bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of bubble growth and detachment in a viscous shear flow
Authors: Duhar Géraldine , Colin Catherine
Published: August 2006

The quasistatic injection of air bubbles at the wall in a viscous shear flow is experimentally investigated by high-speed video pictures and image processing. The bubble radius and center of gravity position are determined throughout the growth. Experimental results are then used to validate a force balance model during bubble growth and after detachment for small bubble Reynolds numbers. The contact angles at the bubble foot calculated from the force balance agree well with those experimentally obtained from image processing. Finally, the force balance model is used to predict the departure bubble radius versus the gas flow rate and liquid flow shear rate.
Structure and grain growth of TiO2 nanoparticles investigated by electron and x-ray diffractions and Ta181 perturbed angular correlations
Authors: Schlabach S. , Szabó D. V. , Vollath D. , de la Presa P. , Forker M.
Published: August 2006

Bare and coated TiO2 nanoparticles with particle sizes d<5nm have been synthesized in a microwave plasma process. Structural properties of these materials have been investigated by transmission electron microscopy, x-ray diffraction, and perturbed angular correlation (PAC) measurements of the electric quadrupole interaction (QI) at the probe nucleus Ta181 on the metal site of TiO2 at temperatures 290⩽T⩽1450K. The electron diffraction of the uncoated nanoparticles in the as-synthesized state reflects long range order in the Ti sublattice. Depending on the particles size, either the anatase or the rutile phase of TiO2 was found. Anatase appears to be the stable form of nanocrystalline TiO2 below d∼10nm. The PAC spectra of these nanocrystalline oxides are characterized by a broad distribution of strong quadrupole interactions, indicating a strongly disordered oxygen environment of the metal sites. Upon annealing, the grain size grows from d<5nm after synthesis to d>100nm after 1300K. PAC spectra taken in the same temperature range show that with increasing temperature, the initially disordered state transforms to well-ordered rutile TiO2. The data suggest a critical grain size of d∼10nm for the onset of the ordering process. The spectra of coarse-grained TiO2 are reached at a particle size d⩾30nm. In n-TiO2 coated with Al2O3 and ZrO2 both the cores and the coatings were found to grow with increasing temperature; the cores of the coated particles, however, grow much less than those of the noncoated particles. The PAC method was used to investigate the QI in both TiO2 cores and in the ZrO2 coating of n-TiO2∕ZrO2 at different temperatures. These data suggest that although the coated particles grow with temperature, the ordering process is obstructed, possibly by a solid state reaction between the TiO2 kernels and the coatings.
Light beating spectroscopy of Brillouin scattering in gases and solids
Authors: Yogi Takeshi , Sakai Keiji , Takagi Kenshiro
Published: August 2006

The experimental system of optical beating spectroscopy was improved, and the increased sensitivity and frequency resolution were demonstrated in the Brillouin scattering experiments in solids, crown glass (BK7), and polymethylmetacrylate (PMMA), and also in gases, air, and nitrogen. For their weak light scattering ability, these substances are very tough specimens for Brillouin scattering studies, and the classical spectrometer of Fabry-Pérot étalon has been so far used for the frequency analysis. The phonon peaks were observed with the present light beating system in BK7 at room temperature over the wave number range from k∕2π=1.03×104to8.84×104m−1 (corresponding frequency from 61.6to548MHz), and the spectra were fitted with Gaussian curves since a condenser lens in the incident light path caused a large instrumental width that overwhelmed the intrinsic phonon width. The spectra in PMMA were analyzed with Voigt functions. The dispersion relations obtained in these solids were in good agreement with the literature values of ultrasonic velocity. The spectra in air were well fitted with Lorentzian curve representing the phonon lifetime, and the absorption as well as the phase velocity was obtained.
Er coordination in Y2O3 thin films studied by extended x-ray absorption fine structure
Authors: Van Trinh Tu , Bargar John R. , Chang Jane P.
Published: August 2006

Extended x-ray absorption fine structure (EXAFS) spectroscopy was employed to study the Er coordination in polycrystalline Y2O3 thin films, which was found to dictate their photoluminescence (PL) properties. Incorporation of Er with concentrations varying from 6to14at.% was achieved by radical-enhanced atomic layer deposition at 350°C. In all samples, Er was found to be in the optically active trivalent state, confirmed by their x-ray absorption near-edge spectroscopy spectra. Modeling of the EXAFS data revealed that the local structure of Er3+ is similar to that of Er3+ in Er2O3. Specifically, Er3+ is coordinated with six O at 2.24 and 2.32Å. Excellent fits to the EXAFS for samples with Er3+ concentration less than 8at.% were achieved when the second coordination shell was modeled as a mixture of Y3+ and Er3+, indicating a complete miscibility of Er3+ in the Y2O3 matrix under these experimental conditions. This behavior is attributed to the almost perfect ionic size match between Y3+ and Er3+, having identical valence state and coordination characteristics. For thin films with higher Er concentrations, the EXAFS analysis revealed an exsolution with Er2O3 domain. Since there is no indication of Er clustering, it is concluded that the PL quenching observed in samples with the Er doping level exceeding 8at.% is likely due to Er ion-ion interaction but not Er immiscibility in Y2O3. Specifically, an increase in the Er3+ concentration implied an increase in the average number of Er3+ in the second coordination shell, thus making ion-ion interaction possible. The critical interionic distance between two Er3+ was determined to be ∼4Å, thus setting an upper limit on the Er3+ concentration in Y2O3 at ∼6×1021cm−3, at least three orders of magnitude higher than the Er3+ solubility limit in the conventional SiO2 host (<1018cm−3).
A numerical study of two-phase Stokes flow in an axisymmetric flow-focusing device
Authors: Jensen Mads Jakob , Stone Howard A. , Bruus Henrik
Published: August 2006

