bluuebunny/crossref_metadata_2025 huggingface dataset:
Calibrated cylindrical Mach probe in a plasma wind tunnel
Authors: Zhang X. , Dandurand D. , Gray T. , Brown M. R. , Lukin V. S.
Published: March 2011

A simple cylindrical Mach probe is described along with an independent calibration procedure in a magnetized plasma wind tunnel. A particle orbit calculation corroborates our model. The probe operates in the weakly magnetized regime in which probe dimension and ion orbit are of the same scale. Analytical and simulation models are favorably compared with experimental calibration.
Simulations of copper single crystals subjected to rapid shear
Authors: Higginbotham Andrew , Bringa Eduardo M. , Marian Jaime , Park Nigel , Suggit Matthew , Wark Justin S.
Published: March 2011

We report on nonequilibrium molecular dynamics simulations of single crystals of copper experiencing rapid shear strain. A model system, with periodic boundary conditions, which includes a single dislocation dipole is subjected to a total shear strain of close to 10% on time-scales ranging from the instantaneous to 50 ps. When the system is strained on a time-scale short compared with a phonon period, the initial total applied shear is purely elastic, and the eventual temperature rise in the system due to the subsequent plastic work can be determined from the initial elastic strain energy. The rate at which this plastic work occurs, and heat is generated, depends on the dislocation velocity, which itself is a function of shear stress. A determination of the stress-dependence of the dislocation velocity allows us to construct a simple analytic model for the temperature rise in the system as a function of strain rate, and this model is found to be in good agreement with the simulations. For the effective dislocation density within the simulations, 7.8×1011cm−2, we find that applying the total shear strain on time-scales of a few tens of picoseconds greatly reduces the final temperature. We discuss these results in the context of the growing interest in producing high pressure, solid-state matter, by quasi-isentropic (rather than shock) compression.
Epitaxial integration of photoresponsive Bi0.4Ca0.6MnO3 with Si(001)
Authors: Yong Grace J. , Kolagani Rajeswari M. , Hofmann Benjamin P. , Adhikari Sanjay , Liang Yong , Smolyaninova Vera N.
Published: March 2011

Previously it has been shown that the resistivity of Bi0.4Ca0.6MnO3 epitaxial thin films on oxide substrates decreases significantly upon illumination with visible light. The resistivity decrease is observed over a wide temperature range and is understood as arising due to the destruction of charge ordering. The light responsivity makes Bi0.4Ca0.6MnO3 thin films attractive for photonic and optoelectronic device applications. In this paper, we report the heteroepitaxy of Bi0.4Ca0.6MnO3 thin films on (001) Si which is relevant for the potential integration of the optoelectronic/photonic functionality of Bi0.4Ca0.6MnO3 with semiconductor electronics. As in the case of other perovskite oxides, heteroepitaxy of Bi0.4Ca0.6MnO3 on Si requires the use of buffer layers to circumvent the problems associated with the presence of an amorphous native silicon dioxide layer and the reactivity of perovskite oxides with Si at high temperatures. We demonstrate that high quality epitaxial thin films of Bi0.4Ca0.6MnO3 can be grown via pulse laser deposition on Si that has been prebuffered with a SrTiO3 layer via a Motorola molecular beam epitaxy process. The magnitude and dynamics of the photoresponse in these films is comparable to that of Bi0.4Ca0.6MnO3 films on oxide substrates.
Two-layer Hall effect model for intermediate band Ti-implanted silicon
Authors: Olea J. , González-Díaz G. , Pastor D. , Mártil I. , Martí A. , Antolín E. , Luque A.
Published: March 2011

Si samples have been implanted with very high Ti doses (over the theoretical Mott limit) to obtain an intermediate band (IB) in the host semiconductor. The electronic transport properties of this material have been analyzed by temperature-dependent sheet resistance and Hall effect measurements in the 7–400 K range. The experimental results are successfully explained by means of an analytical two-layer model, in which the implanted layer and the substrate behave as an IB/n-Si type junction. We deduce that the IB is located at 0.38 eV below the conduction band, which is around one third of the Si bandgap, i.e., theoretically close to the optimum location for an IB. Finally, we obtain that carriers at the IB behave as holes with a mobility of 0.4–0.6 cm2 V−1 s−1. This extremely low mobility is the one expected for a semifilled, metallic band, being this metallic condition of the IB a requirement for IB solar cells.
Molecular dynamics simulations of shock waves in oriented nitromethane single crystals
Authors: He Lan , Sewell Thomas D. , Thompson Donald L.
Published: March 2011

