bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical simulation of mass transport in internal solitary waves
Authors: Salloum Maher , Knio Omar M. , Brandt Alan
Published: January 2012

A computational study of mass transport by large-amplitude, mode-2 internal solitary waves propagating on a pycnocline between two layers of different densities was conducted. The numerical model is based on the simulation of a vorticity-based formulation of the two-dimensional Navier-Stokes equations in the Boussinesq limit. Numerical experiments are conducted of the collapse of an initially mixed region, which leads to the generation of a train of internal solitary waves. The peak wave amplitude, a, is achieved by the leading wave, which encloses an intrusional bulge. The wave amplitude decays as it moves away from the collapsing mixing region. When the amplitude drops below a critical value, the wave is no longer able to transport mass and sharp-nosed intrusion is left behind. Mass transport by the leading wave, and by the trailing wave train and intrusion, is analyzed by tracking the motion of Lagrangian particles initially concentrated in the mixed region. Results indicate that for moderate wave amplitudes, a gradual decay in the wave amplitude occurs as the wave propagates, but the structure of the bulge is essentially maintained during this process. In contrast, for large-amplitude waves, the motion around the bulge is substantially more complex, exhibiting periodic shedding of vortex structures in the wake of the bulge and entrainment of external fluid into its core. It is shown that these motions have substantial impact on mass transport by the wave train, which is quantified through detailed analysis of the Lagrangian particle distributions.
Potential of mean force between hydrophobic solutes in the Jagla model of water and implications for cold denaturation of proteins
Authors: Maiti Moumita , Weiner Saul , Buldyrev Sergey V. , Stanley H. Eugene , Sastry Srikanth
Published: January 2012

Using the Jagla model potential we calculate the potential of mean force (PMF) between hard sphere solutes immersed in a liquid displaying water-like properties. Consistent estimates of the PMF are obtained by (a) umbrella sampling, (b) calculating the work done by the mean force acting on the hard spheres as a function of their separation, and (c) determining the position dependent chemical potential after calculating the void space in the liquid. We calculate the PMF for an isobar along which cold denaturation of a model protein has previously been reported. We find that the PMF at contact varies non-monotonically, which is consistent with the observed cold denaturation. The Henry constant also varies non-monotonically with temperature. We find, on the other hand, that a second (solvent separated) minimum of the PMF becomes deeper as temperature decreases. We calculate the solvent-solvent pair correlation functions for solvents near the solute and in the bulk, and show that, as temperature decreases, the two pair correlation functions become indistinguishable, suggesting that the perturbation of solvent structure by the solute diminishes as temperature decreases. The solvent-solute pair correlation function at contact grows as the temperature decreases. We calculate the cavity correlation function and show the development of a solvent-separated peak upon decrease of temperature. These observations together suggest that cold denaturation occurs when the solvent penetrates between hydrophobic solutes in configurations with favorable free energy. Our results thus suggest that cold denatured proteins are structured and that cold denaturation arises from strong solvent-solute interactions, rather than from entropic considerations as in heat denaturation.
Electron heat conduction under non-Maxwellian distribution in hohlraum simulation
Authors: Yi Huo Wen , Lan Ke , Jun Gu Pei , Yong Heng , Hong Zeng Qing
Published: January 2012

An electron transport model based on the non-Maxwellian distribution f0∝e-νm (NM model), caused by the inverse bremsstrahlung heating, is used in 1-D plane target and 2-D hohlraum simulations. In the NM model, the electron heat flux depends not only on the gradient of electron temperature Te but also on the gradients of electron number density and the index m. From 1-D simulations, the spatial distribution of Te is dune-like and Te decreases obviously in the flux-heated region, which is very different from the flat profile obtained by using the flux limit model (FL model) but similar to the experimental observations [Gregori et al., Phys. Rev. Lett. 92, 205006 (2004)] and the nonlocal results [Rosen et al., High Energy Density Phys. 7, 180 (2011)]. The reason which causes the dune-like profile of Te is discussed in the paper. From 2-D hohlraum simulations, the NM results of the plasma status, the emission peak and profile inside hohlraum are very different from the FL model results. Finally, it is hard to use an average flux limiter in the FL model to obtain the same hohlraum plasma status and emission with those under the NM model.
Saturation of a floating potential of an electron emitting electrode with increased electron emission: A one-dimensional kinetic model and particle-in-cell simulation
Authors: Gyergyek T. , Kovačič J.
Published: January 2012

