bluuebunny/crossref_metadata_2025 huggingface dataset:
Stochastic heat and Burgers equations and their singularities. II. Analytical properties and limiting distributions
Authors: Davies Ian M. , Truman Aubrey , Zhao Huaizhong
Published: March 2005

We study the inviscid limit, μ→0, of the stochastic viscous Burgers equation, for the velocity field vμ(x,t), t>0, x∊Rd, (∂vμ∕∂t)+(vμ·∇)vμ=−∇c(x,t)−ϵ∇k(x,t)Ẇt+(μ2∕2)Δvμ, for small ϵ, with vμ(x,0)≡∇S0(x) for some given S0, Ẇt representing white noise. Here we use the Hopf–Cole transformation, vμ=−μ2∇lnuμ, where uμ satisfies the stochastic heat equation of Stratonovich-type and the Feynmac–Kac Truman–Zhao formula for uμ, where dutμ(x)=[(μ2∕2)Δutμ(x)+μ−2c(x,t)utμ(x)]dt+ϵμ−2k(x,t)utμ(x)∘dWt, with u0μ(x)=T0(x)exp(−S0(x)∕μ2), S0 as before and T0 a smooth positive function. In an earlier paper, Davies, Truman, and Zhao [J. Math. Phys. 43, 3293 (2002)], an exact solution of the stochastic viscous Burgers equation was used to show how the formal “blow-up” of the Burgers velocity field occurs on random shockwaves for the vμ=0 solution of Burgers equation coinciding with the caustics of a corresponding Hamiltonian system with classical flow map Φ. Moreover, the uμ=0 solution of the stochastic heat equation has its wavefront determined by the behavior of the Hamilton principal function of the corresponding stochastic mechanics. This led in particular to the level surface of the minimizing Hamilton–Jacobi function developing cusps at points corresponding to points of intersection of the corresponding prelevel surface with the precaustic, “pre” denoting the preimage under Φ determined algebraically. These results were primarily of a geometrical nature. In this paper we consider small ϵ and derive the shape of the random shockwave for the inviscid limit of the stochastic Burgers velocity field and also give the equation determining the random wavefront for the stochastic heat equation both correct to first order in ϵ. In the case c(x,t)=12xTΩ2x, ∇k(x,t)=−a(t), we obtain the exact random shockwave and prove that its shape is unchanged by the addition of noise, it merely being displaced by a random Brownian vector N(t). By exploiting the Jacobi fields for this problem we obtain the large time limit of the distribution of the Burgers fluid velocity for noises which have infinite time averages, such as almost periodic ones. Here resonance with the underlying ϵ=0 classical problem has an important effect. Imitating these results for the case of a periodic underlying classical problem perturbed by small noise, arming ourselves with some detailed estimates for Green’s functions enables us to make generalizations. In the stochastic case we have also the possibility of “infinitely rapid” changes in the number of cusps on the minimizing level surface of the Hamilton–Jacobi function. This will engender stochastic turbulence in the Burgers velocity field and, due to its stochasticity, may be of an “intermittent” nature. There is no analog of this in the deterministic case.
Experiments on the late-time development of single-mode Richtmyer–Meshkov instability
Authors: Jacobs J. W. , Krivets V. V.
Published: March 2005

The late-time development of Richtmyer–Meshkov instability is studied in shock tube experiments. This investigation makes use of the experimental apparatus and visualization methods utilized in the earlier study of Collins and Jacobs [J. Fluid Mech. 464, 113 (2002)] but employs stronger shocks and initial perturbations with shorter wavelengths to obtain much later-time (in the dimensionless sense) images of the single-mode instability. These modifications produce a very detailed look at the evolution of the late-time single-mode instability, revealing the transition and development of turbulence in the vortex cores that eventually results in the disintegration of the laminar vortex structures into small scale features. Amplitude measurements taken from these images are shown to be effectively collapsed when plotted in dimensionless variables defined using the wave number and the initial growth rate. The amplitude measurements are compared with several late-time nonlinear models and solutions. The best agreement is obtained with the model of Sadot et al. [Phys. Rev. Lett. 80, 1654 (1998)] which can be slightly improved by modifying the expression for the late-time asymptotic growth rate.
Backward displacement of ultrasonic waves reflected from a periodically corrugated interface
Authors: Teklu A. , Breazeale M. A. , Declercq Nico F. , Hasse Roger D. , McPherson Michael S.
Published: March 2005

Experiments using the schlieren technique to image sound incident on a corrugated, water-brass interface show a backward displacement of the reflected beam at an angle of 22.5°, confirming the observations of Breazeale and Torbett [Appl. Phys. Lett. 29, 456 (1976)]. However, the present theory hypothesizes that this beam displacement results from excitation of a type of leaky surface wave. Further experiments with a sufficiently narrow incident beam reveal a backward displacement also at angles around 44°, resulting from excitation of Rayleigh surface waves, as predicted by the theory of Tamir and Bertoni [J. Acoust. Soc. Am. 61, 1397 (1971)]. Thus, a wide beam gives a backward displacement at 22.5° only. A narrow beam gives a backward displacement also at angles around 44°.
On the microstructure of AlxGa1−xN layers grown on 6H-SiC(0001) substrates
Authors: Kröger Roland , Einfeldt Sven , Chierchia Rosa , Hommel Detlef , Reitmeier Zachary J. , Davis Robert F. , Liu Quincy K. K.
Published: March 2005

