bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultrafast carrier dynamics and dephasing in InAs quantum-dot amplifiers emitting near 1.3-μm-wavelength at room temperature
Authors: Borri P. , Schneider S. , Langbein W. , Woggon U. , Zhukov A. E. , Ustinov V. M. , Ledentsov N. N. , Alferov Zh. I. , Ouyang D. , Bimberg D.
Published: July 2002

The carrier dynamics in an electrically pumped InAs quantum-dot amplifier emitting near 1.3-μm-wavelength at room temperature is measured with femtosecond time resolution performing a pump–probe and a four-wave mixing experiment resonant to the dot ground–state transition. In contrast to the dynamics of the absorption bleaching over hundreds of picoseconds, an ultrafast gain recovery is measured, promising for high-speed applications of strongly confined InAs dots. Moreover, a dephasing time of 220 fs is measured in the absorption and of 150 fs in the gain case. This latter value is more than three times longer than our previous finding on less-confined quantum dots [P. Borri et al., Appl. Phys. Lett. 76, 1380 (2000)] indicating that the strong confinement can indeed lower the homogeneous broadening under electrical injection.
Self-assembled heterojunction between electrodeposited PbS nanoparticles and indium tin oxide substrate
Authors: Nanda K. K. , Sahu S. N.
Published: July 2002

Self-assembled heterojunction was fabricated by means of an electrochemical deposition of PbS nanoparticles on indium tin oxide substrate. The current–voltage and capacitance–voltage studies confirmed the formation of a heterojunction. A large current and large capacitance were observed in the case of a device from particle of smaller size which is believed to be due to the large surface area contact. The rectifying behavior of the heterojunction was found to be weak as compared to the usual p–n junction.
An improved reactive flux method for evaluation of rate constants in dissipative systems
Authors: Drozdov Alexander N. , Tucker Susan C.
Published: July 2002

We present an improved method for numerical evaluation of the least nonvanishing eigenvalue in dissipative systems. It is based on a modified version of the reactive flux formula for the rate which eliminates the principal shortcomings of the original formulation. A new fourth-order algorithm is used to efficiently integrate the underlying generalized Langevin equation. The proposed methodology provides accurate eigenvalues with a small computational effort for systems which could not be accurately treated with the standard techniques. Its efficiency is tested for Brownian motion in a symmetric double well subject to an exponential friction kernel. The presented results demonstrate the applicability of the method in all regimes of interest, reaching from underdamped to overdamped Brownian motion.
Rotational and vibrational dynamics of CO2 and N2O in helium nanodroplets
Authors: Nauta K. , Miller R. E.
Published: July 2002

Infrared spectra are reported for carbon dioxide and nitrous oxide solvated in superfluid helium droplets, corresponding to the vibrational excitation of the (0201)/(1001) Fermi diad. Although the rotational constants of these two molecules are similar in the gas phase, they are observed to be quite different in liquid helium, namely, 0.154 cm−1 for CO2 and 0.0717 cm−1 for N2O. In addition, solvation in helium results in shifts in the vibrational origin that are in the opposite directions, −0.42 cm−1, for CO2 and +1.2 cm−1 for N2O. The spectra also show strong droplet size dependence, indicative of the interactions between the molecule and the liquid.
The propagation of a surfactant laden liquid plug in a capillary tube
Authors: Waters S. L. , Grotberg J. B.
Published: July 2002

This paper considers the propagation of a liquid plug, forced by a driving pressure ΔP, within a rigid tube. The tube is already lined with a liquid precursor film of thickness h̄2. Both the plug and the precursor film, as well as the interface, contain small amounts of surfactant whose concentrations are assumed to be near equilibrium. Since the motions are slow, we seek asymptotic solutions for small capillary number, Ca≪1, and also assume that sorption kinetics control the surfactant flux to the interface compared to bulk diffusion. An additional asymptotic assumption is that the Stanton number, St, is sufficiently large such that β∝Ca1/3/St≪1, which relates the importance of sorption kinetics to convection. The surfactant strength is measured by the surface elasticity, E=M/β where M is the Marangoni number. The results of the analysis are that, for a given plug Ca, ΔP increases with increasing E but decreases with increasing h̄2. The trailing film thickness, h̄1, increases with ΔP, but at a slower rate when E is larger. For h̄1<h̄2, criteria for plug rupture are established. This model is relevant to delivery of surfactants into the lung by direct instillation into the bronchial network as is done in surfactant replacement therapy and the use of surfactant solutions to carry other substances (e.g., genetic material) into the airways.
A study of the C(1D)+H2→CH+H reaction: Global potential energy surface and quantum dynamics
Authors: Bussery-Honvault B. , Honvault P. , Launay J.-M.
Published: July 2002

The adiabatic global potential energy surface of the CH2 system for the first singlet state of A′ symmetry (ã 1A′) has been computed. Ab initio, multireference, single and double configuration interaction calculations have been used to characterize this state. This potential energy surface has a calculated well depth of 99.7 kcal/mol relative to the C(1D)+H2 asymptote. The surface has no barrier for the perpendicular C2v geometry, but presents a large barrier (12.35 kcal/mol) for the collinear C∞v geometry. The ab initio calculations were carried out over 1748 geometries and the resulting energies were fitted to a many body expansion. Based on this surface, we have performed the first quantum reactive scattering calculations for the C(1D)+H2(X 1Σg+)→CH(X 2Π)+H(2S) reaction and total angular momentum J=0. The hyperspherical coordinates time-independent method has been used. We note that the state-to-state reaction probabilities as a function of the collision energy show a dense resonance structure which is unusual for this type of atom+diatom reaction. We present also rotational distributions.
Discrete Kaluza–Klein from scalar fluctuations in noncommutative geometry
Authors: Martinetti Pierre , Wulkenhaar Raimar
Published: July 2002

