bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase behavior of Stockmayer fluids confined to a nonpolar porous material
Authors: Spöler C. , Klapp S. H. L.
Published: February 2003

The phase behavior of hard core Stockmayer fluids (i.e., dipolar hard spheres with additional Lennard-Jones interactions) adsorbed into a disordered porous hard sphere matrix is studied by means of replica-integral equations in the reference hypernetted chain approximation. The integral equations are solved for the homogeneous isotropic phase and the low-temperature phase behavior is inferred from those thermal fluctuations which strongly increase when the stability limit of the isotropic phase is approached. Analyzing first number density fluctuations and adsorption isotherms we find that the vapor–liquid transition occuring in bulk Stockmayer fluids still takes place in matrices of sufficiently high porosity. The corresponding critical point is shifted towards a lower temperature and density, in agreement with previous results on simpler fluids. At high fluid densities a diverging dielectric constant indicates the presence of an isotropic-to-ferroelectric transition for all systems considered, and the corresponding transition temperatures are found to increase with increasing matrix density. Comparing the fluid-matrix systems with fully equilibrated mixtures we argue that this enhanced tendency for ferroelectric ordering is essentially due to excluded volume effects.
Three-dimensional band structure and bandlike mobility in oligoacene single crystals: A theoretical investigation
Authors: Cheng Y. C. , Silbey R. J. , da Silva Filho D. A. , Calbert J. P. , Cornil J. , Brédas J. L.
Published: February 2003

Quantum-chemical calculations coupled with a tight binding band model are used to study the charge carrier mobilities in oligoacene crystals. The transfer integrals for all nonzero interactions in four crystalline oligoacenes (naphthalene, anthracene, tetracene, and pentacene) were calculated, and then used to construct the excess electron and hole band structures of all four oligoacene crystals in the tight binding approximation. From these band structures, thermal-averaged velocity–velocity tensors in the constant-free-time and the constant-free-path approximations for all four materials were calculated at temperatures ranging from 2 to 500 K. The bandwidths for these oligoacenes were found to be of the order of 0.1–0.5 eV. Furthermore, comparison of the thermal-averaged velocity–velocity tensors with the experimental mobility data indicates that the simple band model is applicable for temperatures only up to about 150 K. A small-polaron band model is also considered, but the exponential band narrowing effect is found to be incompatible to experimental power law results.
Effects of microscopic disorder in electroabsorption spectroscopy: Orientational disorder in the fullerene crystal
Authors: Eilmes Andrzej , Petelenz Piotr
Published: February 2003

A Merrifield-type Hamiltonian describing the coupling between the Frenkel and charge-transfer (CT) excitations is constructed for a finite cubic cluster of C60 molecules. The model explicitly takes into account the off-diagonal disorder due to a random distribution of the CT integrals, resulting from the orientational disorder in the high-temperature fullerene phase. The Hamiltonian is numerically diagonalized; its eigenvalues and eigenvectors are used to calculate the electroabsorption signal. The spectrum is compared with that obtained for the previously proposed ad hoc model rooted in the mean-field approximation, where the crystal is assumed to have full translational symmetry and the disorder is included only implicitly by introducing effective matrix elements averaged over molecular orientations. The results confirm the validity of the mean-field approach. It is found to be very successful in reproducing the spectrum of the disordered crystal to a good approximation; the new results also justify a minor modification of the effective CT integrals, which still improves the agreement of the calculated spectrum with experiment.
Generalized grazing-incidence-angle x-ray scattering analysis of quantum dots
Authors: Kimura Masao , Acosta Ana , Fujioka Hiroshi , Oshima Masaharu
Published: February 2003

The generalized grazing-incidence-angle x-ray scattering technique has been developed for analysis of near-surface structures, namely, the anisotropy of the structure in the directions perpendicular and parallel to its surface, and its change within the depth. In this method, a specimen is irradiated by an x-ray beam with a grazing-incidence angle as small as the critical angle of the surface, and both in- and out-of-plane scattering are analyzed simultaneously. The technique has been applied to investigate the structures of InAs quantum dots grown on Si with a diameter of 37 nm and a surface coverage of 11%. It has been found that the InAs dots were dilated by 1.7% along the direction perpendicular to the surface and compressed by 1.4% in the lateral direction. The compression in the lateral direction shows a maximum value at the interface, and decreases as the distance from the interface increases. The existence of 1.7% compressive strain means that most of the 11.7% lattice mismatch between the InAs dots and the Si substrate was relaxed during the growth.
Electrically detected and microwave-modulated Shubnikov–de Haas oscillations in an Al0.4Ga0.6N/GaN heterostructure
Authors: Hang D. R. , Liang C.-T. , Juang J.-R. , Huang Tsai-Yu , Hung W. K. , Chen Y. F. , Kim Gil-Ho , Lee Jae-Hoon , Lee Jung-Hee
Published: February 2003

We report the drastic enhancement pattern of Shubnikov–de Haas (SdH) oscillations observed in an AlGaN/GaN heterostructure by microwave modulation. The dependence of the SdH pattern on microwave power and temperature is investigated. The underlying mechanism is attributed to the effect of carrier heating. This technique helps study the transport properties of two-dimensional electrons in many wide-band-gap heterostructures, in which moderate mobilities and heavier electron effective mass (rapidly damping SdH amplitudes) are frequently encountered. In addition, this method has the advantage of keeping the carrier concentration fixed and not requiring expensive high-energy laser facilities compared with carrier-modulated SdH measurements.
Iterative solution of multiple scattering and emission from inhomogeneous scatter media
Authors: Jin Ya-Qiu , Liang Zhichang
Published: February 2003

