482 research outputs found
Position reporting system using small satellites
A system able to provide position reporting and monitoring services for mobile applications represents a natural complement to the Global Positioning System (GPS) navigation system. The system architecture is defined on the basis of the communications requirements derived by user needs, allowing maximum flexibility in the use of channel capacity, and a very simple and low cost terminal. The payload is sketched, outlining the block modularity and the use of qualified hardware. The global system capacity is also derived. The spacecraft characteristics are defined on the basis of the payload requirements. A small bus optimized for Ariane IV, Delta II vehicles and based on the modularity concept is presented. The design takes full advantage of each launcher with a common basic bus or bus elements for a mass production
Normal Mode Determination of Perovskite Crystal Structures with Octahedral Rotations: Theory and Applications
Nuclear site analysis methods are used to enumerate the normal modes of
perovskite polymorphs with octahedral rotations. We provide the modes
of the fourteen subgroups of the cubic aristotype describing the Glazer
octahedral tilt patterns, which are obtained from rotations of the
octahedra with different sense and amplitude about high symmetry axes. We
tabulate all normal modes of each tilt system and specify the contribution of
each atomic species to the mode displacement pattern, elucidating the physical
meaning of the symmetry unique modes. We have systematically generated 705
schematic atomic displacement patterns for the normal modes of all 15 (14
rotated + 1 unrotated) Glazer tilt systems. We show through some illustrative
examples how to use these tables to identify the octahedral rotations,
symmetric breathing, and first-order Jahn-Teller anti-symmetric breathing
distortions of the octahedra, and the associated Raman selection
rules. We anticipate that these tables and schematics will be useful in
understanding the lattice dynamics of bulk perovskites and would serve as
reference point in elucidating the atomic origin of a wide range of physical
properties in synthetic perovskite thin films and superlattices.Comment: 17 pages, 3 figures, 17 tables. Supporting information accessed
through link specified within manuscrip
Trigonal Symmetry Breaking and its Electronic Effects in Two-Dimensional Dihalides and Trihalides
We study the consequences of the approximately trigonal () point
symmetry of the transition metal (M) site in two-dimensional van der Waals
MX dihalides and MX trihalides. The trigonal symmetry leads to a 2-2-1
orbital splitting of the transition metal shell, which may be tuned by the
interlayer distance, and changes in the ligand-ligand bond lengths. Orbital
order coupled to various lower symmetry lattice modes may lift the remaining
orbital degeneracies, and we explain how these may support unique electronic
states using ZrI and CuCl as examples, and offer a brief overview of
possible electronic configurations in this class of materials. By building and
analysing Wannier models adapted to the appropriate symmetry we examine how the
interplay among trigonal symmetry, electronic correlation effects, and -
orbital charge transfer leads to insulating, orbitally polarized magnetic
and/or orbital-selective Mott states. Our work establishes a rigorous framework
to understand, control, and tune the electronic states in low-dimensional
correlated halides. Our analysis shows that trigonal symmetry and its breaking
is a key feature of the 2D halides that needs to be accounted for in search of
novel electronic states in materials ranging from CrI to -RuCl
Strain-controlled band engineering and self-doping in ultrathin LaNiO films
We report on a systematic study of the temperature-dependent Hall coefficient
and thermoelectric power in ultra-thin metallic LaNiO films that reveal a
strain-induced, self-doping carrier transition that is inaccessible in the
bulk. As the film strain varies from compressive to tensile at fixed
composition and stoichiometry, the transport coefficients evolve in a manner
strikingly similar to those of bulk hole-doped superconducting cuprates with
varying doping level. Density functional calculations reveal that the
strain-induced changes in the transport properties are due to self-doping in
the low-energy electronic band structure. The results imply that thin-film
epitaxy can serve as a new means to achieve hole-doping in other (negative)
charge-transfer gap transition metal oxides without resorting to chemical
substitution
Database, Features, and Machine Learning Model to Identify Thermally Driven Metal-Insulator Transition Compounds
Metal-insulator transition (MIT) compounds are materials that may exhibit
insulating or metallic behavior, depending on the physical conditions, and are
of immense fundamental interest owing to their potential applications in
emerging microelectronics. There is a dearth of thermally-driven MIT materials,
however, which makes delineating these compounds from those that are
exclusively insulating or metallic challenging. Here we report a material
database comprising temperature-controlled MITs (and metals and insulators with
similar chemical composition and stoichiometries to the MIT compounds) from
high quality experimental literature, built through a combination of
materials-domain knowledge and natural language processing. We featurize the
dataset using compositional, structural, and energetic descriptors, including
two MIT relevant energy scales, an estimated Hubbard interaction and the charge
transfer energy, as well as the structure-bond-stress metric referred to as the
global-instability index (GII). We then perform supervised classification,
constructing three electronic-state classifiers: metal vs non-metal (M),
insulator vs non-insulator (I), and MIT vs non-MIT (T). We identify two
important descriptors that separate metals, insulators, and MIT materials in a
2D feature space: the average deviation of the covalent radius and the range of
the Mendeleev number. We further elaborate on other important features (GII and
Ewald energy), and examine how they affect classification of binary vanadium
and titanium oxides. We discuss the relationship of these atomic features to
the physical interactions underlying MITs in the rare-earth nickelate family.
