9,723 research outputs found
Magnetic incommensurability and fluctuating charge density waves in the repulsive Hubbard model
Magnetic and charge susceptibilities of the two-dimensional repulsive Hubbard
model are investigated applying a strong coupling diagram technique in which
the expansion in powers of the hopping constants is used. For small lattices
and high temperatures results are in agreement with Monte Carlo simulations.
With the departure from half-filling the low-frequency magnetic
susceptibility becomes incommensurate and the incommensurability parameter
grows with . The incommensurability, its dependence on frequency and on
resemble experimental results in lanthanum cuprates. Also for finite sharp
maxima appear in the static charge susceptibility. The maxima are finite which
points to the absence of the long-range charge ordering (static stripes).
However, for the maxima are located near the momenta
, . In this case an interaction of carriers with
tetragonal distortions can stabilize stripes with the wavelength of four
lattice spacings, as observed in the low-temperature tetragonal phase of
cuprates. As follows from the obtained results, the magnetic incommensurability
is not a consequence of the stripes.Comment: 4 pages, 3 figures, manuscript for proceefings of LT2
Protected polymorphisms and evolutionary stability of patch-selection strategies in stochastic environments
We consider a population living in a patchy environment that varies
stochastically in space and time. The population is composed of two morphs
(that is, individuals of the same species with different genotypes). In terms
of survival and reproductive success, the associated phenotypes differ only in
their habitat selection strategies. We compute invasion rates corresponding to
the rates at which the abundance of an initially rare morph increases in the
presence of the other morph established at equilibrium. If both morphs have
positive invasion rates when rare, then there is an equilibrium distribution
such that the two morphs coexist; that is, there is a protected polymorphism
for habitat selection. Alternatively, if one morph has a negative invasion rate
when rare, then it is asymptotically displaced by the other morph under all
initial conditions where both morphs are present. We refine the
characterization of an evolutionary stable strategy for habitat selection from
[Schreiber, 2012] in a mathematically rigorous manner. We provide a necessary
and sufficient condition for the existence of an ESS that uses all patches and
determine when using a single patch is an ESS. We also provide an explicit
formula for the ESS when there are two habitat types. We show that adding
environmental stochasticity results in an ESS that, when compared to the ESS
for the corresponding model without stochasticity, spends less time in patches
with larger carrying capacities and possibly makes use of sink patches, thereby
practicing a spatial form of bet hedging.Comment: Revised in light of referees' comments, Published on-line Journal of
Mathematical Biology 2014
http://link.springer.com/article/10.1007/s00285-014-0824-
Near-Infrared spectroscopy of the super star cluster in NGC1705
We study the near-infrared properties of the super star cluster NGC1750-1 in
order to constrain its spatial extent, its stellar population and its age. We
use adaptive optics assisted integral field spectroscopy with SINFONI on the
VLT. We estimate the spatial extent of the cluster and extract its K-band
spectrum from which we constrain the age of the dominant stellar population.
Our observations have an angular resolution of about 0.11", providing an upper
limit on the cluster radius of 2.85+/-0.50 pc depending on the assumed
distance. The K-band spectrum is dominated by strong CO absorption bandheads
typical of red supergiants. Its spectral type is equivalent to a K4-5I star.
