bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal conductivity reduction in HgTe/CdTe superlattices
Authors: Bies W. E. , Ehrenreich H. , Runge E.
Published: July 2002

The techniques used previously to calculate the threefold thermal conductivity reduction due to phonon dispersion in GaAs/AlAs superlattices (SLs) are applied to HgTe/CdTe SLs. The reduction factor is approximately the same, indicating that this SL may be applicable both as a photodetector and a thermoelectric cooler.
Growth of GaN free from cracks on a (111)Si substrate by selective metalorganic vapor-phase epitaxy
Authors: Honda Y. , Kuroiwa Y. , Yamaguchi M. , Sawaki N.
Published: July 2002

The selective metalorganic vapor-phase epitaxy of wurtzite GaN was performed on a (111) silicon substrate using SiO2 grid mask pattern. Within window regions of (0.2–0.5) mm×(0.2–0.5) mm, GaN films free from cracks were achieved. The full width at half maximum of the (0004) X-ray rocking curve was as narrow as 388 arcs and that of the band edge emission was 18.6 meV at 77 K. The band edge emission peak energy was redshifted. The redshift is reduced slightly in a sample grown on small windows. This suggests that the biaxial strain due to the thermal expansion coefficient mismatch is partly relaxed on small windows.
Rapid thermal annealing effects on step-graded InAlAs buffer layer and In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor structures on GaAs substrates
Authors: Cui L. J. , Zeng Y. P. , Wang B. Q. , Wu J. , Zhu Z. P. , Lin L. Y.
Published: July 2002

A step-graded InAlAs buffer layer and an In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor (MM-HEMT) structures were grown by molecular beam epitaxy on GaAs (001) substrates, and rapid thermal annealing was performed on them in the temperature range 500–800 °C for 30 s. The as-grown and annealed samples were investigated with Hall measurements, and 77 K photoluminescence. After rapid thermal annealing, the resistivities of step-graded InAlAs buffer layer structures became high. This can avoid leaky characteristics and parasitic capacitance for MM-HEMT devices. The highest sheet carrier density ns and mobility μ for MM-HEMT structures were achieved by annealing at 600 and 650°C, respectively. The relative intensities of the transitions between the second electron subband to the first heavy-hole subband and the first electron subband to the first heavy-hole subband in the MM-HEMT InGaAs well layer were compared under different annealing temperatures.
Polymorphous hydrogenated silicon Schottky diodes: Properties under illumination
Authors: Gauthier Monique
Published: July 2002

This report describes the properties of thick Schottky diodes based on standard device grade amorphous hydrogenated silicon or polymorphous hydrogenated silicon under illumination. It has been observed that the internal collection efficiency in the red part of the spectrum is much higher for the polymorphous diodes than for the standard one. In contrast, at short wavelength there is no significant difference at short circuit, but the maximum output power and the fill factor are much higher in the case of polymorphous samples. Finally, the effect of reverse bias annealing has been examined. The experimental results are discussed, taking into account simulation studies in the literature. In particular it is shown that the collection efficiency at long wavelength is determined mainly by recombination in the bulk, and that the experimental data correlate with the electronic properties of films prepared in the same conditions. At short wavelength, recombination in the front layer seems to be an important carrier loss mechanism in standard diode at low forward bias, while it is negligible in polymorphous samples. Finally, the effect of reverse bias annealing is tentatively attributed to a widening of the valence band tail.
Current-assisted annealing effect in Nd0.5(Ca,Sr)0.5MnO3
Authors: Chang C. W. , Debnath A. K. , Lin J. G.
Published: July 2002

Current–voltage (I–V) relations of Nd0.5(Ca,Sr)0.5MnO3 are investigated near its percolation threshold. Resistivity and I–V relations reveal the unstable nature of Nd0.5(Ca,Sr)0.5MnO3. A large slope change in the I–V curve is observable for low (I<15 mA) applied current by manipulating the applied magnetic field at 85 K. Our results indicate that the external applied current can assist not only an insulator to metal transition but also a metal to insulator transition.
The shear viscosity of molecular fluids: A calculation by reverse nonequilibrium molecular dynamics
Authors: Bordat Patrice , Müller-Plathe Florian
Published: July 2002

The reverse nonequilibrium molecular dynamics [F. Müller-Plathe, Phys. Rev. E 49, 359 (1999)] presented for the calculation of the shear viscosity of Lennard-Jones liquids has been extended to atomistic models of molecular liquids. The method is improved to overcome the problems due to the detailed molecular models. The new technique is besides a test with a Lennard-Jones fluid, applied on different realistic systems: liquid nitrogen, water, and hexane, in order to cover a large range of interactions and systems/architectures. We show that all the advantages of the method itemized previously are still valid, and that it has a very good efficiency and accuracy making it very competitive.
Multiqubit computing and error-avoiding codes in subspace using quantum dots
Authors: Qiao Bi , Ruda H. E. , Wang Jian
Published: July 2002

An approach is presented for directly constructing cn-Not multi-qubit logic gates using quantum dots. In addition, error-avoiding codes in the projected subspace are discussed, offering a potentially robust scheme for protecting against decoherence. The following general conditions were found: (1) a projected subspace can always be constructed, in which decoherence effects can be cancelled by adjusting the interaction coupling time, enabling error-correction to be performed. This subspace is closed for the projected states. (2) The necessary and sufficient conditions for a decoherence-free subspace are that the second term in the subdynamic equation, be equal to zero. Commonly formulated conditions for a decoherence-free subspace are shown to be particular approximations of this condition, including the so-called Born–Markovian approximation.
The onset of Rayleigh–Taylor instabilities in magnetized partially ionized dense dusty plasmas
Authors: Birk Guido T.
Published: July 2002

