Wave Calculations Research Articles

Gravitational radiation from first-order phase transitions in the presence of a fluid

First-order phase transitions are a source of stochastic gravitational radiation. Precision calculations of the gravitational waves emitted during these processes, sourced by both the degrees of freedom undergoing the transition and anisotropic stress of the coupled, ambient constituents, have reached an age of maturity. Here we present high-resolution numerical simulations of a scalar field coupled to a fluid and parameterize the final gravitational wave spectrum as a function of the ratio of the energies of the two sectors and the coupling between the two sectors for a set of models that represent different types of first-order phase transitions. In most cases, the field sector is the dominant source of gravitational radiation, but it is possible in certain scenarios for the fluid to have the most important contribution.

Dependence of the electronic and transport properties ofmetal-MoSe2interfaces on contact structures

Transition metal dichalcogenides (TMDs) are considered as promising candidates for next generation of electronic and optoelectronic devices. To make use of these materials, for instance in field effect transistor applications, it is mandatory to know the detailed properties of contacts of such TMDs with metal electrodes. Here, we investigate the role of the contact structure on the electronic and transport properties of ${\text{metal-MoSe}}_{2}$ interfaces. Two different contact types, namely face and edge contacts, are studied. We consider both low (Sc) and high (Au) work function metals in order to thoroughly elucidate the role of the metal work function and the type of metal. First principles plane wave calculations and transport calculations based on nonequilibrium Green's function formalism reveal that the contact type has a large impact on the electronic and transport properties of ${\text{metal-MoSe}}_{2}$ interfaces. For the Sc electrode, the Schottky barrier heights are around 0.25 eV for face contact and bigger than 0.6 eV for edge contact. For the Au case, we calculate very similar barrier heights for both contact types with an average value of 0.5 eV. Furthermore, while the face contact is found to be highly advantageous as compared to the edge contact for the Sc electrode, the latter contact becomes a better choice for the Au electrode. Our findings provide guidelines for the fabrication of TMD-based devices.

Zero-energy Majorana states in a one-dimensional quantum wire with charge-density-wave instability

One-dimensional lattice with strong spin-orbit interactions (SOI) and Zeeman magnetic field is shown to lead to the formation of a helical charge-density wave (CDW) state near half-filling. Interplay of the magnetic field, SOI constants and the CDW gap seems to support Majorana bound states under appropriate value of the external parameters. Explicit calculation of the quasi-particles' wave functions supports a formation of the localized zero-energy state, bounded to the sample end-points. Symmetry classification of the system is provided. Relative value of the density of states shows a precise zero-energy peak at the center of the band in the non-trivial topological regime.

Tight-Binding study of Boron structures

We have performed Linearized Augmented Plane Wave (LAPW) calculations for five crystal structures (alpha, dhcp, sc, fcc, bcc) of Boron which we then fitted to a non-orthogonal tight-binding model following the Naval Research Laboratory Tight-Binding (NRL-TB) method. The predictions of the NRL-TB approach for complicated Boron structures such as R105 (or β-rhombohedral) and T190 are in agreement with recent first‐principles calculations. Fully utilizing the computational speed of the NRL-TB method we calculated the energy differences of various structures, including those containing vacancies using supercells with up to 5000 atoms.

Clarification of isomeric structures and the effect of intermolecular interactions in blue-emitting aluminum complex Alq3 using first-principles 27Al NMR calculations

We have performed structure analysis of the blue-emitting aluminum complex Alq3 using 27Al NMR and gauge-including projector-augmented wave calculations. The results clearly show that 27Al NMR spectra are insensitive to intermolecular interactions, thus providing a means of carrying out precise intramolecular structure determination. The key determinant of the blue-shifted emission of Alq3 is the facial isomerization.

Effects of silver adatoms on the electronic structure of silicene

Abstract This paper presents the adsorption of Ag adatoms on silicene surface using first-principles plane wave calculations within density functional theory. It is obtained that silver adatoms form strong bonds with the silicene yielding significant binding energies. The bare silicene, which is a nonmagnetic semimetal, becomes either nonmagnetic metal or semiconductor depending on the number of adsorbed silver atoms. Because of the charge transfer from adatoms to silicene, bonding and antibonding π bands crossing linearly at the Fermi level shift 0.35 eV below the Fermi level for both single and trimer Ag adsorption. Ag dimer adsorbed silicene becomes a narrow gap semiconductor with Eg = 0.112 eV.

