bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of oxidation on the thermoelectric properties of PbTe and PbS epitaxial films
Authors: Rogacheva E. I. , Krivulkin I. M. , Nashchekina O. N. , Sipatov A. Yu. , Volobuev V. V. , Dresselhaus M. S.
Published: July 2002

We report on the thickness d dependences of the Seebeck coefficient, electrical conductivity, and Hall coefficient of PbTe and PbS epitaxial thin films (d=5–200 nm), prepared by thermal evaporation in vacuum and deposition on (001) KCl substrates. The oxidation of the films in air at 300 K leads to a sign inversion of the carrier type from n to p in films with d⩽125 and 110 nm for PbTe and PbS, respectively. The observed d dependences are interpreted in terms of compensating acceptor states created by oxygen on the film surface.
Peak effect in laser ablated DyBa2Cu3O7−δ films at microwave frequencies at subcritical currents
Authors: Bhangale A. R. , Raychaudhuri P. , Banerjee T. , Pinto R. , Shirodkar V. S.
Published: July 2002

In this article we report the observation of a peak in the microwave surface resistance (at frequencies ∼10 GHz) of laser ablated DyBa2Cu3O7−δ films in magnetic field ranging from 2 to 9 kOe (∥c) close to the superconducting transition temperature [Tc(H)]. The exact nature of the peak is sample dependent but it follows a general behavior. The peak shifts to lower temperature when the magnetic field is increased. It has strong frequency dependence and the peak is pronounced at frequencies close to the depinning frequency of the flux line lattice. From the observed temperature and field dependence we argue that this peak is associated with the order–disorder transition of the flux line lattice close to Tc(H).
Planar Hall effect and magnetoresistance in spin valve multilayers
Authors: Lu Z. Q. , Pan G. , Li J. , Lai W. Y.
Published: July 2002

Magnetoresistance (MR) and planar Hall effect (PHE) in spin valves were simultaneously measured in fields applied at different angles (α) in the film plane with respect to the easy axis of the free layer. MR curve measurements showed that the MR response to the field was linear in the α angle range from ±60° to ±120°. However, it was found that when the applied field was near perpendicular to the easy axis of the free layer, PHE curves were nonsymmetrical on both sides of the axis along the sample height. The Boltzmann transport equation was used to simulate the MR and PHE curves and determine magnetization orientation of the free layer. Results showed that the nonsymmetrical PHE curves originated from the interaction of the interlayer exchange coupling field between the pinned and free layers. Consequently, the magnetization reversal was a coherent rotation when the applied field angle α>90° and incoherent when α<90°.
Alternating current susceptibility study of the low doped regime of La1−xSrxMnO3 perovskites
Authors: Martı́nez B. , Laukhin V. , Fontcuberta J. , Nogués J. , Skumryev V. , Muñoz J. S. , Pinsard L. , Revcolevschi A.
Published: July 2002

The dynamic response of La1−xSrxMnO3 perovskites has been studied in the low doped regime (x⩽0.14). The frequency dependence of the ac susceptibility gives clear indications in favor of the existence of electronic phase separation not only in the O″ phase but also at temperatures between the ferromagnetic transition temperature TC, and the O′–O″ structural transition occurring at TFI. The influence of the Jahn–Teller distortion on the magnetic susceptibility is also detected as a drop in the magnetic susceptibility on crossing down the O–O′ structural transition, while a jump occurs at O′–O″ transition. A close similarity, both structural and magnetic, between the O and O″ phase is strongly suggested by the experimental results.
Ex situ CrPtMn pinning and ex situ CrPtMn pinned spin valves
Authors: Qian Zhenghong , Tondra Mark , Wang Dexin , Daughton James M. , Brownell David , Nordman Cathy , Schuetz James A.
Published: July 2002

Ex situ CrPtMn pinned bilayers and ex situ CrPtMn pinned spin valves have been investigated by exploring the correlation between the pinning and deposition process. It was found that exchange coupling is strongly related to the deposition condition. The ex situ deposited CrPtMn can only reliably exchange couple to the NiFeCo (or CoFe) when CrPtMn is deposited on it with an applied magnetic field. The exchange coupling is not seen as a strong function of the thickness of the removed NiFeCo (or CoFe) surface layer if the backsputter time is longer than 3 s. However, it is found that the giant magnetoresistance (GMR) value decreases with an increase of backsputter time, which is probably due to interface disruption during backsputter. It is found that 3–5 s backsputter can produce GMR values as high as in situ CrPtMn-pinned spin valves. The GMR value is around 8% for ex situ CrPtMn-pinned spin valves with a configuration of Ta(30 Å)/NiFe(45 Å)/CoFe(10 Å)/Cu(30 Å)/CoFe(41 Å)/CrPtMn(300 Å).
Mechanism of electron-beam writing in passivated gold nanoclusters
Authors: Bedson T. R. , Palmer R. E. , Wilcoxon J. P.
Published: July 2002

We have investigated the mechanism of direct electron-beam writing in thin films of passivated gold nanoclusters. The exposure of films of approximately monolayer thickness (6 nm) was investigated as a function of electron dose on various substrates. Films were obtained on various substrates: graphite, silicon, thermally grown silicon dioxide and sputtered silicon dioxide. The experimental results are compared with Monte Carlo simulations of the electron scattering. We conclude that, in the case of such monolayer films, exposure of the clusters is dominated by electrons scattered in the substrate, so that the properties of the resist depend strongly on the nanocluster/substrate combination.
Structure and chemistry of manganite based tunnel junctions
Authors: Ozkaya D. , Petford-Long A. K. , Jo Moon-Ho , Blamire M. G.
Published: July 2002

