bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic surface displacement measurement in 1-3 and 1-1-3 piezocomposites
Authors: Lian L. , Sottos N. R.
Published: July 2002

The sensitivity of 1-3 piezocomposites can be improved by the addition of a third phase or interlayer around the piezoceramic rods which results in a composite with 1-1-3 connectivity. In the current investigation, dynamic surface displacements of both 1-3 and 1-1-3 piezocomposites were measured using a laser Doppler heterodyne interferometer. Dynamic displacement profiles showing the nonuniform deformation of the piezoelectric rod and the nonpiezoelectric polymer were obtained at 10 Hz, 100 Hz, and 1.0 kHz. Measurements of the displacement at the center of the piezoelectric rod were performed over the frequency range from 10 Hz to 27 kHz. Three different interlayer conditions were considered. The experimental measurements demonstrated that both the surface displacement profile and the maximum rod displacement varied with frequency and interlayer properties. The dynamic displacement profiles were also compared with previously measured static displacement profiles.
Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (011̄2) sapphire by metalorganic chemical vapor deposition
Authors: Gorla C. R. , Emanetoglu N. W. , Liang S. , Mayo W. E. , Lu Y. , Wraback M. , Shen H.
Published: July 2002

High-quality ZnO films are receiving increased interest for use in low-loss high-frequency surface acoustic wave (SAW) devices, acousto-optic and optical modulators, as buffer layers for III-nitride growth, and as the active material in ultraviolet solid state lasers. In this work, high quality epitaxial ZnO films were grown on R-plane sapphire substrates by metalorganic chemical vapor deposition. The structural, piezoelectric, and optical properties of the ZnO films on R sapphire have been investigated. The epitaxial relationship between ZnO and R-Al2O3 was found to be (112̄0) ZnO∥(011̄2) Al2O3, and [0001] ZnO∥[01̄11] Al2O3. The interface between as-grown ZnO and R sapphire was atomically sharp and semicoherent, as evaluated by transmission electron microscopy. On annealing the films at temperatures above 850 °C, a solid state reaction occurred between ZnO and Al2O3, resulting in the formation of ZnAl2O4 (spinel) at the interface. A 15–20 nm spinel layer formed when the ZnO film was annealed at 850 °C for 30 min, whereas a 150 nm layer formed when the film was annealed at 1000 °C for 150 min. To prevent this reaction from occurring, the maximum process temperature should be below 750 °C. The surface acoustic wave properties of the piezoelectric ZnO were evaluated by fabricating SAW devices on (112̄0) ZnO/(011̄2) Al2O3. An effective electromechanical coupling coefficient, keff2, of 6% was achieved for a 1.5 μm thick ZnO film, which is close to the value for bulk single-crystal ZnO. The photoluminescence spectra were obtained both at room temperature and at 11 K. The full width at half maximum of the 3.363 eV band edge emission photoluminescence peak measured at 11 K was 6 meV, which is close to that for single-crystal ZnO. We also evaluated the anisotropic absorption characteristics of the (112̄0) ZnO film, which can be used for a high contrast ultraviolet light modulator.
Electroelastic flexural vibrations of a laminated plate
Authors: Camacho H. , Ramos R. R. , Comas F.
Published: July 2002

In the present work, we study the flexural modes in a thin laminated plate starting from the Kirchhoff’s hypothesis and from the linear approximation for the electric potential. The piezoelectric problem is stated with the aim of obtaining the flexural modes. The investigated system is modeled as a laminated plate of cylindrical shape, composed of 2N layers of piezoelectric materials. We study the effects of the number of layers and the effects of the plate thickness and radius variations on the mentioned modes. Finally, we focus our attention on the influence of electric potential configurations at the bottom and the top faces of the laminated plate on the electroelastic flexural vibrations.
Influence of the substrate thickness and radio frequency on the deposition rate of amorphous hydrogenated carbon a-C:H films
Authors: Balachova O. V. , Alves M. A. R. , Swart J. W. , Braga E. S. , Cescato L.
Published: July 2002

Amorphous hydrogenated carbon a-C:H films were deposited on silicon and quartz substrates in a parallel plate reactor by radio frequency (rf) plasma-enhanced chemical vapor deposition. The deposition rates of the films have been studied in the low-frequency region of the electric field. The Si and quartz substrates of different thickness have been used. The rf and substrate thickness dependencies of the deposition rates are discussed in terms of a theory of ion bombardment.
Tribology of proximity magnetoresistive head–disk interface for 5 Gb/in.2 recording
Authors: Jiang Zhaoguo , Yang Ming M. , Sullivan Mike , Chao James L. , Russak Mike , Chhabra Devendra S. , Fu Yen , Yang Chao-Hui
Published: July 2002

Low fly-height magnetoresistive (MR) sliders and low glide-height laser-texture thin film disks were introduced to meet the tribological challenges of proximity MR recording, with which an areal density of 5 Gb/in.2 has been achieved when using the sliders with dual-stripe MR heads and the disks with low-noise media. The 30% pico sliders employed two air-bearing designs with a fly height around 12.5 nm. The thin film disks used superfinish substrates with a glide-avalanche-height falling well below 10.0 nm. In the contact start/stop (CSS) zones, small crater-shape laser texture bumps were generated to meet both low stiction and low glide-height requirements. An 80 Å thin amorphous nitrogenated carbon was added over the magnetic layers as a protective overcoat. A layer of perfluoropolyether with an additive of phosphazene compounds was applied on the disk surface to improve the reliability of head–disk interfaces. The near contact head–disk interface survived for 20 k-cycle hot/wet and hot/dry CSS tests without wear and high stiction. Twenty-four-hour hot/wet park-stiction values after 20 k CSS cycles were within the acceptable range. Remarkably, thermal-asperity tests showed no hits by using the proximity heads on the ultralow glide-height thin film disks.
Magnetic excitations in the S=1/2 quasi-one-dimensional magnet Sr14−xCaxCu24O41
Authors: Matsuda M. , Katsumata K. , Shapiro S. M. , Shirane G.
Published: July 2002

