bluuebunny/crossref_metadata_2025 huggingface dataset:
Large transport critical currents in dense Fe- and Ni-clad MgB2 superconducting tapes
Authors: Suo HongLi , Beneduce Concetta , Dhallé Marc , Musolino Nicolas , Genoud Jean-Yves , Flükiger René
Published: July 2002

We report on the preparation of dense monofilamentary MgB2/Ni and MgB2/Fe tapes with high critical current densities. In annealed MgB2/Ni tapes, we obtained transport critical current densities as high as 2.3×105 A/cm2 at 4.2 K and 1.5 T, and for MgB2/Fe tapes 104 A/cm2 at 4.2 K and 6.5 T. An extrapolation to zero field of the MgB2/Fe data gives a critical current value of ∼1 MA/cm2, corresponding to a critical current value well above 1000 A. The high jc values obtained after annealing are a consequence of sintering densification and grain reconnection. Fe does not react with MgB2 and is thus an excellent sheath material candidate for tapes with self-field jc values at 4.2 K in excess of 1 MA/cm2.
The dynamics of the internal phonons tris(quinolin-8-olato) aluminum(III) in crystalline β-phase
Authors: Degli Esposti Alessandra , Brinkmann Martin , Ruani Giampiero
Published: July 2002

A new approach to the analysis of the internal phonons of tris(quinolin-8-olato) aluminum(III) is presented, which enlightens the role played by the ligands in determining the vibrational properties of the organometallic compound and evidences the importance of the contributions arising from the coupling terms among the three quinolinato fragments. An accurate exam of the normal modes of the meridianal isomer evidences the role of the interactions among the fragments in the vibrational dynamics of the ground state. Due to the special attention paid to the quinolinato fragments, a preliminary investigation on the vibrational properties of 8-hydroxyquinoline, taken as a model fragment, was also performed. The vibrational properties of the polymorph species β of the organometallic molecule were obtained refining the calculated frequencies, the dipole moment derivative matrix, and the polarizability derivative tensor derived by the hybrid density functional B3LYP/6-31G* comparing with the frequencies and intensities recorded by the infrared and the Raman spectroscopies performed on a polycrystalline sample. One thus obtains the most accurate intramolecular force constants up to date for the meridianal isomer in a crystalline phase.
Improved binary collision approximation ion implant simulators
Authors: Hernández-Mangas J. M. , Arias J. , Bailón L. , Jaraı́z M. , Barbolla J.
Published: July 2002

An efficient binary collision approximation (BCA) ion implant code with good prediction capabilities for semiconductor materials (Si, GaAs, SiC) with only one fitting parameter for low implantation doses is presented. It includes specific interatomic potentials and recent improvements in physical models for inelastic stopping. A periodic ab initio full bond electron density for the target is used. Damage accumulation is supported using a modified Kinchin–Pease model [G. H. Kinchin and R. S. Pease, Rep. Prog. Phys. 18, 1 (1955)]. Also, some of the BCA integration algorithms and target selection procedure have been refined. An algorithm commonly used for statistical noise reduction has been modified to also improve the noise reduction in the lateral and shallow zones. The agreement with experiments is good, even under channeling conditions and for different target materials. A comparison with experimental secondary ion mass spectroscopy results for several projectiles and targets is presented.
Photothermal deflection spectroscopy of chalcogenide glasses
Authors: Tanaka K. , Gotoh T. , Yoshida N. , Nonomura S.
Published: July 2002

Photothermal deflection spectroscopy has been applied to optical absorption measurements in As2S3, Se, and As–S glasses. The spectroscopic method is demonstrated to be suitable for evaluation of small absorption in thin samples, while its accuracy is affected by light scattering. For As2S3, as-evaporated films give higher residual absorption than those of the bulk. In the As–S system, As2S3 shows the most prominent weak absorption tail, and As17S83 exhibits a peculiar spectrum. The origins of these features are discussed.
Populations of coupled electrochemical oscillators
Authors: Kiss István Z. , Wang Wen , Hudson John L.
Published: July 2002

Experiments were carried out on arrays of chaotic electrochemical oscillators to which global coupling, periodic forcing, and feedback were applied. The global coupling converts a very weakly coupled set of chaotic oscillators to a synchronized state with sufficiently large values of coupling strength; at intermediate values both intermittent and stable chaotic cluster states occur. Cluster formation and synchronization were also obtained by applying feedback and forcing to a moderately coupled base state. The three cases differ, however, in other details. The feedback and forcing also produce periodic cluster states and more than two clusters. Configurations of two (chaotic) clusters and two, three, or four (periodic) clusters were observed.
Temperature dependence of stresses in GaN thin films grown on (0001) sapphire: Modeling of thermal stresses
Authors: Keckes J. , Gerlach J. W. , Averbeck R. , Riechert H. , Bader S. , Hahn B. , Lugauer H.-J. , Lell A. , Härle V. , Wenzel A. , Rauschenbach B.
Published: July 2002

Residual stresses and unstressed lattice parameters are characterized in heteroepitaxial GaN thin films grown on (0001) sapphire using three different deposition techniques. X-ray diffraction measurements in the temperature range of 25–600 °C indicate a reversible change of stresses in the films from compressive to tensile state and vice versa. The thermal behavior of stresses in the samples prepared by different methods is comparable. The experimental results are consistent with the model of thermal stresses originating from the mismatch of thermal expansion coefficients of GaN and sapphire.
Multimode quantitative scanning microwave microscopy of in situ grown epitaxial Ba1−xSrxTiO3 composition spreads
Authors: Chang K. S. , Aronova M. , Famodu O. , Takeuchi I. , Lofland S. E. , Hattrick-Simpers J. , Chang H.
Published: July 2002