We compute the metric associated with noncommutative spaces described by a tensor product of spectral triples. Well-known results of the two-sheets model (distance on a sheet, distance between the sheets) are extended to any product of two spectral triples. The distance between different points on different fibers is investigated. When one of the triples describes a manifold, one finds a Pythagorean theorem as soon as the direct sum of the internal states (viewed as projections) commutes with the internal Dirac operator. Scalar fluctuations yield a discrete Kaluza–Klein model in which the extra component of the metric is given by the internal part of the geometry. In the standard model, this extra component comes from the Higgs field.
Highly conductive GaAsNSe alloys grown on GaAs and their nonalloyed ohmic properties
Authors: Uesugi Katsuhiro , Suemune Ikuo
Published: July 2002

Doping properties of Se in GaAsN alloys grown on GaAs (001) substrates by metalorganic-molecular-beam epitaxy were studied. Ditertiarybutylselenide (DtBSe) precursor was used as a Se source. It was found that Se was incorporated into GaAs and GaAsN layers up to a considerable concentration of ∼15%. It was also suggested that the N concentrations in GaAsNSe layers were increased by the DtBSe supply. The GaAsNSe layers were heavily doped n type, and the maximum electron concentration was as high as ∼1×1020 cm−3. With the increase of the carrier concentrations, the resistivity of GaAsNSe dramatically decreased to 1.2×10−4 Ω cm. This made it possible to have ohmic contacts without thermal annealing, which indicates that GaAsNSe alloys are an attractive candidate for the formation of nonalloyed ohmic contacts.
New forcefield parameters for branched hydrocarbons
Authors: Nath Shyamal K. , Khare Rajesh
Published: July 2002

A new set of united-atom force field parameters is proposed for simulating the phase equilibria of branched alkanes. These parameters complement the already existing set of Nath, Escobedo, and de Pablo revised (NERD) [Nath et al., J. Chem. Phys. 105, 4391 (1998); Nath and de Pablo, Mol. Phys. 98, 231 (2000)] force field parameters. The proposed force field is used to study vapor–liquid equilibria for various isomers of alkanes up to C8. Results of simulations are found to be in good agreement with available experimental data.
Catalyzed growth of carbon nanoparticles by microwave plasma chemical vapor deposition and their field emission properties
Authors: Yu J. , Ahn J. , Zhang Q. , Yoon S. F. , Rusli  , Li Y. J. , Gan B. , Chew K. , Tan K. H.
Published: July 2002

Carbon nanoparticles were prepared from H2 and CH4 by microwave plasma chemical vapor deposition at various temperatures as low as 250 °C by using nickel and iron as catalysts. The carbon nanoparticles are well graphitized until a temperature as low as 400 °C, and the degree of graphitization increases with increasing growth temperature. Field emission measurements showed that the carbon nanoparticles are excellent electron field emitters, comparable to carbon nanotubes. Field emission properties became better with increasing growth temperature, and the threshold fields of the carbon nanoparticles deposited at 400, 500, 670 °C, were 3.2, 3, and 1 V/μm, respectively. No emission was observed for the carbon nanoparticles deposited below 400 °C. The low threshold field of the carbon nanoparticles is attributed to field enhancement effect and the higher degree of graphitization.
BaPbO
3
perovskite electrode for lead zirconate titanate ferroelectric thin films
Authors: Luo Yih-Rong , Wu Jenn-Ming
Published: July 2002

BaPbO 3 (BPO) films were prepared by rf-magnetron sputtering at temperatures as low as 350 °C. These films possessed low electrical resistivity of 1.4×10−3 Ω cm, which is appropriate for electrodes. The BPO electrode has an advantage over a Pt electrode in that it lowers the crystallization temperature of Pb(Zr0.53Ti0.47)O3 (PZT) films from 600 to 550 °C. The coercive fields of the PZT films deposited on a BPO/Pt electrode are significantly lower than those deposited on the Pt electrode, but the remanent polarization remained essentially unchanged. The BPO electrode also improved the fatigue resistance and decreased the leakage current of the PZT films deposited.
Sedimentation equilibrium of a suspension of adhesive colloidal particles in a planar slit: A density functional approach
Authors: Choudhury Niharendu , Ghosh Swapan K.
Published: July 2002

The sedimentation equilibrium of a colloidal suspension modeled as a collection of adhesive hard-sphere particles confined in a planar slit pore is studied by using a density functional approach with a newly developed approximation in which the repulsive part of the adhesive hard-sphere interaction potential is treated within weighted density functional theory while the attractive part is evaluated by a third-order perturbative approach. The calculated density profiles obtained from this approach for the adhesive hard-sphere fluid confined in a slit pore in the absence of gravitational field is in excellent agreement with the available computer simulation results for varying pore sizes and stickiness parameters of the potential. The same approach applied to the study of the effect of gravitational field on the equilibrium structure of the system predicts density profiles exhibiting strong oscillation at the bottom of the pore with a decreasing tail approaching the upper wall. The interplay between the packing effect due to confinement by the walls, gravitational field, and attractive forces among the particles determines the structure of the density profiles. The effect of all these parameters on the distribution of the colloidal particles in the confined environment is investigated.
Orientation-controlled nucleation of crystal silicon grains in amorphous silicon on a rolled nickel tape substrate
Authors: Huh Hwang , Shin Jung H.
Published: July 2002