Using evolutionary tracks from the Geneva and Utrecht groups, we determine an
age of 12+/-6 Myr. The large uncertainty is rooted in the large difference
between the Geneva and Utrecht tracks in the red supergiants regime. The
absence of ionized gas lines in the K-band spectrum is consistent with the
absence of O and/or Wolf-Rayet stars in the cluster, as expected for the
estimated age.Comment: 5 pages, 4 figures. Research Note accepted in Astronomy and
Astrophysic
A 1.82 m^2 ring laser gyroscope for nano-rotational motion sensing
We present a fully active-controlled He-Ne ring laser gyroscope, operating in
square cavity 1.35 m in side. The apparatus is designed to provide a very low
mechanical and thermal drift of the ring cavity geometry and is conceived to be
operative in two different orientations of the laser plane, in order to detect
rotations around the vertical or the horizontal direction. Since June 2010 the
system is active inside the Virgo interferometer central area with the aim of
performing high sensitivity measurements of environmental rotational noise. So
far, continuous not attempted operation of the gyroscope has been longer than
30 days. The main characteristics of the laser, the active remote-controlled
stabilization systems and the data acquisition techniques are presented. An
off-line data processing, supported by a simple model of the sensor, is shown
to improve the effective long term stability. A rotational sensitivity at the
level of ten nanoradiants per squareroot of Hz below 1 Hz, very close to the
required specification for the improvement of the Virgo suspension control
system, is demonstrated for the configuration where the laser plane is
horizontal
Simulation study of a highly efficient, high resolution X-ry sensor based on self-organizing aluminum oxide
State of the art X-ray imaging sensors comprise a trade-off between the
achievable efficiency and the spatial resolution. To overcome such limitations,
the use of structured and scintillator filled aluminum oxide (AlOx) matrices
has been investigated. We used Monte-Carlo (MC) X-ray simulations to determine
the X-ray imaging quality of these AlOx matrices. Important factors which
influence the behavior of the matrices are: filling factor (surface ratio
between channels and 'closed' AlOx), channel diameter, aspect ratio, filling
material etc. Therefore we modeled the porous AlOx matrix in several different
ways with the MC X-ray simulation tool ROSI [1] and evaluated its properties to
investigate the achievable performance at different X-ray spectra, with
different filling materials (i.e. scintillators) and varying channel height and
pixel readout. In this paper we focus on the quantum efficiency, the spatial
resolution and image homogeneity
The Physical Properties of LBGs at z>5: Outflows and the "pre-enrichment problem"
We discuss the properties of Lyman Break galaxies (LBGs) at z>5 as determined
from disparate fields covering approximately 500 sq. arcmin. While the broad
characteristics of the LBG population has been discussed extensively in the
literature, such as luminosity functions and clustering amplitude, we focus on
the detailed physical properties of the sources in this large survey (>100 with
spectroscopic redshifts). Specifically, we discuss ensemble mass estimates,
stellar mass surface densities, core phase space densities, star-formation
intensities, characteristics of their stellar populations, etc as obtained from
multi-wavelength data (rest-frame UV through optical) for a subsample of these
galaxies. In particular, we focus on evidence that these galaxies drive
vigorous outflows and speculate that this population may solve the so-called
``pre-enrichment problem''. The general picture that emerges from these studies
is that these galaxies, observed about 1 Gyr after the Big Bang, have
properties consistent with being the progenitors of the densest stellar systems
in the local Universe -- the centers of old bulges and early type galaxies.Comment: 4 pages, to appear in "Pathways Through an Eclectic Universe", J. H.
Knappen, T. J. Mahoney, and A. Vazedekis (Eds.), ASP Conf. Ser., 200
Measures of galaxy dust and gas mass with Herschel photometry and prospects for ALMA
(Abridged) Combining the deepest Herschel extragalactic surveys (PEP,
GOODS-H, HerMES), and Monte Carlo mock catalogs, we explore the robustness of
dust mass estimates based on modeling of broad band spectral energy
distributions (SEDs) with two popular approaches: Draine & Li (2007, DL07) and
a modified black body (MBB). As long as the observed SED extends to at least
160-200 micron in the rest frame, M(dust) can be recovered with a >3 sigma
significance and without the occurrence of systematics. An average offset of a
factor ~1.5 exists between DL07- and MBB-based dust masses, based on consistent
dust properties. At the depth of the deepest Herschel surveys (in the GOODS-S
field) it is possible to retrieve dust masses with a S/N>=3 for galaxies on the
main sequence of star formation (MS) down to M(stars)~1e10 [M(sun)] up to z~1.
At higher redshift (z<=2) the same result is achieved only for objects at the
tip of the MS or lying above it. Molecular gas masses, obtained converting
M(dust) through the metallicity-dependent gas-to-dust ratio delta(GDR), are
consistent with those based on the scaling of depletion time, and on CO
spectroscopy. Focusing on CO-detected galaxies at z>1, the delta(GDR)
dependence on metallicity is consistent with the local relation. We combine
far-IR Herschel data and sub-mm ALMA expected fluxes to study the advantages of
a full SED coverage.Comment: Accepted for publication in Astronomy and Astrophysics. Some figures
have degraded quality for filesize reason
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