A bounded plasma system is studied by a one-dimensional kinetic model and particle-in-cell computer simulation using the XPDP1 code. Three particle species are injected into the system from a planar source, which are the singly charged positive ions and the cool and the hot electrons. All the particle species are injected with half-Maxwellian velocity distributions with different temperatures. From the collector, the emitted electrons are injected, also with a half-Maxwellian velocity distribution, but with a much lower temperature than the cool electrons. As electron emission from the collector is increased, the floating potential of the collector increases also until the boundary of space charge limited emission is achieved. In the simulation, the emission can be increased further and it turns out that the floating potential of the collector remains constant in spite of the increased electron emission. The model on the other hand is valid only up to the boundary of space charge limited emission. The predictions of that limit and of the respective floating potential of the collector by the model are in very good agreement with the simulation. As the criterion for comparison of the model and the simulations propose the matching of the potential, electric field, and density profiles obtained from the simulation and from the numerical solution of the Poisson equation. The matching of potential and electric field profiles is usually almost perfect. On the other hand, the numerical solutions of the Poisson equation give larger ion density at the source and emitted electron density at the collector than obtained from the simulation, but the matching of the particle densities around the inflection point of the potential between the model and the simulation is excellent for all 4 particle species. The same is valid also for the hot electron density at the source. If the potentials and the electric fields are read from the simulation and inserted into the model equations, one obtains an over-determined system of 4 equations for 3 unknown parameters: the ion and the hot electron density at the source and the emitted electron density at the collector. A solution of such a system with the method of least squares is presented. The errors obtained by such a solution can be considered as a measure of how well does the model describe the simulated system.
Cohomology of line bundles: Applications
Authors: Blumenhagen Ralph , Jurke Benjamin , Rahn Thorsten , Roschy Helmut
Published: January 2012

Massless modes of both heterotic and Type II string compactifications on compact manifolds are determined by vector bundle valued cohomology classes. Various applications of our recent algorithm for the computation of line bundle valued cohomology classes over toric varieties are presented. For the heterotic string, the prime examples are so-called monad constructions on Calabi-Yau manifolds. In the context of Type II orientifolds, one often needs to compute cohomology for line bundles on finite group action coset spaces, necessitating us to generalize our algorithm to this case. Moreover, we exemplify that the different terms in Batyrev's formula and its generalizations can be given a one-to-one cohomological interpretation. Furthermore, we derive a combinatorial closed form expression for two Hodge numbers of a codimension two Calabi-Yau fourfold.
Affine reflection groups for tiling applications: Knot theory and DNA
Authors: Bodner M. , Patera J. , Peterson M.
Published: January 2012

We present in this paper some non-conventional applications of affine Weyl groups Waff of rank 2, the symmetry group of the tiling/lattice. We first develop and present the tools for applications requiring tilings of a real Euclidean plane \documentclass[12pt]{minimal}\begin{document}$\mathbb {R}^2$\end{document}R2. We then elucidate the equivalence of these tilings with 2D projections of knots. The resulting mathematical structure provides a framework within which is encompassed recent work utilizing knot theory for modeling the structure and function of genetic molecules, specifically the action of particular enzymes in altering the topology of DNA in site-specific recombination.
Two dimensional nonplanar evolution of electrostatic shock waves in pair-ion plasmas
Authors: Masood W. , Rizvi H.
Published: January 2012

Electrostatic waves in a two dimensional nonplanar geometry are studied in an unmagnetized, dissipative pair-ion plasma in the presence of weak transverse perturbations. The dissipation in the system is taken into account by incorporating the kinematic viscosity of both positive and negative ions in plasmas. The nonplanar Kadomtsev-Petviashvili-Burgers (KPB) as well as the Burgers Kadomtsev-Petviashvili (Burgers KP) equations are derived using the small amplitude expansion method and the range of applicability of both the equations are discussed. The system under consideration is observed to admit compressive rarefactive shocks. The present study may have relevance to understand the formation of two dimensional nonplanar electrostatic shocks in laboratory plasmas.
Coupled modes in magnetized dense plasma with relativistic-degenerate electrons
Authors: Khan S. A.
Published: January 2012

Low frequency electrostatic and electromagnetic waves are investigated in ultra-dense quantum magnetoplasma with relativistic-degenerate electron and non-degenerate ion fluids. The dispersion relation is derived for mobile as well as immobile ions by employing hydrodynamic equations for such plasma under the influence of electromagnetic forces and pressure gradient of relativistic-degenerate Fermi gas of electrons. The result shows the coexistence of shear Alfven and ion modes with relativistically modified dispersive properties. The relevance of results to the dense degenerate plasmas of astrophysical origin (for instance, white dwarf stars) is pointed out with brief discussion on ultra-relativistic and non-relativistic limits.
Density gradient effects in weakly nonlinear ablative Rayleigh-Taylor instability
Authors: Wang L. F. , Ye W. H. , He X. T.
Published: January 2012