The structural relaxation of crystalline nitromethane initially at T = 200 K subjected to moderate (∼15 GPa) supported shocks on the (100), (010), and (001) crystal planes has been studied using microcanonical molecular dynamics with the nonreactive Sorescu–Rice–Thompson force field [D. C. Sorescu, B. M. Rice, and D. L. Thompson, J. Phys. Chem. B 104, 8406 (2000)]. The responses to the shocks were determined by monitoring the mass density, the intermolecular, intramolecular, and total temperatures (average kinetic energies), the partitioning of total kinetic energy among Cartesian directions, the radial distribution functions for directions perpendicular to those of shock propagation, the mean-square displacements in directions perpendicular to those of shock propagation, and the time dependence of molecular rotational relaxation as a function of time. The results show that the mechanical response of crystalline nitromethane strongly depends on the orientation of the shock wave. Shocks propagating along [100] and [001] result in translational disordering in some crystal planes but not in others, a phenomenon that we refer to as plane-specific disordering; whereas for [010] the shock-induced stresses are relieved by a complicated structural rearrangement that leads to a paracrystalline structure. The plane-specific translational disordering is more complete by the end of the simulations (∼6 ps) for shock propagation along [001] than along [100]. Transient excitation of the intermolecular degrees of freedom occurs in the immediate vicinity of the shock front for all three orientations; the effect is most pronounced for the [010] shock. In all three cases excitation of molecular vibrations occurs more slowly than the intermolecular excitation. The intermolecular and intramolecular temperatures are nearly equal by the end of the simulations, with 400–500 K of net shock heating. Results for two-dimensional mean-square molecular center-of-mass displacements, calculated as a function of time since shock wave passage in planes perpendicular to the direction of shock propagation, show that the molecular translational mobility in the picoseconds following shock wave passage is greatest for [001] and least for the [010] case. In all cases the root-mean-square center-of-mass displacement is small compared to the molecular diameter of nitromethane on the time scale of the simulations. The calculated time scales for the approach to thermal equilibrium are generally consistent with the predictions of a recent theoretical analysis due to Hooper [J. Chem. Phys. 132, 014507 (2010)].
Development of a molecular dynamics-based coalescence model for DSMC simulations of ammonia condensate flows
Authors: Li Zheng , Levin Deborah A.
Published: March 2011

A coalescence model for homogeneous condensation of ammonia in supersonic expansions to vacuum has been developed using molecular dynamics trajectory calculations. The MD calculations show that the sticking probability increases as the ammonia cluster size increases or the cluster temperature decreases. In addition, the sensitivity of the sticking probability to cluster size decreases as the temperature decreases. Comparison of the Ashgriz–Poo semiempirical coalescence model with MD simulations show that for cluster sizes larger than 100 the former model may be used. To model homogeneous nucleation in an ammonia jet, direct simulation Monte Carlo (DSMC) simulations were performed for different stagnation pressure conditions using the MD simulation outcomes for smaller cluster–cluster collisions and the Ashgriz–Poo model for cluster sizes larger than 100. We found that, by including the combined coalescence model, the average cluster sizes and size distributions predicted by DSMC agree reasonably well with experiment.
Static contribution to thermal shifts of spectral lines in gadolinium gallium garnet Gd3Ga5O12:Nd3+ laser crystal
Authors: Zheng W. C. , Su P. , Liu H. G.
Published: March 2011

The static contribution due to lattice thermal expansion and the vibrational contribution due to electron–phonon interaction to the thermal shifts of the spectral lines E1(R1 → Z5) and E2 (R2 → Z5) in the high-power laser crystal gadolinium gallium garnet Gd3Ga5O12 (GGG):Nd3+ are studied by analyzing the observed pressure- and temperature-dependence of these spectral lines. It is found that the static contribution due to thermal expansion in magnitude is greater than, and it in sign is opposite to the vibrational contribution due to electron–phonon interaction. So for the theoretical study of thermal shifts of the spectral lines in GGG:Nd3+ and other similar “hard” crystals, the neglect of the contribution due to the lattice thermal expansion suggested in the previous papers is unsuitable.
A fully simultaneously optimizing genetic approach to the highly excited coupled-cluster factorization problem
Authors: Engels-Putzka Anna , Hanrath Michael
Published: March 2011

In this article we report on the coupled-cluster factorization problem. We describe the first implementation that optimizes (i) the contraction order for each term, (ii) the identification of reusable intermediates, (iii) the selection and factoring out of common factors simultaneously, considering all projection levels in a single step. The optimization is achieved by means of a genetic algorithm. Taking a one-term-at-a-time strategy as reference our factorization yields speedups of up to 4 (for intermediate excitation levels, smaller basis sets). We derive a theoretical lower bound for the highest order scaling cost and show that it is met by our implementation. Additionally, we report on the performance of the resulting highly excited coupled-cluster algorithms and find significant improvements with respect to the implementation of Kállay and Surján [J. Chem. Phys. 115, 2945 (2001)]10.1063/1.1383290 and comparable performance with respect to MOLPRO’s handwritten and dedicated open shell coupled cluster with singles and doubles substitutions implementation [P. J. Knowles, C. Hampel, and H.-J. Werner, J. Chem. Phys. 99, 5219 (1993)10.1063/1.465990].
Simultaneous imaging of the topography and dynamic properties of nanomechanical systems by optical beam deflection microscopy
Authors: Kosaka P. M. , Tamayo J. , Gil-Santos E. , Mertens J. , Pini V. , Martínez N. F. , Ahumada O. , Calleja M.
Published: March 2011