Neutron scattering experiments have been performed on Sr14−xCaxCu24O41 (x=3 and 6), which consists of CuO2 chains and Cu2O3 ladders, in order to study the effect of Ca substitution on the dimerized state in the chains. The Ca substitution broadens the excitation peak although the excitation energy is almost unchanged. The effects of Ca and Y substitution on the dimerized state are compared.
Microstructure and magnetic properties of as-quenched and heat-treated (Nd,Dy)FeB powders produced by high pressure gas atomization
Authors: Snyder J. E. , Lo C. C. H. , Fang X. , Kriegermeier B. , Jiles D. C.
Published: July 2002

The magnetic properties and microstructures of a series of as-quenched and heat-treated inert gas atomized (IGA) rare-earth rich (Nd,Dy)–Fe–B particles have been investigated. Heat treatment was found to substantially improve magnetic properties, with effects most pronounced in samples with higher Dy content and higher total rare earth (RE) content. The as-quenched particles consisted of an underquenched dendritic-like structure with the majority phase RE2Fe14B, and a fine network of RE-rich material between the grains. The heat-treated particles showed a change in microstructure which correlated with magnetic property changes. Particles which showed little change in magnetic properties showed no obvious change in microstructure. Particles which showed large changes in magnetic properties showed a large change in microstructure: most of the fine network of RE-rich interdendritic material disappeared, leaving behind only a few small isolated regions. This would seem to indicate that the predominant mechanism determining coercivity in these IGA RE-rich (Nd,Dy)–Fe–B powers is nucleation of reverse domains, rather than domain wall pinning at nonmagnetic intergranular material.
Corrosion protection of Nd–Fe–B magnets by bismaleimide coating
Authors: Cheng F. T. , Man H. C. , Chan W. M. , Cheng C. W. , Chan W. O.
Published: July 2002

Samples of Nd–Fe–B were coated with bismaleimide (BMI), a resin which is well known for its desirable barrier, mechanical, and thermal properties but has not been used as a coating material. The performance of BMI-coated samples (average coating thickness ∼15 μm) was compared with commercial epoxy-coated samples (average coating thickness ∼20 μm) in a series of tests. In the copper-accelerated acetic acid–salt spray test, the epoxy-coated samples failed badly (damaged area >50%) within 20 h while the BMI-coated samples showed no sign of failure up to 200 h. Using the cross-cut tape test for adhesion strength (ASTM D 3359), the commercial epoxy coatings suffered 25% area detachment while for the BMI coatings, no detachment was observed. The scratch resistance of the BMI coatings, measured in terms of scratch depth in the pin scratch test, was about 1.5 that of the commercial epoxy coatings, and the Vickers hardness was higher by a factor of about 2.5. When magnetized under the same magnetizing field, there was no significant difference in the magnetization obtained among bare, epoxy-coated, and BMI-coated samples. These results suggest that BMI coatings are excellent for protection of the Nd–Fe–B magnets against corrosion and function better than the commonly used epoxy resin.
Helical spin-density waves in Fe/Cr trilayers with perfect interfaces
Authors: Fishman R. S.
Published: July 2002

Despite the presence of only collinear, commensurate (C) and incommensurate (I) spin-density waves (SDWs) in bulk Cr, the interfacial steps in Fe/Cr multilayers are now believed to stabilize a helical (H) SDW within the Cr spacer. Yet HSDWs were first predicted in an Fe/Cr trilayer with perfect interfaces when the orientation of the Fe moments does not favor C ordering: if the number of Cr monolayers is even (odd) and the Fe moments are pointing in the same (opposite) direction, then a CSDW does not gain any coupling energy. Under these circumstances, a simple model verifies that H ordering is indeed favored over I ordering provided that the Fermi surface mismatch is sufficiently small or the temperature sufficiently high.
High frequency permeability of patterned spin valve type thin films
Authors: Varga L. , Tanaka A. , Nagasaka K. , Kondo R.
Published: July 2002

The dynamic response of spin valve films were studied and compared to NiFe/CoFe bilayer and NiFe single layer films by measuring their high frequency permeability spectra on sheet and patterned samples. It was found that the high frequency response of these thin films were affected only by the ferromagnetic resonance and the behavior was well described by the calculated Landau–Lifshitz–Gilbert permeability. The resonance frequency of spin valve films was strongly influenced by the interaction field from the pinned layer. Decreasing interlayer thickness of Cu resulted in an increase of the interaction field, which shifted the resonance frequency to higher frequencies while reducing the initial permeability. The shape anisotropy, which was increased by decreasing pattern size, also shifted the films resonance frequency to higher values accompanied by lowered initial permeabilities. Regarding their dynamic response, spin valve films were found to be suitable for ultrahigh recording densities, when the effective anisotropy was sufficiently large.
Thermal reliability of NiMn-based spin valves
Authors: Tsu I-Fei , Duxstad K. J.
Published: July 2002