Onset criteria for unstable Rayleigh–Taylor modes in partially ionized dusty plasmas are derived for different density characteristics of the neutral gas and dust components across the interface where the modes are excited. The influences of a homogeneous equilibrium magnetic field and magnetic perturbations are taken into account. Dust–neutral gas collisions limit the range of unstable wave lengths. Different from (magneto)hydrodynamic modes in one-component neutral fully ionized fluids, increase of the mass density of the heavier fluid does not tend to a destabilization of all excited modes in the stratified fluid layer.
Potential energy surfaces of Lawrencium and Nobelium dihydrides (LrH2 and NoH2)
Authors: Balasubramanian K.
Published: July 2002

It is demonstrated that the compounds of late actinides, namely Lawrencium and Nobelium surprisingly exhibit unusual nonactinide properties in that unlike other actinides the chemistry of these species is principally determined by the 7s and 7p orbitals rather than the 5f or 6d shells. Relativistic computations including electron correlation and spin–orbit effects using the complete-active space multiconfiguration self-consistent field followed by second-order and multireference relativistic configuration interaction (RCI) techniques are considered for the Lawrencium and Nobelium dihydrides as well the atoms. The ground and first excited states of Lawrencium and Nobelium arise from the 7s and 7p shells, and thus the potential energy surfaces of these species are unusual in having considerable 7p characteristics. Both molecules form stable bent ground states reminiscent of sp2 hybridization with equilibrium bond angles near 120°. The Lawrencium compounds exhibit unusual characteristics due to avoided crossings of the potential energy surfaces. As a result of spin–orbit coupling, the B22 state of LrH2 undergoes avoided crossing with the A12 state in the spin double group, which reduces the barrier for insertion of Lr into H2. The Nobelium compounds are shown to be considerably less stable compared to Lawrencium compounds due to the relativistic stabilization of the 7s shell of the Nobelium atom. It is shown that the barrier for insertion of Lr into H2 is lowered by relativity (spin–orbit coupling), while No has to surpass a larger barrier due to the relativistic stabilization of the 7s2 shell, which is not very reactive. Lawrencium is the only element in the actinide series with unusually low ionization potential, and NoH2 has an unusually large dipole moment of 5.9 Debye. It is suggested that the Lawrencium and Nobelium compounds should have periodic similarities to the thallium and radium compounds, respectively.
Ab initio investigation of doping-enhanced electronic and vibrational second hyperpolarizability of polyacetylene chains
Authors: Champagne Benoı̂t , Spassova Milena , Jadin Jean-Benoit , Kirtman Bernard
Published: July 2002

The effect of charging on the longitudinal second hyperpolarizability of polyacetylene (PA) chains containing up to nearly 70 carbon atoms has been investigated ab initio by characterizing chains with and without an explicit alkali atom (Li, Na, K) as dopant. Whereas charging dramatically enhances the static electronic and vibrational hyperpolarizabilities, γLe(0) and γLv, of an isolated chain at intermediate chain lengths, the presence of an alkali atom counterion substantially reduces this effect. As the size of the alkali atom increases, most properties, including the hyperpolarizabilities, approach those of the isolated chain. Detailed analysis shows that the behavior of γLe(0) is most simply explained in terms of a reduced electrostatic pinning potential due to increased distance between chain and counterion. At all chain lengths studied γLe(0) of PA is enhanced by alkali doping. For chains containing 50 carbon atoms (NC=50), the increase due to K doping is about 9×107 a.u., which more than doubles the value for an undoped chain of similar length. The normalized quantity γLe(0)/NC exhibits a maximum for the isolated soliton (at about NC=61) that is over four times that of the infinite undoped (and unbent) chain. When the alkali dopant is taken into account this maximum diminishes considerably and shifts to larger NC than we have considered. In comparison with the maximum for the undoped species (at NC=∞) there is a small enhancement of γLe(0)/NC for K doping, but none for either Li or Na doping at the coupled-perturbed Hartree–Fock (CPHF)/6-31G level of theory. Intermediate length isolated chains bearing a charged soliton show order of magnitude increases in γv for the degenerate four-wave mixing (DFWM) and, especially, electric field-induced second harmonic generation (dc-SHG) processes compared to undoped PA. As in the case of γLe(0) this enhancement persists, but is significantly reduced when the dopant atom is included. Vibrational anharmonicity, which contributes only to the dc Kerr effect at our level of treatment, is much more important in the doped than undoped species.
Analysis of one-sun monocrystalline rear-contacted silicon solar cells with efficiencies of 22.1%
Authors: Dicker J. , Schumacher J. O. , Warta W. , Glunz S. W.
Published: July 2002

