A deconvolution enhancement of the Navier–Stokes-αβ model

A deconvolution enhancement of the Navier–Stokes-αβ model for turbulent flow is introduced. The energy and energy-dissipation rate for the enhanced model are derived. It is also shown that the consistency error, relative to the Navier–Stokes equations, and the microscale of the enhanced model are less than those of the Navier–Stokes-αβ model. The proposed model is used to simulate the Taylor–Green vortex problem and results show a qualitatively improved representation of the mean-square vorticity when compared to the Navier–Stokes-αβ model. Numerical studies of the energy spectrum and the alignment between the vorticity and the eigenvectors of the stretching tensor for three-dimensional turbulent flows with Re = 200 are used to explore the utility of the model. A benchmark problem of a two-dimensional channel flow over a step for Re = 600 also indicates that this model can be applied to more general flows than those involving periodic boundary conditions.     

A spectral-element discontinuous Galerkin lattice Boltzmann method for nearly incompressible flows

We present a spectral-element discontinuous Galerkin lattice Boltzmann method for solving nearly incompressible flows. Decoupling the collision step from the streaming step offers numerical stability at high Reynolds numbers. In the streaming step, we employ high-order spectral-element discontinuous Galerkin discretizations using a tensor product basis of one-dimensional Lagrange interpolation polynomials based on Gauss–Lobatto–Legendre grids. Our scheme is cost-effective with a fully diagonal mass matrix, advancing time integration with the fourth-order Runge–Kutta method. We present a consistent treatment for imposing boundary conditions with a numerical flux in the discontinuous Galerkin approach. We show convergence studies for Couette flows and demonstrate two benchmark cases with lid-driven cavity flows for Re = 400–5000 and flows around an impulsively started cylinder for Re = 550–9500. Computational results are compared with those of other theoretical and computational work that used a multigrid method, a vortex method, and a spectral element model.     

Phonon dispersion of carbon nanotubes

We present the phonon dispersion relations of single-wall carbon nanotubes calculated within a force-constants approach. By using the full symmetry group of the tubes, we are able to calculate the dispersion relations for any chirality starting from one single carbon atom. We find an overbending in the highest optical branch between 6 and 12 cm⁻¹ independent of the tube diameter. The order of the high-energy modes at the Γ-point differs from the results derived from simple zone folding. The splitting between the two Raman active optical modes with A₁ symmetry at the Γ-point of chiral tubes is ≈4 cm⁻¹ for typical diameters; it increases with decreasing tube diameter.     

Large transport critical currents of powder-in-tube Sr0.6K0.4Fe2As2/Ag superconducting wires and tapes

We report the achievement of transport critical currents in Sr_0.6K_0.4Fe₂As₂ wires and tapes with a T_c = 34 K. The wires and tapes were fabricated through an in situ powder-in-tube process. Silver was used as a chemical addition as well as a sheath material. All the wire and tape samples have shown the ability to transport superconducting current. Critical current density J_c was enhanced upon silver addition, and at 4.2 K, a largest J_c of ～1200 A/cm² (I_c = 9 A) was achieved for 20% silver added tapes, which is the highest in iron-based wires and tapes so far. The J_c is almost field independent between 1 T and 10 T, exhibiting a strong vortex pinning. Such a high transport critical current density is attributed to the weak reaction between the silver sheath and the superconducting core, as well as an improved connectivity between grains. We also identify a weak-link behavior from the apparent drop of J_c at low fields and a hysteretic phenomenon. Finally, we found that compared to Fe, Ta and Nb tubes, Ag was the best sheath material for the fabrication of high-performance 122 type pnictide wires and tapes.     

Vortex structure in Sr2RuO4

The vortex structure in p-wave superconductors is investigated by the Bogoliubov–de Gennes theory on a tight-binding model. We calculate the temperature dependence of the electronic state at each site in the vortex lattice state, and show the difference between sin p_x+i sin p_y-wave and sin p_x−i sin p_y-wave superconducting state. Furthermore the relation of the electronic structure and the site-dependence of the nuclear magnetic relaxation time is also discussed.     

Proper general decomposition (PGD) for the resolution of Navier–Stokes equations

In this work, the PGD method will be considered for solving some problems of fluid mechanics by looking for the solution as a sum of tensor product functions. In the first stage, the equations of Stokes and Burgers will be solved. Then, we will solve the Navier–Stokes problem in the case of the lid-driven cavity for different Reynolds numbers (Re = 100, 1000 and 10,000). Finally, the PGD method will be compared to the standard resolution technique, both in terms of CPU time and accuracy.     

