bluuebunny/crossref_metadata_2025 huggingface dataset:
1.54 μm electroluminescence from erbium-doped SiGe light emitting diodes
Authors: Chang Shoou-Jinn , Nayak Deepak K. , Shiraki Yasuhiro
Published: July 2002

Er-doped SiGe light emitting diodes were fabricated by implanting Er3+ ions into SiGe epi-layers. The fabricated SiGe: diodes show good current–voltage characteristics with a typical reverse breakdown voltage between 10 and 14 V. In the forward bias region, the ideality factor η is found to be 1.84, which indicates that the forward current is dominated by space charge recombinations. By injecting minority carriers into the diodes, Er3+ related emission was observed in the 1.54 μm region at 77 K. To obtain the maximum electroluminescence intensity, the post-implantation annealing under vacuum condition was found to be 800 °C for 30 min.
Blueshift of effective band gap in n-i-p-i doping superlattices as a function of optical excitation intensity
Authors: Bastola Subas , Chua S. J. , Xu S. J.
Published: July 2002

It is a well-established fact that the luminescence peak arising from spatially indirect transitions (effective band gap Egeff) of a n-i-p-i structure shifts to a higher energy when the intensity of the excitation is increased. In this article, the quantitative dependence of the effective band gap on the excitation light intensity IW is formulated and verified experimentally. Using AlGaAs based n-i-p-i samples, the plot of ln(IW) versus Egeff gives a slope of value 4×10−21 eV−1 which agrees well with the theoretical value of 5.2×10−21 eV−1. It is found that this blueshift is typically 50 meV for a doubling in the magnitude of the optical excitation intensity from 50 to 100 mW cm−2. Further, it is found that the carrier lifetime decreases exponentially with an increase in the excitation intensity from zero to 500 mW cm−2 by four orders of magnitude which also agrees well with our theoretical model.
Transport property of In0.5Ga0.5P layers grown by liquid phase epitaxy
Authors: Yoon I. T. , Oh S. J. , Park H. L.
Published: July 2002

The temperature dependent Hall mobility and carrier concentration of In0.5Ga0.5P epilayers grown on (100) semi-insulating GaAs substrates by the liquid phase epitaxy technique have been investigated in the range of 77–300 K. The electron mobility and carrier concentration were of the order of 806 cm2 V−1 s−1, and 6.5×1016 cm−3, respectively at 300 K. The model (taking into account ionized impurity, alloy and space-charge scattering mechanisms) is considered in order to properly portray the observed features of the electron mobility results. The theoretical prediction is in good agreement with the observed results. The electron mobility was limited by ionized scattering up to 120 K and was also limited by alloy, space-charge scattering up to 300 K. It was found that the space-charge scattering has a stronger temperature dependence, T−1, than the usual T−0.5 and the In0.5Ga0.5P epilayer was heavily compensated for having an acceptor concentration of NA=8.5×1016 cm−3 with a compensation ratio of 0.5.
Counterdoped very shallow p+/n junctions obtained by B and Sb implantation and codiffusion in Si
Authors: Solmi Sandro
Published: July 2002

In this article we investigate the B and Sb codiffusion upon postimplantation annealing in order to fabricate very shallow p+/n junctions (⩽70 nm), suitable for a complementary metal–oxide–semiconductor technology with a channel length of 0.18 μm. The junctions are prepared by implanting Sb and subsequently BF2, at a higher dose, in an n-type Si substrate. The preamorphization with Sb avoids the B channeling and increases the n-type doping in the junction region, thus confining the depth of the p layer. Furthermore, both the transient enhanced diffusion, being the B implanted in a preamorphized layer, and the standard diffusion, due to the pairing between donors and acceptors, are strongly reduced. This procedure allows us to obtain very shallow junctions even after annealings with relatively high thermal budget, like 800 °C/8 h or 900 °C/1 h, or 950 °C/10 min or 1000 °C/60 s. We verified that dopant diffusion is strongly affected by a direct donor–acceptor interaction, and that good prediction of the experimental results can only be obtained using a simulation code which takes into account the formation of neutral, near immobile, Sb–B pairs.
Analysis of a Mach-Zehnder interferometry measurement of the Pockels coefficients in a poled polymer film with a reflection configuration
Authors: Shin M. J. , Cho H. R. , Han S. H. , Wu J. W.
Published: July 2002

Mach-Zehnder interferometry is employed to measure the Pockels coefficients in a poled thin polymer film, which serves as a reflection mirror in the sample arm of the interferometer. As a complete optical characterization of the electro-optic polymer film, the modulated light intensity of the Mach-Zehnder interferometer is investigated as a function of the optical bias in the reference arm, the modulation voltage applied to the film, the polarization angle of the incident light, and the angle of incidence on the film. The Mach-Zehnder interferometry measurement of the Pockels coefficients in the reflection configuration has an advantage over single-beam polarization interferometry in permitting the independent determination of the Pockels tensor components, r13 and r33. Particularly, in a reflection configuration two-beam interferometric measurement, a proper consideration of the optical path change due to the refractive angle change is found to be critical in determining the absolute values of the Pockels coefficients.
Ensemble Monte Carlo analysis of self-heating effects in graded heterojunction bipolar transistors
Authors: Kumar T. , Cahay M. , Roenker K.
Published: July 2002

