bluuebunny/crossref_metadata_2025 huggingface dataset:
A computer-controlled apparatus for performing pressure–composition–temperature measurements on metal hydrides with protium, deuterium, and tritium
Authors: Nobile A. , Bach H. T. , Romero J. , Basinger R. W.
Published: July 2002

An apparatus for measuring protium, deuterium, and tritium pressure–composition–temperature (PCT) thermodynamic equilibrium behavior on metal hydrides has been designed, constructed, and operated with deuterium at the Weapons Engineering Tritium Facility at Los Alamos National Laboratory. The apparatus is capable of performing PCT measurements with the three hydrogen isotopes at a maximum pressure of 3.45 MPa, and sample temperatures in the range 25–300 °C. A personal computer that controls the system provides the capability for manual or automated operation. Two deuterium PCT isotherms on palladium at 51 and 70 °C were measured with the apparatus in the automated mode to demonstrate its operation. Comparison of the resulting isotherms with literature data showed a plateau pressure at 70 °C in close agreement with the literature value (57.06 kPa versus literature value of 57.33 kPa). The deuterium capacity at 70 °C in palladium at the high end of the isotherm was measured both by the PCT apparatus and by weighing the sample before and after the PCT measurement. The deuterium capacity determined by the PCT apparatus (D/M=0.624 at 144.52 kPa) agreed well with the value determined by the weight difference measurement (D/M=0.628, at 144.52 kPa). These values agreed closely with the literature value (D/M=0.629 at 144.52 kPa).
Numerical Fokker–Planck calculations in nonuniform grids
Authors: Bizarro João P. S. , Rodrigues Paulo
Published: July 2002

A practical, implicit finite-difference scheme, accurate to second order both in time step and mesh size, is presented to solve nonlinear Fokker–Planck (FP) equations in nonuniform grids. Weighting coefficients are introduced for proper equilibration, and the conditions under which the nonuniformity correction is essential in them to ensure physically meaningful solutions are analyzed. Conservation of particle number and energy is addressed, as well as the constraints on the design of nonuniform grids. The proposed scheme is shown to efficiently handle both linear and weakly nonlinear problems and, in addition, its ability to provide solutions to stronger nonlinear situations is demonstrated. Examples of FP equations (describing Coulomb collisions, Compton scattering, and rf heating and current drive) are solved to illustrate the effectiveness of nonuniform grids in reducing computational effort with no significant deterioration in accuracy, and in increasing the latter at no expense of the former.
Size and thickness dependencies of magnetization reversal in Co dot arrays
Authors: Lebib A. , Li S. P. , Natali M. , Chen Y.
Published: July 2002

The magnetization reversal properties of high-resolution Co dot arrays fabricated by nanoimprint lithography have been studied as a function of both diameter and thickness. Both vortex and single-domain states were observed by measuring the hysteresis loops, which result in an experimental phase diagram in the diameter-thickness plane. In the vortex state, magnetization reversal proceeds by vortex nucleation, growth, and subsequent annihilation under relatively high field. The vortex growth has been shown to be reversible in a wide field range, in agreement with micromagnetic simulations. Finally, a distribution of annihilation fields in patterned dot arrays was studied experimentally.
State-specific reactions HBr+(2Πi,v+)+(H2, HBr)→H2Br+ at low collisional energies
Authors: Belikov Andrey E. , Mullen Christopher , Smith Mark A.
Published: July 2002

State-specific ion-molecule reactions of H-atom transfer between the HBr and H2 molecules with HBr+(2Πi,v+) were studied in a free jet flow reactor. The selected spin-orbit and vibrational states of the HBr+ ion were prepared by resonance-enhanced multiphoton ionization. All of the reactant and product ions were monitored using a time-of-flight mass spectrometer. Rate coefficients of the HBr+/H2 reaction vary from <2×10−12 cm3/s for the lowest spin-orbit-vibrational state to 2.1×10−11 cm3/s for the highest. Rates for the HBr+/HBr reaction are ∼1.5×10−9 cm3/s independent of the internal state of the ion.
High-energy plasma acceleration by means of an ideal arcjet thruster
Authors: Hirano Kei-ichi
Published: July 2002

A theoretical study is presented on the entropy acceleration of a plasma by means of an ideal arcjet thruster made up of two major parts; an arc column of a uniform temperature between the cathode and the anode, and the heating zone of the injected cold gas. The plasma flow through the arc column follows the Navier–Stokes, since its dynamic viscosity is quite high. Proper boundary values on the cathode surface give a solution that yields sharp plasma acceleration to a high energy by the viscous force. In the heating zone, the flow is driven up mainly by a large temperature difference between the arc and the injected cold gas. The viscous force plasma driving model is applied to the solar wind; good agreement is confirmed between the theory and the observed data.
The braiding for representations of q-deformed affine sl2
Authors: Beggs E. J. , Johnson P. R.
Published: July 2002

We compute the braiding for the “principal gradation” of Uq(sl2∧) for |q|=1 from first principles, starting from the idea of a rigid braided tensor category. It is not necessary to assume either the crossing or the unitarity condition from S-matrix theory. We demonstrate the uniqueness of the normalization of the braiding under certain analyticity assumptions, and show that its convergence is critically dependent on the number theoretic properties of the number τ in the deformation parameter q=e2πiτ. We also examine the convergence using probability, assuming a uniform distribution for q on the unit circle.
Perturbation theory and the energy spectrum of normal Fermi systems
Authors: Povzner A. Ya. , Kukharenko Yu. A.
Published: July 2002