An iterative layering method is developed to take into account multiple scattering of random media in vector radiative transfer. By stratifying the random media into multiple thin slabs and employing the low-order approximated Mueller matrix solution of each thin slab, high-order scattering of the whole scatter media can be derived. This approach is applied to simulation of the polarized brightness temperature from an inhomogeneous snowpack, which is modeled as a layer of randomly and densely distributed spherical particles with nonuniform profiles of particle size, fractional volume, and physical temperature. Numerical results are compared with a conventional eigen-analysis approach for a uniform scatter medium. The functional dependence of multiple scattering and emission upon the characteristic parameters, such as the profiles of particle size, fractional volume, physical temperature, etc., are discussed. The results are also compared with microwave remote sensing measurements of a multilayered snowpack.
Enhanced optical properties of Si1−xGex alloy nanocrystals in a planar microcavity
Authors: Toshikiyo Kimiaki , Fujii Minoru , Hayashi Shinji
Published: February 2003

The emission properties of Si1−xGex alloy nanocrystals (nc-Si1−xGex) in an optical microcavity were studied, and the results were compared with those of nc-Si in the same structure. The cavity consists of two distributed Si/SiO2 Bragg reflectors (DBRs) sandwiching a thin SiO2 film containing nc-Si1−xGex. The commonly observed cavity effects, that is, spectral narrowing, high directionality, and photoluminescence (PL) enhancement in the normal direction, were observed. In nc-Si1−xGex, PL lifetime was lengthened by cavity formation, while that of nc-Si was shortened. This difference is due to the different dielectric contrast between active layers and DBRs.
Soft magnetic properties of high-temperature nanocrystalline alloys: Permeability and magnetoimpedance
Authors: Blázquez J. S. , Franco V. , Conde A. , Kiss L. F.
Published: February 2003

The technological applicability of FeCoNbBCu alloys is suggested in terms of measurements of room temperature magnetoimpedance and temperature dependence of magnetic permeability μr. Results for the Fe78-xCoxNb6B15Cu1 alloy series show that room temperature soft magnetic properties are enhanced in the lowest Co containing alloy (μr∼10 500 and magnetoimpedance ratio ∼60% at 1 MHz). However, permeability exhibits a smoother thermal dependence in the alloys with medium and high Co content. A tradeoff between magnetic softness and its thermal stability reveals the alloy with 39 at. % Co as the most suitable composition among those studied, characterized by a temperature coefficient of ∼ 0.02%/K from room temperature up to 900 K. This value is 1 order of magnitude smaller than those observed for FeSiBCuNb (FINEMET-type) alloys and Mn ferrites and extended over a much wider temperature range than in these materials.
Observation of coherent optical phonons in GaAs/AlxGa1−xAs single heterojunctions
Authors: Chang Y.-M. , Chang N.-A.
Published: February 2003

Time-resolved second-harmonic generation was used to generate and detect coherent longitudinal optical phonons in GaAs/AlxGa1−xAs single heterojunctions. Three coherent LOGaAs, LOGaAs - like, and LOAlAs - like phonons were launched by transiently screening the depletion field in the GaAs/AlxGa1−xAs interfacial region. The first and the last two phonon modes were assigned to confined LO phonons in the GaAs well and the AlxGa1−xAs barrier, respectively. The initial phase and dephasing time were determined for each phonon mode. The driving mechanism and dephasing process were investigated in the experiments.
Phonon deformation potentials of wurtzite AlN
Authors: Gleize J. , Renucci M. A. , Frandon J. , Bellet-Amalric E. , Daudin B.
Published: February 2003

A strained AlN buffer layer used for the growth of a nitride-based superlattice on silicon carbide was studied by combining x-ray diffraction measurements and Raman spectroscopy. The deformation potentials have been derived from strains and frequency shifts for most long-wavelength optical phonons. The obtained values are compared with recent theoretical calculations and experimental determinations, restricted for the latter to a few accessible modes on account of constraints imposed by the methods of investigation.
Ion energy distribution functions of vacuum arc plasmas
Authors: Byon Eungsun , Anders André
Published: February 2003

The velocity distribution function of vacuum arc ions can be measured by a time-of-flight technique. The measuring principle makes use of the well-justified assumption that the ion drift velocity from the cathode spot region to a collector is approximately constant. It is shown that the negative time derivative of the collector current is directly proportional to the ion distribution function provided that the time-averaged emission of ions from cathode spots is constant until the arc is rapidly switched off. In the experiment, arc termination took about 700 ns, which is much faster than the decay of the ion current measured at the collector placed more than 2 m from the cathode. The experimental distribution functions for most cathode materials show one large peak with a tail and one or more small peaks at higher ion velocities. The typical peak position is at about 104 m/s, with the precise values being material specific. The distribution functions for some materials exhibit not one but several peaks. No conclusive answer can be given about the nature of these peaks. Arguments are presented that the peaks are not caused by different charge states or plasma contamination but rather are due to insufficiently averaged source fluctuations and/or acceleration by plasma instabilities.
Thermal stress at wafer contact points in rapid thermal processing investigated by repeated spike treatment before oxidation
Authors: Hong Chao-Chi , Chang Chang-Yun , Lee Chaung-Yuan , Hwu Jenn-Gwo
Published: February 2003