The thermal reliability of NiMn-based top spin valve was studied at sensor temperatures between 200 and 300 °C by furnace annealing and thermal-electrical stressing. Sensor resistance was measured as a function of heating time up to 1000 h. The sensor resistance change (dR/R) vs time (t) curves showed diffusion-like characteristics. Sensor lifetime for 0.5% resistance increase was determined from the Arrhenius plot to be more than 5 years at 200 °C anneal condition. Temperature- independent linear relationship between resistance change and time was observed in both anneal and bias stress conditions in dR/R=0.2%–3% range. It suggests an interface-controlled kinetics for the sensor failure process. Comparison between the reliability results from two types of stress suggests that device degradation is accelerated by the electrical bias when at the same sensor temperature.
Lead-doped single crystals of Ln1−x−ySrxPbyMnO3
Authors: Lin C. T. , Yan Y. , Bischoff E. , Peters K. , Schönherr E.
Published: July 2002

Single crystals of Ln1−x−ySrxPbyMnO3 (Ln: La, Pr, or Nd) with sizes up to 3 cm3 have been obtained using a flux solution of PbF2 and PbO at the temperature range between 917 and 1300 °C. The content of Pb was found to range between 0.26⩽y⩽0.43, depending upon the growing temperature, i.e., the higher the temperature, the lower the Pb content. Transmission electron microscopy studies provided evidence of a nanostructural behavior for Nd0.67Sr0.01Pb0.32MnO3. The crystals exhibit a sharp transition of ferromagnetic metallic to paramagnetic insulating behavior. Resistance measurement was performed in a magnetic field of 0, 2 and 7 T, respectively.
Distant-neighbor exchange constants in Mn-based dilute magnetic semiconductors
Authors: Bindilatti V. , ter Haar E. , Oliveira N. F. , Shapira Y. , Liu M. T.
Published: July 2002

Individual exchange constants Ji between distant neighbors in Zn1−xMnxX (X=S, Se, Te) were obtained from magnetization steps at 20 mK. The results disagree with all predictions. The principles involved in this experimental work and its analysis are reviewed.
Exchange coupling mediated by a nonmagnetic spacer with a large electron–electron exchange interaction
Authors: Takahashi Y.
Published: July 2002

The exchange effect due to a strong electron–electron Coulomb interaction in a nonmagnetic spacer metal is taken into account in the interlayer exchange coupling theory in magnetic superlattices. A Rudermann–Kittel–Kasuya–Yoshida-like oscillatory behavior is superimposed on the exponentially decreasing ferromagnetic bias which is the contribution from the e–e exchange interaction. Some of the antiferromagnetic exchange couplings in the oscillatory behavior may disappear because of the ferromagnetic bias. The first and second antiferromagnetic exchange couplings disappear and the third one remains in Fe/Pd/Fe magnetic sandwiches in numerical calculations and the result agrees well with the experimental one. In magnetic superlattices with a Pt spacer metal, the first and second antiferromagnetic exchange couplings are clearly shown because of the relatively weak ferromagnetic bias, as observed in Co/Pt superlattices.
Interlayer magnetoresistance peak in β″-(BEDT–TTF)2SF5CH2CF2SO3
Authors: Su X. , Zuo F. , Schlueter J. A. , Williams Jack M.
Published: July 2002

Transport measurements of interlayer magnetoresistance with the field parallel to the current direction have been performed on single crystals of organic superconductor β″-(BEDT–TTF)2SF5CH2CF2SO3. The magnetoresistance is found to display a pronounced peak as a function of magnetic field below the superconducting transition temperature, similar to those of κ-(BEDT–TTF)2X compounds. Analysis of the magnetoresistance peak in terms of a simple pair and quasiparticle tunneling model gives a physically negligible gap energy.
Magnetic properties of Bi-doped Gd2−xBixCuO4 cuprates
Authors: Ku H. C. , Luo H. M. , Hsu Y. Y. , Sarker M. M. , Lee T. J.
Published: July 2002

Structural and magnetic properties of the new Bi-doped Gd2−xBixCuO4 system are reported. With the doping of larger trivalent Bi3+ ions, the T′ phase solubility region is retained in the range of 0⩽x⩽0.1. Weak ferromagnetic or canted antiferromagnetic ordering temperature TN(Cu) of Cu moments decreases from 282 K for Gd2CuO4 to 258 K for Gd1.93Bi0.07CuO4 and 256 K for Gd1.9Bi0.1CuO4. Due to small internal field Hint associated with weak ferromagnetic order, the spin reorientation temperature Tsr(Cu) is field dependent and decreases from 24 K in 1 G to 12 K in 1 kG for Gd1.95Bi0.05CuO4. Preliminary powder x-ray Rietveld refinement analysis using the T′ phase ideal oxygen position indicates a large temperature factor B for the oxygen sites in the Cu–O plane and suggests that the degree of oxygen structural distortion may be closely related to the strength of weak ferromagnetic order.
Torsion measurements using inverse Wiedemann effect in glass covered amorphous wires
Authors: Chiriac H. , Hristoforou E. , Neagu Maria , Barariu Firuta , Ovari T. A.
Published: July 2002