Crystal silicon grains with grain sizes in excess of 10 μm and orientation control over all three directions were produced by depositing an amorphous silicon thin film on a cold-rolled and annealed nickel tape and annealing at 600 °C for 2 h. The needle-like morphology of the grains indicated that the crystallization was mediated by NiSi2. All grains had their [110] axis about 21° off the surface normal. Furthermore, nearly all of them had the same rotation about the [110] axis except for presence of twins and/or type A–B formations. Despite the use of the nickel substrate, the Ni concentration within the Si film was below the detection limit of energy-dispersive x-ray spectroscopy (1019 cm−3). This low-Ni contamination level is attributed to the presence of an oxide layer between the Ni substrate and the Si film.
Grain boundary transport and vapor sensing in α-sexithiophene
Authors: Schön J. H. , Kloc Ch. , Dodabalapur A. , Crone B.
Published: July 2002

The charge transport through an α-sexithiophene bi-crystal exposed to various gaseous agents is investigated in the temperature range from 4 to 300 K. This method allows a comparison between intragrain and grain boundary effects. The importance of grain boundaries for gas sensing applications is demonstrated. Charged trap states are formed at the grain boundaries, which lead to the change of the potential barrier at the boundary. Consequently, the charge transport properties are affected by the gas exposure.
Atomistic simulations of structural relaxation processes in amorphous silicon
Authors: Ishimaru Manabu
Published: July 2002

Structural relaxation processes in amorphous silicon (a-Si) have been examined by molecular-dynamics (MD) simulations using the Tersoff interatomic potential. The a-Si networks generated by rapid quenching from liquid Si were annealed. Structural changes due to the relaxation of a-Si networks were observed. The present MD simulations reproduce well experimental measurements of changes in radial distribution functions, static structure factors, bond angle distributions, and phonon densities of states due to structural relaxation.
Lattice site location of ion-implanted Li8 in Silicon Carbide
Authors: Virdis S. , Vetter U. , Ronning C. , Kröger H. , Hofsäss H. , Dietrich M. ,   ISOLDE Collaboration,
Published: July 2002

The lattice sites of ion-implanted Li atoms in 6H-, 4H-, and 3C-SiC were studied. Radioactive Li8 ions (t1/2=0.84 s) were implanted with 60 keV into the crystalline SiC samples, and the channeling and blocking effects of 1.6 MeV alpha particles emitted in the decay were measured to determine the Li lattice sites. The alpha emission channeling spectra measured along different crystallographic directions reveal that Li occupies mainly interstitial sites with tetrahedral symmetry, centered along the c-axis atom rows in the hexagonal lattices. In the cubic 3C-SiC structure, Li is located on tetrahedral interstitial sites as well. For 6H-SiC, the implantation temperature was varied between 200 and 823 K without observing significant changes in the emission channeling spectra. Thus, Li diffusion or Li defect interaction resulting in a lattice site change does not occur in this temperature regime.
Global phase diagrams for freezing in porous media
Authors: Radhakrishnan Ravi , Gubbins Keith E. , Sliwinska-Bartkowiak Malgorzata
Published: July 2002

Using molecular simulations and free energy calculations based on Landau theory, we show that freezing/melting behavior of fluids of small molecules in pores of simple geometry can be understood in terms of two main parameters: the pore width H* (expressed as a multiple of the diameter of the fluid molecule) and a parameter α that measures the ratio of the fluid-wall to the fluid–fluid attractive interaction. The value of the α parameter determines the qualitative nature of the freezing behavior, for example, the direction of change in the freezing temperature and the presence or absence of new phases. For slit-shaped pores, larger α values lead to an increase in the freezing temperature of the confined fluid, and to the presence of a hexatic phase. For pores that accommodate three or more layers of adsorbate molecules several kinds of contact layer phase (inhomogeneous phases in which the contact layer has a different structure than the inner layers) are observed. Smaller α values lead to a decrease in the freezing temperature. The parameter H* determines the magnitude of shift in the freezing temperature, and can also affect the presence of some of the new phases. Results are presented as plots of transition temperature vs α for a particular pore width. Experimental results are also presented for a variety of adsorbates in activated carbon fibers (ACF) covering a wide range of α values; the ACF have slit-shaped pores with average pore width 1.2 nm. The experimental and simulation results show qualitative agreement.
Field emission characteristics of BN/GaN structure
Authors: Kimura Chiharu , Yamamoto Tomohide , Hori Takamitsu , Sugino Takashi
Published: July 2002

n -type gallium nitride (GaN) layers grown on sapphire substrates by metalorganic chemical vapor deposition are used to examine field emission characteristics. The electron concentration of the GaN is 2×1017 cm−3. In order to enhance the electric field, the GaN surface is roughened by hydrogen (H2) plasma treatment. Boron nitride (BN) films are grown on the roughened surface of the GaN by plasma-assisted chemical vapor deposition. The turn-on electric field between the anode and sample surface is estimated to be 12.4 and 8.8 V/μm from the field emission characteristics of the roughened GaN and the BN/GaN samples, respectively. It is demonstrated that BN coating is effective in improving the field emission characteristics.
Magnetic well and instability thresholds in the TJ-II stellarator
Authors: Castellano J. , Jiménez J. A. , Hidalgo C. , Pedrosa M. A. , Fraguas A. L. , Pastor I. , Herranz J. , Alejaldre C. ,   TJ-II Team
Published: July 2002