We have performed variable-temperature multimode quantitative microwave microscopy of in situ epitaxial Ba1−xSrxTiO3 thin-film composition spreads fabricated on (100) LaA1O3 substrates. Dielectric properties were mapped as a function of continuously varying composition from BaTiO3 to SrTiO3. We have demonstrated nondestructive temperature-dependent dielectric characterization of local thin-film regions. Measurements are simultaneously taken at multiple resonant frequencies of the microscope cavity. The multimode measurements allow frequency dispersion studies. We observe strong composition-dependent dielectric relaxation in Ba1−xSrxTiO3 at microwave frequencies.
An experimental study of the flowing granular layer in a rotating tumbler
Authors: Jain Nitin , Ottino J. M. , Lueptow R. M.
Published: July 2002

Granular flow in a rotating tumbler is of theoretical and industrial significance. However, in spite of its relative simplicity, little is known about the dynamics of the top flowing layer. Here we present an experimental study of the velocity field within the fluidized layer of monodisperse particles in a quasi-2D (two-dimensional) rotating tumbler in the rolling flow regime using particle tracking velocimetry. The granular flow is illuminated by a laser flash and recorded using a charge coupled device camera. Image processing is used to remove the experimental noise and to achieve sub-pixel accuracy in calculating the particle displacements. The ensemble-averaged streamwise and transverse velocity profiles are calculated based on the particle displacements for three angular velocities and three bead sizes. The normalized streamwise velocity profile is linear throughout the fluidized layer, but becomes logarithmic as it enters the “fixed” bed where slow particle rearrangements dominate. The rms velocities appear to be exponentially related to the depth in the layer. Nondimensionalizing the number density in the fluidized layer with a geometric factor based on the square packing results in collapse of the data over a range of bead sizes and angular velocities.
Observations on an equation of state for water confined in narrow slit-pores
Authors: Giaya Arjan , Thompson Robert W.
Published: July 2002

Water may be adsorbed in microporous or mesoporous materials from the liquid phase or from the gaseous, or vapor, phase. In either case, the fluid state of the adsorbed water molecules may be liquid or vapor as well, depending on several factors inherent to the adsorbate itself. This work sought to understand these factors using an equation of state reported in the literature recently. It was determined that the model’s predicted contribution of the hydrogen bonding to the Helmholtz free energy was less than expected. The estimation of the magnitude of hydrogen bonding appears to be more realistic if positive contributions of four neighbors is included rather than one. The model also appears to involve quite sensitive calculations, which may be prone to precision errors.
Low frequency oscillations in a plasma with spatially variable field-aligned flow
Authors: Ganguli G. , Slinker S. , Gavrishchaka V. , Scales W.
Published: July 2002

The effects of a transverse gradient in the plasma flow velocity parallel to the ambient magnetic field are analyzed. A transverse velocity gradient in the parallel ion flow, even in small magnitude, can increase the parallel phase speed of the ion-acoustic waves sufficiently to reduce ion Landau damping. This results in a significantly lower threshold current for the current driven ion acoustic instability. Ion flow gradients can also give rise to a new class of ion cyclotron waves via inverse cyclotron damping. A broadband wave spectrum with multiple cyclotron harmonics is possible. A combination of the multiple cyclotron harmonic waves can result in spiky electric field structures with their peaks separated by an ion cyclotron period. A spatial gradient in the parallel electron flow is also considered but it is found to play a minimal role in the low frequency regime. Relevance of these to natural plasma environments is discussed.
Manipulation of single vortices in YBa2Cu3O6.354 with a locally applied magnetic field
Authors: Gardner Brian W. , Wynn Janice C. , Bonn D. A. , Liang Ruixing , Hardy W. N. , Kirtley John R. , Kogan Vladimir G. , Moler Kathryn A.
Published: July 2002

We demonstrate the controlled, reversible manipulation of individual vortices in a superconductor with a locally applied magnetic field. The local field is supplied by a field coil on a superconducting quantum interference device (SQUID). The SQUID is used to image the vortices before and after moving. This device can be used both to push individual vortices and to create individual vortex–antivortex pairs. We calculate the force applied on a rigid vortex and find that ∼0.5 pN is necessary to move vortices in underdoped single crystals of YBa2Cu3O6.354 with Tc∼ 12 K.
Characterization of direct-drive-implosion core conditions on OMEGA with time-resolved Ar K-shell spectroscopy
Authors: Regan S. P. , Delettrez J. A. , Epstein R. , Jaanimagi P. A. , Yaakobi B. , Smalyuk V. A. , Marshall F. J. , Meyerhofer D. D. , Seka W. , Haynes D. A. , Golovkin I. E. , Hooper C. F.
Published: July 2002

Direct-drive-implosion core conditions have been characterized on the 60-beam OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system with time-resolved Ar K-shell spectroscopy. Plastic shells with an Ar-doped deuterium fill gas were driven with a 23 kJ, 1 ns square laser pulse smoothed with 1 THz smoothing by spectral dispersion (SSD) and polarization smoothing (PS) using birefringent wedges. The targets are predicted to have a convergence ratio of ∼15. The emissivity-averaged core electron temperature (Te) and density (ne) were inferred from the measured time-dependent Ar K-shell spectral line shapes. As the imploding shell decelerates the observed Te and ne increase to 2.0 (±0.2) keV and 2.5 (±0.5)×1024 cm−3 at peak neutron production, which is assumed to occur at the time of the peak emissivity-averaged Te. At peak compression the ne increases to 3.1 (±0.6)×1024 cm−3 and the Te decreases to 1.7 (±0.17) keV. The observed core conditions are close to those predicted by a one-dimensional hydrodynamics code.
Closely stacked InAs/GaAs quantum dots grown at low growth rate
Authors: Heidemeyer H. , Kiravittaya S. , Müller C. , Jin-Phillipp N. Y. , Schmidt O. G.
Published: July 2002