This paper is a study in a possible steady-state perturbation theory for arbitrary excited multiply degenerate states of a normal Fermi system in the statistical limit. An operator technique has been developed to transform the relevant Hamiltonian into an operator that is a function of occupation number operators only. The perturbation theory equations have been proved to be solvable in the space of quasinormal form operators and their formal solutions have been derived. It is shown that the operator series of perturbation theory can be transformed to a linked cluster expansion with the nonphysical powers of volume eliminated. Elimination of nonphysical terms is effected without use of the diagrams technique.
Discrete symmetries in axisymmetric toroidal plasma confinement
Authors: Cohen R. H. , Ryutov D. D.
Published: July 2002

Several discrete symmetry parameters characterizing axisymmetric toroidal plasmas have been introduced. For a device without up–down symmetry such parameters are S1, related to the handedness of the toroidal current, S2, related to the direction of the toroidal velocity, and S3, characterizing the handedness of the toroidal magnetic field. All these parameters can acquire values ±1, thereby making eight different combinations. For a device with up–down symmetry only the mutual orientation of the toroidal current, toroidal velocity, and toroidal magnetic field remain important. Based on the general expression for the Lagrangian for charged particles in an external magnetic field, we identify the invariance properties of this Lagrangian (and, accordingly, of plasma behavior) with respect to these transformations. Reduced plasma models, based on the Maxwell–Boltzmann equations, and on the magnetohydrodynamic equations, are also considered. An analysis of the plasma behavior in the context of the symmetry properties may be helpful in identifying the most probable theoretical models of plasma transport.
Statistical theory of dusty plasmas: Microscopic description and numerical simulations
Authors: Zagorodny A. G. , Sitenko A. G. , Bystrenko O. V. , Schram P. P. J. M. , Trigger S. A.
Published: July 2002

The first principles of statistical mechanics are used to formulate the basic points of kinetic theory of dusty plasmas. Equations for microscopic phase densities of plasma particles and grains are derived with regard for electron and ion collection by dust particles and elastic contact collisions between grains. The Bogolyubov–Born–Green–Kirkwood–Yvon hierarchy is generalized to the case of dusty plasmas and used to derive kinetic equations, taking into account elastic and inelastic particle collisions. An example of such an equation is presented and applied to the calculations of stationary grain velocity and charge grain distributions. The results of Monte Carlo studies of a strongly coupled dusty plasma are also presented. Microscopic simulations of critical behavior of a dusty plasma with regard for the discrete nature of the plasma subsystem are performed. The effect of nonlinear screening of dust particles on the dusty crystal formation is considered as well.
Effective medium calculation of creep of fiber reinforced composites
Authors: Chui S. T.
Published: July 2002

The effective creep rate of fiber reinforced composites in an effective medium approximation is calculated analytically. The fibers are modeled by ellipsoids of a given aspect ratio. Both the case of aligned and randomly oriented ellipsoids are investigated. The elastic strains are calculated along the previous work of [J. D. Eshelby, Proc. R. Soc. London, Ser. A 241, 376 (1957)]. I find that different amounts of the reinforcement is to change the time scale of change. The functional form remains unchanged.
Effect of quench depth on grain structure in quiescently ordered block copolymers
Authors: Kim W. G. , Chang M. Y. , Garetz B. A. , Newstein M. C. , Balsara N. P. , Lee J. H. , Hahn H. , Patel S. S.
Published: July 2002

Grain growth in a polystyrene–polyisoprene block copolymer melt is studied by time-resolved depolarized light scattering after a quiescent quench from the disordered to the ordered state. At shallow quench depths, classical nucleation and growth kinetics are observed. Grains comprising the equilibrated ordered phase nucleate and grow by consuming the surrounding disordered phase. In contrast, deep quenches result in the formation of disorganized grains with an average order parameter that is well below the equilibrium value. Small angle neutron scattering and rheological experiments were conducted to facilitate the interpretation of the light scattering data. We show that the nonequilibrium grain structure formed during deep quenches is due to extremely high nucleation density. Under these circumstances, the space required for the formation of equilibrated grains is unavailable.
Magnetic measurements on the layered III–VI diluted magnetic semiconductor Ga1−xFexSe
Authors: Pekarek T. M. , Fuller C. L. , Garner J. , Crooker B. C. , Miotkowski I. , Ramdas A. K.
Published: July 2002

Magnetic properties of single crystalline Ga1−xFexSe have been measured. This material is in the new class of diluted magnetic semiconductors based on the III–VI semiconductors. The magnetization versus field for an x=0.05 sample deviates from the linear response seen previously in Ga1−xMnxSe and Ga1−xMnxS and reaches a maximum of 0.12 emu/g (<7% of the expected saturation value) at 1.8 K in 7 T. Ga1−xFexSe exhibits an anisotropy below 2 T from 5 to 400 K with the hard axis perpendicular to the GaSe planes. Neither the broad peak observed from 119–195 K in Ga1−xMnxSe nor the Curie–Weiss behavior observed above 75 K in Ga1−xMnxS are observed in Ga1−xFexSe. The sharp cusp at 10.9 K in Ga1−xMnxS (characteristic of longer range ordering) is also not observed in Ga1−xFexSe in temperatures down to 1.8 K. In 0.1 T in temperatures between 50 and 400 K, the magnetization drops to a roughly constant 0.004 emu/g. Below 5 K, the magnetization approaches a constant value of approximately 0.12 emu/g. The magnetic behavior of Ga1−xFexSe is consistent with Van Vleck paramagnetism.
Structure and representation theory for the double group of the four-dimensional cubic group
Authors: Dai Jian , Song Xing-Chang
Published: July 2002