Experimental evidence for the controlled onset of fluctuations by varying the magnetic well in the plasma edge region of the TJ-II stellarator (major radius R=1.5 m, minor radius a<0.22 m, magnetic field B=1 T) has been obtained. The level of fluctuation, the degree of intermittency and the radial correlation of relevant quantities increase as the magnetic well is reduced. TJ-II possesses a high degree of flexibility, making it an ideal device for investigating the characteristics of transport close to instability thresholds.
Ion-source and low-energy beam-transport issues with the front-end systems for the spallation neutron source
Authors: Keller R. , Cheng D. , DiGennaro R. , Gough R. A. , Greer J. , Leung K. N. , Ratti A. , Reijonen J. , Thomae R. W. , Schenkel T. , Staples J. W. , Yourd R. , Aleksandrov A. , Stockli M. P. , Welton R. W.
Published: July 2002

The front-end systems (FES) of the spallation neutron source project are being built by Berkeley Lab and will deliver a pulsed 40 mA H− ion beam at 2.5 MeV energy to the subsequent drift-tube linac. The FES accelerator components comprise a rf driven, volume-production, cesium-enhanced, multicusp ion source; an electrostatic low-energy beam transport (LEBT) that includes provisions for transverse focusing, steering, and beam chopping; a radio-frequency quadrupole accelerator; and a medium-energy beam transport line. The challenges for ion source and LEBT design are the generation of a plasma suitable for creating the required high H− ion density, lifetime of the rf antenna at 6% duty factor, removal of the parasitic electron population from the extracted negative ions, and emittance conservation. The article discusses these issues in detail and highlights key experimental results obtained so far.
Growth-temperature dependence of Er-doped GaN luminescent thin films
Authors: Lee D. S. , Heikenfeld J. , Steckl A. J.
Published: July 2002

Visible photoluminescence (PL) and electroluminescence (EL) emission has been observed from Er-doped GaN thin films grown on (111) Si at various temperatures from 100 to 750 °C in a radio-frequency plasma molecular beam epitaxy system. PL and EL intensities of green emission at 537 nm from GaN:Er films exhibited strong dependence on the growth temperature, with a maximum at 600 °C. Scanning electron and atomic force microscopy showed smooth surfaces at 600 °C and rough surfaces at 100 and 750 °C. X-ray diffraction indicated that the GaN:Er film structure was oriented with the c axis perpendicular to the substrate for all growth temperatures. The crystalline quality initially improves with an increase in growth temperature, and saturates at ∼500 °C. Considering both the luminescence and structural properties of the film, ∼600 °C seems to be the optimal temperature for growth of Er-doped GaN luminescent films on Si substrates.
Influence of the recharging process on the dark current noise in quantum-well infrared photodetectors
Authors: Rehm Robert , Schneider Harald , Walther Martin , Koidl Peter
Published: July 2002

We show that the dark curent noise spectrum of an In0.30Ga0.70As/GaAs quantum-well infrared photodetector (QWIP) is characterized by two plateau-like frequency regions. At high frequencies, the observed noise current is due to the generation–recombination noise of carriers, which have been emitted thermionically from the quantum wells into the continuum. In the low-frequency regime, an additional contribution to the noise current is caused by the redistribution of space charges, that occurs on a time scale similar to the dielectric relaxation time.
X-ray diffraction studies of solutions of lithium in ammonia: The structure of the metal–nonmetal transition
Authors: Hayama S. , Skipper N. T. , Wasse J. C. , Thompson H.
Published: July 2002

The structure of solutions of lithium in ammonia has been studied at 0, 2, 8, and 22 mol % metal (MPM) and 200 K by wide-angle x-ray diffraction. The principal diffraction peak shifts from 2.14(2) Å−1 at 0 MPM to 1.93(3) Å−1 at 22 MPM, reflecting the 30% decrease in overall density as the solution expands to accommodate the excess electrons. We find that the solvent is significantly perturbed over both the short- and intermediate-length scales. The nearest neighbor (N–N) coordination number decreases from 11.8(10) at 0 MPM to 7.6(10) at 22 MPM. In addition, electrostriction around the fourfold coordinated lithium ions causes N–N correlations to become progressively shorter as concentration is increased. At 22 MPM a strong diffraction prepeak is located at 1.05(3) Å−1. Upon dilution to 2 MPM, our experiments find that this feature shifts to 1.29(5) Å−1. We conclude that the prepeak observed in our experiments is a signature of polaronic solvent cavities of approximate radius 2.6 Å. The first solvation shell of an excess electron then contains about 7 ammonia molecules, the second shell about 30 ammonia molecules. This picture is in excellent agreement with interpretation of magnetic resonance data.
Edge turbulence imaging in the Alcator C-Mod tokamak
Authors: Zweben S. J. , Stotler D. P. , Terry J. L. , LaBombard B. , Greenwald M. , Muterspaugh M. , Pitcher C. S. ,   Alcator C-Mod Group, Hallatschek K. , Maqueda R. J. , Rogers B. , Lowrance J. L. , Mastrocola V. J. , Renda G. F.
Published: July 2002