21.4% efficient rear-contacted cells (RCC) with interdigitated contact grids processed at the Fraunhofer ISE on 1.25 Ω cm float-zone (FZ) silicon are analyzed in detail. The comprehensive description does not only include a two-dimensional numerical device simulation, but also a detailed analysis of the optical carrier generation using optical ray tracing and determination of the losses due to distributed metal resistance and perimeter currents employing circuit simulation. Bulk and surface recombination losses are separated, combining carrier lifetime and open-circuit voltage measurements with numerical device simulation. The interface surface recombination velocity of the thermally oxidized emitter covering the front surface is deduced to be 1500 cm/s and the bulk diffusion length within the 1.25 Ω cm FZ silicon base is 1200 μm. Despite this excellent bulk diffusion length, the simulations reveal that at a maximum power point 80% of the total recombination is due to Shockley–Read–Hall recombination in the base. It was determined that losses due to the distributed metal resistance within the contact grid (including nongeneration losses) caused an fill factor decrease of 1% absolute. Loss currents flowing out of the cell perimeter caused an additional fill factor loss of 1.5% absolute. It was predicted that changing the surface concentration of the front and rear emitter diffusion from 5×1018 to 1×1018 cm−3, while keeping the sheet resistance constant, effected a relative improvement of 3% in the efficiency of the RCC structure. In fact, this modification has lead to an increase in the realized cell efficiency from 21.4% to 22.1% (VOC=697.6 mV, JSC=39.8 mA/cm2, and FF=79.4%), i.e., a relative improvement of 3.3%. This cell has an efficiency of 18.6% (VOC=696.5 mV, JSC=33.9 mA/cm2, and FF=78.8%) if illuminated from the rear side.
Enhanced electromagnetic emission from a dusty plasma
Authors: Shukla P. K. , Stenflo L. , Resendes D. P.
Published: July 2002

It is shown how enhanced electromagnetic emission (EEE) is induced when large amplitude electromagnetic waves scatter off acoustic-like modes in a dusty plasma. Following the formalism of standard parametric interactions and using the Rostoker superposition principle, a general expression for the ensemble average of the square of the vector potential of EEE is derived. The result should be useful in deducing the plasma parameters in-situ when intense radar beams are used for the diagnosis of the polar mesospheric summer echos in association with low-frequency irregularities.
Thickness dependence of the V induced magnetic moment in Fe/V/Fe(110) trilayers
Authors: Scherz A. , Poulopoulos P. , Wende H. , Ceballos G. , Baberschke K. , Wilhelm F.
Published: July 2002

We report on in situ x-ray magnetic circular dichroism experiments at the Fe and V L edges on Fe/V/Fe(110) trilayers. Our results show that the induced V magnetic moment, which couples antiparallel to the Fe one, decreases rapidly away from the interface indicating a short-range polarization in the V layers. This finding confirms recent ab initio calculations.
Fast magnetic multipole imaging of elementary sources on the disk
Authors: Kildishev Alexander V. , Nyenhuis John A.
Published: July 2002

The multipole image (MI) is the set of coefficients in a harmonic expansion of the scalar magnetic potential of a magnetic source. The expansion coefficients of the MI can be determined using methods of spatial harmonic analysis. Coefficients of the oblate spheroidal MI provide an improved description of the field near a flattened source. In a simplified model, the source can be simulated as a superposition of elementary sources on the disk. This work focuses on fast algorithms for the oblate spheroidal multipole imaging of the elementary magnetic sources assuming that the elementary sources (magnetic dipoles) are located anywhere on a disk. Recursive relations for the gradient of the expansion terms of the Green function have been derived. The oblate spheroidal multipolar coefficients are then calculated in the similar recursive manner.
Electric fields produced by electron irradiation of insulators in a low vacuum environment
Authors: Toth M. , Phillips M. R. , Craven J. P. , Thiel B. L. , Donald A. M.
Published: July 2002

We report on the properties of electric fields generated as a result of electron irradiation of dielectrics in a low vacuum scanning electron microscope. Individual field components produced by (i) ionized gas molecules located outside the sample surface and (ii) subsurface trapped charge were detected by measurements of changes in (i) primary electron landing energy and (ii) secondary electron (SE) emission current, respectively. The results provide experimental evidence for a recently proposed model of field-enhanced SE emission from electron irradiated insulators in a low vacuum environment [Toth et al., J. Appl. Phys. 91, 4479 (2002)]. Errors introduced into x-ray microanalysis by the electric fields generated by ionized gas molecules can be alleviated by minimizing the steady state ion concentration by the provision of efficient ion neutralization routes. It is demonstrated how this can be achieved using simple sample–electrode geometries.
Dependence of pulsed-laser deposition parameters on the microstructure and magnetic property of Nd–Fe–B thin films grown at high substrate temperature
Authors: Huang X. J. , Xu S. Y. , Ong C. K. , Yang Z. , Si L. , Li Y.
Published: July 2002

We systematically studied the dependencies of substrate temperature, laser fluence, deposition time, ambient gas pressure and laser frequency on the structural and magnetic properties of Nd–Fe–B thin films synthesized by pulsed-laser deposition at high substrate temperature Ts. A coercive force of 2.0–2.4 kOe was obtained on samples with a thickness of 70–90 nm and an average grain size of 50–70 nm grown at Ts of 620–650 °C. We showed a clear trend through statistical analysis that the coercivity decreases with increasing film thickness up to 800 nm due to an increase in the crystalline grain size. Oxidation layer and defects on the grain surface may also have greatly reduced the nucleation field and thus resulted in a low coercivity and low saturation magnetization of the samples. Comparison of the films made by PLD and sputtering under similar conditions has been discussed.
Preparation of polycrystalline boron carbide thin films at room temperature by pulsed ion-beam evaporation
Authors: Suematsu H. , Kitajima K. , Suzuki T. , Jiang W. , Yatsui K. , Kurashima K. , Bando Y.
Published: July 2002

Polycrystalline boron carbide (B4C) thin films have been prepared by a pulsed ion-beam evaporation technique without heating substrates or annealing samples. Here, we clearly demonstrate the possibility of preparing B4C thin films for electronic device applications.
Band structures for nonlinear photonic crystals
Authors: Huttunen A. , Törmä P.
Published: July 2002