Hypercubic groups in any dimension are defined and their conjugate classifications and representation theories are derived. Double group and spinor representation are introduced. A detailed calculation is carried out on the structures of four-dimensional cubic group O4 and its double group, as well as all inequivalent single-valued representations and spinor representations of O4. All representations are derived adopting Clifford theory of decomposition of induced representations. Based on these results, single-valued and spinor representations of the orientation-preserved subgroup of O4 are calculated.
Magnetic and transport properties of III–V diluted magnetic semiconductor Ga1−xCrxAs
Authors: Saito H. , Zaets W. , Akimoto R. , Ando K. , Mishima Y. , Tanaka M.
Published: July 2002

Magnetic and transport properties of Cr-based III–V diluted magnetic semiconductor Ga1−xCrxAs Cr concentrations up to x=0.10 have been investigated. For all the films, no long-range magnetic order was observed down to 2 K. A sign of paramagnetic Curie temperature is positive, indicating that the dominant magnetic interaction between Cr atoms is ferromagnetic. The effective number of Bohr magneton is estimated to be 5.1±0.4, which is close to that of Cr2+ ion. The magnetization curve shows superparamagnetic behavior at low temperatures. The radius of the local spin order in x=0.034 is estimated to be 1.5–2.0 nm at T=5 K and it decreases with increasing temperature. From Hall effect measurements at room temperature, the magnitude of mobility is estimated to be less than 0.5 cm2/V s. This low mobility strongly suggests that the hopping conductivity is dominant in Ga1−xCrxAs films.
Effects of ordering and alloy phase separation on the optical emission characteristics of In1−xGaxAsyP1−y layers grown on GaAs substrates
Authors: Bernussi A. A. , Carvalho W. , Dias Franco Margareth K. K.
Published: July 2002

We investigated the optical emission characteristics of low- and high-arsenic content In1−xGaxAsyP1−y alloys grown on exact-oriented (100) GaAs substrates. Clear evidence of a spontaneously ordering superlattice, even in high-arsenic content quaternary samples, was obtained by synchrotron x-ray diffractometry. Photoluminescence measurements at low temperatures revealed the presence of two well-resolved emission bands: an excitation intensity dependent (low-energy) and an excitation intensity independent (high-energy) transitions. Temperature dependent photoluminescence experiments give qualitative information about localization effects of photoexcited carriers in these samples. The origin of carrier localization is attributed to band gap fluctuations in the real space as a result of alloy phase separation and a distribution of domains with varying size and ordering degree.
Low-resistance IrMn and PtMn tunnel valves for recording head applications
Authors: Childress J. R. , Schwickert M. M. , Fontana R. E. , Ho M. K. , Rice P. M. , Gurney B. A.
Published: July 2002

We have investigated the use of ultrathin Al2O3 barriers to fabricate low-resistance tunnel-valve sensors suitable for recording heads. Tunnel valves of the type underlayer/(IrMn or PtMn)/CoFe/Al2O3/CoFe/NiFe/Cap layer have been fabricated by magnetron sputtering. Tunnel barriers are formed by Al metal deposition followed by in situ oxidation, and tunnel-junction test devices are built by photolithography with areas down to 1×1 μm2. Specific resistances as low as 13 Ω μm2 with 25% tunnel magnetoresistance have been obtained using Al thicknesses of 6–7 Å.
Double-scaling limit of a broken symmetry quantum field theory in dimensions D<2
Authors: Bender Carl M. , Boettcher Stefan , Jones H. F. , Meisinger Peter N.
Published: July 2002

The Ising limit is a correlated limit in which two bare Lagrangian parameters, the coupling constant g and the negative mass squared −m2, both approach infinity with the ratio −m2/g=α>0 held fixed. In a conventional Hermitian parity-symmetric scalar quantum field theory, with interaction term g|φ|N/N, the renormalized mass of the asymptotic theory is finite in this limit, and the limiting theory exhibits universality in N. For a non-Hermitian 𝒫𝒯-symmetric but parity-violating Lagrangian, with interaction term −g(iφ)N/N, the renormalized mass diverges in the same correlated limit. Nevertheless, the asymptotic theory still has interesting properties. In particular, the one-point Green’s function approaches the value −iα1/(N−2) independently of the space–time dimension D for D<2. Moreover, while the Ising limit of a conventional theory is dominated by a dilute instanton gas, the corresponding correlated limit of this 𝒫𝒯-symmetric theory is dominated by a constant-field configuration with corrections determined by a weak-coupling expansion in which the expansion parameter is proportional to an inverse power of g. We thus observe a weak-coupling/strong-coupling duality: the Ising limit itself is a strong-coupling limit, but the expansion about this limit takes the form of a conventional weak-coupling expansion. A possible generalization to dimensions D<4 is briefly discussed.
Formation of n-alcohol crystallites from solution
Authors: Kožı́šek Z. , Koga T. , Sato K. , Demo P.
Published: July 2002

