bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies of thermodynamic and magnetic properties of Ce1−xGdxSn3
Authors: Lin C. L. , Yuen Tan , Jiang G. Y. , Mihalisin T. , Bykovetz N.
Published: July 2002

The temperature dependence of the specific heat, magnetic susceptibility, and 119Sn Mössbauer spectra of Ce1−xGdxSn3 has been measured. An evolution from mixed valent, to enhanced paramagnetic, to spin glass, and finally to a magnetically ordered state is observed as one alloys from pure CeSn3 to GdSn3. The C/T curves show a strong upswing at the lowest temperatures for the enhanced paramagnetic systems with dilute Gd concentrations. In the spin-glass regime, the temperature where the cusp occurs in the zero-field cooled magnetic susceptibility data is lower than the temperature at which the Mössbauer line width starts broadening. For the magnetically ordered state, TN increases with increasing Gd concentration reaching 31 K for pure GdSn3.
Weak ferromagnetism and spin-glass-like behavior in Tb2CuO4
Authors: Tovar M. , Obradors X. , Pérez F. , Oseroff S. B. , Duro R. J. , Rivas J. , Chateigner D. , Bordet P. , Chenavas J.
Published: July 2002

Polycrystalline Tb2CuO4 with tetragonal (T′) structure was synthesized under high pressure. The in-phase component of the alternating-current (ac) susceptibility χ′(T) presents a peak whose onset at TN ≊ 290 K, we associate with antiferromagnetic ordering of the Cu moments. The shape of this peak is strongly dependent on the amplitude of the excitation field, indicating nonlinearity of the magnetization as a function of the applied magnetic field. The frequency dependence of the peak observed in χ′(T) and the nonzero out-of-phase component χ″(T) in the same temperature range indicate the existence of relaxation processes associated with the development of a weak ferromagnetic component. Below ∼100 K, the relaxation becomes much slower than the frequency of the oscillating field and the ac susceptibility is dominated by the paramagnetic response of Tb ions, which follows a Curie–Weiss law. The direct-current (dc) magnetization presents a nonlinear dependence on the applied field for Ha ≲ 2 kOe and shows thermomagnetic irreversibility below a second characteristic temperature Tirr < TN. An additional peak at TL ≊ 10 K indicates a long range magnetic order of the Tb ions with a possible spin reorientation of the Cu moments. The thermal irreversibility and nonlinearity of the dc magnetization below Tirr, the relaxation processes revealed by the ac susceptibility, and the typical logarithmic decay of the remanent magnetization, point to the existence in this material of a weak ferromagnetic state with spin-glass-like characteristics.
Controlled solidification and magnetic properties of Pr-Fe-B-Cu and Nd-Fe-B alloys
Authors: Faria R. N. , Abell J. S. , Harris I. R.
Published: July 2002

The as-cast microstructure of the alloys Pr20.5Fe73.8B3.7Cu2 and Nd16Fe76B8 have been modified using a vertical floating-zone (VFZ) technique. By annealing the as-cast alloys and VFZ material at 1000 °C, the magnetic properties increased substantially for Pr-Fe-B-Cu alloy. VFZ growth rates up to 38 cm/h have not been sufficient to reproduce the as-cast ingot microstructure of the Pr-Fe-B-Cu alloy. Cast magnets prepared in a very small chill mold showed the best magnetic properties, with a (BH)max=98 kJ/m3. Sintered magnets prepared using the hydrogen decrepitation process from the large grained Nd-Fe-B zoned alloys exhibit inferior squareness factors to those prepared from the as-cast ingot.
Magnetoresistance of Co/Pd and Co/Pt multilayer films for magneto-optical data storage applications
Authors: Hajjar Roger A. , Wu Te-ho , Mansuripur M.
Published: July 2002

A study of the galvanomagnetic, magnetic, and magneto-optical properties of multilayered Co/Pd and Co/Pt films is presented. The samples are of different Co content with fixed thickness for the evaporated Co/Pt series and different thickness with fixed composition for the sputtered Co/Pd series. The magnetic and electronic properties of these films are correlated with the magnetoresistance measurements performed with fields applied parallel and perpendicular to the plane of the samples. The s-d scattering observed when the magnetization is aligned with the applied magnetic field is interpreted in terms of the electronic band structure of the materials.
Study of the influence of the magnetoelastic energy in driving the magnetic and structural transition in TbCu
Authors: Ibarra M. R. , Marquina C. , Pavlovic A. S. , Ritter C.
Published: July 2002

TbCu suffers a structural transformation at TN = 117 K, with the establishment of a collinear antiferromagnetic order. On the other hand, YCu is a Pauli paramagnet, which also transforms structurally at Ts = 150 K. In order to test the influence of the magnetoelastic energy as the driving mechanism of the transition in TbCu, we have performed neutron diffraction experiments on the compound Tb0.6Y0.4Cu. From the thermal evolution of the structural and magnetic peaks, we have observed that both, structural (Ts) and magnetic transitions (TN), occur at different temperatures. This separation in the transition temperatures constitutes a strong indication that magnetoelastic energy is not responsible for the transition in TbCu, in spite of the large magnetostriction observed at low temperatures at the metamagnetic field induced transition.
Magnetic susceptibility, specific heat and 119Sn Mössbauer studies of the magnetic system Gd(In1−xSnx)3
Authors: Yuen Tan , Lin C. L. , Mihalisin T. , Bykovetz N.
Published: July 2002