This article presents results concerning inverse Wiedemann effect (IWE) dependence on the torsion applied along the Fe77.5Si7.5B15 glass covered amorphous wires tested before and after glass removal. In the absence of torsion during measurements, the IWE voltage is zero for amorphous wires tested in the as-cast state. In the same condition of measurements a small spontaneous IWE voltage appears for wires after glass removal annealed without torsion, and the voltage reaches its maximum value for torsion annealed wires both before and after glass removal. Increasing the torsion’s value during measurements, for as-cast wires and wires annealed without torsion, the value of the induced voltage increases until it reaches a maximum while for torsion annealed wires a small increase is observed followed by a strong decrease down to that torsion value at which the thermal treatment was performed. In the case of torsion annealed wires a good linearity of the torsion response was obtained for a given range of torsions which depends on the sample’s state (before and after glass removal). The obtained results suggest the possibility to use IWE in Fe77.5Si7.5B15 glass covered amorphous wires in torque sensors.
Long-term stability of a quasiperiodic Ta/Al multilayer: Disintegration at room temperature analyzed by grazing angle x-ray scattering and photoelectron spectroscopy
Authors: Yang P. , Klemradt U. , Tao Y. , Peisl J. , Peng R. W. , Hu A. , Jiang S. S.
Published: July 2002

A three-component Fibonacci (3CF) Ta/Al multilayer has been reinvestigated by specular and diffuse x-ray reflectivity and x-ray photoelectron spectroscopy (XPS) after 41 months of storage at room temperature. The specular reflectivity shows drastically suppressed diffraction peaks, whose positions and intensities are explained by severe interdiffusion of the Ta/Al bilayers building the 3CF sequence. Nonspecular reflectivity scans still indicate a high degree of interfacial roughness correlation in the growth direction that is attributed to the long spatial Fourier components of the interface profiles, which are substantially less affected by interdiffusion. The angle-resolved XPS spectra show that the Ta capping layer is completely oxidized and interdiffused by Al, whereas below the oxide layer Ta and Al coexist in metallic form in the same film. Both x-ray reflectivity and XPS yield an oxide layer of ∼30 Å thickness. Despite the severe structural disintegration, the multilayer diffraction spectrum can still be indexed by means of the projection theory for quasiperiodic sequences, which points to a remarkable stability of quasiperiodic properties against significant disorder. We conclude that Ta/Al bilayers are apparently unsuitable for multilayer applications due to the lack of thermal stability even at room temperature, with grain boundary diffusion pointed out as a possible disintegration mechanism. The design of improved 3CF Ta/Al multilayers is discussed with regard to applications in x-ray optics.
Evolution of the mound morphology in (111) oriented polycrystalline Pd films and Co/Pd multilayered films with Pd underlayers
Authors: Chung I. B. , Koo Y. M. , Kang J.-S. , Hong J. H. , Jeong J. I.
Published: July 2002

Pd thin films and Co/Pd multilayered films with progressively thicker Pd underlayers are prepared by physical vapor depositions. Their growth behaviors are investigated using atomic force microscopy and transmission electron microscopy. We observed that a mound occurs on top of each crystallite of (111) oriented polycrystalline Pd films and that the average mound size increases according to the capillary-induced coalescence mechanism. We attribute this observed growth instability to the step barrier which resists step-down diffusion of deposited atoms. We also observed that the mound slopes of Co/Pd multilayers are smaller than those of their Pd underlayers. We suggest that this results from a downhill current driven by the interface energy between the Co and Pd layers.
Representation of GaP formation by a reduced order surface kinetics model using p-polarized reflectance measurements
Authors: Beeler S. , Tran H. T. , Dietz N.
Published: July 2002

This contribution presents results on the parameter estimation of rate constants and optical response factors in a reduced order surface kinetics (ROSK) model, which has been developed to describe the decomposition kinetics of the organometallic precursors involved and their incorporation into the film deposition. As a real-time characterization technique, we applied p-polarized reflectance spectroscopy (PRS) during low temperature growth of epitaxial GaP heterostructures on Si(001) substrates by pulsed chemical beam epitaxy. The high surface sensitivity of PRS allows us to follow alterations in the composition and thickness of the surface reaction layer as they are encountered during periodic precursor supply. Linkage of the PRS response to the ROSK model provides the base for the parameter estimation of the reduced order surface kinetics model, giving insights into the organometallic precursor decomposition and growth kinetics.
Photoluminescence study of the defects induced by neutron irradiation and rapid annealing in semi-insulating GaAs
Authors: Liu Jian , Wang Pei-xuan
Published: July 2002

The rapid annealing behavior of several kinds of defects in semi-insulating GaAs irradiated with various neutron fluences has been characterized using a photoluminescence technique. In this experiment, transmutation impurities form not only donors, but also acceptors, GeAs (the 1.4783 eV peak). The intensity ratio of the 1.4783 eV peak (GeAs) to the 1.4917 eV peak (CAs) increases with the neutron dose. This finding is consistent with the expected increase of GeAs produced by transmutation. We also see that short time heat treatment leads to the increase of antisite defects GaAs and of complex centers IGa-VAs after neutron irradiation. Based on analysis of the rapid annealing process in comparison with the regular annealing process, it is concluded that the two kinds of defects GaAs and IGa-VAs are the products of defect reactions during the annealing process.
Photoinduced current transient spectroscopy of deep defects in n-type ultrapure germanium
Authors: Blondeel A. , Clauws P.
Published: July 2002

Photoinduced current transient spectroscopy (PICTS) is used to study deep minority carrier traps in n-type ultrapure germanium (shallow concentration of the order 109 cm−3). In this technique, which is a variant of deep level transient spectroscopy (DLTS), a neutral structure with two ohmic contacts applied on two opposite faces of the sample is illuminated through one of the contacts with intrinsic, strongly absorbed light. The current transients which follow after interrupting the photoexcitation are analyzed using classical double lock-in DLTS resulting in the detection of centers in the minority half of the band gap (provided the back ohmic contact is negative). After correcting the PICTS spectra for the temperature dependence of the mobility, six peaks superimposed on a broad background are clearly resolved. The peaks are the same as the ones found earlier in high-purity n-type germanium (shallow concentration of the order 1010 cm−3) using optical DLTS. These peaks are mainly Cu related. A formula to calculate concentrations from the PICTS spectra is deduced and verified experimentally.
Analytical micromagnetics of near single domain particles
Authors: Cowburn R. P. , Welland M. E.
Published: July 2002