Kinetics of heterogeneous nucleation of n-alcohols (C20OH and C22OH) on vapor-deposited thin films of fatty acid is studied both experimentally and theoretically. A kinetic model describing the three-dimensional nucleation and subsequent growth on active centers is solved numerically. The basic characteristics of nucleation process, such as nucleation rate and the total number of nuclei on unit surface, are determined from the distribution function. Depletion of active centers on template film has been incorporated into our computation. The interfacial energy was chosen to get best coincidence between theoretical and experimental data of the total number of nuclei, which gives 0.0029 J m−2 for C20OH and 0.00285 J m−2 for C22OH, respectively.
Magnetic dome configuration for magnetron sputtering
Authors: Ejima Seiki , Shimizu Yoshiaki
Published: July 2002

In order to achieve sputter erosion on the entire target surface in magnetron sputtering devices, we have developed a new “magnetic dome” concept that involves the creation of separatrices and a null point within the sputter discharge volume. In the magnetic dome configuration the field lines coming out of the sputtering target are all bent back onto the target. The unique feature of this dome configuration is that it maximizes the capture efficiency for secondary electrons, i.e., secondary electrons emitted from the target during sputtering events are essentially all captured by the curved field lines and electrostatically trapped by the self-bias sputtering potential of the target sheath. A magnet set has been specially designed and constructed for reactive dc sputtering and used to demonstrate experimentally the erosion of an entire target surface in a magnetic dome.
Studies of a weak polyampholyte at the air–buffer interface: The effect of varying pH and ionic strength
Authors: Cicuta Pietro , Hopkinson Ian
Published: July 2002

We have carried out experiments to probe the static and dynamic interfacial properties of β-casein monolayers spread at the air–buffer interface, and analyzed these results in the context of models of weak polyampholytes. Measurements have been made systematically over a wide range of ionic strength and pH. In the semidilute regime of surface concentration a scaling exponent, which can be linked to the degree of chain swelling, is found. This shows that at pH close to the isoelectric point, the protein is compact. At pH away from the isoelectric pH the protein is extended. The transition between compact and extended states is continuous. As a function of increasing ionic strength, we observe swelling of the protein at the isoelectric pH but contraction of the protein at pH values away from it. These behaviors are typical of a those predicted theoretically for a weak polyampholyte. Dilational moduli measurements, made as a function of surface concentration exhibit maxima that are linked to the collapse of hydrophilic regions of the protein into the subphase. Based on this data we present a configuration map of the protein configuration in the monolayer. These findings are supported by strain (surface pressure) relaxation measurements and surface quasielastic light scattering measurements which suggest the existence of loops and tails in the subphase at higher surface concentrations.
Harmonic and power balance tools for tapping-mode atomic force microscope
Authors: Sebastian A. , Salapaka M. V. , Chen D. J. , Cleveland J. P.
Published: July 2002

The atomic force microscope (AFM) is a powerful tool for investigating surfaces at atomic scales. Harmonic balance and power balance techniques are introduced to analyze the tapping-mode dynamics of the atomic force microscope. The harmonic balance perspective explains observations hitherto unexplained in the AFM literature. A nonconservative model for the cantilever–sample interaction is developed. The energy dissipation in the sample is studied and the resulting power balance equations combined with the harmonic balance equations are used to estimate the model parameters. Experimental results confirm that the harmonic and power balance tools can be used effectively to predict the behavior of the tapping cantilever.
A density functional study on the adsorption of methanethiolate on the (111) surfaces of noble metals
Authors: Akinaga Yoshinobu , Nakajima Takahito , Hirao Kimihiko
Published: July 2002

The adsorption energies and structures of methanethiolate, SCH3, on the (111) surfaces of Au, Ag, and Cu have been studied using a density functional theory. The results obtained for the Au surface are in good agreement with experiments and previous calculations. The strength of the adsorption energies is found to be Cu>Ag>Au, and the nature of the chemisorption bond is discussed. The strong interaction between the SCH3 and Cu surface can be explained in a similar way to that as for the binding energy of SCH3 with metal atoms. Scalar-relativistic effects in the adsorption energies and adsorption structures, which dominate the differences observed between the Ag and Au surfaces, are studied using quasirelativistic and nonrelativistic pseudopotentials. The relativistic effects decrease the adsorption energy of SCH3 on the Au(111) surface, although the binding energy of the AuSCH3 complex is increased by relativity. The unexpected relativistic effects are also discussed.
Self-organized molecular-sized, hexagonally ordered SnOx nanodot superlattices on Pt(111)
Authors: Batzill Matthias , Beck David E. , Koel Bruce E.
Published: July 2002

Complete oxidation of the (√3×√3)R30° Sn/Pt(111) surface alloy or submonolayer amounts of Sn adatoms on Pt(111) under ultrahigh vacuum conditions, forms a highly ordered, lateral superlattice of SnOx islands on the Pt(111) substrate. The island superstructure exhibits a sharp (5×5) low energy electron diffraction pattern. Scanning tunneling microscopy images show islands arranged in a hexagonal lattice, uniformly distributed over the whole sample. This island array is thermally stable up to 1050 K. The coincidence of the island periodicity with a multiple of the supporting substrate, and the same hexagonal symmetry of islands and substrate, suggests a strong island–substrate interaction. We propose that the island formation results from the breakup of a strained SnOx adlayer.
Excitation dynamics of dye doped tris(8-hydroxy quinoline) aluminum films studied using time-resolved photoelectron spectroscopy
Authors: Read K. , Karlsson H. S. , Murnane M. M. , Kapteyn H. C. , Haight R.
Published: July 2002