The microstructural as well as the compositional evolution of AlxGa1−xN (x∼0.15) layers grown on 6H-SiC(0001) substrates by metalorganic vapor phase epitaxy were analyzed by atomic force microscopy, X-ray diffraction, and transmission electron microscopy in conjunction with energy dispersive X-ray spectroscopy. The epitaxial growth was followed from the early nucleation stage on the substrate to the development of a thick bulk-like film. Phase separation was observed during the early stage of growth; that is, islands of two different shapes formed whose Al mole fractions were about 0.035 and 0.18, respectively. The AlxGa1−xN coalesced at a film thickness of about 100 nm with the domains of varying Al content being fully coherent. Such domains were not only found at the film∕substrate interface but also further away from the interface. They were arranged in layers that were shifted laterally against each other; that is, Al-deficient domains formed on top of Al-rich domains and vice versa. Increasing the film thickness to more than 100 nm finally led to a homogeneous Al distribution. Finite-element simulations were performed to calculate the strain distribution in these inhomogeneous systems. They allowed the experimental results to be explained by an interplay of strain minimization in the epitaxial film and growth kinetics.
Vibronic energies and spectra of molecular dimers
Authors: Eisfeld A. , Braun L. , Strunz W. T. , Briggs J. S. , Beck J. , Engel V.
Published: March 2005

We consider three distinct methods of calculating the vibronic levels and absorption spectra of molecular dimers coupled by dipole-dipole interactions. The first method is direct diagonalization of the vibronic Hamiltonian in a basis of monomer eigenstates. The second method is to use creation and annihilation operators leading in harmonic approximation to the Jaynes–Cummings Hamiltonian. The adiabatic approximation to this problem provides insight into spectral behavior in the weak and strong coupling limits. The third method, which serves as a check on the accuracy of the previous methods, is a numerically exact solution of the time-dependent Schrödinger equation. Using these methods, dimer spectra are calculated for three separate dye molecules and show good agreement with measured spectra.
Temperature dependence of transport properties of high mobility holes in Ge quantum wells
Authors: Myronov M. , Irisawa T. , Koh S. , Mironov O. A. , Whall T. E. , Parker E. H. C. , Shiraki Y.
Published: March 2005

The transport properties of a two-dimensional hole gas (2DHG), with very high room-temperature drift mobilities, formed in 7.5- (2540cm2V−1s−1) and 20-nm (2940cm2V−1s−1) strained Ge quantum wells of multilayered Si∕Si0.33Ge0.67∕Ge∕Si0.33Ge0.67∕Si0.73Ge0.27∕Si(001) p-type modulation-doped heterostructures were investigated experimentally in the temperature range of 10–295K. For both samples the drift mobility of the 2DHG behaves in the same way as the Hall mobility, increases as the temperature decreases. The sheet carrier density of the 2DHG increases with decreasing temperature, which is opposite to the behavior of Hall-effect sheet carrier density. We found that the 2DHG formed in the thicker Ge quantum well has a higher drift mobility at 295K but a lower one at low temperatures. For both samples the Hall factor increases with decreasing temperature, reaching unity at 10K, when carriers in parallel conducting layers completely freeze-out. The interpretation of these results is carried out and the scattering mechanisms limiting the 2DHG mobility are discussed.
Phase transition and properties of a ferroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer
Authors: He Xujiang , Yao Kui , Gan Bee Keen
Published: March 2005

The phase transition, ferroelectric, and piezoelectric properties of a poly(vinylidene fluoride-hexafluoropropylene) [PVDF∕HFP (10 mol %)] copolymer have been investigated. A rapid quenching process led to the formation of the ferroelectric β phase in the PVDF∕HFP (10 mol %). An irreversible crystal phase transition from the β phase to the α and γ phases at a temperature from 120 to 130 °C, which is below its melting point 159 °C, was observed through x-ray diffraction (XRD) experiments. Stretching the melt-quenched material or annealing the material at a temperature below the crystal phase transition temperature resulted in a significant increase in the β phase. Based on the XRD results, the stretched samples had oriented β phase, but the amount of β phase is less than that in the annealed samples; however, our ferroelectric property measurements showed that the stretched samples had a higher polarization than the annealed samples. A theoretical analysis was conducted to clarify the effects of the polymer chain orientation in order to explain our ferroelectric measurement results. Assuming the dipoles are rigid and ignoring the effects of the internal electric field, it was found that the polarization of a randomly oriented PVDF-based polymer is about two-thirds of that of a completely oriented polymer when the volume fractions of β phase are same. According to the analysis, polymer chain orientation significantly affects the ferroelectric polarization due to their determinative role on the distribution of the dipole polarization direction, although there is no polarization along the polymer chain direction.
The importance of tunneling in the first hydrogenation step in ammonia synthesis over a Ru(0001) surface
Authors: Tautermann Christofer S. , Clary David C.
Published: March 2005

