1,386 research outputs found
Orbitally-driven Peierls state in spinels
We consider the superstructures, which can be formed in spinels containing on
B-sites the transition-metal ions with partially filled t2g levels. We show
that, when such systems are close to itinerant state (e.g. have an
insulator-metal transition), there may appear in them an orbitally-driven
Peierls state. We explain by this mechanism the very unusual superstructures
observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest
that similar phenomenon should be observed in NaTiO2 and possibly in some other
systems.Comment: 4 pages, 3 figure
Spitzer Constraints on the Stellar Populations of Lyman-Alpha Emitting Galaxies at z = 3.1
We investigate the stellar populations of a sample of 162 Lyman-alpha
emitting galaxies (LAEs) at z = 3.1 in the Extended Chandra Deep Field South,
using deep Spitzer IRAC data available from the GOODS and SIMPLE surveys to
derive reliable stellar population estimates. We divide the LAEs according to
their rest-frame near-IR luminosities into IRAC-detected and IRAC-undetected
samples. About 70% of the LAEs are undetected in 3.6 micron down to [3.6] =
25.2 AB. Stacking analysis reveals that the average stellar population of the
IRAC-undetected sample has an age of ~ 200 Myr and a mass of ~ 3x10^8 solar
masses, consistent with the expectation that LAEs are mostly young and low-mass
galaxies. On the other hand, the IRAC-detected LAEs are on average
significantly older and more massive, with an average age > 1 Gyr and mass ~
10^10 solar masses. Comparing the IRAC colors and magnitudes of the LAEs to z ~
3 Lyman break galaxies (LBGs) shows that the IRAC-detected LAEs lie at the
faint blue end of the LBG color-magnitude distribution, suggesting that
IRAC-detected LAEs may be the low mass extension of the LBG population. We also
present tentative evidence for a small fraction (~ 5%) of obscured AGN within
the LAE sample. Our results suggest that LAEs posses a wide range of ages and
masses. Additionally, the presence of evolved stellar populations inside LAEs
suggests that the Lyman-alpha luminous phase of galaxies may either be a
long-lasting or recurring phenomenon.Comment: Accepted for publication in ApJ; 5 pages, 4 figure
Fast semi-automatic segmentation of focal liver lesions in contrast-enhanced ultrasound, based on a probabilistic model
Periodically kicked turbulence
Periodically kicked turbulence is theoretically analyzed within a mean field
theory. For large enough kicking strength A and kicking frequency f the
Reynolds number grows exponentially and then runs into some saturation. The
saturation level can be calculated analytically; different regimes can be
observed. For large enough Re we find the saturation level to be proportional
to A*f, but intermittency can modify this scaling law. We suggest an
experimental realization of periodically kicked turbulence to study the
different regimes we theoretically predict and thus to better understand the
effect of forcing on fully developed turbulence.Comment: 4 pages, 3 figures. Phys. Rev. E., in pres
A parametric integer programming algorithm for bilevel mixed integer programs
We consider discrete bilevel optimization problems where the follower solves
an integer program with a fixed number of variables. Using recent results in
parametric integer programming, we present polynomial time algorithms for pure
and mixed integer bilevel problems. For the mixed integer case where the
leader's variables are continuous, our algorithm also detects whether the
infimum cost fails to be attained, a difficulty that has been identified but
not directly addressed in the literature. In this case it yields a ``better
than fully polynomial time'' approximation scheme with running time polynomial
in the logarithm of the relative precision. For the pure integer case where the
leader's variables are integer, and hence optimal solutions are guaranteed to
exist, we present two algorithms which run in polynomial time when the total
number of variables is fixed.Comment: 11 page
Shape resonance for the anisotropic superconducting gaps near a Lifshitz transition: the effect of electron hopping between layers
The multigap superconductivity modulated by quantum confinement effects in a
superlattice of quantum wells is presented. Our theoretical BCS approach
captures the low-energy physics of a shape resonance in the superconducting
gaps when the chemical potential is tuned near a Lifshitz transition. We focus
on the case of weak Cooper-pairing coupling channels and strong pair exchange
interaction driven by repulsive Coulomb interaction that allows to use the BCS
theory in the weak-coupling regime neglecting retardation effects like in
quantum condensates of ultracold gases. The calculated matrix element effects
