1,575 research outputs found
Phase diagram of an extended quantum dimer model on the hexagonal lattice
We introduce a quantum dimer model on the hexagonal lattice that, in addition
to the standard three-dimer kinetic and potential terms, includes a competing
potential part counting dimer-free hexagons. The zero-temperature phase diagram
is studied by means of quantum Monte Carlo simulations, supplemented by
variational arguments. It reveals some new crystalline phases and a cascade of
transitions with rapidly changing flux (tilt in the height language). We
analyze perturbatively the vicinity of the Rokhsar-Kivelson point, showing that
this model has the microscopic ingredients needed for the "devil's staircase"
scenario [E. Fradkin et al., Phys. Rev. B 69, 224415 (2004)], and is therefore
expected to produce fractal variations of the ground-state flux.Comment: Published version. 5 pages + 8 (Supplemental Material), 31
references, 10 color figure
Scaling of the von Neumann entropy across a finite temperature phase transition
The spectrum of the reduced density matrix and the temperature dependence of
the von Neumann entropy (VNE) are analytically obtained for a system of hard
core bosons on a complete graph which exhibits a phase transition to a
Bose-Einstein condensate at . It is demonstrated that the VNE undergoes
a crossover from purely logarithmic at T=0 to purely linear in block size
behaviour for . For intermediate temperatures, VNE is a sum of two
contributions which are identified as the classical (Gibbs) and the quantum
(due to entanglement) parts of the von Neumann entropy.Comment: 4 pages, 2 figure
Early Growth and Efficient Accretion of Massive Black Holes at High Redshift
Black-hole masses of the highest redshift quasars (4 <~ z <~ 6) are estimated
using a previously presented scaling relationship, derived from reverberation
mapping of nearby quasars, and compared to quasars at lower redshift. It is
shown that the central black holes in luminous z >~ 4 quasars are very massive
(>~ 10^9 solar masses). It is argued that the mass estimates of the high-z
quasars are not subject to larger uncertainties than those for nearby quasars.
Specifically, the large masses are not overestimates and the lack of similarly
large black-hole masses in the nearby Universe does not rule out their
existence at high-z. However, AGN host galaxies do not typically appear fully
formed or evolved at these early epochs. This supports scenarios in which black
holes build up mass very fast in a radiatively inefficient (or obscured) phase
relative to the stars in their galaxies. Additionally, upper envelopes of
black-hole mass of approximately 10^{10} solar masses and bolometric luminosity
of ~ 10^{48} erg/s are observed at all redshifts.Comment: 17 pages including 7 figures (5 in color) and 1 table. To appear in
ApJ, v600, January 1, 200
Crystal lattice properties fully determine short-range interaction parameters for alkali and halide ions
Accurate models of alkali and halide ions in aqueous solution are necessary
for computer simulations of a broad variety of systems. Previous efforts to
develop ion force fields have generally focused on reproducing experimental
measurements of aqueous solution properties such as hydration free energies and
ion-water distribution functions. This dependency limits transferability of the
resulting parameters because of the variety and known limitations of water
models. We present a solvent-independent approach to calibrating ion parameters
based exclusively on crystal lattice properties. Our procedure relies on
minimization of lattice sums to calculate lattice energies and interionic
distances instead of equilibrium ensemble simulations of dense fluids. The gain
in computational efficiency enables simultaneous optimization of all parameters
for Li+, Na+, K+, Rb+, Cs+, F-, Cl-, Br-, and I- subject to constraints that
enforce consistency with periodic table trends. We demonstrate the method by
presenting lattice-derived parameters for the primitive model and the
Lennard-Jones model with Lorentz-Berthelot mixing rules. The resulting
parameters successfully reproduce the lattice properties used to derive them
and are free from the influence of any water model. To assess the
transferability of the Lennard-Jones parameters to aqueous systems, we used
them to estimate hydration free energies and found that the results were in
quantitative agreement with experimentally measured values. These
lattice-derived parameters are applicable in simulations where coupling of ion
parameters to a particular solvent model is undesirable. The simplicity and low
computational demands of the calibration procedure make it suitable for
parametrization of crystallizable ions in a variety of force fields.Comment: 9 pages, 5 table
Continuum surveys with LOFAR and synergy with future large surveys in the 1-2 GHz band
Radio astronomy is entering the era of large surveys. This paper describes
the plans for wide surveys with the LOw Frequency ARray (LOFAR) and their
synergy with large surveys at higher frequencies (in particular in the 1-2 GHz
band) that will be possible using future facilities like Apertif or ASKAP. The
LOFAR Survey Key Science Project aims at conducting large-sky surveys at 15,
30, 60, 120 and 200 MHz taking advantage of the wide instantaneous field of
view and of the unprecedented sensitivity of this instrument. Four topics have
been identified as drivers for these surveys covering the formation of massive
galaxies, clusters and black holes using z>6 radio galaxies as probes, the
study of the intercluster magnetic fields using diffuse radio emission and
Faraday rotation measures in galaxy clusters as probes and the study of star
formation processes in the early Universe using starburst galaxies as probes.
