3,961 research outputs found
Ancestral Spin Information in Gravitational Waves from Black Hole Mergers
The heaviest black holes discovered through gravitational waves have masses
that are difficult to explain with current standard stellar models. This
discrepancy may be due to a series of hierarchical mergers, where the observed
black holes are themselves the products of previous mergers. Here we present a
method to estimate the masses and spins of previous generations of black holes
based on the masses and spins of black holes in a binary. Examining the merger
GW190521, we find that assuming black hole spins that are consistent with those
of merger remnants will alter the reconstructed ancestral spins when compared
to results with uninformed priors. At the same time, the inclusion of black
hole spins does not significantly affect the mass distributions of the
ancestral black holes.Comment: 4 pages, 2 figure
Implementation of the 64-meter-diameter Antennas at the Deep Space Stations in Australia and Spain
The management and construction aspects of the Overseas 64-m Antenna Project in which two 64-m antennas were constructed at the Tidbinbilla Deep Space Communications Complex in Australia, and at the Madrid Deep Space Communications Complex in Spain are described. With the completion of these antennas the Deep Space Network is equipped with three 64-m antennas spaced around the world to maintain continuous coverage of spacecraft operations. These antennas provide approximately a 7-db gain over the capabilities of the existing 26-m antenna nets. The report outlines the project organization and management, resource utilization, fabrication, quality assurance, and construction methods by which the project was successfully completed. Major problems and their solutions are described as well as recommendations for future projects
Quasi-Objective Eddy Visualization from Sparse Drifter Data
We employ a recently developed single-trajectory Lagrangian diagnostic tool,
the trajectory rotation average , to visualize
oceanic vortices (or eddies) from sparse drifter data. We apply the to two drifter data sets that cover various
oceanographic scales: the Grand Lagrangian Deployment (GLAD) and the Global
Drifter Program (GDP). Based on the , we develop a
general algorithm that extracts approximate eddy boundaries. We find that the outperforms other available single-trajectory-based
eddy detection methodologies on sparse drifter data and identifies eddies on
scales that are unresolved by satellite-altimetry
Fine structure of -excitons in multilayers of transition metal dichalcogenides
Reflectance and magneto-reflectance experiments together with theoretical
modelling based on the approach have been employed to study
the evolution of direct bandgap excitons in MoS layers with a thickness
ranging from mono- to trilayer. The extra excitonic resonances observed in
MoS multilayers emerge as a result of the hybridization of Bloch states of
each sub-layer due to the interlayer coupling. The properties of such excitons
in bi- and trilayers are classified by the symmetry of corresponding crystals.
The inter- and intralayer character of the reported excitonic resonances is
fingerprinted with the magneto-optical measurements: the excitonic -factors
of opposite sign and of different amplitude are revealed for these two types of
resonances. The parameters describing the strength of the spin-orbit
interaction are estimated for bi- and trilayer MoS.Comment: 14 pages, 10 figure
Recurrent Neutrino Emission from Supermassive Black Hole Mergers
The recent detection of possible neutrino emission from the blazar TXS
0506+056 was the first high-energy neutrino associated with an astrophysical
source, making this special type of active galaxies promising neutrino
emitters. The fact that two distinct episodes of neutrino emission were
detected with a separation of around 3 years suggests that emission could be
periodic. Periodic emission is expected from supermassive binary black hole
systems due to jet precession close to the binary's merger. Here we show that
if TXS 0506+056 is a binary source then the next neutrino flare could occur
before the end of 2021. We derive the binary properties that would lead to the
detection of gravitational waves from this system by LISA. Our results for the
first time quantify the time scale of these correlations for the example of TXS
0506+056, providing clear predictions for both the neutrino and
gravitational-wave signatures of such sources.Comment: 6 pages, 3 figures, submitte
Probing and manipulating valley coherence of dark excitons in monolayer WSe
Monolayers of semiconducting transition metal dichalcogenides are
two-dimensional direct-gap systems which host tightly-bound excitons with an
internal degree of freedom corresponding to the valley of the constituting
carriers. Strong spin-orbit interaction and the resulting ordering of the
spin-split subbands in the valence and conduction bands makes the lowest-lying
excitons in WX (X~being S or Se) spin-forbidden and optically dark. With
polarization-resolved photoluminescence experiments performed on a WSe
monolayer encapsulated in a hexagonal boron nitride, we show how the intrinsic
exchange interaction in combination with the applied in-plane and/or
out-of-plane magnetic fields enables one to probe and manipulate the valley
degree of freedom of the dark excitons.Comment: Manuscript: 6 pages, 3 figures; SM: 6 pages, 5 figure
Neutral and charged dark excitons in monolayer WS
Low temperature and polarization resolved magneto-photoluminescence
experiments are used to investigate the properties of dark excitons and dark
trions in a monolayer of WS encapsulated in hexagonal BN (hBN). We find
that this system is an -type doped semiconductor and that dark trions
dominate the emission spectrum. In line with previous studies on WSe, we
identify the Coulomb exchange interaction coupled neutral dark and grey
excitons through their polarization properties, while an analogous effect is
not observed for dark trions. Applying the magnetic field in both perpendicular
and parallel configurations with respect to the monolayer plane, we determine
the g-factor of dark trions to be -8.6. Their decay rate is close to 0.5
ns, more than 2 orders of magnitude longer than that of bright excitons.Comment: 6 pages, 6 figures, supplemental materia
Singlet and triplet trions in WS monolayer encapsulated in hexagonal boron nitride
Embedding a WS monolayer in flakes of hexagonal boron nitride allowed us
to resolve and study the photoluminescence response due to both singlet and
triplet states of negatively charged excitons (trions) in this atomically thin
semiconductor. The energy separation between the singlet and triplet states has
been found to be relatively small reflecting rather weak effects of the
electron-electron exchange interaction for the trion triplet in a WS
monolayer, which involves two electrons with the same spin but from different
valleys. Polarization-resolved experiments demonstrate that the helicity of the
excitation light is better preserved in the emission spectrum of the triplet
trion than in that of the singlet trion. Finally, the singlet (intravalley)
trions are found to be observable even at ambient conditions whereas the
emission due to the triplet (intervalley) trions is only efficient at low
temperatures.Comment: 11 pages, 4 figure
Energy spectrum of two-dimensional excitons in a non-uniform dielectric medium
We demonstrate that, in monolayers (MLs) of semiconducting transition metal
dichalcogenides, the -type Rydberg series of excitonic states follows a
simple energy ladder: , =1,2,\ldots, in which
is very close to the Rydberg energy scaled by the dielectric constant of
the medium surrounding the ML and by the reduced effective electron-hole mass,
whereas the ML polarizability is only accounted for by . This is
justified by the analysis of experimental data on excitonic resonances, as
extracted from magneto-optical measurements of a high-quality WSe ML
encapsulated in hexagonal boron nitride (hBN), and well reproduced with an
analytically solvable Schr\"odinger equation when approximating the
electron-hole potential in the form of a modified Kratzer potential. Applying
our convention to other, MoSe, WS, MoS MLs encapsulated in hBN, we
estimate an apparent magnitude of for each of the studied structures.
Intriguingly, is found to be close to zero for WSe as well as for
MoS monolayers, what implies that the energy ladder of excitonic states in
these two-dimensional structures resembles that of Rydberg states of a
three-dimensional hydrogen atom.Comment: Manuscript: 6 pages, 4 figures; SM: 11 pages, 12 figure
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