bluuebunny/crossref_metadata_2025 huggingface dataset:
Doping in quantum cascade lasers. I. InAlAs–InGaAs∕InP midinfrared devices
Authors: Aellen Thierry , Beck Mattias , Hoyler Nicolas , Giovannini Marcella , Faist Jérôme , Gini Emilio
Published: September 2006

The effect of the doping densities on the performance of 9μm InAlAs–InGaAs∕InP quantum cascade lasers is presented. Doping densities varying between 1.0×1011 and 2.6×1011cm−2 were investigated. In this range, a linear increase in both threshold and maximum current density with sheet carrier density is observed. These effects are explained using a model based on resonant tunneling transport and rate equations.
Investigations of the effects of tempering and composition dependence on charge carrier field effect mobilities in polymer and fullerene films and blends
Authors: von Hauff Elizabeth , Parisi Jürgen , Dyakonov Vladimir
Published: September 2006

In this study we report on field effect mobilities of charge carriers in materials used in polymer photovoltaics. The field effect mobilities of charge carriers in polymer and fullerene films and in various blend compositions were investigated for the effects of tempering. Contact resistances were considered for more accurate mobility estimations. The field effect mobilities of holes in films of the conjugated polymer poly(3-hexylthiophene) (P3HT) were found to be in the 10−3cm2∕Vs range and the field effect mobilities of electrons in films of the methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were found to be in the order of 10−2cm2∕Vs. The concentration of PCBM was then varied in P3HT:PCBM blends, and blend compositions with PCBM contents of 0%, 33%, 40%, 50%, 60%, 67%, and 100% weight percents were measured before and after tempering. Electron mobilities were not detected in untempered blend compositions with PCBM content lower than 67% PCBM. Tempering led to a large increase in the electron mobility in the blends, and electron mobilities were detected in all tempered blends with PCBM content above 40%. Electron and hole field effect mobilities were found to be roughly balanced in the 10−3cm2∕Vs range for the tempered blend with 67% PCBM content.
Self-modification of the remanence distribution of a hollow cylindrical permanent magnet array
Authors: Xu X. N. , Lu D. W. , Yuan G. Q. , Zang W. C. , Jin X.
Published: September 2006

A hollow cylindrical permanent magnet array (HCPMA) used for refrigeration generates a strong magnetic field in the access hole and also inside the permanent magnets. The generated field modifies the remanence distribution of the magnets. Here, we describe a self-consistent calculation method for the remanence vectors and magnetic inductions using the Biot-Savart law. The method takes into account the anisotropic magnetization loops of commercial permanent magnets. Results obtained for the modified remanence distribution of a long HCPMA show demagnetization and deflection, including the reversal of original remanence vectors, when the generated field of the HCPMA exceeds the coercivity of the permanent magnets. The reversed remanence vectors lead to the formation of remanence vortices. In comparison with the field obtained from the ideal model assuming constant remanence vectors, remanence demagnetization and deflection result in obvious decreases in both the field magnitude and field homogeneity in the access hole. However, the field distribution in the access hole can be improved by using the magnet segments of specially high coercivity in places where large inverse or transverse fields exist. The method reported here is applicable for studying the magnetic fields, including the demagnetization, saturation, deflection, and rotation of the remanence vectors, of other permanent magnet arrays.
Rolling-induced texturing in metal-clad MgB2 tapes and magnetoresistivity anisotropy
Authors: Beilin V. , Lapides I. , Roth M. , Dul’kin E. , Mojaev E. , Gerber A. , Riss O.
Published: September 2006

Metal-clad MgB2 tapes with Cu, Ni, Fe, and stainless steel sheaths, fabricated by the powder-in-tube method, have been studied using x-ray diffraction and magnetoresistance measurements. Tapes subjected to different mechanical and thermal processings have been used to probe the ab-plane texturing. Only moderate rolling-induced texturing has been observed experimentally, with a maximal texture factor, ΔF00l, of about 0.22. ΔF00l is found to be dependent on both the sheath material and tape processing prehistory. Electrical resistivity measurements in high magnetic fields (parallel and perpendicular to the tape plane) show that even poor texturing, with ΔF00l=0.065, may result in a significant anisotropy of magnetoresistance. The anisotropy of the upper critical field, Bc2, has been derived from the experimental texturing and magnetoresistance data, with the anisotropy factor of the order 5 at 4.2K. It is shown that a maximal magnetic field shift of the resistively probed superconducting transition associated with the tape core texturing may reach 4.5T at 4.2K.
Poling-assisted bleaching of soda-lime float glasses containing silver nanoparticles with a decreasing filling factor across the depth
Authors: Deparis Olivier , Kazansky Peter G. , Podlipensky Alexander , Abdolvand Amin , Seifert Gerhard , Graener Heinrich
Published: September 2006

