bluuebunny/crossref_metadata_2025 huggingface dataset:
Hot wire chemical vapor deposition of isolated carbon single-walled nanotubes
Authors: Mahan A. H. , Alleman J. L. , Heben M. J. , Parilla P. A. , Jones K. M. , Dillon A. C.
Published: November 2002

Hot wire chemical vapor deposition (HWCVD) has been employed for the continuous generation of carbon single-walled nanotube (SWNT) materials. Interestingly, transmission electron microscopy analyses revealed only the presence of isolated SWNTs, rather than nanotubes existing in bundles. An analysis of the growth mechanism explaining the production of isolated SWNTs is provided. Also, the Raman radial breathing modes (RBMs) of the isolated HWCVD-generated nanotubes are compared to the RBMs of small bundles of nanotubes deposited by a conventional CVD technique having a similar diameter distribution.
Reduction of shock-wave data with mean-field potential approach
Authors: Wang Yi , Ahuja Rajeev , Johansson Börje
Published: November 2002

Based on ab initio calculated 0 K results for the assumed face- or body-centered-cubic ground state, the thermal volume expansion of the principal Hugoniot relative to the 300 K isotherm is calculated using the recently developed mean-field potential approach for Cu, Ta, Mo, Pt, and Au. Neglecting shock melting and phase dependence of the high-temperature equation of state, shock-reduced 300 K isotherms at pressures up to 1 TPa (10 Mbar) are derived for these metals by substracting the calculated thermal volume expansion from the experimental shock-wave data. This approach does not invoke any empirical assumptions regarding the Grüneisen parameters or heat capacities. The excellent agreement between such shock-reduced data and the normal standards or empirical reductions by other authors shows that the reduced results can be used as pressure standards for the widely employed static diamond-anvil-cell experiments.
Studies of the plume accompanying pulsed ultraviolet laser ablation of zinc oxide
Authors: Claeyssens Frederik , Cheesman Andrew , Henley Simon J. , Ashfold Michael N. R.
Published: November 2002

The plume of ejected material accompanying pulsed laser ablation of a ZnO target at 193 nm in vacuum has been investigated using wavelength and spatially resolved optical emission spectroscopy and Langmuir probes. All lines in the observed optical emission spectra are assignable to electronically excited Zn+* cations, and Zn* and O* neutrals, all of which emitting species we attribute to the result of electron–ion recombination processes in the gas phase following material ejection, laser–plume interactions, ionization, and thus, plasma formation. Various contributory components can be identified within the plume. Included among these are: a fast distribution of Zn2+ ions (observed via emission from highly excited states of Zn+*) together with an accompanying subset of fast electrons—the relative importance of which increases with increasing incident fluence on the target; a more abundant slower component involving both Zn+ and O+ ions, which expand in association with the main body of the electron distribution; and a slow moving component of Zn* emitters, which we suggest should be associated with material that has been backscattered from the expanding plasma ball towards the target surface and then rebounded or desorbed into the gas phase. The observation that the postablated target surface is substantially enriched in Zn provides additional support for the importance of material backscattering from within the dense plasma ball, accommodation, and in this case, recondensation on the target. The deduction that the target surface in the vicinity of the irradiated area is Zn rich after just a few laser shots provides an explanation for the oft-reported observation that ZnO films deposited by pulsed laser ablation of ZnO in vacuum are nonstoichiometric, with a Zn:O ratio greater than unity. Such backscattering from the plasma volume and selective recondensation of the less volatile component or components within the plume prior to the next ablation pulse being incident on the target surface appear to account for virtually all reported instances of nonstoichiometric film growth by pulsed laser deposition (PLD) in vacuum. Indeed, given the deduced area of the target surface affected by such redeposition and the target translation speeds typically employed in PLD studies, it would appear that nonstoichiometric film growth is likely to be the norm whenever PLD is carried out in vacuum and at wavelengths and fluences that lead to formation of a sufficiently dense plasma to cause material redeposition on the target.
High-temperature tensile tests and activation volume measurement of free-standing submicron Al films
Authors: Kalkman A. J. , Verbruggen A. H. , Radelaar S.
Published: November 2002

Tensile tests on free-standing, 200-nm-thick Al films were conducted between room temperature and 200 °C. Applied strain rates were in the range 1×10−8–2×10−6/s. At a temperature of 200 °C a saturation of the flow stress was observed. The strain required to achieve steady-state saturation is of the order of 2×10−3, which is much smaller than for bulk Al. From the observed strain rate sensitivity of the saturation stress we calculate an activation volume Ωact=60b3, which is small compared to the value of 700b3 observed in bulk Al (b is the magnitude of Burgers vector).
Raman and cathodoluminescence study of dislocations in GaN
Authors: Lei H. , Leipner H. S. , Schreiber J. , Weyher J. L. , Wosiński T. , Grzegory I.
Published: November 2002

Structural and optical properties of freshly created and in-grown dislocations in GaN single crystal are investigated by Raman and cathodoluminescence (CL) microscopy. The introduction of a high density of dislocations by micro-indentation is accompanied by the generation of intrinsic point defects. A high amount of VGa–impurity complexes is responsible for the decrease in the free electron concentration and the enhanced yellow luminescence around the indentation. A compressive stress induced by deformation is revealed by Raman scattering and CL. In-grown dislocations are decorated with a point defect atmosphere, leading to a reduction in the free carrier concentration around the dislocation.
Structure of amorphous hydrogenated carbon films prepared by radio frequency plasma enhanced chemical vapor deposition. An analogy with the structure zone model developed for metals
Authors: Dumay B. , Finot E. , Theobald M. , Legaie O. , Durand J. , Baclet P. , Goudonnet J. P.
Published: November 2002

