2,828 research outputs found
Localized excited charge carriers generate ultrafast inhomogeneous strain in the multiferroic BiFeO
We apply ultrafast X-ray diffraction with femtosecond temporal resolution to
monitor the lattice dynamics in a thin film of multiferroic BiFeO after
above-bandgap photoexcitation. The sound-velocity limited evolution of the
observed lattice strains indicates a quasi-instantaneous photoinduced stress
which decays on a nanosecond time scale. This stress exhibits an inhomogeneous
spatial profile evidenced by the broadening of the Bragg peak. These new data
require substantial modification of existing models of photogenerated stresses
in BiFeO: the relevant excited charge carriers must remain localized to be
consistent with the data
Deep near-infrared imaging of W3 Main: constraints on stellar cluster formation
Embedded clusters like W3 Main are complex and dynamically evolving systems
that represent an important phase of the star formation process. We aim at the
characterization of the entire stellar content of W3 Main in a statistical
sense to identify possible differences in evolutionary phase of the stellar
populations and find clues about the formation mechanism of this massive
embedded cluster. Methods. Deep JHKs imaging is used to derive the disk
fraction, Ks-band luminosity functions and mass functions for several
subregions in W3 Main. A two dimensional completeness analysis using artificial
star experiments is applied as a crucial ingredient to assess realistic
completeness limits for our photometry. We find an overall disk fraction of 7.7
2.3%, radially varying from 9.4 3.0 % in the central 1 pc to 5.6
2.2 % in the outer parts of W3 Main. The mass functions derived for three
subregions are consistent with a Kroupa and Chabrier mass function. The mass
function of IRSN3 is complete down to 0.14 Msun and shows a break at M
0.5 Msun. We interpret the higher disk fraction in the center as evidence for a
younger age of the cluster center. We find that the evolutionary sequence
observed in the low-mass stellar population is consistent with the observed age
spread among the massive stars. An analysis of the mass function variations
does not show evidence for mass segregation. W3 Main is currently still
actively forming stars, showing that the ionizing feedback of OB stars is
confined to small areas ( 0.5 pc). The FUV feedback might be influencing
large regions of the cluster as suggested by the low overall disk fraction.Comment: 15 pages, 13 figures, accepted by A&
Correlation functions, null polygonal Wilson loops, and local operators
We consider the ratio of the correlation function of n+1 local operators over
the correlator of the first n of these operators in planar N=4 super-Yang-Mills
theory, and consider the limit where the first n operators become pairwise null
separated. By studying the problem in twistor space, we prove that this is
equivalent to the correlator of a n-cusp null polygonal Wilson loop with the
remaining operator in general position, normalized by the expectation value of
the Wilson loop itself, as recently conjectured by Alday, Buchbinder and
Tseytlin. Twistor methods also provide a BCFW-like recursion relation for such
correlators. Finally, we study the natural extension where n operators become
pairwise null separated with k operators in general position. As an example, we
perform an analysis of the resulting correlator for k=2 and discuss some of the
difficulties associated to fixing the correlator completely in the strong
coupling regime.Comment: 34 pages, 6 figures. v2: typos corrected and references added; v3:
published versio
Evidence for Environmentally Dependent Cluster Disruption in M83
Using multi-wavelength imaging from the Wide Field Camera 3 on the Hubble
Space Telescope we study the stellar cluster populations of two adjacent fields
in the nearby face-on spiral galaxy, M83. The observations cover the galactic
centre and reach out to ~6 kpc, thereby spanning a large range of environmental
conditions, ideal for testing empirical laws of cluster disruption. The
clusters are selected by visual inspection to be centrally concentrated,
symmetric, and resolved on the images. We find that a large fraction of objects
detected by automated algorithms (e.g. SExtractor or Daofind) are not clusters,
but rather are associations. These are likely to disperse into the field on
timescales of tens of Myr due to their lower stellar densities and not due to
gas expulsion (i.e. they were never gravitationally bound). We split the sample