The recently discovered poling-assisted bleaching of glass with embedded silver nanoparticles has renewed the interest in thermal poling as a simple, reliable, and low-cost technique for controlling locally the surface-plasmon-resonant optical properties of metal-doped nanocomposite glasses. In the present study, the emphasis is put on the influence of the volume filling factor of metallic clusters on poling-assisted bleaching. Soda-lime silicate glass samples containing spherical silver nanoparticles with a decreasing filling factor across the depth were subject to thermal poling experiments with various poling temperatures, voltages, and times. Optical extinction spectra were measured from ultraviolet to near-infrared ranges and the surface-plasmon-resonant extinction due to silver nanoparticles (around 410nm) was modeled by the Maxwell Garnett [Philos. Trans. R. Soc. London, Ser. A 203, 385 (1904); 205, 237 (1906)] effective medium theory which was adapted in order to take into account the filling factor depth profile. A method was proposed for the retrieval of the filling factor depth profile from optical extinction spectra recorded in fresh and chemically etched samples. A stretched exponential depth profile turned out to be necessary in order to model samples having a high filling factor near the surface. Based on the fact that the electric-field-assisted dissolution of embedded metallic nanoparticles proceeded progressively from the top surface, a bleaching front was defined that moved forward in depth as time elapsed. The position of the bleaching front was determined after each poling experiment by fitting the measured extinction spectrum to the theoretical one. In samples with higher peak value and steeper gradient of the filling factor, the bleaching front reached more rapidly a steady-state depth as poling time increased. Also it increased less strongly with increasing poling voltage. These results were in agreement with the physics of the dissolution process. Finally, clear evidence of injection of hydrogenated ionic species from the atmosphere into the sample during poling was obtained from the growth of the infrared extinction peak associated with OH radicals.
Dynamic photothermal and thermoelastic microscopies described by complex finite element analysis
Authors: Cavallier Bruno , Ballandras Sylvain , Cretin Bernard , Vairac Pascal
Published: September 2006

Photothermal and thermoelastic microscopes are nondestructive apparatus that generally work with lock-in detection. The related magnitude and phase images are obtained for one modulation frequency. These techniques, devoted to nondestructive evaluation, are suitable for inverse problems such as thermoelastic parameter reconstruction. But up to now the thermoelastic models have been limited to simple geometries or have encountered many problems with temporal sampling. In order to overcome these problems we develop a finite element analysis dynamic method, based upon complex analysis, that enables us to directly obtain both thermal and thermoelastic magnitude and phase fields. This method has been applied to thermoelastic microscopy and has shown very good agreement with experiments.
Structural, ferroelectric, and dielectric properties of vanadium-doped Bi4−x∕3Ti3−xVxO12
Authors: Mao X. Y. , He J. H. , Zhu J. , Chen X. B.
Published: September 2006

The microstructure, ferroelectric, and dielectric properties of vanadium-doped Bi4Ti3O12 ceramics have been investigated. V substitution is found to cause a transition from an orthorhombic phase to a tetragonal phase at x∼0.03, and again to an orthorhombic phase at higher V content. The ferroelectric properties of Bi4Ti3O12 were significantly improved by V doping. The 2Pr of Bi4Ti3O12 is 16μC∕cm2, and it reaches a maximum value of 26.4μC∕cm2 when the V content is 0.03. The two relaxation peaks (PI, PII) are observed in the dielectric loss (D) curves for all of the samples. The PI and PII peaks related to oxygen vacancies tend to decrease with V doping, which implies the decreasing of the oxygen vacancy concentration caused by V doping and favors the improvement of 2Pr and 2Ec. On the contrary, Raman spectra reveal the occurrence of Ti vacancies when V content is more than x=0.01, which may be responsible for the variation of the microstructure and the deterioration of 2Pr and 2Ec. The ferroelectricity of V doping Bi4−x∕3Ti3−xVxO12 ceramics is therefore likely dominated by these two competing mechanisms, with the optimal ferroelectric properties appearing at V content x=0.03.
Rotation-translation device for condensed-phase spectroscopy with small sample volumes
Authors: Nuernberger Patrick , Krampert Gerhard , Brixner Tobias , Vogt Gerhard
Published: September 2006

We present and characterize an experimental device for optical spectroscopy with small sample volumes contained in a thin film. Employing rotational and translational motion, the sample transport speeds are high enough to offer a new sample volume for each interaction in time-resolved spectroscopy experiments working with a 1kHz repetition rate. This is especially suited for ultrafast femtosecond spectroscopy such as transient absorption spectroscopy or fluorescence upconversion. To reduce photodegradation and effects from local thermal heating, a large sample area is scanned in contrast to conventional devices with either only rotation or translation movements. For characterization of the setup, transient absorption experiments are carried out using the rotation-translation device and a conventional flow-cell setup, which exhibit similar signal-to-noise ratio in the two cases. The effects of photodegradation and diffusion are also investigated, demonstrating the suitability of the device for time-resolved spectroscopic experiments. The transient absorption data show that the setup is well suited for biomolecular samples, which are often only available in small amounts and are very sensitive to thermal heating.
Rheo-optical determination of flow birefringence and flow dichroism with the pulsed laser method
Authors: Clasen Christian , Kulicke W.-M.
Published: September 2006