The two-dimensional (2D) radial vs poloidal structure of edge turbulence in the Alcator C-Mod tokamak [I. H. Hutchinson, R. Boivin, P. T. Bonoli et al., Nucl. Fusion 41, 1391 (2001)] was measured using fast cameras and compared with three-dimensional numerical simulations of edge plasma turbulence. The main diagnostic is gas puff imaging, in which the visible Dα emission from a localized D2 gas puff is viewed along a local magnetic field line. The observed Dα fluctuations have a typical radial and poloidal scale of ≈1 cm, and often have strong local maxima (“blobs”) in the scrape-off layer. The motion of this 2D structure motion has also been measured using an ultrafast framing camera with 12 frames taken at 250 000 frames/s. Numerical simulations produce turbulent structures with roughly similar spatial and temporal scales and transport levels as that observed in the experiment; however, some differences are also noted, perhaps requiring diagnostic improvement and/or additional physics in the numerical model.
Thermal, structural, and orientational relaxation of supercooled salol studied by polarization-dependent impulsive stimulated scattering
Authors: Glorieux C. , Nelson K. A. , Hinze G. , Fayer M. D.
Published: July 2002

Time-resolved transient grating experiments with various polarizations are used to separate different responses and measure their dynamics in supercooled liquid salol. A contribution to signal from orientational alignment induced by flow that arises from thermal expansion is demonstrated. This contribution is distinct from that due to orientational alignment induced directly by the excitation light through the molecular polarizability anisotropy (i.e., through the optical Kerr effect). It is also distinct from signal contributions due to density modulations induced by thermal expansion. The results offer additional insight into salol dynamics and into time-dependent transient grating measurements of this class. Depending on the light polarizations used, any of the signal contributions can be eliminated or highlighted.
Production of nanostructures of silicon on silicon by atomic self-organization observed by scanning tunneling microscopy
Authors: Jones D. , Palermo V.
Published: July 2002

The possibility of exploiting the self-organization of mobile silicon atoms on silicon surfaces during ultrahigh vacuum thermal annealing for the construction of silicon-on-silicon structures is demonstrated. Rearrangement of the surface during thermal decomposition of an oxide layer can be controlled allowing the growth of primary structures surrounded by voids which can then be seeded by adsorbed gas molecules for the subsequent growth of secondary structures around the primary one. The controlled growth of these structures could find possible applications in Si-based microelectronic devices.
Cross-section set and chemistry model for the simulation of c-C4F8 plasma discharges
Authors: Font G. I. , Morgan W. L. , Mennenga G.
Published: July 2002

Great interest exists in c-C4F8 (octafluorocyclobutane or perfluorocyclobutane) etching plasma discharges due to their selectivity and potential for decreasing global warming gas emissions. In order to allow computational exploration of the discharge physics, a numerical model for a c-C4F8 discharge has been constructed. A set of cross sections has been assembled for electron collisions with c-C4F8 based on a combination of ab initio calculations, beam measurements, and swarm (i.e., electron transport coefficient) analysis. In addition, a chemical reaction set has been proposed and an axisymmetric numerical model has been used to test the cross section and chemical reaction set against experiments. Results show that measured trends are reproduced and absolute values are well represented. A mechanism is suggested for negative atomic fluorine ion (F−) behavior with respect to power.
Analysis of waves near focus: Method and experimental test
Authors: Bosch Salvador , Ferré-Borrull Josep
Published: July 2002

We present an approach to the analysis of scalar wave focusing. A detailed study is made of an optical system where the basic idea is to define the geometry of the semiperiodic zones within the exit pupil, followed by a numerical integration on these zones. This approach clearly illustrates the nature of the phenomena involved and allows exact computations in focal regions. A practical illustration is presented by considering a meniscus lens system. The method may be readily adapted to a wide range of applications in confocal microscopy, or be used in the analysis of an optical instrument’s resolving power.
Thermodynamic properties of mixtures of H2O and D2O in the critical region
Authors: Abdulkadirova Kh. S. , Wyczalkowska A. Kostrowicka , Anisimov M. A. , Sengers J. V.
Published: July 2002

An accurate equation of state is presented for the thermodynamic properties of mixtures of H2O and D2O in the critical region. The equation incorporates a crossover from fluctuation-induced singular behavior near the locus of vapor–liquid critical points to regular behavior outside the critical region. It is a generalization of a crossover equation-of-state previously developed for pure H2O and pure D2O extended to the mixtures on the basis of the principle of isomorphism of critical behavior. The equation represents the thermodynamic properties of mixtures of H2O and D2O in the same range of temperatures and densities as for the pure components. A comparison is made with available experimental data for the pressure, for the isochoric heat capacity, for the enthalpy of mixing, and for the isotope fraction ratio of mixtures of H2O and D2O. An analysis is presented of the near-critical behavior of various thermodynamic properties, such as the isobaric heat capacity, the partial molar volumes, Henry’s constant, and the vapor–liquid distribution ratio (K factor).
Domain wall motion in nanowires using moving grids (invited)
Authors: Forster H. , Schrefl T. , Suess D. , Scholz W. , Tsiantos V. , Dittrich R. , Fidler J.
Published: July 2002