Self-heating effects in graded AlGaAs/GaAs heterojunction bipolar transistors are analyzed using an ensemble Monte Carlo description of electron transport from emitter to subcollector including space-charge effects through a self-consistent solution of Poisson’s equation. For a given collector current density, the temperature rise in the intrinsic portion of the device is calculated using a simple analytical treatment. The temperature dependence of the scattering rates and various device parameters is included in the Monte Carlo simulations. The extent of velocity overshoot in the collector and its dependence on the device temperature is analyzed. The collector delay time is most affected by self-heating effects leading to a degradation of the unity current gain cut-off frequency of graded heterojunction bipolar transistors. For the biasing conditions considered here, the unity current gain cut-off frequency is found to be as much as 18% below its value calculated while neglecting self-heating effects.
Temperature dependence of galvanomagnetic properties for Gd doped and undoped p-type GaSe
Authors: Gürbulak B. , Yildirim M. , Tüzemen S. , Efeoǧlu H. , Yoǧurtçu Y. K.
Published: July 2002

The magnetoresistance and Hall effect measurements were carried out in undoped p-GaSe and Gd doped p-GaSe (p-GaSe:Gd) samples in the temperature range 90–320 K and 60–320 K, respectively. The transverse and longitudinal magnetoresistance coefficients for p-GaSe and p-GaSe:Gd samples decrease with increasing temperatures and were found to be MJ⊥B and MJ∥B∝T−2.37 (90⩽T⩽320 K) for p-GaSe and MJ⊥B and MJ∥B∝T−2.09 (60⩽T⩽320 K) for p-GaSe:Gd. The carrier concentration obtained from the Hall effect measurements in the p-GaSe:Gd sample increases up to 100 K, decreases in the range 100–140 K, increases in the range 140–280 K, and decreases in the range 280–320 K, although the carrier concentration in the p-GaSe sample continuously increases up to 320 K. Impurity energy levels calculated from ln(p/T3/2) vs 103/T for p-GaSe in the range 100–140, 140–220, and 220–320 K are Ec−216 meV, Ev +322 meV, and Ev +573 meV, respectively, and for p-GaSe:Gd in the range 100–140, 140–260, and 280–320 K are Ec−224 meV, Ev+330 meV, and Ec −592 meV, respectively. In the same samples, the temperature dependence of the Hall mobilities of the p-GaSe and p-GaSe:Gd were found to be μH∝T−2.09 (180<T<320) and μH∝T−1.68 (180<T<320), respectively. The Hall mobilities for p-GaSe and p-GaSe:Gd are independent from the magnetic field at all sample temperatures. The electrical conductivities for both samples increase with increasing temperature and the electrical conductivity for p-GaSe is greater than that of p-GaSe:Gd for T>120 K.
Negative magnetoresistance in (InSb)1−xYx at low temperature
Authors: Yang J. , Heremans J. , Partin D. L. , Thrush C. M. , Naik R.
Published: July 2002

We report the low temperature negative magnetoresistance and magnetization of molecular beam epitaxy grown (InSb)1−xYx for a range of yttrium concentrations (0.03%⩽x⩽4.0%). Our experimental results from x-ray diffraction, Hall effect, and magnetization measurements suggest that the yttrium atoms are located as interstitials in the InSb lattice and hence do not hybridize their outer shell d electrons with the InSb band leading to localized moments. Although the magnetization versus magnetic field (B) data do not fit brilliouin function for all the temperatures studied, we clearly observe a correlation between the measured negative magnetoresistance and the magnetic moment. We believe that the s–d exchange interaction between the localized electrons of Y atoms and the conduction electrons gives rise to the large negative magnetoresistance observed. A theoretical model is used to explain the scattering mechanism. According to this model, when the conduction electrons in the lower subband of InSb have energy E smaller than the Zeeman splitting AM, where A is the s–d exchange integral and M is the magnetic moment, the lower subband conduction electrons are scattered from the d spins without changing their spin projections. On the other hand, as E becomes greater than AM, the spin-flip process can be thermally activated. For the conduction electrons in the upper subband, both spin-flip and spin-non-flip coexist. Therefore the model uses two distinctive relaxation times rather than just one. A numerical calculation is performed to fit the relation between the magnetoresistance and the magnetic moment of the yttrium atoms.
Raman investigation of molecular beam epitaxy grown InGaAlAs epilayers lattice matched to InP for low Al concentrations
Authors: Keeler W. J. , Keeler G. A. , Harrison D. A. , Wasilewski Z. R.
Published: July 2002

Room temperature Raman measurements on a series of molecular beam epitaxy grown In0.53Ga0.47−xAlxAs epilayers lattice matched to InP substrates have been carried out for the composition range 0⩽x⩽0.08. This low Al composition range is important for tailoring the well gap energy in communication device fabrication. The Raman data show three-mode phonon behavior for films with Al concentrations as low as x=0.03. In addition to detecting the fundamental one-phonon scattering processes, overtone, and combinations of the GaAs-like and AlAs-like optic phonon modes are recorded. These are then used to determine the composition dependent phonon mode shifts. By comparing the separate estimates of the free carrier band-gap energy using low temperature absorption and photoluminescence measurements, unintentional doping of the epilayers by the substrate is revealed.
Temperature dependence of the magnetic and transport properties of Co15Cu85 magnetic granular alloys
Authors: Yu R. H. , Zhu J. , Zhang X. X. , Tejada J.
Published: July 2002