Rheo-optical measurements of the flow birefringence, flow dichroism and their respective orientations to the flow direction allow for a detailed direct investigation not only of the configuration of polymers in a flow field but also of the behavior of associated structures or multiphase systems under flow. However, the standard setup of an optical train, using phase modulation for the rheo-optical investigation of a chosen flow field, is a rather expensive, complex and delicate structure that needs careful adjustment and calibration. This article presents a new, simple and robust design of a measurement unit, consisting of several pulsed lasers, to determine the desired rheo-optical material functions. The setup uses three laser beams with different polarization planes that pass in the same path through a fluid sample. Modulation patterns that alternately operate only single lasers in the kHz regime and the subsequent determination of intensity changes caused by the anisotropy of the sample allow for the calculation of the flow birefringence or dichroism. Comparison of experimental results to conventional methods show a lowest resolution limit of Δn′∼3.5×10−8 for the birefringence, determined by the signal-to-noise ratio of the pulsed laser system.
Vapor-liquid-solid mechanisms: Challenges for nanosized quantum cluster/dot/wire materials
Authors: Cheyssac P. , Sacilotti M. , Patriarche G.
Published: September 2006

The growth mechanism model of a nanoscaled material is a critical step that has to be refined for a better understanding of a nanostructure’s dot/wire fabrication. To do so, the growth mechanism will be discussed in this paper and the influence of the size of the metallic nanocluster starting point, referred to later as “size effect,” will be studied. Among many of the so-called size effects, a tremendous decrease of the melting point of the metallic nanocluster changes the physical properties as well as the physical/mechanical interactions inside the growing structure composed of a metallic dot on top of a column. The thermodynamic size effect is related to the bending or curvature of chains of atoms, giving rise to the weakening of bonds between them; this size or curvature effect is described and approached to crystal nanodot/wire growth. We will describe this effect as that of a “cooking machine” when the number of atoms decreases from ∼1023at.∕cm3 for a bulk material to a few tens of them in a 1–2nm diameter sphere. The decrease of the number of atoms in a metallic cluster from such an enormous quantity is accompanied by a lowering of the melting temperature that extends from 200 up to 1000K, depending on the metallic material and its size under study. In this respect, the vapor-liquid-solid (VLS) model, which is the most utilized growth mechanism for quantum nanowires and nanodots, is critically exposed to size or curvature effects (CEs). More precisely, interactions in the vicinity of the growth regions should be reexamined. Some results illustrating the growth of micrometer-/nanometer-sized materials are presented in order to corroborate the CE/VLS models utilized by many research groups in today’s nanosciences world. Examples of metallic clusters and semiconducting wires will be presented. The results and comments presented in this paper can be seen as a challenge to be overcome. From them, we expect that in a near future an improved model can be exposed to the scientific community.
Effect of slip on existence, uniqueness, and behavior of similarity solutions for steady incompressible laminar flow in porous tubes and channels
Authors: Chellam Shankararaman , Liu Mei
Published: September 2006

The existence and multiplicity of similarity solutions for steady, fully developed, incompressible laminar flow in uniformly porous tubes and channels with one or two permeable walls is investigated from first principles. A fourth-order ordinary differential equation obtained by simplifying the Navier-Stokes equations by introducing Berman’s stream function [A. S. Berman, J. Appl. Phys. 24, 1232 (1953)] and Terrill’s transformation [R. M. Terrill, Aeronaut. Q. 15, 299 (1964)] is probed analytically. In this work that considers only symmetric flows for symmetric ducts; the no-slip boundary condition at porous walls is relaxed to account for momentum transfer within the porous walls. By employing the Saffman [P. G. Saffman, Stud. Appl. Math. 50, 93 (1971)] form of the slip boundary condition, the uniqueness of similarity solutions is investigated theoretically in terms of the signs of the guesses for the missing initial conditions. Solutions were obtained for all wall Reynolds numbers for channel flows whereas no solutions existed for intermediate values for tube flows. Introducing slip did not fundamentally change the number or the character of solutions corresponding to different sections. However, the range of wall Reynolds numbers for which similarity solutions are theoretically impossible in tube flows was found to be a weak function of the slip coefficient. Slip also weakly influenced the transition wall Reynolds number corresponding to flow in the direction of a favorable axial pressure gradient to one in the direction of an adverse pressure gradient. Momentum transfer from the longitudinal axis to the walls appears to occur more efficiently in porous channels compared to porous tubes even in the presence of slip.
Drag reduction of a bluff body using adaptive control methods
Authors: Beaudoin Jean-François , Cadot Olivier , Aider Jean-Luc , Wesfreid José-Eduardo
Published: September 2006

