bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonequilibrium translational-rotational effects in nucleation
Authors: Reguera D. , Rubı́ J. M.
Published: July 2002

The role that translational-rotational degrees of freedom play in nucleation theories is reconsidered by the introduction of a new formalism that properly accounts for the effects of motion of clusters in nucleation rate. The analysis of the nonequilibrium kinetics of the process, performed by retaining the dynamics of the clusters, enables one to clarify some of the paradoxical aspects that the inclusion of these degrees of freedom has presented.
Structural, magnetic, and transport properties in La0.7Ca0.3Mn1−xScxO3
Authors: Huang Yun-Hui , Yan Chun-Hua , Zhang Lei , Wang Zhe-Ming , Gao Song , Liao Chun-Sheng , Xu Guang-Xian
Published: July 2002

Structural, magnetic, and transport properties of polycrystalline La0.7Ca0.3Mn1−xScxO3 have been experimentally investigated. Single-phase samples are obtained in the range 0⩽x⩽0.25 by the soft chemical synthesis route. The substitution of Sc3+ at the Mn site results in reductions in the Curie temperature Tc and in the magnetic moment per Mn ion. The Tc transition temperature becomes broader with x, and the magnetic spontaneous state has changed into a cluster glass state for the sample of x=0.1. In addition, great enhancement in resistance and magnetoresistance effect is observed in the Sc-doped samples. In our case the notable doping effects can be mainly ascribed to the variation in the Mn3+–O2−–Mn4+ network and to the magnetic disorder caused by the presence of the large and nonmagnetic Sc3+ ions in the Mn sublattice.
Evidence for voltage drops at misaligned wafer-bonded interfaces of AlGaInP light-emitting diodes by electrostatic force microscopy
Authors: O’Shea James J. , Camras Michael D. , Wynne Dawnelle , Höfler Gloria E.
Published: July 2002

Electrostatic force microscopy (EFM) with phase detection has been applied to cleaved cross sections of wafer-bonded transparent substrate (TS) AlGaInP light-emitting diode (LED) structures. EFM was performed with the LED under active bias to image the voltage drops across the device layers. Measurements on a nonwafer-bonded, absorbing substrate (AS) AlGaInP LED wafer, showed a voltage drop only at the p–n junction. A TS wafer with high forward voltage (Vf ) showed a much larger voltage drop at the wafer-bonded interface, compared with a normal TS LED wafer. Secondary ion mass spectrometry profiles of these wafers revealed ∼1×1013 cm−2 of carbon at the bonded interface in the high Vf sample, compared to ∼3×1012 cm−2 in the normal wafer. The unwanted voltage drop at the bonded interface was likely caused by a combination of carbon acting as a p-type dopant and the presence of interface states due to a ∼3° in-plane rotational misalignment at wafer bonding.
Anomalous thermal conductivity enhancement in nanotube suspensions
Authors: Choi S. U. S. , Zhang Z. G. , Yu W. , Lockwood F. E. , Grulke E. A.
Published: July 2002

We have produced nanotube-in-oil suspensions and measured their effective thermal conductivity. The measured thermal conductivity is anomalously greater than theoretical predictions and is nonlinear with nanotube loadings. The anomalous phenomena show the fundamental limits of conventional heat conduction models for solid/liquid suspensions. We have suggested physical concepts for understanding the anomalous thermal behavior of nanotube suspensions. In comparison with other nanostructured materials dispersed in fluids, the nanotubes provide the highest thermal conductivity enhancement, opening the door to a wide range of nanotube applications.
The design and construction of a frequency response apparatus to investigate diffusion in zeolites
Authors: Turner Michael D. , Capron Laurent , Laurence Robert L. , Conner Wm. Curtis
Published: July 2002

A frequency response device has been designed and built based on previous efforts by Y. Yasuda, L. C. V. Rees, F. Meunier, and Ph. Grenier. This design uses the best attributes of the three previous devices and is improved in five specific ways. First, the device makes use of the fast response capacitance manometer. Second, the device uses a servomotor to push and pull a metal bellows pump, which drives the sinusoidal input function, removing the chance of “floating” the bellows. Third, both sinusoidal and step-change volume perturbations can be studied. Fourth, the overall cost of the device was kept below $30 000. Finally, the pressure transducer is mounted only 8 cm from the sample, instead of ∼30 cm, thus removing any averaging of the pressure signal at higher frequencies. The device is currently capable of measuring frequencies between 0.005 and 5 Hz, a range of three full orders of magnitude. The system can operate at temperatures between room temperature and 473 K and pressures between 0.3 and 300 Torr. Two systems were used to test the device, n-hexane/silicalite and methanol/silicalite. The model developed by Yasuda is used to analyze the frequency response of these systems. This model can describe a system as having more than one diffusivity and accounts for the presence of a surface resistance to diffusion. The results for n-hexane/silicalite and methanol/silicalite are compared to those of van den Begin and Nayak, respectively, and are in good agreement. Also, the diffusivities measured using this device are comparable to those estimated in 1997 by Rees using the frequency response method. In addition to measuring the diffusion rate processes, the kinetic parameters associated with a surface resistance to diffusion were also estimated.
Control of circular cylinder wakes using base mass transpiration
Authors: Delaunay Yannick , Kaiktsis Lambros
Published: July 2002