The temperature dependence of magnetic and transport properties of annealed Co15Cu85 alloys has been studied from 2 to 300 K. It was found that the magnetic property and resistivity significantly change with the temperature. Based on the two-current resistance model, we have calculated the electron spin-mixing parameter Q(T), which explains the magnetoresistance change with the temperature for granular Co15Cu85 alloys.
Ultrashallow junctions in Si using decaborane? A molecular dynamics simulation study
Authors: Smith Roger , Shaw Marcus , Webb Roger P. , Foad Majeed A.
Published: July 2002

The feasibility of using decaborane B10H14, for the manufacture of shallow junctions in Si is investigated by means of molecular dynamics simulations. Bombardment energies of 1, 2, and 4 keV are investigated and the simulations run for up to 7 ps in order to ascertain the implantation profiles of the B atoms, the whereabouts of the H from the impacted molecule and the damage to the lattice. The simulations show that if a small binding energy of the B atom in the Si lattice is assumed then most of the B from the cluster is implanted. The implantation distributions are flatter with depth than those for single B interactions and the surface layers undergo damage and amorphisation in the proximity of the impact.
Dependence of carrier lifetime and resistivity on annealing in InP grown by He-plasma-assisted molecular beam epitaxy
Authors: Kang Jin U. , Frankel Michael Y. , Esman Ronald D. , Thompson D. A. , Robinson B. J.
Published: July 2002

We have experimentally investigated the carrier lifetime and resistivity versus anneal temperature in InP films grown by He-plasma-assisted molecular beam epitaxy. This material is shown to exhibit picosecond carrier lifetime with high resistivity even after anneal at temperatures up to 700 °C.
Interface acoustic modes of twisted Si(001) wafers
Authors: Mozhaev V. G. , Tokmakova S. P. , Weihnacht M.
Published: July 2002

Interface acoustic waves at the (001) boundary of twisted single-crystal silicon wafers are studied using analytical and numerical techniques. A secular equation of the problem in an explicit form is derived for the propagation directions along the twist-angle bisectrix and a complete numerical solution is calculated for arbitrary propagation. A new mode of leaky-wave type is predicted. This leaky mode has a low propagation loss and distinct near-interface localization at small twist angles. The leaky wave is purely longitudinal at the interface in the bisectric case. A mode without leakage is found only in the vicinity of the bisectrix directions. Its feature is a rotating movement of interface particles in the plane normal to the propagation direction in the bisectrix case. The sector of azimuthal angles permitted for the propagation of the nonleaky mode has an unusual jumplike behavior as a function of the twist angle.
Integer quantum Hall effect in a high electron density Al0.2Ga0.8As/In0.2Ga0.8As/GaAs quantum well
Authors: Dunford R. B. , Popović Dragana , Pollak Fred H. , Noble Thomas F.
Published: July 2002

A high electron density Al0.2Ga0.8As/In0.2Ga0.8As/GaAs quantum well exhibited the integer quantum Hall effect at even-integer filling factors ν=4,6,.. and 24, only. The energy gaps and pre-exponential factors were determined. A carrier effective mass of m*=0.082me was found. The ratio of the transport relaxation time to the quantum lifetime (α=3.9±0.3) indicated that the dominant scattering mechanism was long-range remote ion scattering. At 30 mK, the carrier density in the dark was 1.35×1012 cm−2, which increased (due to the persistent photoconductivity effect) to 1.76×1012 cm−2 upon illumination, accompanied by a similar improvement in mobility.
Nuclear-induced electrical conductivity in 3He
Authors: Bitteker Leo
Published: July 2002

The results of a computational and experimental investigation of nuclear-induced electrical conductivity enhancement in 3He is discussed with application to magnetohydrodynamic (MHD) energy conversion. Calculations based on the standard model of interactions and conductivity in a gas of partially ionized 3He suggest a region of conditions not previously considered may provide significant conductivity enhancement. Specifically, at densities less than standard atmospheric density and neutron flux greater than 1×1012/cm2 s, conductivity greater than 10 mho/m may be achievable. These calculations also suggest conductivities of several hundred mho/m may be possible for an achievable range of conditions. Additionally, the calculations show that nuclear-induced conductivity is strongly density dependent and weakly temperature dependent. Therefore, higher flow velocities, and hence higher power densities than those typical in traditional MHD channels utilizing thermal ionization would be possible. Experiments were designed to measure the conductivity enhancement in pure 3He for temperatures from 300 to 1500 K, pressure from 3.8×103–7.6×10−2 Torr, and thermal neutron flux from 1010 to 1016/cm2 s. Due to unforeseeable events, the reactor facility was shut down prior to the completion of these experiments resulting in a sparse data set. The data collected indicate the actual conductivity enhancement is approximately three orders of magnitude below the value calculated by the standard model of interactions. Possible reasons for this discrepancy are discussed, and additional calculations and experiments are proposed to resolve the discrepancy.
Comment on “Amorphization and defect recombination in ion implanted silicon carbide” [J. Appl. Phys. 81, 7181 (1997)]
Authors: Heera V.
Published: July 2002

It is demonstrated that the simplified analysis of Rutherford backscattering\channeling data on damage production in SiC performed by Grimaldi et al. [J. Appl. Phys. 81, 7181 (1997)] cannot be used to calculate the atomic displacement energy. The value of 12 eV at which the authors arrive is much too small. Moreover, their conclusion of similar displacement energies in Si and SiC is essentially wrong. The general reasons for that are discussed and illustrated by an example.
Surface effects on the magnetic behavior of antiferromagnetic particles
Authors: Trohidou K. N. , Zianni X. , Blackman J. A.
Published: July 2002