A classical actuator is used to control the drag exerted on a bluff body at large Reynolds number (Re=20000). The geometry is similar to a backward-facing step whose separation point is modified using a rotating cylinder at the edge. The slow fluctuations of the total drag are directly measured by means of strain gauges. As shown by visualizations, the actuator delays the separation point. The size of the low-pressure region behind the body is decreased and the drag reduced. It is found that the faster the rotation of the cylinder, the lower the drag. In a first study, the goal of the control is for the system to reach a drag consign predetermined by the experimentalist. The control loop is closed with a proportional integral correction. This adaptive method is shown to be efficient and robust in spite of the large fluctuations of the drag. In the second method, the system finds itself its optimal set point. It is defined as the lowest cost of global energy consumption of the system (drag reduction versus energy used by the actuator). For this purpose, an extremum seeking control method is applied in order to deal with the large background noise due to turbulence. It consists in a synchronous detection of the response measured in the drag measurements to a modulation of the actuator. The phase shift and amplitude of the modulation estimate the local gradient of the total energy function. With this gradient estimation, the system goes to the minimum of global power consumption by itself. The system is found to be also robust and reacts successfully to changes of the external mean flow. This experiment attests to the real efficiency of local active control in reducing autonomously the global energy consumption of a system under turbulent flow.
Measurement of the electrical and mechanical responses of a force transducer against impact forces
Authors: Fujii Yusaku
Published: September 2006

A method for measuring the electrical and mechanical responses of force transducers to impact loads is proposed. The levitation mass method (LMM) is used to generate and measure the reference impact force used. In the LMM, a mass that is levitated using an aerostatic linear bearing (and hence encounters negligible friction) is made to collide with the force transducer under test, and the force acting on the mass is measured using an optical interferometer. The electrical response is evaluated by comparing the output signal of the force transducer with the inertial force of the mass as measured using the optical interferometer. Simultaneously, the mechanical response is evaluated by comparing the displacement of the sensing point of the transducer, which is measured using another optical interferometer, with the inertial force of the mass. To demonstrate the efficiency of the proposed method, the impact responses of a force transducer are accurately determined.
Gravity-driven film flow with variable physical properties
Authors: Andersson Helge I. , Santra Bidyut , Dandapat Bhabani S.
Published: September 2006

The laminar flow and heat transfer in an accelerating thin liquid film are considered with the view to examine the influence of variable density and transport properties. A new similarity transformation is proposed which exactly transforms the hydrodynamic and thermal boundary layer equations for vertically falling film flow into a coupled set of ordinary differential equations. The resulting two-point boundary value problem is integrated numerically with empirical data for the physical properties of water. For given inflow conditions, the temperature-dependency of the dynamic fluid viscosity makes both the hydrodynamic and thermal boundary layers thinner with increasing wall temperature. The expected thickening of the thermal boundary layer due to the increasing thermal diffusivity is therefore more than outweighed by the decreasing viscosity. The nonlinear variation of the physical properties makes these effects more pronounced at the lower inflow temperature.
Modeling the crystal-field splitting of energy levels of Er3+(4f11) in charge-compensated sites of KPb2Cl5
Authors: Gruber John B. , Yow Raylon M. , Nijjar Anmol S. , Russell Charles C. , Sardar Dhiraj K. , Zandi Bahram , Burger Arnold , Roy Utpal N.
Published: September 2006

A point-charge lattice-sum model corrected for induced multipoles is used to investigate the crystal-field splitting of the LJ2S+1 energy levels of Er3+(4f11) ions that occupy charge-compensated sites in the laser host crystal KPb2Cl5. Spectroscopic data are reported and analyzed for Er3+ between 1550 and 440nm at cryogenic temperatures. The crystal-field (Stark) splitting of the ground-state manifold I15∕24 and the splitting of individual excited manifolds of Er3+ are established from analyses of temperature-dependent (hot band) absorption spectra. The analyses confirm the Stark splitting of the IJ4 and F9∕24 manifolds that are reported in the literature. From an analysis of the data, it appears that only one of the two possible Pb2+ sites serving as a charge-compensated site for Er3+ is involved in the optical activity of the Er3+ ions. From an examination of the crystallographic data of KPb2Cl5, we identify a possible site for the optically active Er3+ ion in the lattice and calculate the lattice-sum components for that site. These components are used to evaluate the crystal-field splitting of the energy levels of Er3+ at that site. The charge-compensation model assumes that Er3+ substitutes for Pb2+ in the Pb(2) site with a vacancy in a nearby K+ site. To obtain the best overall agreement between calculated and experimental levels, the position of the Er3+ ion is treated as an adjustable parameter along the quantization axis. Individual manifold centroids of the Er3+ ion are also varied. From a least-squares fitting analysis, we obtain a rms deviation of 10cm−1 between 48 calculated-to-observed Stark levels.
Absence of ferromagnetism in Co-doped ZnO prepared by thermal diffusion of Co atoms
Authors: Zhang Z. , Chen Q. , Lee H. D. , Xue Y. Y. , Sun Y. Y. , Chen H. , Chen F. , Chu Wei-Kan
Published: September 2006

Thermal diffusion was used to dope Co atoms into ZnO single crystal. Particle-induced x-ray emission combined with channeling technique, x-ray diffraction, and ultraviolet-visible light absorption show that Co-diffused ZnO is pure single-phase alloy. The temperature variation of magnetization can be fitted by a linear combination of the Curie law and the Curie-Weiss law with negative Curie-Weiss temperature (−175K). In the high temperature region, the Co-doped ZnO layer is paramagnetic. In the low temperature region, antiferromagnetism and paramagnetism coexist. Evidence of ferromagnetic hysteresis behavior was not observed down to 5K.
Growth of InP self-assembled quantum dots on strained and strain-relaxed Inx(Al0.6Ga0.4)1−xP matrices by metal-organic chemical vapor deposition
Authors: Zhang X. B. , Heller R. D. , Ryou J. H. , Dupuis R. D. , Walter G. , Holonyak N.
Published: September 2006