The effect of steady base suction and blowing on the stability and dynamics of the cylinder wake is investigated at low Reynolds numbers, using numerical simulation and stability analysis. Simulation results show that, in the supercritical Reynolds number regime (Re>47), slight blowing or high enough suction stabilizes the wake; in the subcritical regime, suction destabilizes the wake and results in vortex shedding, whereas blowing has no detectable effect on the flow stability. At supercritical Reynolds numbers, the transition from unsteady to steady flow at a critical suction flow rate is accompanied by simultaneous symmetry breaking, resulting in strongly asymmetric steady flow. For finite flow domain, the flow undergoes another transition from steady asymmetric to steady symmetric flow at even higher suction flow rates. The dynamics of vortex shedding in the controlled flow can be strongly modified, in comparison to the uncontrolled flow. Global stability analysis confirms the results of numerical simulation, and yields detailed information on the dynamics of linear global modes. Computational domain size can significantly affect the flow transitions, in particular, at high suction flow rates.
Defect-modified exchange bias
Authors: Kim Joo-Von , Stamps R. L.
Published: July 2002

The influence of local defects in exchange biased ferromagnetic/antiferromagnetic bilayers is studied using numerical simulations. The location of defects relative to the interface is found to be the main factor determining the magnitude of coercive and bias field modifications. One consequence is that some types of defects control stability of partial wall structures. This may explain coercivity enhancement as arising from irreversible pinning of partial domain walls in the antiferromagnet.
Simple method for measuring the azimuthal anchoring strength of nematic liquid crystals
Authors: Fonseca João G. , Galerne Yves
Published: July 2002

We present a simple method for measuring the azimuthal anchoring strength and the angular gliding of nematic liquid crystals at the surface of solid substrates. The method only needs symmetrical samples. It uses the averaging of the interference oscillations when using white light, which allows one to measure the azimuthal deviation angle at the surface δφ directly from the polarizers angles at the minimum of light. It is then easy to determine the azimuthal extrapolation length ξφ on varying the cell thickness, e.g., by means of wedge cells. As an example, we measure the azimuthal anchoring strength of 5CB onto self-assembled monolayers deposited onto polished indium–tin–oxide.
Anomalous diffusion and exit time distribution of particle tracers in plasma turbulence model
Authors: Carreras B. A. , Lynch V. E. , Zaslavsky G. M.
Published: July 2002

To explore the character of transport in a plasma turbulence model with avalanche transport, the motion of tracer particles has been followed. Both the time evolution of the moments of the distribution function of the tracer particle radial positions, 〈|r(t)−r(0)|n〉, and their finite scale Lyapunov number are used to determine the anomalous diffusion exponent, ν. The numerical results show that the transport mechanism is superdiffusive with an exponent ν close to 0.88±0.07. The distribution of the exit times of particles trapped into stochastic jets is also determined. These particles have the lowest separation rate at the low resonant surfaces.
Transverse dynamics of dispersive Alfvén waves. I. Direct numerical evidence of filamentation
Authors: Laveder D. , Passot T. , Sulem P. L.
Published: July 2002

The three-dimensional dynamics of a small-amplitude monochromatic Alfvén wave propagating along an ambient magnetic field is simulated by direct numerical integration of the Hall-magnetohydrodynamics equations. As predicted by the two-dimensional nonlinear Schrödinger equation or by more general amplitude equations retaining the coupling to low-frequency magnetosonic waves, the transverse instability of the pump leads to wave collapse and formation of intense magnetic filaments, in spite of the presence of competing, possibly linearly dominant, instabilities that in some instances distort the above structures. In computational boxes, including a large number of pump wavelengths, an early arrest of the collapse is possible under the effect of quasi-transverse instabilities that drive magnetosonic waves and also prescribe the directions of the filaments.
Orientation and velocity effects in the global positioning system
Authors: Krall Alfred H. , Bahder Thomas B.
Published: July 2002

We present a simple model of the signals transmitted by global positioning system (GPS) satellites and of the reception of these signals by a GPS receiver, neglecting atmospheric effects. The model idealizes the receiver and transmitter as electric dipole antennas. We derive the open-circuit voltage induced in the antenna of the receiver as a function of time, position, velocity, and orientation of the respective antenna. As an application of this model, we show that by using a single dipole antenna, a user of the GPS can compute his position, time, and two angles of attitude, by tracking the phases of the open-circuit antenna voltages for six GPS satellites.
Single-simulation determination of phase boundaries: A dynamic Clausius–Clapeyron integration method
Authors: de Koning Maurice , Antonelli Alex , Yip Sidney
Published: July 2002

We present a dynamic implementation of the Clausius–Clapeyron integration (CCI) method for mapping out phase-coexistence boundaries through a single atomistic simulation run. In contrast to previous implementations, where the reversible path of coexistence conditions is generated from a series of independent equilibrium simulations, dynamic Clausius–Clapeyron integration (d-CCI) explores an entire coexistence boundary in a single nonequilibrium simulation. The method gives accurately the melting curve for a system of particles interacting through the Lennard-Jones potential. Furthermore, we apply d-CCI to compute the melting curve of an ab initio pair potential for argon and verify earlier studies on the effects of many-body interactions and quantum effects in the melting of argon. The d-CCI method shows to be effective in both applications, giving converged coexistence curves spanning a wide range of thermodynamic states from relatively short nonequilibrium simulations.
Surface morphology evolution during the overgrowth of large InAs–GaAs quantum dots
Authors: Joyce P. B. , Krzyzewski T. J. , Bell G. R. , Jones T. S.
Published: July 2002