In this research, density gradient effects (i.e., finite thickness of ablation front effects) in ablative Rayleigh-Taylor instability (ARTI), in the presence of preheating within the weakly nonlinear regime, are investigated numerically. We analyze the weak, medium, and strong ablation surfaces which have different isodensity contours, respectively, to study the influences of finite thickness of ablation front on the weakly nonlinear behaviors of ARTI. Linear growth rates, generation coefficients of the second and the third harmonics, and coefficients of the third-order feedback to the fundamental mode are obtained. It is found that the linear growth rate which has a remarkable maximum, is reduced, especially when the perturbation wavelength λ is short and a cut-off perturbation wavelength λc appears when the perturbation wavelength λ is sufficiently short, where no higher harmonics exists when λ<λc. The phenomenon of third-order positive feedback to the fundamental mode near the λc [J. Sanz et al., Phys. Rev. Lett. 89, 195002 (2002); J. Garnier et al., Phys. Rev. Lett. 90, 185003 (2003); J. Garnier and L. Masse, Phys. Plasmas 12, 062707 (2005)] is confirmed in numerical simulations, and the physical mechanism of the third-order positive feedback is qualitatively discussed. Moreover, it is found that generations and growths of the second and the third harmonics are stabilized (suppressed and reduced) by the ablation effect. Meanwhile, the third-order negative feedback to the fundamental mode is also reduced by the ablation effect, and hence, the linear saturation amplitude (typically ~0.2λ in our simulations) is increased significantly and therefore exceeds the classical prediction 0.1λ, especially for the strong ablation surface with a small perturbation wavelength. Overall, the ablation effect stabilizes the ARTI in the weakly nonlinear regime. Numerical results obtained are in general agreement with the recent weakly nonlinear theories and simulations as proposed [J. Sanz et al., Phys. Rev. Lett. 89, 195002 (2002); J. Garnier et al., Phys. Rev. Lett. 90, 185003 (2003); J. Garnier and L. Masse, Phys. Plasmas 12, 062707 (2005)].
Surpassing one x-ray photon per electron in nonlinear Thomson scattering in 180° geometry
Authors: Zhao Shihua , Lv Qingzheng , Yuan Suying , Li Yingjun
Published: January 2012

We have obtained the general analytical expressions of harmonic radiation for Thomson scattering (TS) of arbitrary polarized laser by virtue of generalized Bessel functions and derived the extremum conditions for backscattered harmonics. Especially, for the fundamental backscattered Thomson scattering x-ray yield, we have shown that at the same conditions, the circular polarization reaches maximum while the linear case minimum. This effect is significant when a2≥1. With the assumption that the x-ray photon yield of a realistic focused pulse of energy E, wavelength λ, and Rayleigh range zR is equivalent to a plane wave pulse containing Nl cycles via the relation Nlλ=pzR where the effective factor p is of order one, we applied the plane wave results to realistic laser pulses and deduced that the backscattered x-ray photon number Nf per electron achieves its peak value when the average dimensionless laser intensity a2=0.677 and is irrelevant to the value of p. Since Nf and its maximum Nfmax both scale with the square root of E/λ, it is realizable to attain Nf≥1 using joule-scale laser pulses while a great challenge for Nf>10.
Three-dimensional model and simulation of vacuum arcs under axial magnetic fields
Authors: Wang Lijun , Jia Shenli , Zhou Xin , Wang Haijing , Shi Zongqian
Published: January 2012

In this paper, a three-dimensional (3d) magneto-hydro-dynamic (MHD) model of axial magnetic field vacuum arcs (AMFVAs) is established. Based on this model, AMFVAs are simulated and analyzed. Three-dimensional spatial distributions of many important plasma parameters and electric characteristics in AMFVAs can be obtained, such as ion number density, ion temperature, electron temperature, plasma pressure, current densities along different directions (x, y, and z), ion velocities along different directions, electric fields strength along different directions, and so on. Simulation results show that there exist significant spiral-shaped rotational phenomena in the AMFVAs, this kind of rotational phenomenon also can be verified by the many related experiments (AMFVAs photographs, especially for stronger AMF strength). For current simulation results of AMFVAs, the maximal rotational velocity at anode side is about 1100 m/s. Radial electric field is increased from arc center to arc edge; axial electric field is decreased from cathode side to anode side. Radial electric field at arc edge can be larger than axial electric field. Azimuthal electric field in most regions is much smaller than radial and axial electric field, but it can reach about 1.19 kV/m. Radial magnetic field is the smallest one compared with other components, it reaches to maximum value at the position near to anode, it can influence arc characteristics.
Catalyst solubility and self-doping in ZnS nanostructures
Authors: Hafeez M. , Manzoor U. , Bhatti A. S. , Burak Kaynar M. , Ismat Shah S.
Published: January 2012

We demonstrate that the variation in the solubility of the catalyst during nanostructure growth via vapor-liquid-solid technique is dependent on the catalyst dimensions. This property can be exploited in a simple way for controlled doping of the nanostructures. Specifically, we investigate the role of Au droplet size on its solubility in ZnS nanostructures. The size-dependent variations in the surface tension of the catalyst droplet can change its solubility in the nanostructures. The solubility of the catalyst (Au) has been observed to decrease drastically with the increase in its droplet radius. X ray diffraction and scanning electron microscopy were used to determine the phase, shape, size, and density of nanostructures. XPS results revealed that concentration of Au in the ZnS nanostructures was high for a small Au droplet and dropped considerably with increase in the droplet size. The experimental findings were in good agreement with the theoretical model, based on the thermodynamical equations for the doping in nanowires. The critical size of the catalyst for the surface and core doping is determined for the zinc blende and Wurtzite phases of ZnS. In the end, we show that, by using a suitable catalyst with high miscibility, nanostructures can be doped during growth, thus eliminating a step in the device fabrication.
Thermal conductivity of semiconductor nanowires from micro to nano length scales
Authors: Maldovan Martin
Published: January 2012

