Helium-Iron Compounds at Terapascal Pressures.

We investigate the binary phase diagram of helium and iron using first-principles calculations. We find that helium, which is a noble gas and inert at ambient conditions, forms stable crystalline compounds with iron at terapascal pressures. A FeHe compound becomes stable above 4 TPa, and a FeHe_{2} compound above 12 TPa. Melting is investigated using molecular dynamics simulations, and a superionic phase with sublattice melting of the helium atoms is predicted. We discuss the implications of our predicted helium-iron phase diagram for interiors of giant (exo)planets and white dwarf stars

Breaking the H2 chemical bond in a crystalline environment

Through density functional theory and molecular dynamics calculations, we have analysed various metal polyhydrides to understand whether hydrogen is present in its molecular or atomic form - tetrahydrides of Ba,Sr,Ra, Cs and La; Ba$_8$H$_{46}$ and BaH$_{12}$. We show that, in experimentally reported binary barium hydrides (BaH$_x$), molecular H$_2$ and atomic H$^-$ can coexist with the metallic cations. In this thorough study of differences between BaH$_4$, higher barium hydrides, and other binary tetrahydrides we find the number of atomic hydrogens is equal to the formal charge of the cations. The remaining hydrogen forms molecules in proportions yielding, e.g. BaH$_2$(H$_2)_x$, at pressures as high as 200 GPa. At room temperature these are highly dynamic structures with the hydrogens switching between H$^-$ and H$_2$ while retaining the 2:x ratio. We find some qualitative differences between our static DFT calculations and previously reported structural and spectroscopic experimental results. Two factors allow us to resolve such discrepancies: Firstly, in static relaxation H$_2$ must be regarded as a non-spherical object, which breaks symmetry in a way invisible to X-rays; Secondly the required number of molecules $x$ may be incompatible with the experimental space group (e.g. $BaH_2(H_2)_5$). In molecular dynamics, bond-breaking transitions between various structural symmetry configurations happen on a picosecond timescale via an H$_3^-$ intermediate. Rebonding is slow enough to allow a spectroscopic signal but frequent enough to average out over the lengthscale involved in diffraction

Analysis of wear mechanism in TPU-steel contact pair by means of long stroke tribometer tests

A complete wear characterisation of a thermoplastic polyurethane (TPU) sliding against steel is presented. Wear tests were performed using a long stroke tribometer under sliding reciprocating movement and dry conditions. The dependence of some of the most significant parameters was studied. Before and after wear tests, complementary observations, analyses and measurements were also performed on both contacting materials. These various characterisations involved weight loss of TPU, physico-chemical and thermo- mechanical analyses of TPU, included topographical measurements and morphological observations of worn surfaces of contacting pairs and calculations of the temperature rise generated by friction

Dirac cones in two-dimensional borane

We introduce two-dimensional borane, a single-layered material of BH stoichiometry, with promising electronic properties. We show that, according to Density Functional Theory calculations, two-dimensional borane is semimetallic, with two symmetry-related Dirac cones meeting right at the Fermi energy $E_f$. The curvature of the cones is lower than in graphene, thus closer to the ideal linear dispersion. Its structure, formed by a puckered trigonal boron network with hydrogen atoms connected to each boron atom, can be understood as distorted, hydrogenated borophene (Science \textbf{350}, 1513 (2015)). Chemical bonding analysis reveals the boron layer in the network being bound by delocalized four-center two-electron ${\sigma}$ bonds. Finally, we suggest high-pressure could be a feasible route to synthesise two-dimensional borane.Comment: 5 pages, 3 figures, 1 tabl

High-temperature phase transitions in dense germanium.

Through a series of high-pressure x-ray diffraction experiments combined with in situ laser heating, we explore the pressure-temperature phase diagram of germanium (Ge) at pressures up to 110 GPa and temperatures exceeding 3000 K. In the pressure range of 64-90 GPa, we observe orthorhombic Ge-IV transforming above 1500 K to a previously unobserved high-temperature phase, which we denote as Ge-VIII. This high-temperature phase is characterized by a tetragonal crystal structure, space group I4/mmm. Density functional theory simulations confirm that Ge-IV becomes unstable at high temperatures and that Ge-VIII is highly competitive and dynamically stable at these conditions. The existence of Ge-VIII has profound implications for the pressure-temperature phase diagram, with melting conditions increasing to much higher temperatures than previous extrapolations would imply

High-temperature phase transitions in dense germanium.

Through a series of high-pressure x-ray diffraction experiments combined with in situ laser heating, we explore the pressure-temperature phase diagram of germanium (Ge) at pressures up to 110 GPa and temperatures exceeding 3000 K. In the pressure range of 64-90 GPa, we observe orthorhombic Ge-IV transforming above 1500 K to a previously unobserved high-temperature phase, which we denote as Ge-VIII. This high-temperature phase is characterized by a tetragonal crystal structure, space group I4/mmm. Density functional theory simulations confirm that Ge-IV becomes unstable at high temperatures and that Ge-VIII is highly competitive and dynamically stable at these conditions. The existence of Ge-VIII has profound implications for the pressure-temperature phase diagram, with melting conditions increasing to much higher temperatures than previous extrapolations would imply

Coronagraphs adapted to atmosphere conditions

In this paper we show new ways to improve the performance of ground-based coronagraphy. We introduce adaptive coronagraphic masks whose profile is a binary version of the instantaneous atmospherically degraded star image. We also propose the hyper-Gaussian profile masks obtained by averaging adaptive masks. In addition, adaptive Lyot stops and hyper-Gaussian Lyot stops are analyzed. Computer simulations show that all these masks outperform the circular hard-edged mask and that a proper mask and stop combination significantly reduces the angular separation at which a faint companion can be detected