We present a method for calculating band structures for one-dimensional Kerr nonlinear photonic crystals, which exhibit an optical switching function. The band structure shows the allowed modes for the nonlinear photonic crystal as a function of the magnitude of the nonlinearity. The dielectric band is found to be most suited for the control beam as it is least effected by the nonlinearity. The third band is more sensitive to the nonlinearity and thus suited for the probe beam. Also the wavelength dependence of the switching function can be estimated using our method. The applied Fourier method is found to be robust in describing the effect of the nonlinearity.
Interaction between Γ8 states in hexaborides
Authors: Schlottmann P.
Published: July 2002

Ce ions in (Ce,La)B6 have a Γ8 ground multiplet, which is fourfold degenerate and has orbital and spin content. The interaction between Ce ions is of the Ruderman–Kittel–Kasuya–Yosida (RKKY)-type mediated by conduction states in three approximately ellipsoidal pockets centered at the X points of the cubic lattice. The three pockets interfere yielding a RKKY interaction that depends on the relative position of the ions, as well as on the relative orientation of the line joining two ions to the cubic crystalline field axis. The 16 states of a pair of Ce ions are split by the RKKY interaction into a singlet, a triplet, and a 12-fold degenerate level. The ground state is always either the singlet or a triplet, depending on the sign of the interaction, and as a consequence a pair of Ce ions always has a quadrupolar moment. The interaction strength for ions on the four closest neighbor shells is obtained. The dipolar, quadrupolar, and octupolar content of the interaction along the (1,0,0) direction is discussed.
Photodetachment spectroscopy of the C2nH− (n=2–4) anions in the vicinity of their electron detachment threshold
Authors: Pino T. , Tulej M. , Güthe F. , Pachkov M. , Maier J. P.
Published: July 2002

The electronic spectra of the C2nH(D)−, n=2–4, anions have been observed in the gas phase using photodetachment spectroscopy. These are assigned to Π1←X 1Σ+ electronic transitions. The Π1 excited states possess a dipole bound character indicated by the energetic proximity between the origin of the transitions and electron affinities. The dipole bound states are related to the X 2Π ground states of C6H and C8H whereas for C4H, which has an X 2Σ+ ground state, the Π2 is an excited state. Vibronic coupling through a bending motion of the carbon skeleton is inferred to be the reason of the stabilization of this state.
The origin of bias-voltage dependence in CoFe/SrTiO3/La0.7Sr0.3MnO3 magnetic tunnel junctions
Authors: Hayakawa J. , Ito K. , Kokado S. , Ichimura M. , Sakuma A. , Sugiyama M. , Asano H. , Matsui M.
Published: July 2002

The relationship between the oxidized state in a barrier and the bias-voltage dependence on tunnel magnetoresistance (TMR) effect was examined in Co90Fe10/SrTiO3(STO)/La0.7Sr0.3MnO3(LSMO) magnetic tunnel junctions using single crystalline LSMO electrode and STO barrier. A TMR junction, whose STO barrier was fabricated stoichiometrically, exhibited an asymmetric dependence with regard to the bias direction. However, when the STO barrier had an oxygen deficiency, the TMR ratio was reduced symmetrically. The oxygen deficiency of the STO barrier is probably the cause of the oxygen deficiency in the interfacial LSMO and the electron scattering in the barrier, and they may reduce the symmetric bias-voltage dependence.
Perpendicular electric transport in Fe/X/Fe model heterostructures
Authors: Herper H. C. , Weinberger P. , Szunyogh L. , Sommers C.
Published: July 2002

The electric transport properties of Fe/Xs/Fe model systems have been investigated for current perpendicular to the plane geometry. Three different spacer materials X=Ge, [In, P], [Zn, Se] have been used in order to examine the dependence of the magnetoresistance with respect to the spacer thickness. In the case of binary spacer materials the influence of the termination of the spacer material is also investigated. In addition, some important technical aspects like the variation of the sheet resistance with the number of lead layers are discussed. The investigations have been performed in the framework of the relativistic spin-polarized version of the screened Korringa–Kohn–Rostoker method, and the Kubo–Greenwood formula has been used to describe the transport properties. It has been observed that for all three systems the magnetoresistance becomes constant with increasing spacer thickness, the absolute value depending on the material and the termination of the spacer. Furthermore, we can demonstrate that the dominant contributions to the magnetoresistance arise from the Fe/X and X/Fe interfaces.
Study of low-energy-electron forward focusing effects on highly oriented pyrolytic graphite using scanning auger microscopy
Authors: Cui Nai-Yi , Brown Norman M. D.
Published: July 2002

The effects of the electron diffraction in highly oriented pyrolytic graphite are studied in an incident electron beam of energy range 500–700 eV using scanning Auger microscopy. The intensity of the exiting electrons monitored by a cylindrical-mirror analyzer can be well interpreted as the joint effect of incident and exit forward focusing along the close-packed 〈21̄1̄2〉-type atomic chains, with the contribution from forward focusing along the secondary close-packed atomic chains increasing rapidly as the beam energy increases. Incident beam effects contribute greatly to the variation in the intensity of the exiting electrons. Increasing the beam energy in this range also increases the anisotropy observed in the angular distribution of the intensity of both elastically scattered and Auger electrons.
Bulk nanocomposite permanent magnets produced by crystallization of Fe66.5Co10Pr3.5B20 bulk glassy alloy
Authors: Zhang Wei , Inoue Akihisa
Published: July 2002