In this work, we use excite-probe photoelectron spectroscopy to study the decay of electronic excitation in tris(8-hydroxy quinoline) aluminum (Alq) doped with the organic dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). Ultrashort laser pulses are used to photoexcite electrons into unoccupied molecular orbitals, and the ensuing decay rate is directly observed using photoelectron spectroscopy. Decay of the electronic excitation is studied as a function of DCM doping percentage and excitation intensity. The decay rate is seen to increase with both doping percentage and excitation intensity. These data are explained using a model including Förster transfer, stimulated emission, concentration quenching, and bimolecular singlet–singlet exciton annihilation. In this model, we find that it is necessary to include a very fast (faster than predicted in standard Förster transfer theory) excitation transfer of a fraction of the excitation from the Alq to the DCM, where that fraction corresponds to the approximate nearest-neighbor population.
Molten salt electrolytes. I. Experimental and theoretical studies of LiI/AlCl3
Authors: Lee Yi-Chia , Kolafa Jiřı́ , Curtiss Larry A. , Ratner Mark A. , Shriver Duward F.
Published: July 2002

Molten LiI/AlCl3 with different molar ratios of LiI to AlCl3 were prepared and characterized by Raman spectroscopy, thermal analysis, and impedance measurements. The LiI/AlCl3 adducts melt at 70–80 °C and the Raman spectrum indicates that a variety of haloaluminates exist in the system. The 1:1 adduct has the highest ionic conductivity, 2×10−6 S/cm at 25 °C, and the conductivity increases dramatically as the temperature is increased. Molecular dynamics (MD) simulations suggest that several haloaluminates are present in the adducts. Ab initio calculations were carried out on the species that were predicted by MD simulations and these results were compared with Raman spectra, and good agreement was obtained. Several-ns-long MD simulations allowed us to study the conductivity and relaxation processes in the 1:1 and 1:2 melts at higher temperatures.
Femtosecond energy relaxation of nonthermal electrons injected in p-doped GaAs base of a heterojunction bipolar transistor
Authors: Prabhu S. S. , Vengurlekar A. S.
Published: July 2002

We study femtosecond relaxation of minority carriers (electrons) injected into a heavily p-doped base of a heterojunction bipolar transistor (HBT). Here, we consider the case of p-doped GaAs, to be specific. The electrons are assumed to have a peaked energy distribution at t=0, with kinetic energies a few hundred meV above the conduction band threshold. We solve the time dependent Boltzmann equation governing the dynamics of these electrons. The main feature of this work is a detailed calculation of the time dependent nonthermal, nonequilibrium electron energy distribution, that relaxes due to single particle excitations via electron–hole scattering and interaction with coupled optical phonon-hole plasmon modes in the sub and picosecond time domains. We highlight the significant role that the electron-hole scattering plays in this relaxation. The majority carriers (holes) are assumed to remain in quasiequilibrium with the lattice, taken to be at room temperature (or at 77 K). We present calculations of electron energy relaxation with the hole density varied from 1×1018 to 1×1020 cm−3. In the initial, subpicosecond stages of the relaxation, the energy distribution evolves into two major components: a quasiballistic but broad component, at energies near the injection energy, and an energy relaxed component near E=0. The latter becomes dominant in a picosecond or so. The electrons with an initial mean velocity of ∼1.5×108 cm/s attain a cooler distribution with a mean velocity of ∼4×107 cm/s within about 1 ps for p doping in excess of 1×1019 cm−3. The temporal evolution of average velocity 〈v〉 of the electrons should be useful in obtaining values of the base width suitable for effective operation of HBTs.
Spin-polarized current induced switching in Co/Cu/Co pillars
Authors: Grollier J. , Cros V. , Hamzic A. , George J. M. , Jaffrès H. , Fert A. , Faini G. , Ben Youssef J. , Legall H.
Published: July 2002

We present experiments of magnetization reversal by spin injection performed on pillar-shaped Co/Cu/Co trilayers. The pillars (200×600 nm2) are fabricated by electron beam lithography and reactive ion etching. Our data for the magnetization reversal at a threshold current confirm previous results on similar pillars. In addition, we present another type of experiment that also clearly evidences the control of the magnetic configuration by the current intensity. Our interpretation is based on a version of the Slonczewski model in which the polarization of the current is calculated in the Valet–Fert model of the giant magnetoresistance with current applied perpendicular to plane.
Effect of light on the polarization of a banana-shaped achiral compound doped with a photoactive azobenzene material
Authors: Nair Geetha G. , Prasad S. Krishna , Hiremath Uma S. , Yelamaggad C. V.
Published: July 2002

We report photoinduced effects on a liquid crystal comprising of bent-core molecules, doped with a photoactive azobenzene compound and exhibiting the recently discovered “banana” B2 mesophase. The photoisomerization of the azobenzene molecules, brought about by the UV radiation, hardly changes the B2-isotropic transition temperatures, but has a significant influence on the spontaneous polarization (Ps) as well as the switching time in the B2 phase. The efficiency of this opto-polarization effect has been observed to be strongly dependent on the temperature at which the irradiation is carried out. A possible mechanism responsible for the observed phenomenon has been suggested.
Hydrophobic hydration of argon at high temperatures
Authors: Sullivan D. M. , Neilson G. W. , Fischer H. E.
Published: July 2002