The magnetization reversal process of Co nanowires was investigated using a moving mesh technique. The nucleation and expansion of reversed domains is calculated by solving the Gilbert equation of motion for different damping constants. The adaptive finite element method reduces the total CPU time by more than a factor of 4 as compared to a uniform mesh. Two different domain wall types are observed. For a wire diameter of d=10 nm transverse walls occur and gyromagnetic precession limits the domain wall velocity. The domain wall velocity increases from 50 to 520 m/s as the Gilbert damping constant increases from α=0.05 to α=1 at an applied field of 500 kA/m. For a diameter greater than 20 nm vortex walls are formed. The vortex mobility increases with decreasing damping constant. Thus velocities up to 2000 m/s are reached for a wire diameter of 40 nm, α=0.05, and an applied field of 250 kA/m.
Latching type optical switch
Authors: Didosyan Y. S. , Hauser H. , Fiala W. , Nicolics J. , Toriser W.
Published: July 2002

An optical switch on yttrium orthoferrite crystal is developed. The switch is of latching type and has an operating time of 100 ns. Matrices of switches are feasible due to the dimensions of the optical rotator of less than 2 mm3.
Effects of intentionally introduced hydrogen on the electrical properties of ZnO layers grown by metalorganic chemical vapor deposition
Authors: Theys Bertrand , Sallet Vincent , Jomard François , Lusson Alain , Rommeluère Jean-François , Teukam Zéphyrin
Published: July 2002

Unintentionally doped ZnO layers grown epitaxially on a sapphire substrate have been exposed either to a hydrogen or deuterium plasma. Secondary ion mass spectroscopy measurements performed subsequently showed a rapid diffusion of hydrogen in these layers. Furthermore, the presence of hydrogen in the ZnO samples is found to be responsible for nearly a factor of 3 increase in the free electron concentration. This effect is attributed to the hydrogen passivation of compensating acceptor impurities present in the as-grown ZnO layers.
Study of organic light emitting devices with a 5,6,11,12-tetraphenylnaphthacene (rubrene)-doped hole transport layer
Authors: Aziz Hany , Popovic Zoran D.
Published: July 2002

We investigated the stability of an organic light emitting device (OLED) with structure of indium–tin–oxide (ITO) anode/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine hole transport layer (HTL)/tris(8-hydroxyquinoline) aluminum (AlQ3) electron transport layer/Mg:Ag cathode, in which different portions of the HTL were doped with 5,6,11,12-tetraphenylnaphthacene (rubrene). Compared to undoped devices, the stability of OLEDs in which HTL doping was limited to only a thin interfacial layer at either the ITO or AlQ3 interface was essentially the same, whereas the stability of OLEDs in which a substantial portion of the HTL was doped was about an order of magnitude higher, and approached that of devices in which the whole HTL was doped. The color of the emission depended only on the material in immediate contact with AlQ3. The results demonstrate that increasing the OLED’s stability by means of doping the HTL is associated with changes in bulk HTL properties rather than interfacial properties, and is consistent with the OLED degradation mechanism based on the instability of AlQ3 cationic species.
Efficient polymer light-emitting diodes using conjugated polymer blends
Authors: He Gufeng , Liu Jie , Li Yongfang , Yang Yang
Published: July 2002

Photoluminescence studies have been performed on poly [2,5-bis(cheolestranoxy)-1,4-phenylene vinylene] (BCHA) blends with poly(9,9-dioctylfluorene) (PF). The composition-independent, distinctive emission of BCHA in the BCHA/PF blends suggests the nonexistence of interchain species in the blends, due to polymer chain separation by bulky side groups. The electroluminescence (EL) efficiencies of devices made with BCHA/PF blends were also found to be superior to the BCHA-only devices. For example, the EL efficiency of the BCHA (10%)/PF device was 3.0 cd/A, which is about five times higher than a BCHA device. The basis for this enhancement has been attributed to improved charge injection and charge trapping in BCHA/PF blends. Holes are injected in these blends through the PF component and then transferred and trapped in BCHA, where radiative recombination subsequently occurs. As a result of improved minority-carrier injection, the turn-on voltages of the BCHA/PF devices are also much lower than the BCHA devices.
Phase separation of a binary fluid in the presence of immobile particles: A lattice Boltzmann approach
Authors: Suppa Domenico , Kuksenok Olga , Balazs Anna C. , Yeomans J. M.
Published: July 2002

Using a lattice Boltzmann model, the phase separation of a binary fluid in the presence of immobile, penetrable particles is studied in two dimensions. The particles are preferentially wetted by one of the fluid components. At early times, the hydrodynamic flow promotes the growth of the fluid domains. At later times, the domains are pinned to a finite size if there is a sufficiently strong interaction between the particles and the compatible fluid. The final size of the domains depends on the specific strength of the particle–fluid interaction and on the particle concentration. These results indicate that the domain size can be tailored by varying the chemical nature and the number of the particles.
Molar volume dependence of the thermal conductivity in mixed cryocrystals
Authors: Konstantinov V. A. , Orel E. S. , Revyakin V. P.
Published: July 2002

The dependence of the thermal conductivity Λ of solid solutions of cryocrystals on the molar volume is investigated theoretically in the Callaway approach under the assumption that the heat is transferred by mobile low-frequency phonons and, above the phonon mobility edge, by “localized” modes which migrate randomly from site to site. The phonon mobility edge ω0 is found from the condition that the phonon mean free path, which is governed by umklapp processes and scattering on point defects, cannot become smaller than one-half the phonon wavelength. The Bridgman coefficient g=−(∂ ln Λ/∂ ln V)T is the weighted mean over these modes, which have strongly different volume dependences. For the example of the solid solution Kr1−c(CH4)c, where c is the mole fraction of the components, it is shown that with increasing methane concentration in krypton the Bridgman coefficient decreases, in good agreement with experiment, from g≈9, characteristic for the pure crystals, to g≈4.
Self-consistent-field theory for interacting polymeric assemblies. II. Steric stabilization of colloidal particles
Authors: Roan Jiunn-Ren , Kawakatsu Toshihiro
Published: July 2002