In a structure zone model (SZM), the temperature and the sputtering gas pressure have been proposed as major factors explaining the structural properties of both evaporated and sputtered metals. Nevertheless, there is no evidence that such models can be applied to amorphous hydrogenated carbon films synthesized by radio frequency plasma enhanced chemical vapor deposition (rf PECVD). This paper reports on an original experiment designed to test by atomic force microscopy the hypothesis of an analogy between the rf PECVD amorphous materials and the existing SZM model for metals (amorphous and crystalline). Our results show that four zone morphologies called 1, M, T, and 2 can be obtained for amorphous materials according to the adatom mobility conditions. The parameters that control these morphologies are the plasma pressure, the surface temperature, and the plasma composition. The SZM model enables control of the a-C:H and a-C:D growth process. Zone T was found particularly interesting for the preparation of an ultra-smooth surface regardless of the film thickness. This result is of fundamental importance for the next “megajoule” laser facility, in which a-C:H or a-C:D capsules of 175 μm in thickness and 20 nm in surface roughness are expected.
Protein–trehalose–water structures in trehalose coated carboxy-myoglobin
Authors: Cottone Grazia , Ciccotti Giovanni , Cordone Lorenzo
Published: November 2002

Some organisms can survive complete dehydration and/or high temperature in a state of suspended animation called anydrobiosis, in which all metabolic processes are “switched off” however, upon rehydration, their normal life cycle is restored, without formation of irreversible damages. A common feature of these organisms, when in anhydrobiosis, is the presence of large amounts of sugar, particularly trehalose, which has been found to protect most effectively biomaterials. Several studies have attempted to understand how trehalose interacts with biomolecules. To address this problem, we performed molecular dynamics simulations of carboxy-myoglobin embedded in a trehalose aqueous solution and in a trehalose–water plasticized amorphous matrix. The results show that, in an aqueous solution, trehalose is excluded from the protein domain. This behavior extends also to the trehalose–water plasticized amorphous matrix, where we find sugar–water–protein structures with more water molecules that those derived from system concentration, and only few trehalose molecules bound to the protein, mainly through single hydrogen bonds.
Stability analysis of thermosolutal convection in a vertical packed porous enclosure
Authors: Mamou Mahmoud
Published: November 2002

A linear stability theory is performed to investigate the stability of the quiescent state and fully developed thermosolutal convection within a vertical porous enclosure subject to horizontal opposing gradients of temperature and solute. The fluid motion is modeled using the unsteady form of Darcy’s law coupled with energy and species conservation equations. The effect of different thermal and solutal boundary conditions is considered. The linearized governing equations are solved numerically using a finite element method. The thresholds for oscillatory and stationary convection are determined as functions of the governing parameters. It is concluded that the porosity and the acceleration parameter of the porous medium have a strong effect on the onset of overstability for a confined enclosure and on the wave number for an infinite enclosure. The stability analysis of fully developed flows within a slender enclosure reveals that an increase in the porosity and the acceleration parameter of the porous media delays the appearance of oscillatory finite amplitude flows. A nonlinear numerical solution is also computed by solving the full governing equations using a finite element method. Within the overstable regime, nonlinear traveling waves exist within slender enclosures, subject to Dirichlet thermal and solutal boundary conditions.
Rotational spectrum of the weakly bonded C6H6–H2S dimer and comparisons to C6H6–H2O dimer
Authors: Arunan E. , Emilsson T. , Gutowsky H. S. , Fraser Gerald T. , de Oliveira G. , Dykstra C. E.
Published: November 2002

Two symmetric-top, ΔJ=1 progressions were observed for the C6H6–H2S dimer using a pulsed nozzle Fourier transform microwave spectrometer. The ground-state rotational constants for C6H6–H2S are B=1168.53759(5) MHz, DJ=1.4424(7) kHz and DJK=13.634(2) kHz. The other state observed has a smaller B of 1140.580(1) MHz but requires a negative DJ=−13.80(5) kHz and higher order (H) terms to fit the data. Rotational spectra for the isotopomers C6H6–H234S, C6H6–H233S, C6H6–HDS, C6H6–D2S and CC513H6–H2S were also obtained. Except for the dimer with HDS, all other isotopomers gave two progressions like the most abundant isotopomer. Analysis of the ground-state data indicates that H2S is located on the C6 axis of the C6H6 with a c.m. (C6H6)–S distance of 3.818 Å. The angle between the a axis of the dimer and the C2v axis of the H2S is determined to be 28.5°. The C6 axis of C6H6 is nearly coincident with a axis of the dimer. Stark measurements of the two states led to dipole moments of 1.14(2) D for the ground state and 0.96(6) D for the other state. A third progression was observed for C6H6–H2S which appear to have K≠0 lines split by several MHz, suggesting a nonzero projection of the internal rotation angular momentum of H2S on the dimer a axis. The observation of three different states suggests that the H2S is rotating in a nearly spherical potential leading to three internal rotor states, two of which have Mj=0 and one having Mj=±1,Mj being the projection of internal rotational angular momentum on to the a axis of the dimer. The nuclear quadrupole hyperfine constant of the S33 nucleus in the dimer is determined for the two symmetric-top progressions and they are −17.11 MHz for the ground state and −8.45 MHz for the other state, consistent with the assignment to two different internal-rotor states. The O17 quadrupole coupling constant for the two states of C6H6–H2O were measured for comparison and it turned out to be nearly the same in the ground and excited internal rotor state, −1.89 and −1.99 MHz, respectively. The rotational spectrum of the C6H6–H2S complex is very different from that of the C6H6–H2O complex. Model potential calculations predict small barriers of 227, 121, and 356 cm−1 for rotation about a, b and c axes of H2S, respectively, giving quantitative support for the experimental conclusion that H2S is effectively freely rotating in a nearly spherical potential. For the C6H6–H2O complex, the corresponding barriers are 365, 298 and 590 cm−1.
Adaptive quantum control of DCM fluorescence in the liquid phase
Authors: Lee Sung-Hae , Jung Kyung-Hoon , Sung Jae Hee , Hong Kyung-Han , Nam Chang Hee
Published: November 2002