We present a systematic study of closely stacked InAs/GaAs quantum dots (QDs) grown at low growth rates. Transmission electron microscopy reveals that for thin spacer layers vertically aligned QDs merge into one large QD. After capping the initial QD layer the GaAs surface is decorated with well-developed nanostructures, which act as nucleation centers for the QDs deposited in the second layer. Despite the size increase, photoluminescence (PL) experiments show a systematic blueshift up to 103 meV of the QD related signal with decreasing spacer thickness. We explicitly show that this significant blueshift cannot fully be ascribed to specific growth phenomena, but instead is caused by the actual presence of the second dot layer. We report a PL linewidth as narrow as 16 meV at low temperature for a sample with 5 nm spacer thickness.
Deceleration phase of inertial confinement fusion implosions
Authors: Betti R. , Anderson K. , Goncharov V. N. , McCrory R. L. , Meyerhofer D. D. , Skupsky S. , Town R. P. J.
Published: July 2002

A model for the deceleration phase and marginal ignition of imploding capsules is derived by solving a set of ordinary differential equations describing the hot-spot energy balance and the shell dynamics including the return shock propagation. It is found that heat flux leaving the hot spot goes back in the form of internal energy and PdV work of the material ablated off the inner-shell surface. Though the hot-spot temperature is reduced by the heat conduction losses, the hot-spot density increases due to the ablated material in such a way that the hot-spot pressure is approximately independent of heat conduction. For hot-spot temperatures exceeding approximately 7 keV, the ignition conditions are not affected by heat conduction losses that are recycled into the hot spot by ablation. Instead, the only significant internal energy loss is due to the hot-spot expansion tamped by the surrounding shell. The change of adiabat induced by the shock is also calculated for marginally igniting shells, and the relation between the in-flight and stagnation adiabats is in general agreement with the numerical fit of LASNEX simulations by Herrmann et al. [Nucl. Fusion 41, 99 (2001)] and the self-similar solution of Kemp et al. [Phys. Rev. Lett. 15, 3336 (2001)]. The minimum kinetic energy required for ignition is also calculated from the same model and shown to be in good agreement with the numerical fit of LASNEX simulations. It is also found that mass ablation leads to a significant reduction of the deceleration phase Rayleigh–Taylor instability growth rates and to the suppression of short wavelength modes.
Electrical resistivity study of La, B doped nanocrystalline superconducting vanadium nitride
Authors: Ningthoujam R. S. , Sudhakar N. , Rajeev K. P. , Gajbhiye N. S.
Published: July 2002

A class of nanocrystalline superconducting vanadium nitrides has been prepared by a chemical synthesis route. This method involves the simultaneous decomposition of [VO(NH2O)2Gly]⋅H2O complex and nitridation at 973 K in ammonia atmosphere resulting pure and doped VN with controlled stoichiometry. The structure and stoichiometry of these samples were characterized by x-ray diffraction, scanning electron microscope, energy dispersive analysis of x-ray, transmission electron microscope, and chemical analysis. The VN materials crystallize in the cubic structure. The crystallite size was found to decrease with doping. We obtained samples having crystallite sizes between 8 and 32 nm. The electrical resistivity variation in these materials was carefully measured and the results are presented. The resistivity data in the range Tc<T< 30 K have been fitted to the power law ρ=ρ0+ATn and analyzed the results in the light of the existing theories. It was found that the resistivity follows a T3 law rather than the expected T2 behavior.
Direct measurement of the guided modes in LiNbO3 waveguides
Authors: Campillo A. L. , Hsu J. W. P. , White C. A. , Jones C. D. W.
Published: July 2002

Mode profiles of the light coming out of Ti diffused waveguides formed in LiNbO3 crystals were imaged using a near-field scanning optical microscope (NSOM). We find that the transverse magnetic mode has a smaller spatial dimension and is closer to the surface of the waveguide than the transverse electric mode. We also measured the phase and intensity distributions of the guided optical modes directly by incorporating the NSOM into an interferometer. The effective index (neff) is determined from the spacings of phase fronts, which equals λ/neff. The experimental results of mode profiles and neff are in good agreement with theoretical simulations. However, an unexpected nonuniform intensity distribution was observed, which could be linked to defects in the waveguide.
Positive phase-velocity tapering of broadband helix traveling-wave tubes for efficiency enhancement
Authors: Jung Sun-Shin , Soukhov Andrei V. , Jia Baofu , Park Gun-Sik
Published: July 2002

A positive phase-velocity tapering of 1.5 octave broadband helix traveling-wave tubes for efficiency enhancement, where the phase velocity is linearly increased in the output section, was studied by using the one-dimensional nonlinear theory. At high frequencies, the electromagnetic wave in the positively tapered section traps the fastest electrons in the decelerating electric field, extracting more energy from the electron beam. At low frequencies, a decreased velocity difference between the electron beam and the electromagnetic wave destroys the phase condition for second-harmonic generation, retaining fundamental wave efficiency as well as reducing its second-harmonic power.
Electrical conductivity control in transparent p-type (LaO)CuS thin films prepared by rf sputtering
Authors: Hiramatsu Hidenori , Orita Masahiro , Hirano Masahiro , Ueda Kazushige , Hosono Hideo
Published: July 2002