The effects of GaAs overgrowth on the structural properties of large low-growth-rate InAs quantum dots (LGR-QDs) grown on GaAs(001) are examined using in situ scanning tunneling microscopy. Strongly anisotropic surface diffusion produces a characteristic valley-ridge structure above the LGR-QDs and the surface is not planarized even after a cap thickness >400 Å. The evolution of surface morphology proceeds very differently to the case of smaller conventional growth rate QDs capped under the same conditions, due to the different initial strain states of the QDs.
Analysis of cancellation in two-dimensional magnetohydrodynamic turbulence
Authors: Sorriso-Valvo L. , Carbone V. , Noullez A. , Politano H. , Pouquet A. , Veltri P.
Published: July 2002

A signed measure analysis of two-dimensional intermittent magnetohydrodynamic turbulence is presented. This kind of analysis is performed to characterize the scaling behavior of the sign-oscillating flow structures, and their geometrical properties. In particular, it is observed that cancellations between positive and negative contributions of the field inside structures are inhibited for scales smaller than the Taylor microscale, and stop near the dissipative scale. Moreover, from a simple geometrical argument, the relationship between the cancellation exponent and the typical fractal dimension of the structures in the flow is obtained.
An ab initio study of water molecules in the bromide ion solvation shell
Authors: Raugei Simone , Klein Michael L.
Published: July 2002

The solvation shell structure and dynamics of Br− in an aqueous solution of HBr (1.6 M) was studied, under ambient conditions, by ab initio molecular dynamics using the Car–Parrinello approach. The present calculations indicate that Br− is asymmetrically solvated by water. The calculated residence time of water in the inner Br− anion solvation shell agrees well with the most recent experimental estimate. Analysis indicates that the large anion scarcely influences the molecular dipole moment of solvation shell molecules.
Growth and bonding structure of hard hydrogenated amorphous carbon thin films deposited from an electron cyclotron resonance plasma
Authors: Durand-Drouhin O. , Lejeune M. , Benlahsen M.
Published: July 2002

Analysis of hard hydrogenated amorphous carbon films (a-C:H) deposited from an electron cyclotron resonance radio frequency discharge of methane–argon (5%) mixture at low pressure is reported. The properties of films were determined in their as deposited state using elastic recoil detection analysis, infrared absorption, Raman spectroscopy, transmission spectroscopy, photothermal deflexion spectroscopy, and residual stress measurements. The microstructural changes (i.e., hydrogen content and C-sp3/C-sp2 ratio) have been explained qualitatively in terms of a balance between implantation and relaxation processes. A good correlation is observed between the variation of Raman features and the optical gap as a function of the self-bias substrate. The residual stress versus bias plot shows behavior similar to that already obtained for tetrahedral amorphous carbon films and the optimum energy, which corresponds to films of maximum C–C sp3, is similar to those obtained in the literature.
Effect of aluminum substitution in La3Ga5SiO14 crystals on their structure and piezoelectricity
Authors: Takeda Hiroaki , Kumatoriya Makoto , Shiosaki Tadashi
Published: July 2002

We report La3Ga5−xAlxSiO14 (0⩽x⩽1) crystals, with a Ca3Ga2Ge4O14-type structure, suitable for piezoelectric application. A single-crystal x-ray structure analysis reveals that Al atoms are distributed in all cation sites except for the decahedral one occupied by La, by rather favoring the smallest tetrahedral one. The piezoelectric modulus |d11| increased ∼1.3% and d14 decreased ∼7.7% with the increasing Al content, x, up to 1.0. We discuss the possible correlation between the atomic displacement and the observed compositional dependence of the piezoelectric modulus |d11| by referring to other isomorphs. The optimal solid-solution compositions and the use of Al substitution for obtaining the maximum electromechanical coupling factors (k12, k25, and k26) are advanced.
Organic field-effect transistors with polarizable gate insulators
Authors: Katz Howard E. , Hong X. Michael , Dodabalapur Ananth , Sarpeshkar Rahul
Published: July 2002

A quasi-stable threshold voltage (Vt) shift is imparted onto field-effect transistors (FETs) with organic semiconductors and polymer dielectrics. Adjustment of Vt from accumulation mode to zero or depletion mode is demonstrated for both p-channel and n-channel FETs, and is accomplished by applying a depletion voltage to the gate prior to device operation. Hydrophobic dielectrics and dopant-resistant semiconductors were advantageous. A pixel circuit that utilizes this nonvolatile memory element is proposed.
Advances in the development of the magnetoquenched superconducting valve: Integrated control lines and a Nb-based device
Authors: Clinton T. W. , Broussard P. R. , Johnson Mark
Published: July 2002