Half-metallic manganite (La0.7Ca0.3MnO3) based spin tunnel junctions with two barrier layers (NdGaO3 and La0.45Ca0.55MnO3) have been investigated structurally as well as chemically using high spatial resolution electron microscopy techniques. Both junctions were grown on a NdGaO3 substrate, which has better lattice match compared to a more traditional substrate such as strontium titanate (SrTiO3). Devices with a NdGaO3 barrier showed tunnel magnetoresistance (TMR) values up to 85% at 77 K, whereas the devices with a La0.45Ca0.55MnO3 barrier layer showed only 18% TMR at 77 K. The electron microscopy data show much clearer, sharper interfaces, structurally as well as chemically, for NdGaO3 than for the La0.45Ca0.55MnO3 barrier layer.
Universal and specific contributions in the density-like functionals: Application to transition metals
Authors: Zein N. E. , Antropov V. P.
Published: July 2002

We present an approach for obtaining generating functionals for density-functional-like theory, which takes into account the structure of electronic levels near Fermi surface and density variations at distances exceeding the screening radius in metals. Materials-specific information is contained mainly in these structure-dependent terms, while other terms depend mainly on the local density. We used proposed method to study the electronic structure of ferromagnetic Fe and Ni and obtain reasonable agreement with previous local density calculations and experiment.
Magnetic properties of high-oxygen pressure prepared PrBa2Cu3O7−y cuprates
Authors: Lin Y. H. , Lin B. N. , Lin Y. X. , Hsu Y. Y. , Hsu T. I. , Ku H. C.
Published: July 2002

High oxygen pressure annealed PrBa2Cu3O7−y (y∼0) cuprates were prepared in order to study the effect of oxygen stoichiometric parameter y on the unusual Pr/Cu magnetic properties and/or recently reported superconductivity. The oxygen-rich orthorhombic 123-chain phase is highly unstable under high-oxygen pressure synthesis and decomposes completely in 10 bar pressure. For a smaller 2 bar prepared sample a relatively clean phase was preserved with an oxygen parameter y=0.05, as compared with y=0.11 from a conventional 1 bar flowing oxygen method. No superconductivity can be detected for all high-oxygen pressure prepared samples. Instead, Mott-insulator behavior with anomalous high Pr ordering TN(Pr)=19 K was observed for PrBa2Cu3O6.95. Comparison with other Pr/Ba intersubstituted Pr1−xBa2−xCu3O7−y cuprates is discussed.
Muon spin relaxation study on the heavy fermion compound YbSb
Authors: Hashi Kenjiro , Watanabe Isao , Oyamada Akira , Nagamine Kanetada
Published: July 2002

The magnetic properties of the phase transitions in the heavy fermion compound YbSb are studied by muon spin relaxation measurements at the RIKEN-RAL Muon facility. In this compound, previous neutron diffraction measurements did not detect any long-range magnetic ordering down to 7 mK. On the other hand, Mössbauer spectroscopy measurements on the same sample have reported the presence of two phase transitions. Recently, we have measured the specific heat of YbSb and found two peaks at 0.5 and 5 K. However, the temperature dependence of the muon-spin relaxation rate shows no anomaly at transition points. This result indicates that these phase transitions are not magnetic.
First-order antiferromagnetic and structural transition in Sr-rich Pr1−xSrxMnO3
Authors: Kolesnik S. , Dabrowski B. , Chmaissem O. , Bukowski Z. , Mais J.
Published: July 2002

We present magnetization and resistivity data on polycrystalline samples of Pr1−xSrxMnO3 (0.58⩽x⩽1.00). All the samples are antiferromagnetic insulators at low temperatures. For 0.78<x<0.90, and for x=0.58, both resistivity and magnetization show a clear hysteresis which indicates the first-order nature of the antiferromagnetic transition. Neutron diffraction data show that the paramagnetic-to-antiferromagnetic transition is accompanied by a cubic-to-tetragonal structural transition for 0.78<x<0.90, and tetragonal-to-orthorombic transition for x=0.58. The other samples exhibit continuous transitions to the antiferromagnetic state. Our results show that the structural transitions are the origin of the discontinuous magnetic and resistive transitions. A moderate magnetoresistance effect can be observed for a lower Sr content possibly due to ferromagnetic fluctuations.
Paramagnetic susceptibility and ferromagnetism in Sr2FeMoO6 perovskites
Authors: Navarro J. , Balcells Ll. , Martı́nez B. , Fontcuberta J.
Published: July 2002

In this article, we report a detailed study of the paramagnetic susceptibility χ of Sr2FeMoO6 ceramic samples prepared under various conditions. We show that χ−1(T,H) has a rather unconventional behavior in the 800 K>T>TC(≈400 K) temperature range, namely, a field-dependent χ−1(T) slope which is also sensitive to the synthesis conditions. Detailed analysis and comparison of samples allow us to conclude that this behavior is nonintrinsic but rather reflects the sample quality. We quantitatively show that small amounts of ferromagnetic impurities (<0.1%) can account for the observed behavior. In spite of this, we demonstrate that the dominant magnetic interactions in this system are ferromagnetic, thus strongly supporting a double-exchange mechanism for magnetic interactions.
Magnetotransport and Mössbauer study of Fe3O4/γFe2O3 granular thin films
Authors: Taniyama T. , Kitamoto Y. , Yamazaki Y.
Published: July 2002