Antiferromagnetic particles can exhibit the phenomenon of superparamagnetism if the number of spins in the two sublattices is unequal (“uncompensated spins”). Monte Carlo simulations have been performed on spherical particles in which the spins interact with classical Heisenberg interactions. Anisotropy effects are also included with distinct forms in the core and at the surface. The magnetization was calculated as a function of both temperature and applied magnetic field for a range of particle sizes. Different types of behavior are found which depend on the number of uncompensated spins. Certain field dependent behavior is shown to scale with the ratio of the number of uncompensated to the total number of spins.
Structural and electronic properties of boron nitride thin films containing silicon
Authors: Ronning C. , Banks A. D. , McCarson B. L. , Schlesser R. , Sitar Z. , Davis R. F. , Ward B. L. , Nemanich R. J.
Published: July 2002

The incorporation of silicon into boron nitride films (BN:Si) has been achieved during ion beam assisted deposition growth. A gradual change from cubic boron nitride (c-BN) to hexagonal boron nitride (h-BN) was observed with increasing silicon concentration. Ultraviolet photoelectron spectroscopy, field emission, and field emission electron energy distribution experiments indicated that the observed electron transport and emission were due to hopping conduction between localized states in a band at the Fermi level for the undoped c-BN films and at the band tails of the valence band maximum for the BN:Si films. A negative electron affinity was observed for undoped c-BN films; this phenomenon disappeared upon silicon doping due to the transformation to h-BN. No shift of the Fermi level was observed in any BN:Si film; thus, n-type doping can be excluded.
Quantum size effects and magnetoresistance in spin-valved Co/Cu/Co trilayer structures
Authors: Diao Z. T. , Tsunashima S. , Jimbo M.
Published: July 2002

Electron transport and the magnetoresistance of magnetron sputtered ultrathin Co(M1)/Cu/Co(M2) trilayer structures that are of comparable structural perfection are presented as a function of magnetic and nonmagnetic layer thicknesses. We apply the quantum well states model to the interpretation of the electron transport data, while also considering the shunting effects or classic diffuse bulk scattering effect on electron transport in these metallic trilayer structures. This approach represents a beyond free-electron approximation that takes into model calculations details of the electronic band structure of the trilayers and spin-dependent electron scattering by impurities and/or at interfaces. A concurrent description of both the resistivity and magnetoresistance data can be achieved, as distinguished from such general semiclassical ones as Camley–Barnas’s and its deviations that account for the magnetoresistance well, but fail to describe electron transport of these layered structures in the ultrathin layer thickness limit.
A note on the semiconductor laser equivalent circuit
Authors: Flynn E. J.
Published: July 2002

The small-signal rate equations for a semiconductor laser can be represented as a network of electrical components, as shown by Tucker and Kaminow [R. S. Tucker and I. P. Kaminow, J. Lightwave Technol. LT-2, 385 (1984)] for the case of finite gain compression ε>0. However, a calculation of small-signal response curves δS/δI vs I derived from this equivalent circuit exhibit a linear decrease (“droop”) above threshold that is inconsistent with the dc solutions of the original rate equations. This problem flags a subtle flaw in the formulation of the circuit model. We observe, moreover, that the expression for the resonance damping parameter used throughout the laser literature follows exactly from this same RLC circuit model. The derivation of the circuit model from the rate equations is revisited. Consistency between the dc and ac solutions is achieved when the differential of the net gain δ{G/(1+εS)} is appropriately represented as a shunt element with zero admittance. The corrected circuit model resolves the “droop” problem in the small-signal δS/δI vs I relation. As expected, the new circuit model leads to a modified expression for the damping parameter.
Evaluating the impedance field through several transport models: A comparison
Authors: Bonani F. , Ghione G. , Houlet P. , Varani L. , Aboubacar M. , Vaissière J. C. , Nougier J. P. , Starikov E. , Gruz̆inskis V. , Shiktorov P.
Published: July 2002

The article presents a critical discussion on the evaluation of the impedance field for noise analysis by means of several transport models. The models considered are the drift-diffusion model, the full hydrodynamic model, and the scattered packet method for the direct solution of the Boltzmann transport equation. The comparison is carried out on a canonical Si p+pp+ diode at ambient temperature. The agreement found between the approaches is good, thus suggesting that the evaluation of the impedance field does not require any additional care with respect to other, more conventional operating conditions.
A study of various oxide/silicon interfaces by Ar+ backsurface bombardment
Authors: Lai P. T. , Li G. Q. , Huang M. Q. , Zeng S. H. , Cheng Y. C.
Published: July 2002

A low-energy (550 eV) argon beam is used to bombard the backsurfaces of 6 kinds of metal–oxide–semiconductor capacitors, and the resulting effects on their interface characteristics are then investigated. The gate oxide of these capacitors includes thermal oxide, trichloroethyene (TCE) oxide, NH3-nitrided oxide, reoxidized-nitrided oxide, rapid-thermal-nitrided oxide, and N2O-nitrided oxide. Measurements show that for bombardment times up to 45 min the interface-state density of all the devices, in general, decreases with increasing bombardment time/dose, and the midgap energy at the silicon surface tends to rise. Moreover, the bombardment is more effective in reducing acceptor-type than donor-type interface states. On the other hand, the change of fixed-charge density is more complex. For TCE, N2O-nitrided and reoxidized-nitrided oxides, fixed-charge density decreases initially with increasing bombardment time, but then increases, while the trend is reversed for the other gate oxides. A model with stress compensation and weak bond breaking is suggested to explain the results.
The strain dependence of the critical properties of Nb3Sn conductors
Authors: ten Haken B. , Godeke A. , ten Kate H. H. J.
Published: July 2002

