fe/Pd superlattices with periodic modulation wavelength Λ ranging from 33.2 to 188.4 Å. Long-range structural coherency is inferred from several x-ray diffraction techniques. Fe and Pd were found to grow (110) and (111) parallel to the substrate, respectively. The in-plane electrical resistivity was measured in the range 7.5–300 K by the four-point probe method. The dependence of the resistivity on Λ will be explained by a model using interfacial scattering of electrons. Room-temperature magnetic properties of the samples were measured using conversion electron Mössbauer spectroscopy and vibrating sample magnetometry. We found all the samples to be ferromagnetic. Both the intrinsic hyperfine magnetic field and the saturation magnetization show an increase as Λ decreases. We found that both of these physical quantities strongly depend on the number of atomic layers of (110) Fe in Λ. We did not find any induced magnetism in (111) Pd to within the experimental error. We determined the magnetic moment profile in the growth direction using neutron scattering.
Multicritical point in the magnetic phase diagram of CsNiCl3 (abstract)
Authors: Caillé A. , Plumer M. L. , Hood Kevin
Published: July 2002

A previously developed1 Landau-type free-energy functional forms the basis of a theory for the low-temperature magnetic properties of CsNiCl3 under the influence of an applied magnetic field. Three magnetically ordered phases, as well as the paramagnetic phase, are found to all coexist at a single multicritical point representing the intersection of one line of first-order phase transitions and three lines of second-order transitions in the H-T diagram. The temperature dependencies of the critical fields are in good agreement with corresponding experimental data.2
Phase transition in Ising ferromagnetic lattices with fixed spins (abstract)
Authors: Labarta A. , Marro J. , Tejada J. , Martínez B.
Published: July 2002

In this paper we report a study of the influence of the symmetry which characterizes the quenched defects (namely, 1/2 spins fixed either ‘‘up’’ or ‘‘down’’) distributions into the nature of the involved phase transitions. We have performed a Monte Carlo simulation involving L3 cubic lattices with L≤40 and, in order to conclude above the phase transition nature, a detailed finite size scaling analysis has been done. It is concluded that the transition remains second order with pure critical exponents for regularly spaced defects. Any small randomness in the location of the defects, however, makes the transition first order.
Thermodynamics of free electrons in helical magnetic fields
Authors: Calvo Miguel
Published: July 2002

The low-temperature thermomagnetic properties of a free-electron gas coupled to an external helical magnetic field of variable strength and pitch are calculated. The diamagnetic behavior of the system is shown to exhibit some novel remarkable effects which differ substantially from the constant field Landau theory. In the strong field regime (B∼104 G) the magnetization, susceptibility, and helicity modulus are evaluated, yielding some new physical results.
Electromagnetic gauge invariance and orbital motion in field-induced spin-density-wave collective modes: Magnetorotons (abstract)
Authors: Lederer P. , Poilblanc D.
Published: July 2002

The metallic ground state of strongly anisotropic metals with an open Fermi surface is known to be destroyed by a magnetic field. A field-induced spin-density-wave (FISDW) phase appears. As the field varies a cascade of transitions between SDW subphases is observed. The physics of FISDW is characterized by two very different lengths. One, the modulation wavelength, is of the order of the interatomic distance. The other is the magnetic length x0, which is much larger and which is connected to electronic orbital motion in the presence of the magnetic field. The order parameters breaks the continuous translation and spin rotational symmetries. We present a theory of the collective excitation spectrum of the FISDW phases. We find that this spectrum exhibits, beside the Goldstone bosons related to the two continuous broken symmetries, a series of rotonlike modes within the single particle electronic gap at the Fermi level, both in the transverse (to the order parameter) spin-wave spectrum and the phase fluctuation spectrum. The magnetoroton minima are centered at quantized values of the wave vector in terms of x−10. The degeneracy between the modes is lifted by the interaction between fluctuations transverse and parallel to the magnetic field. The rotonlike structure is connected to the interplay between orbitally broken translation invariance and electromagnetic gauge invariance. The spectrum of collective modes proves that FISDW is a new type of electron-hole condensate. The theory applies to Bechgaard’s salts.
Spin fluctuations in EuS above T
C: Comparison with asymptotic renormalization-group theory
Authors: Böni P. , Shirane G. , Bohn H. G. , Zinn W.
Published: July 2002