into two discrete fields (inner and outer regions of the galaxy) and search for
evidence of environmentally dependent cluster disruption. Colour-colour
diagrams of the clusters, when compared to simple stellar population models,
already indicate that a much larger fraction of the clusters in the outer field
are older by tens of Myr than in the inner field. This impression is quantified
by estimating each cluster's properties (age, mass, and extinction) and
comparing the age/mass distributions between the two fields. Our results are
inconsistent with "universal" age and mass distributions of clusters, and
instead show that the ambient environment strongly affects the observed
populations.Comment: 6 pages, 3 figures, MNRAS in pres
Fishing for carbon nanotubes with a photonic metamaterial net
We experimentally demonstrate that plasmonic metamaterials can optically harvest single-walled carbon nanotubes from colloidal suspension. We investigate whether this phenomenon could provide a new technique for nanotube purification via plasmon-exciton resonance interactions
CHY representations for gauge theory and gravity amplitudes with up to three massive particles
We show that a wide class of tree-level scattering amplitudes involving
scalars, gauge bosons, and gravitons, up to three of which may be massive, can
be expressed in terms of a Cachazo-He-Yuan representation as a sum over
solutions of the scattering equations. These amplitudes, when expressed in
terms of the appropriate kinematic invariants, are independent of the masses
and therefore identical to the corresponding massless amplitudes.Comment: 20 pages, 1 figure; v2: minor typos corrected, published versio
Metamaterial enhancement of metal-halide perovskite luminescence
Metal-halide perovskites are rapidly emerging as solution-processable optical materials for light-emitting applications. Here, we adopt a plasmonic metamaterial approach to enhance photoluminescence emission and extraction of methylammonium lead iodide (MAPbI3) thin films based on the Purcell effect. We show that hybridization of the active metal-halide film with resonant nanoscale sized slits carved into a gold film can yield more than 1 order of magnitude enhancement of luminescence intensity and nearly 3-fold reduction of luminescence lifetime corresponding to a Purcell enhancement factor of more than 300. These results show the effectiveness of resonant nanostructures in controlling metal-halide perovskite light emission properties over a tunable spectral range, a viable approach toward highly efficient perovskite light-emitting devices and single-photon emitter
LEGUS and Halpha-LEGUS Observations of Star Clusters in NGC 4449: Improved Ages and the Fraction of Light in Clusters as a Function of Age
We present a new catalog and results for the cluster system of the starburst
galaxy NGC 4449 based on multi-band imaging observations taken as part of the
LEGUS and Halpha-LEGUS surveys. We improve the spectral energy fitting method
used to estimate cluster ages and find that the results, particularly for older
clusters, are in better agreement with those from spectroscopy. The inclusion
of Halpha measurements, the role of stochasticity for low mass clusters, the
assumptions about reddening, and the choices of SSP model and metallicity all
have important impacts on the age-dating of clusters. A comparison with ages
derived from stellar color-magnitude diagrams for partially resolved clusters
shows reasonable agreement, but large scatter in some cases. The fraction of
light found in clusters relative to the total light (i.e., T_L) in the U, B,
and V filters in 25 different ~kpc-size regions throughout NGC 4449 correlates
with both the specific Region Luminosity, R_L, and the dominant age of the
underlying stellar population in each region. The observed cluster age
distribution is found to decline over time as dN/dt ~ t^g, with g=-0.85+/-0.15,
independent of cluster mass, and is consistent with strong, early cluster
disruption. The mass functions of the clusters can be described by a power law
with dN/dM ~ M^b and b=-1.86+/-0.2, independent of cluster age. The mass and
age distributions are quite resilient to differences in age-dating methods.
There is tentative evidence for a factor of 2-3 enhancement in both the star
and cluster formation rate ~100 - 300 Myr ago, indicating that cluster
formation tracks star formation generally. The enhancement is probably
associated with an earlier interaction event
- …