The hydrogenation of nitrogen (Nads+Hads→NHads) on metal surfaces is an important step in ammonia catalysis. We investigate the reaction dynamics of this hydrogenation step by time independent scattering theory and variational transition state theory (VTST) including tunneling corrections. The potential energy surface is derived by hybrid density functional theory on a model cluster composed of 12 ruthenium atoms resembling a Ru(0001) surface. The scattering calculations are performed on a reduced dimensionality potential energy hypersurface, where two dimensions are treated explicitly and all others are included implicitly by the zero-point correction. The VTST calculations include quantum effects along the reaction coordinate by applying the small curvature tunneling scheme. Even at room temperature (where ruthenium already shows catalytic activity) we find rate enhancement by tunneling by a factor of ≈70. Inspection of the reaction probabilities shows that the major contribution to reactivity comes from the vibrational ground state of the reactants into vibrationally excited product states. The reaction rates are higher than determined in previous studies, and are compatible with experimental overall rates for ammonia synthesis.
Electrical activation characteristics of silicon-implanted GaN
Authors: Irokawa Y. , Fujishima O. , Kachi T. , Nakano Y.
Published: March 2005

Electrical activation studies of Si-implanted GaN layers on sapphire were made as a function of annealing temperature (1100–1400°C). For an ion dose of 1.0×1014cm−2, the optimum annealing temperature was 1400°C, exhibiting a nearly 100% electrical activation efficiency and a low sheet resistance of ∼450Ω∕square at room temperature. From variable temperature Hall-effect measurements, Si-implanted GaN films annealed below 1200°C displayed deep ionization levels of ∼280meV, whereas samples annealed above 1300°C had shallow ones of ∼11meV. For lateral Schottky diodes fabricated on Si-implanted GaN layers annealed below 1200°C, capacitance frequency and thermal admittance measurements showed a typical dispersion effect characteristic of a single deep donor with an activation energy of ∼133meV. These results illustrate that deep donor levels created by the Si implantation in GaN layers apparently annihilate and transit to shallow levels produced by the Si ion substitution for Ga in the GaN lattice (SiGa) by annealing at temperature greater than ∼1300°C at these doses.
Temperature dependence of interfacial fluctuations of polymerized fatty acid salt multilayers
Authors: Hu Shu-Xin , Li Xiu-Hong , Jia Quan-Jie , Mai Zhen-Hong , Li Ming
Published: March 2005

X-ray scattering was used to study the temperature dependence of the profile structure of polymerized 10,12-tricosadiynoic acid salt multilayers. The stacking periodicity of the multilayers was found to decrease with increasing temperature due to the conformational changes of the alkyl chains. When the samples were fully hydrated in water, the reflectivity measurement showed that the thermal fluctuations of the interfaces are enhanced with temperature, resulting in reduced ordering. Meanwhile, the diffuse scattering indicated that the thermal fluctuations renormalize the elasticity of the multilayers; both the bending and the compression moduli are reduced. Similar measurements performed in air, however, do not show this thermal enhancement although the stacking periodicity decreases in the same manner. It is implied that water might weaken the interaction between the carboxyl groups and the metal ions so that the polymerized bilayers are softened in water.
Molecular simulation of surfactant-assisted protein refolding
Authors: Lu Diannan , Liu Zheng , Liu Zhixia , Zhang Minlian , Ouyang Pingkai
Published: March 2005

Protein refolding to its native state in vitro is a challenging problem in biotechnology, i.e., in the biomedical, pharmaceutical, and food industry. Protein aggregation and misfolding usually inhibit the recovery of proteins with their native states. These problems can be partially solved by adding a surfactant into a suitable solution environment. However, the process of this surfactant-assisted protein refolding is not well understood. In this paper, we wish to report on the first-ever simulations of surfactant-assisted protein refolding. For these studies, we defined a simple model for the protein and the surfactant and investigated how a surfactant affected the folding behavior of a two-dimensional lattice protein molecule. The model protein and model surfactant were chosen such that we could capture the important features of the folding process and the interaction between the protein and the surfactant, namely, the hydrophobic interaction. It was shown that, in the absence of surfactants, a protein in an “energy trap” conformation, i.e., a local energy minima, could not fold into the native form, which was characterized by a global energy minimum. The addition of surfactants created folding pathways via the formation of protein-surfactant complexes and thus enabled the conformations that fell into energy trap states to escape from these traps and to form the native proteins. The simulation results also showed that it was necessary to match the hydrophobicity of surfactant to the concentration of denaturant, which was added to control the folding or unfolding of a protein. The surfactants with different hydrophobicity had their own concentration range on assisting protein refolding. All of these simulations agreed well with experimental results reported elsewhere, indicating both the validity of the simulations presented here and the potential application of the simulations for the design of a surfactant on assisting protein refolding.
Phase behavior and local structure of a binary mixture in pores: Mean-field lattice model calculations for analyzing neutron scattering data
Authors: Woywod Dirk , Schemmel Sebastian , Rother Gernot , Findenegg Gerhard H. , Schoen Martin
Published: March 2005

We investigate the phase behavior of an asymmetric binary liquid A−W mixture confined between two planar homogenous substrates (slit pore). Molecules of species W interact preferentially with the solid walls via a long-range potential. Assuming nearest-neighbor attractions between the liquid molecules, we employ a lattice-gas model and a mean-field approximation for the grand potential. Minimization of this potential yields the density profiles of thermodynamically stable phases for fixed temperature, chemical potentials of both species, pore width and strengths of attraction. This model is used to analyze experimental small-angle neutron-scattering (SANS) data on the microscopic structure of the binary system isobutyric acid (iBA)+heavy water (D2O) inside a mesoscopic porous matrix (controlled-pore glass of about 10 nm mean pore width). Confinement-independent model parameters are adjusted so that the theoretical liquid-liquid coexistence curve in the bulk matches its experimental counterpart. By choosing appropriate values of the pore width and the attraction strength between substrates and water we analyze the effect of confinement on the phase diagram. In addition to a depression of the liquid-liquid critical point we observe surface induced phase transitions as well as water-film adsorption near the walls. The temperature dependence of the structure of water-rich and iBA-rich phases of constant composition are discussed in detail. The theoretical predictions are consistent with results of the SANS study and assist their interpretation.
An abstract interface to higher spin gauge field theory
Authors: Bengtsson A. K. H.
Published: March 2005