Magnetic susceptibility, specific heat and 119Sn Mössbauer measurements have been carried out in the Gd(In1−xSnx)3 system. The Gd moments order magnetically across the entire series. The Néel temperature shows a W-shaped variation with concentration. A region (0.45≤x≤0.70) is found in which the alloys exhibit a second magnetic transition at temperatures below TN, similar to those found in some other RX3 compounds. The nature of the magnetic structures is discussed.
Thermodynamic measurements on geometrically frustrated magnets (invited)
Authors: Ramirez A. P.
Published: July 2002

Geometrical frustration occurs in antiferromagnetic systems with triangular coordination symmetry. An additional requirement for strong frustration is high local symmetry. We discuss the class of compounds which meet these two requirements and exhibit strong frustration. Novel cooperative effects related to geometrical frustration are discussed for specific compounds, SrCr9pGa12−9pO19, RCuO2 (R=rare earth) and Gd3Ga5O12.
Pressure-induced Mott transition in transition-metal iodides (invited)
Authors: Pasternak M. P. , Taylor R. D. , Jeanloz Raymond
Published: July 2002

Many of the transition-metal (TM) compounds, because of exchange and correlation interactions within the narrow and poorly overlapping d bands, become antiferromagnetic insulators, the Mott insulators (MI). The properties of the MI and their gradual transition into the noncorrelated metallic state (the Mott transition) are of crucial importance for the elucidation of high-temperature superconducting materials features in particular and to magnetism in general. The transition of the MI into a metal can be achieved either by doping or by high pressure. The first method is definitely inappropriate for studying the nature of the Mott transition; for the narrow-band materials the electronic and structural disorder inherent in doping has a strongly perturbing effect. To yield the definitive data on the Mott transition, high-pressure work on well-characterized materials should be sought. The present studies provide for the first time extensive information on the Mott–Hubbard gap closure induced by high pressure. High-pressure studies using diamond anvil cells were conducted with several (TM)I2 compounds. They all have layered structures and order antiferromagnetically at ambient pressure. 129I Mössbauer spectroscopy (MS) was used to study the properties of the (TM)2+ sublattice magnetization as a function of pressure and temperature, and x-ray diffraction was used to look for possible crystallographic transitions and to obtain the equation of state. Results show that the high-pressure transition at Pc from a magnetic to a nonmagnetic state is not accompanied by crystallographic changes. Previous studies [M. P. Pasternak et al., Phys. Rev. Lett. 65, 790 (1990)] with NiI2 have confirmed the presence of a metallic state at P ≳ Pc. Inherent to the pressure behavior of the magnetic state is the gradual increase of TN in all cases and a slight increase in the TM2+ moments with pressure increase. The collapse of the magnetic state is abrupt for some cases (NiI2) and gradual for others (CoI2), indicative of different band-overlap mechanisms.
Magnetocrystalline and magnetoelastic anisotropy in epitaxial Co/Pd superlattices
Authors: Engel Brad N. , England Craig D. , Van Leeuwen Robert A. , Wiedmann Michael H. , Falco Charles M.
Published: July 2002

We have used molecular-beam epitaxy (MBE) to grow Co/Pd superlattices along the three high-symmetry crystal axes: [001], [110], and [111]. Identical conditions were maintained for all depositions, and a series of samples of fixed Pd thickness (tPd = 10 ± 1 Å) and varying Co thickness (2 Å ≤ tCo ≤ 22 Å) were prepared for each orientation. A variety of in situ and ex situ characterization studies were made, which confirm these superlattices are single crystalline for all growth directions. The dependence of the uniaxial magnetic anisotropy energy on the Co thickness in these superlattices showed significant systematic differences for each of the three crystal orientations. These variations result entirely from differences in the volume contribution to the anisotropy. Estimates of the magnetocrystalline and magnetoelastic contributions for the (111) and (001) samples are in good agreement with the measured anisotropy energies of these oriented superlattices.
Heat treatment dependences of the ultrasound velocities in the Fe79Cr6.6B14.4 metallic glasses produced in magnetic fields
Authors: Kaczkowski Zbigniew , Kisdi-Koszó Éva , Potockỳ Ladislav
Published: July 2002

The aim of the investigations was to determine the influence of the magnetic and thermal treatments and of the magnetic bias field on the ultrasound velocities c in Fe79Cr6.6B14.4 metallic glass ribbons prepared in a longitudinal or transverse magnetic field. The values of c for the points near the demagnetization state and near the magnetic saturation were changing from about 4550–4600 m/s for as-cast state to 4700–4900 m/s after the last annealing at 360 °C. After the annealings between 280 and 350 °C the minimum values of c dropped to 4360 m/s and this phenomenon is connected with the ΔE effect and and with increasing of the magnetomechanical coupling coefficient k with annealing (up to about 0.25 after annealing at 350 °C from km = 0.1 for as-cast state). This material may be useful in magnetostrictive delay line applications.
Josephson magnetization of Y–Ba–Cu–O superconductors near the critical temperature
Authors: Ivanchenko Yu. M. , Lisyansky A. A. , Tsindlekht M. I.
Published: July 2002