The glass-forming ability, thermal stability, crystallized structure, and magnetic properties have been investigated for an Fe66.5Co10Pr3.5B20 glassy alloy with a large supercooled liquid region of 43 K before crystallization and a high reduced glass transition temperature of 0.56. The glassy phase is formed in a rod form with a diameter of 0.5 mm by copper mold casting. The crystallized nanocomposite structure consists of Fe3B, α-Fe and Pr2Fe14B phases, and the remanence (Br), coercivity (iHc), and maximum energy product (BH)max are 1.23 T, 225 kA/m, and 95.9 kJ/m3, respectively, for the cast rod annealed at 863 K for 600 s. The present study shows the possibility of producing bulk Pr2Fe14B/(Fe3B,α-Fe) nanocomposite permanent magnets by the simple process of copper mold casting and heat treatment.
An accurate head-positioning signal for perpendicular recording using a dc-free servo pattern
Authors: Hamaguchi Takehiko , Ichihara Takayuki , Takano Hisashi
Published: July 2002

We devised a dc-free servo pattern for perpendicular recording to prevent distortion of the readback wave form from the servo area. This servo pattern remarkably improves the servo sector error rate, and shows the same linearity of position error signals as does that from a conventional pattern. An experimental 2.5 in hard disk drive (HDD) with a trapezoidal-shaped single pole type (SPT) head shows good following performance within a wide range of yaw angle.
Minority carrier diffusion and defects in InGaAsN grown by molecular beam epitaxy
Authors: Kurtz Steven R. , Klem J. F. , Allerman A. A. , Sieg R. M. , Seager C. H. , Jones E. D.
Published: July 2002

To gain insight into the nitrogen-related defects of InGaAsN, nitrogen vibrational mode spectra, Hall mobilities, and minority carrier diffusion lengths are examined for InGaAsN (1.1 eV band gap) grown by molecular beam epitaxy (MBE). Annealing promotes the formation of In–N bonding, and lateral carrier transport is limited by large scale (≫mean free path) material inhomogeneities. Comparing solar cell quantum efficiencies with our earlier results for devices grown by metalorganic chemical vapor deposition (MOCVD), we find significant electron diffusion in the MBE material (reversed from the hole diffusion in MOCVD material), and minority carrier diffusion in InGaAsN cannot be explained by a “universal,” nitrogen-related defect.
Self-oscillation mode induced in an atomic force microscope cantilever
Authors: Kim Kiwoong , Lee Soonchil
Published: July 2002

We developed a technique to excite the self-oscillation of a commercially available metal-coated cantilever and detect the deflection simultaneously with only a single optical fiber. The technique is based on the fact that the light transmitted to a cantilever in the fiber-optic interferometer induces the photothermal positive feedback to the cantilever oscillation. The model analysis on the cantilever oscillation shows that the product of the slope of an intensity curve and the thermal coefficient of deflection of the cantilever should be greater than the product of the damping parameter and the dynamic heat capacity to excite the self-oscillation, which was confirmed by experiment. It can be used either to excite the self-oscillation in the frequency modulation atomic force microscopy or to avoid it in normal force microscopy.
Magnetic force microscopy characterization of a first-order transition: Magnetic-martensitic phase transformation in Gd5(SixGe1−x)4
Authors: Leib J. , Snyder J. E. , Lo C. C. H. , Paulsen J. A. , Xi P. , Jiles D. C.
Published: July 2002

The magnetic-martensitic phase transformation of Gd5(SixGe1−x)4 (x≈0.5), which occurs close to room temperature, has been observed for the first time using a magnetic force microscope (MFM) equipped with a heating–cooling stage. MFM images obtained from a polycrystalline Gd5(Si2.09Ge1.91) sample and single crystal Gd5(Si1.95Ge2.05) and Gd5(Si2Ge2) samples showed transition to a domain structure at low temperatures indicative of a ferromagnetic phase. Some samples exhibited complex domain structures, suggesting that Gd5(SixGe1−x)4 (x≈0.5) has a strong magnetic anisotropy. As the sample temperature increased the domain structure diminished, reflecting the transformation from ferromagnetic to paramagnetic state. On cooling the sample the domain structure reappeared, but at a lower transformation temperature than on heating. This magnetic phase transformation is highly unusual because it is an “order–disorder” phase transition, which is normally second order, but in this case the “order–disorder” (ferromagnetic to paramagnetic) transition exhibits hysteresis in temperature, indicating that it is first order. Such thermal hysteresis in phase transformation was also observed in thermal expansion experiments. The transformation temperatures obtained in the MFM study are in good agreement with those determined from thermal expansion data.
High confinement plasmas in the Madison Symmetric Torus reversed-field pinch
Authors: Chapman B. E. , Almagri A. F. , Anderson J. K. , Biewer T. M. , Chattopadhyay P. K. , Chiang C.-S. , Craig D. , Den Hartog D. J. , Fiksel G. , Forest C. B. , Hansen A. K. , Holly D. , Lanier N. E. , O’Connell R. , Prager S. C. , Reardon J. C. , Sarff J. S. , Wyman M. D. , Brower D. L. , Ding W. X. , Jiang Y. , Terry S. D. , Franz P. , Marrelli L. , Martin P.
Published: July 2002