The emission efficiency, (dye fluorescence)/(excitation intensity), by multi-photon excitation of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethyl-amino) styryl]-4H-pyran in methanol solution was maximized using an evolutionary algorithm. Without any a priori knowledge on the molecular system, the optimization goal was automatically achieved by improving spectral phases through feedback of experimental outputs. The spectral structure of the optimized pulse was found to have a negative chirp. It is proposed that the efficiency change induced by the negative chirp is due to wavepacket localization on locally excited states.
Photoconductive Z-scan measurement of multiphoton absorption in GaN
Authors: Pačebutas V. , Krotkus A. , Suski T. , Perlin P. , Leszczynski M.
Published: November 2002

A study of photoconductivity excited by picosecond laser pulses in bulk GaN samples is presented. Photoconductivity traces measured by moving the sample across the laser beam waist (photoconductive Z scan) were used for investigating two- and three-photon absorption processes. The value of the three-photon absorption coefficient β3=0.012 cm3/GW2 was estimated from a comparison of the traces measured by exciting the sample with the first and second harmonics of Nd:glass laser. Nonlinear photoconductivity was also used for the determination of the laser pulse duration by the autocorrelation.
Wave-sustained discharges in helium–argon gas mixtures
Authors: Berndt J. , Makasheva K. , Schlüter H. , Shivarova A.
Published: November 2002

A numerical fluid-plasma model of diffusion-controlled surface-wave-sustained discharges in helium–argon gas mixtures is presented. Owing to a detailed description of the ion dynamics, with effective ion-mobility and ion ambipolar-diffusion coefficients introduced, variation of the gas components from 100% argon up to 100% helium is permitted. After dealing with the radial structure of the discharge, the model finally provides results for the interrelated axial variations of the discharge characteristics: wave number and wave space damping rate, power absorbed on average by an electron, electron density and electron temperature, as well as ion densities, which are additional characteristics compared to discharges in pure gases. Because of both applications and spectroscopy diagnostics, a 2.45 GHz discharge at a total gas pressure p=1 Torr is chosen for discussions. The influence of the ratio of atom concentration of the gases in the mixture on the discharge structure is shown. Although only slightly modifying the gradient of the axial profile of the electron density, the proportion of the atom concentrations has a strong impact on the electron temperature and the power absorbed on average by an electron, and thus, on the power necessary for the discharge maintenance.
Magnetization of ferromagnetic material surfaces by tribological process
Authors: Mishina H.
Published: November 2002

Experimental results showing that the surface of ferromagnetic materials is spontaneously magnetized by tribological actions such as friction and wear in the absence of external magnetic fields are presented. The magnetization mechanism of the frictional surface and wear particles is discussed based on the results of measurements of the magnetic flux density as well as observations using an atomic force microscope (AFM). In particular, the AFM observations performed on the frictional surface reveal that extremely fine particles form during the initial stage of wear processes, and the diameter of the debris is observed to be 16–25 nm. The particles are determined to be elemental debris, which is the most fundamental debris composing wear particles. Furthermore, the observed elemental debris formed during the initial stage of tribological actions is of the same order as the single magnetic domain particle that is given by the theoretical calculation and experimental results. The phenomenon of tribomagnetization, i.e., the magnetization of frictional surface by the tribological actions, is discussed in view of the relationship between the formation of single magnetic domain particles and the elemental debris of wear particles in tribological processes.
Micro-Raman investigation of thin lateral epitaxial overgrown GaN/sapphire(0001) films
Authors: Chaldyshev V. V. , Pollak Fred H. , Pophristic M. , Guo S. P. , Ferguson I.
Published: November 2002

Using micro-Raman spectroscopy we have investigated the n dopant and strain distribution in lateral epitaxial overgrowth technique GaN films grown by metalorganic chemical vapor deposition on the sapphire (0001) substrates with SiNx masks. The widths of the mask stripes were 2, 4, 8, or 16 μm, while the mask windows were always 4 μm wide. In the case of narrow stripes (2 and 4 μm), when the overgrowth wings were well coalesced, the films were found to be fairly uniform with a background n doping of (4±2)×1017 cm−3. The GaN wings in the samples with 8 and 12 μm stripes did not coalesce, leaving “V”-shaped and trapezoidal grooves, respectively. In the latter case, additional doping [n=(6.5±0.6)×1017 cm−3] of the wing area was revealed, which may be due to surface diffusion of Si atoms from the SiNx mask to the GaN growth front and their incorporation into the growing film.
Many optical absorption peaks observed in electron-irradiated n-type Si
Authors: Suezawa M. , Fukata N. , Mchedlidze T. , Kasuya A.
Published: November 2002

