778 research outputs found
The Gradients in the 47 Tuc Red Giant Branch Bump and Horizontal Branch are Consistent With a Centrally-Concentrated, Helium-Enriched Second Stellar Generation
We combine ground and space-based photometry of the Galactic globular cluster
47 Tuc to measure four independent lines of evidence for a helium gradient in
the cluster, whereby stars in the cluster outskirts would have a lower initial
helium abundance than stars in and near the cluster core. First and second, we
show that the red giant branch bump (RGBB) stars exhibit gradients in their
number counts and brightness. With increased separation from the cluster
center, they become more numerous relative to the other red giant (RG) stars.
They also become fainter. For our third and fourth lines of evidence, we show
that the horizontal branch (HB) of the cluster becomes both fainter and redder
for sightlines farther from the cluster center. These four results are
respectively detected at the 2.3, 3.6, 7.7 and
4.1 levels. Each of these independent lines of evidence is found to be
significant in the cluster-outskirts; closer in, the data are more compatible
with uniform mixing. Our radial profile is qualitatively consistent with but
quantitatively tighter than previous results based on CN absorption. These
observations are qualitatively consistent with a scenario wherein a second
generation of stars with modestly enhanced helium and CNO abundance formed deep
within the gravitational potential of a cluster of previous generation stars
having more canonical abundances.Comment: 20 pages, 6 figures, 1 table, submitted to The Astrophysical Journa
Heisenberg Uncertainty Principle as Probe of Entanglement Entropy: Application to Superradiant Quantum Phase Transitions
Quantum phase transitions are often embodied by the critical behavior of
purely quantum quantities such as entanglement or quantum fluctuations. In
critical regions, we underline a general scaling relation between the
entanglement entropy and one of the most fundamental and simplest measure of
the quantum fluctuations, the Heisenberg uncertainty principle. Then, we show
that the latter represents a sensitive probe of superradiant quantum phase
transitions in standard models of photons such as the Dicke Hamiltonian, which
embodies an ensemble of two-level systems interacting with one quadrature of a
single and uniform bosonic field. We derive exact results in the thermodynamic
limit and for a finite number N of two-level systems: as a reminiscence of the
entanglement properties between light and the two-level systems, the product
diverges at the quantum critical point as . We
generalize our results to the double quadrature Dicke model where the two
quadratures of the bosonic field are now coupled to two independent sets of two
level systems. Our findings, which show that the entanglement properties
between light and matter can be accessed through the Heisenberg uncertainty
principle, can be tested using Bose-Einstein condensates in optical cavities
and circuit quantum electrodynamicsComment: 7 pages, 3 figures. Published Versio
Time-reversal symmetry breaking Abelian chiral spin liquid in Mott phases of three-component fermions on the triangular lattice
We provide numerical evidence in favor of spontaneous chiral symmetry
breaking and the concomitant appearance of an Abelian chiral spin liquid for
three-component fermions on the triangular lattice described by an SU(3)
symmetric Hubbard model with hopping amplitude () and on-site
interaction . This chiral phase is stabilized in the Mott phase with one
particle per site in the presence of a uniform -flux per plaquette, and in
the Mott phase with two particles per site without any flux. Our approach
relies on effective spin models derived in the strong-coupling limit in powers
of for general SU and arbitrary uniform charge flux per plaquette,
which are subsequently studied using exact diagonalizations and variational
Monte Carlo simulations for , as well as exact diagonalizations of the
SU() Hubbard model on small clusters. Up to third order in , and for
the time-reversal symmetric cases (flux or ), the low-energy
description is given by the - model with Heisenberg coupling and real
ring exchange . The phase diagram in the full - parameter range
contains, apart from three already known, magnetically long-range ordered
phases, two previously unreported phases: i) a lattice nematic phase breaking
the lattice rotation symmetry and ii) a spontaneous time-reversal and parity
symmetry breaking Abelian chiral spin liquid. For the Hubbard model, an
investigation that includes higher-order itinerancy effects supports the
presence of a phase transition inside the insulating region, occurring at
[] between the
three-sublattice magnetically ordered phase at small and this Abelian
chiral spin liquid.Comment: 21 pages, 23 figure
High-contrast imaging of 180{\deg} ferroelectric domains by optical microscopy using ferroelectric liquid crystals
Ferroelectric liquid crystals (FLCs) couple the direction of their
spontaneous electric polarization to the direction of tilt of their optic axis.
