11,546 research outputs found
Expected spectral characteristics of (101955) Bennu and (162173) Ryugu, targets of the OSIRIS-REx and Hayabusa2 missions
NASA's OSIRIS-REx and JAXA's Hayabusa2 sample-return missions are currently
on their way to encounter primitive near-Earth asteroids (101955) Bennu and
(162173) Ryugu, respectively. Spectral and dynamical evidence indicates that
these near-Earth asteroids originated in the inner part of the main belt. There
are several primitive collisional families in this region, and both these
asteroids are most likely to have originated in the Polana-Eulalia family
complex. We present the expected spectral characteristics of both targets based
on our studies of our primitive collisional families in the inner belt:
Polana-Eulalia, Erigone, Sulamitis, and Clarissa. Observations were obtained in
the framework of our PRIMitive Asteroids Spectroscopic Survey (PRIMASS). Our
results are especially relevant to the planning and interpretation of in-situ
images and spectra to be obtained by the two spacecraft during the encounters
with their targets.Comment: 22 pages, 11 figures. Accepted for publication in Icarus on May 11,
201
Ostrogradski Formalism for Higher-Derivative Scalar Field Theories
We carry out the extension of the Ostrogradski method to relativistic field
theories. Higher-derivative Lagrangians reduce to second differential-order
with one explicit independent field for each degree of freedom. We consider a
higher-derivative relativistic theory of a scalar field and validate a powerful
order-reducing covariant procedure by a rigorous phase-space analysis. The
physical and ghost fields appear explicitly. Our results strongly support the
formal covariant methods used in higher-derivative gravity.Comment: 22 page
High-Pressure Phase Diagram in the Manganites: a Two-site Model Study
The pressure dependence of the Curie temperature in manganites,
recently studied over a wide pressure range, is not quantitatively accounted
for by the quenching of Jahn-Teller distortions, and suggests the occurrence of
a new pressure-activated localizing processes. We present a theoretical
calculation of based on a two-site double-exchange model with
electron-phonon coupling interaction and direct superexchange between the core spins. We calculate the pressure dependence of and compare
it with the experimental phase diagram. Our results describe the experimental
behavior quite well if a pressure-activated enhancement of the
antiferromagnetic superexchange interaction is assumed
An Infrared Determination of the Reddening and Distance to Dwingeloo 1
We present for the first time infrared observations of the nearby highly
obscured galaxy Dwingeloo 1 (Dw1), including deep H-band imaging covering a
total of 4.9x4.9 arcmin, together with J and Ks imaging of the central 2.5x2.5
arcmin. We used the small dispersion of the intrinsic infrared colors of spiral
galaxies to determine an infrared H-band extinction of A_H = 0.47+/-0.11 mag
towards Dw1. In using infrared colors, the uncertainties in the reddening and
distance are reduced by a factor of three. The H-band magnitude corrected for
extinction and the infrared Tully-Fisher relation are then used to estimate a
distance modulus of (m-M)_0 = 28.62+/-0.27, and thus a distance of d = 5.3
{+0.7/-0.6} Mpc, which puts Dw1 at the far end of the IC342/Maffei 1 & 2 group.
Our result is largely independent of the nature of the reddening law because we
estimated both the reddening and the distance at the same wavelength range.Comment: 20 pages, 2 figures, see
http://nicmos2.as.arizona.edu/~aalonso/Dw1/dw1_paper.htm
The interplay between double exchange, super-exchange, and Lifshitz localization in doped manganites
Considering the disorder caused in manganites by the substitution of Mn by Fe
or Ga, we accomplish a systematic study of doped manganites begun in previous
papers. To this end, a disordered model is formulated and solved using the
Variational Mean Field technique. The subtle interplay between double exchange,
super-exchange, and disorder causes similar effects on the dependence of T_C on
the percentage of Mn substitution in the cases considered. Yet, in
LaCaMnGaO our results suggest a quantum
critical point (QCP) for , associated to the localization of
the electronic states of the conduction band. In the case of
LaCaMnFeO (with ) no such QCP is expected.Comment: 6 pages + 3 postscript figures. Largely extended discussio
Aspectos comportamentais da interação entre Telenomus podisi (Hymenoptera: Scelionidae) e Tibraca limbativentris (Heteroptera: Pentatomidae).
O objetivo desse estudo é conhecer aspectos comportamentais do parasitismo de Telenomus podisi, sem e com competição intraespecífica, em ovos de Tibraca limbativentris.Pôster - graduação
Island phases and charge order in two-dimensional manganites
The ferromagnetic Kondo lattice model with an antiferromagnetic interaction
between localized spins is a minimal description of the competing kinetic t and
magnetic K energy terms which generate the rich physics of manganite systems.
Motivated by the discovery in one dimension of homogeneous ``island phases'',
we consider the possibility of analogous phases in higher dimensions. We
characterize the phases present at commensurate fillings, and consider in
detail the effects of phase separation in all filling and parameter regimes. We
deduce that island and flux phases are stable for intermediate values of K/t at
the commensurate fillings n = 1/4, 1/3, 3/8, and 1/2. We discuss the connection
of these results to the charge and magnetic ordering observed in a wide variety
of manganite compounds.Comment: 13 pages, 17 figure
Gauge Fixing in Higher Derivative Gravity
Linearized four-derivative gravity with a general gauge fixing term is
considered. By a Legendre transform and a suitable diagonalization procedure it
is cast into a second-order equivalent form where the nature of the physical
degrees of freedom, the gauge ghosts, the Weyl ghosts, and the intriguing
"third ghosts", characteristic to higher-derivative theories, is made explicit.
The symmetries of the theory and the structure of the compensating
Faddeev-Popov ghost sector exhibit non-trivial peculiarities.Comment: 21 pages, LaTe
Raman phonons as a probe of disorder, fluctuations and local structure in doped and undoped orthorhombic and rhombohedral manganites
We present a rationalization of the Raman spectra of orthorhombic and
rhombohedral, stoichiometric and doped, manganese perovskites. In particular we
study RMnO3 (R= La, Pr, Nd, Tb, Ho, Er, Y and Ca) and the different phases of
Ca or Sr doped RMnO3 compounds as well as cation deficient RMnO3. The spectra
of manganites can be understood as combinations of two kinds of spectra
corresponding to two structural configurations of MnO6 octahedra and
independently of the average structure obtained by diffraction techniques. The
main peaks of compounds with regular MnO6 octahedra, as CaMnO3, highly Ca doped
LaMnO3 or the metallic phases of Ca or Sr doped LaMnO3, are bending and tilt
MnO6 octahedra modes which correlate to R-O(1) bonds and Mn-O-Mn angles
respectively. In low and optimally doped manganites, the intensity and width of
the broad bands are related to the amplitude of the dynamic fluctuations
produced by polaron hopping in the paramagnetic insulating regime. The
activation energy, which is proportional to the polaron binding energy, is the
measure of this amplitude. This study permits to detect and confirm the
coexistence, in several compounds, of a paramagnetic matrix with lattice
polaron together with regions without dynamic or static octahedron distortions,
identical to the ferromagnetic metallic phase. We show that Raman spectroscopy
is an excellent tool to obtain information on the local structure of the
different micro or macro-phases present simultaneously in many manganites.Comment: Submitted to PR
Parasitismo de ovos de Tibraca limbativentris (Hemiptera: Pentatomidae) por Telenomus podisi (Hymenoptera:Scelionidae).
O objetivo desse estudo é verificar a potencialidade de Tibraca limbativentris como hospedeiro alternativo de T. podisi, de acordo com aspectos comportamentais.Pôster - graduação
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