We present an optical microscopy technique based on the scanning of a laser beam across the surface of a sample and the measurement of the deflection of the reflected laser beam in two dimensions. The technique is intended for characterization of nanomechanical systems. It provides the height of a nanomechanical system with sub-nanometer vertical resolution. In addition, it simultaneously provides a complete map of the resonant properties. We demonstrate the capability of the technique by analyzing the residual stress and vibration mode shape of a system consisting of two elastically coupled nanocantilevers. The technique is simple, allows imaging in air, vacuum and liquids, and it is unique in providing synchronized information of the static and dynamic out-of-plane displacement of nanomechanical systems.
Generation of shear Alfvén waves by a rotating magnetic field source: Three-dimensional simulations
Authors: Karavaev A. V. , Gumerov N. A. , Papadopoulos K. , Shao Xi , Sharma A. S. , Gekelman W. , Wang Y. , Van Compernolle B. , Pribyl P. , Vincena S.
Published: March 2011

The paper discusses the generation of polarized shear Alfvén waves radiated from a rotating magnetic field source created via a phased orthogonal two-loop antenna. A semianalytical three-dimensional cold two-fluid magnetohydrodynamics model was developed and compared with recent experiments in the University of California, Los Angeles large plasma device. Comparison of the simulation results with the experimental measurements and the linear shear Alfvén wave properties, namely, spatiotemporal wave structure, a dispersion relation with nonzero transverse wave number, the magnitude of the wave dependences on the wave frequency, show good agreement. From the simulations it was found that the energy of the Alfvén wave generated by the rotating magnetic field source is distributed between the kinetic energy of ions and electrons and the electromagnetic energy of the wave as: ∼1/2 is the energy of the electromagnetic field, ∼1/2 is the kinetic energy of the ion fluid, and ∼2.5% is the kinetic energy of electron fluid for the experiment. The wave magnetic field power calculated from the experimental data and using a fluid model differ by ∼1% and is ∼250 W for the experimental parameters. In both the experiment and the three-dimensional two-fluid magnetohydrodynamics simulations the rotating magnetic field source was found to be very efficient for generating shear Alfvén waves.
The effect of excitation wavelength on dynamics of laser-produced tin plasma
Authors: Harilal S. S. , Sizyuk T. , Hassanein A. , Campos D. , Hough P. , Sizyuk V.
Published: March 2011

We investigated the effect of the excitation wavelength on the density evolution of laser-produced tin plasmas, both experimentally and numerically. For producing plasmas, Sn targets were excited with either 10.6 μm CO2 laser or 1.06 μm Nd:yttrium aluminum garnet laser; both are considered to be potential excitation lasers for extreme ultraviolet lithography laser-produced plasma light sources. The electron density of the plasma during the isothermal expansion regime was estimated using an interferometric technique. The Stark broadening of isolated singly-ionized emission was employed for deducing the density during the plasma adiabatic expansion regime. Our results indicate that the excitation source wavelength determines the initial density of the plasma, as well the plume expansion dynamics. Numerical simulation using HEIGHTS simulation package agrees well with the experimentally measured density profile.
Combined experimental and modeling studies of microwave activated CH4/H2/Ar plasmas for microcrystalline, nanocrystalline, and ultrananocrystalline diamond deposition
Authors: Richley James C. , Fox Oliver J. L. , Ashfold Michael N. R. , Mankelevich Yuri A.
Published: March 2011

A comprehensive study of microwave (MW) activated CH4/H2/Ar plasmas used for diamond chemical vapor deposition is reported, focusing particularly on the effects of gross variations in the H2/Ar ratio in the input gas mixture (from H2/Ar mole fraction ratios of > 10:1, through to ∼1:99). Absolute column densities of C2(a) and CH(X) radicals and of H(n = 2) atoms have been determined by cavity ringdown spectroscopy, as functions of height (z) above a substrate and of process conditions (CH4, H2, and Ar input mole fractions, total pressure, p, and input microwave power, P). Optical emission spectroscopy has also been used to explore the relative densities of electronically excited H atoms, and CH, C2, and C3 radicals, as functions of these same process conditions. These experimental data are complemented by extensive 2D (r, z) modeling of the plasma chemistry, which provides a quantitative rationale for all of the experimental observations. Progressive replacement of H2 by Ar (at constant p and P) leads to an expanded plasma volume. Under H2-rich conditions, > 90% of the input MW power is absorbed through rovibrational excitation of H2. Reducing the H2 content (as in an Ar-rich plasma) leads to a reduction in the absorbed power density; the plasma necessarily expands in order to accommodate a given input power. The average power density in an Ar-rich plasma is much lower than that in an H2-rich plasma operating at the same p and P. Progressive replacement of H2 by Ar is shown also to result in an increased electron temperature, an increased [H]/[H2] number density ratio, but little change in the maximum gas temperature in the plasma core (which is consistently ∼3000 K). Given the increased [H]/[H2] ratio, the fast H-shifting (CyHx + H ↔ CyHx−1 + H2; y = 1−3) reactions ensure that the core of Ar-rich plasma contains much higher relative abundances of “product” species like C atoms, and C2, and C3 radicals. The effects of Ar dilution on the absorbed power dissipation pathways and the various species concentrations just above the growing diamond film are also investigated and discussed.
Speed of traveling fronts in a sigmoidal reaction-diffusion system
Authors: Zemskov E. P. , Kassner K. , Tsyganov M. A. , Epstein I. R.
Published: March 2011

