Structural Properties and Relative Stability of (Meta)Stable Ordered, Partially-ordered and Disordered Al-Li Alloy Phases

We resolve issues that have plagued reliable prediction of relative phase stability for solid-solutions and compounds. Due to its commercially important phase diagram, we showcase Al-Li system because historically density-functional theory (DFT) results show large scatter and limited success in predicting the structural properties and stability of solid-solutions relative to ordered compounds. Using recent advances in an optimal basis-set representation of the topology of electronic charge density (and, hence, atomic size), we present DFT results that agree reasonably well with all known experimental data for the structural properties and formation energies of ordered, off-stoichiometric partially-ordered and disordered alloys, opening the way for reliable study in complex alloys.Comment: 7 pages, 2 figures, 2 Table

The Structure of Barium in the hcp Phase Under High Pressure

Recent experimental results on two hcp phases of barium under high pressure show interesting variation of the lattice parameters. They are here interpreted in terms of electronic structure calculation by using the LMTO method and generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II the dramatic drop in c/a is an instability analogous to that in the group II metals but with the transfer of s to d electrons playing a crucial role in Ba. Meanwhile in phase V, the instability decrease a lot due to the core repulsion at very high pressure. PACS numbers: 62.50+p, 61.66Bi, 71.15.Ap, 71.15Hx, 71.15LaComment: 29 pages, 8 figure

Sophisticated Inference.

Active inference offers a first principle account of sentient behavior, from which special and important cases-for example, reinforcement learning, active learning, Bayes optimal inference, Bayes optimal design-can be derived. Active inference finesses the exploitation-exploration dilemma in relation to prior preferences by placing information gain on the same footing as reward or value. In brief, active inference replaces value functions with functionals of (Bayesian) beliefs, in the form of an expected (variational) free energy. In this letter, we consider a sophisticated kind of active inference using a recursive form of expected free energy. Sophistication describes the degree to which an agent has beliefs about beliefs. We consider agents with beliefs about the counterfactual consequences of action for states of affairs and beliefs about those latent states. In other words, we move from simply considering beliefs about "what would happen if I did that" to "what I would believe about what would happen if I did that." The recursive form of the free energy functional effectively implements a deep tree search over actions and outcomes in the future. Crucially, this search is over sequences of belief states as opposed to states per se. We illustrate the competence of this scheme using numerical simulations of deep decision problems

Spin configuration in a frustrated ferromagnetic/antiferromagnetic thin film system

We have studied the magnetic configuration in ultrathin antiferromagnetic Mn films grown around monoatomic steps on an Fe(001) surface by spin-polarized scanning tunneling microscopy/spectroscopy and ab-initio-parametrized self-consistent real-space tight binding calculations in which the spin quantization axis is independent for each site thus allowing noncollinear magnetism. Mn grown on Fe(001) presents a layered antiferromagnetic structure. In the regions where the Mn films overgrows Fe steps the magnetization of the surface layer is reversed across the steps. Around these defects a frustration of the antiferromagnetic order occurs. Due to the weakened magnetic coupling at the central Mn layers, the amount of frustration is smaller than in Cr and the width of the wall induced by the step does not change with the thickness, at least for coverages up to seven monolayers.Comment: 10 pages, 5 figure

Energetics of Hydrogen Chemisorbed on Cu(110): A First Principlies Calculations Study

In the current study we present a potential energy surface (PES)for atomic hydrogen chemisorbed on Cu(110)at Θ=1/8 monolayer ~ML! obtained from a plane-wave, gradient-corrected, density functional calculation. This PES is markedly different from and significantly more complex than that predicted by empirical embedded atom method (EAM) calculations. Our results, for example, suggest strongly that the hollow (HL)site is not the preferred binding site for this system. In our calculations, both the short bridge (SB)and pseudo-threefold sites are energetically more favorable than the hollow (HL)site. Energetically, we find the SB site to be slightly lower (30 meV)than the pseudo-threefold site. We also find, however, that the calculated vibrational frequencies for the pseudo-threefold site agree more closely with experimental electron energy loss data than for the SB site. In view of the relatively flat region between adjacent pseudo-threefold sites along the cross-channel [001]direction, we speculate that the hydrogen atom motion at low coverages may be two-dimensional rather than quasi-one-dimensional in character

Optical interconnect solution with plasmonic modulator and Ge photodetector array

We report on an optical chip-to-chip interconnect solution, thereby demonstrating plasmonics as a solution for ultra-dense, high-speed short-reach communications. The interconnect comprises a densely integrated plasmonic Mach-Zehnder modulator array that is packaged with standard driving electronics. On the receiver side, a germanium photodetector array is integrated with trans-impedance amplifiers. A multicore fiber provides a compact optical interface to the array. We demonstrate 4 × 20 Gb/s on-off keying signaling with direct detection.ISSN:1041-1135ISSN:1941-017

