16,951 research outputs found
Exceptionally large room-temperature ferroelectric polarization in the novel PbNiO3 multiferroic oxide
We present a study based on several advanced First-Principles methods, of the
recently synthesized PbNiO3 [J. Am. Chem. Soc 133, 16920 (2011)], a
rhombohedral antiferromagnetic insulator which crystallizes in the highly
distorted R3c crystal structure. We find this compound electrically polarized,
with a very large electric polarization of about 100 (\muC/cm)^2, thus even
exceeding the polarization of well-known BiFeO3. PbNiO3 is a proper
ferroelectric, with polarization driven by large Pb-O polar displacements along
the [111] direction. Contrarily to naive expectations, a definite ionic charge
of 4+ for Pb ion can not be assigned, and in fact the large Pb 6s-O 2p
hybridization drives the ferroelectric distortion through a lone-pair mechanism
similar to that of other Pb- and Bi-based multiferroic
PPAK Wide-field Integral Field Spectroscopy of NGC 628: I. The largest spectroscopic mosaic on a single galaxy
We present a wide-field IFS survey on the nearby face-on Sbc galaxy NGC 628,
comprising 11094 individual spectra, covering a nearly circular field-of-view
of ~6 arcmin in diameter, with a sampling of ~2.7 arcsec per spectrum in the
optical wavelength range (3700--7000 AA). This galaxy is part of the PPAK IFS
Nearby Galaxies Survey, (PINGS, Rosales-Ortega et al. 2009). To our knowledge,
this is the widest spectroscopic survey ever made in a single nearby galaxy. A
detailed flux calibration was applied, granting a spectrophotometric accuracy
of \,0.2 mag.
The age of the stellar populations shows a negative gradient from the inner
(older) to the outer (younger) regions. We found an inversion of this gradient
in the central ~1 kpc region, where a somewhat younger stellar population is
present within a ring at this radius. This structure is associated with a
circumnuclear star-forming region at ~ 500 pc, also found in similar spiral
galaxies. From the study of the integrated and spatially resolved ionized gas
we found a moderate SFR of ~ 2.4 Msun yr. The oxygen abundance shows a a
clear gradient of higher metallicity values from the inner part to the outer
part of the galaxy, with a mean value of 12~+~log(O/H) ~ 8.7. At some specific
regions of the galaxy, the spatially resolved distribution of the physical
properties show some level of structure, suggesting real point-to-point
variations within an individual \hh region. Our results are consistent with an
inside-out growth scheme, with stronger star formation at the outer regions,
and with evolved stellar populations in the inner ones.Comment: 31 pages, 22 Figuras, Accepted for Publishing in MNRAS (corrected
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K-Chameleon and the Coincidence Problem
In this paper we present a hybrid model of k-essence and chameleon, named as
k-chameleon. In this model, due to the chameleon mechanism, the directly strong
coupling between the k-chameleon field and matters (cold dark matters and
baryons) is allowed. In the radiation dominated epoch, the interaction between
the k-chameleon field and background matters can be neglected, the behavior of
the k-chameleon therefore is the same as that of the ordinary k-essence. After
the onset of matter domination, the strong coupling between the k-chameleon and
matters dramatically changes the result of the ordinary k-essence. We find that
during the matter-dominated epoch, only two kinds of attractors may exist: one
is the familiar {\bf K} attractor and the other is a completely {\em new},
dubbed {\bf C} attractor. Once the universe is attracted into the {\bf C}
attractor, the fraction energy densities of the k-chameleon and
dust matter are fixed and comparable, and the universe will undergo
a power-law accelerated expansion. One can adjust the model so that the {\bf K}
attractor do not appear. Thus, the k-chameleon model provides a natural
solution to the cosmological coincidence problem.Comment: Revtex, 17 pages; v2: 18 pages, two figures, more comments and
references added, to appear in PRD, v3: published versio
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