A formalism based on the perturbation theory is described which allows for an analytical approximation to the solution of the micromagnetic equations. The approximation is valid in the limit of small deviations of the magnetization from the uniformly magnetized state. These small deviations can lead to large effective anisotropy fields in magnetic elements, which can be large enough to dominate the spin reversal behavior. We demonstrate the formalism by calculating the energy of the single domain state in nanometer scale square magnetic prisms. Good agreement is found with values calculated by conventional finite element numerical methods.
Grain-boundary magnetic properties of ball-milled nanocrystalline Fex Rh100−x alloys
Authors: Navarro E. , Hernando A. , Yavari A. R. , Fiorani D. , Rosenberg M.
Published: July 2002

Fe x Rh 100−x  (x=65, 50, 28, and 26) fcc nanocrystalline alloys have been produced using the high-energy ball-milling technique. All the compositions investigated exhibited a maximum in the alternative and zero field cooled susceptibilities in the low temperature regime. The temperature at which this maximum occurs, Tmax, weakly depends on both the frequency and the applied field, and increases with the grain size. These features suggest the appearance of a spin-glass ground state for all the compositions analyzed. The Mössbauer spectra at T<Tmax shows a broad distribution of hyperfine fields similar to that of an amorphous magnet. Shifted hysteresis loops as well as a considerable irreversibility at high fields indicate that Tmax corresponds to the freezing temperature of the strongly deformed grain boundaries, which behave as spin-glass-like phase.
Interfacial electronic density of states in phthalocyanine derivative Langmuir–Blodgett films determined by surface potential measurement
Authors: Kokubo Haruo , Oyama Yutaka , Majima Yutaka , Iwamoto Mitsumasa
Published: July 2002

Surface potentials of phthalocyanine derivative Langmuir–Blodgett (LB) films on metal electrodes were studied. Two kinds of phthalocyanine derivatives (C6PcCu and CuttbPc) were examined as a function of the number of deposited layers. The distributions of spatial excess charge density and electronic density of states at the metal/film interface were determined. The difference in the electrostatic phenomena observed in these two phthalocyanine LB films was attributed to the difference in the density of surface states of these two films.
Effect of Fe doping in La1−xSrxMnO3
Authors: Wen-xu Xianyu , Bao-he Li , Zheng-nan Qian , Han-min Jin
Published: July 2002

The effects of Fe doping on the magnetic, transport, and magnetoresistance properties are studied for La0.7Sr0.3Mn1−yFeyO3 (y=0–0.18) and La1−xSrxMn0.88Fe0.12O3 (x=0.1–0.50). For increasing y, a peak in the thermal dependence of the resistivity appears and develops at the temperature TPl lower than TC, while the another peak near TC becomes smaller and then disappears. At 78 K the La1−xSrxMn0.88Fe0.12O3 oxides are insulators for x<0.2 and x>0.5 and only the lower-temperature metal–insulator transition peak appears for 0.25<x<0.45. With increase of x from 0.25, TPl increases, becomes maximum at x=0.4 and then decreases. The temperature dependence of resistivity for the doped samples is quite different from that of the undoped La1−xSrxMnO3 series.
Femtosecond pulse evolution in GaAs crystal
Authors: Kumar J. , Sen Pratima
Published: July 2002

Evolution of an ultrashort pulse having frequency spectrum above the band edge in a sample of GaAs has been investigated. The evolution operator technique is used for solving the wave equation under slowly varying envelope approximation coupled with the dynamic response of the medium. The pulse duration has been taken to be less than the dephasing time of the material so that there is no k mixing. Severe distortion and attenuation of pulses in the linear regime is predicted. The pulse is found to stabilize in the nonlinear regime.
Simulation of ion transport in an extended electron cyclotron resonance plasma
Authors: Liu Minghai , Hu Xiwei , Wu Hanming , Wu Qinchong , Yu Guoyang , Ren Zhaoxing
Published: July 2002

A two-dimensional hybrid model for an electron cyclotron resonance (ECR) plasma reactor is improved by taking into account the microwave absorption. With this code, the velocity and angle distribution function of ions in the source and downstream areas, and the ion current density at the substrate surface in an extended ECR plasma reactor were investigated. The spatial profiles as well as the dependence of these distribution functions on pressure and microwave power were discussed. It is shown that the results are in good agreement with many experimental measurements.
Light scattering and atomic force microscopy study of InAs island formation on InP
Authors: Rasnik I. , Brasil M. J. S. P. , Cerdeira F. , Mendonça C. A. C. , Cotta M. A.
Published: July 2002

Some aspects of the morphology of InAs island formation on InP have been studied by atomic force microscopy, photoluminescence, photoluminescence excitation spectroscopy, and Raman scattering. The InAs layer is grown by chemical beam epitaxy on top of InP surfaces with sawtooth-like channels. The deposition of a thin InAs layer results in quantum dots strongly aligned along the InP channels. Subsequent annealing in an arsenic atmosphere produces growth and loss of coherency of the islands. Atomic force microscopy shows the changes in size and alignment of the islands. Optical measurements serve to give quantitative estimates of the strain distribution among the top of the InP buffer layer, the wetting layer and the islands for the differently treated samples.
A rate equation model for the growth of GaN on GaN(0001̄) by molecular beam epitaxy
Authors: Held R. , Ishaug B. E. , Parkhomovsky A. , Dabiran A. M. , Cohen P. I.
Published: July 2002