We present a numerical investigation of the time-dependent dynamics of the creation of gas bubbles in an axisymmetric flow-focusing device. The liquid motion is treated as a Stokes flow, and using a generic framework we implement a second-order time-integration scheme and a free-surface model in MATLAB, which interfaces with the finite-element software FEMLAB. We derive scaling laws for the volume of a created bubble and for the gas flow rate, and confirm them numerically. Our results are consistent with existing experimental results by Garstecki et al. [Phys. Rev. Lett. 94, 164501 (2005)], and predict a scaling yet to be observed: the bubble volume scales with the outlet channel radius to the power of 4 and the surface tension. Our axisymmetric simulations further show that the collapse of the gas thread before bubble snap-off is different from the recent experimental results. We suggest that this difference is caused by differences in geometry between experiments and the simulations.
An experimental study of contact effects in organic thin film transistors
Authors: Gundlach D. J. , Zhou L. , Nichols J. A. , Jackson T. N. , Necliudov P. V. , Shur M. S.
Published: August 2006

We report on parasitic contact effects in organic thin film transistors (OTFTs) fabricated with pentacene films. The influence on the OTFT performance of the source and drain contact metal and the device design was investigated. Top contact (TC) and bottom contact (BC) gated transmission line model (gated-TLM) test structures were used to extract the combined parasitic contact resistance as a function of gate voltage swing and drain-source voltage for OTFTs with gold source and drain contacts. For comparison BC test structures with palladium contacts were studied. Differences in the bias dependence of the contact resistance for TC and BC OTFTs indicate that charge injection and device performance are strongly affected by the device design and processing. The results from this investigation show that TC and BC device performances may be contact limited for high mobility OTFTs with channel lengths less than 10μm.
Evolution of erbium lattice locations in silicon: Effects of thermal annealing and codoped impurities (carbon, nitrogen, oxygen, and fluorine)
Authors: Ren X. T. , Huang M. B.
Published: August 2006

The effects of thermal annealing and codoped impurities including carbon, nitrogen, oxygen, and fluorine, on the occupation of erbium lattice locations in Si, have been investigated in detail. Ion channeling measurements indicate that ion-implanted Er can mainly occupy two distinct lattice locations in silicon, i.e., the tetrahedral (T) and hexagonal (H) interstitial sites, with their respective population depending on thermal annealing and codoped impurity contents. Interestingly, all four codopant species under study give rise to strikingly similar effects on the occupation of Er lattice locations: to promote and stabilize Er on the H site. Our work suggests that interactions among Er, codoped impurities, and crystal defects are cooperative in determining the Er lattice locations in Si.
Rotation of the methyl groups and thermal conductivity of molecular crystals: ethane
Authors: Konstantinov V. A. , Revyakin V. P. , Sagan V. V.
Published: August 2006

The thermal conductivity of solid ethane (C2H6) is investigated for three samples of different density in the temperature range from 35K to the onset of melting. In all cases the isochoric thermal conductivity in the α and β phases decreases with increasing temperature by a dependence weaker than Λ∝1∕T. Such behavior is typical for orientationally ordered phases of molecular crystals and does not reveal any clear features due to rotation of the methyl groups. The thermal conductivities of ethane and some of its halogen derivatives (freons) are compared. The experimental results are discussed in the framework of a model in which heat is transported by low-frequency phonons, and, above the mobility edge, by high-frequency “diffusive” modes.
Influence of plastic deformation on the temperature dependence of the thermopowerin copper
Authors: Svetlov V. N. , Stepanov V. B.
Published: August 2006

A shift of the minimum on the temperature dependence of the thermopower of copper at low temperatures is observed experimentally upon plastic deformation and is related to the dislocation and impurity structures of the deformed sample. To estimate the temperature interval in which the dislocation contribution is predominant, the experimental results are compared with the published results of theoretical calculations.
Synchrotron x-ray scattering study of thin epitaxial Pr2O3 films on Si(001)
Authors: Guo Xiangxin , Braun Wolfgang , Jenichen Bernd , Kaganer Vladimir M. , Tinkham Brad P. , Trampert Achim , Ploog Klaus H.
Published: August 2006

We investigate the structural and interfacial properties of thin Pr2O3 films on Si(001) substrates grown by molecular beam epitaxy using synchrotron grazing incidence x-ray diffraction and reflectivity measurements in ultrahigh vacuum. The epitaxial films consist of two orthogonal [101]-oriented cubic domains, with equal proportion. The average in-plane domain sizes are larger than the film thickness. Scans along crystal truncation rods confirm the cubic Mn2O3 structure of the epitaxial layer. A small amount of hexagonal Pr2O3 is found in thin films. Its fraction increases with increasing layer thickness indicating that it is not confined to the interface. Reflectivity measurements reveal an additional layer at the Pr2O3∕Si(001) interface, which is extended by in situ annealing. Transmission electron microscopy of the samples confirms the structural properties of the films found by the x-ray measurements and shows that the interfacial layers are nonuniform with a coexistence of crystalline and amorphous regions.
Transformation behavior of room-temperature-stable metastable defects in hydrogen-implanted n-type silicon studied by isothermal deep-level transient spectroscopy
Authors: Tokuda Yutaka
Published: August 2006

