20 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$_6$ phase

We report the discovery of a new allotrope of iron by first principles
calculations. This phase has $Pmn2_1$ symmetry, a six-atom unit cell (hence the
name Fe$_6$), and the highest magnetization density (M$_s$) among all known
crystalline phases of iron. Obtained from the structural optimizations of the
Fe$_3$C-cementite crystal upon carbon removal, $Pmn2_1$ Fe$_6$ 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$_{6}$ could become the
basis material for high M$_s$ 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$_s$ of Fe$_{16}$N$_2$, 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

### Instability of the ACW model, and problems with massive vectors during inflation

We prove that the anisotropic inflationary background of the
Ackerman-Carroll-Wise model, characterized by a fixed-norm vector field, is
unstable. We found the instability by explicitly solving the linearized
equations for the most general set of perturbations around this background, and
by noticing that the solutions diverge close to horizon crossing. This happens
because one perturbation becomes a ghost at that moment. A simplified
computation, with only the perturbations of the vector field included, shows
the same instability, clarifying the origin of the problem. We then discuss
several other models, with a particular emphasis on the case of a nonminimal
coupling to the curvature, in which vector fields are used either to support an
anisotropic expansion, or to generate cosmological perturbations on an
isotropic background. In many cases, the mass term of the vector needs to have
the ``wrong'' sign; we show that, as a consequence, the longitudinal vector
mode is a ghost (a field with negative kinetic term, and negative energy; not
simply a tachyon). We comment on problems that arise at the quantum level. In
particular, the presence of a ghost can be a serious difficulty for the UV
completion that such models require in the sub-horizon regime.Comment: 22 pages, 4 figure