Thin films of wide-gap p-type (LaO)CuS were prepared by rf sputtering followed by postannealing. Undoped (LaO)CuS films are electrically insulating and emit ultraviolet (UV) light arising from excitons at room temperature upon photoexcitation. The hole carrier concentration in the film was controlled in the range ∼1015 to ∼1020 cm−3 by doping Sr2+ ions to replace La3+. The hole concentration reached a maximum of 2.7×1020 cm−3 at a doping level of 3 at. % Sr2+ ions, and then the carrier generation efficiency was ∼50%. These results demonstrate that the (La1−xSrxO)CuS films are potentially applicable to optoelectronic devices operating at short wavelengths because a transparent p electrode, as well as UV-emitting layers, may be fabricated using this material by controlling the doping level.
Surface chemistry of CuxS and CuxS/CdS determined from x-ray photoelectron spectroscopy
Authors: Partain L. D. , Schneider R. A. , Donaghey L. F. , McLeod P. S.
Published: July 2002

X-ray photoelectron spectroscopy spectra measured on copper sulfide (CuxS) films showed that a thin surface reaction product containing Cu in the +2 valence state was formed on CuxS films exposed to air for 46 h at 40 °C and 90% relative humidity. An entirely different CuxS surface reaction product layer was formed in dry air at 170 °C for 30 min and it contained sulfur in the +6 valence state. The copper (Cu) valence state in CuxS was not found to be +2 even when the x value was less than 1.9. When the argon sputter-cleaned surface of CuxS or CuxS/CdS films was exposed to room-temperature air for 10 min, cadmium (Cd) atoms appeared on the CuxS surface. X-ray powder diffraction patterns showed that CuO and CdS reacted at 500 °C in flowing nitrogen to form Cu2S and CdO. This cation exchange between CdS and copper oxide may explain the surface Cd on the CuxS films. The standard free energy of reaction between CuO and CdS is positive while that between Cu2O and CdS is negative. These results indicate a method for stabilizing CuxS/CdS solar cells against degradation.
Secondary electron yield and Auger electron spectroscopy measurements on oxides, carbide, and nitride of niobium
Authors: Garwin E. L. , Hoyt E. W. , Kirby R. E. , Momose T.
Published: July 2002

Secondary electron yield measurements before and after Ar ion sputter cleaning were made on Nb and Nb compounds of interest for rf superconducting cavities. Total secondary electron yields (σ) for primary energies 20–1500 eV were measured for solid Nb (σmax=1.3 at 300 eV), anodized Nb2O5 (σmax=1.2 at 300 eV), and powders of Nb(σmax=1.0 at 400 eV), NbO (σmax=0.9 at 400 eV), NbO2 (σmax=1.0 at 400 eV), Nb2O5 (σmax=0.95 at 400 eV), NbC(σmax=0.8 at 400 eV), and NbN (σmax=0.8 at 500 eV). Determinations were made for Auger elemental sensitivities, and the relationship between Auger peak heights and oxide stoichiometry is discussed. The sputter etch rate of anodized Nb2O5 was measured by depth profiling anodic coatings of known thickness.
Preferential site occupation and magnetic structure of Nd2(CoxFe1−x)14B systems
Authors: Herbst J. F. , Yelon W. B.
Published: July 2002

Rietveld analyses of room-temperature neutron diffraction data for seven Nd2(CoxFe1−x)14B alloys (x=0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) are reported. Throughout the entire composition range the Nd2Fe14B-type tetragonal crystal structure is maintained, with the lattice constants decreasing significantly as the Co concentration x increases. We find that the j2-type transition-metal sites are preferentially occupied by Fe ions in the pseudoternary systems, a result which is analogous to the preferential Fe occupation of c sites we previously observed in hexagonal Nd2(CoxFe1−x)17 alloys.
Hysteresis loops modeled from coercivity, anisotropy, and microstructure parameters
Authors: Dionne Gerald F. , Weiss Jerald A. , Allen Gary A.
Published: July 2002

By modifying the microstructure (a/H) and magnetocrystalline anisotropy (b/H2) terms of the approach-to-saturation (ATS) law, the range of fields over which these terms remain finite is extended to low fields and a good approximation to the hysteresis loop may be constructed in its entirety. The model involves the introduction of the coercive field Hc, the anisotropy field HK, and a reverse domain nucleation field Hn as bias fields that are included in the denominators of the a and b terms. With these phenomenological refinements, the remanent magnetization becomes a function of a/(Hn+Hc), from which Hn may be obtained once a and Hc are determined. Initial results from several different magnetic materials, particularly garnets and spinel ferrites with square hysteresis loops, indicate good agreement with experiment. In cases where ATS analysis is not available because of inadequate sensitivity or inability to reach sufficiently high fields, curve fitting to a lower-field portion of the hysteresis loop is an alternative method of estimating the a and b coefficients.
Thin-film garnet materials with zero linear birefringence for magneto-optic waveguide devices (invited)
Authors: Wolfe R. , Fratello V. J. , McGlashan-Powell M.
Published: July 2002