The properties of many optical absorption peaks in electron-irradiated n-type Si crystals were studied. Specimens were prepared from various Si crystals. After chemical etching, they were irradiated with 3 MeV electrons at room temperature (RT). Their optical absorption spectra were measured with a Fourier transform infrared spectrometer at temperatures in the range of 7 K to RT with a resolution of 0.25 cm−1. Many optical absorption peaks were observed in the wave number range between 850 and 1600 cm−1 only in n-type (phosphorus-doped) Si crystals. Hence, they are due to donors. Such peaks were very weak in Czochralski-grown Si (Cz Si) crystals. This suggests that these peaks are due to complexes of phosphorus and vacancies since most vacancies in Cz Si form pairs (A center) with oxygen because of the very high concentration of oxygen in those crystals. They disappeared as the result of annealing at above 175 °C. Temperature dependencies of intensities of various peaks were studied.
Spontaneous emission in highly excited semiconductors: Saturation of the radiative recombination rate
Authors: Bourdon G. , Robert I. , Sagnes I. , Abram I.
Published: November 2002

The spontaneous emission rate of semiconductors displays a saturation at high carrier injection densities due to the filling of the conduction and valence bands. We have carried out time-resolved experiments, whereby a semiconductor quantum well at room temperature is excited optically by a short laser pulse and the time decay of its luminescence is measured for different injected carrier densities. The luminescence decay rate, plotted as a function of injected carrier density, deviates strongly from the well-known quadratic recombination law, valid at low carrier densities, displaying a saturation. We have developed a simple analytic theory that accounts for this saturation and can describe adequately the recombination kinetics of highly excited semiconductor quantum wells.
Nonlinear electrokinetic ejection and entrainment due to polarization at nearly insulated wedges
Authors: Thamida Sunil Kumar , Chang Hsueh-Chia
Published: November 2002

We examine a singular electrokinetic flow around a corner or a wedge in micro-channels constructed from dielectric materials whose permittivity is small but finite compared to that of the electrolyte. When the wedge angle is less than 180°, the applied electric field, which is tangential far from the corner, develops a normal surface component that becomes singular at the corner. This normal field leakage causes opposite polarization at the two sides of the wedge and produces a converging singular tangential electrokinetic flow that ejects liquid from the tip. By expanding in cylindrical harmonics, we estimate this ejecting flow as a function of the permittivity ratio, applied electric field, angle of the wedge and the microscopic corner curvature that suppresses the singularity. The ejecting flow entrains tangential flow on the front side of the wedge and produces a vortex on the downstream side. This entrainment offers a long-range attractive hydrodynamic force that complements short-range electrostatic DLVO (Derjaguin–Landau–Verwey–Overbeek) and dielectrophoretic forces to enhance corner deposition and aggregation of colloids and proteins during electrophoresis/electro-osmosis.
Influence of initial conditions on transient two-dimensional thin-film flow
Authors: Khayat Roger E. , Kim Kyu-Tae
Published: November 2002

This study examines theoretically the influence of initial conditions on the development of early transients for pressure-driven planar flow of a thin film over a stationary substrate, emerging from a channel. The flow is governed by the thin-film equations of the boundary-layer type. The wave and flow structure are examined for various initial conditions of flow and film profile. It is found that, depending on the initial film profile and velocity distribution, the steady state may or may not be reached. Alternatively, the instability of the steady state is shown to be closely linked to the existence of a gradient catastrophe.
Focusing neutrons with tapered capillary optics
Authors: Mildner D. F. R. , Chen-Mayer H. H. , Gibson W. M.
Published: November 2002

The transmission characteristics of radiation through hollow tapered capillaries may be described by expressions derived for straight capillaries but with a reduced critical angle. The size of the focused beam for recent transmission measurements through a tapered polycapillary focusing optic performed at various different wavelengths gives a measure of the reduced critical angle. We derive the transmission characteristics of the focusing lens from the measured data and compare these with the analytic results. A similar comparison is made for a polychromatic incident beam.
Parametric analysis of micromachined reflex klystrons for operation at millimeter and submillimeter wavelengths
Authors: Garcı́a-Garcı́a J. , Martı́n F. , Miles R. E. , Steenson D. P. , Chamberlain J. M. , Fletcher J. R. , Thorpe J. R.
Published: November 2002

A tool for the simulation of high frequency reflex klystrons is presented. Based on the Monte Carlo technique, this simulation includes loss mechanisms (due to the opacity of the grids) and takes into account the main peculiarities expected for device operation at millimeter and submillimeter wave frequencies. The tool has been used to study the effects of device parameters which are critical to the optimization of output power in a prototype 100 GHz micromachined structure based on a field emission electron source, namely, the electric field in the drift region and the grid distance. The results also show that this device can be used for direct generation of useful amounts of power at these frequencies.
Energy transfer from organics to rare-earth complexes
Authors: Dı́az-Garcı́a Marı́a A. , De Ávila Susana Fernández , Kuzyk Mark G.
Published: November 2002