Reduction of core-resonant m=1 magnetic fluctuations and improved confinement in the Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch have been routinely achieved through control of the surface poloidal electric field, but it is now known that the achieved confinement has been limited in part by edge-resonant m=0 magnetic fluctuations. Now, through refined poloidal electric field control, plus control of the toroidal electric field, it is possible to reduce simultaneously the m=0 and m=1 fluctuations. This has allowed confinement of high-energy runaway electrons, possibly indicative of flux-surface restoration in the usually stochastic plasma core. The electron temperature profile steepens in the outer region of the plasma, and the central electron temperature increases substantially, reaching nearly 1.3 keV at high toroidal plasma current (500 kA). At low current (200 kA), the total beta reaches 15% with an estimated energy confinement time of 10 ms, a tenfold increase over the standard value which for the first time substantially exceeds the constant-beta confinement scaling that has characterized most reversed-field-pinch plasmas.
Cluster demagnetizing effects in particulate dispersions
Authors: Blackwell J. J. , O’Grady K. , Verdes C. , Chantrell R. W.
Published: July 2002

We describe a study of a dispersion of metal pigments. We have used a high field pulsed magnetometer to examine the acquisition of remanence and have observed anomalous behavior as a function of both the magnitude of the pulsed field and its duration. The remanence increases monotonically until a critical field or a critical time is reached and then suddenly decreases at a highly reproducible value of field, and then subsequently increases when a larger pulsed field is applied. At very high fields a second drop in remanence is observed. By comparison with a computer simulation we infer that this behavior may arise due to strong demagnetising dipolar interactions in aggregates of a certain critical size.
Depth profiling of silicon–hydrogen bonding modes in amorphous and microcrystalline Si:H thin films by real-time infrared spectroscopy and spectroscopic ellipsometry
Authors: Fujiwara Hiroyuki , Kondo Michio , Matsuda Akihisa
Published: July 2002

Real-time infrared attenuated total reflection (ATR) spectroscopy and spectroscopic ellipsometry (SE) have been applied to assess depth profiles of Si–Hn (n=1–2) bonding modes in Si:H thin films. We have established an ATR data analysis procedure and the depth profiles of the Si–Hn bonding modes were readily obtained from incorporation rates of Si–Hn bonds at each film thickness estimated by SE. In an attempt to investigate the nucleation reaction of microcrystalline silicon (μc-Si:H) from the hydrogenated amorphous silicon (a-Si:H) phase, a series of Si:H thin films were prepared by an rf plasma-enhanced chemical vapor deposition using various hydrogen dilution ratios R=[H2]/[SiH4]. Real-time ATR and SE revealed that hydrogen content in the a-Si:H layers is uniform toward the growth direction. On the contrary, in the μc-Si:H film deposited at high R, a drastic reduction in the SiH hydrogen content is observed after the μc-Si:H nucleation, whereas the SiH2 hydrogen content is not sensitive to the phase transition. We find a weak variation in the hydrogen content in the a-Si:H layers deposited at various R, and no significant changes are observed in the a-Si:H bulk layers formed at onsets of μc-Si:H nucleation. The above results suggest that the μc-Si:H nucleation reaction occurs near the a-Si:H surface, but not in the a-Si:H bulk layer.
Nonideal fishbone instability
Authors: Marchenko V. S. , Günter S. , Kolesnichenko Ya. I.
Published: July 2002

It is shown that there exists a nonideal collisionless fishbone mode in toroidal plasmas, which may lead to the formation of the magnetic islands and thermal crashes during fishbone activity. The mode arises because of the resonance interaction between the energetic trapped ions and the collisionless m=1 nonideal kink mode. The theory predictions are found to be consistent with experimental observations of fishbone oscillations accompanied by thermal crashes on the Axisymmetric Divertor Experiment Upgrade tokamak [Günter et al., Nucl. Fusion 39, 1535 (1999)].
Microstructures of GaN islands on a stepped sapphire surface
Authors: Kim C. C. , Je J. H. , Ruterana P. , Degave F. , Nouet G. , Yi M. S. , Noh D. Y. , Hwu Y.
Published: July 2002

We investigated the structural evolution of GaN nucleation layers in the initial growth stages on commercial c-plane sapphires with atomic steps at the surface, using field-emission scanning electron microscopy, synchrotron x-ray scattering, and high-resolution electron microscopy. GaN nucleates into islands preferentially on the atomic steps. The initial small islands of 25 Å high have well-ordered cubic sequences and nearly coherent interfacial structures with a large compressive strain of ∼10%. As the islands grow to 50 Å high, the strain is drastically reduced, to less than 1%, by generating misfit dislocations at the interface and forming the six-to-seven matched interfacial structure. Interestingly, stacking faults are developed from the GaN/sapphire interface, which induces a cubic–hexagonal transformation. The changes in the stacking order during the initial growth are investigated quantitatively.
Probing surface properties with hyperthermal polyatomic ions: Scattering of C60+ from highly oriented pyrolitic graphite, Au (111), and n-alkylthiol self-assembled monolayers
Authors: Hillenkamp Matthias , Jester Stefan-Sven , Kappes Manfred M.
Published: July 2002