Neutron diffraction experiments were carried out on a mixture of argon in water under a pressure of 300 bar and a temperature of 318 °C. The data were analyzed by standard procedures and showed that the argon atom is hydrated with 9 (1) water molecules, a substantial reduction from the value of 16 (2) at ambient temperature and a pressure of 250 bar. This result is in excellent agreement with the computer simulation study of Guissani and Guillot, which was carried out on an argon/water mixture at high temperatures and pressures.
Luminescent properties of local atomic order of Er3+ and Yb3+ ions in aluminophosphate glasses
Authors: d’Acapito F. , Mobilio S. , Bruno P. , Barbier D. , Philipsen J.
Published: July 2002

We report on an extended x-ray absorption fine structure (EXAFS) investigation on the local atomic order around rare earth (RE) ions in Er+Yb doped aluminophosphate glasses. Samples were obtained from the melt and the RE concentration ranged from 0.5 to 5 wt. %. EXAFS data were analyzed within the multiple scattering formalism providing a detailed description of the atomic neighborhood in the first two shells. In particular, each RE in was found to be surrounded by ∼7 PO4 tetrahedra linked by corner O atoms. For RE concentrations below and above the luminescence “quenching” value, no detectable variations are reported leading us to exclude the formation of RE clusters within the sensitivity of the technique. From the analysis of the edge region a description of the density of free 5d states (linked to the radiative emission probability) is also obtained, which again shows no difference at the various RE concentrations.
High power radiation from ionization fronts in a static electric field in a waveguide
Authors: Hoffman J. R. , Muggli P. , Liou R. , Gundersen M. , Yampolsky J. , Katsouleas T. , Joshi C. , Mori W. B.
Published: July 2002

The radiation produced when a relativistically moving plasma/gas boundary (i.e., an ionization front) passes between alternatively biased capacitor electrodes is studied. Results of an experiment based on a design which incorporates the capacitor electrodes into an X band waveguide are presented. The waveguided design effectively couples nearly three orders of magnitude more power into the output than the previously unguided designs. Linear theory is extended to include the depletion of the laser energy as it propagates through the ionizable gas (i.e., laser depletion), and the effect of finite output pulse duration.
Rotational echo double resonance in ISN spin networks: Deconvolution of multiple dipole–dipole couplings
Authors: Liivak Oskar , Zax David B.
Published: July 2002

In a recent paper we have demonstrated how a simple modification to the standard rotational echo double resonance pulse sequence, where the flip angle of the pulse applied to the S spin is varied, can be used to separate the measurement of the magnitude of I–S dipole–dipole couplings from that of their relative orientation. An equivalent result can be achieved via phase modulation which labels and differentiates between different evolving coherences in the density matrix based on the number of S spins participating—exactly as is done in multiple quantum spectroscopy. As phase modulation can be effected on modern instruments with much higher precision, in this paper we explore the experimental implementation of this method in the IS2 spin system glycine–C213–N15, and discuss generalizations of this technique to larger spin systems.
Growth of Si0.75Ge0.25 alloy layers grown on Si(001) substrates using step-graded short-period (Sim/Gen)N superlattices
Authors: Rahman M. M. , Matada H. , Tambo T. , Tatsuyama C.
Published: July 2002

Short-period (Sim/Gen)N superlattices (SSLs) are grown step by step on a Si(001) substrates by solid source molecular beam epitaxy. Using the step-graded SSLs as buffer layers, 2000 Å uniform Si0.75Ge0.25 alloy layers are grown on the same substrates. The growth temperature of the SSLs and uniform layers is 500 °C. In the SSLs layers, m and n are the number of monolayers of Si and Ge, respectively. N is the period of (Sim/Gen) bilayers. The samples grown are characterized by x-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM) as a function of the step number of SSL layers. The SSLs show very smooth surfaces [the root-mean-square (rms) surface roughness is between 7 and 12 Å]. A dramatic decrease in roughness is observed in the uniform Si0.75Ge0.25 alloy layers, when even a one-step SSL is used as a buffer layer. A noticeable increase in rms roughness is seen in both SSL and alloy layers when the number of Ge monolayers is changed from one to two. AFM observation shows that the rms surface roughness behavior of the SSLs is reflected to their corresponding top alloy layers. The residual strains in alloy layers are considerably lower, with a maximum relaxation rate of about 80% for the sample with a seven-step SSL buffer. Cross-sectional TEM images show that strained SSL buffer layers effectively deflect threading dislocations in the substrate or confine the dislocations in the SSL buffer layers.
Grazing incidence x-ray scattering study of the structure of epitaxial Cr/Sn multilayers
Authors: Gupta Ajay , Paul Amitesh , Mukhopadhyay Sanghamitra , Mibu Ko
Published: July 2002

The structure of epitaxial Cr/Sn multilayers has been studied experimentally using x-ray reflectivity and x-ray diffuse scattering measurements, as well as theoretically using linear muffin-tin orbital (LMTO) calculations. Measurements show a distinct variation in the structure of the multilayers as the Sn layer thickness increases from 0.4 to 0.6 nm. A decrease in the electron density of the Sn layer and an increase in the jaggedness of the interfaces accompany a partial transformation of the Sn layer from an epitaxial bcc structure to a β-Sn structure, as observed using in situ reflection high energy electron diffraction measurements [K. Mibu, S. Tanaka, and T. Shinjo, J. Phys. Soc. Jpn. 67, 2633 (1998)]. Present measurements along with the LMTO calculations support a structure for the multilayer in which Sn layers grow epitaxially with Cr in a bcc structure with a finite density of steps at the interfaces which causes the average electron density of a Sn layer to decrease. Diffuse scattering measurements give the average distance between steps to be about 50–100 nm. Taking the structure of the epitaxial Sn layer to be bcc, the calculated variation in the lattice strain and Sn layer energy with the thickness of Cr and Sn layers explains qualitatively the range of Cr layer thicknesses for which epitaxial growth is observed.
Effects of solvent damping on side chain and backbone contributions to the protein boson peak
Authors: Tarek Mounir , Tobias Douglas J.
Published: July 2002