The many advantages of thin-film waveguide magneto-optic devices have been difficult to realize because of the linear birefringence of these films, which makes it very difficult to achieve the required Faraday rotation. In epitaxial garnet films, this linear birefringence (the difference between the refractive indices of the TE and TM modes) can be reduced to small values by (1) growing single-mode multilayer films to minimize the shape effect, (2) growing the films in compression to control the photoelastic effect, and (3) annealing at high temperatures to eliminate the growth-induced effect. The remaining birefringence can be reduced to zero by growing the top active layer too thick so that the shape effect is smaller in magnitude than the stress effect, and then thinning it by chemical etching until the effects exactly cancel each other at a particular wavelength. An alternative method is to grow the top layer too thin and then to deposit a dielectric layer such as silicon nitride of the proper thickness to reduce the shape effect so that it exactly cancels the stress effect. Theoretical results on multilayer magneto-optic waveguides and successful etch-tuning experiments at 1.32 and 1.51 μm are presented. These tuning procedures can produce polarization-independent waveguide devices such as isolators or polarization rotators which may replace bulk or thick-film devices.
The effect of oxygen on the properties of AlGaAs solar cells grown by molecular-beam epitaxy
Authors: Amano Chikara , Ando Koshi , Yamaguchi Masafumi
Published: July 2002

The effect of oxygen, which is unintentionally incorporated into AlGaAs films during epitaxial growth, on AlGaAs solar cell characteristics is investigated using molecular-beam epitaxy. It is found that oxygen concentration strongly influences minority-carrier diffusion length and that solar cell efficiency greatly decreases as oxygen concentration increases. A model to describe the effect of oxygen on solar cell characteristics is suggested.
Analysis of the electrical resistivity of Ti, Mo, Ta, and W monocrystalline disilicides
Authors: Nava F. , Mazzega E. , Michelini M. , Laborde O. , Thomas O. , Senateur J. P. , Madar R.
Published: July 2002

The electrical resistivity of monocrystalline TiSi2, TaSi2, MoSi2, and WSi2 has been measured from 4.2 to 1100 K. These disilicides are metallic, yet there is a remarkable difference in the temperature dependence of their intrinsic resistivities. TiSi2 and TaSi2 are found to exhibit a T5 dependence in the temperature range of 13<T<30 K and 15<T<28 K, respectively, while MoSi2 and WSi2 show a T3.8 dependence from 15 to 40 K. For TiSi2, along the three crystallographic directions 〈100〉, 〈010〉, and 〈001〉, the phonon contribution to the resistivity was found to be linear in temperature above 300 K. The same behavior was observed for TaSi2 along the 〈0001〉 axis, while a negative deviation from the linearity followed by a quasisaturation was observed with the current, parallel to the 〈101̄0〉 axis. The resistivity data of WSi2 and MoSi2 with the current parallel to 〈001〉 and 〈110〉 crystallographic directions showed a positive deviation from linearity. The data are fitted to several theoretical expressions at low temperatures and in the full range of temperatures. The results are discussed in light of these theories.
Fresnel diffraction theory for steady-state thermal lens measurements in thin films
Authors: Wu Shaole , Dovichi Norman J.
Published: July 2002

Fresnel diffraction theory is combined with a three-dimensional temperature model to predict accurately the beam intensity profile produced by a steady-state thermal lens in 250-μm liquid films. In the comparison of theory and data, the optical parameters of the thermal lens instrument and the thermo-optical properties of the sample were introduced into the model; it was not necessary to adjust any parameter in the theory to produce good agreement between the predicted and observed beam profile. Because no parameters are adjusted to match theory to data, the thermal lens measurement provides an absolute measurement of sample absorbance. The model is used to predict the behavior of the thermal lens in thin films with respect to sample absorbance, sample thickness, laser power, and optical alignment. The new model is compared with several models from the literature; it is necessary to utilize the three-dimensional heat-flow analysis and Fresnel diffraction theory to obtain an accurate description of the behavior of the thermal lens in measurements of thin films.
Effective conductivity of hard-sphere dispersions
Authors: Miller C. A. , Torquato S.
Published: July 2002

Three-point bounds on the effective conductivity σe of isotropic two-phase composites, that improve upon the well-known two-point Hashin–Shtrikman bounds [J. Appl. Phys. 23, 779 (1962)], depend upon a key microstructural parameter ζ2. A highly accurate approximation for σe developed by Torquato [J. Appl. Phys. 58, 3790 (1985)] also depends upon ζ2. This paper reports a new and accurate algorithm to compute the three-point parameter ζ2 for dispersions of hard spheres by Monte Carlo simulation. Data are reported up to values of the sphere volume fraction φ2 near random close-packing and are used to assess the accuracy of previous analytical calculations of ζ2. A major finding is that the exact expansion of ζ2 through second order in φ2 provides excellent agreement with the simulation data for the range 0≤φ2 ≤0.5, i.e., this low-volume-fraction expansion is virtually exact, even in the high-density region. For φ2 >0.5, this simple quadratic formula is still more accurate than other more sophisticated calculations of ζ2. The linear term of the quadratic formula is the dominant one. Using our simulation data for ζ2, we compute three-point bounds on the conductivity σe and Torquato’s approximation for σe .
Ellipsometric study of AlInAs and AlGaP alloys
Authors: Rodríguez J. M. , Armelles G.
Published: July 2002

Spectroscopic ellipsometry has been used for determining optical direct interband transitions in a set of AlxIn1−xAs and AlxGa1−xP alloys. From these experimental results, the dependence of the E0 and E1 transitions is established as a function of the Al composition and the bowing parameters are obtained.
Charge storage in a nitride-oxide-silicon medium by scanning capacitance microscopy
Authors: Barrett R. C. , Quate C. F.
Published: July 2002