The influence of surface properties on the kinematics associated with hyperthermal grazing incidence scattering (Ecoll=200–1000 eV, θin=75°) of large polyatomic ions was studied using the model projectile C60+. Surfaces employed were highly oriented pyrolitic graphite (HOPG), thin gold films on mica and self-assembled monolayers (SAMs) of n-alkylthiols on gold (of various chain lengths). Scattering kinematics was probed by measuring product angular and kinetic energy distributions. The internal energy distribution of scattered C60+* was determined with the help of unimolecular rate theory fits of resulting fragment mass spectra. HOPG and gold targets show qualitatively comparable scattering behavior, i.e., both collision energy dependent subspecular scattering and partitioning of collision energy into possible degrees of freedom are similar. SAMs, comprised of molecules which may be deformed easily with respect to each other and to the surface, behave quite differently. Specular scattering as well as uptake of a significantly larger amount of the incident kinetic energy (Ecoll) into the surface is observed—independent of chain length or chemical composition. Coupling of Ecoll into scattered projectile vibrations is found to be chain length dependent, with two classes of behavior observed.
Alpha particle physics in a tokamak burning plasma experiment
Authors: Heidbrink W. W.
Published: July 2002

Much is known about the behavior of energetic ions in tokamak devices but much remains to be understood. Single-particle effects are well understood and provide a firm basis for extrapolation to a burning plasma. In contrast, collective effects involving fast ions are more poorly understood and extrapolations are unreliable. Collective modes of concern include toroidicity-induced and ellipticity-induced Alfvén eigenmodes, kinetic ballooning modes, and internal kink modes. In addition to these magnetohydrodynamic normal modes, there are also energetic particle modes characterized by strong dependence on the fast-ion distribution function. Although many issues are important areas of study in current experiments, five issues distinguish burning plasma experiments. First, the energetic alphas are not the dominant source of free energy for the instabilities unless the fusion power exceeds the heating power by a factor of 10. Second, the damping of the instabilities depends sensitively on mode coupling to other heavily-damped waves. The magnitude of this coupling is expected to depend on the normalized thermal gyroradius, which is much smaller in a reactor. Third, in a reactor, both the radial extent of the instabilities and the fast-ion orbit contract relative to current experiments, so the fast-ion transport will change. Fourth, when instability occurs, a larger number of modes are unstable, so the mechanism of nonlinear saturation could shift from fast-ion transport to mode coupling. Fifth, because of the extreme sensitivity of energetic particle modes to the distribution function, an isotropic alpha particle distribution function differs from anisotropic fast-ion populations.
Stochastic acceleration by intense laser fields
Authors: Nakamura Tatsufumi , Kato Susumu , Tamimoto Mitsumori , Kato Tomokazu
Published: July 2002

A stochastic acceleration mechanism which is a direct acceleration mechanism effective for the intense laser case is studied by the Fokker–Planck approach. The Fokker–Planck equation of the electron distribution function is derived from the equation of motion of electrons which interact with filamented laser fields. The Fokker–Planck equation contains nonlinear coefficients and gives an anisotropic distribution in momentum space. The strong directionality of the acceleration is explained. The accelerated electrons tend to be collimated towards the direction of the wave vector. The effective temperature scales as T∝tβ with β≃1.
Surface stress induced deflections of cantilever plates with applications to the atomic force microscope: V-shaped plates
Authors: Sader John Elie
Published: July 2002

Surface stress measurements using atomic force microscopy (AFM) require theoretical knowledge of the surface stress induced deformation of AFM cantilever plates. In a companion paper [J. E. Sader, J. Appl. Phys. 89, 2911 (2001)], a detailed theoretical study of the effects of homogeneous surface stress on rectangular AFM cantilever plates was presented. Since cantilevers of both rectangular and V-shaped geometries are used widely in practice, a corresponding theoretical study for V-shaped cantilevers is presented here. In line with the companion study, an assessment of Stoney’s equation is given, together with the presentation of greatly improved analytical formulas and rigorous finite element results.
The coupled modified Korteweg–de Vries equations: Similarity reduction, Lie–Bäcklund symmetries and integrability
Authors: Sahadevan R. , Kannagi N.
Published: July 2002

The Lie point symmetries of N-coupled modified Korteweg–de Vries equations (N-cmKdV) is derived and shown that the similarity reduction associated with symmetries passes the Painlevé property indicating its integrability. The nonclassical symmetry analysis of Bluman and Cole and the direct method of Clarkson and Kruskal to N-cmKdV equations show that there exists no new similarity reductions. A sequence of Lie–Bäcklund symmetries for N-cmKdV equations is derived explicitly, establishing its complete integrability. The question of constructing the recursion operator of N-cmKdV, a characteristic of integrable systems, is also discussed.
Magnetization reversal in antiferromagnetically coupled magnetic layers
Authors: Wang Y. J. , Wang J. P. , Wong Catherine Y. C.
Published: July 2002

On the basis of a micromagnetic model, magnetization reversal in antiferromagnetically coupled (AFC) binary and ternary layers has been represented by computer simulation. Optimization of media parameters of the AFC layers has been done, especially for the effect of parameters, such as anisotropy and coupling magnitude, on the reversal. The results specify that easy axis dispersion rather than random anisotropy and a reasonably small anisotropy magnitude of the recording layer are required for obtaining large coercivity, small saturation field, and thermal stability in the AFC layers. The difference between the AFC binary and ternary structures is discussed briefly.
Structure conserving parametrization of Feynman diagrams
Authors: Mendels E.
Published: July 2002

Using x-space parameters instead of Feynman parameters, dimensionally regularized Feynman diagrams are expanded with respect to the external momenta, the internal masses, the logarithms of these masses and the regularization parameter (ν-12n). A general formula for an arbitrary Feynman diagram is obtained for any dimension n. All ultraviolet divergences appear in a direct and transparent way as poles. Their residues in the limit (ν-12n)→0 are subseries that are recognized as subdiagrams. Relations between diagrams are transparent in all orders of perturbation theory.
Geometric quantization of mechanical systems with time-dependent parameters
Authors: Giachetta G. , Mangiarotti L. , Sardanashvily G.
Published: July 2002