Superconducting properties of the pyrochlore oxide Cd2Re2O7

We report the superconducting properties of the pyrochlore oxide Cd₂Re₂O₇. The bulk superconducting transition temperature T_c is about 1.0 K, and the upper critical field H_c2 determined by the measurement of specific heat under magnetic fields is 0.29 T. The superconducting coherence length is estimated to be 34 nm. Specific heat data measured on single crystals suggest that the superconducting gap of Cd₂Re₂O₇ is nodeless.     

Dynamics of vortices in type-II superconductors with periodic square columnar pins

The dynamics of a two-dimensional vortex system with strong periodic square columnar pins is investigated. For the case vortex number matching pinning number, we find that the vortex liquid is frozen into square lattice via a continuous transition, and the freezing (melting) temperature T_m is the same as the thermal depinning temperature of vortices, which are different from the first-order phase transition at weak pinning. The zero-temperature critical depinning force F_c0 is exactly the same as the maximum pinning force, and the depinning property at T = 0 can be expressed by scaling v ～ (F ? F_c0)^β with the exponent β close to 0.5. The v–F curves at temperatures below T_m show that vortices are pinned at small driving force.     

Two-level discretizations of nonlinear closure models for proper orthogonal decomposition

Proper orthogonal decomposition has been successfully used in the reduced-order modeling of complex systems. Its original promise of computationally efficient, yet accurate approximation of coherent structures in high Reynolds number turbulent flows, however, still remains to be fulfilled. To balance the low computational cost required by reduced-order modeling and the complexity of the targeted flows, appropriate closure modeling strategies need to be employed. Since modern closure models for turbulent flows are generally nonlinear, their efficient numerical discretization within a proper orthogonal decomposition framework is challenging. This paper proposes a two-level method for an efficient and accurate numerical discretization of general nonlinear closure models for proper orthogonal decomposition reduced-order models. The two-level method computes the nonlinear terms of the reduced-order model on a coarse mesh. Compared with a brute force computational approach in which the nonlinear terms are evaluated on the fine mesh at each time step, the two-level method attains the same level of accuracy while dramatically reducing the computational cost. We numerically illustrate these improvements in the two-level method by using it in three settings: the one-dimensional Burgers equation with a small diffusion parameter ν = 10^?3, the two-dimensional flow past a cylinder at Reynolds number Re = 200, and the three-dimensional flow past a cylinder at Reynolds number Re = 1000.     

Insulation room for aero-acoustic experiments at moderate Reynolds and low Mach numbers

A non-expensive insulation box for aero-acoustic experiments at moderate Reynolds numbers Re < 2 × 10⁴ and low Mach numbers M < 0.2 is presented. Its performance is evaluated with particular attention to unwanted noise sources inherent to the flow facility. Objective acoustic parameters of the insulation box are assessed.     

Fermi surface of Ba1−xKxFe2As2 as probed by angle-resolved photoemission

Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba_1?xK_xFe₂As₂ (T_c = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X-symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in Γ?X and Γ?Y directions.     

Dynamic evolution of Riemann–Silberstein vortices for Gaussian vortex beams

The dynamic evolution of Riemann–Silberstein (RS) vortices for Gaussian vortex beams with topological charges m = ± 1 in free space is studied. It is shown that for Gaussian on-axis vortex beams there exist both RS vortex with m = + 2 and circular edge dislocation. For Gaussian off-axis vortex beams the circular edge dislocation splits into two RS vortices with opposite topological charges m = ± 1 and the RS vortex with m = + 2 decays into two vortices with same topological charges m = + 1. The motion of RS vortices takes place by varying the propagation distance, waist width, off-axis parameter, or topological charge. RS vortices for Gaussian vortex-free beams can be treated as a special case. The results are illustrated analytically and numerically.     

Interfacial energies of carbon tetrabromide

The equilibrated grain boundary groove shapes for solid carbon tetrabromide (CTB) in equilibrium with its melt were directly observed by using a horizontal temperature gradient stage. From the observed grain boundary groove shapes, Gibbs–Thomson coefficient (Γ) and solid–liquid interfacial energy (σ_SL) and grain boundary energy (σ_gb) of CTB have been determined to be (7.88 ± 0.8) × 10⁻⁸ K m, (6.91 ± 1.04) × 10⁻³ J m⁻² and (13.43 ± 2.28) × 10⁻³ J m⁻², respectively. The ratio of thermal conductivity of equilibrated liquid phase to solid phase for CTB has also been measured to be 0.90 at its melting temperature. The value of σ_SL for CTB obtained in present work was compared with the values of σ_SL determined in the previous works for same material and it was seen that the present result is in good agreement with previous works.     