Magnetotransport nature of Fe3O4/γFe2O3 granular thin films is examined in the transformation process from Fe3O4 to γFe2O3. The electrical and magnetoresistive features systematically change around the percolation threshold of ferrimagnetic Fe3O4 metal. The electrical conduction can be described on the basis of small-polaron hopping and variable range hopping with coulomb gap in the Fe3O4/γFe2O3 thin films. Possible causes for the change in the magnetoresistance are also addressed.
Current-modulating magnetic force microscope probe
Authors: Wang Frank Z. , Helian Na , Clegg Warwick W , Windmill James F. C. , Jenkins David
Published: July 2002

A new current-modulating probe for the magnetic force microscope (MFM) is proposed in this article. The magnetic field, which will be used to interact with a magnetic specimen’s stray field, is induced on the sharp tip of the conical magnetic core surrounded by a microfabricated single turn conductive coil. The reciprocity principle is used to obtain the force acting on the probe due to the specimen’s stray field when scanned over a magnetic specimen. The magnetic field intensity is adjustable by control of the applied current. Images of specimens have been modeled using this probe. The suitability to different specimens is seen to be the biggest advantage of this scheme over the conventional probe designs.
Magnetic characterization of Fe60Sm40/(Fe96.7Ta3.3)N multilayer films
Authors: Weston J. L. , Yan S.-S. , Zangari G. , Barnard J. A.
Published: July 2002

FeSm/FeTaN hard/soft multilayers with well-defined in-plane uniaxial anisotropy have been grown. Exchange spring behavior is observed and its occurrence and character is correlated with component layer thicknesses and volume fractions. The dc demagnetization remanence technique is used to determine the irreversible changes in the magnetization of the exchange spring structure.
Dependence of the coercive field on the Cu overlayer thickness in thin Co/Cu(001) and Ni/Cu(001) fcc epitaxial films
Authors: Vaz C. A. F. , Bland J. A. C.
Published: July 2002

We have studied the effect of the Cu overlayer thickness tCu on the coercive field Hc of epitaxial ultrathin fcc Co/Cu(001) and Ni/Cu(001) films by magneto-optic Kerr effect magnetometry. The Cu/40 Å Ni/Cu(001) system shows perpendicular magnetic anisotropy (PMA) for the whole tCu range studied (20–180 Å) with Hc showing a large, linear increase with tCu (from ∼280 to ∼360 Oe). On the other hand, the Cu/36 Å Co/Cu(001) structure has dominant in-plane anisotropy and in contradistinction with the Ni case the coercive field decreases rapidly with tCu (from 140 Oe for tCu=20 Å to ∼90 Oe at tCu∼180 Å). The variation in coercivity is described in terms of the magnetoelastic energy contribution to the magnetic anisotropy induced by the Cu overlayers (via strain). In particular, the increase in the Hc of Ni is attributed to an increase in the PMA while the relation between the magnetic anisotropy and coercivity in the case of Co remains unclear.
Effect of hydrostatic pressure on degradation of CdTe/CdMgTe heterostructures grown by molecular beam epitaxy on GaAs substrates
Authors: Wasik D. , Baj M. , Siwiec-Matuszyk J. , Gronkowski J. , Jasiński J. , Karczewski G.
Published: July 2002

We have shown that external hydrostatic pressure leads to the creation of structural defects, mainly in the vicinity of the II–VI/GaAs interface in the CdTe/Cd1−xMgxTe heterostructures grown by the molecular beam epitaxy method on GaAs substrates. These defects propagating across the epilayer cause permanent damage to the samples from the point of view of their electrical properties. In contrast, photoluminescence spectra are only weakly influenced by pressure. Our results shed light on the degradation process observed even without pressure in II–VI-based heterostructures.
Magnetic measurements and numerical simulations of interacting nanometer-scale particles
Authors: Christoph V. , Wirth S. , von Molnár S.
Published: July 2002

Magnetization processes in arrays of ferromagnetic, nanometer-scale iron particles were investigated experimentally and by numerical simulations. The arrays typically consisted of several hundred, pillar-shaped particles (diameter about 13 nm) in square or triangular arrangement. The particles were grown directly onto a thin soft-magnetic Permalloy film to enhance their interactions. A modified boundary-element method was developed to calculate the magnetization pattern of the particle-Permalloy heterostructures with a proper discretization of the system. The calculated magnetic behavior of the Permalloy film, the particles, as well as the coupled heterostructure for applied fields are compared to the experimental magnetization curves. The complex magnetic behavior observed in these heterostructures (e.g., the formation of domain walls within the Permalloy due to the particle magnetization) was also found numerically. These interaction effects may become relevant for ever smaller and more dense magnetic storage media.
Two field-emission states of single-walled carbon nanotubes
Authors: Collazo R. , Schlesser R. , Sitar Z.
Published: July 2002

Two field-emission states of single-walled carbon nanotubes have been identified according to their respective emission current levels. The state yielding increased emission current has been attributed to the presence of adsorbates on the nanotubes. It was realized that, by application of high electric fields inducing large emission currents, a transition between the two states could be induced. For the high current state, field-emitted electrons originated from states located 1 eV below the Fermi level, as was determined by field-emission energy distribution measurements. This suggested that adsorbates introduced a resonant state on the surface that enhanced the tunneling probability of the electrons. These states are removed when the nanotubes are cleaned by application of a large electric field, thus, decreasing the field-emitted current.
Remanent state studies of truncated conical magnetic particles
Authors: Hwang M. , Redjdal M. , Humphrey F. B. , Ross C. A.
Published: July 2002