Consequently, reversal of the electric polarization by an electric field gives
rise to an immediate and lasting optical response when an appropriately aligned
FLC is observed between crossed polarizers, with one field direction yielding a
dark image, and the opposite direction yielding a bright image. Here this
peculiar electro-optic response is used to image, with high optical contrast,
180{\deg} ferroelectric domains in a crystalline substrate of magnesium-doped
lithium niobate. The lithium niobate substrate contains a few domains with
upwards electric polarization surrounded by regions with downward electric
polarization. In contrast to a reference non-chiral liquid crystal that is
unable to show ferroelectric behavior due to its high symmetry, the FLC, which
is used as a thin film confined between the lithium niobate substrate and an
inert aligning substrate, reveals ferroelectric domains as well as their
boundaries, with strong black and white contrast. The results show that FLCs
can be used for non-destructive read-out of domains in underlying
ferroelectrics, with potential applications in e.g. photonic devices and
non-volatile ferroelectric memories.Comment: 12 pages, 3 figure
Optimal Survey Strategies and Predicted Planet Yields for the Korean Microlensing Telescope Network
The Korean Microlensing Telescope Network (KMTNet) will consist of three 1.6m
telescopes each with a 4 deg^{2} field of view (FoV) and will be dedicated to
monitoring the Galactic Bulge to detect exoplanets via gravitational
microlensing. KMTNet's combination of aperture size, FoV, cadence, and
longitudinal coverage will provide a unique opportunity to probe exoplanet
demographics in an unbiased way. Here we present simulations that optimize the
observing strategy for, and predict the planetary yields of, KMTNet. We find
preferences for four target fields located in the central Bulge and an exposure
time of t_{exp} = 120s, leading to the detection of ~2,200 microlensing events
per year. We estimate the planet detection rates for planets with mass and
separation across the ranges 0.1 <= M_{p}/M_{Earth} <= 1000 and 0.4 <= a/AU <=
16, respectively. Normalizing these rates to the cool-planet mass function of
Cassan (2012), we predict KMTNet will be approximately uniformly sensitive to
planets with mass 5 <= M_{p}/M_{Earth} <= 1000 and will detect ~20 planets per
year per dex in mass across that range. For lower-mass planets with mass 0.1 <=
M_{p}/M_{Earth} < 5, we predict KMTNet will detect ~10 planets per year. We
also compute the yields KMTNet will obtain for free-floating planets (FFPs) and
predict KMTNet will detect ~1 Earth-mass FFP per year, assuming an underlying
population of one such planet per star in the Galaxy. Lastly, we investigate
the dependence of these detection rates on the number of observatories, the
photometric precision limit, and optimistic assumptions regarding seeing,
throughput, and flux measurement uncertainties.Comment: 29 pages, 31 figures, submitted to ApJ. For a brief video explaining
the key results of this paper, please visit:
https://www.youtube.com/watch?v=e5rWVjiO26
Protected quantum computation with multiple resonators in ultrastrong coupling circuit QED
We investigate theoretically the dynamical behavior of a qubit obtained with
the two ground eigenstates of an ultrastrong coupling circuit-QED system
consisting of a finite number of Josephson fluxonium atoms inductively coupled
to a transmission line resonator. We show an universal set of quantum gates by
using multiple transmission line resonators (each resonator represents a single
qubit). We discuss the intrinsic 'anisotropic' nature of noise sources for
fluxonium artificial atoms. Through a master equation treatment with colored
noise and manylevel dynamics, we prove that, for a general class of anisotropic
noise sources, the coherence time of the qubit and the fidelity of the quantum
operations can be dramatically improved in an optimal regime of ultrastrong
coupling, where the ground state is an entangled photonic 'cat' state.Comment: Added results with N = 3,4,5 Josephson atoms and different anisotropy
ratios for the decoherence channels in the new figures 2 and
Magnetic properties of the honeycomb oxide NaCoTeO
We have studied the magnetic properties of NaCoTeO, which
features a honeycomb lattice of magnetic Co ions, through macroscopic
characterization and neutron diffraction on a powder sample. We have shown that