A comprehensive approach to the theory of higher spin gauge fields is proposed. By explicitly separating out details of implementation from general principles, it becomes possible to focus on the bare minimum of requirements that such a theory must satisfy. The abstraction is based on a survey of the progress that has been achieved since relativistic wave equations for higher spin fields were first considered in the 1930s. As a byproduct, a formalism is obtained that is abstract enough to describe a wide class of classical field theories. The formalism, viewed as syntax, can then be semantically mapped to a category of strongly homotopy Lie algebras, thus showing that the theory in some sense exists, at least as an abstract mathematical structure. Still, a concrete physicslike, implementation remains to be constructed. Lacking deep physical insight into the problem, an implementation in terms of generalized vertex operators is set up within which a brute force iterative determination of the first few orders in the interaction can be attempted.
Effect of oxygen content on piezoresistivity of indium tin oxide thin films prepared by pulsed laser deposition
Authors: Fang H. , Miller T. , Rogers B. R. , Magruder R. H. , Weller R. A.
Published: March 2005

The piezoresistivity of thin films of indium tin oxide prepared by pulsed laser deposition has been measured as a function of the O-to-(In+Sn) atom ratio. The oxygen-to-metal atom ratio was determined through Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy analyses. Gauge factors, defined as the fractional change of the film resistance to the applied strain, increase with the film’s oxygen content. The deposition under 50 mTorr oxygen pressure resulted in the film with the largest oxygen-to-metal atom ratio, 1.92, and a gauge factor of −14.5. A model based on hopping conduction is proposed. Results from this model are consistent with the sign and magnitude of the observed gauge factors.
Dynamical scaling of single chains on adsorbing substrates: Diffusion processes
Authors: Descas Radu , Sommer Jens-Uwe , Blumen Alexander
Published: March 2005

We study the dynamics of tethered chains of length N on adsorbing surfaces, considering the dilute case; for this we use the bond fluctuation model and scaling concepts. In particular, we focus on the mean-square displacement of single monomers and of the center of mass of the chains. The characteristic time τ of the fluctuations of a free chain in a good solvent grows as τ∼Na, where the coefficient a obeys a=2ν+1. We show that the same coefficient also holds at the critical point of adsorption. At intermediate time scales single monomers show subdiffusive behavior; this concurs with the behavior calculated from scaling arguments based on the dynamical exponent a. In the adsorbed state τ⊥, the time scale for the relaxation in the direction perpendicular to the surface, becomes independent of N;τ⊥ is then the relaxation time of an adsorption blob. In the direction parallel to the surface the motion is similar to that of a two-dimensional chain and is controlled by a time scale given by τ‖∼N2ν2+1L−2Δν∕ν, where ν2 is the Flory exponent in two dimensions, ν is the Flory exponent in three dimensions, and Δν=ν2−ν. For the motion parallel to the surface we find dynamical scaling over a range of about four decades in time.
Magnetron sputter epitaxy of wurtzite Al1−xInxN(0.1<x<0.9) by dual reactive dc magnetron sputter deposition
Authors: Seppänen T. , Persson P. O. Å. , Hultman L. , Birch J. , Radnóczi G. Z.
Published: March 2005

Ternary wurtzite Al1−xInxN thin films with compositions throughout the miscibility gap have been grown onto seed layers of TiN and ZrN by magnetron sputter epitaxy (MSE) using dual reactive direct current magnetron sputter deposition under ultra high vacuum conditions. The film compositions were calculated using Vegard’s law from lattice parameters determined by x-ray diffraction (XRD). XRD showed that single-phase Al1−xInxN alloy films in the wurtzite structure with [0.10<x<0.90] could be obtained at substrate temperatures up to 600°C by heteroepitaxial growth. Epitaxial growth at 600°C gave the crystallographic relations Al1−xInxN(0001)∕∕TiN,ZrN(111) and Al1−xInxN⟨10-10⟩∕∕TiN,ZrN⟨110⟩. At higher substrate temperatures almost pure AlN was formed. The microstructure of the films was also investigated by high-resolution electron microscopy. A columnar growth mode with epitaxial column widths from 10to200nm was observed. Rocking curve full-width-at-half-maximum measurements revealed highly stressed lattices for growth onto TiN at 600°C. Pseudobinary MSE growth phase field diagrams for Al1−xInxN onto ZrN and TiN were established for substrate temperatures up to 1000°C. Large regimes for single-phase solid solutions were thus identified with In being the diffusing species.
Electrical switching in sol–gel derived Ag–SiO2 nanocomposite thin films
Authors: Sarkar D. K. , Cloutier F. , El Khakani M. A.
Published: March 2005