Experimental and theoretical data for the rectifying properties in a ceramic Y–Ba–Cu–O cylindrical sample are presented. A sample is subjected to the influence of a very small perturbation just to have the linear rectifying regime. Theoretical analysis using the Josephson medium concept gives qualitative agreement with the observed experimental results.
Atomic structure in amorphous and multilayered films: Origins of magnetic anisotropy (invited) (abstract)
Authors: Yan X.
Published: July 2002

To understand the microscopic mechanism for the magnetic anisotropy, a description of the atomic structure, particularly structural anisotropy, is a crucial first step. Here is described an observation of structural anisotropy by synchrotron x-ray-diffraction experiments on two notable magneto-optical materials having perpendicular anisotropy, namely sputter-deposited amorphous TbFeCo films and 〈111〉-oriented Co/Pt multilayered films. In the as-deposited Tb26Fe62Co12 films, the scattering intensity I(Q) were measured with the scattering vector Q nearly perpendicular and nearly parallel to the film plane. Structural anisotropy was observed to be uniaxial and anelastic, suggesting the presence of the bond-orientational anisotropy, or an anisotropic distribution of the nearest-neighbor bond orientation. In a multilayered Co (3 Å)/Pt (18 Å) film epitaxially grown on a GaAs 〈111〉 substrate, scattering intensity was measured for the scattering vector Q covering a plane with a major component perpendicular and a minor component parallel to the film plane. The diffraction peaks were found to have a large width parallel to the film plane, indicating that there is strong in-plane disorder. Atomic interdiffusion was evaluated from the first two superlattice peaks by integrating the intensity in the film plane of the 〈111〉-oriented film, and was found to be similar to the 〈001〉-oriented film where magnetic easy axis is in the film plane. In the 〈111〉-oriented Co/Pt, however, a Lorentzian peak shape of the intensity distribution in the film plane was observed, indicating the presence of uniaxial structural defects creating 1/r strain field in the film plane. Both types of structural anisotropy could be responsible for the perpendicular magnetic anisotropy.
X-ray reflectometry on (SimGen)p short-period superlattices
Authors: Baribeau J.-M.
Published: July 2002

Grazing incidence x-ray reflectometry on molecular beam epitaxy grown (SimGen)p short-period superlattices is reported. Reflectivity curves are analyzed to estimate the structural perfection of samples grown on (100)Si and Ge substrates. Samples grown on Si with n<4 were found to have sharp interfaces (half a monolayer wide) but a structure with n≊8 had rough interfaces (two monolayers wide) consistent with a transition to a three-dimensional growth mode. Superlattices grown on Ge, have sharper interfaces than equivalent (with Si and Ge layer thicknesses reversed) structures on Si.
On the origin of Franz–Keldysh oscillations in AlGaAs/GaAs modulation-doped heterojunctions
Authors: Novellino R. A. , Vazquez-López C. , Bernussi A. A. , Schmidt C. , Cerdeira F. , Motisuke P. , Pollak F. H. , Meseguer F. , Ploog K.
Published: July 2002

We have performed a series of photoreflectance measurements in a modulation-doped AlGaAs/GaAs heterojunction containing a high mobility two-dimensional electron gas. Measurements were performed as a function of temperature in the range 2 K≤T≤300 K. We studied the Franz–Keldysh oscillations associated with the E0 transition of both the GaAs and AlGaAs. The fields obtained from these oscillations for both sides of the heterojunction are quite different. Also, the temperature dependence of these fields are radically different. In fact, the temperature dependence of the field in the GaAs side of the modulation-doped heterojunction sample is very similar to that of the field in a single undoped GaAs film deposited on a GaAs substrate, where no two-dimensional electron gas is present. This shows that the field producing the observed oscillations on the GaAs side of the modulation-doped heterojunction sample is not related to the field that confines the two-dimensional electron gas.
Correlation of time-resolved electroluminescence and cathodoluminescence measurements on quantum well light emitters with varying barrier widths
Authors: Krahl M. , Kirstaedter N. , Bauer R. K. , Bimberg D. , Meier H. P. , Harder C.
Published: July 2002

We present results of a comparative study of the time-dependent luminescence properties of multiple quantum well structures with varying barrier widths which are embedded in the active area of a light-emitting device. The carrier kinetics is investigated by different experimental approaches: Cathodoluminescence and electroluminescence experiments where excitation is on/off-modulated for the purpose of time-resolved measurements and time-resolved electroluminescence experiments in the small signal regime which allow for observation of the carrier kinetics under flatband conditions. Due to the exact determination of the excess carrier density the latter technique provides a sensitive tool for a precise estimation of the mono- and bimolecular recombination coefficients. Comparison with light output data yields radiative and nonradiative parts. We find that coupling of quantum wells dramatically favors nonradiative interface recombination as expected from a theoretical model accounting for the superlattice wavefunctions. On the other hand, the bimolecular recombination rate remains unaltered even when the barrier width is lowered from 18 to 0.9 nm. In contrast, on/off modulated experiments reveal that luminescence decay is strongly influenced by carrier drift out of the active area. A barrier width dependent carrier mobility in growth direction accounts for these results if phonon assisted hopping rather than Bloch transport is presumed. Thus, an estimation of device quality of quantum well light emitters by conventional time-resolved cathodo- (or photo-) luminescence experiments is found to be possible if internal field induced carrier drift processes are taken into account.
The effects of majority carrier diffusion on the minority carrier radiative lifetime in thin layer surface free GaAs/AlGaAs structures
Authors: Rimmer J. S. , Hawkins I. D. , Hamilton B.
Published: July 2002