The remanent state of truncated conical particles is investigated as a function of their size, aspect ratio, and anisotropy, using a micromagnetic model based on the Landau–Lifshitz–Gilbert equation. Particles with a base diameter smaller than three times the exchange length show a “flower” state, while larger particles show a “vortex” magnetization state. The critical size for this transition increases with increasing anisotropy. Small flower-state particles show abrupt reorientation from out-of-plane to in-plane magnetization at a critical aspect ratio of 0.9. For vortex-state particles, the axial remanence gradually increases as the aspect ratio increases, and high aspect ratio particles have significant remanence even at larger diameters.
Enhanced magnetoresistance in Ag-doped granular La2/3Sr1/3MnO3 thin films prepared by dual-beam pulsed-laser deposition
Authors: Li J. , Huang Q. , Li Z. W. , You L. P. , Xu S. Y. , Ong C. K.
Published: July 2002

In this article, we report our investigations on the doping effect metallic Ag on the intergrain magnetoresistance phenomenon. La2/3Sr1/3MnO3 (LSMO) thin films with Ag doping were prepared on (001) LaAlO3 at different substrate temperatures (Ts) by the dual-beam pulsed-laser deposition technique. Grown at Ts=750 °C, the films are perfectly epitaxial with their c axis perpendicular to the film surface. Ag dopant cannot substitute into the LSMO lattice, thus showing no obvious effect on the magnetotransport properties of the film, though it did impair the film in-plane epitaxy and improve the intergrain diffusion of the lattice atoms. However, grown at lower Ts(400 °C), the films are granular with c-axis texture. The Ag dopant exists at GBs and helps to increase the local Mn spin disorder at GBs and the phase interfaces, thus enhancing the eMR value by a factor of 2 compared with the undoped film. Experiment data also suggest that the transport mechanism underlying the Ag-doping enhanced eMR is spin-dependent scattering.
Representation of hysteresis with wipe-out memory
Authors: Friedman G. , Cha K.
Published: July 2002

A model representing scalar hysteretic systems with wipe-out memory is proposed. In this model a hysteresis operator is represented as a power series expansion containing an infinite number of terms in general. It is shown that this representation converges to any given hysteresis relation having wipe-out memory as long as the output of the given hysteresis varies sufficiently smoothly with input history.
Variation of magnetoresistive sensitivity maps of patterned giant magnetoresistance sensors
Authors: Foss-Schroeder Sheryl , van Ek Johannes , Song Dian , Louder Darrell , Al-Jumaily Ghanim , Ryan Pat , Prater Craig , Hachfeld Ed , Wilson Matt , Tench Robert
Published: July 2002

A scanning probe microscope which combines probe contacts for the supply of current with a magnetic force microscope (MFM) for fully automated imaging of electrically active, patterned sensor-like devices across a wafer was developed. This was used for magnetoresistive sensitivity mapping (MSM) of giant magnetoresistive sensors with different stabilization schemes. Multiple measurements of sensors showed that the MSM images were very repeatable. The complex image patterns varied significantly from sensor to sensor across a wafer. With MFM tips magnetized perpendicular to the ferromagnetic films in the sensor, MSM signals at the top and bottom of the sensor were significantly more intense than signals at the sensor interior. Results from micromagnetic calculations were found to be consistent with the experimental observations.
Invariant integration on classical and quantum Lie supergroups
Authors: Scheunert M. , Zhang R. B.
Published: July 2002

Invariant integrals on Hopf superalgebras, in particular, the classical and quantum Lie supergroups, are studied. The uniqueness (up to scalar multiples) of a left integral is proved, and a Z2-graded version of Maschke’s theorem is discussed. A construction of left integrals is developed for classical and quantum Lie supergroups. Applied to several classes of examples the construction yields the left integrals in explicit form.
Initial experiments in the Idaho Dusty Plasma Device
Authors: Gandy Rex , Willis Shawn , Shimoyama Hiro
Published: July 2002

The Idaho Dusty Plasma Laboratory has recently begun operation. Work to date has used an argon dc glow plasma in which dust particles are levitated. Langmuir probe measurements indicate plasma densities in the range 1014–1015 m−3 and electron temperatures in the range 1–5 eV depending on plasma conditions and spatial location. To date monodispersive particles with a diameter range of 20–30 μm have been used. The observed dust clouds have dust densities in the range 1010–1012 m−3. Dust particle average kinetic energies are typically near the electron temperature in magnitude. The bulk of the dust cloud is in a crystalline solid configuration. The motion of the dust can best be characterized as collective normal mode activity. Spectra of the particle motion reveal the distinction between collective and random motion. The dust charge has been measured to be on order of −104e. The variations of particle energy and charge within the dust cloud have been measured.
Coherent versus incoherent dynamics in InAs quantum-dot active wave guides
Authors: Borri P. , Langbein W. , Hvam J. M. , Heinrichsdorff F. , Mao M.-H. , Bimberg D.
Published: July 2002

Coherent dynamics measured by time-resolved four-wave mixing is compared to incoherent population dynamics measured by differential transmission spectroscopy on the ground-state transition at room temperature of two types of InAs-based quantum dots with different confinement energies. The measurements are performed with heterodyne detection on quantum-dot active wave guides to enhance the light–matter interaction length. An elastic nature of the measured dephasing is revealed which is independent of the dot energy level scheme.
Symmetry transformations in extended irreversible thermodynamics
Authors: Chen M.
Published: July 2002