The magnetoquenched superconducting valve is a nonvolatile switch consisting of a ferromagnetic film that spans a superconducting strip. A device state is dictated by the magnetization orientation of the ferromagnetic element and the corresponding magnetic fringe fields at its edge which control the critical current in the underlying superconductor. A Pb-based device has been fabricated with a “write wire” inductively coupled to the ferromagnetic layer as an integrated control line. A current pulse through the control line produces a small local magnetic field that is larger than the coercivity Hc of the ferromagnet and which orients the magnetization, thus switching the device state. A comparison of device switching is made between use of a current pulse through the control line and use of an external magnetic field pulse. There is good agreement between the two methods, and device switching is demonstrated with fields as small as 20 Oe. In a further effort to develop a practical device, the quenchvalve effect is demonstrated in Nb, the superconducting digital electronics “industry-standard” material. A two-fold modulation in the critical current is observed at T=4.2 K, and the effect increases as the temperature approaches Tc=9 K. These results further demonstrate the promise of this device for application as a storage cell in a high-density superconducting random access memory.
Traps at the bonded interface in silicon-on-insulator structures
Authors: Antonova I. V. , Naumova O. V. , Nikolaev D. V. , Popov V. P. , Stano J. , Skuratov V. A.
Published: July 2002

In this study, we compared the trap density distributions, Dit, in the band gap of silicon at the Si/thermal SiO2 interface and at the bonded interface of the silicon–on–insulator structure, deduced from deep level transient spectroscopy measurements. The trap energies for the bonded Si/SiO2 interface are localized in the range from Ec–0.17 to Ec–0.37 eV. The lack of the transient SiOx layer at the bonded interface is suggested to lead to a relatively narrow interval of trap energies.
Stabilization of ion–ion instability with the aim of improvement transportation characteristics of a negative ion beam
Authors: Goretsky V. P. , Soloshenko I. A.
Published: July 2002

In the transportation of ion beams in a rarefied gas the plasma is formed due to the ionization of gas particles by the beam ions, and this plasma partially compensates the space charge of the ions (and thus improves the conditions of beam transportation). Moreover, in the case of a negative ion beam at a certain critical value of gas pressure, the electric field in the beam changes its sign so that at high pressure the beam is transported in the focusing field. The regime of high pressures is attractive also for the reason that ion–electron and ion–ion instabilities, which develop at that, are not harmful—they may cause just a little growth of the beam emittance. However, in this regime losses of the beam ions due to recharging are essential. As to pressures lower than the critical one, at which these losses are not so essential, the beam transportation is practically impossible due to large-scale ion instability which in certain time points results in almost complete decompensation of the beam space charge. In the present proceeding the way of stabilization of this instability is proposed, which gives the possibility of negative ion beam transportation at low pressures. Mechanism of the stabilization consists in diminishing the wavelength of unstable oscillations by means of the shielding action of the electrons introduced from the heated emitter and held in the beam by a periphery magnetic field. Experiments performed with the beam of hydrogen negative ions with 30 mA current and 10 keV energy have shown that introducing the electrons into the beam, which is transported at low pressure, really provides a essential reduction of ion–ion oscillations amplitude and, correspondingly, improves transportation characteristics of the beam.
Control of molecular handedness using pump-dump laser pulses
Authors: Hoki Kunihito , González Leticia , Fujimura Yuichi
Published: July 2002

A theoretical method for controlling handedness of preoriented enantiomers starting from an equal mixture of right(R)-handed and left(L)-handed molecules using linearly polarized femtosecond laser pulses is presented. The essence of the method lies in the fact that the molecular handedness of oriented enantiomers is reflected in the direction of the electronic transition moment vector. A pump-dump control scheme via an electronic excited state is considered for controlling molecular handedness in a femtosecond time scale. The direction of the polarization vector of the pump pulse and that of the dump pulse are determined in such a way that there is the largest interaction between the laser and the L-(R-) handed molecules, while the interaction with R-(L-) handed ones vanishes. In the case in which both the pump and dump pulses are independent of each other with no overlap between them, an analytical expression for the yield of molecular handedness is derived by solving the equation of motion of the density matrix. This expression shows that both the pump and dump lasers with π-pulse area produce the maximum transfer yield of molecular handedness. The effectiveness of the laser control method is demonstrated by numerical simulation of dynamic chirality of pre-oriented H2POSH in a racemic mixture via the first electronic singlet excited state.
Effects of absorption and desorption on the chemical stress field
Authors: Zhang Tong-Yi , Ko Sun-Chien , Lee Sanboh
Published: July 2002

The present work studies the effects of absorption and desorption on the chemical stress field in a membrane during permeation or both-side charging of a diffusing species. The permeation and both-side charging processes are analyzed with the consideration of the absorption and desorption processes, wherein the flux continuity boundary conditions are adopted. The results show that the chemical stresses are compressive near the surfaces and tensile in the center of the membrane. The maximum magnitude of stress occurs at the entry surface for the permeation process and at both surfaces for the both-side charging process. For the permeation process, the compressive stress at the entry surface increases with time to a maximum and then decreases with time gradually to zero. A similar phenomenon is found at both surfaces in the both-side charging process. In both processes, the stress magnitude depends upon the ratio of the drift velocity through surface to the drift velocity through bulk. When this ratio approaches infinity, the flux continuity boundary conditions reduce to the concentration boundary condition and the highest chemical stress field will be produced. Therefore, reducing the drift velocity through surface would be an efficient means to reduce the strength of the chemical stress. In general, designed surface alloying can introduce diffusing species traps at the surfaces of a membrane, which may efficiently decrease the drift velocity through surface.
Polarizability of molecular clusters as calculated by a dipole interaction model
Authors: Jensen Lasse , Åstrand Per-Olof , Osted Anders , Kongsted Jacob , Mikkelsen Kurt V.
Published: July 2002