GaN(0001̄) films were grown by molecular beam epitaxy using ammonia and elemental Ga. The surface reactivity and growth kinetics of GaN(0001̄) were investigated as a function of growth parameters using desorption mass spectroscopy. Growth proceeds either by island nucleation or by step flow, depending on the steady state surface coverage of Ga. Three Ga adsorption states were found on the surface, one chemisorption and two weak states. One of the weak states corresponds to Ga adsorbed on a gallided surface, while the other corresponded to an intrinsic physisorption state on a hydrogen-passivated, nitrided surface. An abrupt growth mode transition between excess Ga and excess nitrogen was found as a function of growth parameters. The transition was modeled by rate equations based on growth at step edges and the three types of adsorption states.
Interfacial electronic structure for Ca and an electroluminescent polymer: Poly (2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene)
Authors: Rajagopal Aparna , Koch Norbert , Ghijsen Jacques , Johnson Robert L. , Kaeriyama K. , Leising Günther , Pireaux Jean-Jacques
Published: July 2002

X-ray photoemission spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) have been used to investigate the evolution of the interfacial electronic structure of a blue light-emitting polymer: poly (2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene) (PDHPT) as a function of Ca metal coverage. UPS results show a decrease in intensity of the highest molecular orbital-derived feature of the polymer upon Ca deposition and a decrease in work function. XPS data suggest that Ca interacts preferentially with S atoms in the thienylene functional group, and with the aromatic carbon atoms of the π-conjugated backbone. Both sets of experiments are compatible with a significant perturbation of electronic structure of the polymer at the Ca–PDHPT interface. Widths and energy positions of the C 1s, S 2p, and Ca 2p spectra suggest cluster formation and island growth of Ca on PDHPT. There is diffusion of Ca into PDHPT with time, and heating the Ca-covered PDHPT to 100 °C in ultrahigh vacuum leads to improved wetting of the surface. These results provide further insight into the performance of an organic light-emitting device consisting of a Ca–PDHPT interface.
Selective fabrication of InGaN nanostructures by the focused ion beam/metalorganic chemical vapor deposition process
Authors: Lachab M. , Nozaki M. , Wang J. , Ishikawa Y. , Fareed Q. , Wang T. , Nishikawa T. , Nishino K. , Sakai S.
Published: July 2002

In the present article we report on the selective fabrication of InGaN nanostructures on Si coated GaN/sapphire substrates using the focused ion beam (FIB)/metalorganic chemical vapor deposition (MOCVD) process. InGaN quantum dots and InGaN quantum wires have been fabricated. The process combines window openings in the Si mask layer with localized highly energetic Ga+ FIB irradiation, subsequent photo-assisted wet chemical etching in a solution of KOH:H2O2 (3:1 by mole) and finally the growth of InGaN/GaN nanostructures using MOCVD. This technique proved to be efficient in realizing practically damage-free etching, hence preventing the deterioration of the nanostructure’s crystal quality. The density, size, and positions of the nanostructures could be well designed and controlled using the above process. Structural characterization by transmission electron microscopy, atomic force microscopy observations, and optical investigation by cathodoluminescence were carried out.
Hysteresis with nonequilibrium characteristics in sidegating effect of GaAs devices
Authors: Zhao Fu-Chuan , Ding Yong , Xia Guan-Qun , Tan Hui-Zu
Published: July 2002

Hysteresis was observed with nonequilibrium characteristics in the sidegate voltage dependence of drain current when measuring the threshold behavior of sidegating effect in GaAs metal–semiconductor field-effect transistors in the voltage-controlled condition. Once the sidegate voltage is varied in a quasi-steady state, hysteresis disappears immediately. A new mechanism is presented to explain the phenomenon that hysteresis are related to electron capture and emission from EL2 deep center on the substrate side of channel-substrate junction.
Power self-regulation in double-pass high-gain laser amplifiers
Authors: Casperson Lee W. , Casperson Janet M.
Published: July 2002

Double-pass laser amplifiers can provide automatic passive regulation of the power in an optical signal. This regulation can significantly reduce the amplitude noise on a laser beam that is intended as a continuous wave light source. Analytic expressions are derived to describe the optical-noise-reduction region of double-pass amplifier operation and the dependence of the self-regulation properties on gain, saturation, and mirror reflectivity.
High-quality Si-implanted In0.53Ga0.47As epitaxial layers and their application to n+p junction devices
Authors: Blanco M. N. , Redondo E. , Calle F. , Mártil I. , González-Díaz G.
Published: July 2002

Si implantations into undoped In0.53Ga0.47As have been carried out to obtain n-layers suitable for device applications. Different doses and energies have been analyzed. After rapid thermal annealing at 850–875 °C for 10–20 s, electrical activations of about 100%, and mobilities as high as 4000 cm2/V s were obtained. Different Hall measurements show that there is no redistribution of the dopants. Photoluminescence measurements demonstrate the satisfactory recrystallization of the lattice and the excellent activation of the dopants. Electrical characteristics of n+p junctions made by Si implantation into Zn-doped In0.53Ga0.47As are described. Junction behavior at forward bias could be explained by recombination in the space-charge zone mechanisms, whereas different tunneling processes dominate at reverse bias.
Ballistic electron magnetic microscopy studies of magnetization reversal in Co/Cu/Co trilayer films
Authors: Rippard W. H. , Buhrman R. A.
Published: July 2002