Isothermal deep-level transient spectroscopy (DLTS) with a single pulse has been used to study the transformation behavior of hydrogen-related metastable defects labeled EM1 (Ec−0.28eV) and EM2 (Ec−0.37eV), which are observed in n-type silicon implanted with hydrogen ions at 88K and subsequently heated to room temperature. EM1 shows the anomalous filling behavior that its isothermal DLTS peak height decreases exponentially with filling pulse duration time in the range from 1msto1000s. A corresponding exponential increase in EM2 peak height is found. This indicates that EM1 filled with electrons is transformed into EM2 during the application of filling pulse. The dependence of EM1 and EM2 peak heights on the emission time between two adjacent filling pulses reveals the transformation from EM2 to EM1 with fast rates after electron emission of EM2. This shows that EM1 and EM2 are different configurations of the same defect and are stable under reverse bias and zero bias, respectively. The rate equations governing the emission, capture, and transformation kinetics for EM1 and EM2 are solved to extract those parameters. The electron emission rate of EM2 and the transformation rate from EM1 to EM2 are found to be dependent on electric field. It is suggested that the hydrogen-related metastable defect is donorlike.
Electromigration and critical product in eutectic SnPb solder lines at 100°C
Authors: Agarwal R. , Ou Shengquan E. , Tu K. N.
Published: August 2006

Due to a significant change in electromigration behavior of eutectic SnPb at different temperatures, it is critical to understand it at the device temperature, i.e., 100°C. In the present study, the electromigration behavior of e-SnPb is investigated at 100°C and at different current densities, varying within the range of 5×103–5×104A∕cm2. The test samples are prepared by reflowing solder into V grooves etched on (001) Si wafers. The electromigration is observed at the current densities higher than 1×104A∕cm2. The dominant diffusing species at 100°C is found to be Pb. The threshold current density for electromigration to occur in e-SnPb comes out to be 5×103A∕cm2. We also notice Pb rich phase coarsening, which becomes significant at high current densities. The results are compared to that in preannealed samples. The effects of the changes in microstructure on the electrical reliability are also discussed. The interface diffusion is dominant when the grain size is small. Subsequently, as the grain size increases, it is reduced to the order of the lattice diffusion, and therefore the electromigration is impeded. The critical product for e-SnPb solder is estimated to be 150 experimentally, and is also compared to the theoretically calculated value of 8. The reason for the discrepancy is discussed.
Characterization of helium bubble size and faceting by electron holography
Authors: Snoeck E. , Majimel J. , Ruault M. O. , Hÿtch M. J.
Published: August 2006

Iron-9% chromium alloy of controlled purity and microstructure was helium implanted and studied using transmission electron microscopy and electron holography (EH). Fresnel imaging is compared with electron holography for a set of identical helium bubbles of size 2–4nm. Electron holography reveals that the bubbles are, in fact, faceted and the lateral and projected dimensions of the cavities are measured. Current measurements allow the detection of 1nm diameter bubbles using EH. The merits of conventional and holographic techniques are discussed and future improvements anticipated. Electron holography seems to be the ideal technique for investigating the morphology of helium bubbles and cavities in general.
Transmission properties of perturbed optical Cantor multilayers
Authors: Chiadini Francesco , Scaglione Antonio , Fiumara Vincenzo
Published: August 2006

Fractal multilayers of two distinct dielectric materials are structures exhibiting stop bands and transmission peaks in their electromagnetic response. In this work we analyze how the transmission spectrum of a Cantor fractal multilayer changes when a defect, consisting of a dielectric layer different from those constituting the multilayer, is inserted midway through its thickness. Unlike a periodic multilayer structure, no transmission peaks (defect modes) appear in the stop band by inserting a defect in a Cantor multilayer. We show that a defect in the fractal structure results in a frequency shift of its transmission spectrum and in a change of the width of both stop bands and transmission peaks. The extent of such spectrum modifications depends on either the thickness and the refractive index of the defect layer. By suitably modulating these parameters a tunable narrow band filter can be realized to cover a desired range of frequency. This feature may hold attraction for certain applications such as dense wavelength division multiplexing systems and sensors.
Thermal stress analysis for GaInAsP multiple quantum well wafer chemically bonded to Si (100)
Authors: Zhao Hong-Quan , Yu Li-Juan , Huang Yong-Zhen
Published: August 2006

An n-InP-based InGaAsP multiple-quantum-well wafer was bonded with p-Si by chemical surface activated bonding at 70°C, and then annealed at 450°C. Different thermal expansion coefficients between InP and Si will induce thermal stresses in the bonded wafer. Planar and cross-sectional distributions of thermal stress in the bonded InP–Si pairs were analyzed by a two-dimensional finite element method. In addition, the normal, peeling, and shear stresses were calculated by an analytic method. Furthermore, x-ray double crystalline diffraction was applied to measure the thermal strain and the strain caused by the mismatching of the crystalline orientation between InP (100) and Si (100). The wavelength redshift of the photoluminescence (PL) spectrum due to thermal strain was investigated via the calculation of the band structure, which is in agreement with the measured PL spectra.
Nanoindentation of plasma-deposited nitrogen-rich silicon nitride thin films
Authors: Soh Martin T. K. , Fischer-Cripps A. C. , Savvides N. , Musca C. A. , Faraone L.
Published: August 2006