Repeated spike treatment (RST), which was characterized by setting the temperature to ramp up and down repeatedly, was employed to study the stress effect on wafers by annealing the silicon wafers in ambient N2, followed by oxidation in O2. These RST+O samples had three apparent local thick oxide regions that adjoined the contacts of the three-pin quartz holder, as opposed to the typical+O samples annealed with a conventional temperature profile before the same oxidation process. It was observed that defects could be created on the silicon surface due to the high thermal stress at contacts during RST, and that the oxidation rate of these damaged zones was greatly increased. I–V data show that for a similar oxide thickness, the leakage currents in RST+O samples are higher and more scattered than those in typical+O samples. C–V measurements illustrate that RST+O samples have higher interface state densities than typical+O samples. These enhanced degradation phenomena could be caused by the RST, which resulted in a rough Si/SiO2 interface and nonuniform oxide thickness.
Kinetics of NO nitridation in SiO2/4H–SiC
Authors: McDonald K. , Feldman L. C. , Weller R. A. , Chung G. Y. , Tin C. C. , Williams J. R.
Published: February 2003

The kinetics of nitrogen incorporation in SiO2/4H–SiC using NO are presented. Samples were annealed in NO at pressures of 100 Torr and 1 atm, at temperatures from 1050 to 1175 °C, and for times from 0.5 to 6 h. Annealing in NO incorporates ∼1014 cm−2 of nitrogen at the SiO2/SiC interface. The nitrogen content initially increases with time and temperature, but nitrogen is removed at later times at temperatures above 1050 °C. This nitrogen removal, and the associated oxide growth in the SiC substrate, is caused by O2 formed by the thermal decomposition of NO. Eventually, the nitridation and oxidation reactions reach equilibrium, and the nitrogen content saturates as the oxide thickness increases.
Simulation of optical pulse propagation in a two-dimensional photonic crystal waveguide using a high accuracy finite-difference time-domain algorithm
Authors: Yamada S. , Watanabe Y. , Katayama Y. , Cole J. B.
Published: February 2003

Propagation properties of optical pulses in a two-dimensional photonic crystal with a straight waveguide structure imbedded were examined using a high accuracy finite-difference time-domain (FDTD) algorithm based on nonstandard finite differences. A tunable and significantly large group velocity dispersion was found even for photonic crystal structures as small as 10 unit cells long. Detailed calculations indicated that a very small photonic crystal with an imbedded waveguide can be used to control pulse dispersion, i.e., a just 25 μm long photonic crystal with waveguide can compress a 1% up-chirped pulse to the Fourier transform limit. Further, our FDTD calculations showed excellent agreement with the prediction of photonic band calculations on infinite structures.
Extended test-particle method for predicting the inter- and intramolecular correlation functions of polymeric fluids
Authors: Yu Yang-Xin , Wu Jianzhong
Published: February 2003

The Percus’ test-particle method is extended to predict the inter- and intramolecular correlation functions of polymeric fluids using a density functional theory developed earlier [J. Chem. Phys. 117, 2368 (2002)]. The calculated inter- and intramolecular distribution functions as well as the site–site correlation functions agree well with the results from Monte Carlo simulation for freely jointed hard-sphere chains. Compared with the integral-equation approaches and alternative density functional theories, the present method is free of molecular simulations as input and has the advantage of self-consistency among inter- and intramolecular correlation functions and thermodynamic properties.
Investigating pure vibrational dephasing of I3− in solution: Temperature dependence of T2* for the fundamental and first harmonic of ν1
Authors: Gershgoren Erez , Wang Zhaohui , Ruhman Sanford , Vala Jiri , Kosloff Ronnie
Published: February 2003

Pure ν1 vibrational dephasing of triiodide is recorded in ethanol and methyl-tetrahydrofurane solutions from 300 to 100 K, for the vibrational fundamental and its first overtone. Using impulsive Raman spectroscopy, dephasing is demonstrated to be homogeneous throughout the temperature range studied. Independent measures of T1 prove that population relaxation contributes negligibly to the dephasing rates. The reduction in temperature gradually leads to a ∼2-fold decrease in the rate of pure dephasing. With cooling the ratio of T2(n=1)*/T2(n=2)* reduces slightly but remains in the range of 2.7 to 2. These results are discussed in terms of Kubo lineshape and Poisson dephasing theories. Neither of these consistently explains the experimental observations assuming reasonable intensities and rates of intermolecular encounters in the solutions.
On the efficient representation of comprehensive, precise spectroscopic data sets: The A state of I2
Authors: Tellinghuisen Joel
Published: February 2003