We study the influence of phonon boundary scattering on the transport of thermal energy in semiconductor nanowires from micro to nano length scales. We use a kinetic theory model based on the Boltzmann transport equation that accurately calculates the reduction of the phonon mean free paths by considering their directional dependence and the fundamental statistical definition of the mean free path. As a result, our model does not use phenomenological formulas to account for the reduction of the phonon mean free paths due to boundary scattering. The transport of thermal energy is also fully divided into that carried by different polarizations by separating phonon group velocities and relaxation times for transverse and longitudinal phonons. We study the correctness of using frequency independent versus frequency dependent models for describing the specularity of the nanowire boundary. We also examine the validity of the assumption that phonons in the semiconductor nanowire maintain their bulk phonon dispersion relations and that modifications to the dispersion relations due to phonon confinement effects can be neglected. The thermal conductivities of silicon nanowires are calculated for different length scales and temperatures and good agreement is obtained with experiments. The theoretical results in this paper can be used to understand and quantitatively predict heat transport in nanowires, which is critical for increasing the efficiency of thermoelectric and electronic devices.
Correlated nucleation and self-accommodating kinetic pathway of ferroelectric phase transformation
Authors: Zhou Jie E. , Cheng Tian-Le , Wang Yu U.
Published: January 2012

Mechanisms of nucleation and growth of domains during ferroelectric phase transformation are investigated by using theoretical and computational approaches. It is shown that ferroelectric phase transformation exhibits some peculiar behaviors due to strong long-range dipole-dipole-like interactions involved in the system. Incorporation of electrostatic and elastostatic energies into the classical Landau-Ginzburg-Devonshire theory effectively modifies the coefficients of the polynomial free energy function and introduces extra energy barrier for ferroelectric phase transformation. It is found that independent nucleation of ferroelectric phase in the context of classical nucleation theory is prohibited, because electrostatic interaction generates an insurmountable energy barrier to isolated nucleus. Phase field modeling and computer simulation reveals that, in order to circumvent such an energy barrier, ferroelectric nucleation exhibits strong spatial correlation and self-organization behaviors from the very beginning, and ferroelectric phase transformation proceeds via spatial and temporal evolution of self-accommodating domains that provide a low-energy kinetic pathway throughout the phase transformation process. Theoretical analysis of the critical wavelength of correlated nucleation agrees with the computer simulation. Heterogeneous nucleation as induced by externally applied local electric field and subsequent polarization evolution process is also simulated to further demonstrate such self-organized pattern formation behaviors.
Dynamics of ferroelectric switching of [H3CNH3]5[Bi2Br11]
Authors: Wojtaś Maciej , Reece Michael J.
Published: January 2012

The switching behavior of the organic, order-disorder ferroelectric [H3CNH3]5[Bi2Br11] has been studied. It exhibits a complex behavior that arises from the different ferroelectric domain structures that dominate in different time scales. An analysis of the dynamics of the switching of [H3CNH3]5[Bi2Br11] using the theory of rates of reactions has shown that the activation volumes and energies associated with the switching nuclei of critical size are very different at low (10 Hz) and high (100 Hz) frequencies. In both cases, it was found that the contribution from the thermal fluctuations to the formation of the switching nuclei is much larger than the contribution from the work done by the applied electric field.
Computational study of energy filtering effects in one-dimensional composite nano-structures
Authors: Kim Raseong , Lundstrom Mark S.
Published: January 2012

Possibilities to improve the Seebeck coefficient S versus electrical conductance G trade-off of diffusive composite nano-structures are explored using an electro-thermal simulation framework based on the non-equilibrium Green’s function method for quantum electron transport and the lattice heat diffusion equation. We examine the role of the grain size d, potential barrier height ΦB, grain doping, and the lattice thermal conductivity κL using a one-dimensional model structure. For a uniform κL, simulation results show that the power factor of a composite structure may be improved over bulk with the optimum ΦB being about kBT, where kB and T are the Boltzmann constant and the temperature, respectively. An optimum ΦB occurs because the current flow near the Fermi level is not obstructed too much while S still improves due to barriers. The optimum grain size dopt is significantly longer than the momentum relaxation length λp so that G is not seriously degraded due to the barriers, and dopt is comparable to or somewhat larger than the energy relaxation length λE so that the carrier energy is not fully relaxed within the grain and |S| remains high. Simulation results also show that if κL in the barrier region is smaller than in the grain, S and power factor are further improved. In such cases, the optimum ΦB and dopt increase, and the power factor may improve even for ΦB (d) significantly higher (longer) than kBT (λE). We find that the results from this quantum mechanical approach are readily understood using a simple, semi-classical model.
The ultraviolet photodissociation of CS2: The S(1D2) channel
Authors: Brouard M. , Campbell E. K. , Cireasa R. , Johnsen A. J. , Yuen W.-H.
Published: January 2012