The critical current (Ic) of six different Nb3Sn multifilamentary wires is investigated as a function of temperature, magnetic field, and strain. A relation for a critical temperature (Tc) that depends on the deviatoric strain is proposed and applied to interpret the results. First, a short review is given on the flux-pinning relations that are used to introduce a strain dependent Tc in a relation for the Ic as a function of field and temperature. The conductor samples are investigated in two different deformation states, namely, in a spiraled shape on a Ti sample holder and a straight section soldered onto a brass substrate. The brass substrate is used to apply a compressive or tensile axial strain to the conductor. The Ic in the different samples prepared from a single conductor type can be described very well with a single set of critical properties and strain parameters. In particular, in the strain regime where the matrix deformation is limited and the superconductor is axially compressed, the proposed strain relation is very accurate. The small variation in the strain parameter between the six conductors investigated suggests that this strain parameter is an intrinsic property of Nb3Sn.
Legendre polynomial approach for modeling free-ultrasonic waves in multilayered plates
Authors: Lefebvre J. E. , Zhang V. , Gazalet J. , Gryba T.
Published: July 2002

Modeled on the Laguerre polynomial approach used for surface acoustic waves, a formulation is presented for the study of free-acoustic wave propagation in layered plates. It uses the Legendre polynomials. Each layer can be of arbitrary anisotropy and piezoelectricity with arbitrary crystal orientation with the only restriction that the parameters of the constituent materials are close to each other. Formulations are given for open-circuit and short-circuit surfaces. Phase velocity dispersion curves, attenuations, power distributions, and field profiles are easily obtained from an algorithm easily implemented on a computer using commercial software. Numerical results are given for AlAs/GaAs multilayered structures which illustrate the capabilities of the described method. Its major advantages are: (i) in a unified formulation, all types of modes, surface modes, interface modes, and plate modes are naturally encompassed. (ii) Large values of the frequency-thickness product as compared to the values allowed by the usual methods may be dealt with. (iii) For propagation loss calculation, the usual multivariable search is avoided since the proposed method naturally works in the complex plane.
Analysis of local mechanical stresses in and near tungsten lines on silicon substrate
Authors: De Wolf Ingrid , Ignat Michel , Pozza Georges , Maniguet Laurent , Maes Herman E.
Published: July 2002

The mechanical stresses in tungsten lines on a silicon substrate were determined by x-ray diffraction (XRD). The stresses are found to be plane and tensile. The stresses induced in the silicon substrate are compressive under the lines, but tensile in between the lines. This stress state in the silicon substrate was studied by micro-Raman spectroscopy (μRS). A good correlation is found between the XRD results and the μRS results. The stresses determined by XRD and μRS were compared to the stresses calculated using two analytical models and a numerical approach. The best fit to the experimental results was obtained when using the distributed edge force model. It is shown that the combination of both XRD and μRS provides a useful way to test the validity of the assumptions which are used in the stress models.
Unicrystal Co–alloy media on Si(110)
Authors: Yang Wei , Lambeth David N. , Laughlin David E.
Published: July 2002

Unicrystal Co–alloy/Cr/Ag films, which exhibit a single, in-plane easy axis orientation, were epitaxially grown on hydrofluoric acid etched Si(110) single crystal substrates by sputter deposition for the purpose of the systematic study of Co–alloy magnetocrystalline anisotropy. The orientation relationship was studied by x-ray θ/2θ diffraction, pole figure φ scan, and transmission electron microscopy, and it was determined to be Co(101̄0)[0001]‖Cr(112)[11̄0]‖Ag(110)[001]‖Si(110)[001]. The φ scan also showed two twin-related orientations of Cr grains. The easy axis hysteresis loops had a square shape, while the hard axis loops showed zero openness. The uniaxial anisotropy constants K1 and K2 of the unicrystal Co and CoCrTa films were determined from torque and hard axis hysteresis loop measurements.
Drude-like behavior of Ge:Sb:Te alloys in the infrared
Authors: Mendoza-Galván A. , González-Hernández J.
Published: July 2002

The optical transmittance of Ge:Sb:Te evaporated thin film alloys with different compositions was measured in the wavelength range from 2.5 to 34 μm. The as-deposited films with an amorphous structure were thermally crystallized to achieve the two known crystalline phases: the fcc and the hexagonal at higher treatment temperatures. There are drastic changes in the transmittance spectra of samples with different structural phase, which reflects the differences in their optical constants. For amorphous films, only the frequency independent term in Drude’s formula is required to properly describe their transmittance spectra. For samples in the hexagonal and the fcc structures, the transmittance spectra show a metallic and semimetallic behavior, respectively; therefore, the full Drude expression has been used as the general dispersion relation for the optical constants of the films. The electrical conductivity deduced from the Drude expression shows a dependence on Sb concentration in good agreement with previously reported values measured on bulk single crystals. Furthermore, the n and k spectra obtained in the measured infrared range are in good agreement with the values in the visible-UV region, previously measured using spectroscopic ellipsometry.
On the linearity of holographic recording in amorphous As2S3 films
Authors: Spektor Boris , Lisiansky Michael , Shamir Joseph , Klebanov Matvei , Lyubin Victor
Published: July 2002