Mixed representations—polynomials in (v+1/2) at low v, near-dissociation expansions (NDEs) in (vD−v) at high v—are tested for their ability to fit a comprehensive and precise data set for the A state of I2. The combined functions for the vibronic energy Tv and the rotational constant Bv are rendered smooth at the point of switchover through two approaches: the use of Lagrange’s method of undetermined multipliers to incorporate exactly satisfied constraints for continuity in the functions and their first derivatives, and the use of a smooth switching function. As compared with a previously reported pure NDE analysis [Appadoo et al., J. Chem. Phys. 104, 903 (1996)], both approaches yield significantly reduced chi-square and a more realistic extrapolation of Bv from the highest analyzed level (v=35) to dissociation. The switching-function method has a number of advantages over the constraint method, and is thus recommended as the preferred approach for fitting to mixed representations.
Infrared emission spectra of BeH2 and BeD2
Authors: Shayesteh A. , Tereszchuk K. , Bernath P. F. , Colin R.
Published: February 2003

High resolution infrared emission spectra of beryllium dihydride and dideuteride have been recorded with a Fourier transform spectrometer. The molecules were generated in a discharge-furnace source, at 1500 °C and 333 mA discharge current, with beryllium metal and a mixture of helium and hydrogen or deuterium gases. The antisymmetric stretching modes (ν3) of BeH2 and BeD2, as well as several hot bands involving ν1, ν2, and ν3, were rotationally analyzed and spectroscopic constants were determined. The equilibrium rotational constant (Be) of BeH2 was found to be 4.753 66(2) cm−1, and the equilibrium bond distance (Re) of 1.326 407(3) Å was determined for BeH2.
Effects of competitive carbon monoxide adsorption on the hydrogen response of metal–insulator–semiconductor sensors: the role of metal film morphology
Authors: Will Medlin J. , McDaniel Anthony H. , Allendorf Mark D. , Bastasz Robert
Published: February 2003

We have investigated the effects of carbon monoxide coadsorption on the performance of hydrogen-sensitive palladium metal–insulator–semiconductor (MIS) sensors, and have found that device performance is highly dependent on the morphology of the metal film. On smooth, continuous films, experiments utilizing H2/CO mixtures show that CO induces a time-delayed increase in response to H2. Based on a kinetic model of CO and hydrogen adsorption on the Pd film, this increase is interpreted as being due to trapping of H atoms at the Pd–SiO2 interface by CO adsorbed on the metal surface. However, MIS sensors fabricated so that a large concentration of voids exist in the metal film respond in a markedly different fashion: on these devices, the addition of CO causes a time-delayed decrease in the sensor response. Possible explanations for this attenuating CO effect are discussed. The experimental and modeling results presented here suggest that sensor behavior in the presence of competitively adsorbing mixtures can be quite complex, with film imperfections playing an important role in determining the response.
Enhancement of Jc by doping silver in grain boundaries of YBa2Cu3Oy polycrystals with solid-state diffusion method
Authors: Cheng C. H. , Zhao Y.
Published: February 2003

Grain boundaries (GBs) of YBa2Cu3Oy (YBCO) are preferentially doped with Ag by the solid-state diffusion method. The distribution of Ag is highly localized in the GB. As a result of the Ag doping, the Jc-H behavior of YBCO textured polycrystalline samples is significantly improved, and at 60 K and 7 T, Jc is tripled. It is proposed that Ag could partially replace the Cu in CuO2 planes near GBs, therefore reducing the geometric distortion of Cu-O bonds and making the effective GBs thinner.
Time-resolved emittance of a bismuth ion beam from a pulsed vacuum arc ion source
Authors: Anders André , Chacon-Golcher Edwin
Published: February 2003

The emittance of a pulsed bismuth vacuum arc ion source has been measured using a time-resolving double-slit emittance scanner. Each arc pulse was 250 μs long. Ion extraction was performed with 20 μs pulses 200 μs after arc triggering, when Bi ions are almost exclusively singly charged. This was motivated by a possible use of the ion source for heavy ion fusion. Pulsed extraction implies perveance mismatch during extractor voltage rise and fall time. The emittance increased with arc current. The beam current is very constant when operating in the slightly overdense plasma regime because fluctuations of the ion supply current are partially offset by extraction optics. For an extraction voltage of 35 kV, a bismuth current density of up to 18 mA/cm2 with a normalized emittance of 0.006 π mm mrad was obtained for a beamlet of 2 mm nominal diameter.
Theoretical study of radiation effects on GaAs/AlGaAs and InGaAsP/InP quantum-well lasers
Authors: Gilard O.
Published: February 2003

Radiation hardness assurance of light emitters such as laser diodes is a primary concern before using such devices in space. In this article, both gallium arsenide (GaAs) and indium phosphide (InP) based laser diodes are theoretically analyzed from a radiation hardness point of view. It has been found that InP based lasers are less sensitive to radiation than GaAs lasers. We will also demonstrate that threshold carrier density is the key parameter to consider when assessing the radiation sensitivity of laser diodes.
Effect of molecular elongation on the thermal conductivity of diatomic liquids
Authors: Tokumasu Takashi , Ohara Taku , Kamijo Kenjiro
Published: February 2003