The photodissociation of CS2 has been investigated using velocity-map ion imaging of the S(1D2) atomic photofragments following excitation at 193  nm and at longer wavelengths close to the S(1D2) channel threshold. The experiments probe regions both above and below the energetic barrier to linearity on the \documentclass[12pt]{minimal}\begin{document}$^{1}\Sigma _{u}^{+}(^{1}B_{2})$\end{document}Σu+1(B21) potential energy surface. The imaging data in both regions indicate that the electronic angular momentum of the S(1D2) atom products is unpolarized, but also reveal different dissociation dynamics in the two regions. Excitation above the barrier to linearity yields an inverted CS(1Σ+) vibrational population distribution, whereas the long-wavelength state-to-state results following excitation below the barrier reveal CS(1Σ+)(v, J) coproduct state distributions which are consistent with a statistical partitioning of the energy. Below the barrier, photofragment excitation spectra point to an enhancement of the singlet channel for K = 1, relative to K = 0, where K is the projection of the angular momentum along the principal axis, in agreement with previous work. However, the CS cofragment product state distributions are found to be insensitive to K. It is proposed that dissociation below the barrier to linearity occurs primarily on a surface with a significant potential energy well and without an exit channel barrier, such as that for the ground electronic state. However, oscillatory structure is also observed in the kinetic energy release distributions, which is shown to be consistent with a mapping of parent molecule bending motion. This could indicate the operation of competing direct and indirect dissociation mechanisms below the barrier to linearity.
Multiple zonal jets on the polar beta plane
Authors: Slavin A. G. , Afanasyev Y. D.
Published: January 2012

The mechanism of the formation of multiple quasi-zonal jets as well as their meridional scaling is investigated for a range of control parameters including the radius of deformation and the parameter which characterizes the polar β-plane. Flows in a rotating cylindrical domain where the depth of the fluid layer varies quadratically with distance from the center are studied using numerical simulations and laboratory experiments. The flows are generated by a spatially localized heater. The westward radiation of barotropic Rossby waves is shown to be a major factor contributing to the formation of jets such that their meridional scaling is in good agreement with a modified Rhines scaling.
Single charge sensing and transport in double quantum dots fabricated from commercially grown Si/SiGe heterostructures
Authors: Payette C. , Wang K. , Koppinen P. J. , Dovzhenko Y. , Sturm J. C. , Petta J. R.
Published: January 2012

We perform quantum Hall measurements on three types of commercially available modulation-doped Si/SiGe heterostructures to determine their suitability for depletion gate defined quantum dot devices. By adjusting the growth parameters, we are able to achieve electron gases with charge densities 1–3 × 1011/cm2 and mobilities in excess of 100 000 cm2/Vs. Double quantum dot devices fabricated on these heterostructures show clear evidence of single charge transitions as measured in dc transport and charge sensing and exhibit electron temperatures of 100 mK in the single quantum dot regime.
Increasing the applicability of density functional theory. II. Correlation potentials from the random phase approximation and beyond
Authors: Verma Prakash , Bartlett Rodney J.
Published: January 2012

Density functional theory (DFT) results are mistrusted at times due to the presence of an unknown exchange correlation functional, with no practical way to guarantee convergence to the right answer. The use of a known exchange correlation functional based on wave-function theory helps to alleviate such mistrust. The exchange correlation functionals can be written exactly in terms of the density-density response function using the adiabatic-connection and fluctuation-dissipation framework. The random phase approximation (RPA) is the simplest approximation for the density-density response function. Since the correlation functional obtained from RPA is equivalent to the direct ring coupled cluster doubles (ring-CCD) correlation functional, meaning only Coulomb interactions are included, one can bracket RPA between many body perturbation theory (MBPT)-2 and CCD with the latter having all ring, ladder, and exchange contributions. Using an optimized effective potential strategy, we obtain correlation potentials corresponding to MBPT-2, RPA (ring-CCD), linear-CCD, and CCD. Using the suitable choice of the unperturbed Hamiltonian, Kohn-Sham self-consistent calculations are performed. The spatial behavior of the resulting potentials, total energies, and the HOMO eigenvalues are compared with the exact values for spherical atoms. Further, we demonstrate that the self-consistent eigenvalues obtained from these consistent potentials used in ab initio dft approximate all principal ionization potentials as demanded by ionization potential theorem.
Stabilization and growth of non-native nanocrystals at low and atmospheric pressures
Authors: Pandey Mohnish , Pala Raj Ganesh S.
Published: January 2012

The stabilization and growth of nanocrystals in “non-native” structures is explored via density functional calculations. Non-native and “native” bulk structures differ in their discrete translational symmetry. Computations suggest that the lower surface energy of the non-native structures always facilitates their stabilization in the early stages of crystal growth. In the compound semiconductors considered here, the transition pathways between non-native and native structures involve planar or near-planar depolarized layers and the growth conditions have significant effects on the stabilization and growth of non-native structures. The findings of this study help in identifying heuristics for the synthesis of non-native nanocrystals.
Numerical investigation of the ion temperature effect in magnetized plasma sheath with two species of positive ions
Authors: Shaw A. K. , Kar S. , Goswami K. S. , Saikia B. J.
Published: January 2012

The effect of ion temperature, magnitude of magnetic field and its orientation on a magnetized plasma sheath consisting of electrons and two species of positive ions are investigated. Using three-fluid hydrodynamic model and some dimensionless variables, the dimensionless equations are obtained and solved numerically. It is found that with the increase of the ion temperature and magnetic field strength there is a significant change in ion densities and energies in the sheath. It is also noticed that increase of magnetic field angle enhances the ion density near the sheath edge for a constant ion temperature. With increase in ion temperature and magnetic field angle, the lighter ion density near the sheath edge enhances and reverses for the heavier ion species.
Exact self-similar solutions for the magnetized Noh Z pinch problem
Authors: Velikovich A. L. , Giuliani J. L. , Zalesak S. T. , Thornhill J. W. , Gardiner T. A.
Published: January 2012