Optical recording in amorphous arsenic trisulfide was investigated for amplitude to phase conversion to be exploited in applications such as holographic recording and the fabrication of diffractive optical elements. The main result of this study is the demonstration that it is possible to obtain a linear relation between the spatial variation of the recording intensity and the recorded phase modulation. The extent of the linear recording region depends strongly on the recording wavelength. At short wavelength (458 nm in these experiments), where the rate of the photoinduced structural changes is strong, the recording is practically linear from zero exposure up to about 25% of the maximum value of the refractive index change. With higher exposure, saturation effects start to play an important role and degrade the linearity of the response. Recording with longer wavelength (488 and 514.5 nm) is less effective (absorption becomes weaker) and then relaxation processes in the material become significant. If the competition between photoinduced processes and relaxation processes is strong, the linear relationship between exposure and refractive index change can no longer be linear. Nevertheless, even for recording with the longer wavelengths, a region of exposures can be found within which variations of intensity can be converted into variations of phase with a linear relationship.
Effect of electron–hole interaction on electron spin relaxation in GaAs/AlGaAs quantum wells at room temperature
Authors: Gotoh H. , Ando H. , Sogawa T. , Kamada H. , Kagawa T. , Iwamura H.
Published: July 2002

We investigated electron spin relaxation with respect to the spatial electron–hole separation in GaAs/AlGaAs quantum wells at room temperature. The polarization dependent time-resolved photoluminescence method was used to measure the electron spin relaxation time by applying an electric field perpendicular to the quantum wells. The spin relaxation time had a strong electric field dependence and largely increased with an increase in the spatial electron–hole separation. These results cannot be explained by only the D’yakonov–Perel process, which has often been considered the cause of spin relaxation. We discuss the possible mechanisms that cause the spin relaxation by taking into account the electron–hole exchange interaction.
Spin dynamics and absence of a central peak anomaly in La0.67Ca0.33MnO3
Authors: Rhyne J. J. , Kaiser H. , Stumpe L. , Mitchell J. F. , McCloskey T. , Chourasia A. R.
Published: July 2002

Low-angle inelastic neutron scattering was used to study the temperature and wave vector dependence of the spin waves in La0.67Ca0.33MnO3 perovskite-based colossal magnetoresistance material. At low q the spin waves show Heisenberg ferromagnetic dispersion (E=Dq2+Δ) where D is the spin stiffness, q is the wave vector, and Δ is the energy gap. However, the temperature renormalization of the spin stiffness D is anomalous, and as T increases toward Tc, D does not show the expected power law collapse, but rather exhibits a sudden sharp drop suggestive of a first-order phase transition. Detailed neutron measurements of the order parameter in zero applied field showed a similar first-order-like transition. However, no temperature hysteresis was observed in either D or in the magnetization.
Analysis of power loss in Ni–Zn ferrites
Authors: Kondo K. , Chiba T. , Yamada S. , Otsuki E.
Published: July 2002

The power loss (Pcv) was analyzed by combining two methods. The first, Pcv, is divided into hysteresis loss (Ph) and residual loss (Pr) from the frequency dependence of the power loss according to the method of Otsuki et al., and, second, the loss factors are attributed to domain wall motion (Pw) and the rotation magnetization (Prot) by adapting the method proposed by Visser et al. It was found that Pw coincides with Ph in the lower frequency range, but the difference between them becomes significant as frequency goes up. The higher value of Pw, in comparison with Ph in the higher frequency range, can be attributed to the enhancement of loss due to the dynamic motion of the domain wall (Pwd) by raising the frequency. The hysteresis loss dominates Pcv in the frequency range below 500 kHz, while Pwd becomes predominant factor in the higher frequency range more than 500 kHz.
Enhanced robustness and tunnel magnetoresistance in artificial ferrimagnet based tunnel junctions
Authors: Dimopoulos T. , Tiusan C. , Ounadjela K. , Hehn M. , van den Berg H. A. M. , Henry Y. , da Costa V.
Published: July 2002

Magnetic tunnel junctions have been fabricated, that use Co/Ru/Co and Co/Ru/CoFe artificial ferrimagnet (AFi) systems as hard magnetic electrodes. The ∼22% tunnel magnetoresistance signal obtained, at room temperature, for junctions with Co/Ru/Co was increased to ∼30% by taking advantage of the higher spin polarization of CoFe in contact with the barrier. Along with this improvement, a different reversal behavior for the AFi was obtained. Together with the enhanced thermal stability, up to 350 °C, of the junctions involving the Co/Ru/CoFe system potential use as magnetic sensors is discussed.
Domain wall trapping in controlled submicron ferromagnetic elements
Authors: Hirohata A. , Xu Y. B. , Yao C. C. , Leung H. T. , Lee W. Y. , Gardiner S. M. , Hasko D. G. , Bland J. A. C. , Holmes S. N.
Published: July 2002

The domain configuration in permalloy wires (30 nm thick, 10 μm wide, and 205 μm long) with a wide size range of a narrow central bridge (5 μm long and w μm wide; 0.5⩽w⩽10 μm) were investigated in both their demagnetized and remanent states using magnetic force microscopy and the results were confirmed by micromagnetic calculations. At the bridge region, domain walls were found to be shifted by a small external field. Scanning magneto-optical Kerr effect revealed that the coercivity in these structures are the same as that in a straight wire, suggesting that domain wall movement is the dominant process in the magnetization reversal of these structures.
Identification of the switching field function in vector hysteresis models
Authors: Huang Y. , Friedman G. , Obaidat I. , Kouvel J.
Published: July 2002