In this paper we consider symmetry transformations that preserve the first law and the second law of thermodynamics, as well as the pseudo-Riemannian structure of the Legendre manifold defined in terms of the second derivative matrix of the entropy function in extended irreversible thermodynamics.
Steady streaming in an oscillatory inviscid flow
Authors: Riley N. , Trinh E. H.
Published: July 2002

Steady streaming, arising within a fluctuating flow field, is commonly associated with attenuation due to viscosity. In this paper we show that such streaming may be induced within an inviscid fluid when the underlying fluctuating flow is the result of a nonconservative body force. In such a situation the induced streaming is significantly greater than that in a viscous fluid brought about by a fluctuating conservative body force. The vehicle we choose to illustrate this is a unidirectional buoyancy force, otherwise known as g-jitter. In particular the flow about a heated sphere in an otherwise gravity-free environment is analyzed. The relationship with the analogous viscous-flow problem is investigated.
Evolution of particle-velocity correlations in sedimentation
Authors: Guazzelli Élisabeth
Published: July 2002

The velocity field of a dilute monodisperse sedimenting suspension is measured as a function of time with the Eulerian particle image velocimetry technique. The sedimentation process is observed to be dominated by vortices of the size of the container in the initial moments after the cessation of mixing, which diminish with time and seem to reach an ultimate size ≈15–20 mean interparticle distances.
Computer simulation of adsorption of a Stockmayer molecule chlorodifluoromethane in activated carbon slit pores
Authors: Jin Wenzheng , Wang Wenchuan
Published: July 2002

The adsorption recovery of HCFC-22 is an urgent task for environment protection. In this work we use the grand canonical Monte Carlo (GCMC) method to simulate the adsorption of HCFC-22 in terms of a slit-like activated carbon adsorbent. In our simulation the molecule of HCFC-22 is modeled by using the effective Stockmayer potential developed by this group. Heterogeneous activated sites with three different densities are imposed on the carbon walls. Three types of simulations are carried out: (1) The Gibbs ensemble MC method is used to test the Stockmayer potential parameters recommended here. (2) The Widom test particle method is used for determining the relationship of the chemical potential and the bulk phase pressure. (3) The GCMC method is used for adsorption simulations. Simulation results suggest that the optimum carbon slit pore is of width 1.75 nm and the activated site density is 0.8 sites/nm2, when the adsorption is conducted at ambient temperature and pressure, and the exhaustion pressure is 0.011 MPa. In this case, the maximum amount of HCFC-22 would be recovered.
A class of integrable Davey–Stewartson type systems
Authors: Maccari Attilio
Published: July 2002

A new integrable Davey–Stewartson type class of systems of partial differential equations in 2+1 dimensions is derived from a previously known integrable equation by means of an asymptotically exact nonlinear reduction method based on Fourier expansion and spatio–temporal rescaling. The integrability by the inverse scattering method is explicitly demonstrated, because the reduction technique is applied to the Lax pair of the starting equation and the corresponding Lax pair of the class of systems of equations is found. The particular characteristics of the reduction method imply that the new systems are likely to be of applicative relevance.
High-quality porous-silicon buried waveguides
Authors: Rossi Andrea M. , Amato Giampiero , Camarchia Vittorio , Boarino Luca , Borini Stefano
Published: July 2002

This letter reports a method to produce porous-silicon waveguides by means of a laser local oxidation process. The estimated losses of the waveguides are below 1 dB/cm. This demonstrates the applicability of this material in integrated optics and telecommunications. Moreover, our results disclose the opportunity to integrate optoelectronic devices onto Si substrates. The laser writing method is achievable at low laser power, thus it is highly efficient and achievable with the standard equipment present in most laboratories. Another advantage is that oxidation is achieved without heating the complete chip, thus simplifying the integration process, i.e., the oxidation is inherently local through the direct-write process. This method opens the opportunity to build microstructures, like channel and membrane filters, in a flexible manner by R&D laboratories.
Spins and microstructure of hydrogenated amorphous carbon: A multiple frequency electron paramagnetic resonance study
Authors: von Bardeleben H. J. , Cantin J. L. , Zeinert A. , Racine B. , Zellama K. , Hai P. N.
Published: July 2002

Polymer-like and diamond-like hydrogenated amorphous carbon films, characterized by high spin concentrations of 1020 cm−3, have been studied by multiple frequency electron paramagnetic resonance (EPR) spectroscopy at 9, 35, and 94 GHz. Whereas the low-frequency measurements show only one single symmetric EPR line, the high-frequency measurements reveal the anisotropic nature of the g tensor. They show the presence of two different paramagnetic centers with anisotropic g tensors of g∥(1)=2.005, g⊥(1)≈2.003, and g∥(2)=2.010, g⊥(2)≈2.003. We attribute both to localized π states on nanosized sp2 graphitic clusters. The g∥ values of the g tensors, which are correlated to the size of the graphite like carbon clusters, indicate two different cluster sizes with values below 50 Å.
Response to “Comment on ‘Role of quasineutrality in drift waves’ ” [Phys. Plasmas 8, 3519 (2001)]
Authors: Saleem H. , Haque Q.
Published: July 2002