Constant-energy scans in neutron scattering provide a very sensitive tool for testing theoretical line shapes in detail. In order to verify the predictions of asymptotic renormalization-group theory for isotropic ferromagnets above TC we measured the temperature dependence of the paramagnetic scattering from the Heisenberg ferromagnet EuS. The width of the constant-E peaks increases with increasing temperature as predicted. Surprisingly the positions do not scale. The nonuniversal behavior is attributed to the crossover from isotropic to dipolar dynamics.
Infrared investigation of the presence of Fe in B-doped polycrystalline silicon
Authors: Borghesi A. , Geddo M. , Stella A. , Chen-Jia Chen
Published: July 2002

Boron-doped polycrystalline silicon samples containing variable amounts of iron have been investigated by means of transmission measurements, from 230 to 360 cm−1, in order to study the effect of the presence of iron on boron acceptor excitation spectrum lines. The first absorption measurements in the far infrared which indicate the formation of Fe-B complexes and the accumulation of Fe at the grain boundaries in such samples are reported here.
Conductivity in concentrated disordered dispersions of spheres
Authors: Franklin J. G. , Javanaud C.
Published: July 2002

An effective medium approach is described for calculating the conductivity in a disordered dispersion of highly conducting spheres close to their maximum packing fraction. The dispersion is modeled by a network of resistances whose values follow a probability distribution reflecting the distribution of gaps between neighboring spheres. The effective conductivity of the medium increases by a few percent as the distribution of gap sizes broadens. The shape of the distribution—rectangular or triangular—has less effect. Results are presented for the simple cubic lattice in two and three dimensions. The two-dimensional results are found to agree well with those from simulations performed on the same resistor network.
Surface effect as a limitation on the performance of polycrystalline Si thin-film transistors
Authors: Kung K. T-Y. , Reif R.
Published: July 2002

In a previous article [J. Appl. Phys. 62, 1503 (1987)], we demonstrated how the average grain size and the {110} fiber texture of a polycrystalline Si film could affect the performance of metal-oxide-semiconductor thin-film transistors fabricated in the film. We showed that, for grain sizes of the order of 1–2 μm, a strong {110} texture was more effective than a larger average grain size in improving the threshold voltages and field-effect carrier mobilities of both the n- and p-channel transistors fabricated. In this article, we propose an interpretation of these results, and offer supporting evidences, to relate the crystallographic texture of a polycrystalline Si film to thin-film transistor performance. We show that a {110}-oriented surface exhibits less surface effect than a randomly oriented surface on average and, consequently, it is more desirable for thin-film transistor applications.
Optoelectronic sequential waveform generation in a microstrip directional coupler
Authors: Buck J. A. , Neifeld M. A. , Bowers M. W.
Published: July 2002

A continuous sequential waveform generation scheme based on an optically switched microstrip directional coupler is described. Experimental results are presented which demonstrate feasibility of both forward and backward coupling at frequencies near 100 MHz. Extension to higher frequencies is discussed.
Bistability and intensity noise of semiconductor lasers due to weak optical feedback
Authors: Sato Hisanao , Matsui Yasushi , Ohya Jun , Serizawa Hiroyuki
Published: July 2002

It is shown theoretically that semiconductor lasers become bistable with weak optical feedback and that the intensity noise is increased by transitions between the bistable states due to steady-state fluctuations. The bistability occurs in a narrow range of the feedback magnitude, depending on the phase of feedback light at the semiconductor facet. This is in good agreement with observed noise increase at low frequencies caused by external feedback.
p-on-n heterojunctions of (Hg,Cd)Te by molecular-beam epitaxy: Controlled silver doping and compositional grading
Authors: Peterman D. J. , Wroge M. L. , Morris B. J. , Leopold D. J. , Broerman J. G.
Published: July 2002