We have developed and investigated a dipole interaction model for calculating the polarizability of molecular clusters. The model has been parametrized from the frequency-dependent molecular polarizability as obtained from quantum chemical calculations for a series of 184 aliphatic, aromatic, and heterocyclic compounds. A damping of the interatomic interaction at short distances is introduced in such a way as to retain a traceless interaction tensor and a good description of the damping over a wide range of interatomic distances. By adopting atomic polarizabilities in addition to atom-type parameters describing the damping and the frequency dependence, respectively, the model is found to reproduce the molecular frequency-dependent polarizability tensor calculated with ab initio methods. A study of the polarizability of four dimers has been carried out: the hydrogen fluoride, methane, benzene, and urea dimers. We find in general good agreement between the model and the quantum chemical results over a wide range of intermolecular distances. To demonstrate the power of the model, the polarizability has been calculated for a linear chain of urea molecules with up to 300 molecules and one- and two-dimensional clusters of C60 with up to 25 molecules. Substantial intermolecular contributions are found for the polarizability anisotropy, whereas the effects are small for the mean polarizability. For the mean polarizability of C60, we find good agreement between the model and experiments both in the case of an isolated molecule and in a comparison of a planar cluster of 25 C60 molecules with experimental results on thin films.
Direct-current glow discharges in atmospheric pressure air plasmas
Authors: Yu Lan , Laux Christophe O. , Packan Denis M. , Kruger Charles H.
Published: July 2002

Investigations have been conducted to experimentally validate the mechanisms of ionization in two-temperature atmospheric pressure air plasmas in which the electron temperature is elevated with respect to the gas temperature. To test a predicted S-shaped dependence of steady-state electron number density on the electron temperature and its macroscopic interpretation in terms of current density versus electric field, direct-current (dc) glow discharge experiments have been conducted in flowing low temperature, atmospheric pressure air plasmas. These experiments show that it is feasible to create stable diffuse glow discharges with electron number densities in excess of 1012 cm−3 in atmospheric pressure air plasmas. Electrical characteristics were measured and the thermodynamic parameters of the discharge were obtained by spectroscopic measurements. The measured gas temperature is not noticeably affected by whether or not the dc discharge is applied. The discharge area was determined from spatially resolved optical measurements of plasma emission during discharge excitation. The measured discharge characteristics are found to be in good agreement with the predicted electrical characteristics.
Simulations of binary hard-sphere crystal-melt interfaces: Interface between a one-component fcc crystal and a binary fluid mixture
Authors: Sibug-Aga Rachel , Laird Brian B.
Published: July 2002

The crystal-melt interfaces of a binary hard-sphere fluid mixture in coexistence with a single-component hard-sphere crystal is investigated using molecular-dynamics simulation. In the system under study, the fluid phase consists of a two-component mixture of hard spheres of differing size, with a size ratio α=0.414. At low pressures this fluid coexists with a pure fcc crystal of the larger particles in which the small particles are immiscible. For two interfacial orientations, [100] and [111], the structure and dynamics within the interfacial region is studied and compared with previous simulations on single component hard-sphere interfaces. Among a variety of novel properties, it is observed that as the interface is traversed from fluid to crystal the diffusion constant of the larger particle vanishes before that of the small particle, defining a region of the interface where the large particles are frozen in their crystal lattice, but the small particles exhibit significant mobility. This behavior was not seen in previous binary hard-sphere interface simulations with less asymmetric diameters.
Stationary tempering and the complex quadrature problem
Authors: Sabo Dubravko , Doll J. D. , Freeman David L.
Published: July 2002

In the present paper we describe a stochastic quadrature method that is designed for the evaluation of generalized, complex averages. Motivated by recent advances in sparse sampling techniques, this method is based on a combination of parallel tempering and stationary phase filtering methods. Numerical applications of the resulting “stationary tempering” approach are presented. We also examine inherent structure decomposition from a probabilistic clustering perspective.
Kinetics of phase transformations in a model with metastable fluid–fluid separation: A molecular dynamics study
Authors: Costa D. , Ballone P. , Caccamo C.
Published: July 2002