In this paper we describe a variant of the scanning capacitance microscope (SCaM) which is based on the atomic force microscope. Our SCaM involves a cantilever beam that is used to press a conducting tip against a conducting substrate coated with a dielectric film. A capacitance sensor is then used to measure the tip-sample capacitance as a function of lateral position. The deflection of the cantilever can also be used to measure independently the surface topography. This microscope can be used to measure electrical properties of dielectric films and their underlying substrates. We have applied this microscope to the study of the nitride-oxide-silicon (NOS) system. This system has been studied extensively because of its ability to store information by trapping charge in the silicon nitride. Commercial semiconductor nonvolatile memories have been designed using this NOS technology. We have used the SCaM tip to apply a localized bias to the NOS sample, causing charge to tunnel through the oxide layer and to be trapped in the nitride film. This trapped charge induces a depletion region in the silicon substrate, which can be detected by the resulting depletion capacitance between the tip and sample. The stored charge can be interpreted as a digital memory. Bit sizes as small as 750 Å full width at half maximum have been stored using this technique. The stored charge has been observed to be stable over a period of seven days. The stored charge can be removed by applying a reverse bias to the region, and the bit can be subsequently rewritten. By simultaneously measuring capacitance and topography images, we have demonstrated that the stored information is not the result of any topographic change to the surface. Simulations of the potential distributions resulting from this trapped charge have been performed and are compared with the experiments. Finally, a discussion is presented on the ultimate density and speed limits of such a storage technology.
Overview of the status of and prospects for high-T
c wire and tape development (invited)
Authors: Maley Martin P.
Published: July 2002

Recent world-wide progress in the effort to develop long continuous lengths of conductor based on high-Tc materials with high critical currents in substantial magnetic fields offers promise for a variety of applications. Two major problems, weak links and weak flux pinning, have plagued conductor development efforts from the beginning. Recent advances in mechanical processing of tape conductors based on BSCCO show great promise in overcoming weak links, with low-temperature Jc values≳105 A/cm2 extending to magnetic fields≳25 T. Unfortunately, these materials continue to exhibit weak flux pinning at moderate temperatures for magnetic fields aligned along the crystalline c axis. This manifests itself in a Jc that declines precipitously with magnetic field for T≳30 K. By contrast, conductors based on YBCO exhibit much stronger pinning at high temperature with smaller sensitivity to magnetic field orientation, but remain severely limited by weak links. The poor performance of BSCCO- and TBCCO-based materials appears to be strongly correlated with the high degree of intrinsic anisotropy. Phenomenological models describing this behavior and prospects for further progress are examined.
Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single-crystal films (invited)
Authors: Sugita Y. , Mitsuoka K. , Komuro M. , Hoshiya H. , Kozono Y. , Hanazono M.
Published: July 2002

Single-crystal Fe16N2 films have been grown epitaxially on Fe(001)/InGaAs(001) and InGaAs(001) substrates by molecular beam epitaxy (MBE). Saturation flux density Bs of Fe16N2 films has been demonstrated to be 2.8–3.0 T at room temperature, which is very close to the value obtained by Kim and Takahashi using polycrystalline evaporated Fe–N films. Temperature dependence of Bs has been measured. Bs changed with temperature reversibly up to 400 °C, while beyond 400 °C, Bs decreased irreversibly. X-ray diffraction showed that Fe16N2 crystal is stable up to 400 °C, while beyond 400 °C, Fe16N2 dissolves into Fe and Fe4N, and also some chemical reactions between Fe16N2 and the substrate occurs. This caused the temperature dependence of Bs mentioned above. From the temperature dependence of Bs up to 400 °C, the Curie temperature of Fe16N2 is estimated to be around 540 °C by using the Langevin function. The above mentioned Bs of 2.9 T at room temperature and 3.2 T at −268 °C corresponded to an average magnetic moment of 3.2μB per Fe atom and 3.5μB, respectively. These values of the magnetic moment of Fe atoms are literally giant, far beyond the Slater–Pauling curves. The origin of the giant magnetic moment has been discussed based on the calculation carried out by Sakuma. However, there was a significant disagreement between experimental values and calculated ones, so the origin remained to be clarified. Also, magneto-crystalline anisotropy of Fe16N2 films has been investigated.
Liquid metal expulsion during laser irradiation
Authors: Basu S. , DebRoy T.
Published: July 2002

During laser assisted materials processing such as welding, cutting, drilling, or surface alloying, the processing conditions are adjusted to either achieve or avoid liquid metal expulsion. Currently, there is no theoretical model to determine, from fundamental principles, the operating conditions for the initiation of liquid metal expulsion during laser irradiation. Processing conditions necessary for the initiation of liquid metal expulsion during pulsed laser irradiation have been investigated experimentally and theoretically. Lead, titanium, and stainless steel samples were irradiated by single and multiple pulses of varying pulse durations to investigate conditions for liquid metal expulsion. It is demonstrated that using theoretically computed transient spatial temperature profiles, and by balancing surface tension and recoil forces, the conditions for the initiation of liquid metal expulsion can be determined.
A Monte Carlo simulation model for plasma source ion implantation
Authors: Wang Dezhen , Ma Tengcai , Gong Ye
Published: July 2002