Nanoindentation was performed on plasma-deposited nitrogen-rich silicon nitride thin films deposited on various substrates between 150 and 300°C. A very simple and effective depth-profiling method is introduced, which involves indentation of thin films deposited on substrates with different mechanical properties. The primary advantage of this method is that it avoids the complications associated with many of the complex mathematical models available to deconvolve thin film mechanical properties, while nevertheless allowing the user to visually identify thin film properties. This method is demonstrated on our thin films, which have a hardness between 14 and 21GPa, and reduced modulus between 120 and 160GPa. The initial rise in hardness at low contact depths, commonly attributed to an indentation-size effect, is shown to be due to elastic contact between the indenter and thin film surface. This demonstrates the perils of blindly following the 10% rule for hardness calculation. The contribution of elastic and plastic deformations from nanoindentation is used to clarify the physical meaning of hardness and reduced modulus.
Higher lying conduction band in GaN and AlN probed by photoluminescence spectroscopy
Authors: Nepal N. , Nam K. B. , Li J. , Nakarmi M. L. , Lin J. Y. , Jiang H. X.
Published: August 2006

Photoluminescence spectroscopy has been employed to study the band edge emissions in GaN and AlN epilayers up to 800K. Two distinctive activation processes have been observed in both GaN and AlN. The first process occurring below Tt=325K (Tt=500K) for GaN (AlN) is due to the activation of free excitons to free carriers, whereas the second occurring above Tt with an activation energy of 0.29eV (0.3eV) for GaN (AlN) is believed to be associated with a higher lying conduction band (Γ3) at about 0.3eV above the conduction band minimum (Γ1). An emission line at about 0.29eV above the dominant transition in GaN was also observed at 700K, corroborating the assignment of Γ3. The values of Tt are a direct measure of the onset temperature at which free excitons dissociate into free carriers.
Microscopic simulations of an exothermic chemical wave front: Departure from the continuity equations
Authors: Hansen J. S. , Nowakowski B. , Lemarchand A.
Published: August 2006

We perform microscopic simulations of a reactive dilute gas and study the propagation of an exothermic chemical wave front in an infinite, one-dimensional medium. The simulation results concerning front propagation speed and concentrations, temperature and stream velocity profiles are compared with the results of the integration of the macroscopic continuity equations in the case of fast reactions. The discrepancies between the two approaches are related to a chemically induced departure from local equilibrium.
Quantum corrections to classical evaluation of nonadiabatic transition rates
Authors: Kim Hyojoon , Rossky Peter J.
Published: August 2006

A recently developed quantum correction approach is applied to evaluating the nonadiabatic quantum-mechanical transition rate between Born-Oppenheimer states of a subsystem embedded in a thermal bath of harmonic oscillators. In the first-order perturbation theory, the nonadiabatic rate can be expressed in terms of a quantum-mechanical correlation function, which can be estimated directly from classical data. Application to a popular spin-boson model shows that our results are in excellent agreement with the exact quantum-mechanical results.
Modified fundamental-measure theory for additive hard-disk fluids
Authors: Kim Soon-Chul , Lee Song Hi
Published: August 2006

A modified fundamental-measure theory (MFMT), which is based on the semiempirical equation of state for the hard-disk mixtures, has been proposed for studying the uniform and nonuniform structural properties of hard-disk fluids. The calculated results show that the MFMT theory yields in an excellent agreement with the computer simulations and is better than the original FMT proposed by [Rosenfeld and co-workers [Phys. Rev. A42, 5978 (1990); Phys. Rev. E55, 4245 (1997)]..
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double heterojunction bipolar transistor with GaAs∕Al0.11Ga0.89As∕GaInP composite collector
Authors: Poh Z. S. , Yow H. K. , Houston P. A. , Krysa A. B. , Ong D. S.
Published: August 2006

Ga In P ∕ Ga As ∕ Ga In P double heterojunction bipolar transistor (DHBT) with an Al0.11Ga0.89As layer within lowly doped GaAs–GaInP composite collector was characterized. In comparison to an abrupt GaInP∕GaAs∕GaInP DHBT with saturation voltages in excess of 20V, current gains of 25 at high biases, and breakdown voltages in the range of 22V, the DHBT incorporating GaAs–Al0.11Ga0.89As–GaInP composite collector has demonstrated lower saturation voltages of less than 6V and high current gains of 50 without compromising the breakdown voltages of the GaInP collector. Al0.11Ga0.89As layer can thus provide an alternative design to effectively minimize the potential spike effects at the GaAs∕GaInP heterojunction.
Lasing features in scattering gain media and amplified spontaneous emission systems
Authors: de Souza M. A. F. , Lencina A. , Vaveliuk P.
Published: August 2006