By molecular dynamics (MD) simulations we study the crystallization process in a model system whose particles interact by a spherical pair potential with a narrow and deep attractive well adjacent to a hard repulsive core. The phase diagram of the model displays a solid–fluid equilibrium, with a metastable fluid–fluid separation. Our computations are restricted to fairly small systems (from 2592 to 10368 particles) and cover long simulation times, with constant energy trajectories extending up to 76×106 MD steps. By progressively reducing the system temperature below the solid–fluid line, we first observe the metastable fluid–fluid separation, occurring readily and almost reversibly upon crossing the corresponding line in the phase diagram. The nucleation of the crystal phase takes place when the system is in the two-fluid metastable region. Analysis of the temperature dependence of the nucleation time allows us to estimate directly the nucleation free energy barrier. The results are compared with the predictions of classical nucleation theory. The critical nucleus is identified, and its structure is found to be predominantly fcc. Following nucleation, the solid phase grows steadily across the system, incorporating a large number of localized and extended defects. We discuss the relaxation processes taking place both during and after the crystallization stage. The relevance of our simulation for the kinetics of protein crystallization under normal experimental conditions is discussed.
Understanding the effect of the tape surface on the metal-particle tape medium noise
Authors: Roesler Alex , Zhu Jian-Gang
Published: July 2002

Spacing fluctuations resulting from the head–tape interface dominate the metal-particle tape medium noise. Utilizing narrow-gap record heads, the spatial correlation of the head-medium interface has been completely and accurately characterized using a magnetic recording measurement. The measurements show that the spacing fluctuations damp with shorter wavelength, or in other words, the tape head-medium noise possesses a long spatial correlation length. In addition, the results demonstrate that the roughness-induced spacing fluctuations originate from portions of the tape surface that contact the tape, i.e., the load-bearing surface. This understanding of the head-medium noise reveals that the majority of the tape medium noise occurs during the recording process, and not during signal playback. Spatial correlation measurements on low-density (5 kfci) recordings provide verification of these conclusions. The study offers a thorough understanding of the surface roughness role on producing the head-medium noise.
Room temperature magnetoresistance in La0.67Sr0.33Mn1−xCoxO3
Authors: Hu Jifan , Qin Hongwei , Chen Juan , Zheng R. K.
Published: July 2002

The substitution of Co for Mn in La0.67Sr0.33Mn1−xCoxO3 lowers the Curie temperature TC and the metal–insulator transition temperature TMI, accompanying the increase of the resistivity due to the weakening of the double-exchange interaction. The difference value between the TC and the TMI increases with Co substitution which may be attributed to local inhomogeneities in magnetic and electronic transport properties within the doped sample. There is an enhancement of the room temperature magnetoresistance ΔR/R0 induced by the substitution of Co for Mn in La0.67Sr0.33Mn1−xCoxO3, which may be connected with the shift of the Curie temperature TC and metal–insulator transition temperature TMI to the near room temperature through the substitution. Values of the room temperature magnetoresistance ΔR/R0 for La0.67Sr0.33Mn1−xCoxO3 depend not only on the TC but also on the TMI.
Electron imaging of dielectrics under simultaneous electron–ion irradiation
Authors: Toth M. , Phillips M. R. , Thiel B. L. , Donald A. M.
Published: July 2002

We demonstrate that if charging caused by electron irradiation of an insulator is controlled by a defocused flux of soft-landing positive ions, secondary electron (SE) images can contain contrast due to lateral variations in (i) changes in the SE yield caused by subsurface trapped charge and (ii) the SE-ion recombination rate. Both contrast mechanisms can provide information on microscopic variations in dielectric properties. We present a model of SE contrast formation that accounts for localized charging and the effects of gas ions on the SE emission process, emitted electrons above the sample surface, and subsurface trapped charge. The model explains the ion flux dependence of charge-induced SE contrast, an increase in the sensitivity to surface contrast observed in SE images of charged dielectrics, and yields procedures for identification of contrast produced by localized sample charging.
Gap state distribution in amorphous hydrogenated silicon carbide films deduced from photothermal deflection spectroscopy
Authors: Chew K. , Rusli  , Yoon S. F. , Ahn J. , Zhang Q. , Ligatchev V. , Teo E. J. , Osipowicz T. , Watt F.
Published: July 2002

The density of gap states distribution in silicon (Si) rich hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films with varying carbon (C) fraction (x) is investigated by the photothermal deflection spectroscopy (PDS). The films are grown using the Electron Cyclotron Resonance Chemical Vapor Deposition (ECR-CVD) technique. By using different methane-to-silane gas flow ratios, a-Si1−xCx:H with x ranging from 0 to 0.36 are obtained. A deconvolution procedure is performed based on a proposed DOS model for these Si rich a-Si1−xCx:H. Good fits between the simulated and experimental spectra are achieved, thus rendering support to the model proposed. Deduction of the DOS enables us to obtain various parameters, including the optical gap and the valence band tail width. The fitted mobility gap Eg is found to be well correlated to the Tauc gap Etauc and E04 gap deduced from the optical absorption spectra. A correlation is also seen between the fitted valence band tail width Evu, the Urbach energy Eu and the defect density. All these parameters are seen to increase with C alloying. A shift in the defect energy level in the midgap with increasing C incorporation is observed, together with a broadening of the defect distribution and a stronger correlation between the defect bands, which can be accounted for in terms of the influence of C dangling bonds on the deep defect density distribution.
Designing a magnetoacoustic emission measurement configuration for measurement of creep damage in power plant boiler tubes
Authors: Augustyniak B. , Chmielewski M. , Piotrowski L. , Sablik M. J.
Published: July 2002