We study a sigmoidal version of the FitzHugh–Nagumo reaction-diffusion system based on an analytic description using piecewise linear approximations of the reaction kinetics. We completely describe the dynamics of wave fronts and discuss the properties of the speed equation. The speed diagrams show front bifurcations between branches with one, three, or five fronts that differ significantly from the classical FitzHugh–Nagumo model. We examine how the number of fronts and their speed vary with the model parameters. We also investigate numerically the stability of the front solutions in a case when five fronts exist.
The particle distributions of asymmetric kinetic electrostatic structures
Authors: Nocera L. , Palumbo L. J.
Published: March 2011

We give the energy distributions of electrons and ions supporting a steady state electrostatic structure in a collisionless plasma. The electric potential of the structure is skew asymmetrically distributed in space. We show that the jump discontinuous, logarithmically singular electron and ion distributions may be reduced to elliptic integrals. We give the coefficients of the logarithmic terms and the jumps at the discontinuities and we show that they are reciprocally proportional. We calculate bounds for the potential skew asymmetry and show that these bounds are regulated by the boundary conditions of the particle distributions. Despite singularities, our treatment reproduces a smooth space distribution of the potential amplitude and electron and ion distributions that are smooth at one of the boundaries of the electrostatic structure.
Investigation of mode coupling in a magnetized plasma column using fast imaging
Authors: Oldenbürger S. , Brochard F. , Bonhomme G.
Published: March 2011

Spectral analysis and bicoherence analysis are applied to fast camera data in order to study the nonlinear mode coupling in the low-β cylindrical magnetized plasma device Mirabelle [T. Pierre et al., Rev. Sci. Instrum. 58, 6 (1987)]. The camera provides a measurement of electron plasma density fluctuations in a complete plasma column cross-section. By extraction of virtual probe data, bicoherence in the frequency domain is obtained and compared to probe signal bicoherence. Two-dimensional camera measurements make it possible to compute the mode number bicoherence as well as the related amplitude weighted phase coherence B⋆, which conserves time resolution. Investigation of temporal evolution of this quantity and power spectra shows typical sequences where a summed normalized B⋆ peak on a specific mode precedes a gain in spectral energy. The assumption of an energy transfer is confirmed by the value of the phase of B⋆. In addition to temporal dynamics, bidimensional data make it possible to study the localization of nonlinear couplings. Highest values are detected at a radial position where most of the present modes coexist. Fast camera imaging is a powerful, nonperturbative diagnostic providing two-dimensional time-resolved measurements. An adapted utilization of these data helps to get a better insight in drift wave turbulence and coupling mechanisms.
Nonlinear collisionless plasma wakes of small particles
Authors: Hutchinson I. H.
Published: March 2011

The wake behind a spherical particle smaller than the Debye length (λDe) in flowing plasma is calculated using a particle-in-cell code. The results with different magnitudes of charge reveal substantial nonlinear effects down to values that for a floating particle would correspond to a particle radius ∼10−2λDe. The peak potential in the oscillatory wake structure is strongly suppressed by nonlinearity, never exceeding ∼0.4 times the unperturbed ion energy. By contrast, the density peak arising from ion focusing can be many times the ambient. Strong heating of the ions occurs in the nonlinear regime. Direct ion absorption by the particle is not important for the far wake unless the radius exceeds 10−1λDe, and is therefore never significant (for the far wake) in the linear regime. Reasonable agreement with full-scale linear response calculations are obtained in the linear regime. The wake wavelength is confirmed and an explanation, in terms of the conical potential structure, is proposed for experimentally-observed oblique alignment of different-sized grains.
Improved dielectric properties and their temperature insensitivity in multilayered Ba0.8Sr0.2TiO3/ZrO2 thin films
Authors: Sahoo S. K. , Misra D. , Sahoo M. , MacDonald C. A. , Bakhru H. , Agrawal D. C. , Mohapatra Y. N. , Majumder S. B. , Katiyar R. S.
Published: March 2011

The electrical and dielectric properties of Ba0.8Sr0.2TiO3 thin films and Ba0.8Sr0.2TiO3/ZrO2 multilayer thin films deposited on Pt/Ti/SiO2/Si substrates by sol-gel method are studied. The temperature dependence of the dielectric properties for pure Ba0.8Sr0.2TiO3 film and Ba0.8Sr0.2TiO3/ZrO2 multilayer films has been studied in the temperature range from 90 to 500 K. Both dielectric constant and dielectric loss exhibit minimal dispersion as a function of temperature in this range. It is observed that dielectric constant, dielectric loss, and tunability are reduced for multilayer thin films. Additionally, the ferroelectricity disappears in multilayer thin films. Our results show that the multilayered Ba0.8Sr0.2TiO3/ZrO2 design has excellent dielectric properties and they are stable over a broad temperature range (90–500 K), thereby making them excellent candidates for the next generation of enhanced performance temperature stable microwave devices.
A photoelectron velocity map imaging spectrometer for experiments combining synchrotron and laser radiations
Authors: O’Keeffe P. , Bolognesi P. , Coreno M. , Moise A. , Richter R. , Cautero G. , Stebel L. , Sergo R. , Pravica L. , Ovcharenko Y. , Avaldi L.
Published: March 2011