We report a MD simulation study of the behavior of the boson peak of a globular protein in realistic powder environments corresponding to conditions of neutron scattering studies (hydrated at 150 K, dry at 150 K, and dry at 300 K). The temperature and hydration dependence of the boson peak, an excess of inelastic scattering intensity over the harmonic background at low frequency, are in excellent agreement with neutron scattering data on powder samples of several proteins. To gain further insight into the nature of boson peak, and its relation to hydration water, we have decomposed the inelastic spectrum into contributions from the protein backbone, nonpolar side chains in the interior of the protein, and polar side chains exposed to the solvent. We find that the boson peak arises from motions distributed throughout the protein, regardless of the conditions of temperature and hydration. Furthermore, the relative contribution from each part of the protein considered shows a similar temperature and hydration dependence. This demonstrates that the damping of the boson peak upon hydration is not solely due to the damping of the water-coupled motion of exposed polar side chains, but rather propagates through the whole protein.
Computer modeling of dual-band HgCdTe photovoltaic detectors
Authors: Jóźwikowski K. , Rogalski A.
Published: July 2002

The performance of middle wavelength/long wavelength (MW/LW) dual-band HgCdTe photovoltaic detectors was examined theoretically. An original iteration scheme was used to solve the system of nonlinear continuity equations and the Poisson equation. The effect of composition and doping profiles on the heterojunction detector parameters is presented. It is assumed that the performance of photodiodes is due to thermal generation governed by the Auger mechanism. All quantities are functions of the electric potential and Fermi quasi-levels. The results of calculations are presented as maps showing spatial distribution of electrical potential, photoelectrical gain, sensitivity, and density of noise generation. The theoretical predictions of heterojunction device parameters are compared with available experimental data.
Interesting trends in direct current electrical conductivity of chemical vapor deposited diamond sheets
Authors: Sikder A. K. , Misra D. S. , Palnitkar Umesh , Shirodkar V. S.
Published: July 2002

Self-supported diamond sheets of the thickness ranging from 15 to 30 μm were prepared using hot filament chemical vapor deposition technique. The controlled variation of the deposition parameters resulted in the sheets with varying amount of nondiamond impurities. Routine characterization of the sheets was carried out using scanning electron microscopy, x-ray diffractometry, Raman spectroscopy, Fourier transform infrared spectroscopy, and Positron annihilation spectroscopy techniques. Detailed measurements of room temperature electrical conductivity (σ300), current–voltage (I–V) characteristics, and annealing studies on the sheets deposited with various structural disorder have yielded useful information about the electrical conduction in this interesting material. σ300 and I–V characteristic measurements were done in sandwiched configuration taking care off the surface effects. The diamond sheets deposited at low deposition pressure (Pd<60 Torr) contain negligible nondiamond impurities and show σ300≅10−6–10−7 S.cm−1. The I–V characteristics in these sheets show space charge limited conduction behavior with I∝Vn and n>1, in high voltage range. In contrast the sheets deposited at higher pressure (60 Torr and higher), containing high concentration of nondiamond impurities, show a sharp reduction in the values of σ300. Interestingly, the conduction in these sheets is ohmic with n values nearly equal to unity. Similarly the sheets deposited with nitrogen also show a sharp reduction in σ300. Annealing of all types of diamond sheets results in a decrease in σ300 values by several orders of magnitude. In the sheets deposited at low Pd, the n values increase sharply with annealing. On the other hand the values of n in the sheets deposited at higher pressure remain constant with annealing. The above results are explained in terms of hydrogen abstraction from the traps and compensation of donor–acceptor pairs.
Carbon delta-doped AlGaAs grown by metalorganic vapor phase epitaxy
Authors: Tribuzy C. V. B. , Butendeich R. , Pires M. P. , Souza P. L. , Henriques A. B.
Published: July 2002

Good-optical-quality C delta-doped AlGaAs layers grown by metalorganic vapor phase epitaxy using CBr4 with the impurities confined over not more than 5 Å and with doping levels above 1×1012 cm−2 are obtained. Such layers are found to be adequate for use in the fabrication of nipi superlattices for amplitude modulation. Yet, little flexibility is found in the growth conditions, in particular for the V to III fluxes ratio, for obtaining such layers.
Mechanism of mechanical crystallization of amorphous Fe–Mo–Si–B alloy
Authors: Yao Bin , Liu Shu-E , Liu Li , Si Lun , Su Wen-Hui , Li Yi
Published: July 2002