in the pairing interaction are shown to yield a complex physics near the
particular quantum critical points due to Lifshitz transitions in multigap
superconductivity. Strong deviations of the ratio from the
standard BCS value as a function of the position of the chemical potential
relative to the Lifshitz transition point measured by the Lifshitz parameter
are found. The response of the condensate phase to the tuning of the Lifshitz
parameter is compared with the response of ultracold gases in the BCS-BEC
crossover tuned by an external magnetic field. The results provide the
description of the condensates in this regime where matrix element effects play
a key role.Comment: 12 pages, 6 figure
Evidence for Ubiquitous, High-EW Nebular Emission in z~7 Galaxies: Towards a Clean Measurement of the Specific Star Formation Rate using a Sample of Bright, Magnified Galaxies
Growing observational evidence now indicates that nebular line emission has a
significant impact on the rest-frame optical fluxes of z~5-7 galaxies observed
with Spitzer. This line emission makes z~5-7 galaxies appear more massive, with
lower specific star formation rates. However, corrections for this line
emission have been very difficult to perform reliably due to huge uncertainties
on the overall strength of such emission at z>~5.5. Here, we present the most
direct observational evidence yet for ubiquitous high-EW [OIII]+Hbeta line
emission in Lyman-break galaxies at z~7, while also presenting a strategy for
an improved measurement of the sSFR at z~7. We accomplish this through the
selection of bright galaxies in the narrow redshift window z~6.6-7.0 where the
IRAC 4.5 micron flux provides a clean measurement of the stellar continuum
light. Observed 4.5 micron fluxes in this window contrast with the 3.6 micron
fluxes which are contaminated by the prominent [OIII]+Hbeta lines. To ensure a
high S/N for our IRAC flux measurements, we consider only the brightest
(H_{160}<26 mag) magnified galaxies we have identified in CLASH and other
programs targeting galaxy clusters. Remarkably, the mean rest-frame optical
color for our bright seven-source sample is very blue, [3.6]-[4.5]=-0.9+/-0.3.
Such blue colors cannot be explained by the stellar continuum light and require
that the rest-frame EW of [OIII]+Hbeta be greater than 637 Angstroms for the
average source. The bluest four sources from our seven-source sample require an
even more extreme EW of 1582 Angstroms. Our derived lower limit for the mean
[OIII]+Hbeta EW could underestimate the true EW by ~2x based on a simple
modeling of the redshift distribution of our sources. We can also set a robust
lower limit of >~4 Gyr^-1 on the specific star formation rates based on the
mean SED for our seven-source sample. (abridged)Comment: 9 pages, 6 figures, 1 table, submitted to the Astrophysical Journa
A Census of Star-Forming Galaxies in the z~9-10 Universe based on HST+Spitzer Observations Over 19 CLASH clusters: Three Candidate z~9-10 Galaxies and Improved Constraints on the Star Formation Rate Density at z~9
We utilise a two-color Lyman-Break selection criterion to search for z~9-10
galaxies over the first 19 clusters in the CLASH program. A systematic search
yields three z~9-10 candidates. While we have already reported the most robust
of these candidates, MACS1149-JD, two additional z~9 candidates are also found
and have H_{160}-band magnitudes of ~26.2-26.9. A careful assessment of various
sources of contamination suggests <~1 contaminants for our z~9-10 selection. To
determine the implications of these search results for the LF and SFR density
at z~9, we introduce a new differential approach to deriving these quantities
in lensing fields. Our procedure is to derive the evolution by comparing the
number of z~9-10 galaxy candidates found in CLASH with the number of galaxies
in a slightly lower redshift sample (after correcting for the differences in
selection volumes), here taken to be z~8. This procedure takes advantage of the
fact that the relative volumes available for the z~8 and z~9-10 selections
behind lensing clusters are not greatly dependent on the details of the lensing
models. We find that the normalization of the UV LF at z~9 is just
0.28_{-0.20}^{+0.39}\times that at z~8, ~1.4_{-0.8}^{+3.0}x lower than
extrapolating z~4-8 LF results. While consistent with the evolution in the UV
LF seen at z~4-8, these results marginally favor a more rapid evolution at z>8.
Compared to similar evolutionary findings from the HUDF, our result is less
insensitive to large-scale structure uncertainties, given our many independent
sightlines on the high-redshift universe.Comment: 22 pages, 11 figures, 5 tables, accepted for publication in the
Astrophysical Journal, updated to include the much deeper Spitzer/IRAC
observations over our three z~9-10 candidate
- …