The sol–gel technique has been used to produce Ag–SiO2 nanocomposite thin films consisting of silver nanoparticles embedded in a SiO2 matrix. The size of the silver nanoparticles is of about (4±0.2)nm when the firing temperature is in the (500–700°C) range, as determined from ultraviolet-visible spectroscopy analysis. The increase of the firing temperature beyond 300°C, was found to lead to an outer diffusion of Ag toward the surface. As a consequence the surface atomic concentration of Ag is found to increase from 1.3% to 12% for the as-dried (at 120°C) and for those further fired at 600°C, respectively. On the other hand, it is shown that the electrical behavior of these Ag–SiO2 nanocomposite films can drastically change from highly insulating to conducting depending on the firing temperature used. In the intermediate firing temperature (300–400°C) range, the films were found to exhibit a reversible switching behavior with a resistivity transition of about 7 orders of magnitude between the insulating (OFF) and the conducting (ON) state. Such an electrical switching could be explained by a change in the predominant conduction mechanism of the films.
Ab initio study of the gas phase nucleation mechanism of GaN
Authors: Moscatelli Davide , Caccioppoli Pierpaolo , Cavallotti Carlo
Published: March 2005

The mechanism of formation of gas phase adducts in a Ga(CH3)3–NH3 atmosphere was investigated using density functional theory and fluid dynamic simulations. We found that the GaN gas phase chemistry is dominated by a radical chain mechanism, started by the CH3· radicals originated by the decomposition of Ga(CH3)3, and resulting in the fast formation of linear covalently bonded R(GaN)x adducts. As the number of GaN units in the adduct reaches three, the molecule cyclises. A kinetic mechanism suitable to study the formation of GaN adducts was then proposed and used to simulate the chemistry in a GaN metalorganic chemical vapor deposition reactor. It was found that GaNH2 is among the most important precursors to the film growth.
Reconciliation of luminescence and Hall measurements on the ternary semiconductor CuGaSe2
Authors: Siebentritt Susanne , Beckers Inge , Riemann Till , Christen Jürgen , Hoffmann Axel , Dworzak Matthias
Published: March 2005

Previous Hall and photoluminescence investigations on CuGaSe2 yielded conflicting results: defect depths determined by luminescence are lower than the ionization energy found in electrical measurements. In this contribution, we present time and spatially resolved cathodoluminescence measurements that necessitate a new interpretation of the luminescence data, leading to a consistent model for luminescence and Hall data with three acceptors with ionization energies of 60, 100, and 150meV. Luminescence decay times are long, in the range of 100ns, indicating strong capture. Luminescence is spatially inhomogeneous, indicating inhomogeneous distribution of defects.
Conductivity switching characteristics and reset currents in NiO films
Authors: Seo S. , Lee M. J. , Seo D. H. , Choi S. K. , Suh D.-S. , Joung Y. S. , Yoo I. K. , Byun I. S. , Hwang I. R. , Kim S. H. , Park B. H.
Published: March 2005

Conductivity switching phenomena controlled by external voltages have been investigated for various NiO films deposited by dc reactive sputtering methods. Pt∕NiO∕Pt capacitor structures with top electrodes of different diameters have showed increasing off-state current with the diameter of a top electrode and nearly the same on-state current independent of the diameter. Local conductivity switching behaviors have been observed in a series structure consisting of two Pt∕NiO∕Pt capacitors with different resistance values. By reasoning out conductivity switching mechanisms from the switching characteristics and introducing multilayers consisting of NiO layers with different resistance values, we have reduced the reset current by two orders of magnitude.
Method for trapping and manipulating nanoscale objects in solution
Authors: Cohen Adam E. , Moerner W. E.
Published: March 2005

We present a device that allows a user to trap a single nanoscale object in solution at ambient temperature, and then to position the trapped object with nanoscale resolution. This anti-Brownian electrophoretic trap (ABEL trap) works by monitoring the Brownian motion of the particle (via fluorescence microscopy), and then applying a feedback voltage to the solution so that the electrophoretic drift exactly cancels the Brownian motion. The ABEL trap works on any object that can be imaged optically and that acquires a charge in water. The ABEL trap is noninvasive, is gentle enough to handle biological molecules, and can trap objects far smaller than can be trapped with laser tweezers. Our proof-of-principle device can trap fluorescent polystyrene nanospheres with diameters down to 20nm.
A 1:1 complex between a hydroxyl radical and ozone
Authors: Engdahl Anders , Nelander Bengt
Published: March 2005

A hydrogen bonded complex between a hydroxyl radical and ozone has been found in argon matrices at 9 K. The shift of the OH stretch (−12.6cm−1) indicates that this complex is somewhat weaker than the OH–CO complex (−21.8cm−1, D0⩽430cm−1, 1.2 kcal/mol).
Robustness of nanotube electronic transport to conformational deformations
Authors: Léonard F. , Jones F. E. , Talin A. A. , Dentinger P. M.
Published: March 2005

We present experimental observation and theoretical analysis of looping carbon nanotubes connecting two electrodes. The measured conductance of the nanotubes is not strongly affected by the presence of these conformational defects, a result that is confirmed by quantum transport calculations. Our work indicates that solution-based fabrication methods for carbon nanotube devices can have high conformational defect tolerance, except for defects with 5–10nanometer bending radius.
Performance evaluation of ballistic silicon nanowire transistors with atomic-basis dispersion relations
Authors: Wang Jing , Rahman Anisur , Ghosh Avik , Klimeck Gerhard , Lundstrom Mark
Published: March 2005