The self-consistent-field (SCF) theory developed in Part I [J. Chem. Phys. 116, 7283 (2002), preceding paper] is employed to compute the interaction between particles coated by end-grafted homopolymers in good solvent, where the particles and the homopolymers have comparable sizes. The result shows that, contrary to the prediction of the conventional theory for colloidal stabilization and previous SCF studies, the interaction is attractive, repulsive, and attractive at large, intermediate, and small distances, respectively, for densely grafted particles, while it is purely attractive for sparsely grafted particles. The attractive interaction is a consequence of two important factors that were ignored in previous studies: (i) the sphere–sphere geometry of the system and (ii) the segment density associated with individual particle being deformed anisotropically, with respect to the particle, under the perturbation of other particles. We argue that the conventional wisdom that end-grafted homopolymers in good solvent always impart stability indeed is correct only in a kinetic sense and that our result will become more observable in systems composed of nanoparticles. Limitations of our prediction and considerations that must be carefully taken into account when generalizing our result to micron-sized particles and star polymers are discussed.
Gas-phase thermodynamic models of nitrogen-induced nanocrystallinity in chemical vapor-deposited diamond
Authors: Corvin Raymond B. , Harrison Joseph G. , Catledge Shane A. , Vohra Yogesh K.
Published: July 2002

Gas-phase thermodynamic equilibrium calculations involving H2/CH4/N2 mixtures were performed to investigate the chemical interactions leading to nitrogen-induced nanocrystallinity in microwave plasma chemical vapor deposition of diamond films. The strong influence of the CN radical in causing nanocrystallinity is confirmed by the correlation of its modeled composition in the gas phase with the degree of nanocrystallinity as determined experimentally for diamond films grown with different N2 additions. For a given CH4 feedgas concentration, there exists a critical N2 feedgas concentration, above which the change in the CH3/CN ratio is minimal and further induced nanocrystallinity is diminished. This is verified experimentally where it is observed that the same critical N2 feedgas concentration exists, above which a further decrease in diamond crystallinity and surface roughness of the grown diamond films is minimal.
Optical thin films consisting of nanoscale laminated layers
Authors: Zaitsu Shin-ichi , Jitsuno Takahisa , Nakatsuka Masahiro , Yamanaka Tatsuhiko , Motokoshi Shinji
Published: July 2002

The control of the refractive index of laminated coatings consisting of alternating stacks of nanoscale Al2O3 and TiO2 sublayers grown by atomic layer deposition has been achieved. The refractive index of the coating linearly changed from 1.870 to 2.318 as the thickness of the single TiO2 sublayer was varied from 2.0 to 39 Å while that of the single Al2O3 sublayer was kept constant at 5.5 Å. The refractive index could be varied by adjusting only the number of growth cycles of each material. This approach will have potential applications to optical multilayer coatings consisting of well-controlled extremely thin layers.
Nanopatterning of alkyl monolayers covalently bound to Si(111) with an atomic force microscope
Authors: Ara Masato , Graaf Harald , Tada Hirokazu
Published: July 2002

Alkyl monolayers covalently bound to silicon were prepared through the reaction between 1-alkene molecules and hydrogen-terminated Si. The surfaces were anodized in nanometer scale with a contact-mode atomic force microscope (AFM) by applying positive bias voltage to the surface with respect to a conducting cantilever under ambient conditions. Following the anodization, patterned areas were selectively modified by chemical etching and coating with different molecules. The alkyl monolayers showed high resistance against chemical etching and protected Si surfaces from oxidation. AFM lithography of monolayers on Si was found to be useful for nanofabrication of organic/inorganic interfaces based on the Si–C covalent bond.
Microscopic surface photovoltage spectroscopy
Authors: Saraf S. , Shikler R. , Yang J. , Rosenwaks Y.
Published: July 2002

We present a microscopic surface photovoltage spectroscopy method. It is based on a tunable illumination system combined with a kelvin probe force microscope, which measures the contact potential difference between a sample surface and a tip of an atomic force microscope. By measuring the contact potential difference as a function of illumination wavelength, the whole surface photovoltage spectrum of a semiconductor sample is obtained with submicrometer spatial resolution. This resolution can be as high as 100 nm, in regions where the minority carrier transport is controlled by drift rather than by diffusion.
The Meyer–Neldel rule for a property determined by two transport mechanisms
Authors: Widenhorn Ralf , Rest Armin , Bodegom Erik
Published: July 2002

We propose that the Meyer–Neldel rule (MNR) arises naturally for a quantity where both an intrinsic process as well as a process involving impurities contribute. The strength of the latter depends solely on the density of the impurities. This leads to a spread in the apparent activation energy of the measured quantity and the observation of the MNR, even though the intrinsic processes have fixed activation energies. A consequence of the MNR is the occurrence of a temperature TMN where a measured parameter is independent of the activation energy. For the system studied, the MNR does not accurately predict the results at temperatures larger than TMN. Our model for the MNR is supported by experimental data and it also can explain the inverse MNR for low activation energies.
Longitudinal Excitons in GaN
Authors: Reynolds D. C. , Jogai B. , Collins T. C.
Published: July 2002