InP self-assembled quantum dots (QDs) were deposited on Inx(Al0.6Ga0.4)1−xP matrices grown on GaAs (001) substrates by metal-organic chemical vapor deposition. By adjusting the indium concentration in the Inx(Al0.6Ga0.4)1−xP matrices, the effects of strain and its relaxation in the matrix on the cathodoluminescence (CL) properties and morphology of grown QDs were studied. We found that the CL intensity of the InP QDs increases with an increase in the indium composition in the Inx(Al0.6Ga0.4)1−xP matrix. For QDs grown on strain-relaxed matrices, CL imaging studies show that the CL of QDs close to dislocation lines in tensile-strained Inx(Al0.6Ga0.4)1−xP matrices is much weaker than in the surrounding strained areas, while in the compressively strained matrices, the CL of the QDs close to dislocations is much brighter. The morphology of uncapped QD samples shows that QDs on top of (or close to) dislocations are not well developed in the tensile matrices but are much larger and have a higher density in compressively strained matrices as compared to the surrounding strained areas. Details of the strain effect on the CL properties and on the nucleation of QDs are discussed.
A cross-sectional scanning tunneling microscopy study of a quantum dot infrared photodetector structure
Authors: Ouattara L. , Mikkelsen A. , Lundgren E. , Höglund L. , Asplund C. , Andersson J. Y.
Published: September 2006

We report on cross-sectional scanning tunneling microscopy studies of a quantum dot infrared photodetector structure consisting of multiple InGaAs quantum wells containing InAs quantum dots and separated by GaAs. We have investigated the composition and size distribution of the InAs quantum dots in this structure. Using cross-sectional scanning tunneling microscopy images displaying atomic resolution, we reveal that the InAs quantum dots are strongly intermixed with the InGaAs quantum well layer. We estimate the size distribution of the dots and find this to be nonuniform. Further, defects related to dopants introduced in the structure are identified and their concentration is estimated.
Collisionless electron power absorption in capacitive radio-frequency plasma sheaths
Authors: Gahan D. , Hopkins M. B.
Published: September 2006

A capacitively coupled, rf biased electrode mounted in an inductively coupled plasma reactor is used to investigate rf power coupling through the resulting capacitive sheath. Carefully calibrated current and voltage probes provide the electrical measurements necessary for this investigation. An external negative dc bias enables control over the electron component of the total current arriving at the electrode through the capacitive sheath. Sheath models generally assume that electron loss at the end of the rf cycle is negligible but recent theoretical work indicates that there is a significant difference in power coupled through the sheath when electron loss is permitted. The experimental study presented here confirms this result. Retardation of electron loss to the electrode is accompanied with an increase in rf power absorbed by the electrons in the capacitive sheath. Comparing the current signals with and without electron loss establishes that the measured increase in power is associated with the electron loss phase of the rf cycle, which is consistent with predictions of the earlier theoretical work.
Stochastic thermal transport of nanoparticle suspensions
Authors: Xuan Yimin , Li Qiang , Zhang Xing , Fujii Motoo
Published: September 2006

Both the Langevin equation of the Brownian motion and the concept of the stochastic thermal process are adopted to describe the temperature fluctuation of the nanoparticles suspended in carrier liquids. The heat transfer chain between the nanoparticles and the ambient liquid is analyzed. Based on the superposition principle and the Green-Kubo theorem, a thermal conductivity model that is able to account for the effects of the volume fraction and sizes of nanoparticles has been developed. Comparisons show that the results predicted by the present model are well coincident with the experimental data.
Assessing the role of filler atoms on the thermal conductivity of filled skutterudites
Authors: Nolas G. S. , Fowler G. , Yang J.
Published: September 2006

We investigate the lattice thermal conductivity of a series of La- and Yb-filled skutterudite antimonides by modeling different phonon-scattering parameters using the Debye approximation. We also investigate the contributions of the different phonon-scattering mechanisms in understanding the relative importance of the different scattering mechanisms on filled skutterudites. We find that increasing the filler atom concentration effectively increases both point defect scattering and resonance scattering of phonons. In addition, point defect scattering can have a substantial effect in lowering the thermal conductivity of skutterudites. These results help elucidate the role of the different scattering mechanisms on the thermal conductivity of these materials.
Effective thermal conductivity and thermal diffusivity of nanofluids containing spherical and cylindrical nanoparticles
Authors: Zhang Xing , Gu Hua , Fujii Motoo
Published: September 2006