Energy transfer from several semiconducting organic molecules and polymers to rareearth complexes in the form of spin-cast films is reported. Energy transfer is observed in poly(N-vinylcarbazole) (PVK) films doped with various europium and samarium complexes. Polystyrene films containing the hole-transporting organic molecule 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and the electron-transporting molecule N,N′-bis(3-methylphenyl)-N,N-diphenylbenzidine also show energy transfer to the europium complexes, but not to the samarium ones. We show that energy transfer from PBD to the europium complexes is more efficient than from PVK. These films are semiconducting and easy to process, so they are candidates for monochromatic light-emitting diodes.
Monte-Carlo simulation of electron conductance and magnetoresistance in magnetic polaron systems
Authors: Liu J.-M. , Zhou X. H. , Chen X. Y. , Liu Z. G. , Du Y. W. , Ming N. B.
Published: November 2002

The electron conductance and magnetoresistive effect in phase-separation-induced single-carrier magnetic polaron systems is studied by Monte-Carlo simulation. The simulation reveals the co-occurrence of ferromagnetic transition and insulating metallic transition as a function of temperature. The resistivity above the Curie point can be explained by the small-polaron mechanism and variable-range-hopping picture, while the resistivity far below the Curie point shows the contribution of two-magnon scattering. The electron tunneling behavior is confirmed by the nonlinear I–V behaviors and the as-induced magnetoresistance is discussed.
Separation of photonic crystal waveguides modes using femtosecond time-of-flight
Authors: Netti M. C. , Finlayson C. E. , Baumberg J. J. , Charlton M. D. B. , Zoorob M. E. , Wilkinson J. S. , Parker G. J.
Published: November 2002

We demonstrate that ultrabroadband ultrashort-pulse white light supercontinua can be used to track the group velocity of photons in optical waveguides using a Kerr gate technique. Results on silicon nitride slab waveguides show both polarization birefringence and multimode dispersion, which vanish at critical wavelengths. When photonic crystals are embedded in the waveguides, the higher order modes are excited within the band-gap region, demonstrating the need to control their dispersion to make effective use of photonic crystal waveguide devices.
Dynamics of the tip–sample interaction in near-field scanning optical microscopy and the implications for shear force as an accurate distance measure
Authors: Jahncke C. L. , Huerth S. H. , Clark Beverly , Hallen H. D.
Published: November 2002

Near-field scanning optical microscopy uses shear-force feedback as the primary method to control the probe–sample distance. We describe the nonlinear interaction between the tip and sample with a simple truncated driven harmonic oscillator model. The model accurately describes the measured dynamics of this system. Insights are gained into the mechanism behind this interaction, and we give strong evidence that the probe taps on sample surface adlayers in normal operation, but will tap the underlying sample surface when the oscillation is nearly quenched.
Time dependence of laser-induced thermoelectric voltages in La1−xCaxMnO3 and YBa2Cu3O7−δ thin films
Authors: Zhang P. X. , Lee W. K. , Zhang G. Y.
Published: November 2002

The time dependence of laser-induced thermoelectric voltage (LITV) in La1−xCaxMnO3 (LCMO) or YBa2Cu3O7−δ (YBCO) thin films grown on a vicinal SrTiO3 substrate is derived. From the fitted curve of the measured time evolution of the LITV, several parameters of a LCMO thin film are deduced.
Dimensional crossover in cylindrical quantum-box structures formed by penetrable barriers
Authors: Souma Satofumi , Lee Seung Joo , Kim Nammee , Kang Tae Won
Published: November 2002

We study the density of states (DOS) in a cylindrical quantum box formed by a cylindrical penetrable barrier and a planer penetrable double barrier perpendicular to the cylinder. The control of the cylindrical barrier thickness or height is found to cause the dimensional transitions from a three-dimensional (3D) DOS to a quasi-one-dimensional (Q1D) DOS and from a quasi-two-dimensional (Q2D) DOS to a quasi-zero-dimensional (Q0D) DOS, if the strength of the planer double barrier is zero and infinity, respectively. In between the 3D and the Q1D (Q2D and Q0D) limiting regimes, we found the presence of a Q2D (Q1D) like crossover regime.
Modeling the role of oxygen vacancy on ferroelectric properties in thin films
Authors: Lo Veng Cheong
Published: November 2002

The presence of oxygen vacancies is considered to be the cause of various phenomena in ferroelectric thin films. In this work, the role of oxygen vacancies is theoretically modeled. Various properties are numerically simulated using the two-dimensional Ising model. In the presence of an oxygen vacancy in a perovskite cell, the octahedral cage formed by oxygen ions is distorted so that the potential energy profile for the displacement of the titanium ion becomes asymmetric. It requires additional energy to move from the lower minimum position to the higher one. Moreover, space charges are also developed by trapping charge carriers into these vacancies. The combination of the pinning effect induced by the distorted octahedral cage and the screening of the electric field in the presence of space charges results in phenomena such as fatigue and imprint.
A generalized soft-sphere model for Monte Carlo simulation
Authors: Fan Jing
Published: November 2002