A self-similar solution is derived for a radially imploding cylindrical plasma with an embedded, azimuthal magnetic field. The plasma stagnates through a strong, outward propagating shock wave of constant velocity. This analysis is an extension of the classic Noh gasdynamics problem to its ideal magnetohydrodynamics (MHD) counterpart. The present exact solution is especially suitable as a test for MHD codes designed to simulate linear Z pinches. To demonstrate the application of the new solution to code verification, simulation results from the cylindrical R-Z version of Mach2 and the 3D Cartesian code Athena are compared against the analytic solution. Alternative routines from the default ones in Athena lead to significant improvement of the results, thereby demonstrating the utility of the self-similar solution for verification.
Numerical demonstration of the reciprocity among elemental relaxation and driven-flow problems for a rarefied gas in a channel
Authors: Takata Shigeru , Oishi Masashi
Published: January 2012

Relaxations from a uniform mass/heat flow and flows driven by an external force/temperature-gradient for a rarefied gas between two parallel plates are studied on the basis of the kinetic theory of gases. By numerical computations of the linearized Bhatnagar–Gross–Krook model of the Boltzmann equation, it is demonstrated that the reciprocity among these elemental flows derived from a general reciprocity theory for time-dependent problems [S. Takata, J. Stat. Phys. 140, 985 (2010)] holds at any time and any Knudsen numbers. Moreover, a propagation of the discontinuity of the velocity distribution function (VDF) in the relaxation problems and that of the derivative discontinuity of the VDF in the driven-flow problems are demonstrated. Their relation is also clarified.
Influence of structure in heterodyne electrophoretic light scattering
Authors: Medebach Martin
Published: January 2012

Heterodyne light scattering is employed to probe the electrophoretic flow behavior of a concentrated, deionized model colloidal suspension. Both for the fluid and crystalline regime, the power spectra are well described as velocity distributions, which let assume that the power spectra are dominated by the self-dynamic structure factor. Furthermore, all measured power spectra show an increase of the frequency integrated intensity with the electric field. Since the number of particles should remain constant this finding is unexpected. These effects are rationalized on the basis of a new theoretical model including the influence of the structure factor and of the mobility polydispersity on the power spectra.
Polarization dependent resonant x-ray emission spectroscopy of D2O and H2O water: Assignment of the local molecular orbital symmetry
Authors: Tokushima Takashi , Horikawa Yuka , Arai Hidemi , Harada Yoshihisa , Takahashi Osamu , Pettersson Lars G. M. , Nilsson Anders , Shin Shik
Published: January 2012

The polarization dependence of the split two peaks in the lone-pair region in the x-ray emission spectra has been determined at several different excitation energies for both D2O and H2O water. In contrast to predictions based on a narrow range of local water structures where the two peaks would be of different molecular orbital symmetry and arise from, respectively, intact and dissociated molecules, we show that the two peaks in the lone-pair region are both of lone-pair 1b1 orbital symmetry. The results support the interpretation that the two peaks appear due to fluctuations between two distinct different main structural environments.
Nanosecond in situ transmission electron microscope studies of the reversible Ge2Sb2Te5 crystalline ⇔ amorphous phase transformation
Authors: Santala M. K. , Reed B. W. , Topuria T. , Raoux S. , Meister S. , Cui Y. , LaGrange T. , Campbell G. H. , Browning N. D.
Published: January 2012

Chalcogenide-based phase-change materials have wide use in optical recording media and are growing in importance for use in non-volatile electronic memory. For both applications, rapid switching between the amorphous and crystalline phases is necessary, and understanding the changes during rapidly driven phase transitions is of scientific and technological significance. Laser-induced crystallization and amorphization occur rapidly and changes in atomic structure, microstructure, and temperature are difficult to observe experimentally and determine computationally. We have used nanosecond-scale time-resolved diffraction with intense electron pulses to study Ge2Sb2Te5 during laser crystallization. Using a unique and unconventional specimen geometry, cycling between the amorphous and crystalline phases was achieved, enabling in situ transmission electron microscope (TEM) study of both microstructural and crystallographic changes caused by repeated switching. Finite element analysis was used to simulate interactions of the laser with the nano-structured specimens and to model the rapidly changing specimen temperature. Such time-resolved experimental methods combined with simulation of experimentally inaccessible physical characteristics will be fundamental to advancing the understanding of rapidly driven phase transformations.
Weak ferromagnetic ordering in Ca doped polycrystalline BiFeO3
Authors: Ramachandran B. , Dixit A. , Naik R. , Lawes G. , Ramachandra Rao M. S.
Published: January 2012