An experimental study was made on lasing features in both scattering gain media and amplified spontaneous emission systems from a rhodamine-6G xanthene dye in ethanol solution. The emission intensity, spectral line-half-width, and lasing threshold are investigated in terms of dye concentration, cell thickness, and pump energy. The analysis shows that both systems behave quite different. It was found that the scattering gain media present best lasing properties which are enhanced to smaller cell thickness where the backscattering effect plays a significant role. In addition, its laserlike characteristics were found slightly dependent on dye concentration within the range in which the dimer species influence is absent. In return, the amplified spontaneous emission systems were observed strongly dependent on the product of cell thickness and dye concentration, namely, optical density. These systems present lasing properties only around an optimized value of optical density. Within this range, their emission intensity, spectral line-half-width, and threshold could overcome those obtained from scattering gain media.
The magnetotransport properties of La0.7Sr0.3MnO3∕BaTiO3 superlattices grown by pulsed laser deposition technique
Authors: Murugavel P. , Prellier W.
Published: August 2006

We have investigated the magnetotransport properties of La0.7Sr0.3MnO3∕BaTiO3 superlattices, grown on SrTiO3 substrate by pulsed laser deposition technique, both with current-in-plane and current-perpendicular-to-the-plane directions. Several features indicate the presence of magnetic inhomogeneities at the interfaces, which are independent of BaTiO3 layer thickness variation. First, the magnetic property in the superlattices decreases. Second, a hysteresis in magnetoresistance due to the relaxation of the resistive state is observed. Third, a threshold under an applied magnetic field in the magnetoresistance is seen. Such behaviors are in agreement with the phase separation scenario, which could be the possible reason for these magnetic inhomogeneities at the interfaces. On the contrary, the magnetoresistance with the current-perpendicular-to-the-plane direction is mostly attributed to the tunneling effect along with the ordering of the spin at the interface. This study confirms the importance of the interfaces in superlattices that can be used to control physical properties in oxide materials.
Temperature and wavelength dependence of Fermi-tail photoemission and two-photon photoemission from multialkali semiconductors
Authors: Booth Mark C. , Saleh Bahaa E. A. , Sergienko Alexander V. , Teich Malvin C.
Published: August 2006

Two-photon photoemission is a useful technique for examining interface, surface, and image-potential states in various materials. We report the temperature and wavelength dependences of the two-photon photoemission yield for several multialkali semiconductors used as photocathode materials in commercially available photomultiplier tubes. We also report the dependence on temperature and wavelength of one-photon photoemission associated with the Fermi tail of the electron-occupancy probability distribution, which can mask two-photon photoemission. The results are expected to be of use in entangled-photon photoemission experiments, for which a large value of the two-photon photoemission yield is required.
Color-center waveguides in low-energy electron-bombarded lithium fluoride
Authors: Rocchetti Alessio , Assanto Gaetano , Montereali Rosa Maria , Nichelatti Enrico , Somma Fabrizia
Published: August 2006

We employ a differential version of m-line spectroscopy through grating coupling in order to measure the refractive index of low-energy electron irradiated lithium fluoride crystal channel waveguides for broadband emission. Using photoresist films and a holographic setup for grating fabrication, we perform an accurate characterization of bidimensional structures and a direct quantitative comparison between treated and blank materials.
Frequency mixing of off-axis focused Gaussian beams: An approach to measure the phase velocity distribution
Authors: Lu Daquan , Qian Liejia , Li Yongzhong , Fan Dianyuan
Published: August 2006

In this letter we theoretically study the frequency mixing of two off-axis focused Gaussian beams. The optimal phase mismatch corresponding to maximized conversion efficiency depends on the separation of off-axis beams and can be expressed in terms of the phase velocities analytically, which provides an effective approach to measure the phase velocity distribution of a Gaussian beam.
Fiber-coupled single-photon detectors based on NbN superconducting nanostructures for practical quantum cryptography and photon-correlation studies
Authors: Słysz W. , Węgrzecki M. , Bar J. , Grabiec P. , Górska M. , Zwiller V. , Latta C. , Bohi P. , Milostnaya I. , Minaeva O. , Antipov A. , Okunev O. , Korneev A. , Smirnov K. , Voronov B. , Kaurova N. , Gol’tsman G. , Pearlman A. , Cross A. , Komissarov I. , Verevkin A. , Sobolewski Roman
Published: August 2006

We have fabricated and tested a two-channel single-photon detector system based on two fiber-coupled superconducting single-photon detectors (SSPDs). Our best device reached the system quantum efficiency of 0.3% in the 1540-nm telecommunication wavelength with a fiber-to-detector coupling factor of about 30%. The photoresponse consisted of 2.5-ns-wide voltage pulses with a rise time of 250ps and timing jitter below 40ps. The overall system response time, measured as a second-order, photon cross-correlation function, was below 400ps. Our SSPDs operate at 4.2K inside a liquid-helium Dewar, but their optical fiber inputs and electrical outputs are at room temperature. Our two-channel detector system should find applications in practical quantum cryptography and in antibunching-type quantum correlation measurements.
Microwave spectroscopy on magnetization reversal dynamics of nanomagnets with electronic detection
Authors: Grollier J. , Costache M. V. , van der Wal C. H. , van Wees B. J.
Published: August 2006