This paper reports on measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al2O3 thin film and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Au/toluene, Al2O3/water, and carbon nanofiber (CNF)/water nanofluids are measured and the effects of the volume fraction and thermal conductivity of the nanoparticles and temperature are clarified. The average diameters of Au and Al2O3 spherical particles are 1.65 and 20nm, respectively. The average length and diameter of CNFs are 10μm and 150nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancements and can be predicted accurately by the model equation of Hamilton and Crosser [Ind. Eng. Chem. Fundam. 1, 187 (1962)] for the spherical nanoparticles and by the unit-cell model equation of Yamada and Ota [Waerme-Stoffuebertrag. 13, 27 (1980)] for carbon nanofibers.
Interfacial oxide growth at silicon∕high-k oxide interfaces: First principles modeling of the Si–HfO2 interface
Authors: Hakala M. H. , Foster A. S. , Gavartin J. L. , Havu P. , Puska M. J. , Nieminen R. M.
Published: September 2006

We have performed first principles calculations to investigate the structure and electronic properties of several different Si–HfOx interfaces. The atomic structure has been obtained by growing HfOx layer by layer on top of the Si(100) surface and repeatedly annealing the structure using ab initio molecular dynamics. The interfaces are characterized via their geometric and electronic properties, and also using electron transport calculations implementing a finite element based Green’s function method. We find that in all interfaces, oxygen diffuses towards the interface to form a silicon dioxide layer. This results in the formation of dangling Hf bonds in the oxide, which are saturated either by hafnium diffusion or Hf–Si bonds. The generally poor performance of these interfaces suggests that it is important to stabilize the system with respect to lattice oxygen diffusion.
Porous fiber formation in polymer-solvent system undergoing solvent evaporation
Authors: Dayal Pratyush , Kyu Thein
Published: September 2006

Temporal evolution of the fiber morphology during dry spinning has been investigated in the framework of Cahn-Hilliard equation [J. Chem. Phys. 28, 258 (1958)] pertaining to the concentration order parameter or volume fraction given by the Flory-Huggins free energy of mixing [P. J. Flory, Principles of Polymer Chemistry (Cornell University Press, Ithaca, NY, 1953), p. 672] in conjunction with the solvent evaporation rate. To guide the solvent evaporation induced phase separation, equilibrium phase diagram of the starting polymer solution was established on the basis of the Flory-Huggins free energy of mixing. The quasi-steady-state approximation has been adopted to account for the nonconserved nature of the concentration field caused by the solvent loss. The process of solvent evaporation across the fiber skin-air interface was treated in accordance with the classical Fick’s law [R. B. Bird et al., Transport Phenomena (J. Wiley, New York, 1960), p. 780]. The simulated morphologies include gradient type, hollow fiber type, bicontinuous type, and host-guest type. The development of these diverse fiber morphologies is explicable in terms of the phase diagram of the polymer solution in a manner dependent on the competition between the phase separation dynamics and rate of solvent evaporation.
The effect of substrate on high-temperature annealing of GaN epilayers: Si versus sapphire
Authors: Pastor D. , Cuscó R. , Artús L. , González-Díaz G. , Iborra E. , Jiménez J. , Peiró F. , Calleja E.
Published: September 2006

We have studied the effects of rapid thermal annealing at 1300°C on GaN epilayers grown on AlN buffered Si(111) and on sapphire substrates. After annealing, the epilayers grown on Si display visible alterations with craterlike morphology scattered over the surface. The annealed GaN∕Si layers were characterized by a range of experimental techniques: scanning electron microscopy, optical confocal imaging, energy dispersive x-ray microanalysis, Raman scattering, and cathodoluminescence. A substantial Si migration to the GaN epilayer was observed in the crater regions, where decomposition of GaN and formation of Si3N4 crystallites as well as metallic Ga droplets and Si nanocrystals have occurred. The average diameter of the Si nanocrystals was estimated from Raman scattering to be around 3nm. Such annealing effects, which are not observed in GaN grown on sapphire, are a significant issue for applications of GaN grown on Si(111) substrates when subsequent high-temperature processing is required.
Extended Hückel theory for band structure, chemistry, and transport. II. Silicon
Authors: Kienle D. , Bevan K. H. , Liang G.-C. , Siddiqui L. , Cerda J. I. , Ghosh A. W.
Published: September 2006

In this second paper, we develop transferable semiempirical extended Hückel theoretical (EHT) parameters for the electronic structure of another technologically important material, namely, silicon. The EHT parameters are optimized to experimental target values of the band dispersion of bulk silicon. We quantitatively benchmark our parameters to bulk electronic properties such as band edge energies and locations, effective masses, and spin-orbit coupling parameters, competitive with a nearest-neighbor sp3d5s* orthogonal tight-binding model for silicon of T. Boykin et al. [Phys. Rev. B 69, 115201 (2004)] that has been widely used to model silicon-based devices (see, e.g., A. Rahman et al. [Jpn. J. Appl. Phys. Part I 44, 2187 (2005)] and J. Wang et al. [Appl. Phys. Lett. 86, 093113 (2005)]). The transferability of the parameters is checked for multiple physical and chemical configurations, specifically, two different reconstructed surfaces, Si(100)-(2×1) and Si(111)-(2×1). The robustness of the parameters to different environments is demonstrated by comparing the surface band structures with density functional theory GW calculations and photoemission/inverse photoemission experiments. We further apply the approach to calculate the one-dimensional band dispersion of an unrelaxed rectangular silicon nanowire and explore the chemistry of surface passivation by hydrogen. Our EHT parameters thus provide a quantitative model of bulk silicon and silicon-based interfaces such as contacts and reconstructed surfaces, which are essential ingredients towards a quantitative quantum transport simulation through silicon-based heterostructures.
Thermal lensing effect in ridge structure InGaN multiple quantum well laser diodes
Authors: Li D. Y. , Huang Y. Z. , Zhu J. J. , Zhao D. G. , Liu Z. S. , Zhang S. M. , Ye X. J. , Chong M. , Chen L. H. , Yang H. , Liang J. W.
Published: September 2006