Two important quantities in determining the electrical properties of a heterojunction are the dopant and compositional grading profiles. Although molecular-beam epitaxy (MBE) has been used to grow high-quality layers of (Hg,Cd)Te, there has been no report of attempts to control these important quantities by use of conventional MBE growth techniques. We report the identification of a reliable p-type impurity dopant that can be used for MBE-(Hg,Cd)Te and we demonstrate, by use of secondary-ion mass spectroscopy, that conventional MBE growth techniques can be used with silver to produce the required dopant and compositional grading profiles to fabricate p-on-n heterojunctions.
Semiconductor superlattices with periodic disorder
Authors: Jiang H. X. , Lin J. Y.
Published: July 2002

For a real superlattice, fluctuations are always presented in the period lengths. The band structure of semiconductor superlattices under the effect of this periodic disorder has been investigated in this paper. The zone center and zone edge of the first subband of electrons and holes and the effective energy gap as functions of this fluctuation have been calculated. We discuss the dependence of the band offset on this fluctuation. Our calculated results can be used to explain some of the experimental observations.
A model for the resistive critical current transition in composite superconductors
Authors: Warnes William H.
Published: July 2002

Much of the research investigating technological type-II superconducting composites relies on the measurement of the resistive critical current transition. We have developed a model for the resistive transition which improves on older models by allowing for the very different nature of monofilamentary and multifilamentary composite structures. The monofilamentary model allows for axial current flow around critical current weak links in the superconducting filament. The multifilamentary model incorporates an additional radial current transfer between neighboring filaments. The development of both models is presented. It is shown that the models are useful for extracting more information from the experimental data than was formerly possible. Specific information obtainable from the experimental voltage-current characteristic includes the distribution of critical currents in the composite, the average critical current of the distribution, the range of critical currents in the composite, the field and temperature dependence of the distribution, and the fraction of the composite dissipating energy in flux flow at any current. This additional information about the distribution of critical currents may be helpful in leading toward a better understanding of flux pinning in technological superconductors. Comparison of the models with several experiments is given and shown to be in reasonable agreement. Implications of the models for the measurement of critical currents in technological composites is presented and discussed with reference to basic flux pinning studies in such composites.
Optical studies on the electric field dependence of excited donor states in silicon
Authors: Larsson K. , Grimmeiss H. G.
Published: July 2002

The effect of strong electric fields on some excited states of the substitutional selenium donor in silicon has been investigated with the aid of a new method for determining Fourier transform photoresponse spectra of impurity centers in the depletion regions of diodes. The response concerned is the current in the external circuit of the diode arising from centers in a certain zone, this zone being controlled by trap-filling pulses applied to the diode in synchronism with the movement of the spectrometer’s scanning mirror. The measurements show that the 1s1T2 and 2p0 states of the selenium donor are subject to quadratic Stark shifts of 2.1×10−16 and 3.0×10−15 eV m2 V−2, respectively. Further, thermal emission from these states to the conduction band is found to be enhanced by the field. Finally, it is found that at high fields transitions from the ground state of this neutral double donor to the spin-triplet component 1s3T2 may occur, a fact which has not been directly observed previously.
Chiral order in a two-dimensional X
Y spin glass
Authors: Kawamura Hikaru , Tanemura Masaharu
Published: July 2002

Ordering phenomena in the two-dimensional XY (n=2) spin glass have been studied by Monte Carlo simulations with a focus on chirality, an Ising-like, multispin variable associated with a broken reflection symmetry. The chiral and n=2 spin-glass orientational degrees of freedom order in a markedly different way: indeed, the chiral susceptibility can be mapped onto the spin-glass susceptibility of a pure Ising (n=1) spin glass.
Overlayer metallicity and Fermi-level pinning at the Ca-GaAs(110) interface
Authors: Mao D. , Young K. , Stiles K. , Kahn A.
Published: July 2002