We have used ballistic electron magnetic microscopy to image, with nanometer resolution, the magnetization behavior of Co/Cu/Co trilayer films in the presence of a magnetic field. Films prepared both by thermal evaporation and by magnetron sputtering have been studied. In each case we have observed both large, ∼500 nm, domain structures, and much smaller, apparently randomly dispersed, regions of magnetic misalignment between the Co layers that persist to fields >100 Oe. We find the details of the ballistic electron transport through the films to be different on small length scales, ∼50 nm, for the two types of growth methods.
Anisotropy in magnetic and mechanical properties in textured Hiperco® FeCoV alloys
Authors: Shang Chang He , Weihs T. P. , Cammarata R. C. , Ji Yi , Chien C. L.
Published: July 2002

Hiperco ® FeCoV alloys are soft magnetic materials suitable for applications at high temperatures and under high stresses. We have studied the evolution of the rolling texture and its effects on the mechanical and magnetic properties after annealing the Hiperco® sheets at high temperatures. X-ray pole-figure measurements reveal that the main texture of the as-rolled HA50HS and HA50 sheets is {001}〈11̄0〉. This rolling texture causes the mechanical and magnetic properties to exhibit a uniaxial anisotropy, which is reduced by annealing.
Study of shield effect on magnetoresistive head performance using magnetoresistive sensitivity map and spin-stand tester
Authors: Han Dehua , Hansen Michael E. , Ding Juren , Fernandez-de-Castro Juan J.
Published: July 2002

The effect of the bottom shield on magnetoresistive (MR) head read-back performance was systematically studied by using a spin-stand tester and a magnetic force microscope with magnetoresistive sensitivity map (MSM) capability. Two groups of high density MR heads with identical design, processes and materials, except for their bottom shields, were tested. It was found that the higher the initial permeability of the shield, the smaller the PW50, the steeper the 50% height slope and the smaller the shoulder in the corresponding side of the isolated transition read-back pulse. The MSM images and analysis not only corroborated the spin stand results, but they also gave additional information about the isolated pulse shape and cross-track profile. The MSM was found to be a useful tool in studying the shield effects on isolated pulses as well as their impact on microtrack profiles.
Magneto-impedance of glass-coated Fe–Ni–Cu microwires
Authors: Wiggins J. , Srikanth H. , Wang K.-Y. , Spinu L. , Tang J.
Published: July 2002

The magneto-impedance (MI) of glass-coated Fe–Ni–Cu microwires was investigated for longitudinal radio-frequency (rf) currents up to a frequency of 200 MHz using a rf lock-in amplifier method. The MI, defined as ΔZ/Z=[Z(H)−Z(H=0.3 T)]/Z(H=0.3 T), displays a peak structure (negative MI) at zero field for rf currents with frequencies less than 20 MHz and this crosses over to a sharp dip (positive MI) at higher frequencies. This crossover behavior is ascribed to the skin-depth-limited response primarily governed by the field-dependence of the permeability. Large saturation fields (300 to 600 Oe) and other anomalies indicate the possible influence of giant magneto-resistance (GMR) on the MI.
Ultrafast intra- and intermolecular energy transfer in solutions after selective infrared excitation
Authors: Scherer P. O. J. , Seilmeier A. , Kaiser W.
Published: July 2002

Energy is supplied to the solvent or solute molecules via resonantly absorbed infrared photons. The long wavelength tail of the S1 absorption band of a probe molecule is monitored on a picosecond time scale. This technique is used to study energy transfer from the solvent to the probe molecule and vice versa. Exciting a vibrational mode of the probe molecules, the energy is rapidly redistributed via intramolecular processes followed by an intermolecular dissipation to the solvent with time constants of ∼10 ps. Intermolecular energy transfer from the solvent to the probe molecules is observed within ≲5 ps, independent of the solvent.
Approaching the exact energy for H2: Bond functions vs polarization functions
Authors: Wright James S. , Barclay V. J.
Published: July 2002

Configuration interaction calculations are reported for hydrogen molecule. The relative efficiency of nuclear−centered polarization functions, verses mid−bond functions, in lowering the absolute energy of hydrogen molecule is examined.(AIP)
Freezing in the density functional approach: Effect of third-order contributions
Authors: Curtin W. A.
Published: July 2002

The widely used Ramakrishnan–Youssouff density functional theory of freezing (a functional expansion, usually truncated at second order, of the excess Helmholtz free energy Fex of the solid about Fex of the coexisting liquid) is extended in a straightforward manner to third order to study the convergence of the theory. The required three-particle direct correlation function of the liquid c(3) is obtained by using the weighted-density functional approximation to Fex as a generating functional. A freezing transition in the hard sphere system is obtained but the predicted coexistence conditions are much worse than those of the second-order theory and in poor agreement with simulations. The magnitude of the third-order contribution to the solid grand potential is typically about one-half of the second-order contribution. Expanding about the equal density liquid rather than the coexisting liquid includes a subset of terms exactly to all orders but fails to predict freezing at either second or third order. These results indicate that the convergence of the functional expansion is not sufficiently rapid to justify truncation at low orders. The generally good agreement predicted by the usual second-order theory is thus apparently fortuitous.
Interaction of oxygen with a Pd(110) surface. I. Structures and coverages
Authors: He Jian-Wei , Memmert Ulrich , Griffiths Keith , Norton Peter R.
Published: July 2002