Plasma source ion implantation is a process in which a target is immersed in a plasma and a series of large negative-voltage pulses are applied to it to extract ions from the plasma and implant them into the target. A Monte Carlo simulation model is developed to study the energy and angle distributions of ions at the planar target for higher pressures of the neutral gas. Cross sections of the charge exchange and momentum transfer that depend on the ion energy are taken into account precisely. The energy and angle distributions of N2+ at the target during the sheath edge evolution for the different pressures are determined.
Surface properties of perovskites and their response to ion bombardment
Authors: Mukhopadhyay Sharmila M. , Chen Tim C. S.
Published: July 2002

X-ray photoelectron spectroscopy has been used to study the surface composition and chemistry of two perovskites: SrTiO3 and Pb(Zr,Ti)O3 (commonly known as PZT). It is seen that ion bombardment, which is a common surface modification technique, can cause substantial changes in these oxides. The PZT surface undergoes surface depletion of lead along with chemical reduction of the Pb2+ ion to its metallic state. The Zr/(Ti+Zr) ratio also changes with sputtering, but the total oxygen to cation ratio is unchanged. On the other hand, the surface stoichiometry of SrTiO3 is almost unaffected by ion bombardment. In all the perovskites, irrespective of whether the composition changes or not, a substantial amount of surface Ti is reduced to a lower valency state on sputtering. Most of this component is restored back to the original Ti4+ state when Ni is evaporated on these surfaces, indicating that the reduced state is associated with a damaged outermost surface that can be repaired with an adsorbate. The implication of these results to the bonding properties of these materials have been discussed.
Mode-mismatched thermal lens determination of temperature coefficient of optical path length in soda lime glass at different wavelengths
Authors: Baesso M. L. , Shen J. , Snook R. D.
Published: July 2002

The time-resolved and steady-state mode-mismatched thermal lens technique has been used to determine the temperature coefficient of optical path length ds/dT at different wavelengths of soda lime glasses (72 wt % SiO2, 18 wt % Na2O, 10 wt % CaO, and 70.5 wt % SiO2, 17.5 wt % Na2O, 10 wt % CaO, 2 wt % Fe2O3). The aberrant model, which takes into account the thickness change of the sample and is more realistic than the parabolic treatment when used to describe the thermal lens effect, was used as the theoretical model. The results showed that ds/dT is (2.1±0.1)×10−6 K−1 at 632.8 nm and (2.4±0.1)×10−6 K−1 at 442 nm for the undoped sample and (4.66±0.03)×10−6 K−1 at 632.8 nm and (6.1±0.1)×10−6 K−1 at 442 nm for the iron-doped sample. The greater value of this parameter found for the doped sample at the absorption band (442 nm) was associated with the bigger value of the temperature coefficient of the electronic polarizability caused by the presence and environment of Fe3+ iron in the structure of the glass.
Raman spectra in a broad frequency region of p− type porous silicon
Authors: Zhang Shu-Lin , Wang Xin , Ho Kouk-san , Li Jinjian , Diao Peng , Cai Shengmin
Published: July 2002

Throughout the 50–1050 cm−1 range, in addition to the first order Raman peak of optical phonon, a weak structure in the vicinity of 147 cm−1 and two intense peaks (bands) in the vicinities of 632 and 956 cm−1 have been observed. The structure at 147 cm−1 is believed to be mainly due to scattering by the transverse acoustic single phonon mode; the intense peak at 632 cm−1 is thought to be mainly due to the transverse acoustic and optical combination phonon modes and the intense peak at 956 cm−1 to the transverse optical overtone double phonon mode of the p− type porous silicon (PS) layer. A comparison of spectral features indicates that, in PS Raman spectra, there is no substantial contribution due to the a-Si component and for PS the crystalline characteristic still remains to a great extent. Contrary to what would be expected using the conventional and simplified microcrystal model, Raman frequency downshifts of the aforementioned structures near 632 and 956 cm−1 were clearly observed, which indicates that, unlike the case of the first order Raman spectrum of optical phonon, the microcrystal model is too simple to give an interpretation of PS second order Raman spectra.
Initial growth stage and optical properties of a three-dimensional InAs structure on GaAs
Authors: Nabetani Y. , Ishikawa T. , Noda S. , Sasaki A.
Published: July 2002

A few mololayers of InAs is heteroepitaxially grown on GaAs substrate by molecular-beam epitaxy. Structure and optical properties are investigated. Reflection high-energy electron-diffraction observation reveals that an InAs layer forms a three-dimensional structure with specific facets after two-dimensional growth. The transmission electron microscope observation shows that these structures have structural anisotropy in the growth plane. Photoluminescense spectroscopy shows that the luminescence from the InAs structures exhibits the polarization property caused by the quantum dot effect of the structural anisotropy.
Alloy-scattering dependence of electron mobility in the ternary gallium, indium, and aluminum nitrides
Authors: Chin V. W. L. , Zhou Bing , Tansley T. L. , Li Xin
Published: July 2002

Based on Phillips’ electronegativity theory [Rev. Mod. Phys. 42, 317 (1970)] we have determined the alloy scattering potential for the ternary nitrides, Ga1−xAlxN, In1−xGaxN, and In1−xAlxN, and hence the 300 and 77 K electron mobilities through a variational principle calculation. Alloy scattering is important in In1−xGaxN, and In1−xAlxN, both of which show a significant composition ‘‘bowing’’ in electron drift mobility, even at 300 K. This contribution is not important in Ga1−xAlxN. Acoustic phonon scattering is also significant at 300 K, in contrast to the situation obtained in GaAs-based ternaries.
Magnetism of the solid solution GdGa1−xGex (abstract)
Authors: Leithe-Jasper A. , Hiebl K.
Published: July 2002