This article deals with a large class of magnetic hysteresis models where bistable elements are the basis of hysteretic behavior. The switching field function characterizes the curves that have to be crossed by the magnetic field vector in order for the bistable elements to switch. This article reports experiments that use observations of the aggregate magnetization to identify the switching field function of the bistable elements in two dimensions.
A Preisach analysis of a reentrant ferromagnet Fe0.65Ni0.23Cr0.12
Authors: Song T. , Roshko R. M. , Dan Dahlberg E.
Published: July 2002

Measurements of the initial magnetizing curve, the magnetizing remanence, the descending branch of the major hysteresis loop, and the demagnetizing remanence have been performed on a reentrant ferromagnet Fe0.65Ni0.23Cr0.12, at several temperatures in the vicinity of the postulated ferromagnetic-spin glass “reentrant transition.” Henkel plots of the principal magnetizing and demagnetizing remanences show a systematic change in curvature with temperature, from a “high temperature” regime T⩾8 K where the curvature is predominantly magnetizing-like to a “low temperature” regime T⩽6 K where the curvature is purely demagnetizing-like. Numerical simulations based on a Preisach model which includes thermal fluctuations show that this behavior is consistent with a physical picture of coupled bistable subsystems with a temperature dependent distribution of interaction fields, which evolves with decreasing temperature from a regime dominated by long range ferromagnetic interactions to a regime in which the interaction fields fluctuate randomly about a zero mean.
Longitudinally driven giant magnetoimpedance effect in stress-annealed Fe-based nanocrystalline ribbons
Authors: Wang Z. C. , Gong F. F. , Yang X. L. , Zeng L. , Chen G. , Yang J. X. , Qian S. M. , Yang D. P.
Published: July 2002

A high-frequency longitudinally driven giant magnetoimpedance (GMI) effect has been measured in stress-annealed Fe73Cu1Nb1.5V2Si13.5B9 nanocrystalline ribbons. Based on how the impedance phase varies with the external magnetic field, it becomes clear that the imaginary part of the complex permeability, μ″, which is related to magnetic losses, plays an important role in the high-frequency longitudinally driven GMI effect. The transverse anisotropy field Hk can be readily determined by a sharp minimum in the curve of the impedance phase as a function of the external magnetic field. This provides a new method for measuring the magnetic anisotropy field in such systems.
Energy band structures of the low-dimensional antiferromagnets Sr2CuO3 and Sr2CuO2Cl2
Authors: Wu Hua , Zheng Qing-qi , Lin H. Q.
Published: July 2002

Calculated in this paper are the band structures of both the one-dimensional (1D) antiferromagnet Sr2CuO3 and the two-dimensional one Sr2CuO2Cl2 by using the local-spin-density approximation (LSDA) and the on-site Coulomb interaction U correction (LSDA+U). The pd hybridization is weaker in 1D Sr2CuO3 than in 2D Sr2CuO2Cl2, thus favoring a spin-polarized and insulating solution for the former but a nonmagnetic and metallic state for the latter. While the strong d–d electron correlations are responsible for the O 2p-Cu 3d charge-transfer insulating character of these two cuprates. The calculated band gaps and spin moments are well comparable with the experimental and theoretical values.
Magnetic dichroism in core-level photoemission from Gd(0001)
Authors: Morais J. , Fecher G. H. , Denecke R. , Hussain Z. , Fadley C. S.
Published: July 2002

This work reports on magnetic dichroism in photoemission from the Gd 4l(l=s,p,d,f ) core level of in-plane magnetized Gd films excited with linearly, circularly, and unpolarized light. Thin Gd films of about 100 ML thickness were evaporated onto a W(110) substrate at room temperature, and subsequently annealed to 700 K to form an ordered layer exhibiting a (0001) surface. The dichroism measurements were performed by physically rotating the sample azimuth so as to change the direction of the magnetization axis relative to the incident light. The magnetic dichroism asymmetries were obtained by calculating the normalized difference between spectra obtained with the two mutually orthogonal orientations of the magnetization. Strong asymmetries were observed, even if exciting the spectra by means of unpolarized light. The experimental results are compared to three-step photoemission cluster calculations based on a relativistic full potential algorithm respecting magnetic exchange.
Magnetic properties of (Pr1−xCex)6Fe13Al compounds
Authors: Wang Jing-yun , Wang Fang-wei , Liang Bing , Zhang Shao-ying , Yan Qi-wei , Shen Bao-gen , Zhan Wen-shan , Liu J. P.
Published: July 2002

Samples of (Pr1−xCex)6Fe13Al (x=0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) were prepared by arc melting and annealed for two weeks at 973 K. The x-ray diffraction patterns show that all the samples crystallize with the Nd6Fe13Si-type structure. The unit-cell volume increases first and then decreases as the Ce content increases. The Curie temperature and the spontaneous magnetization decrease almost linearly with increasing Ce concentration, supporting the existence of a mixed valence of Ce3+ and Ce4+. The series is ferromagnetic at high temperature; however, the peak at low temperature of the M–T curve most likely corresponds to spin-glass behavior.
Anomalous magnetic properties of the compound SmCo3B2
Authors: Ido H. , Yamada M. , Nashima O. , Ito T. , Yoshida H.
Published: July 2002