A new collision model, called the generalized soft-sphere (GSS) model, is introduced. It has the same total cross section as the generalized hard-sphere model [Phys. Fluids A 5, 738 (1993)], whereas the deflection angle is calculated by the soft-sphere scattering model [Phys. Fluids A 3, 2459 (1991)]. In virtue of a two-term formula given to fit the numerical solutions of the collision integrals for the Lennard-Jones (6-12) potential and for the Stockmayer potential, the parameters involved in the GSS model are determined explicitly that may fully reproduce the transport coefficients under these potentials. Coefficients of viscosity, self-diffusion and diffusion for both polar and nonpolar molecules given by the GSS model and experiment are in excellent agreement over a wide range of temperature from low to high.
Transient growth of stationary disturbances in a flat plate boundary layer
Authors: White Edward B.
Published: November 2002

Theoretical and direct numerical simulation models of transient algebraic growth in boundary layers have advanced significantly without an adequate, parallel experimental effort. Experiments that feature disturbances excited by high levels of freestream turbulence or distributed surface roughness show behavior consistent with optimal-disturbance theories but cannot address the theories’ key predictions concerning the growth and decay of disturbances at specific spanwise wavenumbers. The present experiment seeks to provide such data for a flat plate boundary layer using a spanwise roughness array to excite controlled stationary disturbances. The results show that although general trends and qualitative behaviors are correctly captured by optimal-disturbance theories, significant quantitative differences exist between the theories’ predictions and the current experimental measurements. Discrepancies include the location of the wall-normal disturbance profiles’ maxima and the streamwise location of the maximum energy growth. While these discrepancies do not argue against the validity of transient-growth theory in general, they do indicate that correct modeling of receptivity to realistic disturbances is critical and that realistic stationary disturbances can exhibit strongly nonoptimal behavior.
Detection of thermal acoustic radiation from laser-heated deep tissue
Authors: Krotov Eugene V. , Zhadobov Maxim V. , Reyman Alexander M. , Volkov Grigory P. , Zharov Vladimir P.
Published: November 2002

A multichannel acoustic antenna with a linear piezoelectric transducer array was developed to detect thermal acoustic radiation from a deep-lying sample zone heated by laser. A cw Nd:YAG laser (wavelength, 1064 nm; power, 1 W) was used as the thermal source. An optical fiber was inserted into the sample (beef liver) to deliver laser radiation into the deep zone and to heat the area around the fiber tip through absorption. The experiments demonstrated the capability of a new device for real-time monitoring of internal temperature at depths up to 3.5 cm with a spatial resolution of about 5 mm and a sensitivity of about 0.5 K. It is shown that experimental data can be used to visualize heat diffusion effects in deep-lying zones and the spatial position of the fiber tip, and to quantitatively estimate thermal diffusivity and acoustic and optical absorption coefficients. In addition to its laser applications, the described technique could be a promising tool for monitoring temperature distribution at the deposition of an alternative external energy (microwave, focused ultrasound, etc.) in deep-lying tissues.
Static properties and nonlinear dynamics of domain walls with a vortexlike internal structure in magnetic films (Review)
Authors: Filippov B. N.
Published: November 2002

The causes of the formation of domain walls with a vortex structure, their energy, and their static properties are analyzed as functions of the film parameters. The influence of the surface anisotropy and external fields on these properties is elucidated. It is it is shown that that both one-and two-vortex symmetric and asymmetric walls can be stable. The experimental evidence for the existence of asymmetric walls is presented. A review is given of the dynamic properties of walls with a vortex structure. The fields at which catastrophic changes of their structure occur with changing field (the bifurcation fields) are indicated. The mechanisms of dynamic transformation of the internal structure of the walls, in particular, the mechanisms involving the creation and annihilation of vortexlike wall formations, are examined, and the tunneling of these formations through the center of the wall is discussed. The transformations of the wall due to the onset of subperiodic magnetization oscillations are described. The period of the dynamic transformations and the character of its singularity near a bifurcation point are investigated. The nonmonotonic dependence of the bifurcation field on the film thickness, saturation induction, and exchange field and also the dependence of the bifurcation field on the damping parameter and anisotropy field are investigated.
Low-dose radiation effects in thin films of high-temperature superconducting YBa2Cu3O7−x irradiated by 1-MeV electrons
Authors: Fedotov Yu. V. , Danilchenko B. A. , Rogutskiĭ I. S.
Published: November 2002

Radiation effects are investigated in thin epitaxial films of the high-Tc superconductor (HTSC) YBa2Cu3O7−x irradiated by low doses of 1-MeV electrons. The maximum radiation dose (4×1016 electrons/cm2) is chosen from the condition that the defects formed as a result of electron–nucleus collisions cause a negligible lowering of the critical temperature of YBa2Cu3O7−x. Under this condition the main source of radiation effects in HTSC films can be processes involving excitation of the electronic subsystem of YBa2Cu3O7−x. When YBa2Cu3O7−x films are irradiated by doses of (1–4)×1016 electrons/cm2 their critical temperature Tc is observed to increase (in contrast to published reports of a decrease in Tc at irradiation doses greater than 1018 electrons/cm2) and then, after the irradiation has stopped, to relax over time to its original value. These effects are similar to those observed in the photoexcitation of the electronic subsystem of YBa2Cu3O7−x (photostimulated superconductivity). A decrease of the critical current density in the irradiated YBa2Cu3O7−x films is also observed, which is due to radiation-stimulated changes of the transmissivity to supercurrent of the dislocation walls in low-angle interblock boundaries.
Magnetic phases in La0.66Ba0.34MnO3: effects of temperature and elastic strains
Authors: Beznosov A. B. , Eremenko V. V. , Fertman E. L. , Desnenko V. A. , Khalyavin D. D.
Published: November 2002