We discuss design features that are needed for magnetoacoustic emission (MAE) measurement of creep damage in the outer walls of boiler tubes. MAE is used because it decreases monotonically with increasing creep damage. Features of magnet design for boiler tube inspection are presented. Relationship of total MAE to Barkhausen noise path integrals is discussed. Also, dependence of MAE on frequency and tube wall thickness is delineated. Finally, measurements are discussed which show how azimuthal asymmetry in creep damage is echoed in azimuthal asymmetry in MAE. All tests were performed on 2Cr–1Mo Polish steel tube specimens.
Magnetic domain imaging using orthogonal fluxgate probes
Authors: Terashima Y. , Sasada I.
Published: July 2002

This article presents an application of a miniature orthogonal fluxgate magnetometer to magnetic domain imaging of coated grain-oriented silicon steels. The method is based on a common technique which maps stray magnetic fields emerging from the surface of the silicon steel. An orthogonal fluxgate made of a thin amorphous wire core is bent in a J-shape so that it may work as a gradiometer, in which the longer leg of the J-shaped wire core is made sensitive to stray magnetic fields whereas the shorter one is effectively located at a distance from them. Experiments were carried out with a thin Co-based amorphous wire used as a wire core and a highly grain oriented silicon steel as a sample on which the domain refinement by laser irradiation is made. Well-defined stripe domains with some spike domains, and grain boundaries were clearly imaged with this method without any assisting magnetic field to enhance stray magnetic fields.
Study of the effect of natural oxidation and thermal annealing on microstructures of AlOx in the magnetic tunnel junction by high-resolution transmission electron microscopy
Authors: Bae Jun Soo , Shin Kyung Ho , Lee Taek Dong , Lee Hyuck Mo
Published: July 2002

In a magnetic tunnel junction, the formation of an insulator is sensitive and critical to the stable performance and reproducibility of the junction. The oxidation path and the microstructural change with time of the insulator in natural oxidation have been studied by the high-resolution transmission electron microscopy. It has been observed that the oxidation path is primarily through the grain boundary at an early stage of oxidation and then through the grains at a later stage. The morphology of the oxide layer was rugged and modulated. There also occurred an isotropic volume expansion with increased oxidation. It was observed that the ferromagnetic Co layer below an insulator was partially oxidized because of the preferred grain boundary oxidation. When this multilayer was annealed, the locally oxidized Co layer was reduced and the metallic layer formed as a continuous film type, thereby improving the interface.
Electronic structure study of Nd0.5Ca0.5MnO3 with a charge-exchange-type antiferromagnetic ordering
Authors: Hu W. Y. , Zheng Q. Q. , Lin H. Q. , Lau W. M.
Published: July 2002

The electronic structures and magnetic properties of manganites Nd0.5Ca0.5MnO3 with a charge-exchange-type antiferromagnetic ordering are studied by a modified linear combination of the atomic orbital (LCAO) band method within and beyond the local spin density approximation (LSDA and LSDA+U). The known crystal structure with Jahn–Teller distortion is modeled by a large supercell consisting of 80 atoms. Orbital-ordered states like those observed experimentally are obtained in both the LSDA and LSDA+U approaches. However, the LSDA fails in getting the experimentally confirmed charge-ordered insulating ground state. Although the inclusion of the Coulomb interaction, U, in the LSDA+U approach does reduce significantly the density of states of Mn ions around the Fermi level, it does not lead to the formation of an insulating gap. The LSDA+U calculation also shows that with the inclusion of U, the 4f of Nd ions become the majority component of the residual density of states near the Fermi level. In addition, the calculation shows that both the electronic states and magnetic moments of Mn ions are sensitive to many body effects.
Head-to-disk spacing control in contact recording studies
Authors: You Dan , Zheng Yuankai , Qiu Jinjun , Guo Zaibing , Wu Yihong
Published: July 2002

Recording studies with variable head-to-disk spacings have been carried out on a static test stand with a nanometer scale position accuracy. The head-to-disk spacing control was achieved through selective deposition of either supporting pads on the air bearing surface of commercial sliders or additional nonmagnetic layers (in this study, Cr) on commercial post-sputtering disks or the combination of these two. The additional supporting pads not only helped us solve the stiction problem but also allowed us to study the effect of head-to-disk spacing on the readout performance in a contact fashion. A series of read/write experiments have been carried out to study how the half-amplitude pulse width (PW50) parameter is dependent on the head-to-disk spacing by varying the height of the supporting pads or the thickness of the additional nonmagnetic layers or both using a commercial giant magnetoresistance head. It was found that some of the theoretical equations are able to predict the dependence of PW50 on the head-to-disk spacing, but some of them do not. The results may be useful for performing scaling analysis studies of future high- density recording systems.
Magnetic field scaling of relaxation curves in small particle systems
Authors: Iglesias Òscar , Labarta Amı́lcar
Published: July 2002