Magnetization measurements for the compound SmCo3B2 have been performed on loose powder in magnetic fields to 5 T and in the temperature region from 4.2 to 60 K. A small saturation moment of 0.18 μB/SmCo3B2 at T=4.2 K and its variation with increasing temperature suggest ferrimagnetic ordering in SmCo3B2, which is explained as the effect of a higher order crystalline field coefficient. A magnetic first order transition occurs at T=43 K and metamagnetism occurs in the narrow temperature region between 43 and 47 K, for which some possible discussions are given.
The dielectric constant of water. Computer simulations with the MCY potential
Authors: Neumann Martin
Published: July 2002

Despite the sizable literature on computer simulations of water and aqueous solutions, very little is known about one of the most interesting properties of water, namely its dielectric constant. In this paper it is demonstrated that the methodological as well as technological problems that have hitherto impeded the calculation of dielectric properties of realistic model systems have now been overcome. Using a small dedicated array processor, a series of extensive simulations with the MCY (Matsuoka–Clementi–Yoshimine) potential for water have been performed in reaction field geometry, and it is shown that consistent (i.e., free from boundary effects) and accurate results may be obtained for the full frequency-dependent dielectric constant, from the static up to the submillimeter range. At the same time it is found that the rather popular MCY model is not able to satisfactorily reproduce the dielectric properties of real water: Both the static dielectric constant and the principal dielectric relaxation time are much lower than the experimental values, and the temperature dependence of the Kirkwood g factor has the wrong sign. It is concluded that in the future the calculation of the dielectric constant, being a measure of orientational correlations between molecules, will play an important part in assessing the reliability of model potentials for water.
Chain organization and thermodynamics in micelles and bilayers. I. Theory
Authors: Ben-Shaul A. , Szleifer I. , Gelbart W. M.
Published: July 2002

Starting from the partition function of a micellar aggregate, the various assumptions involved in decomposing the aggregate’s standard chemical potential into surface and core terms are explicitly stated and discussed. The conformational statistics of the amphiphiles’ hydrocarbon chains (tails) composing the hydrophobic core is assumed to be governed by the hard core repulsive interactions between chain segments. The density within the core is assumed uniform and liquid-like. By appropriate expansion of the aggregate’s configurational integral, explicit expressions are derived for the (singlet) distribution function of chain conformations and the chain’s conformational partition function (free energy). These quantities depend on the thickness and curvature (geometry) of the hydrophobic core via the lateral pressures representing the geometric packing constraints. (The same distribution function has been previously derived by us using the maximal entropy formalism.) It is argued that the variations in the conformational contribution to the aggregate’s chemical potential may be comparable to those due to the surface term. (In the prevailing models of amphiphile aggregation only the latter are included.) Detailed numerical analyses for model chains packed in spherical, cylindrical, and planar aggregates are presented in the subsequent paper (part II). One of the major conclusions from the calculations is that geometric packing constraints rather than internal energy (gauche–trans) effects are the dominant factors determining chain statistics.
Carbon-13 NMR of nematic and smectic liquid crystals with magic-angle spinning
Authors: Fung B. M. , Gangoda Mahinda
Published: July 2002

Carbon-13 NMR of liquid crystals with magic-angle spinning (MAS) has been studied. Nematic liquid crystals with Δχ>0 orient parallel to the field at low spinning rates, and orient along the spinning axis at high spinning rates (ν). At a field of 7.05 T, the C-13 peaks of EBBA are very sharp and there are no spinning sidebands at ν≳1000 Hz. Spin lattice relaxation time (T1) and nuclear Overhauser effect (NOE) of individual carbon atoms of EBBA have been measured and the results give valuable information on the motional behavior of EBBA in its nematic phase. Nematic liquid crystals with Δχ<0 show rather broad peaks even at high ν. Smectic liquid crystals do not orient along the spinning axis, but can be made to do so when they are cooled through the nematic phase while the sample is being spun. With MAS at high ν, cross polarization is ineffective for nematic liquid crystals, and is not very effective for smectic liquid crystals.
Stabilization of unstable states, relaxation, and critical slowing down in a bistable system
Authors: Kramer Jesse , Ross John
Published: July 2002

The unstable branch of stationary states in an illuminated thermochemical bistable system is stabilized with an external feedback loop which does not otherwise alter the autonomous system. The technique allows the measurement of the entire set of stationary states available to the system. Trajectories of transition between branches of stationary states (relaxation) are investigated theoretically and experimentally. Critical slowing down in the speed of the trajectories is predicted and measured as the points of marginal stability are approached. Trajectories are observed to spread as they depart from the unstable stationary state and then to coalesce as they approach a stable stationary state. These results are obtained due to experimental spread in initial conditions and not from noise along the relaxation trajectory.
On nickel–sulfate contacts and SO=4–H2O interactions in aqueous solutions
Authors: Caminiti R.
Published: July 2002

NiSO4 aqueous solutions are examined by x-ray diffraction. The peaks present in the G(r) correlation functions at about 2.05, 3.80, and 4.20 Å clearly indicate the existence of SO=4–H2O contacts and of two hydration shells around nickel. The absence of any peaks in the range 3.2–3.5 Å, indicates no quantitatively detectable inner sphere ion pairing between nickel and sulfate anion.
Ewald sum of the Rotne–Prager tensor
Authors: Beenakker C. W. J.
Published: July 2002

The lattice sum of the Rotne–Prager hydrodynamic mobility tensor is cast into a rapidly converging form by an Ewald summation technique. The result has a direct application to the problem of how to deal with the long range of hydrodynamic interactions in computer simulations of macromolecular solutions.