A velocity map imaging/ion time-of-flight spectrometer designed specifically for pump–probe experiments combining synchrotron and laser radiations is described. The in-house built delay line detector can be used in two modes: the high spatial resolution mode and the coincidence mode. In the high spatial resolution mode a kinetic energy resolution of 6% has been achieved. The coincidence mode can be used to improve signal-to-noise ratio for the pump–probe experiments either by using a gate to count electrons only when the laser is present or by recording coincidences with the ion formed in the ionization process.
Quantum mechanical theory of dynamic nuclear polarization in solid dielectrics
Authors: Hu Kan-Nian , Debelouchina Galia T. , Smith Albert A. , Griffin Robert G.
Published: March 2011

Microwave driven dynamic nuclear polarization (DNP) is a process in which the large polarization present in an electron spin reservoir is transferred to nuclei, thereby enhancing NMR signal intensities. In solid dielectrics there are three mechanisms that mediate this transfer—the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Historically these mechanisms have been discussed theoretically using thermodynamic parameters and average spin interactions. However, the SE and the CE can also be modeled quantum mechanically with a system consisting of a small number of spins and the results provide a foundation for the calculations involving TM. In the case of the SE, a single electron–nuclear spin pair is sufficient to explain the polarization mechanism, while the CE requires participation of two electrons and a nuclear spin, and can be used to understand the improved DNP enhancements observed using biradical polarizing agents. Calculations establish the relations among the electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) frequencies and the microwave irradiation frequency that must be satisfied for polarization transfer via the SE or the CE. In particular, if δ, Δ < ω0I, where δ and Δ are the homogeneous linewidth and inhomogeneous breadth of the EPR spectrum, respectively, we verify that the SE occurs when ωM = ω0S ± ω0I, where ωM, ω0S and ω0I are, respectively, the microwave, and the EPR and NMR frequencies. Alternatively, when Δ > ω0I > δ, the CE dominates the polarization transfer. This two-electron process is optimized when $\omega _{0S_1 } - \omega _{0S_2 } = \omega _{0I}$ω0S1−ω0S2=ω0I and $\omega _M \sim\omega _{0S_1 }$ωM∼ω0S1 or $\omega _{0S_2 }$ω0S2, where $\omega _{0S_1 }$ω0S1 and $\omega _{0S_2 }$ω0S2 are the EPR Larmor frequencies of the two electrons. Using these matching conditions, we calculate the evolution of the density operator from electron Zeeman order to nuclear Zeeman order for both the SE and the CE. The results provide insights into the influence of the microwave irradiation field, the external magnetic field, and the electron−electron and electron−nuclear interactions on DNP enhancements.
Water bag modeling of a multispecies plasma
Authors: Morel P. , Gravier E. , Besse N. , Klein R. , Ghizzo A. , Bertrand P. , Bourdelle C. , Garbet X.
Published: March 2011

We report in the present paper a new modeling method to study multiple species dynamics in magnetized plasmas. Such a method is based on the gyrowater bag modeling, which consists in using a multistep-like distribution function along the velocity direction parallel to the magnetic field. The choice of a water bag representation allows an elegant link between kinetic and fluid descriptions of a plasma. The gyrowater bag model has been recently adapted to the context of strongly magnetized plasmas. We present its extension to the case of multi ion species magnetized plasmas: each ion species being modeled via a multiwater bag distribution function. The water bag modelization will be discussed in details, under the simplification of a cylindrical geometry that is convenient for linear plasma devices. As an illustration, results obtained in the linear framework for ion temperature gradient instabilities are presented, that are shown to agree qualitatively with older works.
Novel plasma immersion ion implantation and deposition hardware and technique based on high power pulsed magnetron discharge
Authors: Wu Zhongzhen , Tian Xiubo , Shi Jingwei , Wang Zeming , Gong Chunzhi , Yang Shiqin , Chu Paul K.
Published: March 2011

A novel plasma immersion ion implantation technique based on high power pulsed magnetron sputtering (HPPMS) discharge that can produce a high density metal plasma is described. The metal plasma is clean and does not suffer from contamination from macroparticles, and the process can be readily scaled up for industrial production. The hardware, working principle, and operation modes are described. A matching circuit is developed to modulate the high-voltage and HPPMS pulses to enable operation under different modes such as simultaneous implantation and deposition, pure implantation, and selective implantation. To demonstrate the efficacy of the system and technique, CrN films with a smooth and dense surface without macroparticles were produced. An excellent adhesion with a critical load of 59.9 N is achieved for the pure implantation mode.
Study of Nα-benzoyl-L-argininate ethyl ester chloride, a model compound for poly(ester amide) precursors: X-ray diffraction, infrared and Raman spectroscopies, and quantum chemistry calculations
Authors: Fonseca A. C. , Jarmelo S. , Silva M. Ramos , Beja A. M. Matos , Fausto R. , Gil M. H. , Simões P. N.
Published: March 2011