Photoemission spectroscopy is used to study chemistry and band bending at the Ca-GaAs(110) interface as a function of metal coverage. An intermediate position of the Fermi level (EF ) resulting from the formation of adsorbate-induced states and native defects is found at low coverage at 0.75–0.9 eV above the top of the valence-band maximum (VBM). An additional abrupt shift of EF leading to a final position 0.55 eV above VBM takes place when metallicity develops in the overlayer. The results for this interface supports Schottky-barrier models based on gap states induced or modified by the metal.
High-power, high-speed 1.3-μm semi-insulating-blocked distributed-feedback lasers
Authors: Koren U. , Koch T. L. , Corvini P. J. , Miller B. I. , Eisenstein G. , Tucker R. S. , Su Y. K. , Capik R. J.
Published: July 2002

We describe a high-performance 1.3-μm InGaAsP/InP distributed feedback buried heterostructure laser which is compatible with all-vapor-phase growth technology. Current confinement is provided by metalorganic vapor-phase growth of semi-insulating InP blocking layers. The laser has a small-signal bandwidth of 12.5 GHz, a large signal digital capability at 16 Gb/s, and maintains single-frequency operation to output powers as high as 23 mW.
Photoluminescence tomography as a method to image point-defect distributions in crystals: Nitrogen-vacancy pairs in synthetic diamonds
Authors: van Enckevort W. J. P. , Lochs H. G. M.
Published: July 2002

The distribution of grown-in nitrogen-vacancy pairs in synthetic Ib diamond crystals has been imaged by means of a new photoluminescence tomographic technique. Using this method, local changes in luminescence intensity introduced by these defect centers were mapped by means of a laser-scanning microtomographic arrangement. It was found that the spatial variation in nitrogen-vacancy pair concentrations within a crystal qualitatively correlates with the nitrogen impurity distribution.
Valence-band effective masses of GaAs
Authors: Lowney Jeremiah R. , Kahn Arnold H.
Published: July 2002

The density-of-states effective masses for the heavy-hole, light-hole, and split-off valence bands of GaAs have been calculated as a function of energy for each band. The calculations are based on a full k⋅p theory with the most recent values used for the matrix elements. Provision has been made for the effect of the split-off energy on the matrix elements of the split-off band. The results show important nonparabolicities which should be taken into account in modeling the valence band, and rational polynomial fits have been made for ease of computation.
Aspects of energy transport in a vortex-stabilized arc
Authors: Pearson J. B. , Curzon F. L.
Published: July 2002

By measuring the axial profiles of thermal wall loading and radiation output in a dc vortex-stabilized arc, it has been shown that the vortex significantly influences the energy transport in the arc. The arc was operated at 225 A in an argon vortex at a pressure of 5.5 atm. The measurements show that there is a rapid onset of thermal wall loading immediately adjacent to the upstream electrode. Furthermore, there is a significant amount of thermal wall loading upstream of the arc column, which is found to scale with the flowrate through the vortex. This fact, taken together with observations of the tracks produced by tungsten droplets on the arc wall, and photographs of the electrode regions indicate that reverse axial flow exists in the core of the arc vortex. Near the electrodes there is a region of locally increased thermal wall loading and radiation output, but between these regions there exists a region where the wall loading and radiation are axially invariant. In this region the measured wall loading is as much as 100% larger than that predicted using laminar heat transport. The scaling of this wall loading with gas flowrate in the vortex has been measured, and it has been shown that if the data are extrapolated to the no-flow condition, the wall loading is in much better agreement with the laminar heat transport model. The existence of turbulence or Taylor–Görtler vorticies in the stabilizing gas flow are proposed as mechanisms for the enhanced thermal wall loading.