Crystallization processes of an amorphous (Fe0.99, Mo0.01)78Si9B13 alloy induced by mechanical milling and annealing at pressures from 0 to 7.0 GPa were studied. It is found that the milling time needed for the crystallization of the amorphous alloy and its crystallization products are related to the milling intensity. The crystallization products are an α-Fe(Mo, Si) disordered solid solution at a lower milling intensity, while at a higher milling intensity they are α-Fe(Mo, Si), Fe–Si–B, and Fe2B phases. By comparing the mechanical crystallization of the amorphous alloy with its high pressure crystallization, it is suggested that crystallization of the amorphous alloy driven by ball milling results from the simultaneous action of local pressure (4–6 GPa) and local temperature (600–700 K), which are produced by the collision of steel balls. The local pressure decreases the thermodynamic potential barrier of nucleation and increases the diffusion activation energy in the process of mechanical crystallization, leading to that crystallization of amorphous alloy is depressed when the crystallization needs a long-range atomic diffusion and is promoted when the crystallization needs atomic diffusionless or short-range diffusion. The local temperature plays the same role in the mechanical crystallization as the annealing temperature in the thermal crystallization.
Electrical and pyroelectric properties of in-plane polarized lead lanthanum titanate thin film
Authors: Song Z. T. , Chong N. , Chan H. L. W. , Choy C. L.
Published: July 2002

Pb 0.9 La 0.1 Ti 0.975 O 3 (PLT10) thin films were deposited on SiO2/Si(100) substrates coated with a ZrO2 buffer layer. Studies by x-ray diffraction and scanning electron microscopy reveal that the ZrO2 film consists of both tetragonal and monoclinic phases, with the tetragonal phase being the dominant one. The PLT10 film has a perovskite structure and the grains in the film have a rather uniform size of about 50 nm. By using interdigital transducer (IDT) electrodes the in-plane electrical properties, hysteresis loop, and pyroelectric coefficient of the PLT10 film were measured. The dielectric constant and loss factor vary only slightly with frequency in the range 103–106 Hz, with the loss factor being less than 0.01 over the entire range. The leakage current density is lower than 2×10−8 A/cm2 at a bias field of 5 kV/cm. The remnant polarization and coercive field are 12.6 μC/cm2 and 9.93 kV/cm, respectively. The film exhibits a reasonably high pyroelectric coefficient (95 μC/m2 K) after it has been poled by applying 120 V ac at 0.1 Hz across the IDT electrodes.
Nonlinear magneto-optical Kerr effect in hyper-Rayleigh scattering from layer-by-layer assembled films of yttrium iron garnet nanoparticles
Authors: Murzina T. V. , Nikulin A. A. , Aktsipetrov O. A. , Ostrander J. W. , Mamedov A. A. , Kotov N. A. , Devillers M. A. C. , Roark J.
Published: July 2002

Magnetization-induced second-harmonic generation (SHG) in layer-by-layer assembled films containing yttrium iron garnet nanoparticles is observed. The SHG intensity, including its magnetoinduced component, linearly increases with the number of self-assembling layers, which indicates along with diffuseness of the SHG radiation that SHG occurs in its incoherent form: hyper-Rayleigh scattering (HRS). The nonlinear magneto-optical Kerr effect in HRS has a noticeable magnetic contrast brought about by the internal homodyne mechanism.
Study of exciton–longitudinal optical phonon coupling in quantum wells for optoelectronic applications
Authors: Shen W. Z.
Published: July 2002

The study of exciton–longitudinal optical (LO) phonon coupling in semiconductor quantum well (QW) structures and their corresponding bulk materials reveals four distinct features: a large difference between (i) III–V and II–VI QW structures, (ii) multiple QW and single QW structures, as well as (iii) QW structures and their corresponding bulk materials, and its linear dependence on well width in QW structures. A quantitatively theoretical approach is presented, which can explain well all the experimental observations and can clarify the controversy in the literature. The effects of alloy disorder and strain in QW structures on exciton–LO phonon coupling are also discussed.
Orientation and nonlinear optical properties of N-4-nitrophenyl-(L)-prolinol crystals on nanostructured poly(tetrafluoroethylene) substrates
Authors: Vallée R. , Damman P. , Dosière M. , Toussaere E. , Zyss J.
Published: July 2002

Thin crystalline films of N-4-nitrophenyl-(L)-prolinol (NPP) were grown on nanostructured highly oriented poly(tetrafluoroethylene) (PTFE) substrates. Depending on the growth conditions, two types of crystalline orientation were observed. Thinnest samples (<0.2 μm), directly grown from the vapor phase, are made of numerous highly ordered crystallites presenting a (110)NPP∥(100)PTFE interfacial relationship and an almost perfect azimuthal orientation. Thicker layers (>1 μm) could be obtained from deposition–annealing procedures and exhibit a (101)NPP planar orientation but with a lack of azimuthal orientation. The full characterization of these various samples was performed by analysis of x-ray diffraction data obtained in θ–2θ and grazing incidence geometry, the latter being used to determine the azimuthal orientation. Furthermore, the proposed orientations were fully consistent with the nonlinear optical (NLO) properties of the layers, deduced from optical second harmonic generation measurements. The combination of x-ray diffraction and nonlinear ellipsometry technique thus proved once again to be a powerful method to analyze oriented structures obtained with NLO molecules.
Higher order derivatives for nuclear indexes in the framework of density functional theory
Authors: Chamorro E. , Fuentealba P. , Contreras R.
Published: July 2002

General and exact relationships for higher order derivatives of the nuclear Fukui function with respect to the electron number at constant external potential have been explicitly derived in the framework of the four Legendre transformed ensembles of density functional theory. These relationships complement and extend to nuclear reactivities some developments [F. De Proft, S. Liu, and P. Geerlings, J. Chem. Phys. 108, 7549 (1998)] and recent advances [E. Chamorro, R. Contreras, and P. Fuentealba, J. Chem. Phys. 113, 10861 (2000)] found for the high order electron responses introduced in the framework of a nonlocal (pair-site) reactivity formalism.