Calculation of leaky Lamb waves with a semi-analytical finite element method

A semi-analytical finite element method (SAFE) has been widely used for calculating dispersion curves and mode shapes of guided waves as well as transient waves in a bar like structures. Although guided wave inspection is often conducted for water-loaded plates and pipes, most of the SAFE techniques have not been extended to a plate with leaky media. This study describes leaky Lamb wave calculation with the SAFE. We formulated a new solution using a feature that a single Lamb wave mode generates a harmonic plane wave in leaky media. Dispersion curves obtained with the SAFE agreed well with the previous theoretical studies, which represents that the SAFE calculation was conducted with sufficient accuracy. Moreover, we discussed dispersion curves, attenuation curves, and displacement distributions for total transmission modes and leaky plate modes in a single side and both two side water-loaded plate.

EFFECTIVE COLLISION STRENGTHS FOR FINE-STRUCTURE TRANSITIONS IN Si VII

The effective collision strengths for electron-impact excitation of fine-structure transitions in Si vii are calculated as a function of electron temperature in the range 5000–2,000,000 K. The B-spline Breit–Pauli R-matrix method has been used to calculate collision strengths by electron impact. The target wave functions have been obtained using the multi-configuration Hartree–Fock method with term-dependent non-orthogonal orbitals. The 92 fine-structure levels belonging to the 46 LS states of 2s22p4, 2s2p5, 2p6, 2s22p33s, 2s22p33p, 2s22p33d, and 2s2p43s configurations are included in our calculations of oscillator strengths and collision strengths. There are 4186 possible fine-structure allowed and forbidden transitions among the 92 levels. The present excitation energies, oscillator strengths, and collision strengths have been compared with previous theoretical results and available experimental data. Generally, a good agreement is found with the 6 LS-state close-coupling approximation results of Butler & Zeippen and the 44 LS-state distorted wave calculation of Bhatia & Landi.

Large orbital magnetic moment in Pt13 clusters

We present an extensive study of Pt13 clusters embedded in a Na-Y zeolite, by comparing calculations for isolated clusters to experimental data. We perform structural refinements for various geometries involving the isolated clusters and calculate the corresponding x-ray absorption and magnetic circular dichroism spectra, from the joint perspective of pseudopotential plane wave calculations and real space multiple scattering theory. Taking into account the spin–orbit coupling significantly improves the previous scalar relativistic predictions of magnetic properties. The ensemble of embedded Pt13 is found to be dominated by a non-magnetic cuboctahedral geometry. One of the implications is that the ground state of Pt13 clusters in the zeolite environment is different from that of isolated particles. We investigate several isomers that yield a magnetic signature. Furthermore, their abundance was estimated by direct comparison with experiment. We found that one third of the magnetic moment of Pt13 comes from the orbital contribution, in agreement with the experimental value. We therefore provide theoretical proof of the extraordinary orbital magnetization in Pt13 clusters.

Electron-impact ionization of 4d-shell xenon and tin ions

Electron-impact single ionization of xenon and tin ions for charge states where the 4d-subshell is the outermost has been investigated. Measured cross sections have been analyzed in detail by comparing with configuration-averaged distorted wave (CAWD) calculations. Contributions of different direct- and indirect ionization processes have thus been quantitatively revealed.

Electron emission from dressed projectiles in collision with atoms and molecules at intermediate energies

Angular and energy distributions of electron emitted from dressed ions and neutral atoms in collisions with atoms and molecules were measured for intermediate incident energies. Effects of the initial state of the projectile electron were studied using distorted wave and CTMC calculations. Experiments carried out on collisions with He, Li, H, Ne, and Ar show that the electron loss distributions depend very strongly on the target.