It has been recently reported that the minority carrier radiative lifetime in thin layers sandwiched between degenerately doped barrier regions or heterojunction barriers is much reduced by the diffusion of majority carriers into the active layer from the barrier regions. We show here that the radiative lifetime is determined by the average majority carrier concentration which is dependent on active layer thickness and may be significantly higher than the dopant concentration in this layer. Our experimental results are in close agreement with this model. We have simulated a number of structures where the minority carriers are confined by either homojunctions or heterojunctions and have derived the radiative lifetime as a function of active layer thickness. This dependence of radiative lifetime on layer thickness is not embraced in the analysis normally used for example in interface studies but is shown to have a significant effect on the interpretation of results obtained from measurements of interface recombination velocity leading to erroneous conclusions if not taken into account.
Particulate recording tapes studied with neutron depolarization
Authors: Rosman R. , Rekveldt M. Th.
Published: July 2002

The size and shape of the magnetic inhomogeneities in various particulate recording tapes have been studied using neutron depolarization (ND). The tapes are in the remanent magnetic state either during the dc-demagnetization reversal process or after bulk erasure, ac erasure, or thermal annealing. The magnetic correlation length ζ measured along the direction perpendicular to the plane of the tape (Z direction) is at most twice the particle diameter in the dc-(de)magnetized tapes, indicating the existence of clusters and switched volumes containing only a few particles along Z. The switched volumes are the magnetic inhomogeneities in the demagnetized state. A cluster is a group of particles in which the mean particle orientation or density differs from the mean particle orientation and density within the tape. The size along Z of the switched volumes after bulk or ac erasure is slightly smaller than that of switched volumes after dc demagnetization. After thermal annealing, the magnetic correlation length is smallest and corresponds either to magnetically uncorrelated particles or to weak negative magnetic correlations along Z. The latter would indicate a weak antiparallel orientation of the magnetization in neighboring particles in the Z direction. From ND experiments in which the direction of the neutron beam is varied in the YZ plane (length direction tape along Y), it follows that the switched volumes and clusters are elongated along Y, with aspect ratios generally larger than ten. The clusters in Philips CrO2 video tapes are oriented mainly in the YZ plane. The switched volumes in ac- or bulk-erased TDK Co-modified γ-Fe2O3 video tapes closely resemble the particle volume.
Magnetic field of a probe head of general shape with ideal windings
Authors: Owen D.
Published: July 2002

The magnetic field of probe heads of general shape with ideal windings is computed for heads of any thickness and distance of the windings from the curvilinear region of the head. This work closely follows and complements that of Minuhin [J. Appl. Phys. 57, 4006 (1985)].
Electrical conduction in impregnants for all-film power capacitors
Authors: Brosseau Christian
Published: July 2002

The conduction properties of some impregnants (monodibenzyltoluene and phenylxylylethane) used in the all-film capacitors technology are investigated. The intent of this research contribution is to perform detailed investigations of these properties in a substantial range of electric fields (from 10−2 to 102 V μm−1). Measurements of the macroscopic charge transport properties (conductivity, mobility, and viscosity) at low field are reported for the temperature range 293–373 K. Evidence of ionic conduction is found from these results. It is possible to evaluate the ionic dissociation parameter of ionic species [e.g., TBAP (tetrabutylammonium picrate)] by studying solutions of known electrolytes in well-purified solvents. Combining this information with the corresponding investigation in well-purified liquids, it is found that the residual ionic conductivity of these fluids corresponds to 10−10–10−11 mol/l equivalent TBAP. In the high-field region, particular emphasis is given to the quantitative interpretation of the current (electrical losses) versus applied field characteristics. Bulk effects associated with the field-enhanced ionic dissociation (Onsager theory) are observed when the impregnant contains an ionic surfactant such as sodium di-(2-ethylhexyl) sulphosuccinate, while unipolar injection in the neighborhood of the metallic electrodes is evidenced in solutions containing small amounts of ammonium picrate electrolytes (TBAP, triisoamylammonium picrate). It is found here that the high-field conduction mechanisms in purified liquids can be attributed to a charge injection phenomena. A physical analysis that satisfactorily predicts the high-field conduction behavior of these fluids is further proposed and discussed in connection with recent numerical simulations.
Impurity-related optical absorption spectra of GaAs-Ga1−xAlxAs cylindrical quantum-well wires
Authors: Porras-Montenegro N. , Latgé A. , Oliveira Luiz E.
Published: July 2002

The optical-absorption spectra associated with transitions between the n=1 valence subband and the donor-impurity band and between the acceptor-impurity band and the n=1 conduction subband were calculated for GaAs-Ga1−xAlxAs cylindrical quantum-well wires of different radii. The distribution of impurities in the wire was assumed to be homogeneous and interaction between the impurities was neglected. The main features in the theoretical spectra were an absorption edge associated with transitions involving impurities at the center of the wire and a peak related to impurities at the edge of the wire. Calculations were performed for both infinite and finite potential barriers, and were compared with previous results in quantum wells of comparable dimensions.
Experimental and computational analysis of ionized cluster beam deposition
Authors: Turner Dave , Shanks Howard
Published: July 2002