The fourth topic is the exploration of new parameter space for serendipitous
discovery taking advantage of the new observational spectral window open up by
LOFAR. Here, we briefly discuss the requirements of the proposed surveys to
address these (and many others!) topics as well as the synergy with other wide
area surveys planned at higher frequencies (and in particular in the 1-2 GHz
band) with new radio facilities like ASKAP and Apertif. The complementary
information provided by these surveys will be crucial for detailed studies of
the spectral shape of a variety of radio sources (down to sub-mJy sources) and
for studies of the ISM (in particular HI and OH) in nearby galaxies.Comment: to appear in the proceedings of "Panoramic Radio Astronomy:
Wide-field 1-2 GHz research on galaxy evolution", G. Heald and P. Serra eds.,
8 pages, 3 figure
Models of electrolyte solutions from molecular descriptions: The example of NaCl solutions
We present a method to derive implicit solvent models of electrolyte
solutions from all-atom descriptions; providing analytical expressions of the
thermodynamic and structural properties of the ions consistent with the
underlying explicit solvent representation. Effective potentials between ions
in solution are calculated to perform perturbation theory calculations, in
order to derive the best possible description in terms of charged hard spheres.
Applying this method to NaCl solutions yields excellent agreement with the
all-atom model, provided ion association is taken into account.Comment: 4 pages, 5 figure
Adhesive Contact to a Coated Elastic Substrate
We show how the quasi-analytic method developed to solve linear elastic
contacts to coated substrates (Perriot A. and Barthel E. {\em J. Mat. Res.},
{\bf 2004}, {\em 19}, 600) may be extended to adhesive contacts. Substrate
inhomogeneity lifts accidental degeneracies and highlights the general
structure of the adhesive contact theory. We explicit the variation of the
contact variables due to substrate inhomogeneity. The relation to other
approaches based on Finite Element analysis is discussed
HST Imaging of the Host Galaxies of High Redshift Radio-Loud Quasars
We present rest-frame UV and Ly-alpha images of spatially-resolved structures
around five high-redshift radio-loud quasars obtained with the WFPC2 camera on
the Hubble Space Telescope. We find that all five quasars are extended and this
"fuzz" contains ~5-40% of the total continuum flux and 15-65% of the Ly-alpha
flux within a radius of about 1.5 arcsec. The rest-frame UV luminosities of the
hosts are log lambda P_lambda = 11.9 to 12.5 solar luminosities (assuming no
internal dust extinction), comparable to the luminous radio galaxies at similar
redshifts and a factor 10 higher than both radio-quiet field galaxies at z~2-3
and the most UV-luminous low redshift starburst galaxies. The Ly-alpha
luminosities of the hosts are (in the log) approximately 44.3-44.9 erg/s which
are also similar to the those of luminous high redshift radio galaxies and
considerably larger than the Ly-alpha luminosities of high redshift field
galaxies. To generate the Ly-alpha luminosities of the hosts would require
roughly a few percent of the total observed ionizing luminosity of the quasar.
We find good alignment between the extended Ly-alpha and the radio sources,
strong evidence for jet-cloud interactions in two cases, again resembling radio
galaxies, and what is possibly the most luminous radio-UV synchrotron jet in
one of the hosts at z=2.110.Comment: 36 pages (latex, aas macros), 3 figures (3 gif and 10 postscript
files), accepted for publication in the the Astrophysical Journal Supplement
Serie
The quasi-periodic Bose-Hubbard model and localization in one-dimensional cold atomic gases
We compute the phase diagram of the one-dimensional Bose-Hubbard model with a
quasi-periodic potential by means of the density-matrix renormalization group
technique. This model describes the physics of cold atoms loaded in an optical
lattice in the presence of a superlattice potential whose wave length is
incommensurate with the main lattice wave length. After discussing the
conditions under which the model can be realized experimentally, the study of
the density vs. the chemical potential curves for a non-trapped system unveils
the existence of gapped phases at incommensurate densities interpreted as
incommensurate charge-density wave phases. Furthermore, a localization
transition is known to occur above a critical value of the potential depth V_2
in the case of free and hard-core bosons. We extend these results to soft-core
bosons for which the phase diagrams at fixed densities display new features
compared with the phase diagrams known for random box distribution disorder. In
particular, a direct transition from the superfluid phase to the Mott
insulating phase is found at finite V_2. Evidence for reentrances of the
superfluid phase upon increasing interactions is presented. We finally comment
on different ways to probe the emergent quantum phases and most importantly,
the existence of a critical value for the localization transition. The later
feature can be investigated by looking at the expansion of the cloud after
releasing the trap.Comment: 19 pages, 20 figure
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