Quantum systems with adiabatic classical parameters are widely studied, e.g., in the modern holonomic quantum computation. We here provide complete geometric quantization of a Hamiltonian system with time-dependent parameters, without the adiabatic assumption. A Hamiltonian of such a system is affine in the temporal derivative of parameter functions. This leads to the geometric Berry factor phenomena.
Super-Grassmannian and large N limit of quantum field theory with bosons and fermions
Authors: Konechny Anatoly , Turgut O. Teoman
Published: July 2002

We study a large Nc limit of a two-dimensional Yang–Mills theory coupled to bosons and fermions in the fundamental representation. Extending an approach due to Rajeev we show that the limiting theory can be described as a classical Hamiltonian system whose phase space is an infinite-dimensional super-Grassmannian. The linear approximation to the equations of motion and the constraint yields the ’t Hooft equations for the mesonic spectrum. Two other approximation schemes to the exact equations are discussed.
New Liouville integrable noncanonical Hamiltonian systems from the AKNS spectral problem
Authors: Błaszak Maciej , Ma Wen-Xiu
Published: July 2002

Liouville integrable noncanonical Hamiltonian systems with variable coefficient symplectic forms are generated from the AKNS 3×3 matrix Lax pairs by binary symmetry constraints of the AKNS hierarchy. These integrable systems provide new integrable factorization for every AKNS system in the hierarchy.
Enhanced ferroelectric properties of Pb(Zr0.53Ti0.47)O3 thin films on SrRuO3/Ru/SiO2/Si substrates
Authors: Wang Y. K. , Tseng T. Y. , Lin Pang
Published: July 2002

Highly (110)-oriented Pb(Zr0.53Ti0.47)O3 (PZT) films were deposited on SrRuO3 (SRO)/Ru/SiO2/Si substrates using a sol–gel method, in which SrRuO3 films were deposited at various substrate temperatures. The crystallinity of the PZT films was improved after the annealing process. The leakage current, dielectric constant, and polarization versus electric-field characteristics of PZT films were strongly dependent on the annealing and deposition temperatures of the SRO films. The 650 °C annealed PZT film grown on SRO has a leakage current of 9×10−7 A/cm2 at an applied field of 500 kV/cm, dielectric constant of 1306, remanent polarization (Pr) of 40.1 μC/cm2 and coercive field (Ec) of 78.5 kV/cm at an applied voltage of 5 V. The PZT films indicated fatigue-free characteristics up to ∼1.0×1012 switching cycles under 5 V bipolar pulse.
Effect of domain structure on thermal stability of nanoscale ferroelectric domains
Authors: Woo Jungwon , Hong Seungbum , Min Dong Ki , Shin Hyunjung , No Kwangsoo
Published: July 2002

We performed piezoelectric force microscopy (PFM) measurements on arrays of poled nanosize domains in 150 nm thick 〈111〉 preferentially oriented Pb(Zr0.4Ti0.6)O3 films grown by chemical solution deposition. Each domain of arrays was characterized by dot, egg, and ring structures depending on the pulse width ranging from 1 to 5 ms. The PFM measurements were followed by heat treatments at successively higher temperatures of 100 °C, 130 °C, and 160 °C for 30 min with rapid cooling down to room temperature before each measurement. The retention loss phenomena of the bits were characterized by measuring the number and dimension change of the remaining data bits. It was concluded that the ring-structured bit arrays, which represent fully penetrating domains through the film thickness, were the most stable form in terms of retention loss characteristics.
First-principles calculation of the piezoelectric tensor d⇊ of III–V nitrides
Authors: Bernardini Fabio , Fiorentini Vincenzo
Published: July 2002

We report direct first-principles density-functional calculations of the piezoelectric tensor d⇊ relating polarization to applied stress for the binary compounds AlN, GaN, and InN. The values of d⇊ are rather sensitive to the choice of the exchange-correlation functional, and results are presented for both the local-density and gradient approximations. A comparison with experiment and with values predicted indirectly from the elastic C⇊ and piezoconstant e⇊ tensors is also presented.
Cycle expansion for the Lyapunov exponent of a product of random matrices
Authors: Mainieri Ronnie
Published: July 2002

Using cycle expansion for the thermodynamic zeta function, a formula is derived for the Lyapunov exponent of a product of random hyperbolic matrices chosen from a discrete set. This allows for an accurate numerical solution of the Ising model in one dimension with quenched disorder. The formula is compared with weak disorder expansions and with the microcanonical approximation and shown to apply to matrices with degenerate eigenvalues.
Effects of an inorganic coating on the structure and magnetic properties of a 1 wt. % silicon steel
Authors: Rastogi Prabhat K. , Kim Yong-Wu
Published: July 2002

Various organic and inorganic coatings are used for electrical steels to improve their punchability and interlaminar resistance. However, very little information is available in the literature on the effect of these coatings on structure and magnetic properties of electrical steels. In this study, the influence of a commercial mono-aluminum-phosphate coating was investigated for a 1.0 wt. % silicon steel. It was noted that this type of coating is responsible for the following: (1) rate of decarburization is essentially unaffected, (2) the growth of subsurface oxide is substantially reduced, (3) 1.5 T permeability is improved via reduction in the subsurface oxide layer, and (4) core loss is decreased by reducing anomalous loss. A possible mechanism to suppress the subsurface oxidation by phosphorus segregation is suggested on the basis of results obtained by AES analysis.