10,015 research outputs found
Exoplanet Characterization by Multi-Observatory Transit Photometry with TESS and CHEOPS
Space-based photometric surveys have discovered large numbers of planets
transiting other stars, but these observe in a single band-pass and yield only
the planet radius, orbital period, and transit duration. Information on the
masses, compositions, and any atmospheres of these planets requires additional
observations from the ground or space. The Transiting Exoplanet Survey
Satellite (TESS) will yield thousands of planets around bright stars suitable
for such follow-up. In the absence of spectroscopy or spectrophotometry from
space, observations through the different pass-bands of multiple space
telescopes provide some spectral information useful for identifying false
positive signals, distinguishing between reflected light and thermal emission
from hot Jupiters, and detecting Rayleigh scattering by planetary atmospheres.
We calculated the expected difference in transit depths measured by the TESS
and Characterizing Exoplanets Satellites (CHEOPS) missions, which will be more
sensitive to redder and bluer optical wavelengths, respectively. The difference
due to companion or background stars is small (<3% for main sequence
companions) and likely to be negligible and undetectable. For only a few "hot"
Jupiters, can combined photometry disambiguate between the reflected and
thermal signals from planets. However, Rayleigh scattering by hazy atmospheres
with particles sizes near 0.04 m and at pressure altitudes above ~1 mbar
can be detected for ~100 transiting planets, assuming every planet has such an
atmosphere. Hazes with this characteristic particle size do not obscure
observations at longer (near-infrared) wavelengths; CHEOPS follow-up of
TESS-detected planets could thus identify candidates suitable for further study
with the James Webb Space Telescope.Comment: MNRAS, in pres
Constraints on short, hard gamma-ray burst beaming angles from gravitational wave observations
The first detection of a binary neutron star merger, GW170817, and an associated short gamma-ray burst confirmed that neutron star mergers are responsible for at least some of these bursts. The prompt gamma-ray emission from these events is thought to be highly relativistically beamed. We present a method for inferring limits on the extent of this beaming by comparing the number of short gamma-ray bursts (SGRBs) observed electromagnetically with the number of neutron star binary mergers detected in gravitational waves. We demonstrate that an observing run comparable to the expected Advanced LIGO (aLIGO) 2016–2017 run would be capable of placing limits on the beaming angle of approximately \theta \in (2\buildrel{\circ}\over{.} 88,14\buildrel{\circ}\over{.} 15), given one binary neutron star detection, under the assumption that all mergers produce a gamma-ray burst, and that SGRBs occur at an illustrative rate of . We anticipate that after a year of observations with aLIGO at design sensitivity in 2020, these constraints will improve to \theta \in (8\buildrel{\circ}\over{.} 10,14\buildrel{\circ}\over{.} 95), under the same efficiency and SGRB rate assumptions
Low effective mass leading to high thermoelectric performance
High Seebeck coefficient by creating large density-of-states effective mass through either electronic structure modification or manipulating nanostructures is commonly considered as a route to advanced thermoelectrics. However, large density-of-state due to flat bands leads to large transport effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of such a high effective mass is a lower thermoelectric figure of merit, zT, when the carriers are predominantly scattered by phonons according to the deformation potential theory of Bardeen–Shockley. We demonstrate that the beneficial effect of light effective mass contributes to high zT in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction
Weak electron–phonon coupling contributing to high thermoelectric performance in n-type PbSe
PbSe is a surprisingly good thermoelectric material due, in part, to its low thermal conductivity that had been overestimated in earlier measurements. The thermoelectric figure of merit, zT, can exceed 1 at high temperatures in both p-type and n-type PbSe, similar to that found in PbTe. While the p-type lead chalcogenides (PbSe and PbTe) benefit from the high valley degeneracy (12 or more at high temperature) of the valence band, the n-type versions are limited to a valley degeneracy of 4 in the conduction band. Yet the n-type lead chalcogenides achieve a zT nearly as high as the p-type lead chalcogenides. This effect can be attributed to the weaker electron–phonon coupling (lower deformation potential coefficient) in the conduction band as compared with that in the valence band, which leads to higher mobility of electrons compared to that of holes. This study of PbSe illustrates the importance of the deformation potential coefficient of the charge-carrying band as one of several key parameters to consider for band structure engineering and the search for high performance thermoelectric materials
Uncertainty relation in Schwarzschild spacetime
We explore the entropic uncertainty relation in the curved background outside
a Schwarzschild black hole, and find that Hawking radiation introduces a
nontrivial modification on the uncertainty bound for particular observer,
therefore it could be witnessed by proper uncertainty game experimentally. We
first investigate an uncertainty game between a free falling observer and his
static partner holding a quantum memory initially entangled with the quantum
system to be measured. Due to the information loss from Hawking decoherence, we
find an inevitable increase of the uncertainty on the outcome of measurements
in the view of static observer, which is dependent on the mass of the black
hole, the distance of observer from event horizon, and the mode frequency of
quantum memory. To illustrate the generality of this paradigm, we relate the
entropic uncertainty bound with other uncertainty probe, e.g., time-energy
uncertainty. In an alternative game between two static players, we show that
quantum information of qubit can be transferred to quantum memory through a
bath of fluctuating quantum fields outside the black hole. For a particular
choice of initial state, we show that the Hawking decoherence cannot counteract
entanglement generation after the dynamical evolution of system, which triggers
an effectively reduced uncertainty bound that violates the intrinsic limit
. Numerically estimation for a proper choice of initial state shows
that our result is comparable with possible real experiments. Finally, a
discussion on the black hole firewall paradox in the context of entropic
uncertainty relation is given.Comment: 11 pages, 2figures. Minor typos corrected, references and comment on
the black hole firewall added. Matches the version to appear in Physics
Letters
Necessary and sufficient conditions for local creation of quantum correlation
Quantum correlation can be created by a local operation from some initially
classical states. We prove that the necessary and sufficient condition for a
local trace-preserving channel to create quantum correlation is that it is not
a commutativity-preserving channel. This condition is valid for arbitrary
finite dimension systems. We also derive the explicit form of
commutativity-preserving channels. For a qubit, a commutativity-preserving
channel is either a completely decohering channel or a mixing channel. For a
three-dimensional system (qutrit), a commutativity-preserving channel is either
a completely decohering channel or an isotropic channel.Comment: Theorem 2 has been modifie
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