We demonstrate a detection method for microwave spectroscopy on magnetization reversal dynamics of nanomagnets. Measurement of the nanomagnet anisotropic magnetoresistance was used for probing how magnetization reversal is resonantly enhanced by microwave magnetic fields. We used Co strips of 2μm×130nm×40nm, and microwave fields were applied via an on-chip coplanar waveguide. The method was applied for demonstrating single domain-wall resonance, and studying the role of resonant domain-wall dynamics in magnetization reversal.
Phase transitions and magnetic entropy change in Mn-rich Ni2MnGa alloys
Authors: Albertini F. , Paoluzi A. , Pareti L. , Solzi M. , Righi L. , Villa E. , Besseghini S. , Passaretti F.
Published: August 2006

Thermomagnetic and magnetocaloric (MC) properties of Ni2MnGa Heusler alloys are found to be very sensitive to small composition variations. It is shown that the temperature behavior of ac susceptibility mimics that of MC properties of the samples. The coincidence of magnetic and structural transformations is found for a Mn-rich composition. As in the case of Ni-rich alloys, the magnetocaloric effect is enhanced at the transitions co-occurrence. The increase of Mn content, with respect to the stoichiometric value, rapidly decreases anisotropy field and saturation magnetization.
Defects and phase distribution in epitaxial ZnMnSe layers analyzed by transmission electron microscopy
Authors: Litvinov D. , Gerthsen D. , Daniel B. , Klingshirn C. , Hetterich M.
Published: August 2006

Our work is concerned with the occurrence and distribution of the sphalerite, wurtzite, and rocksalt phases, which can be present in the ZnMnSe system, and the analysis of structural defects. For this purpose, ZnMnSe layers with thicknesses between 700 and 1000nm and Mn concentrations of 0%, 4%, 11%, 14%, 17%, 29%, 31%, 43%, 50%, 70%, 85%, and 100% were deposited by molecular-beam epitaxy on GaAs (001) substrates. The structure analyses were performed by transmission electron microscopy. A high density of stacking faults exceeding 109cm−2 is already present for a Mn concentration of 14% suggesting that lower Mn concentrations should be used for spin-aligning layers. A significant volume fraction of twinned regions is contained in the Zn0.69Mn0.31Se layer. ZnMnSe layers with Mn concentrations of 43%, 50%, and less than 30% consist exclusively of the sphalerite phase. The sphalerite and a small volume fraction of the wurtzite phase are contained in the sample with 31% of manganese. A mixture of the sphalerite, wurtzite, and rocksalt phases is observed for Mn concentrations of 70% and 85%. The presence of the wurtzite phase is correlated with regions, which contain particularly high densities of stacking faults. Only the rocksalt and the sphalerite phases remain for pure MnSe.
Thermoelectric properties of CuSbSe2 and its doped compounds by Ti and Pb at low temperatures from 5to310K
Authors: Li D. , Qin X. Y.
Published: August 2006

Transport and thermoelectric properties of CuSbSe2 and CuM0.02Sb0.98Se2 (M=Ti,Pb) have been investigated at the temperatures from 5to310K. The results indicated a good linear relationship between logarithm of dc resistivity and reciprocal of temperature existed for CuSbSe2 at the temperatures above ∼73K and below ∼62K, respectively, from which two activation energies ΔE1 (66meV) and ΔE2 (0.16meV) were derived. Only ΔE2 was changed after Ti substitution, while Pb substitution for Sb only changed ΔE1, both of which could be ascribed, respectively, to the substitution of Ti3+ and Pb4+ for Sb3+. The thermopower of CuPb0.02Sb0.98Se2 decreased substantially as compared with that of CuSbSe2, which can be explained as large increase in electron concentration due to substitution of Pb4+ for Sb3+, leading to transport of double carriers (electrons and holes) in this p-type semiconductor. Experiments showed that CuM0.02Sb0.98Se2 has much smaller (lattice) thermal conductivity as compared with CuSbSe2, coming presumably from the strong phonon scattering by the impurities (Ti or Pb). As compared with that of CuSbSe2 the figure of merit ZT of CuTi0.02Sb0.98Se2 has been improved substantially at T>∼150K due to large decrease in both the resistivity and thermal conductivity. Specially, its ZT value is about twice as large as that of CuSbSe2 at 300K, suggesting that Ti substitution is a promising way to improve its thermoelectric properties.
Gravitational Aharonov-Bohm effect due to weak fields
Authors: Marques Geusa de A. , Fernandes Sandro G. , Bezerra V. B.
Published: August 2006

We study the behavior of relativistic quantum particles in the space-times generated by a rotating massive body and a moving mass current, in the weak field approximation. We solve the Dirac equation in these gravitational fields and calculate the currents associated with the particles. It is shown that these solutions and the currents depend on the angular momentum and on the velocity of the sources, in the cases of a massive rotating body and a moving mass current, respectively. These effects may be looked upon as a gravitational analog of the Aharonov-Bohm effect.
Confined LO-phonon assisted tunneling in a parabolic quantum well with double barriers
Authors: Gong J. , Liang X. X. , Ban S. L.
Published: August 2006

The spatial dependence of the wave functions for electrons in a parabolic quantum well with double barriers has been calculated by using a transfer-matrix approach and compared with that of a rectangular quantum well. The phonon assisted tunneling currents are studied theoretically based on a dielectric continuum model. Only confined LO phonons are considered due to the destroyed interface character. It is found that the results are helpful in observing the current peaks for phonon assisted tunneling as expected. It indicates that the confined LO-phonon peaks are higher than that for rectangular wells. The influence on the current peaks of tunneling from phonons in the emitter barrier is more obvious for a parabolic well with a wider width.
Optical properties of inorganic AgSb recording thin film
Authors: Fang Y. H. , Kuo P. C. , Chen P. W. , Hsu Wei-Chih , Chou C. Y. , Wu T. H.
Published: August 2006