Ionized cluster beam (ICB) deposition has received considerable attention since its introduction in 1972 by Takagi et al. at Kyoto University because of its potential for low-temperature film growth. While further investigation of many aspects of ICB deposition is warranted, it is first necessary to determine with some certainty whether large clusters are being produced. A complete analysis of the Eaton ICB source involving computer calculation of the potential fields and computer simulation of the electrons and ions as they react to and influence these fields provides an in-depth understanding of the dynamics that influence the final ion beam characteristics. A high-resolution time-of-flight mass spectrometer was developed to experimentally investigate the cluster size distribution. No evidence of large clusters was found down to a level more than two orders of magnitude below what the Kyoto University group has reported [T. Takagi, Ionized-Cluster Beam Deposition and Epitaxy, (Noyes, New Jersey, 1988)]. A computer analysis of the three Kyoto University cluster size experiments that form the foundation of ICB has shown that the potential fields in the ionization areas are critically distorted by either space-charge effects or design flaws, both of which are serious enough to invalidate the experiments. The theory behind large cluster production and the body of indirect evidence attributed to the presence of large clusters are not convincing by themselves so it is concluded that a Takagi-type source does not produce large clusters in quantities capable of affecting film growth.
Uncertainties in visibility and phase response functions of optical single-particle counters using the laser Doppler velocimeter optical configuration
Authors: Son Jung-Young , Giel T. V.
Published: July 2002

Particle sizing uncertainties were calculated for the visibility and phase response functions of a dual-beam laser Doppler velocimeter. This uncertainty primarily results from the nonmonotonic behavior of the response functions versus particle size and varies with particle index of refraction and the velocimeter optical configuration, in particular the receiving lens diameter and location. An exact Mie-scattering algorithm predicted the velocimeter response functions for a number of particle indices of refraction and a variety of receiving lens configurations. Uncertainties were determined from the predicted particle size parameter increments corresponding to unit increments in the associated response functions. When examining uncertainties for particles with different refractive indices, the minimum and maximum response function values were used to obtain the associated size parameter increments. Particle sizing uncertainties tend to increase with the particle index of refraction for both visibility and phase. Receiving lens locations for minimal size measurement uncertainties are generally in the backscatter direction for visibility, and in the forward scattering direction near the incident beam plane, with a zenith angle of about 60°, for phase. A large receiving lens (f/♯≊1) minimizes uncertainties for visibility while a moderate lens (f/♯≊2) minimizes uncertainties for phase. Phase generally provides small uncertainties for particles with the same index of refraction but visibility provides smaller uncertainties when the particles have a range of refractive indices.
Analysis of visible surface-emitting second-harmonic generators
Authors: Vakhshoori Daryoosh
Published: July 2002

Analysis of blue/green/red coherent light radiated by surface-emitting nonlinear generators is presented. It is shown that there are possibilities of obtaining efficient mW visible light out of these devices. Several problems in the calculation of the efficiency of these structures are discussed. These include the incorporation of the second-harmonic reflections at the layer boundaries, the field and power continuity requirements, relation of device performances on different substrate orientations due to symmetry of the III-V crystals, and an analysis of active surface-emitting second-harmonic generators.
A new functional, resonant-tunneling bipolar transistor with a superlattice emitter
Authors: Liu Wen-Chau , Lour Wen-Shiung
Published: July 2002

We report a new resonant-tunneling bipolar transistor (RBT), in which electrons are injected from emitter to base by resonant-tunneling through the minibands in the i-AlGaAs/n+-GaAs superlattice emitter. The main feature of the device is the significant double negative-differential-resistance both at room temperature and low temperature. Two high peak-to-valley (PV) current ratios of 4:1 and 2.6:1 were obtained at 77 K. In the common emitter configuration, bistable output currents exist as controlled by the base-emitter voltage at critical values of VBE = 2.5 V and VBE = 3 V, respectively. In addition, two negative transconductance operation regions were obtained when the base-emitter voltage was applied. Yet, three different transistor action regions occur as the control base current is being applied. For base currents that are not high enough to bring the base-emitter junction to flat-band condition, common-emitter current gain up to 65 was obtained. This is the highest value ever reported in an RBT device using GaAs based materials. As control base current increases sufficiently to cause the base-emitter voltage drop beyond the flat-band condition, two different transistor action regions with smaller current gains of 38 and 35 are found. Furthermore, the first peak current is nearly equal to the second peak current and much larger than the second valley current. Thus the proposed device is attractive for multiple-valued logic circuits and frequency multipliers.
Scanning tunneling microscopy on rough surfaces-quantitative image analysis
Authors: Reiss G. , Brückl H. , Vancea J. , Lecheler R. , Hastreiter E.
Published: July 2002

In this communication, the application of scanning tunneling microscopy (STM) for a quantitative evaluation of roughnesses and mean island sizes of polycrystalline thin films is discussed. Provided strong conditions concerning the resolution are satisfied, the results are in good agreement with standard techniques as, for example, transmission electron microscopy. Owing to its high resolution, STM can supply a better characterization of surfaces than established methods, especially concerning the roughness. Microscopic interpretations of surface dependent physical properties thus can be considerably improved by a quantitative analysis of STM images.
An electroreflectance study of n- and p-type gallium arsenide in aqueous electrolytes
Authors: Batchelor R. A. , Hamnett A. , Peat R. , Peter L. M.
Published: July 2002