The effect of molecular elongation on the thermal conductivity of diatomic liquids has been analyzed using a nonequilibrium molecular dynamics (NEMD) method. The two-center Lennard-Jones model was used to express the intermolecular potential acting on liquid molecules. The simulations were performed using the nondimensional form of the potential so that the molecular elongation, d/σ, was the only parameter varied in the simulation. The simulations were performed for five values of this parameter. First, the equation of state of each liquid was obtained using equilibrium molecular dynamics simulation, and the critical temperature, density, and pressure of each liquid were determined. Then, NEMD simulations of heat conduction in the five liquids were performed using values for temperature and density which were identical among the five liquids when they were reduced by their respective critical temperature and density (T=0.7 Tcr and ρ=2.24 ρcr). Obtained thermal conductivities were reduced by the critical temperature, density, and molecular mass of each compound, and these values were compared with each other. It was found that the reduced thermal conductivity increased as molecular elongation increased. Detailed analysis of the molecular contribution to the thermal conductivity revealed that (a) the contribution of the heat flux caused by energy transport and by translational energy transfer to the thermal conductivity is independent of the molecular elongation, and (b) the contribution of the heat flux caused by rotational energy transfer to the thermal conductivity increases with the increase in the molecular elongation.
Magnetic influence on frequency of soft-phonon mode in incipient ferroelectric EuTiO3
Authors: Jiang Qing , Wu Hua
Published: February 2003

The dielectric constant of the incipient ferroelectric EuTiO3 exhibits a sharp decrease at about 5.5 K, at which temperature antiferromagnetic ordering of the Eu spins simultaneously appears, indicating coupling between the magnetism and dielectric properties. This may be attributed to the modification of the soft-phonon mode T1μ, which is the main contribution to the large dielectric constant, by the Eu spins (7μB per Eu). By adding the coupling term between the magnetic and electrical subsystems as −g∑l∑〈i, j〉ql2S⃗i⋅S⃗j we show that the variation of the frequency of soft-phonon mode depends on the spin correlation between the nearest-neighbors Eu spins and is substantially changed under a magnetic field.
Ultrafast optical Kerr effect of Ag–BaO composite thin films
Authors: Zhang Q. F. , Liu W. M. , Xue Z. Q. , Wu J. L. , Wang S. F. , Wang D. L. , Gong Q. H.
Published: February 2003

We report the ultrafast optical Kerr effect of Ag–BaO composite thin films by the femtosecond time-resolved pump-probe technique. The Ag–BaO thin films with Ag nanoparticles embedded into the BaO semiconductor matrix were prepared using a vacuum evaporation-deposition multimetallic layer method. The third-order nonlinear optical susceptibility of the thin films with the thickness of approximately 300 nm and the volume fraction of Ag nanoparticles in the thin films of about 25% was estimated to be 4.8×10−10 esu at the incident laser wavelength of 820 nm. The response time, i.e., the full width at half maximum of the Kerr signal, was as fast as 210 fs. The intrinsic third-order optical nonlinearity, or the optical Kerr effect of the thin films, can be attributed to the change of refractive index due to the intraband transition of electrons from the occupied state near the Fermi level to the unoccupied state in the Ag nanoparticles. Such nonlinearity is further enhanced by the local field effect that is present when the metallic nanoparticles are embedded into the semiconductor matrix.
Acoustic nonlinearity parameter measurements in solids using the collinear mixing of elastic waves
Authors: Jacob Xavier , Barrière Christophe , Royer Daniel
Published: February 2003

An alternative method for measuring the nonlinearity parameter β of longitudinal acoustic waves propagating in solids is presented. The method is based on the detection of the phase modulation resulting from the parametric interaction between a high frequency acoustic wave and a lower frequency acoustic pulse. Results are reported for various materials: fused quartz, duraluminum, titanium, and polystyrene.
Parameters that control pulsed electron beam ablation of materials and film deposition processes
Authors: Strikovski Mikhail , Harshavardhan K. S.
Published: February 2003

Conditions for ablation of materials and film deposition were analyzed for a pulsed (∼100 ns) electron beam produced by a channel-spark source. For dielectric materials, we found the existence of an optimal source voltage related to the saturation of the pulse current. Our analysis indicates a larger ablated mass, smaller optimal deposition rates (∼0.25 Å/pulse), and a larger optimal target to substrate distance relative to pulsed laser deposition process.
On the suprathermal distribution in an anisotropic phonon system in He II
Authors: Adamenko I. N. , Nemchenko K. E. , Wyatt A. F. G.
Published: February 2003

The equation that describes the suprathermal distribution of high-energy phonons (h phonons) created in anisotropic phonon systems in superfluid helium is obtained. The solution of this equation enables the derivation of the value of suprathermal ratio S as the ratio of the actual distribution to the Bose–Einstein one, its dependences on the momentum of the h phonons, the anisotropy parameters, and the temperature of the low-energy phonons from which the h phonons are created. We analyze this equation to obtain an estimate of the value of the ratio between the h-phonon number density in anisotropic and isotropic phonon systems and draw conclusions about the dependence of S on the relevant parameters.
Odd resistive response in superconductors with bianisotropic pinning
Authors: Shklovskij V. A. , Soroka A. A.
Published: February 2003

A theoretical study is made of the odd resistive response (i.e., having odd parity with respect to inversion of the magnetic field) of a superconductor in the mixed state in the presence of bianisotropic pinning and a small isotropic Hall effect. The components of the odd magnetoresistivity in the directions longitudinal and transverse to the current are obtained in a two-dimensional stochastic model of bianisotropic pinning based on the Fokker–Planck equations in the approximation of noninteracting vortices and to a first approximation in the small Hall constant. Both naturally occurring and artificially produced realizations of this model are possible. It is shown that the nonlinear anisotropic properties of the magnetoresistivities are naturally related to the principal critical currents and saturation currents of the system under study. Scaling relations for the Hall conductivity in terms of the longitudinal and transverse magnetoresistivities obtained are discussed, and scaling and its stability in the basal X and Y geometries of the problem are examined.
Single-particle scenario of the metal–insulator transition in two-dimensional systems at T=0
Authors: Tarasov Yu. V.
Published: February 2003