50 nm  Ag1−xSbx (x=10.8–25.5) thin films were prepared by magnetron sputtering. Thermal analysis shows that the phase change occurs around 250°C. The optical property analysis show that the as-deposited Ag80.9Sb19.1 films have high reflectivity of about 62%–73%. After heat treatment at 300°C, the contrast of Ag80.9Sb19.1 film is 12.5%–17% for wavelengths between 400 and 800nm. Dynamic test shows that using the Ag80.9Sb19.1 film as the memory layer of write once optical disk, a carrier-to-noise ratio of about 45dB can be achieved at λ=657nm, numerical aperture of 0.65, and a linear velocity of 3.5m∕s.
Internal wave instability: Wave-wave versus wave-induced mean flow interactions
Authors: Sutherland B. R.
Published: August 2006

In continuously stratified fluid, vertically propagating internal gravity waves of moderately large amplitude can become unstable and possibly break due to a variety of mechanisms including (with some overlap) modulational instability, parametric subharmonic instability (PSI), self-acceleration, overturning, and convective instability. In PSI, energy from primary waves is transferred, for example, to waves with half frequency. Self-acceleration refers to a mechanism whereby a wave packet induces a mean flow (analogous to the Stokes drift of surface waves) that itself advects the waves until they become convectively unstable. The simulations presented here show that self-acceleration dominates over parametric subharmonic instability if the wave packet has a sufficiently small vertical extent and sufficiently fast frequency.
Morphologies in solvent-annealed thin films of symmetric diblock copolymer
Authors: Peng Juan , Kim Dong Ha , Knoll Wolfgang , Xuan Yu , Li Binyao , Han Yanchun
Published: August 2006

We have systematically studied the thin film morphologies of symmetric poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer after annealing to solvents with varying selectivity. Upon neutral solvent vapor annealing, terraced morphology is observed without any lateral structures on the surfaces. When using PS-selective solvent annealing, the film exhibits macroscopically flat with a disordered micellar structure. While PMMA-selective solvent annealing leads to the dewetting of the film with fractal-like holes, with highly ordered nanoscale depressions in the region of undewetted films. In addition, when decreasing the swelling degree of the film in the case of PMMA-selective solvent annealing, hills and valleys are observed with the coexistence of highly ordered nanoscale spheres and stripes on the surface, in contrast to the case of higher swelling degree. The differences are explained qualitatively on the basis of polymer-solvent interaction parameters of the different components.
Stationary regimes of axisymmetric thermal wake interaction of two buoyant drops at low Reynolds and high Peclet number
Authors: Frolovskaya O. , Nir A. , Lavrenteva O. M.
Published: August 2006

Axisymmetric motion of a leading fluid drop and a trailing gas bubble (or thermally nonconducting drop) in a viscous fluid under the combined effect of gravity and thermocapillarity is considered under the assumption of negligible inertia effects and of nondeformable interfaces. The ambient fluid far from the inclusions is isothermal and the temperature of the leading particle differs from that of the continuous medium. At large Peclet number, thermal boundary layers are present along the fluid-liquid and the gas-liquid interfaces, and thermal wakes are formed downstream from the particles. The interaction of the thermal wake, shed from the leading inclusion, with the thermal boundary layer on the surface of the trailing one causes a nonuniform temperature distribution on the surface of the latter. The induced Marangoni flow results in the change of the flow pattern, the velocity of both particles, and the equilibrium separation distance. In the present paper, the influence of the Marangoni effect on the drag force and the rate of heat transfer from the drops translating at given velocity is studied as well as on the equilibrium velocities and separation distance of the drops freely migrating under the action of gravity. The analysis considers particles at arbitrary separation distance and takes full account of thermal and hydrodynamic interactions.
Lubricant dynamics under sliding condition in disk drives
Authors: Wu Lin
Published: August 2006

In this paper, we develop a two-dimensional flow model for the lubricant flow dynamics under a sliding head in disk drives. Our two-dimensional model includes important physics such as viscous force, external air shearing stress, air bearing pressure, centrifugal force, disjoining pressure, and surface tension. Our analysis shows that the lubricant flow dynamics under the sliding condition is a fully two-dimensional phenomenon and the circumferential lubricant flow is strongly coupled to the radial flow. It is necessary to have a two-dimensional flow model that couples the circumferential and radial flows together and includes all important physics to achieve realistic predictions. Our results show that the external air shearing stress has a dominant effect on the lubricant flow dynamics. Both velocity slippage at wall and Poiseuille flow effects have to be considered in the evaluation of the air shearing stress under the head. The nonuniform air bearing pressure has a non-negligible effect on the lubricant film dynamics mostly through the Poiseuille flow effect on the air shearing stress but not from its direct pushing or sucking effect on the lubricant surface. Prediction of the formation of lubricant depletion tracks under a sliding head using the two-dimensional model agrees reasonably well with the existing experimental measurements.
Electrokinetic particle aggregation patterns in microvortices due to particle-field interaction
Authors: Hou Diana , Chang Hsueh-Chia
Published: August 2006