The static magnetization MN and dynamic magnetic susceptibility χac of polycrystalline manganite La0.66Ba0.34MnO3 are investigated in the temperature region 5 K<T<350 K. Measurements are made on a sample with a demagnetizing factor N=5.4 in static magnetic fields H in the range 0–120 Oe and on a toroidal sample with N=0 in fields with frequencies of 120 and 1000 Hz and amplitudes from 8×10−3 to 4.1 Oe. The effect of a uniaxial pressure P=0.1 kbar on the static susceptibility χN=MN/H is investigated in the region 5 K<T<270 K. It is found that in the magnetically ordered state (below 314 K) χac is independent of the field amplitude in the interval 1.5–4.1 Oe and independent of temperature between 250 and 302 K. At a temperature Tp=250 K a second-order phase transition (discontinuity of the derivative ∂χac/∂T) is observed, and at Ti=177.5 K—a first-order phase transition (discontinuity of χac(T)). The uniaxial pressure leads to an anomalous lowering of the susceptibility in the region 160–250 K with a sharp minimum at 200 K. The temperature and field dependence of the magnetic susceptibility outside the phase transition region are interpreted in terms of ideas about the formation of a helicoidal magnetic structure in the system in low fields at a temperature below 302 K.
Low-temperature inversion of the magnetoresistance in charge-ordered layered superstructures
Authors: Gorskyi P. V.
Published: November 2002

The nonoscillatory (with respect to magnetic field) part of the magnetoresistance of layered charge-ordered crystals is determined under the condition that the relaxation time of the charge carriers is inversely proportional to their density of states. It is demonstrated that inversion of the magnetoresistance can occur as a function of the carrier concentration, temperature, magnetic field, and value of the effective attractive interaction that leads to the charge ordering.
On the deviations from Matthiessen’s rule in quasi-one-dimensional conductors
Authors: Kopeliovich A. I. , Mamalui A. A. , Petrenko L. G. , Shelest T. N.
Published: November 2002

An explanation is proposed for the large deviation from Matthiessen’s rule observed experimentally in the quasi-one-dimensional metal NbSe3, the Fermi surface of which contains several sheets which are isolated from each other. The deviation from Matthiessen’s rule arises as a result of the nonadditive influence on the electron distribution function from processes of electron scattering on lattice defects (Se vacancies) and phonons. It is shown that the experimentally obtained temperature dependence of the electrical conductivity of NbSe3 can be described satisfactorily in the framework of a simplified model of a one-dimensional Fermi surface in the form two pairs of symmetric points in quasimomentum space. Large deviations from Matthiessen’s rule can be explained by the different character of the scattering on lattice defects and on phonons for electrons localized on different structural chains of the quasi-one-dimensional metal.
Microscopic nature of Pr3+ optical centers in Y2SiO5, Lu2SiO5, and Gd2SiO5 crystals
Authors: Malyukin Yu. V. , Zhmurin P. N. , Grinev B. V. , Seminozhenko V. P. , Znamenskiı̆ N. V. , Manykin É. A. , Petrenko E. A. , Yukina T. G.
Published: November 2002

The results of investigations of the energy spectrum, energy relaxation channels, and dynamics of electronic excitations of Pr3+ optical centers in Y2SiO5, Lu2SiO5, and Gd2SiO5 crystals are reviewed. It is shown that the crystal field of the ligands of nonequivalent cationic sites in these crystals preserves the quasisymmetry of distorted octahedra and tetrahedra; the Pr3+ activator ions are nonuniformly distributed over the nonequivalent cationic sites in Y2SiO5. For high concentrations of activator ions (>1at.%) in Y2SiO5 crystals, the luminescence is quenched as a result of the migration and trapping of the electronic excitations energy of optical centers by traps. For Y2SiO5: Pr3+ the low-temperature mechanism of dephasing of the optical transitions of Pr3+, which is not characteristic for crystals and is due to the interaction of impurity ions with two-level systems of a multiwall adiabatic potential, is analyzed.
Measurement of the densities of liquids and gases under pressure using magnetic levitation of a standard sample
Authors: Panfilov A. S. , Pushkar’ Yu. Ya.
Published: November 2002

A simple method is proposed for measuring the pressure dependence of the density of gases and liquids in a wide temperature range. The method is based on the principle of free suspension of a standard paramagnetic sample in a nonuniform magnetic field, when the magnetic force compensates the weight of the sample, taking account of the hydrostatic support of the medium. As an example of the application of the method, the density of helium gas up to pressure ∼2.5 kbar at temperatures 20.4 and 77.3 K is determined.
Hydrogen-related mobile charge in the phosphosilicate glass–SiO2–Si structure
Authors: Oborina Elena , Campbell Scott , Hoff Andrew M. , Gilbert Richard , Persson Eric
Published: November 2002