We study the effects of the magnetic field on the relaxation of the magnetization of small monodomain noninteracting particles with random orientations and distribution of anisotropy constants. Starting from a master equation, we build up an expression for the time dependence of the magnetization which takes into account thermal activation only over barriers separating energy minima, which, in our model, can be computed exactly from analytical expressions. Numerical calculations of the relaxation curves for different distribution widths, and under different magnetic fields H and temperatures T, have been performed. We show how a T ln(t/τ0) scaling of the curves, at different T and for a given H, can be carried out after proper normalization of the data to the equilibrium magnetization. The resulting master curves are shown to be closely related to what we call effective energy barrier distributions, which, in our model, can be computed exactly from analytical expressions. The concept of effective distribution serves us as a basis for finding a scaling variable to scale relaxation curves at different H and a given T, thus showing that the field dependence of energy barriers can be also extracted from relaxation measurements.
Effect of Cu doping on the magnetoresistive behavior of double perovskite Sr2FeMoO6 polycrystals
Authors: Yuan C. L. , Zhu Y. , Ong P. P.
Published: July 2002

The electrical, magnetic, and transport properties of Cu-doped polycrystalline samples Sr2Fe1−xCuxMoO6 with ordered double perovskite structure have been investigated systematically. Analysis of the x-ray powder diffraction pattern based on the Rietveld analysis indicates that the substitution of Fe3+ ions by Cu2+ ions enhances the site location order of Fe, Cu, and Mo on the B site for the high-doping-level samples (x=0.20, 0.25, 0.30). With increasing doping level, a transition from semiconductor to metal behavior was also found to occur. Furthermore, the transition temperature was found to decrease either by the application of a magnetic field or by increasing the doping level. It can be concluded that the existence of Cu2+ ions induces the occurrence of Fe3+δ ions and the double exchange interaction in Fe3+–O–Mo–O–Fe3+δ. The transport mechanism in these samples can be attributed to the competition between the metal phase and the semiconductor phase arising from the doping of Cu2+ ions. Both the semiconductor-to-metal transition and the magnetoresistive behavior can be explained by the percolation threshold model.
Stress-based prediction of dislocation generation in GaN during lateral epitaxial overgrowth
Authors: Feng Z. , Lovell E. , Engelstad R. , Kuech T. , Babcock S.
Published: July 2002

Finite element models were developed to simulate the stress history during an entire gallium nitride lateral epitaxial overgrowth process, from seed layer deposition on a substrate to GaN overgrowth above an SiO2 mask at 1100 °C and then cooldown to 20 °C. Lattice and thermal expansion mismatches between GaN and sapphire were considered simultaneously. Shearing stresses on a series of crystal planes were transiently analyzed. Based on the computational results, it is possible to predict the locations of high-density dislocations and when they develop, as well as Burgers vectors, dislocation types and their directions.
Minor hysteresis loop phenomena in particulate recording media
Authors: El-Hilo M. M. , Al-Momnee A. L. , Bakar S. A.
Published: July 2002

In this article measurements of minor hysteresis loops in substituted BaFe powder systems with different particle concentrations have been made. The area of minor loop is observed to decrease with decreasing particle concentration. In addition, the variation of magnetization with time along the upper and lower sections of the minor loop have also been measured and show an inflection point at particular field points. The field at which the rate of magnetization changes sign is believed to be equal to a positive (magnetizing) local interaction field, i.e., the negative applied field is balanced by a local positive field. These results suggest that minor hysteresis loops originate from interactions driving irreversible changes of magnetization during the cycling process of the magnetic field. Hence the opening of minor loops in these systems is governed by the ratio of interaction to anisotropy fields.
Electron transport in Ge nanocrystalline films deposited using the cluster beam evaporation technique
Authors: Banerjee Souri , Nozaki Shinji , Morisaki Hiroshi
Published: July 2002

The carrier transport properties across Ge nanocrystalline films (hereafter referred to as nanofilms) deposited by the cluster beam evaporation technique have been thoroughly studied. A thin nanofilm deposited at liquid nitrogen substrate temperature (Ge-LNT) exhibits the Coulomb blockade characteristic at low temperatures, while that deposited at room temperature (Ge-RT) does not show any evidence of single electron tunneling. This is explained by the difference in the nature of the overall electron transport due to a structural difference between the two types of nanofilms. The electron transport between two adjacent nanocrystals in both types of nanofilms was also studied in detail. It was found that, under relatively low electric fields, a Ge-LNT nanofilm exhibited T−1/2 conductivity dependence, which could be explained by the variable range hopping model of Efros and Shklovskii [A. L. Efros and B. I. Shklovskii, J. Phys. C 8, L49 (1975)], while in the Ge-RT nanofilms, the conductivity-temperature dependence was explained by the variable range hopping mechanism as proposed by Mott and Greaves [N. F. Mott and E. A. Davis, Electronic Process in Non-Crystalline Materials (Clarendon, Oxford, 1979); G. N. Greaves, J. Non-Crystalline Solids 11, 427 (1973)] in the given temperature range.
Controlling pulse propagation speed in CH3F far-infrared amplifier
Authors: Lee Kijoon , Lawandy Nabil M.
Published: July 2002

We calculated the group velocity of a probe pulse located at the gain center of five-level CH3F far-infrared molecular gas amplifier as a function of pump beam intensity and pressure. The results indicate this large class of optically pumped molecular system can exhibit significant group velocity reduction over a broad range of the electromagnetic spectrum. The group velocity can be further controlled by using the rotational sublevel splitting in a Stark static electric field in conjunction with optical pumpings.