this material orders in a zig-zag antiferromagnetic structure. In addition to
allowing a linear magnetoelectric coupling, this magnetic arrangement displays
very peculiar spatial magnetic correlations, larger in the honeycomb planes
than between the planes, which do not evolve with the temperature. We have
investigated this behavior by Monte Carlo calculations using the
-- model on a honeycomb lattice with a small interplane
interaction. Our model reproduces the experimental neutron structure factor,
although its absence of temperature evolution must be due to additional
ingredients, such as chemical disorder or quantum fluctuations enhanced by the
proximity to a phase boundary.Comment: 9 pages, 13 figure
Red Giant Branch Bump Brightness and Number Counts in 72 Galactic Globular Clusters Observed with the Hubble Space Telescope
We present the broadest and most precise empirical investigation of red giant
branch bump (RGBB) brightness and number counts ever conducted. We implement a
new method and use data from two \textit{Hubble Space Telescope (HST)} globular
cluster (GC) surveys to measure the brightness and star counts of the RGBB in
72 GCs. The brightness is measured to a precision better than 0.01 mag while
the precision in number counts reaches 10%. The position of the main-sequence
turnoff (MSTO) and the number of horizontal branch (HB) stars are used as
comparisons where appropriate. Several independent scientific conclusions are
newly possible with our parametrization of the RGBB. Both brightness and number
counts are shown to have second parameters in addition to their strong
dependence on metallicity. The RGBBs are found to be anomalous in the GCs NGC
2808, 5286, 6388 and 6441, likely due to the presence of multiple populations.
Finally, we use our empirical calibration to predict the properties of the
Galactic bulge RGBB if the assumption of similar stellar physics for the bulge
and Galactic GC system holds. The RGBB properties for the bulge are shown to
differ from those of the Galactic GC system, with the former having lower
number counts, a lower brightness dispersion and a brighter peak luminosity
than would be expected from the latter. This discrepancy is well explained by
the Galactic bulge having a higher helium abundance than expected from GCs,
Y0.06 at the median metallicity.Comment: 66 pages, 21 figures, 7 tables, Accepted for publication in The
Astrophysical Journal, modified following referee repor
Experimental study of super-rotation in a magnetostrophic spherical Couette flow
We report measurements of electric potentials at the surface of a spherical
container of liquid sodium in which a magnetized inner core is differentially
rotating. The azimuthal angular velocities inferred from these potentials
reveal a strong super-rotation of the liquid sodium in the equatorial region,
for small differential rotation. Super-rotation was observed in numerical
simulations by Dormy et al. [1]. We find that the latitudinal variation of the
electric potentials in our experiments differs markedly from the predictions of
a similar numerical model, suggesting that some of the assumptions used in the
model - steadiness, equatorial symmetry, and linear treatment for the evolution
of both the magnetic and velocity fields - are violated in the experiments. In
addition, radial velocity measurements, using ultrasonic Doppler velocimetry,
provide evidence of oscillatory motion near the outer sphere at low latitude:
it is viewed as the signature of an instability of the super-rotating region
Abstract robust coarse spaces for systems of PDEs via generalized eigenproblems in the overlaps
Coarse spaces are instrumental in obtaining scalability for domain decomposition methods for partial differential equations (PDEs). However, it is known that most popular choices of coarse spaces perform rather weakly in the presence of heterogeneities in the PDE coefficients, especially for systems of PDEs. Here, we introduce in a variational setting a new coarse space that is robust even when there are such heterogeneities. We achieve this by solving local generalized eigenvalue problems in the overlaps of subdomains that isolate the terms responsible for slow convergence. We prove a general theoretical result that rigorously establishes the robustness of the new coarse space and give some numerical examples on two and three dimensional heterogeneous PDEs and systems of PDEs that confirm this property
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