Poly(ester amide)s (PEAs) are lacking in structural and spectroscopic information. This paper reports a structural and spectroscopic characterization of Nα-benzoyl-L-argininate ethyl ester chloride (BAEEH+·Cl−), an important amino acid derivative and an adequate PEAs’ model compound. Crystals of BAEEH+·Cl− obtained by slow evaporation in an ethanol/water mixture were studied by different complementary techniques. X-ray analysis shows that BAEEH+·Cl− crystallizes in the chiral space group P21. There are two symmetry independent cations (and anions) in the unit cell. The two cations have different conformations: in one of them, the angle between the least-squares planes of the phenyl ring and the guanidyl group is 5.1(2)º, and in the other the corresponding angle is 13.3(2)º. There is an extensive network of H-bonds that assembles the ions in layers parallel to the ab plane. Experimental FT-IR and Raman spectra of BAEEH+·Cl− were recorded at room temperature in the 3750–600 cm−1 and 3380–100 cm−1 regions, respectively, and fully assigned. Both structural and spectroscopic analysis were supported by quantum chemistry calculations based on different models (in vacuo and solid-state DFT simulations).
Relation of open circuit voltage to charge carrier density in organic bulk heterojunction solar cells
Authors: Rauh Daniel , Wagenpfahl Alexander , Deibel Carsten , Dyakonov Vladimir
Published: March 2011

The open circuit voltage Voc and the corresponding charge carrier density were measured in dependence of temperature and illumination intensity by current–voltage and charge extraction measurements for poly(3-hexylthiophene-2,5-diyl) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and P3HT:bisPCBM solar cells. At lower temperatures a saturation of Voc was observed which can be explained by energetic barriers at the contacts (metal–insulator–metal model). Such injection barriers can also influence Voc at room temperature and limit the performance of the working solar cell, as was assured by macroscopic device simulations on temperature dependent IV characteristics. However, under most conditions—room temperature and low barriers—Voc is given by the effective band gap.
A solid-mounted resonator-oscillator-based 4.596 GHz frequency synthesis
Authors: Boudot R. , Li M. D. , Giordano V. , Rolland N. , Rolland P. A. , Vincent P.
Published: March 2011

This paper describes a 4.596 GHz frequency synthesis based on a 2.1 GHz solid mounted resonator (SMR) voltage-controlled oscillator (VCO). The SMR oscillator presents a chip size lower than 2 mm2, a power consumption of 18.2 mW, and exhibits a phase noise of −89 dBc/Hz and −131 dBc/Hz at 2 kHz and 100 kHz offset frequencies, respectively. The VCO temperature–frequency dependence is measured to be −14 ppm/°C over a range of −20 ○C to 60 ○C. From this source, a low noise frequency synthesizer is developed to generate a 4.596 GHz signal (half of the Cs atom hyperfine transition frequency) with a phase noise of −81 dBc/Hz and −120 dBc/Hz at 2 kHz and 100 kHz from the carrier. The frequency synthesis output is used as a local oscillator in a Cs vapor microcell-based compact atomic clock. Preliminary results are reported and discussed. To the authors knowledge, this is the first development of a SMR-oscillator-based frequency synthesizer for miniature atomic clocks applications.
Electron flow stability in magnetically insulated vacuum transmission lines
Authors: Rose D. V. , Genoni T. C. , Clark R. E. , Welch D. R. , Stygar W. A.
Published: March 2011

We evaluate the stability of electron current flow in high-power magnetically insulated transmission lines (MITLs). A detailed model of electron flow in cross-field gaps yields a dispersion relation for electromagnetic (EM) transverse magnetic waves [R. C. Davidson et al., Phys. Fluids 27, 2332 (1984)] which is solved numerically to obtain growth rates for unstable modes in various sheath profiles. These results are compared with two-dimensional (2D) EM particle-in-cell (PIC) simulations of electron flow in high-power MITLs. We find that the macroscopic properties (charge and current densities and self-fields) of the equilibrium profiles observed in the simulations are well represented by the laminar-flow model of Davidson et al. Idealized simulations of sheared flow in electron sheaths yield growth rates for both long (diocotron) and short (magnetron) wavelength instabilities that are in good agreement with the dispersion analysis. We conclude that electron sheaths that evolve self-consistently from space-charged-limited emission of electrons from the cathode in well-resolved 2D EM PIC simulations form stable profiles.
Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures
Authors: Huang Tsang-Min , McCreary Kathleen , Garg Shila , Kyu Thein
Published: March 2011

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory–Huggins free energy for isotropic mixing, Maier–Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar–Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal–smectic, smectic–nematic, and nematic–isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.
Embedded eigenvalues and the nonlinear Schrödinger equation
Authors: Asad R. , Simpson G.
Published: March 2011

A common challenge in proving asymptotic stability of solitary waves is understanding the spectrum of the operator associated with the linearized flow. The existence of eigenvalues can inhibit the dispersive estimates key to proving stability. Following the work of Marzuola and Simpson [Nonlinearity 52, 389 (2011)]10.1088/0951-7715/24/2/003, we prove the absence of embedded eigenvalues for a collection of nonlinear Schrödinger equations, including some one and three dimensional supercritical equations, and the three dimensional cubic–quintic equation. Our results also rule out nonzero eigenvalues within the spectral gap and end point resonances. The proof is computer assisted as it depends on the signs of certain inner products which do not readily admit analytic representations. Our source code is available for verification at http://hdl.handle.net/1807/26121.
Lie symmetry analysis and exact solutions of the quasigeostrophic two-layer problem
Authors: Bihlo Alexander , Popovych Roman O.
Published: March 2011