Adsorption and desorption of oxygen on a Pd(110) surface have been investigated by low energy electron diffraction (LEED), nuclear reaction analysis (NRA), and Rutherford backscattering (RBS). Exposure of Pd(110) to O2 at 100 K and subsequent heating, produces the following surface phases in sequence: pseudo-(2×1), c(2×6), and c(2×4). Exposure at 280 K produces only the c(2×4) phase. Above 400 K the (2×3)-1D and c(2×4) form in sequence upon O2 exposure. The phase transition (2×3)-1D to c(2×4) is accompanied by the continuous movement of the third order beams in (2×3)-1D to the half order positions. The transition between the c(2×4) and (2×3)-1D is reversible above 400 K, and depends only on the oxygen coverage. NRA indicates that the saturation coverage of oxygen with exposure to O2 at 100 K is close to 1 ML (1 ML=9.4×1014 O atoms cm−2 ). The c(2×4) and (2×3)-1D phases have surface coverages of 0.50±0.05 and 0.23±0.05 ML, respectively. The RBS results show that the surface exposed at low temperature, the (2×3)-1D and the c(2×4) phases all involve reconstruction of the Pd surface. In the low temperature exposed surface, more than a monolayer of Pd atoms are displaced laterally from their bulk-like locations in the surface region, indicating that there are displacements in at least the second layer. In the c(2×4) phase more than 0.6 ML of Pd atoms are laterally displaced. Models for the two phases are tentatively proposed.
Study of the universality class of the gel network system
Authors: Li Yong , Tanaka Toyoichi
Published: July 2002

The critical isobar and the specific heat Cv of the N-isopropylacrylamide gel along the critical isobar have been measured. Two critical exponents were obtained from which the conventionally defined exponents are derived. Kinetics of the temperature relaxation process was also studied, which showed an anomaly near the critical point. Fisher renormalization of the critical exponents was observed in the specific heat experiment. The experimental results indicate that the gel system is in the Ising universality class.
Role of the long-range oscillatory tail of the effective pair potential in determining the structure of liquid metals
Authors: Matsuda Naoshi , Hoshino Kozo , Watabe Mitsuo
Published: July 2002

To investigate the role of the long-range oscillatory part of the effective pair potential in determining structural features of liquid metals, the structure factor S(q) and the pair correlation function g(r) calculated for the full pair potential are compared with those obtained for various cutoff forms of the pair potential. The calculations are performed by employing the modified hypernetted-chain approximation, which is one of the best approximations in the integral equation theory of liquid. The results for liquid cesium are presented in detail. It is shown that the peak positions of g(r) and S(q) are affected rather significantly by the long-range oscillatory tail of the pair potential, the contribution from the short-range attractive part being especially important. These features are quite different from the results for the Lennard-Jones liquid, whose structure is known to be dominantly determined by the short-range repulsive part of the pair potential.
Collisional deactivation of K(7s 2S) and K(5d 2D) by H2
Authors: Chang Hai-Chou , Luo Yeung-Long , Lin King-Chuen
Published: July 2002

The radiative lifetimes and total cross sections for deactivation of K(7 2S ) and K(5 2D ) by collisions with H2 have been studied. In the sample vapor, the K atoms were prepared in either the 7 2S or the 5 2D state by two-photon absorption using a dye laser. The decay signal of the time-resolved fluorescence from the 7 2S–4 2P1/2 or 5 2D–4 2P3/2 transition was then monitored. Based on the Stern–Volmer equation, the radiative lifetimes are (157±5) ns for the 7 2S state and (569±12) ns for the 5 2D state. The total cross sections for deactivation of excited K atoms by means of collisions with H2 are (150±2) A2 for the 7 2S state and (39±4) A2 for the 5 2D state. The radiative lifetimes in the absence of H2 collisions agree with those previously reported. The cross sections for quenching by H2 have been measured for the first time and are interpreted in terms of the harpoon mechanism. The cross section for the transition 7 2S–5 2D has been measured; this transition has been proved to be the predominant channel for the depopulation of the 7 2S state.
Diffuse x-ray scattering study of toluene and polybromostyrene PBrS/toluene solutions
Authors: Zhao W. , Zhao X. , Sokolov J. , Rafailovich M. H. , Sanyal M. K. , Sinha S. K. , Cao B. H. , Kim M. W. , Sauer B. B.
Published: July 2002

We report measurements and calculations of x-ray diffuse scattering from the liquid–vapor interface of toluene and polybromostyrene(PBrS)/toluene solutions for polymer molecular weights 90 K and 1 M at concentrations up to 11.7 volume %, well into the entangled semidilute regime. We have calculated the static structure factor S(k) and equal time height–height correlation function C(R) for surface hydrodynamic modes based on a coupled two-fluid model where the polymer response is taken to be that of a Maxwell viscoelastic material [Harden et al., J. Chem. Phys. 94, 5208 (1991)]. We obtain the leading correction term to the capillary-wave result for C(R) dependent on the solution shear modulus E0, as well as an analytic approximation valid for large E0, including the case of pure polymer melt.
Electronic mechanisms of Penning ionization involving excited alkali atoms
Authors: Djerad M. T. , Gounand F. , Kumar A. , Cheret M.
Published: July 2002

Penning ionization (PI) occurring during the collision of a Rydberg atom Rb(nl) with a resonant state K(4P) or Rb(5P) is analyzed theoretically. By using a semiclassical approach, we calculate the rate coefficients corresponding to both the direct and exchange decay channels. We implement the method by deriving various approximations for the exchange and direct decay widths. The radiative mechanism is found to dominate the thermal-energy quenching of alkali atoms by PI, the exchange being negligible. The calculated values are in reasonable agreement with the available experimental results.