Electroreflectance was used to study the variation of the space-charge voltage with applied potential for n- and p-type GaAs in a series of aqueous electrolytes. The results were modeled by an intermediate-field Franz–Keldysh theory that included the electric-field variation within the depletion layer. Comparison of the theoretical and experimental spectra effectively allowed the space-charge voltage to be measured at each applied potential. In all cases the space-charge voltage was found to vary much more slowly than the applied potential on approach to the expected flatband potential and it was possible to characterize in some detail the changes in the ac and dc potential distribution that occurred. In addition, the rapid Fermi-level pinning of p-type GaAs in 0.1 mol KOH was found to be passivated by small concentrations of polysulfide ions. Spectra that were measured under either acidic or alkaline conditions showed quite large changes in line shape with applied potential, as expected. However, for n-type GaAs in pH-5 0.1 mol Na2SO4, little line-shape variation occurs and the space-charge voltage is shown to be fixed close to 0.3 V over a large range of applied potential.
The measurement of second-harmonic generation in novel ferroelectric liquid crystal materials
Authors: Liu Jian-Yu , Robinson Michael G. , Johnson Kristina M. , Walba David M. , Ros M. Blanca , Clark Noel A. , Shao Renfan , Doroski David
Published: July 2002

We present the analysis of experimental results on second-harmonic generation in a novel ferroelectric liquid crystal (FLC) material, o- nitroalkoxyphenyl biphenylcarboxylate 1, synthesized specifically for large nonlinear coefficients. All nonzero dij coefficients have been measured, with the largest being d22 = 0.6 ± 0.3 pm/V. Relative values of refractive indices are also given suggesting uniaxial optical properties with large birefringence (nz − nx≊ 0.20 at λ=1.064 μm). This material demonstrates the flexibility of FLC molecular synthesis to form thermodynamically stable macroscopically aligned materials for second-order nonlinear optics applications.
In situ neutron diffraction study of lattice deformation during oxygen precipitation in silicon
Authors: Liss K. D. , Magerl A. , Schneider J. R. , Zulehner W.
Published: July 2002

We present the first in situ measurement of the evolution of strain fields due to oxygen precipitation in silicon single crystals by means of high-resolution neutron backscattering. The integrated reflecting power R and the lattice parameter variations Δd/d which are directly related to the strain fields have been measured as a function of temperature and annealing time. In the temperature range from 300 to 1185 K, high purity float zone crystals maintain the R-values characteristic for perfect crystals. In contrast, Czochralski-grown crystals which contain on the order of 1018 oxygen atoms cm−3 (20 ppm), show a steep increase in reflectivity starting at 1160 K, and which goes through a maximum at 1350 K. At 1456 K, partial annealing occurs with a time constant of several hours.
Maximized photorefractive holographic storage
Authors: Maniloff Eric S. , Johnson Kristina M.
Published: July 2002

Recording angularly multiplexed holograms in photorefractive media requires an understanding of both the recording and erasure dynamics. In this paper a coupled-wave analysis is used to describe both the recording and erasure processes. This analysis has been applied to the recording of multiplexed holograms, resulting in a procedure to record maximum diffraction efficiency holograms. Experimental results confirming the theory for both the dynamics of a single exposure and the efficiencies of multiple exposures are presented. Using an uncoupled recording wave analysis, an expression for the dependence of the diffraction efficiency on the number of exposures in the case of equal record/erase time constants is presented. Approximate results are presented for the case of unequal time constants. This serves to set an upper limit on the diffraction efficiencies for a given saturation index of refraction modulation.
Electromagnetic waves in a suddenly created Lorentz medium and plasma
Authors: Stanić Božidar V.
Published: July 2002

The dispersion relations for the new waves excited by a suddenly created Lorentz media are derived and presented by the use of a Brillouin-like diagram. The amplitude and power transmission and reflection coefficients for these new modes are derived and discussed. It is shown that less than 4% of the pump wave power is transferred to the reflected modes.
Hall-effect measurement and a defect model analysis of surface-potential changes in GaAs
Authors: Miller W. R. , Stillman G. E.
Published: July 2002

Recently, increases of the surface potential in n-type epitaxial GaAs after NH4OH treatment studied using Hall-effect measurements, were reported. In this work, complementary results for p-type samples show decreases in band bending, again in agreement with the predictions of the advanced unified defect (AUD) model for the surface-interface states. From this and other data, a working mathematical model based on the AUD model is developed. Probable values for its parameters are determined which suggest a physically reasonable mechanism to explain the surface-potential changes.
Optical gain control model of the quantum-well laser diode
Authors: Ahn D. , Yoo T.-K. , Mendez E. , Chuang S. L.
Published: July 2002

The effects of various structure parameters such as graded potential, strain, quantum-well size, barrier heights, and temperature on the optical gain of single quantum-well lasers are studied for a potential solution to the problem of device optimization from the unified point of view with a rigorous model for gain. This study on the structure parameter dependence of the gain with the valence band mixing as well as intraband relaxation is new and presents a unified scheme for the optical gain control. Significant enhancement of gain in the graded quantum well as compared with that of the ordinary square quantum well is predicted. With the uniaxial stress, the TE mode gain is suppressed while the TM mode gain is enhanced due to the change of valence band structure. Calculated results show that the roles played by structure parameters such as well width, barrier heights, and temperature on the optical gain becomes of great importance for the device optimization.
Oriented PbTiO3 films from metalorganic precursors
Authors: Wang Yuguo , Zhang Peilin , Qu Baodong , Zhong Weilie
Published: July 2002