The quasigeostrophic two-layer model is of superior interest in dynamic meteorology since it is one of the easiest ways to study baroclinic processes in geophysical fluid dynamics. The complete set of point symmetries of the two-layer equations is determined. An optimal set of one- and two-dimensional inequivalent subalgebras of the maximal Lie invariance algebra is constructed. On the basis of these subalgebras, we exhaustively carry out group-invariant reduction and compute various classes of exact solutions. Wherever possible, reference to the physical meaning of the exact solutions is given. In particular, the well-known baroclinic Rossby wave solutions in the two-layer model are rediscovered.
Analytical treatment of biased diffusion in tubes with periodic dead ends
Authors: Berezhkovskii Alexander M. , Dagdug Leonardo
Published: March 2011

Effective mobility and diffusion coefficient of a particle in a tube with identical periodic dead ends characterize the motion on large time scale, when the particle displacement significantly exceeds the tube period. We derive formulas that show how these transport coefficients depend on the driving force and the geometric parameters of the system. Numerical tests show that values of the transport coefficients obtained from Brownian dynamics simulations are in excellent agreement with our theoretical predictions.
On the upper limit for optical spin pumping in III-V semiconductor quantum dots
Authors: Vulović Boris M. , Ovchinnikov Igor , Wang Kang L.
Published: March 2011

We studied the optical steady-state generation of spin in the III-V semiconductor quantum dots, where an external laser field couples to the quantum dot only via the electric dipole interband transition and generation of spin in the conduction band is due to the optical transition from the spin-split valence band. In the weak field regime, the optical spin generation is second order in the Rabi frequency. At the intense laser fields, the spin-orbit interaction is of no importance and the optical spin generation is absent. Therefore, there should be a region featured by intermediate Rabi frequencies where the optical spin generation reaches its maximum. We demonstrated this upper limit using a simple model of the III-V quantum dot. The maximum corresponds to the optical interaction energy equal to the spin-orbit interaction energy in the valence band. The work addressed here sets the upper limit for steady-state optical spin generation in the III-V semiconductor quantum dots and completes the physical picture of this process for all optical field strengths.
Entanglement of polar molecules in pendular states
Authors: Wei Qi , Kais Sabre , Friedrich Bretislav , Herschbach Dudley
Published: March 2011

In proposals for quantum computers using arrays of trapped ultracold polar molecules as qubits, a strong external field with appreciable gradient is imposed in order to prevent quenching of the dipole moments by rotation and to distinguish among the qubit sites. That field induces the molecular dipoles to undergo pendular oscillations, which markedly affect the qubit states and the dipole–dipole interaction. We evaluate entanglement of the pendular qubit states for two linear dipoles, characterized by pairwise concurrence, as a function of the molecular dipole moment and rotational constant, strengths of the external field and the dipole–dipole coupling, and ambient temperature. We also evaluate a key frequency shift, △ω, produced by the dipole–dipole interaction. Under conditions envisioned for the proposed quantum computers, both the concurrence and △ω become very small for the ground eigenstate. In principle, such weak entanglement can be sufficient for operation of logic gates, provided the resolution is high enough to detect the △ω shift unambiguously. In practice, however, for many candidate polar molecules it appears a challenging task to attain adequate resolution. Simple approximate formulas fitted to our numerical results are provided from which the concurrence and △ω shift can be obtained in terms of unitless reduced variables.
NMR shielding tensors from auxiliary density functional theory
Authors: Zuniga-Gutierrez Bernardo , Geudtner Gerald , Köster Andreas M.
Published: March 2011

The working equations for the calculation of NMR shielding tensors in the framework of auxiliary density functional theory are derived. It is shown that in this approach the numerical integration over gauge-including atomic orbitals can be avoided without the loss of accuracy. New integral recurrence relations for the required analytic electric-field-type integrals are derived. The computational performance of the resulting formalism permits shielding tensor calculations of systems with more than 1000 atoms and 15 000 basis functions.
Pulsed thrust measurements using electromagnetic calibration techniques
Authors: Tang Haibin , Shi Chenbo , Zhang Xin'ai , Zhang Zun , Cheng Jiao
Published: March 2011

A thrust stand for accurately measuring impulse bits, which ranged from 10–1000 μN s using a noncontact electromagnetic calibration technique is described. In particular, a permanent magnet structure was designed to produce a uniform magnetic field, and a multiturn coil was made to produce a calibration force less than 10 mN. The electromagnetic calibration force for pulsed thrust measurements was linear to the coil current and changed less than 2.5% when the distance between the coil and magnet changed 6 mm. A pulsed plasma thruster was first tested on the thrust stand, and afterward five single impulse bits were measured to give a 310 μN s average impulse bit. Uncertainty of the measured impulse bit was analyzed to evaluate the quality of the measurement and was found to be 10 μN s with 95% credibility.
A high-sensitivity angle and energy dipersive multichannel electron momentum spectrometer with 2π angle range
Authors: Tian QiGuo , Wang KeDong , Shan Xu , Chen XiangJun
Published: March 2011