It is asserted that the Comment of Shukla et al. [Phys. Plasmas 8, 3519 (2001)] does not focus on errors or mistakes in the original paper. It reproduces several results already quoted in the literature. Some of these results are not consistent. In the original paper only a procedural flaw in the previous investigations was pointed out just for the sake of clarity. It does not point out any big change in the results.
Suppression of phase separation in (AlAs)2 ML(InAs)2 ML superlattices using Al0.48In0.52As monolayer insertions
Authors: Lee S. R. , Reno J. L. , Follstaedt D. M.
Published: July 2002

Al 0.48 In 0.52 As monolayers (ML) are inserted at the binary-compound interfaces of (AlAs)2 ML(InAs)2 ML short-period superlattices (SPSs) during growth on (001) InP. The insertion of Al0.48In0.52As interlayers greater than 2 ML thick tends to suppress the phase separation that normally occurs during molecular beam epitaxy of the SPS. The degree of suppression is a sensitive function of both the monolayer-scale thickness, and the intraperiod growth sequence, of the interlayers in the SPS. Given this sensitivity to monolayer-scale variations in the surface-region composition, we propose that cyclical phase transition of the reconstructed surface initiates SPS decomposition.
Nonlinear hydromagnetic waves in a nonideal magnetohydrodynamic plasma: Exact axially symmetric solutions
Authors: Hamabata Hiromitsu
Published: July 2002

The time evolution of axially symmetric hydromagnetic waves in a nonideal magnetohydrodynamic (MHD) plasma is investigated. Exact time-dependent wave solutions of the nonlinear resistive and dissipative MHD equations for the plasma within a cylindrical annulus are presented. It is shown that the magnetic field and the velocity have different damping time and that in the asymptotic limit t→∞ the plasma relaxes toward the equilibrium state involving helical magnetic and velocity fields or uniform axial flow with no magnetic field for the plasma in which the kinematic viscosity is equal or not equal to the magnetic diffusivity.
Effect of cross-field flow on inertial Alfvén waves of small transverse scale
Authors: Drozdenko T. , Morales G. J.
Published: July 2002

This analytic study examines the effect of cross-field flow on a microscopic current channel. To illustrate the subtle interplay between finite electron inertia and flows, a simple model of excitation of a microscopic current channel is considered. The model consists of a slab antenna exciter with a transverse width on the order of the electron skin-depth driven at a single frequency. The antenna is fixed in the laboratory frame and embedded within a plasma that has a uniform drift across the confining magnetic field. The combined effects of the plasma flow and the intrinsic, collisionless, cross-field expansion of the current channel lead to standing wave structures across the confining magnetic field. The resulting parallel electric fields generate an array of current filaments of alternating polarity which individually have transverse width smaller than the original channel. These results may help interpret laboratory and spacecraft measurements of Alfvénic turbulence and could lead to the development of a diagnostic tool to map plasma flows.
Total-reflection x-ray microscopy
Authors: Jibaoui H. , Erre D. , Cazaux J.
Published: July 2002

This contribution describes a new microscopy based on the total reflection of grazing-incident x rays. The corresponding instrument permits, for the first time, acquiring x-ray total-reflection images of surfaces and interfaces. It is a laboratory instrument based on the modification of a scanning electron microscope equipped with a special mechanical setup and a charge-coupled-device camera but without any focusing element for the x rays. The principle of such microscopy is given with a description of the corresponding instrument. The various parameters characterizing its performance are given and are illustrated by some selected images of surfaces acquired in a few seconds and related to surface areas (of the imaged region) of about 1 mm2. It is, in particular, a lateral resolution restricted to the micrometer range but associated with a relief resolution in the nanometer range and to a rapid detection of isolated details on very flat surfaces. Special attention is devoted to interface imaging.
Time-resolved gain-spectrum measurement scheme using two potential laser media and a streak camera
Authors: Kozeki Toshimasa , Suzuki Yuji , Liu Zhenlin , Sarukura Nobuhiko , Shimamura Kiyoshi , Fukuda Tsuguo , Hirano Masahiro , Hosono Hideo
Published: July 2002

A two-crystal, pump and probe scheme is established allowing the evaluation of potential laser materials with unknown gain and loss properties. Using a streak camera coupled to a spectrograph, the time-resolved gain or loss spectra are obtained using a probe light from an identical gain crystal. This method facilitates the evaluation of preliminary laser materials in a variety of wavelength regions. A new ultraviolet tunable amplification medium, Ce3+:LiSr0.8Ca0.2AlF6, is demonstrated to possess a broad gain spectrum of 18 nm, utilizing this newly proposed scheme.
Structural characteristics and magnetic properties of λ-MnO2 films grown by plasma-assisted molecular beam epitaxy
Authors: Guo L. W. , Peng D. L. , Makino H. , Hanada T. , Hong S. K. , Sumiyama K. , Yao T. , Inaba K.
Published: July 2002

High-quality λ-MnO2 single-crystal films were fabricated on MgO (001) substrates by plasma-assisted molecular beam epitaxy. The structural characteristics and magnetic properties of the films were studied. Two magnetic transition temperatures, T1 and T2, were observed from the films. Their magnetic behavior differed from that of bulk polycrystalline samples synthesized by removing Li ions from the parent LiMn2O4. The first, T1, setting at around 20 K, corresponded to long-range antiferromagnetic ordering similar to that observed in bulk at 32 K. T2, set at 43 K, corresponded to weak long-range ferromagnetic ordering among net magnetization of clusters, in which short-range antiferromagnetic ordering existed. T1 shifted to lower temperature with decreasing film thickness, which indicated that the proximity of the interface and strain played a key role in determining the transition temperature.
Optical characterization of two-dimensional photonic crystal cavities with indium arsenide quantum dot emitters
Authors: Yoshie Tomoyuki , Scherer Axel , Chen Hao , Huffaker Diana , Deppe Dennis
Published: July 2002