PbTiO3 films on various substrates were prepared by a metalorganic decomposition process using a multilayer spinning technique. Films on LaAlO3 and MgO substrates were preferentially c-axis oriented, and those on SrTiO3 have a completely c-axis orientation. The mechanism leading to the orientation was discussed. Contrary to the c-axis orientation, a-axis orientation can be induced on otherwise random films if they were annealed under pressure.
Investigation of crystallization of a mechanically alloyed Sm-Fe alloy
Authors: Lue M. Q. , Wang K. Y. , Miao W. F. , Song Q. H. , Sun W. S. , Wei W. D. , Wang L. B.
Published: July 2002

The crystallization of a mechanically alloyed Sm-Fe alloy was investigated. The results show that the Sm-Fe alloy prepared by mechanical alloying consists of amorphous Sm-Fe phase and crystalline α-Fe phase. The composition of the alloy is inhomogeneous, i.e., the surface of the as-milled powder is relatively poor in iron. The crystallization process involves the long-range diffusion of iron atoms and solid state reaction. After proper crystallization, the as-milled powder transforms into a Sm2Fe17 phase completely; no distinguishable crystalline α-Fe phase can be found. A metastable phase, which may be a Sm2Fe17 phase with the structure of hexagonal Th2Ni17 type, appears during the crystallization process.
Limits to the fundamental measurement accuracy in Josephson junction array voltage intercomparisons
Authors: Dunlap D. H. , Duncan R. V.
Published: July 2002

The dc voltage output from a hysteretic Josephson junction which is locked to an ac frequency source differs from the ideal Josephson relation if the junction drives a current about a closed superconducting circuit. The difference in voltage ΔV from two hysteretic Josephson junctions driven in series opposition is proportional to the difference in their driving frequencies Δω if the junctions are each biased to the nth voltage step. It is shown here, however, that ΔV is systematically smaller than the voltage difference ΔV0 predicted by the ideal junction relation ΔV0=(nℏ/2e)Δω. If the loop inductance approaches zero, the smallest detectable voltage difference ΔV0 between two junctions is limited by the intrinsic Josephson inductance. For arrays of more than one junction, however, ΔV0 remains proportional to the loop inductance.
Measurement of the third-order electronic susceptibility of lanthanum-neodymium hexa-aluminate crystals
Authors: Agnesi A. , Gabetta G. , Reali G. C.
Published: July 2002

The third-order susceptibility of lanthanum-neodymium hexa-aluminate (LNA) crystals has been measured by the z-scan technique to be (0.13±0.02)×10−13 esu. Thermal contributions are estimated to be two orders of magnitude lower than the measured value. The nonlinearity is believed to be due to bound electronic effects, and the linear absorption spectrum of LNA supports the interpretation of a nonresonant nonlinear refractivity casual consequence of two-photon absorption.
Low-pressure plasma columns sustained by traveling electromagnetic surface waves in the dipolar (m=1) mode
Authors: Benova E. , Zhelyazkov I. , Ghanashev I.
Published: July 2002

An extension of a theoretical study of a plasma column surrounded by vacuum and sustained by an electromagnetic surface wave in the dipolar mode [E. Benova, I. Ghanashev, and I. Zhelyazkov, J. Plasma Phys. 45, 137 (1991)] is presented. Following the same formalism the plasma column and the wave characteristics have been calculated taking into account the presence of a dielectric container in order to improve the agreement between theory and experiment.
Mechanisms for GaAs surface passivation by a molecular beam epitaxial cap layer grown at 200 °C
Authors: Look D. C. , Walters D. C. , Stutz C. E. , Evans K. R. , Sizelove J. R.
Published: July 2002

A thin, undoped, molecular beam epitaxial (MBE) GaAs cap layer grown on top of an n-type conductive layer significantly reduces the free-electron depletion from the latter. By analyzing electron transfer to surface, interface, and bulk acceptor states in the cap, as a function of cap thickness, we show that either (1) the usual EC−0.7 eV surface states are absent, (2) a dense donor near EC−0.4 eV exists or (3) a high donor interface charge (∼5×1012 cm−2) is present. Any of these conclusions constitutes an important new aspect of low-temperature MBE GaAs.
Characterization of thin Al films using grating coupling to surface plasma waves
Authors: Zaidi Saleem H. , Reicher D. W. , Draper B. L. , McNeil J. R. , Brueck S. R. J.
Published: July 2002