Many of the previous investigations of longitudinal excitons have involved reflection and absorption measurements rather than emission. In these measurements it is more difficult to resolve the longitudinal exciton from the Γ5 and Γ6 free excitons in wurtzite material. The longitudinal excitons have energies and oscillator strengths that depend on the direction of propagation and they are not observable along the principal axis of the crystal. In the wurtzite structure, such as GaN, the Γ5 exciton is the pure transverse mode, whereas the longitudinal is a mixed mode going from pure longitudinal, for the propogation direction K perpendicular to C, to pure transverse for K parallel to C. If more than one orientation is present in the sample, it is clear that more than one longitudinal exciton may be seen since it is a mixed mode. In the current experiment we observe more than one mode, which we associate with more than one crystal orientation. This may result from the columnar growth often observed in GaN.
Extremely high room-temperature two-dimensional hole gas mobility in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures
Authors: Myronov M. , Irisawa T. , Mironov O. A. , Koh S. , Shiraki Y. , Whall T. E. , Parker E. H. C.
Published: July 2002

To extract the room-temperature drift mobility and sheet carrier density of two-dimensional hole gas (2DHG) that form in Ge strained channels of various thicknesses in Ge/Si0.33Ge0.67/Si(001) p-type modulation-doped heterostructures, the magnetic field dependences of the magnetoresistance and Hall resistance at temperature of 295 K were measured and the technique of maximum entropy mobility spectrum analysis was applied. This technique allows a unique determination of mobility and sheet carrier density of each group of carriers present in parallel conducting multilayers semiconductor heterostructures. Extremely high room-temperature drift mobility (at sheet carrier density) of 2DHG 2940 cm2 V−1 s−1 (5.11×1011 cm−2) was obtained in a sample with a 20 nm thick Ge strained channel.
Effect of nonmesogenic solute on the nematic-smectic-A phase transition
Authors: Mukherjee Prabir K.
Published: July 2002

The effect of nonmesogenic solute on the nematic-smectic-A (NA) transition is examined within the Landau phenomenological theory. When nonmesogenic solute are added to a mesogenic compound consisting of both nematic and smectic-A phases, the NA transition temperature is depressed and a two phase region is formed due to the presence of the impurities of the solute. Decreasing the nonmesogenic solute to the pure compound, the first order NA transition becomes second order at the tricritical point in Landau approximation. The effects of nonmesogenic solute on the Frank elastic constants are also discussed.
Time-resolved photoluminescence studies of free excitons in CuInS2 crystals
Authors: Wakita Kazuki , Nishi Kazuhito , Ohta Yoshihiko , Nakayama Naoji
Published: July 2002

The time-resolved photoluminescence of free excitons in bulk single-crystal CuInS2 grown by the traveling heater method is examined. The decay of free exciton emission exhibits a double exponential curve at low temperature. The decay-time constant of the fast component increases monotonically with excitation density, whereas that of the slow component appears to be independent of excitation density. From this result, the fast and slow components are attributed to nonradiative and radiative recombination processes, respectively. The radiative lifetime of free excitons is estimated to be 320±30 ps at 10 K.
Fourier transform millimeter-wave spectroscopy of the deuterated vinyl radical, C2D3
Authors: Kim Eunsook , Yamamoto Satoshi
Published: July 2002

The 101–000 rotational transition of the C2D3 radical in the ground electronic state has been detected for the first time with the Fourier transform millimeter-wave spectrometer. The C2D3 radical is produced by discharging the C2D3Br gas diluted in Ar. Thirty-two fine and hyperfine components of the 101–000 transition are measured in the frequency region around 44.4 GHz. The rotational constant (B+C)/2, the spin-rotation interaction constant, and the hyperfine interaction constants for the s and a states split by the tunneling motion associated with the CDα rocking mode are determined by a least-squares analysis. The energy splitting due to the tunneling motion is discussed on the basis of the hyperfine interactions. The angle between the CDα bond and the a-axis is estimated to be 148.5° from the nuclear quadrupole interaction constant χaa.
All YBa2Cu3O7−δ trilayer junctions with YBa2Cu3O7−δ wiring layers
Authors: Fujimoto E. , Sato H. , Yamada T. , Akoh H.
Published: July 2002

We have fabricated all YBa2Cu3O7−δ (YBaCuO) trilayer junctions with YBaCuO wiring layers using CeO2 insulating layers. The trilayer junctions consist of YBaCuO/PrBa2Cu3O7−δ (PrBaCuO)/YBaCuO structures on (100) MgO substrate. Both YBaCuO wiring layer and base-YBaCuO electrode have the critical temperature Tc of ⩾87 K. A 5×5-μm2 junction with a PrBaCuO barrier thickness of 25 nm shows a resistively-shunted-junction (RSJ)-like current–voltage (I–V) characteristic with a critical current density Jc of 1.7×102 A/cm2 and a critical current×normal resistance (IcRn) product of 0.40 mV, and no contact resistance at 4.2 K. The magnetic field dependence of Ic for the junction shows a Fraunhofer interference pattern with a modulation period of 3.5 mT, which is in good agreement with the value estimated from the junction size of 5 μm. In addition, the YBaCuO wiring layers have no structures in the I–V characteristics due to grain–boundary junctions. These facts confirm that the observed Josephson properties come from the YBaCuO/PrBaCuO/YBaCuO trilayer structure.