The conductance of disordered electron systems of finite size is calculated by reducing the initial dynamical problem of arbitrary dimensionality to strictly one-dimensional problems for single-particle mode propagators. It is shown that the metallic ground state of two-dimensional conductors, considered as a limiting case of three-dimensional quantum waveguides, is due to their multimode nature. As the thickness of the waveguide is decreased, e.g., with the aid of a “pressing” potential, the electron system undergoes a sequence of continuous quantum phase transitions involving a discrete change in the number of extended modes. The closing of the last current-carrying mode is interpreted as a phase transition of the electron system from the metallic to an insulator state. The results agree qualitatively with the observed “anomalies” of the resistance of various two-dimensional electron and hole systems.
Effective equations of motion for solitons in two-sublattice isotropic magnets
Authors: Galkina E. G. , Ivanov B. A. , Murav’yov V. M.
Published: February 2003

A solution of the problem of magnon scattering on Belavin–Polyakov solitons in two-dimensional magnets is constructed in the framework of a generalized σ model. This model can serve as a basis for describing both ferromagnets and antiferromagnets, and it can also describe ferrimagnets near the point of compensation of the sublattice spins. The problem of magnon scattering on a soliton is formulated‘ for this model, and its exact solution is obtained for a partial mode with azimuthal quantum number m=1. It is shown that in a linear approximation this mode completely describes the dynamics of the center of the soliton in a magnet of finite size. Effective equations of motion for solitons in different magnets are constructed on the basis of this analysis.
Atomic-level study of ion-induced nanoscale disordered domains in silicon carbide
Authors: Gao F. , Weber W. J.
Published: February 2003

Atomic-level simulations have been employed to study the nanoscale disordering induced in 3C–SiC by energetic Si and Au ions (up to 50 keV). Topologically disordered nanoscale domains are generated with low frequency in the cascades produced by Au ions, whereas Si ions create only a few small defect clusters, with most defects being single interstitials and monovacancies. The structural image simulations of the nanoscale domains provide for atomic-level insights into disordered states. The simulations suggest that it is possible to design and fabricate nanoscale optoelectronic devices based on SiC using ion-beam-induced order–disorder transformation.
Thermal stability of ion-irradiated InGaAs with (sub-) picosecond carrier lifetime
Authors: Joulaud L. , Mangeney J. , Lourtioz J.-M. , Crozat P. , Patriarche G.
Published: February 2003

Pump-probe experiments have been used to measure the (sub-) picosecond carrier lifetimes in Au+- and proton-irradiated InGaAs samples, subsequently annealed at various temperatures. For both types of irradiation, the carrier lifetime increases with the annealing temperature. After 600 °C annealing, the defects are totally recovered in proton-irradiated samples, whereas they are still present in Au+-irradiated samples. The defect annealing kinetics observed in proton-irradiated samples is described well by a Frenkel pair recombination model, thereby indicating the dominance of isolated point defects. In contrast, the model is not adapted to describe the thermal behavior of Au+-irradiation-induced defects that are clusters of point defects as observed via transmission electronic microscopy. It is clearly shown that heavy-ion irradiated InGaAs exhibits a much higher thermal stability than proton-irradiated InGaAs.
Surface-induced phase separation in manganites: A microscopic origin for powder magnetoresistance
Authors: Bibes M. , Balcells Ll. , Fontcuberta J. , Wojcik M. , Nadolski S. , Jedryka E.
Published: February 2003

Through the analysis of the magnetic properties and of the nuclear magnetic resonance response of La2/3Ca1/3MnO3 ceramics with different grain sizes, we have found that poorly conducting regions, some ferromagnetic and some weakly magnetic, are located at the surface of the grains. We state that these regions constitute the tunnel barrier responsible for the low-field magnetoresistance usually observed in powders of half-metallic oxides. In addition, the spin disorder accompanying the coexistence of phases with different magnetoelectronic character could contribute to the large high-field magnetoresistance also typical of such ceramic samples. From a more general perspective, these findings can be of relevance to understand the microscopic origin of phase separation in manganites.
Temperature dependence of the surface roughness evolution during hydrogenated amorphous silicon film growth
Authors: Smets A. H. M. , Kessels W. M. M. , van de Sanden M. C. M.
Published: February 2003

The scaling behavior of the surface morphology of hydrogenated amorphous silicon deposited from a SiH3 dominated plasma has been studied using atomic force microscopy and in situ ellipsometry. The observed substrate temperature dependence of growth exponent β reflects a crossover behavior from random deposition at 100 °C to a surface diffusion controlled smoothening around 250 °C to full surface relaxation around 500 °C. This crossover behavior has been reproduced by Monte Carlo simulations assuming a site dependent surface diffusion process, revealing an activation energy of ∼1.0 eV for the ruling surface smoothening mechanism. The implications for a-Si:H growth are discussed.
Characterization of the nanostructures of a lithographically patterned dot array by x-ray pseudo-Kossel lines
Authors: Lee D. R. , Chu Y. S. , Choi Y. , Lang J. C. , Srajer G. , Sinha S. K. , Metlushko V. , Ilic B.
Published: February 2003