Continuum elastic sphere vibrations as a model for low-lying optical modes in icosahedral quasicrystals

The nearly dispersionless, so-called "optical" vibrational modes observed by inelastic neutron scattering from icosahedral Al-Pd-Mn and Zn-Mg-Y quasicrystals are found to correspond well to modes of a continuum elastic sphere that has the same diameter as the corresponding icosahedral basic units of the quasicrystal. When the sphere is considered as free, most of the experimentally found modes can be accounted for, in both systems. Taking into account the mechanical connection between the clusters and the remainder of the quasicrystal allows a complete assignment of all optical modes in the case of Al-Pd-Mn. This approach provides support to the relevance of clusters in the vibrational properties of quasicrystals.Comment: 9 pages without figure

First principles modelling of magnesium titanium hydrides

Mixing Mg with Ti leads to a hydride Mg(x)Ti(1-x)H2 with markedly improved (de)hydrogenation properties for x < 0.8, as compared to MgH2. Optically, thin films of Mg(x)Ti(1-x)H2 have a black appearance, which is remarkable for a hydride material. In this paper we study the structure and stability of Mg(x)Ti(1-x)H2, x= 0-1 by first-principles calculations at the level of density functional theory. We give evidence for a fluorite to rutile phase transition at a critical composition x(c)= 0.8-0.9, which correlates with the experimentally observed sharp decrease in (de)hydrogenation rates at this composition. The densities of states of Mg(x)Ti(1-x)H2 have a peak at the Fermi level, composed of Ti d states. Disorder in the positions of the Ti atoms easily destroys the metallic plasma, however, which suppresses the optical reflection. Interband transitions result in a featureless optical absorption over a large energy range, causing the black appearance of Mg(x)Ti(1-x)H2.Comment: 22 pages, 9 figures, 4 table

Spatially continuous snow depth mapping by aeroplane photogrammetry for annual peak of winter from 2017 to 2021 in open areas

Information on snow depth and its spatial distribution is important for numerous applications, including natural hazard management, snow water equivalent estimation for hydropower, the study of the distribution and evolution of flora and fauna, and the validation of snow hydrological models. Due to its heterogeneity and complexity, specific remote sensing tools are required to accurately map the snow depth distribution in Alpine terrain. To cover large areas (&gt;100 km2), airborne laser scanning (ALS) or aerial photogrammetry with large-format cameras is needed. While both systems require piloted aircraft for data acquisition, ALS is typically more expensive than photogrammetry but yields better results in forested terrain. While photogrammetry is slightly cheaper, it is limited due to its dependency on favourable acquisition conditions (weather, light conditions). In this study, we present photogrammetrically processed high-spatial-resolution (0.5 m) annual snow depth maps, recorded during the peak of winter over a 5-year period under different acquisition conditions over a study area around Davos, Switzerland. Compared to previously carried out studies, using the Vexcel UltraCam Eagle Mark 3 (M3) sensor improves the average ground sampling distance to 0.1 m at similar flight altitudes above ground. This allows for very detailed snow depth maps in open areas, calculated by subtracting a snow-off digital terrain model (DTM, acquired with ALS) from the snow-on digital surface models (DSMs) processed from the airborne imagery. Despite challenging acquisition conditions during the recording of the UltraCam images (clouds, shaded areas and fresh snow), 99 % of unforested areas were successfully photogrammetrically reconstructed. We applied masks (high vegetation, settlements, water, glaciers) to increase the reliability of the snow depth calculations. An extensive accuracy assessment was carried out using check points, the comparison to DSMs derived from unpiloted aerial systems and the comparison of snow-free DSM pixels to the ALS DTM. The results show a root mean square error of approximately 0.25 m for the UltraCam X and 0.15 m for the successor, the UltraCam Eagle M3. We developed a consistent and reliable photogrammetric workflow for accurate snow depth distribution mapping over large regions, capable of analysing snow distribution in complex terrain. This enables more detailed investigations on seasonal snow dynamics and can be used for numerous applications related to snow depth distribution, as well as serving as a ground reference for new modelling approaches and satellite-based snow depth mapping.</p

Clustering and the hyperbolic geometry of complex networks

Clustering is a fundamental property of complex networks and it is the mathematical expression of a ubiquitous phenomenon that arises in various types of self-organized networks such as biological networks, computer networks or social networks. In this paper, we consider what is called the global clustering coefficient of random graphs on the hyperbolic plane. This model of random graphs was proposed recently by Krioukov et al. as a mathematical model of complex networks, under the fundamental assumption that hyperbolic geometry underlies the structure of these networks. We give a rigorous analysis of clustering and characterize the global clustering coefficient in terms of the parameters of the model. We show how the global clustering coefficient can be tuned by these parameters and we give an explicit formula for this function.Comment: 51 pages, 1 figur