Time-resolved light-current curves, spectra, and far-field distributions of ridge structure InGaN multiple quantum well laser diodes grown on sapphire substrate are measured with a temporal resolution of 0.1ns under a pulsed current condition. Results show that the thermal lensing effect clearly improves the confinement of the higher order modes. The thermal lens leads to a lower threshold current for the higher order modes, a higher slope efficiency, and a change in the lasing mode of the device. The threshold current for the higher modes decreases by about 5mA in every 10ns in a pulse, and the slope efficiency increases by 7.5 times on the average when higher modes lase.
Influence of geometric shape of optically trapped particles on the optical rotation induced by vortex beams
Authors: Tao S. H. , Yuan X.-C. , Lin J. , Sun Y. Y.
Published: September 2006

Particles optically trapped and rotated in fluidic medium by a vortex beam receive a torque exerted by orbital angular momentum of the beam. Size and shape of the trapped particles are found to affect the rotation remarkably; e.g., a lump of particles is easier to be set in rotation than a single particle, and a row of particles stuck to each other will rotate faster than isolated particles. Influences of the size and arranging manner of a group of particles on the rotation are investigated analytically and experimentally.
Fluctuation of magnetic induction in von Kármán swirling flows
Authors: Volk Romain , Odier Philippe , Pinton Jean-François
Published: September 2006

Studies of magnetic induction in von Kármán swirling flows have so far linked the time-averaged induced magnetic field to the structure of the mean flow. They have evidenced the Omega and Parker mechanism generated, respectively, by the flow differential rotation and helicity, which underly the Dudley and James [N. L. Dudley and R. W. James, Proc. R. Soc. London, Ser. A 425, 407 (1989)] dynamos. Using an array of Hall probes we study here the dynamical regime. In the experimental flow, turbulence is fully developed and large fluctuations are observed in the magnetic induction processes. We find that the large scale turbulent fluctuations have different characteristics when induction results from the differential rotation or from the dynamics of the stagnation point in the midplane of the von Kármán flow. Symmetry considerations indicate that the dynamical flow spends half of its time away from the time-averaged structure. The consequences of these observations for dynamo experiments are discussed.
Organic light-emitting devices using polyacene derivatives as a hole-transporting layer
Authors: Okumoto Kenji , Kanno Hiroshi , Hamada Yuji , Takahashi Hisakazu , Shibata Kenichi
Published: September 2006

Three polyacene derivatives, 2-tert-butyl-9,10-di(2-naphthyl)anthracene (TBADN), 9,9′,10,10′- tetraphenyl-2,2′-bianthracene, and 5,6,11,12-tetraphenylnaphthacene (rubrene), are found to successfully function as hole-transporting layers (HTLs) in red organic light-emitting devices (OLEDs) using tetraphenyldibenzoperiflanthene as an emitter. Compared with an OLED with the most widely used HTL material, the arylamine derivative, 4,4′-bis(N-phenyl-1-naphthylamino)biphenyl (NPB), the OLEDs with the polyacene derivatives exhibit advantageous performance such as lower driving voltage, higher electroluminescence efficiency, or longer luminance lifetime, depending on the employed HTL material. Current efficiency of the red OLED with a TBADN HTL is 5.5cd∕A with Commission Internationale de L’Eclairage chromaticity coordinates of (x=0.66, y=0.34), which is much higher than the value of 2.1cd∕A with the same chromaticity for the OLED with a NPB HTL. The driving voltage and luminance lifetime of the red OLED with rubrene HTL are improved compared with the OLED with a NPB HTL. The hole injection properties of the proposed HTL materials are discussed. The results indicate that the polyacene derivatives are promising for use in OLEDs as a class of HTLs, expanding the variety of HTL materials available for optimizing the performance of OLEDs.
Electronic and optical properties of 1.3μm GaAsSbN∕GaAs quantum well lasers
Authors: Park Seoung-Hwan
Published: September 2006

The electronic and optical properties of GaAsSbN∕GaAs quantum well (QW) laser are investigated using the multiband effective mass theory and the non-Markovian gain model. The results are compared with those of type II GaAsSb∕GaAs QW laser using a self-consistent method. The GaAsSbN∕GaAs QW structure shows that a relatively low compressive strain is required to obtain 1.3μm wavelength compared to the GaAsSb∕GaAs QW structure. The reduction effect of the compressive strain is shown to be dominant for QW structure with a larger N composition or a thick well width. We know that the GaAsSbN∕GaAs QW structure has significantly larger optical gain than the GaAsSb∕GaAs QW structure. This is because the interband matrix element of the former is much larger than that of the latter. We expect that the GaAsSbN∕GaAs QW structure has the improved lasing characteristic compared to the GaAsSb∕GaAs QW structure.
Infrared photoluminescence from erbium-doped spark-processed silicon
Authors: Kim Kwanghoon , Hummel Rolf E.
Published: September 2006