Grazing x-ray scattering from a nanofabricated periodic dot array exhibits an interesting diffraction pattern, resembling x-ray Kossel lines, due to the anisotropic x-ray resolution function. We demonstrate that the unique diffraction pattern can be used for precise characterization of the deep nanostructures, which cannot be obtained accurately by microscopy techniques.
Orientation of self-assembled block copolymer cylinders perpendicular to electric field in mesoscale film
Authors: Elhadj S. , Woody J. W. , Niu V. S. , Saraf R. F.
Published: February 2003

The possibility of using self-assembled films of block polymers as templates to fabricate nanoscale structures for devices has attracted great attention towards this class of material. Self-assembly of a block copolymer can be directed by using an electric field to orient features (i.e., ∼10-nm-diam cylinders) parallel to the electric field, making the material more attractive as a nanoscale lithography mask. In this letter we describe an approach to influence the electric field orientation phenomena by interfacial effects. As a result, the 15-nm-diam polystyrene cylinders of the polystyrene–polyisoprene–polystyrene triblock copolymer orient perpendicular to the electric field. The described approach along with the previous method can produce a directed self-assembly to fabricate complex nanoscale structures with orthogonally oriented nanoscale width lines.
Converse piezoelectric effect and photoelastic effect in InGaN/GaN multiple quantum wells
Authors: Lin T. Y.
Published: February 2003

We present microphotoluminescence and microRaman measurements with different optical excitation intensities in InGaN/GaN multiple quantum wells (MQWs). When the optical excitation density was increased, the InGaN A1(LO) phonon was found to show a redshift in frequency and a blueshift in photoluminescence spectra has been observed. The change in the refractive index of MQWs was found to be strongly related to the blueshift of photoluminescence spectra and the redshift of the InGaN A1(LO) phonon. Our results firmly establish that a converse piezoelectric effect responsible for a considerable photoelastic effect (change in refractive index produced by a strain) arising from the optical modulation does exist in InGaN/GaN MQWs.
Formation of nanofiber crossbars in electrospinning
Authors: Zussman E. , Theron A. , Yarin A. L.
Published: February 2003

In this letter, we report on a technique for the hierarchical assembly of nanofibers into crossbar nanostructures. An electrospinning process is used to create polymer-based nanofibers with diameters ranging from 10–180 nm and lengths of up to several centimeters. By controlling the electrostatic field and the polymer rheology, the nanofibers can be assembled into parallel periodic arrays. We also propose a theoretical model for the process.
GaN growth using a low-temperature GaNP buffer on sapphire by metalorganic chemical vapor deposition
Authors: Tsukihara M. , Naoi Y. , Sakai S. , Li H. D.
Published: February 2003

We developed a buffer layer to grow GaN epilayers by metalorganic chemical vapor deposition. The buffer layer consists of a thin GaN-rich GaNP layer deposited at low temperature (LT) (500 °C) on sapphire substrate, using phosphine (PH3) as the phosphorus source. For high-temperature GaN epilayers grown on this type of buffer, full-width at half maximum values from both (0002) and (101̄2) x-ray rocking curves decrease as phosphorus composition in the GaNP buffer increases; a dislocation density observed by transmission electron microscopy is as low as 5×108 cm−2, which is a factor of 2 less compared to that in a conventional GaN buffer epilayer. These results reveal that LT GaNP can be used as an appropriate buffer for further improving quality of GaN-based films.
Photocurrent method for characterizing the interface of hydrophobically bonded Si wafers
Authors: Yu L. S. , Mages P. , Qiao D. , Jia L. , Yu P. K. L. , Lau S. S. , Suni T. , Henttinen K. , Suni I.
Published: February 2003

We report a simple and nondestructive photocurrent method for characterizing the bonding interface between two hydrophobically bonded Si wafers. The relationship of photocurrent versus wavelength was measured and analyzed. The direction of the photocurrent indicates the band-bending direction, and thus, the donor or acceptor nature of the interface states. The photocurrent is proportional to the electric field at the interface induced by band bending. Our results showed that the Si pairs bonded in air have much larger band bending at the interface than those bonded in dry nitrogen, and that both have donor-like interface states.
Open-type hybrid magnetic shield using high-TC superconducting wire and flexible magnetic sheets
Authors: Seki Yusuke , Suzuki Daisuke , Ogata Kuniomi , Tsukada Keiji
Published: February 2003

An open-ended, cylindrical magnetic shield using high-TC superconducting rings and flexible magnetic sheets has been developed. The superconducting ring is made of Bi2Sr2Ca2Cu3Ox tape wire, and the flexible magnetic sheet is made of Fe–Cu–Nb–Si–B nanocrystalline alloy, which has superior soft magnetic properties. The superconducting rings are set near the open ends of a ferromagnetic cylinder. When a magnetic field is induced in the rings, a shielding current flows so as to keep the magnetic flux through the ring constant. It is concluded that the superconducting rings effectively increase the shielding factor of a magnetic shield.
Enhancement of room-temperature magnetoresistance in Sr2FeMoO6 by reducing its grain size and adjusting its tunnel-barrier thickness
Authors: Yuan C. L. , Zhu Y. , Ong P. P.
Published: February 2003