Structural and magnetic properties of polycrystalline BiFeO3, Bi0.9Ca0.1FeO2.95, Bi0.9Ba0.05Ca0.05FeO2.95, and Bi0.9Ba0.1FeO2.95 ceramic samples were studied to establish the effects of doping in BiFeO3 on the magnetic property. X-ray diffraction data of the undoped and doped BiFeO3 samples were refined to a rhombohedral structure with space group R3c. X-ray photoelectron spectroscopy study showed the formation of a single-phase in both the undoped and doped BiFeO3 ceramics with Fe in the 3+ valence state. Ca doped and Ba-Ca co-doped BiFeO3 ceramic samples show weak ferromagnetic ordering at room temperature. This observation makes Ca doped and Ba-Ca co-doped BiFeO3 samples an interesting material system for magnetoelectric coupling studies.
Thermosize effects and thermodynamic analysis of a macro/nano scaled refrigerator cycle
Authors: Wang Hao , Wu Guoxing , Chen Xinbin
Published: January 2012

We have established a macro/nano refrigerator model. A possibly new thermodynamic refrigerator based on thermosize effects is analyzed. Expressions for some important parameters, such as the refrigeration load, coefficient of performance, and work input, are derived. By means of numerical calculation and illustration, the influence of the thermosize effects and the additional regenerative loss on the performance of the cycle are discussed and evaluated in detail. Some fundamental optimal relations and general performance characteristic curves of the cycle are obtained. The results obtained here will provide theoretical guidance for the design of some new macro/nano scaled devices.
A technique for physical realization of anisotropic density matrices with application to acoustic beam shifters
Authors: Akl Wael , Baz Amr
Published: January 2012

Physical realization of acoustic metamaterials is so far at its initial stages, with attempts to use periodic patterns of engineered material structures to achieve various material properties that would yield the required wave propagation patterns. Phononic crystals are by far the most advanced technique for such realization. In the current work, a new technique for developing a realizable engineered periodic material structure is developed, where the anisotropic material density matrix arising from the linear coordinate transformation of the acoustic domain is rotated to align with its principal axis, resulting in a decoupled diagonal density matrix with entries representing the eigenvalues of the original anisotropic matrix. This allows for selecting feasible material properties that would achieve the targeted wave propagation patterns. The proposed approach is then utilized in modeling an acoustic wave shifting device with controllable directivity and dispersion characteristics. The linear coordinate transformation of the proposed wave shifter is augmented with an additional degree of freedom to simultaneously control the directivity and dispersion characteristics. The resulting anisotropic density matrix is manipulated using the developed technique and corresponding isotropic phononic cell structure is developed. Implementing the new isotropic phononic cell structure resulted in achieving the targeted wave control patterns. The theory governing the design of this class of acoustic metamaterials is introduced, and the parameters that control the tuning of the directivity and dispersion characteristics are presented in detail. With such capabilities, the proposed feasibly realizable acoustic metamaterials will be capable of controlling the wave propagation both in the spatial and spectral domains.
Origin of the enhanced polarization in La and Mg co-substituted BiFeO3 thin film during the fatigue process
Authors: Ke Qingqing , Kumar Amit , Lou Xiaojie , Zeng Kaiyang , Wang John
Published: January 2012

We have studied the polarization fatigue of La and Mg co-substituted BiFeO3 thin film, where a polarization peak is observed during the fatigue process. The origin of such anomalous behavior is analyzed on the basis of the defect evolution using temperature-dependent impedance spectroscopy. It shows that the motion of oxygen vacancies (VO••) is associated with a lower energy barrier, accompanied by the injection of electrons into the film during the fatigue process. A qualitative model is proposed to explain the fatigue behavior, which involves the modification of the Schottky barrier upon the accumulation of VO•• at the metal-dielectric interface.
Surface structure and phase transition of K adsorption on Au(111): By ab initio atomistic thermodynamics
Authors: Gan Li-Yong , Tian Ren-Yu , Yang Xiao-Bao , Zhao Yu-Jun
Published: January 2012

We studied the interactions between atomic potassium (K) and Au(111) at a range of coverage (i.e., ΘK = 0.11–0.5 monolayer (ML)) by ab initio atomic thermodynamics. For K on-surface adsorption, we found that K energetically favors the three-fold hollow sites (fcc or hcp), while the most significant surface rumpling was obtained at the atop sites. The incorporation of gold atoms in the adsorbate layer gradually becomes energetically favorable with increasing K coverage. We proposed a possible model with a stoichiometry of K2Au for the (2 × 2)−0.5 ML phase observed in lower energy electron diffraction (LEED): one K at atop site and the other K as well as one Au adatom at the second-nearest fcc/hcp and hcp/fcc, respectively. Clear theoretical evidences were given for the ionic interaction of K on Au surface. Additionally, phase transitions were predicted based on chemical potential equilibrium of K, largely in line with the earlier reported LEED observations: the clean surface → (√3 × √3)R30° → (2 × 2), and (2 × 2) → (√3 × √3)R30° reversely at an elevated temperature.
Transient electron transport properties of multiple quantum dots systems
Authors: Taranko E. , Wiertel M. , Taranko R.
Published: January 2012

We study theoretically the transient electron transport in the multiple quantum dots (QDs) systems forced by the suddenly applied bias voltage. Depending on the quantum dots configuration and parameters describing the considered system different beat patterns of the resulting transient current are observed. We find out that the careful inspection of the transient current beat patterns can provide the information about the inter-dot hopping amplitudes, quantum dots energy levels and their occupancies before the abrupt change of the bias voltage is made.
Negative resistance phenomenon in dual-frequency capacitively coupled plasma-enhanced chemical vapor deposition system for photovoltaic manufacturing process
Authors: Kwon H. C. , Aman-ur-Rehman  , Won I. H. , Park W. T. , Lee J. K.
Published: January 2012