We have characterized the modes within two-dimensional photonic crystal nanocavities with self-organized indium arsenide quantum dots as an active material. Highly localized donor mode resonances with 3 to 5 nm linewidth were observed when spatially selective optical pumping the cavities. These modes could be lithographically tuned from 1100 to 1300 nm. Other, more extended modes, were also characterized and exhibited narrower resonance linewidths ranging from 0.6 to 2 nm.
Nonresonant suppression of the sawtooth instability by passing ions in tokamaks
Authors: Lazaros Avrilios
Published: July 2002

A new interpretation is presented for the well-known observation of sawteeth stabilization, during ion cyclotron resonant heating, in the Joint European Tokamak [Campbell et al., Phys. Rev. Lett. 60, 2148 (1988)] and the Tokamak Fusion Test Reactor [Phillips et al., Phys. Fluids B 4, 2155 (1992)]. It is shown that the radial fluxes of superthermal and thermal ions exchange a finite amount of power with the m=1 mode (associated with the sawtooth instability) which is suppressed. The dominant contribution to this effect is provided by the passing ions, which experience in the presence of the mode a much faster (than the trapped ions) radial diffusion. This is a nonresonant electrostatic interaction between the passing ions and the finite parallel electric field (along the perturbed magnetic field line), which takes place at any mode frequency. For the requirements of the present theory, the concepts of the finite parallel electric field, and the diffusion of passing ions, in the presence of a magnetic mode are also developed. The present work also provides the interpretation for the suppression of the sawtooth instability by passing ions, during tangential neutral beam injection, in the Japan Atomic Energy Research Institute Tokamak Upgrade (JT-60U) [Kramer et al., Nucl. Fusion 40, 1383 (2000)].
Modeling the test SGS tensor Tij: An issue in the dynamic approach
Authors: Brun Christophe , Friedrich Rainer
Published: July 2002

Based on Taylor series expansions for the filtered velocity field, we present an analytical study of the “Germano identity” used in the dynamic sub-grid scale (SGS) model for large eddy simulation (LES). We consider the case of explicit filtering defined in physical space and show that the choice of the length scale which characterizes the test SGS tensor Tij is a main issue in modeling the “Germano identity.” In the field of eddy-viscosity models, we demonstrate the accuracy of a Gaussian formulation lfg2=lf2+lg2, based on a priori tests of fully developed pipe flow. More than a generalization of formal results [B. Vreman, B. Geurts, and H. Kuerten, J. Fluid Mech. 339, 357 (1997)], our study reveals constraints and limitations of the dynamic approach.
Spatial theory of optimal disturbances in boundary layers
Authors: Tumin A. , Reshotko E.
Published: July 2002

A spatial theory is proposed for the linear transient growth of disturbances in a parallel boundary layer. Following from the consideration of a signaling problem, the spatial development of disturbances downstream of a source may be presented as a sum of decaying eigenmodes and Tollmien–Schlichting (TS) like instability modes. Therefore, the problem of optimal disturbances may be considered as an initial value problem on the subset of the decaying eigenmodes and a TS wave, and a standard optimization procedure may be applied for evaluation of the optimal transient growth. The results indicate that the most significant transient growth is associated with stationary streamwise vortices. Numerical examples illustrate that favorable pressure gradient decreases the overall amplification. Effects of compressibility and the wall cooling are investigated as well.
Magnetic entropy change in La0.54Ca0.32MnO3−δ
Authors: Xu Q. Y. , Gu K. M. , Liang X. L. , Ni G. , Wang Z. M. , Sang H. , Du Y. W.
Published: July 2002

A La-deficient manganite perovskite sample La0.54Ca0.32MnO3−δ was prepared by conventional solid-state reaction method. The Curie temperature TC is 272 K, about 10 K higher than that of La1−xCaxMnO3. A large magnetic entropy change has been observed and the maximum −ΔSM≈2.9 J/kg K appears at its Curie temperature upon a 0.9 T magnetic field change. The easy fabrication and higher chemical stability make La0.54Ca0.32MnO3−δ a suitable candidate as a working substance in magnetic refrigeration technology.
Shape evolution of Ge/Si(001) islands induced by strain-driven alloying
Authors: Huang C. J. , Zuo Y. H. , Li D. Z. , Cheng B. W. , Luo L. P. , Yu J. Z. , Wang Q. M.
Published: July 2002

The shape evolution of Ge/Si(001) islands grown by ultrahigh vacuum chemical vapor deposition were investigated by atomic force microscopy at different deposition rates. We find that, at low deposition rates, the evolution of islands follows the conventional pathway by which the islands form the pyramid islands, evolve into dome islands, and dislocate at a superdome shape with increasing coverage. While at a high deposition rate of 3 monolayers per minute, the dome islands evolve towards the pyramids by a reduction of the contact angle. The presence of the atomic intermixing between the Ge islands and Si substrate at high deposition rate is responsible for the reverse evolution.
A practical and inexpensive design for measuring the radiation patterns and luminescent spectra of optoelectronic devices
Authors: Ho Ching Hwa
Published: July 2002