Computation of steady gravity–capillary waves on deep water based on the pseudo-arclength continuation method

Although the pseudo-arclength continuation method is classical, this method has not been widely or effectively applied in the area of the numerical calculation of nonlinear waves in fluids. The author believes that the present study is the first work that provides detailed numerical procedures incorporating the pseudo-arclength continuation method with the computation of steady solutions of nonlinear model equations for both 2-D and 3-D viscous gravity–capillary waves on deep water generated by a moving pressure forcing. The basic mathematical concept of the pseudo-arclength continuation method, which originated from the study of solving a nonlinear ordinary differential equation (ODE), is introduced, and the associated numerical procedure in solving the present partial differential equation (PDE) is subsequently presented. Numerical results are shown in terms of steady response diagrams and associated nonlinear wave solution profiles to understand the interrelated effect of forcing, nonlinearity, and viscous damping. In the steady response diagram, once an initial point is determined, then all of the possible nonlinear wave solutions are found, making a complex branch in the diagram. In particular, if the viscous effect is included, however small, then the branch shows a zig-zag pattern, i.e., several turning points, in which multiple steady wave solution profiles are possible for a certain forcing speed. The proposed method can also be applied to any types of PDEs or ODEs in other areas where nonlinearities cannot be neglected.

Magnon spectra and strong spin-lattice coupling in magnetically frustratedMnB2O4(B=Mn,V): Inelastic light-scattering studies

The ferrimagnetic spinels MnB2O4 (B = Mn,V) exhibit a similar series of closely spaced magnetic and structural phase transitions at low temperatures, reflecting both magnetic frustration and a strong coupling between the spin and lattice degrees of freedom. Careful studies of excitations in MnB2O4 (B = Mn,V), and the evolution of these excitations with temperature, are important for obtaining a microscopic description of the role that magnetic excitations and spin-lattice coupling play in the low temperature phase transitions of these materials. We report an inelastic light (Raman) scattering study of the temperature and magnetic field dependences of one- and two-magnon excitations in MnV2O4 and Mn3O4. We observe a pair of q=0 one-magnon modes at 74 cm^{-1} and 81 cm^{-1} in MnV2O4, which is in contrast with the single 80 cm^{-1} q=0 magnon that has been reported for MnV2O4 based on previous neutron scattering measurements and spin wave calculations. Additionally, we find that the two-magnon energy of MnV2O4 decreases ("softens") with decreasing temperature below T_{N}, which we attribute to strong coupling between magnetic and vibrational excitations near the zone boundary.

Lattice and spin excitations in multiferroich-YMnO3

We used Raman and terahertz spectroscopies to investigate lattice and magnetic excitations and their cross-coupling in the hexagonal YMnO3 multiferroic. Two phonon modes are strongly affected by the magnetic order. Magnon excitations have been identified thanks to comparison with neutron measurements and spin wave calculations but no electromagnon has been observed. In addition, we evidenced two additional Raman active peaks. We have compared this observation with the anti-crossing between magnon and acoustic phonon branches measured by neutron. These optical measurements underly the unusual strong spin-phonon coupling.

Late-Holocene storm imprint in a coastal sedimentary sequence (Northwest Iberian coast)

A combination of sedimentological, geomorphological, and pedological methods has been used to study a late-Holocene sedimentary sequence in a rock coast sector from NW Spain, with the aim of relating it to storm events and their morphodynamic effects. The sequence contains two coarse beach layers at an elevation of 2.8–3.5 m above the present highest astronomical tide (HAT). Radiocarbon dating revealed that their deposition had begun during 1735–1590 cal. yr BP and has continued until the present. The entire beach system experienced considerable morphological change after 1320–1230 cal. yr BP, with a westward displacement of the beach and a retreat of the sedimentary cliff in the eastern section. The two beach layers seem to have been formed by vertical aggradation of clasts and sand during high-energy storm events, highlighting the role of these events in the formation of sedimentary sequences on the rocky coasts of mid-Atlantic Europe. The effects of a recent storm event, recorded in March 2008, and the results of wave calculations suggest that long swell waves were needed for the accretion of the clasts. Using a hindcast model of wave data, we found a positive correlation between the winter North Atlantic Oscillation (WNAO) index and the winter monthly mean wave height and peak period. While the 14C chronology of beach sedimentation coincides with known climatic periods dominated by a positive NAO index, these results point to the importance of high-energy events and the synergies between past and present processes in the recent evolution and the morphodynamics of rock coast environments.