Last, we implement an online version of the classifiers, enabling quick
probabilistic class predictions by uploading a crystallographic structure file
Electronic properties of bulk and thin film SrRuO: a search for the metal-insulator transition
We calculate the properties of the 4 ferromagnet SrRuO in bulk and
thin film form with the aim of understanding the experimentally observed metal
to insulator transition at reduced thickness. Although the spatial extent of
the 4 orbitals is quite large, many experimental results have suggested that
electron-electron correlations play an important role in determining this
material's electronic structure. In order to investigate the importance of
correlation, we use two approaches which go beyond the conventional local
density approximation to density functional theory (DFT): the local spin
density approximation + Hubbard (LSDA+) and the pseudopotential
self-interaction correction (pseudo-SIC) methods. We find that the details of
the electronic structure predicted with the LSDA do not agree with the
experimental spectroscopic data for bulk and thin film SrRuO. Improvement
is found by including electron-electron correlations, and we suggest that bulk
orthorhombic SrRuO is a {\it weakly strongly-correlated} ferromagnet whose
electronic structure is best described by a 0.6 eV on-site Hubbard term, or
equivalently with corrections for the self-interaction error. We also perform
{\it ab initio} transport calculations that confirm that SrRuO has a
negative spin polarization at the Fermi level, due to the position of the
minority Ru 4 band center. Even with correlations included in our
calculations we are unable to reproduce the experimentally observed
metal-insulator transition, suggesting that the electronic behavior of
SrRuO ultra-thin films might be dominated by extrinsic factors such as
surface disorder and defects.Comment: 15 pages, 12 figures, 3 table
A Surface Reconstruction with a Fractional Hole: LaAlO (001)
The structure of the reconstruction of
LaAlO (001) has been determined using transmission electron diffraction
combined with direct methods. The structure is relatively simple, consisting of
a lanthanum oxide termination with one lanthanum cation vacancy per surface
unit cell. The electronic structure is unusual since a fractional number of
holes or atomic occupancies per surface unit cell are required to achieve
charge neutrality. Density functional calculations indicate that the charge
compensation mechanism occurs by means of highly delocalized holes. The surface
contains no oxygen vacancies and with a better than 99% confidence level, the
holes are not filled with hydrogen. The reconstruction can be understood in
terms of expulsion of the more electropositive cation from the surface followed
by an increased covalency between the remaining surface lanthanum atoms and
adjacent oxygen atoms.Comment: 4 Pages, 3 Figure
Recommended from our members
Computationally Directed Discovery of MoBi\u3csub\u3e2\u3c/sub\u3e
Incorporating bismuth, the heaviest element stable to radioactive decay, into new materials enables the creation of emergent properties such as permanent magnetism, superconductivity, and nontrivial topology. Understanding the factors that drive Bi reactivity is critical for the realization of these properties. Using pressure as a tunable synthetic vector, we can access unexplored regions of phase space to foster reactivity between elements that do not react under ambient conditions. Furthermore, combining computational and experimental methods for materials discovery at high-pressures provides broader insight into the thermodynamic landscape than can be achieved through experiment alone, informing our understanding of the dominant chemical factors governing structure formation. Herein, we report our combined computational and experimental exploration of the Mo–Bi system, for which no binary intermetallic structures were previously known. Using the ab initio random structure searching (AIRSS) approach, we identified multiple synthetic targets between 0–50 GPa. High-pressure in situ powder X-ray diffraction experiments performed in diamond anvil cells confirmed that Mo–Bi mixtures exhibit rich chemistry upon the application of pressure, including experimental realization of the computationally predicted CuAl2-type MoBi2 structure at 35.8(5) GPa. Electronic structure and phonon dispersion calculations on MoBi2 revealed a correlation between valence electron count and bonding in high-pressure transition metal–Bi structures as well as identified two dynamically stable ambient pressure polymorphs. Our study demonstrates the power of the combined computational–experimental approach in capturing high-pressure reactivity for efficient materials discovery
Assessing the socioeconomic and infrastructure development disparity–a case study of city districts of Punjab, Pakistan
- …