Design and implementation of an inexpensive system capable of swift mapping of the radiation patterns and luminescent spectra of optoelectronic devices are presented. The spectrometer is programmed and monitored by a personal computer. Detailed design diagrams of the mechanical structure and electronic hardware of the instrument are described. Measurements of some selective light emitting diode and laser diode optoelectronic components are presented and the experimental analysis demonstrated the intended functions of the system.
Surface plasmon resonance enhanced optical absorption spectroscopy for studying molecular adsorbates
Authors: Wang S. , Boussaad S. , Tao N. J.
Published: July 2002

We present an automated setup to measure the surface plasmon resonance (SPR)-enhanced optical absorption spectra of molecular adsorbates. The setup detects the reflectivity at the SPR resonance angle as a function of the incident light wavelength. Because the resonance angle is wavelength dependent, a feedback mechanism adjusts the photodetector position to follow the resonance angle when the wavelength varies. Both theoretical calculations and experimental measurements show a signal enhancement of up to ∼40 times over the conventional absorption spectroscopy. The SPR-based absorption spectroscopy is surface specific because the optical field is localized near the surface at resonance. In addition, the SPR angular shift is simultaneously measured, which provides adsorbate coverage and adsorption kinetic information. We anticipate that with our automated system, the method could be used in the study of adsorbed molecules and in chemical and biosensor applications.
Cryogenic scanning tunneling microscope for quantum dot spectroscopy
Authors: Chang T. H. , Yang C. H. , Yang M. J. , Dottellis J. B.
Published: July 2002

We have designed and fabricated a cryogenic scanning tunneling microscope for probing lithography defined nanometer-scale devices. The piezoelectric double tube is capable of scanning an area up to 22 μm×22 μm, while maintaining atomic resolution. In addition, the sample mount has a 5 mm×4 mm traveling range. Most importantly, the system is compact and, as a result, it can be inserted into the bore of a superconducting magnet. In this work, we demonstrate a unique application of scanning tunneling system, i.e., the scanning tip is in direct contact with the sample. The spectroscopic information therefore reflects the true characteristics of the devices under test, unlike the typical case where the tunneling barrier through vacuum imposes a large series resistance, on the order of 109 Ω. The design as well as the operation of this compact scanning tunneling microscope is described.
Observability of resonance optical structure in fractal metallic clusters
Authors: Pustovit Vitaly N. , Niklasson Gunnar A.
Published: July 2002

We investigate optical properties of the deterministic network model for fractal metallic clusters at the percolation threshold. As input data we used the experimental bulk dielectric function of gold, aluminum, and nickel. Sharp absorption resonances in the optical spectra are observed only for gold. They are pronounced only for clusters consisting of particles with sizes larger than the mean free path of the conduction electrons. In smaller particles the peaks are heavily damped due to scattering of electrons by the particle surfaces.
X-ray crystal spectroscopy with stationary-state single-crystal utilizing Laue diffraction
Authors: Honda Kazumasa , Ohchi Tadayuki , Kojima Isao , Hayashi Shigeki
Published: July 2002

A new type of x-ray crystal spectrometry has been designed to characterize a hard x-ray spectrum of each pulse emission from a laser-induced plasma x-ray source. Utilizing the Laue diffraction and a two-dimensional detector, it makes possible the measurement of diffracted monochromatic x rays without any time difference using a single crystal in a stationary state. Thus, a hard x-ray spectrum in the range of a wide wavelength can be processed in a very short time period, although the obtained spectrum consists of discrete values. A demonstration of the performance of the proposed spectroscopy was carried out using a conventional x-ray source and a diffractometer.
Lightweight long-hold-time Dewar
Authors: Fixsen D. J. , Cheng E. S. , Crawford T. M. , Meyer S. S. , Wilson G. W. , Oh E. S. , Sharp E. H.
Published: July 2002

We describe the design, operation, and performance of a light-weight, long-hold-time 3He cryostat used to cool the bolometric detectors of a far-infrared radiometer. A small internally pumped 3He reservoir, supported by 4He and liquid-nitrogen baths, is maintained at 0.25 K for more than a week. The cryostat has a total volume of 30 l and weights 10 kg.
Apparatus for low-frequency study of high-loss dielectrics
Authors: Janik P. , Tomawski L. , Ziolo J.
Published: July 2002

The measurement setup for studying complex dielectric permittivity in a region of frequency between 10−3 to 105 Hz. Loss factor, tan δ of the sample can be very high, from 10 to 104, because the loss resistance is compensated by negative resistance, realized by a negative impedance converter. When loss resistance is compensated by negative resistance then oscillations are excited in the circuit. An inductance is simulated by electronic inductance simulator, which quality factor Q is high and the region of setting value of inductance is eight orders wide. The unknown value of capacitance of the sample is calculated from the oscillations frequency and the unknown value of parallel loss resistance from the parameters setting on the negative impedance converter.
Modeling the rapid pressure-strain correlation in homogeneous flows
Authors: Knoell Jens , Taulbee Dale B.
Published: July 2002

For homogeneous flows the rapid pressure-strain correlation tensor can be written in terms of the mean velocity gradient and an integral of the two-point correlation tensor. In the present study the two-point correlation tensor is modeled and analytically integrated to obtain the pressure-strain model coefficients, which can then be expressed in terms of closed integrals of the two-point correlation model coefficients. Therefore, deficiencies of the pressure-strain models can be detected and resolved on the two-point correlation level. As an example for the new approach, direct numerical simulations results for a homogeneous shear flow are analyzed in detail and the resulting coefficients are compared to existing model coefficients for linear and nonlinear pressure-strain models.