Effective atomic orbitals: A tool for understanding electronic structure of molecules

It is discussed that one can obtain effective atomic orbitals (AOs) in quite different theoretical frameworks of Hilbert‐space and three‐dimensional (3D) analyses. In all cases, one can clearly distinguish between the orbitals of an effective minimal basis set and orbitals which are only insignificantly occupied. This observation makes a solid theoretical basis beyond our qualitative picture of molecular electronic structure, described in terms of minimal basis AOs having decisive participation in bonding, and may be considered as a quantum chemical manifestation of the octet rule. For strongly positive atoms like the hypervalent sulfur, some weakly occupied orbitals reflecting “back donation” can also be identified. From the conceptual point of view, it is very important that AOs of characteristic shape can be obtained even by processing the results of plane wave calculations in which no atom‐centered basis orbitals were applied. The different types of analyses (Hilbert‐space and 3D) can be done on equal footing, performing quite analogous procedures, and they exhibit an unexpected interrelation, too. © 2014 Wiley Periodicals, Inc.

Infrared measurements and simulations of metal meshes in a focused beam

Infrared transmittance measurements of quasioptical filters are often restricted to a focused beam due to the optical design of the spectrometer. In contrast, numerical simulations assume an incident plane wave, which makes it difficult to compare theory with experimental data. We compare transmittance measurements with numerical simulations of square arrays of circular holes in 3-μm thick Cu sheets at angles of incidence from 0° to 20° for both s and p polarizations. These simple structures allow detailed tests of our electromagnetic simulation methods and show excellent agreement between theory and measurement. Measurements in a focused beam are accurately simulated by combining plane wave calculations over a range of angles that correspond to the focal ratio of the incident beam. Similar screens have been used as components of narrow bandpass filters for far-infrared astronomy, but these results show that the transmittance variations with angle of incidence and polarization limit their use to collimated beams at near normal incidence. The simulations are accurate enough to eliminate a costly trial-and-error approach to the design of more complex and useful quasioptical infrared filters and to predict their in-band performance and out-of-band blocking in focused beams.

A model for vibrational properties of the atomic interface in A/B fcc metals

The motivation of this theoretical work is to introduce a model calculation for the elastic waves scattering and coherent phonon transport at an atomic nanojunction between face-centered cubic (fcc) leads. The model system A/B consists of two perfect semi-infinite fcc leads A and B, oriented in the same direction and joined by an atomic interface. It is applied to the system Cu/Ni and its inverse Ni/Cu. A theoretical approach based on the matching method is used to study the dynamics of the system A/B.The interface observables are numerically calculated, in the harmonic approximation, to investigate how the interface properties can respond to changes in the elastic force constants of the sub-lattice from where the elastic waves are incident. We have calculated the phonon scattering and the total coherent transmission via the fcc interface as elements of a Landauer–Büttiker-type scattering matrix. The calculated properties are presented for all accessible frequencies in the propagating interval and for the elastic force constants of the leads A and B. The system dynamics, the scattering, and transmission spectra via the atomic interface are analyzed as a function of the incident frequency per propagating mode of the perfect fcc waveguides. Our results show that the atomic interface is an effective phonon splitter and suggest that its characteristics may be controlled by varying its nanometric parameters. In addition, the localized phonon modes associated with the interface domain interact with propagating modes of the perfect fcc systems, located on both sides, leading to interference effects.

Formation of NV centers in diamond: A theoretical study based on calculated transitions and migration of nitrogen and vacancy related defects

Formation and excitation energies as well charge transition levels are determined for the substitutional nitrogen (N${}_{\mathrm{s}}$), the vacancy (V), and related point defects (NV, NVH, N${}_{2}$, N${}_{2}$V, and V${}_{2}$) by screened nonlocal hybrid density functional supercell plane wave calculations in bulk diamond. In addition, the activation energy for V and NV diffusion is calculated. We find good agreement between theory and experiment for the previously well-established data and predict missing ones. Based on the calculated properties of these defects, the formation of the negatively charged NV center is studied, because it is a prominent candidate for application in quantum information processing and for nanosensors. Our results indicate that NV defects are predominantly created directly by irradiation, while simultaneously produced vacancies will form V${}_{2}$ pairs during postirradiation annealing. Divacancies may pin the Fermi level, making the NV defects neutral.

Positron interactions with water-total elastic, total inelastic, and elastic differential cross section measurements.

Utilising a high-resolution, trap-based positron beam, we have measured both elastic and inelastic scattering of positrons from water vapour. The measurements comprise differential elastic, total elastic, and total inelastic (not including positronium formation) absolute cross sections. The energy range investigated is from 1 eV to 60 eV. Comparison with theory is made with both R-Matrix and distorted wave calculations, and with our own application of the Independent Atom Model for positron interactions.