In this letter, we explore the band structure effects on the performance of ballistic silicon nanowire transistors (SNWTs). The energy dispersion relations for silicon nanowires are evaluated with an sp3d5s* tight binding model. Based on the calculated dispersion relations, the ballistic currents for both n-type and p-type SNWTs are evaluated by using a seminumerical ballistic model. For large diameter nanowires, we find that the ballistic p-SNWT delivers half the ON-current of a ballistic n-SNWT. For small diameters, however, the ON-current of the p-type SNWT approaches that of its n-type counterpart. Finally, the carrier injection velocity for SNWTs is compared with those for planar metal-oxide-semiconductor field-effect transistors, clearly demonstrating the impact of quantum confinement on the performance limits of SNWTs.
Using image resonances to probe molecular conduction at the n-heptane∕Au(111) interface
Authors: Lindstrom C. D. , Quinn Daniel , Zhu X.-Y.
Published: March 2005

The binding energies and lifetimes of the n=1 image resonance on Au(111) are measured as a function of n-heptane layer thickness by femtosecond time-resolved two-photon photoemission (TR-2PPE) spectroscopy. The lifetime of the image resonance dramatically increases from ∼4fs on clean Au(111) to 1.6ps with three layers of n-heptane. Because the image resonance is above the L1 band edge of Au, this increase in lifetime is attributed to the tunneling barrier presented by the σ-σ* band gap of the n-heptane film. We use the one-dimensional dielectric continuum model (DCM) to approximate the surface potential and to determine the binding energies and the lifetimes of the image resonances. The exact solution of the DCM potential is determined in two ways: the first by wave-packet propagation and the second by using a tight-binding Green’s function approach. The first approach allows band-edge effects to be treated. The latter approach is particularly useful in illustrating the similarity between TR-2PPE and conductance measurements.
Numerical simulation of head-on droplet collision: Effect of viscosity on maximum deformation
Authors: Dai Meizhong , Schmidt David P.
Published: March 2005

Numerical simulation of head-on collision of two equal-size droplets is conducted to observe the effect of viscosity on the maximum deformation amplitude using a moving-mesh finite-volume method. Recent experimental results by Willis and Orme [Exp. Fluids 34, 28 (2003)] have shown that the maximum deformation amplitude depends on the viscosity coefficient, and thus the percentage of energy that is dissipated until the instant of maximum deformation increases with the increasing fluid viscosity. This observation contradicts previous results by Jiang, Umemura, and Law [J. Fluid Mech. 234, 171 (1992)]. The numerical results in this Letter show that the dissipated energy and the maximum deformation depend on the collision Reynolds number, which is consistent with Willis and Orme (2003). However, this dependence decreases with increasing Reynolds number, which suggests that the effect caused by viscosity on maximum deformation becomes insignificant at sufficiently high Reynolds number.
Imaging of out-of-plane interfacial strain in epitaxial PbTiO3∕SrTiO3 thin films
Authors: van Helvoort A. T. J. , Dahl Ø. , Soleim B. G. , Holmestad R. , Tybell T.
Published: March 2005

In this study, we rely on low-angle annular dark-field scanning transmission electron microscopy to probe the interface strain profile in epitaxial PbTiO3∕SrTiO3 thin-films. All samples displayed a compressively strained layer at the PbTiO3∕SrTiO3 interface, with the strain vector parallel to the polarization direction. The width of the strained layer was found to be ∼15–30Å, dependent on the electrode environment. Our findings open a perspective to use interface strain engineering in combination with control of electrostatic boundary conditions as a tool to monitor the effective interface polarization. These findings have implications for future use of ferroelectrics in electronic and mechanical devices.
Physical model for frequency-dependent dynamic charge trapping in metal-oxide-semiconductor field effect transistors with HfO2 gate dielectric
Authors: Shen C. , Li M. F. , Yu H. Y. , Wang X. P. , Yeo Y.-C. , Chan D. S. H. , Kwong D.-L.
Published: March 2005

In this letter, we report on a physical model to explain the frequency dependence of dynamic charge trapping in metal-oxide-semiconductor (MOS) transistors with ultrathin HfO2 gate dielectrics. For transistors operating in a complementary MOS inverter circuit with a given gate voltage amplitude, we observed a reduction of charge trapping when the stress frequency is increased. This can be explained by the traps in the high-k HfO2 dielectric have the property of negative-U centers. One trap can capture two electrons sequentially, and the trap energy is reduced as a result of lattice relaxation. Results of calculation using the model show excellent agreement with all experiment data.
Selective internal manipulation of a single molecule by scanning tunneling microscopy
Authors: Soukiassian Laetitia , Mayne Andrew J. , Comtet Geneviève , Hellner Lucette , Dujardin Gérald , Gourdon André
Published: March 2005

We have studied the adsorption of the polyaromatic molecule 1,4″-paratriphenyldimethylacetone, which we have nicknamed Trima. The originality of this linear molecule is that it was designed and synthesized to have two functionalities. First, chemisorb itself to the surface by its two ends rather like a bridge. Second, the central part of the molecule could then be rotated by injecting electrons with the tip of the scanning tunneling microscope (STM). The length of the molecule corresponds exactly to the spacing between five dimers in a row on the Si(100)-2×1 surface. We found that the molecule adsorbs as expected on the clean silicon surface by using complementary STM and synchrotron radiation studies. Manipulation of individual molecules with the STM tip showed selective internal modifications that were highly voltage dependent. These manipulations were found to be compatible with an electronic excitation of the π-π* transition of the molecule.
Structure and luminescence of BaFBr:Eu2+ and BaFBr:Eu2+, Tb3+ phosphors and thin films
Authors: Chen Wei , Wang Shaopeng , Westcott Sarah L. , Zhang Jun , Dou Kai , Joly Alan G. , McCready David E.
Published: March 2005

