219 research outputs found
Multi Shell Model for Majumdar-Papapetrau Spacetimes
Exact solutions to static and non-static Einstein-Maxwell equations in the
presence of extremely charged dust embedded on thin shells are constructed.
Singularities of multi-black hole Majumdar-Papapetrou and Kastor-Trashen
solutions are removed by placing the matter on thin shells. Double spherical
thin shell solution is given as an illustration and the matter densitiies on
the shells are derived.Comment: To appear in Physical Review
First principles study of electronic and structural properties of CuO
We investigate the electronic and structural properties of CuO, which shows
significant deviations from the trends obeyed by other transition-metal
monoxides. Using an extended Hubbard corrective functional, we uncover an
orbitally ordered insulating ground state for the cubic phase of this material,
which was expected but never found before. This insulating state results from a
fine balance between the tendency of Cu to complete its d-shell and Hund's rule
magnetism. Starting from the ground state for the cubic phase, we also study
tetragonal distortions of the unit cell (recently reported in experiments), the
consequent electronic reorganizations and identify the equilibrium structure.
Our calculations reveal an unexpected richness of possible magnetic and orbital
orders, relatively close in energy to the ground state, whose stability depends
on the sign and entity of distortion.Comment: 9 pages, 9 figure
Scalar-Scalar, Scalar-Tensor, and Tensor-Tensor Correlators from Anisotropic Inflation
We compute the phenomenological signatures of a model (Watanabe et al' 09) of
anisotropic inflation driven by a scalar and a vector field. The action for the
vector is U(1) invariant, and the model is free of ghost instabilities. A
suitable coupling of the scalar to the kinetic term of the vector allows for a
slow roll evolution of the vector vev, and hence for a prolonged anisotropic
expansion; this provides a counter example to the cosmic no hair conjecture. We
compute the nonvanishing two point correlation functions between physical modes
of the system, and express them in terms of power spectra with angular
dependence. The anisotropy parameter g_* for the scalar-scalar spectrum
(defined as in the Ackerman et al '07 parametrization) turns out to be negative
in the simplest realization of the model, which, therefore, cannot account for
the angular dependence emerged in some analyses of the WMAP data. A g_* of
order -0.1 is achieved when the energy of the vector is about 6-7 orders of
magnitude smaller than that of the scalar during inflation. For such values of
the parameters, the scalar-tensor correlation (which is in principle a
distinctive signature of anisotropic spaces) is smaller than the tensor-tensor
correlation
Searching for high magnetization density in bulk Fe: the new metastable Fe phase
We report the discovery of a new allotrope of iron by first principles
calculations. This phase has symmetry, a six-atom unit cell (hence the
name Fe), and the highest magnetization density (M) among all known
crystalline phases of iron. Obtained from the structural optimizations of the
FeC-cementite crystal upon carbon removal, Fe is shown to
result from the stabilization of a ferromagnetic FCC phase, further strained
along the Bain path. Although metastable from 0 to 50 GPa, the new phase is
more stable, at low pressures, than the other well-known HCP and FCC allotropes
and smoothly transforms into the FCC phase under compression. If stabilized to
room temperature, e.g., by interstitial impurities, Fe could become the
basis material for high M rare-earth-free permanent magnets and high-impact
applications such as, light-weight electric engine rotors or high-density
recording media. The new phase could also be key to explain the enigmatic high
M of FeN, which is currently attracting an intense research
activity.Comment: 7 pages, 7 figure
Role of electronic localization in the phosphorescence of iridium sensitizing dyes
In this work we present a systematic study of three representative iridium
dyes, namely, Ir(ppy)3, FIrpic and PQIr, which are commonly used as sensitizers
in organic optoelectronic devices. We show that electronic correlations play a
crucial role in determining the excited-state energies in these systems, due to
localization of electrons on Ir d orbitals. Electronic localization is captured
by employing hybrid functionals within time-dependent density-functional theory
(TDDFT) and with Hubbard-model corrections within the delta-SCF approach. The
performance of both methods are studied comparatively and shown to be in good
agreement with experiment. The Hubbard-corrected functionals provide further
insight into the localization of electrons and on the charge-transfer character
of excited-states. The gained insight allows us to comment on envisioned
functionalization strategies to improve the performance of these systems.
Complementary discussions on the delta-SCF method are also presented in order
to fill some of the gaps in the literature.Comment: 15 pages, 14 figure
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Structural and electronic properties of SrZrO3 and Sr(Ti,Zr) O3 alloys
Using hybrid density functional calculations, we study the electronic and structural properties of SrZrO3 and ordered Sr(Ti,Zr)O3 alloys. Calculations were performed for the ground-state orthorhombic (Pnma) and high-temperature cubic (Pm3m) phases of SrZrO3. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered SrTixZr1-xO3 structures with x=0, 0.25, 0.5, 0.75, and 1. As Ti is added to SrZrO3, the lattice parameter is reduced and closely follows Vegard's law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For x=0.5, we find that arranging the Ti and Zr atoms into a 1Ă—1SrZrO3/SrTiO3 superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure SrTiO3. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti 3dxy orbitals. The lifting of the t2g orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period SrZrO3/SrTiO3 superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk SrTiO3
Non-gaussianity from the trispectrum and vector field perturbations
We use the \delta N formalism to study the trispectrum T_\zeta of the
primordial curvature perturbation \zeta when the latter is generated by vector
field perturbations, considering the tree-level and one-loop contributions. The
order of magnitude of the level of non-gaussianity in the trispectrum,
\tau_{NL}, is calculated in this scenario and related to the order of magnitude
of the level of non-gaussianity in the bispectrum, f_{NL}, and the level of
statistical anisotropy in the power spectrum, g_\zeta. Such consistency
relations will put under test this scenario against future observations.
Comparison with the expected observational bound on \tau_{NL} from WMAP, for
generic inflationary models, is done.Comment: LaTeX file, 12 pages, no figures. v2: Minor changes, conclusions
unchanged. v3: Version accepted for publication in Physics Letters
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