A high-sensitivity angle and energy dispersive multichannel electron momentum spectrometer with simultaneous detection in 2π angle range is presented. A newly designed double half wedge and strip anode position-sensitive detector is employed to collect the ionized and scattered electrons passing through a 90° sector, 2π spherical electrostatic analyzer over azimuthal angle range of about 150° for each. Experimental results on argon are presented to exhibit the performance of the spectrometer.
Deassociate the initial temporal phase deviation provided by photoelastic modulator for stroboscopic illumination polarization modulated ellipsometry
Authors: Tsai Hsiu-Ming , Chen Chih-Wei , Tsai Tsung-Han , Chao Yu-Faye
Published: March 2011

In addition to operating the imaging ellipsometric measurements by four-specific temporal phases in the photoelastic modulated ellipsometry, we added the fifth one to solve the initial phase of the photoelastic modulator. This methodology has been developed to conquer the slow imaging processing of charge-coupled device camera for the stroboscopic illumination in the polarization modulated imaging ellipsometry. Without any calibration in its initial phase, we can perform the ellipsometric measurement by the measurements of intensity at five-specific temporal phases. The intensities of a full cycle for a point on SiO2/Si thin film were measured and analyzed for verifying this algorithm. The five stroboscopic illuminations were performed to measure the two-dimensional distribution of the same SiO2/Si thin film.
Effect of pressure on thermal conductivity and pressure collapse of ice in a polymer-hydrogel and kinetic unfreezing at 1 GPa
Authors: Andersson Ove , Johari G. P.
Published: March 2011

We report a study of aqueous solutions of poly(vinylalcohol) and its hydrogel by thermal conductivity, κ, and specific heat measurements. In particular, we investigate (i) the changes in the solution and the hydrogel at 0.1 MPa observed in the 350–90 K range and of the frozen hydrogel at 130 K observed in the range from 0.1 MPa to 1.3 GPa, and (ii) the nature of the pressure collapse of ice in the frozen hydrogel and kinetic unfreezing on heating of its high density water at 1 GPa. The water component of the polymer solution on cooling either first phase separates and then freezes to hexagonal ice or freezes without phase separation and the dispersed polymer chains freeze-concentrate in nanoscopic and microscopic regions of the grain boundaries and grain junctions of the ice crystals in the frozen state of water in the hydrogel. The change in κ with temperature at 1 bar is reversible with some hysteresis, but not reversible with pressure after compression to 0.8 GPa at 130 K. At high pressures the crystallized state collapses showing features of κ and specific heat characteristic of formation of high density amorphous solid water. The pressure of structural collapse is 0.08 GPa higher than that of ice at 130 K. The slowly formed collapsed state shows kinetic unfreezing or glass–liquid transition temperature at 140 K for a time scale of 1 s. Comparison with the change in the properties observed for ice shows that κ decreases when the polymer is added.
Vector-magneto-optical generalized ellipsometry
Authors: Mok K. , Du N. , Schmidt H.
Published: March 2011

We present the setup of a variable-angle vector-magneto-optical generalized ellipsometer (VMOGE) in the spectral range from 300 to 1100 nm using an octupole magnet, and demonstrate VMOGE measurements of the upper 3 × 4 submatrix of the Mueller matrix in a magnetic field of arbitrary orientation and magnitude up to 0.4 T at room temperature. New “field orbit” measurements can be performed without physically moving the sample, which is useful to study magnetic multilayer or nanostructure samples. A 4 × 4 matrix formalism is employed to model the experimental VMOGE data. Searching the best match model between experimental and calculated VMOGE data, the magneto-optical dielectric tensor \documentclass[12pt]{minimal}\begin{document}$\varepsilon ^{\text{MO}}$\end{document}ɛMO of each layer in a multilayer sample system can be determined. In this work, we assume that the nonsymmetric terms of \documentclass[12pt]{minimal}\begin{document}$\varepsilon ^{\text{MO}}$\end{document}ɛMO are induced by an external magnetic field and depend linearly on the sample magnetization. Comparison with vector magnetometer measurements can provide the anisotropic magneto-optical coupling constants Qx, Qy, Qz.
Gyrokinetic analysis of tearing instabilities in a collisionless plasma
Authors: Sundaram A. K. , Sen A.
Published: March 2011

Using a gyrokinetic description, an analytic investigation of tearing instabilities is carried out for a collisionless tokamak plasma, with particular emphasis on delineating the effects associated with Landau and ∇B resonances. The linear characteristics of Δ′-driven tearing modes are studied by including short wavelength variations across the confining magnetic field and long wavelength variations along the field. For the case when electrons are adiabatic and ions are fluidlike, the dispersion relation is solved analytically for mode widths lying between electron and ion excursion lengths. It is shown that electron Landau damping effect can significantly influence the tearing mode growth rate by making it proportional to (Δ′)1/2 in contrast to earlier kinetic results, which show a linear dependence on Δ′. The growth rate can further slow down when compressional mode coupling effects are taken into account. Likewise, analytic conditions for the growth of the gyrokinetic tearing mode in the presence of electron ∇B resonance effect are obtained for both the Δ′ driven global mode as well as the large Δ′ branch of this instability and expressions for the real frequency and growth rate of the modes are given. Our analytic results, besides providing physical insights into the influence of these ‘resonance’ effects, can also serve as useful benchmark signatures to look for in large scale numerical gyrokinetic simulations.