The intergrain magnetoresistance (IMR) of polycrystalline Sr2FeMoO6 is known to depend on its grain size and the tunnel barrier thickness formed during fabrication. Further enhancement of the IMR of Sr2FeMoO6 is found possible by further decreasing its grain size using high-energy ball milling, as well as by judiciously adjusting the tunnel barrier thickness. The tunnel barrier thickness depends on the amount of insulating nonmagnetic SrMoO4 impurity formed during fabrication, which in turn can be controlled by varying the mixture ratio of the ambient gaseous H2–Ar stream during annealing. With smaller grain size, the magnetic domain size decreases correspondingly and so can be more easily rotated to produce a lower coercive field. The increase in the SrMoO4 impurity phase increases the number of grain boundary barriers in the Sr2FeMoO6 matrix, leading to a larger low-field magnetoresistance effect (LFMR). A significant enhancement of the LFMR was found when the SrMoO4 impurity was close to the conduction threshold. Compared to the artificial fabrication of multilayer structures, our method is much simpler and offers an enhanced technology to fabricate suitable tunneling barriers in magnetoresistive devices.
Active planar optical waveguide made from luminescent silicon nanocrystals
Authors: Valenta J. , Pelant I. , Luterová K. , Tomasiunas R. , Cheylan S. , Elliman R. G. , Linnros J. , Hönerlage B.
Published: February 2003

We show experimentally that a layer of silicon nanocrystals, prepared by the Si-ion implantation (with the energy of 400 keV) into a synthetic silica slab and exhibiting room-temperature red photoluminescence, can serve simultaneously as a single-mode planar optical waveguide. The waveguide is shown to self-select guided transverse electric and transverse magnetic modes from the broad photoluminescence emission of the nanocrystals resulting in a substantially narrower emission spectrum for these modes. We further report on an investigation of optical gain in a sample implanted to a dose of 4×1017 cm−2. Despite the occurrence of strong waveguiding, results of the variable stripe length method turned out not to be able to give unambiguous evidence for optical gain.
Binding and surface diffusion of SiH3 radicals and the roughness of hydrogenated amorphous silicon
Authors: Dewarrat R. , Robertson J.
Published: February 2003

Local density formalism pseudopotential calculations find that the growth radical SiH3 binds to the hydrogen-terminated (111)Si surface. The bound site is not the three-center Si–H–Si bridging site previously assumed. It has a direct Si–Si bond between the SiH3 and the surface Si, and the terminal hydrogen is displaced to a bond center of a lateral surface Si–Si bond. This site is more stable as the unpaired electron can delocalize over more Si–Si bonds. A bound site validates the standard model of the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline Si, in which a mobile growth species allows surface diffusion and creates smooth surfaces.
Direct observation of a concealed glass transition in a Mg–Ni–Nd metallic glass
Authors: Lu Z. P. , Liu C. T. , Kam C. H. , Li Y.
Published: February 2003

Glass transition and crystallization behavior of Mg75Ni10Nd15 metallic glass has been studied by a temperature-modulated differential scanning calorimeter (TMDSC) and a normal differential scanning calorimeter (DSC). The truly vitreous state of the as-spun Mg75Ni10Nd15 alloy was confirmed by the direct observation of the glass transition through TMDSC measurements, which was not perceptible in the normal DSC heating scans. It was concluded that the glass transition signal was concealed by the relatively strong concurrent signal of the primary crystallization. This was further confirmed by the fact that the glass transition was revealed by both TMDSC and DSC for the samples with negligible heat release from the primary crystallization.
Thermal conductivity of elemental crystalline silicon clathrate Si136
Authors: Nolas G. S. , Beekman M. , Gryko J. , Lamberton G. A. , Tritt T. M. , McMillan P. F.
Published: February 2003

The thermal conductivity and heat capacity of a guest-free polycrystalline silicon clathrate with the type-II hydrate crystal structure is reported. The magnitude of the thermal conductivity at room temperature is only slightly larger than that of vitreous silica, and is thirty times lower than that of diamond structured Si. The temperature dependence of the thermal conductivity of Si136 follows the well-known Debye form, and is dissimilar to that of clathrates with “guest” atoms inside their polyhedra. The Debye temperature of Si136, estimated from low temperature heat capacity measurements, is 470 K. The potential of guest-free clathrates for thermoelectric applications is discussed.
Alternative-precursor metalorganic chemical vapor deposition of self-organized InGaAs/GaAs quantum dots and quantum-dot lasers
Authors: Sellin R. L. , Kaiander I. , Ouyang D. , Kettler T. , Pohl U. W. , Bimberg D. , Zakharov N. D. , Werner P.
Published: February 2003

Metalorganic chemical vapor deposition of laser diodes based on triple stacks of self-organized InxGa1−xAs/GaAs quantum dots (QDs) as active medium using the alternative precursor tertiarybutylarsine (TBAs) is reported. Epitaxy of monodispersed QDs using TBAs is demonstrated. Due to the high cracking efficiency of TBAs at low temperatures, the crucial growth parameters V/III ratio and temperature can be tuned almost independently. Ridge-waveguide QD lasers show a transparency current of 29.7 A/cm2—equivalent to 9.9 A/cm2 per QD layer—an internal quantum efficiency of 91.4%, and an internal optical loss of 2.2 cm−1.