This article demonstrates that mobile charge, Qm, of a nonsodium nature may exist in the thin film stack structure of phosphosilicate glass–SiO2–Si. The behavior of the Qm in this structure has been investigated using noncontact corona charging of the dielectric surface and measurement of the surface potential with a contact potential difference, probe. Qm levels greater than 1012 cm−2 have been measured in as-deposited samples whereas high temperature anneal reduced this level by half. The nonsodium nature of Qm was confirmed by surface analysis measurement. A model based on hydrogen cycling between the two interfaces of a buried oxide layer is used to explain the observed results. The experimental data suggest that charge traps exist at each interface of the buried oxide and that they control the charge release allowing it to move to the opposite interface. The activation energy for charge release from these traps depends on the nature of the interface with the lowest energy estimated near 0.22 eV.
Film heterostructure with soft ferromagnetics to enhance low-field magnetoresistance
Authors: Yuan G. L. , Liu J.-M. , Zhou L. , Zhang S. T. , Chen X. Y. , Liu Z. G.
Published: November 2002

A heterostructure composed of two NiFe films and a perovskite La0.7Ca0.3MnO3 film is prepared using pulsed-laser deposition and magnetron sputtering. An enhanced effective low-field magnetoresistance (LFMR) is observed in this heterostructure because the NiFe thin films concentrate magnetic induction lines onto the small-area La0.7Ca0.3MnO3 film. The dimension of this heterostructure can be reduced to several micrometers by lithography technique, and a great potential for enhancing the effective LFMR for La0.7Ca0.3MnO3 at room temperature is predicted.
Fast switching in a single-domain particle under sub-Stoner–Wohlfarth switching fields
Authors: Gao Kai-Zhong , Boerner Eric D. , Bertram H. Neal
Published: November 2002

We utilize an energy surface model and micromagnetic simulations to study fast switching in a single-domain particle under sub-Stoner–Wohlfarth switching fields. We show that for small damping and instantaneous field reversal, the switching field can be lower than the Stoner–Wohlfarth switching fields. For small field angles with respect to the anisotropy direction, the switching time decreases with decreasing field magnitude. The results are consistent with both micromagnetic simulations and experimental data. A simple physical explanation based on an energy surface model and the Landau–Lifshitz–Gilbert equation is given to explain these phenomena.
All-solid-state electrochromic device composed of WO3 and Ni(OH)2 with a Ta2O5 protective layer
Authors: Ahn Kwang-Soon , Nah Yoon-Chae , Sung Yung-Eun , Cho Ki-Yun , Shin Seung-Shik , Park Jung-Ki
Published: November 2002

An all-solid-state electrochromic device composed of WO3 and Ni(OH)2 with a Ta2O5 protective layer was prepared by rf magnetron sputtering and lamination with a proton-conducting solid polymer electrolyte. This device had good durability, high transmittance modulation (18%–74%) and coloration efficiency (about 84 cm2 C−1), and good response times (8.5 and 18 s, respectively, during the bleaching and coloring processes). This indicates that Ta2O5 layers are electrochemically stable and can be used as protective layer for Ni(OH)2 as well as WO3.
Scanning photoelectron microscopy study of laser-induced surface reactions in Pt/Si(001)
Authors: Nelson A. J. , Danailov M. , Barinov A. , Kaulich B. , Gregoratti L. , Kiskinova M.
Published: November 2002

Synchrotron radiation scanning photoemission spectroscopy has been used to study thin platinum silicide layers produced by pulsed-laser annealing of vacuum deposited Pt films on Si(001). High-resolution photoemission measurements on the Pt 4f and Si 2p core lines were used to evaluate morphological changes and interfacial reactions. Distinct regions were identified with varying PtSi thickness and mixed Pt-rich silicides. The lateral distribution and the chemical phases formed within the laser-irradiated spots indicate that the local laser-induced temperature rise controls the interfacial processes.
Ambipolar field-effect transistor behavior of Gd@C82 metallofullerene peapods
Authors: Shimada T. , Okazaki T. , Taniguchi R. , Sugai T. , Shinohara H. , Suenaga K. , Ohno Y. , Mizuno S. , Kishimoto S. , Mizutani T.
Published: November 2002

Transport properties of C60 fullerene peapods and Gd@C82 metallofullerene peapods are investigated by using the field effect in a gated structure. The results show that C60 peapods exhibit unipolar p-type characteristics, whereas Gd@C82 peapods show ambipolar both p- and n-type characteristics. This difference in transport behavior can be explained in terms of a bandgap narrowing of the peapods. One of the important findings is that tunable electronic properties of peapods are achieved by using the different types of encapsulated fullerene molecules.
High-efficiency electrophosphorescent organic light-emitting diodes with double light-emitting layers
Authors: Zhou X. , Qin D. S. , Pfeiffer M. , Blochwitz-Nimoth J. , Werner A. , Drechsel J. , Maennig B. , Leo K. , Bold M. , Erk P. , Hartmann H.
Published: November 2002

We demonstrate high-efficiency electrophosphorescent organic light-emitting diodes (PHOLEDs) with double light-emitting layers (D–EMLs) by doping both hole and electron transport hosts with fac tris(2-phenylpyridine)iridium [Ir(ppy)3] simultaneously. The D–EMLs PHOLEDs show significantly improved efficiency (peak external quantum efficiency of about 12.6%, corresponding to a current efficiency of 44.3 cd/A) compared to the conventional PHOLEDs with a single EML and either hole or electron transport host doped with Ir(ppy)3. We attribute this improvement mainly to reduced losses of triplet excitons into regions that are not doped by phosphorescent emitter molecules.