The inclusion of Tb3+ ions doped into the storage phosphor BaFBr:Eu2+ results in increased purity of the host material as well as an improvement in the linearity of the photostimulated luminescence (PSL) intensity versus the x-ray irradiation dose. Films produced by pulsed laser deposition (PLD) also show similar results, indicating that PLD is a powerful technique for producing high quality, high purity thin films. As a result, the PSL dose response is more linear in the thin films than in the powder samples. The emission intensity of Eu2+ in the phosphors increases with the increasing time of x-ray irradiation while the emission intensity of Tb3+ remains almost constant. This result may be explained by considering the reduction of impurity Eu3+ ions following trapping of the photoexcited electrons. The results demonstrate that these materials are not only potentially useful for medical imaging but also show promise for use in radiation dosimetry.
Polarization of III-nitride blue and ultraviolet light-emitting diodes
Authors: Shakya J. , Knabe K. , Kim K. H. , Li J. , Lin J. Y. , Jiang H. X.
Published: March 2005

Polarization-resolved electroluminescence studies of III-nitride blue and ultraviolet (UV) light-emitting diodes (LEDs) were performed. The LEDs were fabricated on nitride materials grown by metalorganic chemical vapor deposition on sapphire substrates (0001). Transverse electric (TE) polarization dominates in the InGaN∕GaN quantum-well (QW) blue LEDs (λ′=458nm), whereas transverse magnetic (TM) polarization is dominant in the AlInGaN QW UV LEDs (λ=333nm). For the case of edge emission in blue LEDs, a ratio (r=I⊥∕I‖) of about 1.8:1 was observed between the EL intensities with polarization E⊥c (TE mode) and E‖c (TM mode), which corresponds to a degree of polarization ∼0.29. The UV LEDs exhibit a ratio r of about 1:2.3, corresponding to a degree of polarization ∼0.4. This is due to the fact that the degree of polarization of the bandedge emission of the AlxInyGa1−x−yN active layer changes with Al concentration. The low emission efficiency of nitride UV LEDs is partly related to this polarization property. Possible consequences and ways to enhance UV emitter performances related to this unique polarization property are discussed.
Effective plasma inductance computation
Authors: Maslovsky Dmitry A. , Boozer Allen H.
Published: March 2005

The calculation of the resistive wall modes and their feedback stabilization is greatly simplified by splitting the plasma response from that of the external conductors. Existing calculations either consider highly simplified models of the external conductors or assume the plasma response can be approximated by a single rigid mode. The full response of an ideal, nonrotating plasma to perturbations that evolve slowly compared to the Alfvén time is contained in the matrix for the effective plasma inductance Λ⃡. This matrix can be used in a code that accurately calculates the effects of the external conductors to obtain an essentially complete description of both plasma and the conducting structures. Calculations of Λ⃡ for a number of tokamak equilibria are given together with an explanation of how the calculations are made and why they are important.
Flow-induced vibrations of high mass ratio flexible filaments freely hanging in a flow
Authors: Schouveiler Lionel , Eloy Christophe , Le Gal Patrice
Published: March 2005

The behavior of high mass ratio flexible filaments freely hanging in steady horizontal uniform flows is experimentally and theoretically investigated. When the flow velocity is small, static equilibrium states, where the filaments are inclined to the flow, are observed. Then, above a critical value of the wind velocity, the filaments exhibit periodic oscillations in the vertical plane. The problem is theoretically addressed considering the beam theory equations for the filament dynamics where the action of the flowing fluid is modeled using semiempirical expressions. These equations are first solved for the stationary equilibrium states. Then, the stability of these steady solutions relative to small perturbations is analyzed. A good agreement between experimental and theoretical results is found.
High-power gyrotron traveling-wave amplifier with distributed wall losses and attenuating severs
Authors: Yeh Y. S. , Shin Y. Y. , You Y. C. , Chen L. K.
Published: March 2005

Distributed-loss gyrotron traveling-wave amplifiers (gyro-TWTs) with high-gain, broadband, and millimeter-wave capabilities have been demonstrated. Most structures with distributed wall losses are stabilized in gyro-TWTs that operate at low beam currents. Attenuating severs are added to the interaction circuit of a distributed-loss gyro-TWT to prevent high beam currents that result in mode competition. Simulation results show that gyrotron backward-wave oscillations (gyro-BWOs) are not effectively suppressed by the lossy section; in contrast, the severed sections can effectively enhance the start-oscillation threshold of gyro-BWOs in the proposed gyro-TWT. Meanwhile, localized reflective oscillations seem not to occur in the gyro-TWT unless it operates at a high magnetic field or with a high interaction length. The stable gyro-TWT, operating in the low-loss TE01 mode, is predicted to yield a peak output power of 405 kW at 33 GHz with an efficiency of 20%, a saturated gain of 77 dB and a 3 dB bandwidth of 2.5 GHz for a 100 kV, 20 A electron beam with an axial velocity spread of Δvz∕vz=5%.
Ultrafast resonant optical switching in isolated polyfluorenes chains
Authors: Virgili Tersilla , Marinotto Daniele , Lanzani Guglielmo , Bradley Donal D.C.
Published: March 2005