The infrared (IR) photoluminescence (PL) of erbium-doped spark-processed silicon (sp-Si) was investigated. In addition to IR photoluminescence of undoped sp-Si (which peaks near 945nm), the erbium insertion into the sp-Si matrix causes additional structure near 1540nm, that is, in a wavelength range which is important for telecommunication purposes. Erbium was deposited on a silicon wafer followed by spark processing, which enables diffusion of some erbium into the SiOx matrix, thus achieving optoelectronically active spark-processed silicon. Rapid thermal annealing enhances the 1.54μm wavelength intensity. The optimal processing conditions that result in the most efficient photoluminescence have been established and are presented. In contrast to erbium-doped crystalline silicon, whose light emission is highly affected by temperature (103 times reduction in intensity when heating from 12to150K), the intensity of erbium-doped spark-processed silicon decreases by only a factor of 4 when heated from 15to300K. The room temperature PL efficiency was found to be 0.3% in contrast to erbium-doped crystalline Si whose PL efficiency is known to be 0.05%. The experimental findings are interpreted by postulating a photoluminescence mechanism with energy transfer from spark-processed silicon to the Er3+ ions and by light emission from intrashell energy transition from I13∕24→I15∕24.
Reshock response of shock deformed aluminum
Authors: Huang H. , Asay J. R.
Published: September 2006

The effects of shock loading on compressive yield strength are presented for several aluminum materials. These include commercially pure aluminum 1060, ultrapure aluminum, pure aluminum single crystal of [100] and [111] orientations, and aluminum alloy 6061-T6. The yield strength of these materials was measured by a cyclic shock loading technique in which the sample was first shocked to longitudinal stresses ranging from 4to22GPa, followed by complete release and then reshocked to final states of 4–31GPa. The results reveal that the yield strength of 6061-T6 alloy, as determined through measurement of its Hugoniot elastic limit during reshocking, remains unchanged after first shock loading and release. In contrast, the yield strength of all pure aluminum materials was found to increase during reshocking. In addition, plastic strain hardening was observed in all reshock wave profiles after the initial elastic response, as observed through substantially higher reshock wave speeds. This results in substantially higher reshock Hugoniot stresses than predicted by the original Hugoniot of aluminum at the same compression. The compressive yield strength at the reshock Hugoniot stress was estimated by comparing the first and second Hugoniot states and correcting for thermal effects. The results suggest that strain hardening during second shock compression is independent of first shock amplitudes over the range of 6–22GPa and appears to saturate for a first shock stress greater than 6GPa. The temperature correction for the Hugoniot mean stress shows that the increased temperature produced by second shock compression significantly reduces the flow strength during reshocking and makes the resultant strength strongly dependent on loading history.
Piezoelectric nonlinearity in ferroelectric thin films
Authors: Bassiri Gharb Nazanin , Trolier-McKinstry Susan , Damjanovic Dragan
Published: September 2006

The piezoelectric properties of {001} oriented 0.5Pb(Yb1∕2Nb1∕2)O3–0.5PbTiO3 films were studied. It was found that the films show Rayleigh-like behavior in that the magnitude of the piezoelectric coefficient depends linearly on the amplitude of the exciting ac electric field used for the measurement. The reversible and irreversible Rayleigh piezoelectric coefficients decreased logarithmically with frequency. The frequency dependence of the Rayleigh coefficients measured under applied bias was identical for the dielectric and piezoelectric responses, suggesting that the same interface motion is responsible for both phenomena. It is demonstrated that dynamic poling of the material by the external ac field self-consistently explains both the Rayleigh-like response and the higher harmonics in the strain response.
A free-boundary theory for the shape of the ideal dripping icicle
Authors: Short Martin B. , Baygents James C. , Goldstein Raymond E.
Published: September 2006

The growth of icicles is considered as a free-boundary problem. A synthesis of atmospheric heat transfer, geometrical considerations, and thin-film fluid dynamics leads to a nonlinear ordinary differential equation for the shape of a uniformly advancing icicle, the solution to which defines a parameter-free shape which compares very favorably with that of natural icicles. Away from the tip, the solution has a power-law form identical to that recently found for the growth of stalactites by precipitation of calcium carbonate. This analysis thereby explains why stalactites and icicles are so similar in form despite the vastly different physics and chemistry of their formation. In addition, a curious link is noted between the shape so calculated and that found through consideration of only the thin coating water layer.
Lagrangian modeling of scalar statistics in a double scalar mixing layer
Authors: Sawford Brian L.
Published: September 2006

Statistics of scalar concentration in a double scalar mixing layer are calculated using a Lagrangian stochastic model coupled to the interaction by exchange with the conditional mean micromixing model. Excellent agreement is obtained with recent direct numerical simulation results. The model accurately reproduces a wide variety of behavior across both physical and concentration space and over both unconditional and conditional scalar statistics.