Ternary metal excess rare-earth gallides and germanides have been extensively investigated concerning their interesting physical properties with respect to magnetic and electronic correlations, such as magnetic ordering, intermediate valency, Kondo or heavy fermion behavior. A solid solution of the binary phases GdGa and GdGe was found investigating the phase equilibria in the pseudobinary system Gd-Ga-Ge at T=800 °C. Samples with the composition GdGa1−xGex (x=0, 0.1, 0.15, 0.25, 0.5) were prepared by arc melting the elements under inert atmosphere and subsequent annealing (350 h) in evacuated silica capsules. The crystal structure and unit-cell dimensions were determined by x-ray powder analyses (Guinier technique, Cu Kα1 radiation). Powder patterns were indexed on the basis of a C-centered orthorhombic unit cell. No deviations of the structure type α-ITI (CrB) have been encountered. A monotonic decrease of the unit-cell volume versus Ge concentration is observed. However, a significant decrease of the b and c parameters is thereby compensated by an increase of the a parameter. Magnetic measurements were performed by SQUID and ac techniques over a temperature range T=5–300 K and in external fields up to 6 T. All samples exhibit ferromagnetic order with slightly decreasing Tc as well as values of magnetization (at T=5 K and H=0.1 T) with increasing concentration x. However, metamagnetic transitions are revealed for all samples with x≳0. The results are discussed within the concept of indirect exchange interaction (RKKY-type interaction).
Densified silica glass after shock compression
Authors: Sugiura H. , Ikeda R. , Kondo K. , Yamadaya T.
Published: July 2002

Silica glasses were repeatedly densified by multiple shock reverberations in stainless-steel capsules. The limit of increase in density was about 2.47 Mg/m3 after the first shock loading. Further increase in density was observed after duplicate shock loading, but limited at about 2.55 Mg/m3. Triplicate shock loading was not so effective for the increase in density. The Raman spectra of recovered silica glasses had characteristics of densified silica glass with much higher density. The remarkable one was the enhancement of the line at 605 cm−1 called the D2 line, which was one of two narrow lines peculiar to the Raman spectrum of silica glass.
MeV ion implantation induced damage in relaxed Si1−xGex
Authors: Nylandsted Larsen A. , O’Raifeartaigh C. , Barklie R. C. , Holm B. , Priolo F. , Franzo G. , Lulli G. , Bianconi M. , Nipoti R. , Lindner J. K. N. , Mesli A. , Grob J. J. , Cristiano F. , Hemment P. L. F.
Published: July 2002

The damage produced by implanting, at room temperature, 3-μm-thick relaxed Si1−xGex alloys of high crystalline quality with 2 MeV Si+ ions has been studied as a function of Ge content (x=0.04, 0.13, 0.24, or 0.36) and Si dose in the dose range 1010–2×1015 cm−2. The accumulation of damage with increasing dose has been investigated by Rutherford backscattering spectrometry, optical reflectivity depth profiling, and transmission electron microscopy. An enhanced level of damage, and a strong decrease in the critical dose for the formation of a buried amorphous layer in Si1−xGex is observed with increasing x. Electron paramagnetic resonance studies show that the dominant defects produced by the implantation are Si and Ge dangling bonds in amorphouslike zones of structure similar to a-Si1−xGex films of the same x, and that the effect of increasing the ion dose is primarily to increase the volume fraction of material present in this form until a continuous amorphous layer is formed. A comparative study of the optically determined damage in the alloys with the use of a damage model indicates a significant increase in the primary production of amorphous nuclei in the alloys of Ge content x>0.04.
Effect of oxygen stoichiometry on the electrical properties of La0.5Sr0.5CoO3 electrodes
Authors: Madhukar Sucharita , Aggarwal S. , Dhote A. M. , Ramesh R. , Krishnan A. , Keeble D. , Poindexter E.
Published: July 2002

We report on the metal-insulator transition of La0.5Sr0.5CoO3 thin films deposited by pulsed laser ablation on LaAlO3 substrates. The films were cooled in oxygen partial pressures between 760 and 10−5 Torr and electrical resistivity of the films was measured as a function of cooling oxygen pressure. La0.5Sr0.5CoO3 films changed from metallic to insulating behavior depending on their oxygen content. A defect model has been proposed to explain this transition and the change in conductivity is related to the change in the oxidation state of the cobalt ions. The model explains the relationship between oxygen partial pressure and electrical conductivity in La0.5Sr0.5CoO3, which describes the experimental dependence reasonably well. Positron annihilation studies were also done on the same set of samples and the S parameter was seen to increase by 8% from a fully oxygenated sample to a sample cooled in 10−5 Torr.
Characterization of FeSe thin films prepared on GaAs substrate by selenization technique
Authors: Takemura Y. , Suto H. , Honda N. , Kakuno K. , Saito K.
Published: July 2002

FeSe thin films were prepared on GaAs(100) substrate by the selenization of Fe films using molecular-beam epitaxy. FeSe compound thin films were obtained at a substrate temperature above 380 °C. From the depth profiles of Fe and Se in the selenized film measured by Auger electron spectroscopy, it was confirmed that an FeSe layer with a constant ratio of Fe/Se was formed. The measured composition ratio of Fe/Se in the film was 1/3. It was different from the composition in Fe3Se4 or Fe7Se8, which is a stable bulk FeSe compound. From the measured M–H curve, it was found that the obtained FeSe film consisted of two phases with different magnetic properties.