A detailed characterization of the optical, microstructural, and electrical properties of thin (5–50 nm) Al films grown by thermal evaporation, magnetron sputtering, and ion-assisted sputtering (IAS), is reported. Dielectric-function measurements were carried out by using grating coupling to surface plasma waves (SPW) and, for comparison, ellipsometric measurements were also performed. Scanning electron microscope (SEM) studies of film microstructure as well as dc electrical resistivity measurements were carried out and correlated with the optical data. Using the Bruggeman effective media approximation, good agreement was obtained for thicker films (30–50 nm), but not for thinner films (<30 nm). SEM and resistivity measurements suggest that conditions of film growth influence the behavior of individual grains, resulting in increased electron reflectance at the grain boundaries with increasing energy delivered to the substrate during deposition. This resulted in lower electrical resistivities for evaporated films than for IAS films. Finally, the influence of 5–20 Å Al2O3 on thick Al films was investigated: Both SPW and resistivity measurements suggest that the oxide film was not confined to film surface, but had penetrated inside the film leading to much higher electrical resistivities than would be otherwise expected.
The time evolution of the ion density and the gas temperature in pulsed radio frequency plasmas
Authors: Visser R. J. , Baggerman J. A. G. , Poppelaars J. P. J. , Collart E. J. H.
Published: July 2002

The kinetics of the formation and decay of pulsed nitrogen rf plasmas have been investigated with time and spatially resolved optical emission and laser induced fluorescence spectroscopy. At a gas pressure of 3 Pa the decay of the N2+ ions, after the power is switched off, is caused by dissociative recombination and ambipolar diffusion to the wall. At higher pressures (30 Pa) the formation of N4+ has to be taken into account. An accurate method to determine the ion density from the second order decay is presented. A numerical model of the time evolution of the ion concentration profile is developed, which renders good agreement with the experimental data. Kinetic measurements of the rotational (Tr) and vibrational (Tv) temperature of N2+ show that Tr and Tv of N2+ are in equilibrium with Tr and Tv of the N2 gas. The vibrational temperature is, however, much higher than the rotational temperature. This observation can be readily explained on the basis of the results of the kinetic measurements. The experimental excitation and cooling rates for the translational, rotational, and vibrational energy levels of the N2 molecules are compared to simulations.
Behavior of excess As in nonstoichiometric Si-doped GaAs
Authors: Winer K.
Published: July 2002

The Si doping efficiency (≡[e−]/[Si]) in homoepitaxial GaAs has been measured as a function of molecular beam epitaxial growth conditions (As pressure, substrate temperature, and growth rate) and post-growth thermal annealing conditions. The doping efficiency decreases from one under optimal growth conditions to almost zero: (1) exponentially with inverse substrate temperature below 400 °C and (2) as a power law in the As4-to-Ga beam-equivalent fluence ratio above unity. The doping efficiency of nonoptimally grown films increases from almost zero to near one upon post-growth thermal annealing above 650 °C with slower than exponential kinetics. These changes are attributed to charge trapping into and release from, respectively, excess-As-related defects, which are incorporated during growth and removed by out-diffusion during annealing.
Direct laser indiffusion of Ti into LiNbO3 single crystals
Authors: Bertolotti M. , Rossi M. , Ferrari A. , Craciun V. , Mihailescu I. N. , Ursu I. , Mikhailova G. N. , Dhaul A. , Chander R.
Published: July 2002

The successful indiffusion of Ti into single-crystalline LiNbO3 substrates is reported as a result of high-intensity ruby laser irradiation of LiNbO3 samples coated with films of Ti of 400 Å thickness. The experimental data are compared to the results of cw CO2 or Ar ion laser irradiations. We conclude that the Ti indiffusion process starts with the oxidation of Ti and continues in a liquid phase.
Effects of plasma and/or 193 nm excimer-laser irradiation in chemical-vapor deposition of boron films from B2H6+He
Authors: Komatsu Shojiro , Kasamatsu Mitsuo , Yamada Kawakatsu , Moriyoshi Yusuke
Published: July 2002

Plasma-enhanced chemical-vapor deposition (PECVD) with the surface irradiated by a 193 nm, 50 Hz pulsating laser was performed to synthesize boron films from B2H6+He at a pressure of 200 Pa, where the plasma was employed to generate precursor radicals for the growth while the irradiation was intended for photochemical enhancement of the surface processes such as migration and growth reactions. In addition to the PECVD, PECVD without the irradiation as well as pyrolytic CVD with and without the irradiation were done so as to clarify the effects of the plasma and the laser in the CVD. Micromorphological boron columns were found to grow toward the laser light, and this indicated directly that the surface growth reactions were enhanced photochemically at the laser energy density of 170 mJ/cm2 per pulse. Heating of the surface by the irradiation at this energy density was estimated to be negligible. In the pyrolytic CVD, where the surface migration of the precursor radicals was considered to be relatively hindered according to measured activation energies, a morphological change was found to indicate irradiation-enhanced migration at the laser energy density of 3 mJ/cm2 per pulse. Semiempirical molecular orbital calculations predicted that borane molecules (BH3) should be the dominant precursor in the pyrolytic CVD while the counterpart could be BH2 radicals in the PECVD: This prediction supports a hypothetical photoinduced growth reaction such as BH*2+ nhν = B(s) + H2 (n=1,2, ...) where BH*2 signifies a BH2 radical chemisorbed to a site for growth reaction and B(s) does a boron atom incorporated into the solid structure. This prediction was also consistent with the experimental result that crystalline boron films have grown only in the PECVD while the pyrolytic CVD yielded only amorphous growth at substrate temperatures between 690 and 890 °C. The predicted precursor BH3 for the pyrolytic CVD suggested its photoinduced migration mechanism similar to the photolysis of diborane.