The validity of effective frequency concept is investigated for dual-frequency (DF) capacitively coupled plasma (CCP) discharges by using particle-in-cell/Monte Carlo collision simulations. This concept helps in analyzing DF CCP discharges in a fashion similar to single-frequency (SF) CCP discharges with effective parameters. Unlike the driving frequency of SF CCP discharges, the effective frequency in DF CCP is dependent on the ratio of the two driving currents (or voltages) and this characteristic makes it possible to control the ion flux and the ion bombardment energy independently. This separate control principally allows to increase the ion flux and plasma density for high deposition rates, while keeping the ion mean energy constant at low values to prevent the bombardment of highly energetic ions at the substrate surface to avoid unwanted damage in the solar cell manufacturing. The abrupt transition of the effective frequency leads to the phenomenon of negative resistance which is one of the several physical phenomena associated uniquely with DF CCP discharges. Using effective frequency concept, the plasma characteristics have been investigated in the negative resistance regime for solar cell manufacturing.
Spin transport in memristive devices
Authors: Jang Hyuk-Jae , Kirillov Oleg A. , Jurchescu Oana D. , Richter Curt A.
Published: January 2012

We report on electron spin transport through electrochemically precipitated copper filaments formed in TaOx memristive devices consisting of Co/TaOx/Cu/Py with crossbar-type electrode geometry. The devices show memristive behavior having a typical OFF/ON resistance ratio of 105. Magnetoresistance measurements performed by sweeping an external magnetic field clearly indicate spin transport through an electrochemically formed copper nano-filament as long as 16 nm in the memristive ON-state at 77 K. Spin transport vanishes in the OFF-state. These data are strong evidence that the fundamental switching mechanism in these metal-oxide devices is the formation of continuous metallic conduction paths.
Ultraviolet photodissociation dynamics of the phenyl radical
Authors: Song Yu , Lucas Michael , Alcaraz Maria , Zhang Jingsong , Brazier Christopher
Published: January 2012

Ultraviolet (UV) photodissociation dynamics of jet-cooled phenyl radicals (C6H5 and C6D5) are studied in the photolysis wavelength region of 215–268 nm using high-n Rydberg atom time-of-flight and resonance enhanced multiphoton ionization techniques. The phenyl radicals are produced from 193-nm photolysis of chlorobenzene and bromobenzene precursors. The H-atom photofragment yield spectra have a broad peak centered around 235 nm and are in good agreement with the UV absorption spectra of phenyl. The H + C6H4 product translational energy distributions, P(ET)’s, peak near ∼7 kcal/mol, and the fraction of average translational energy in the total excess energy, 〈fT〉, is in the range of 0.20–0.35 from 215 to 268 nm. The H-atom product angular distribution is isotropic. The dissociation rates are in the range of 107–108 s−1 with internal energy from 30 to 46 kcal/mol above the threshold of the lowest energy channel H + o-C6H4 (ortho-benzyne), comparable with the rates from the Rice–Ramsperger–Kassel–Marcus theory. The results from the fully deuterated phenyl radical are identical. The dissociation mechanism is consistent with production of H + o-C6H4, as the main channel from unimolecular decomposition of the ground electronic state phenyl radical following internal conversion of the electronically excited state.
Note: Percolation in two-dimensional flexible chains systems
Authors: Pawłowska Monika , Żerko Szymon , Sikorski Andrzej
Published: January 2012

The structure of a two-dimensional film formed by adsorbed polymer chains was studied by means of Monte Carlo simulations. The polymer chains were represented by linear sequences of lattice beads and positions of these beads were restricted to vertices of a two-dimensional square lattice. Two different Monte Carlo methods were employed to determine the properties of the model system. The first was the random sequential adsorption (RSA) and the second one was based on Monte Carlo simulations with a Verdier-Stockmayer sampling algorithm. The methodology concerning the determination of the percolation thresholds for an infinite chain system was discussed. The influence of the chain length on both thresholds was presented and discussed. It was shown that the RSA method gave considerably lower thresholds for longer chains. This behavior can be explained by a different pool of chain conformations used in the calculations in both methods under consideration.
Side-dependent electron escape from graphene- and graphane-like SiC layers
Authors: Gori Paola , Pulci Olivia , Marsili Margherita , Bechstedt Friedhelm
Published: January 2012

The structural and electronic properties of SiC-based two-dimensional (2D) crystals are studied by means of density functional theory and many-body perturbation theory. Such properties cannot simply be interpolated between graphene and silicene. The replacement of half of the C atoms by Si atoms opens a large direct electronic gap and destroys the Dirac cones. Hydrogenation further opens the gap and significantly reduces the electron affinity to 0.1 or 1.8 eV in dependence on the carbon or silicon termination of the 2D crystal surface, thus showing a unique direction dependent ionization potential. This suggests the use of 2D-SiC:H as electron or hole filter.