A complex and dynamic particle banding phenomenon in an electric field is reported. A single cylindrical vortex flow is generated by a dc-biased ac electro-osmotic flow on parallel electrodes. Charged particles are attracted to the vortex by positive dielectrophoresis (DEP) to form rotating cylindrical structures. As the particle concentration increases, the cylinder undergoes longitudinal symmetry breaking, producing concentrated rotating bands caused by field screening effects. The focusing of the particles into bands is shown to obey negative diffusion dynamics of a long-wave instability. Funnels and butterfly-like patterns also form because of secondary longitudinal DEP forces from nonuniform screening effects.
Dynamics of vortex lock-on in a perturbed cylinder wake
Authors: Kim Wontae , Yoo Jung Yul , Sung Jaeyong
Published: August 2006

The dynamics of vortex lock-on, downstream of a circular cylinder in an oscillatory flow with nonzero mean velocity, is investigated by applying a time-resolved particle image velocimetry technique at the Reynolds number 360. Since the lock-on occurs when the near wake behind a cylinder is perturbed at twice the natural shedding frequency, we interrogate the wake regions of mean recirculation and vortex formation, and analyze the dynamic behavior of the shed vortices, the coherent structure, the phases of vortex evolution, and the Reynolds stress fields. It is shown that due to vortex lock-on the mean recirculation and vortex formation regions are considerably reduced, which is consistent with previous studies. Besides, the kinetic energy in the near wake is more concentrated near the cylinder base. A dramatic change is demonstrated in the trajectory of the shed vortices, which is evaluated from a hybrid method based on complex eigenvalue and vortex centroid. A novel method to identify the phases of the vortex evolution is proposed in accordance with the distribution of the coherent Reynolds shear stress. Then, it is shown that two flow fields in the natural shedding and lock-on states have a phase difference of about π∕4. As a result of vortex lock-on, the distributions of the Reynolds stresses exhibit not only a stronger Kármán vortex, but also a more synchronized pattern with a shortened wake region. From the streamwise force balance on the wake bubble, it is noted that an outstanding decrease (a reversal) of the shear stress by the lock-on results in an increase of the drag force and that the energy transfer from random component to coherent component decreases the three dimensionality of the cylinder wake, inducing the above-mentioned shedding of stronger Kármán vortices.
Refinements on the quasi-geostrophic ellipsoidal vortex model
Authors: Li Yingtai , Taira Hiroshi , Takahashi Naoya , Miyazaki Takeshi
Published: August 2006

Refinements on the quasi-geostrophic ellipsoidal moment model are proposed, where each coherent vortex is modeled by an ellipsoid of uniform potential vorticity. The dynamics of N ellipsoidal vortices can be described by a canonical Hamiltonian system of 3N degrees of freedom. We introduce a new set of “nearly” canonical variables in addition to those from the refined wire-vortex (spheroidal) model with 2N degrees of freedom. We replace the moment expansions used in the previous ellipsoidal moment model by the Gaussian integration method in the evaluation of the mutual interaction energy integrals. The refined model describes the interaction of two vortices very well and predicts the critical merger distance that was observed by direct simulation more accurately than the refined wire-vortex (spheroidal) model.
Quantized time correlation function approach to nonadiabatic decay rates in condensed phase: Application to solvated electrons in water and methanol
Authors: Borgis Daniel , Rossky Peter J. , Turi László
Published: August 2006

A new, alternative form of the golden rule formula defining the nonadiabatic transition rate between two quantum states in condensed phase is presented. The formula involves the quantum time correlation function of the energy gap, of the nonadiabatic coupling, and their cross terms. Those quantities can be inferred from their classical counterparts, determined via molecular dynamics simulations. The formalism is applied to the problem of the nonadiabatic p→s relaxation of an equilibrated p-electron in water and methanol. We find that, in both solvents, the relaxation is induced by the coupling to the vibrational modes and the quantum effects modify the rate by a factor of 2–10 depending on the quantization procedure applied. The resulting p-state lifetime for a hypothetical equilibrium excited state appears extremely short, in the sub-100fs regime. Although this result is in contrast with all previous theoretical predictions, we also illustrate that the lifetimes computed here are very sensitive to the simulated electronic quantum gap and to the strongly correlated nonadiabatic coupling.
Permeability of the fractal disk-shaped branched network with tortuosity effect
Authors: Xu Peng , Yu Boming , Feng Yongjin , Zou Mingqing
Published: August 2006

In this Brief Communication, the effective permeability of the fractal disk-shaped branched networks is derived and the relationship between the effective permeability and the geometry structures is analyzed in detail. It is found that the tortuosity has significant influence on transport properties of the network and that small variations in the geometrical structures can induce very large variations in the effective permeability. The effective permeability approaches zero as the diameter ratio β<0.5 for different length ratios γ. A comparison of the fractal disk-shaped branched network with the traditional parallel net indicates that the fractal disk-shaped branched network has a much stronger capability of fluid transport. The conductivity scaling law for the fractal disk-shaped branched network is obtained to be Ke∼Va, where the scaling exponent a=−(1∕2)ln(N∕γ)∕ln(γβ2), and the scaling exponent a depends on the microstructures of the network in a very wide range and a=1∕2 under the area-preserving condition.