Time dependence of isothermal magnetization in single-phase B1 MoCy encapsulated in multiwall carbon nanocages

Isothermal magnetization M(t) in nanocrystalline single-phase B1 MoC_y encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 5000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U_eff increases linearly with increasing temperature T, and decreases linearly with increasing J. The behaviors of J(t), U_eff(T), and U_eff(J) can be described by the Anderson–Kim flux-creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U_eff. The slower relaxation of current density above the broad peak field in the isothermal magnetization curves suggests that the peak is a result of vortex dynamics.     

DFT study of carbon monoxide adsorption on α-Al2O3(0001)

The adsorption of CO on α-Al₂O₃(0001) was studied using the DFT-GGA computational method and on α-Al₂O₃ powder experimentally by Infra red spectroscopy. The core and valence level regions of α-Al₂O₃(0001) single crystal surface were also studied experimentally. Ar ions sputtering of the surface results in a slight but reproducible decrease in the XPS O2p lines in the valence band regions due to preferential removal of surface (and near surface) O atoms. Core level XPS O1s and Al2p further confirmed oxygen depletion with an associated surface stoichiometry close to Al₂O_2.9. The adsorption energy of CO was computed and found equal to 0.52 eV for θ = 0.25, it decreased to 0.42 eV at θ = 1. The IR frequency of νCO was also computed and in all cases it was blue shifted with respect to gas phase CO. The shift, Δν, decreased with increasing coverage where it was found equal to 56 cm^? 1 for θ = 0.25 and decreased to 30 cm^? 1 for θ = 1. Structural analyses indicated that the change in the adsorption energy and the associated frequency shift is due to surface relaxation upon adsorption. Experimentally the adsorption of CO gave rise to one main IR peak at 2154 cm^? 1 at 0.3 Torr and above. Two far smaller peaks are also seen at lower pressures of 0.03–0.2 Torr at 2189 and 2178 cm^? 1. The isosteric heat of adsorption was computed for the IR band at 2154 cm^? 1 and was found equal to 0.2 eV which did not change with coverage in the investigated range up to θ = 0.6.     

Near limit behavior of the detonation velocity

The behavior of the detonation velocity near the limits is investigated. Circular tubes of diameters 65, 44 and 13 mm are used. To simulate a quasi two-dimensional rectangular geometry thin annular channels are also used. The annular channels are formed by a 1.5 m long insert of a smaller diameter tube into the larger outer diameter detonation tube. Premixed mixtures of C₂H₂ + 2.5O₂ + 70%Ar, CH₄ + 2O₂ and C₂H₂ + 5N₂O + 50%Ar are used in the present study. The high argon dilution stoichiometric C₂H₂ + 2.5O₂ mixture has a regular cell size and piecewise laminar reaction zone and thus referred to as “stable”. The other two mixtures give highly irregular cell pattern and a turbulent reaction zone and are hence, referred to as “unstable” mixtures. Pressure transducers and optical fibers spaced 10 cm apart along the tube are used for pressure and velocity measurements. Cell size of the three mixtures studied is also determined using smoked foils in both the circular tubes and annular channels. The ratio d/λ (representing the number of cells across the tube diameter) is found to be an appropriate sensitivity parameter to characterize the mixture. The present results indicate that well within the limit, the detonation velocity is generally a few percent below the theoretical Chapman–Jouguet (CJ) value. As the limit is approached, the velocity decreases rapidly to a minimum value before the detonation fails. The narrow range of values of d/λ of the mixture where the velocity drops rapidly is found to correspond to the range of values for the onset of single headed spinning detonations. Thus we may conclude that the onset of single headed spin can be used as a criterion for defining the limits. Spinning detonations are also observed near the limits in annular channels.     

Geometry dependent resistivity behavior in mesoscopic Bi2Sr2CaCu2O8+δ single crystals

In order to investigate the interaction between Josephson vortices and pancake vortices in a layered Bi₂Sr₂CaCu₂O_8+δ superconductor, c-axis resistivity measurements were carried out on fabricated mesoscopic single crystals for two experimental geometries, when an applied high magnetic field of 14 T is perpendicular (B_ex⊥L) and parallel (B_ex∥L) to the width of sample. In the first case, the angular dependence of the resistance obtained near the ab-plane at T = 30 K exhibited hysteretic non-monotonic behavior with local maxima at θ = 0.6° away from the ab-plane and dips and spikes at θ = 0.2° marking a vortex lock-in transition. In the second geometry (B_ex∥L), instead of a local maximum, peculiar resistance shoulders were observed. At higher temperatures, hysteresis is reduced, while the vortex lock-in transition becomes considerably broader indicating the interaction of pancake and Josephson vortices.     