Using ultrafast spectroscopy, we report evidence of monodimensional photophysics coming from isolated chains of poly(9,9-dioctylfluorene) dispersed in a polymethylmethacrylate matrix. We identify intrachain charge carriers dynamics, the decay taking place in about 1 ps. Using a three-pulse experiment we bring evidence for charges generation from higher lying states and we report an ultrafast optical gain switching in the stimulated emission region over a broad wavelength range (around 100 nm). Our experiments suggest that the expected maximum rate of on–off switching in such a molecular device can be as high as 300 GHz.
Variations of differential capacitance in SrBi2Ta2O9 ferroelectric films induced by photoperturbation
Authors: Leu Ching-Chich , Chien Chao-Hsin , Chen Chih-Yuan , Chang Mao-Nan , Hsu Fan-Yi , Hu Chen-Ti , Chen Yung-Fu
Published: March 2005

In this letter, we demonstrated the impact of illumination on the differential capacitance variation of a strontium bismuth tantalite (SBT) capacitor during scanning capacitance microscopy measurements. It was found that illumination with a stray light of laser in an atomic force microscope could perturb the dC∕dV signals of the samples. We attribute this phenomenon to the generation of free carriers by the photon absorptions via defect traps in the SBT thin film. Therefore, this present work suggests that the effect of laser illumination must be carefully taken into consideration whenever a field-sensitive technique is employed to analyze the properties of a ferroelectric material.
Effective plasma frequency in one-dimensional metallic-dielectric photonic crystals
Authors: Xu Xiaochuang , Xi Yonggang , Han Dezhuan , Liu Xiaohan , Zi Jian , Zhu Ziqiang
Published: March 2005

Photonic band structures of one-dimensional (1D) metallic-dielectric photonic crystals (MDPCs) are studied theoretically. We show that a 1D MDPC can be considered as an effective metallic medium with a well-defined effective plasma frequency. This effective plasma frequency is found to be inversely proportional to the optical thickness of the dielectric layer and is independent of either the constituent metal or the thickness of the metallic layer. By increasing the optical thickness of the dielectric layer, the effective plasma frequency of a 1D MDPC can be depressed into extremely low frequencies such as far infrared or even below.
High-performance type-II InAs/GaSb superlattice photodiodes with cutoff wavelength around 7 μm
Authors: Wei Y. , Hood A. , Yau H. , Yazdanpanah V. , Razeghi M. , Tidrow M. Z. , Nathan V.
Published: March 2005

We report the most recent result in the area of type-II InAs/GaSb superlattice photodiodes that have a cutoff wavelength around 7 μm at 77 K. Superlattice with a period of 40 Å lattice matched to GaSb was realized using GaxIn1−x type interface engineering technique. Compared with significantly longer period superlattices, we have reduced the dark current density under reverse bias dramatically. For a 3 μm thick structure, using sulfide-based passivation, the dark current density reached 2.6×10−5A∕cm2 at −3 V reverse bias at 77 K. At this temperature the photodiodes have R0A of 9300Ωcm2 and a thermally limited zero bias detectivity of 1×1012cmHz1∕2∕W. The 90%–10% cutoff energy width was only 16.5 meV. The devices did not show significant dark current change at 77 K after three months storage in the atmosphere.
Second-order optical nonlinear and luminescent properties of Ba2TiSi2O8 nanocrystallized glass
Authors: Takahashi Yoshihiro , Kitamura Kenji , Benino Yasuhiko , Fujiwara Takumi , Komatsu Takayuki
Published: March 2005

The optical properties of Ba2TiSi2O8 nanocrystals in transparent crystallized glasses were investigated. The transparent nanocrystallized glass with Ba2TiSi2O8 crystals having a particle size of ∼200nm was fabricated by crystallization (at 760 °C for 1 h) of a glass with the stoichiometric composition of Ba2TiSi2O8, i.e., 40BaO∙20TiO2∙40SiO2 glass. Both visible second-harmonic generation (green light, 532 nm) and luminescence (blue light, ∼470nm) could be observed in the transparent Ba2TiSi2O8 nanocrystallized glass, demonstrating the optical multifunctional nanocrystallized material. It was suggested that the blue luminescence from the transparent Ba2TiSi2O8 nanocrystallized glass originated from the oxygen-related defects of the SiO4 unit in the Ba2TiSi2O8 nanocrystals.
Evaporation and condensation of a binary mixture of vapors on a plane condensed phase: Numerical analysis of the linearized Boltzmann equation
Authors: Yasuda Shugo , Takata Shigeru , Aoki Kazuo
Published: March 2005

Half-space problem of evaporation and condensation of a binary mixture of vapors is investigated on the basis of the linearized Boltzmann equation for hard-sphere molecules with the complete condensation condition. The problem is analyzed numerically by a finite-difference method, in which the complicated collision integrals are computed by the extension of the method proposed by Y. Sone, T. Ohwada, and K. Aoki [“Temperature jump and Knudsen layer in a rarefied gas over a plane wall: Numerical analysis of the linearized Boltzmann equation for hard-sphere molecules,” Phys. Fluids A 1, 363 (1989)] to the case of a gas mixture. As a result, the behavior of the mixture is clarified not only at the level of the macroscopic quantities but also at the level of the velocity distribution function. In addition, accurate formulas of the temperature, pressure, and concentration jumps caused by the evaporation and condensation are constructed for arbitrary values of the concentration of the background reference state by the use of the Chebyshev polynomial approximation.