Theoretical study of optoelectronic properties of GaAs1−xBix alloys using valence band anticrossing model

The (12 × 12) and (14 × 14) valence band anticrossing (V-BAC) models were applied to calculate the electronic band structure of GaAs_1−xBi_x dilute alloys along Δ-, Λ- and Σ-directions at room temperature. A comparative study based on these models was performed in terms of energy levels, optical transitions, spin–orbit splitting and effective mass. We found a significant reduction of the band-gap energy E_g by roughly 81 meV/%Bi accompanied by an increase in the spin–orbit splitting Δ_so+ by about 56 meV/%Bi. Furthermore, Δ_so+ does come into resonance with E_g at ∼12%Bi for resonance energy equal to 0.73 eV. An excellent agreement has occurred between the (14 × 14) V-BAC model predictions and experimental results reported in the literature. In addition, we have investigated the Bi composition and k-directions dependence of the effective mass at Γ point. A slight increase of the holes effective mass with x can affect the holes transport properties of GaAsBi. The intrinsic carrier density increases with both x and the temperature T, but it remains below 10¹⁰ cm⁻³ for x ⩽ 5% and T ⩽ 300 K.     

Modelling the directional and energy dependence of 5–10 keV Ar+ ion-induced secondary electron yields from Cu crystals

The primary ion directional effects observed in secondary electron yields induced by ion bombardment [5 keV Ar⁺ → Cu(100)] are simulated using a semi-empirical molecular dynamics model. The directional effects are presumed to arise from inelastic energy transfers that take place in close binary atomic encounters. The latter are estimated using the Oen-Robinson model, in combination with a critical apsidal distance (R_c). The connection between the measured kinetic electron emission (KEE) yields (γ_KEE) and the predicted inelastic energy loss in a binary atomic collision (ΔE_i) is established through a semi-empirical fitting procedure involving R_c and other parameters in the following model: γ_e = γ₀ + γ_KEE = γ₀ + 〈ΔE_i(z)exp(? z/λ)〉, where z is the collision depth. The directional effects are best reproduced by fitting the model to Ar–Cu inelastic collisions for two azimuthal incident directions: R_c is estimated to be 0.47 ± 0.03 Å; the parameter, λ (an effective electron attenuation length), is estimated to be 18 ± 2 Å. The same model also describes the γ_KEE energy dependence for 5–10 keV Ar⁺ normally incident on low-index Cu crystal targets [Phys. Rev. 129 (1963) 2409]. The spatial and temporal distributions of the hard collisions that initiate KEE are discussed on the basis of the model.     

Point-contact Andreev-reflection spectroscopy in ReFeAsO1−xFx (Re = La,Sm): Possible evidence for two nodeless gaps

A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO_1?xF_x (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection spectroscopy experiments performed on LaFeAsO_1?xF_x (La-1111) polycrystals with T_c ～ 27 K and SmFeAsO_0.8F_0.2 (Sm-1111) polycrystals with T_c ～ 53 K gave differential conductance curves exhibiting two peaks at low bias and two additional structures (peaks or shoulders) at higher bias voltages, an experimental situation quite similar to that observed by the same technique in pure and doped MgB₂. The single-band Blonder–Tinkham–Klapwijk model is totally unable to properly fit the conductance curves, while the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV vs. V curves. These results give direct evidence of two nodeless gaps in the superconducting state of ReFeAsO_1?xF_x (Re = La, Sm): a small gap, Δ₁, smaller than the BCS value (2Δ₁/k_BT_c ～ 2.2–3.2) and a much larger gap Δ₂ which gives a ratio 2Δ₂/k_BT_c ～ 6.5–9.0. In Sm-1111 both gaps close at the same temperature, very similar to the bulk T_c, and follow a BCS-like behaviour, while in La-1111 the situation is more complex, the temperature dependence of the gaps showing remarkable deviations from the BCS behaviour at T close to T_c.The normal-state conductance reproducibly shows an unusual, but different, shape in La-1111 and Sm-1111 with a depression or a hump at zero bias, respectively. These structures survive in the normal state up to T¹ ～ 140 K, close to the temperatures at which structural and magnetic transitions occur in the parent, undoped compound.