3,049 research outputs found
Phonon Dispersion Effects and the Thermal Conductivity Reduction in GaAs/AlAs Superlattices
The experimentally observed order-of-magnitude reduction in the thermal
conductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n)
superlattices is investigated theoretically for (2x2), (3x3) and (6x6)
structures using an accurate model of the lattice dynamics. The modification of
the phonon dispersion relation due to the superlattice geometry leads to
flattening of the phonon branches and hence to lower phonon velocities. This
effect is shown to account for a factor-of-three reduction in the thermal
conductivity with respect to bulk GaAs along the growth direction; the
remainder is attributable to a reduction in the phonon lifetime. The
dispersion-related reduction is relatively insensitive to temperature (100 < T
< 300K) and n. The phonon lifetime reduction is largest for the (2x2)
structures and consistent with greater interface scattering. The thermal
conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs
force constant differences than to those associated with molecular masses.Comment: 5 figure
Strong XUV irradiation of the Earth-sized exoplanets orbiting the ultracool dwarf TRAPPIST-1
We present an XMM-Newton X-ray observation of TRAPPIST-1, which is an
ultracool dwarf star recently discovered to host three transiting and temperate
Earth-sized planets. We find the star is a relatively strong and variable
coronal X-ray source with an X-ray luminosity similar to that of the quiet Sun,
despite its much lower bolometric luminosity. We find L_x/L_bol=2-4x10^-4, with
the total XUV emission in the range L_xuv/L_bol=6-9x10^-4, and XUV irradiation
of the planets that is many times stronger than experienced by the present-day
Earth. Using a simple energy-limited model we show that the relatively close-in
Earth-sized planets, which span the classical habitable zone of the star, are
subject to sufficient X-ray and EUV irradiation to significantly alter their
primary and any secondary atmospheres. Understanding whether this high-energy
irradiation makes the planets more or less habitable is a complex question, but
our measured fluxes will be an important input to the necessary models of
atmospheric evolution.Comment: 5 pages, published as a letter in MNRAS (accepted 16 September 2016
Microscopic gauge-invariant theory of the c-axis infrared response of bilayer cuprate superconductors and the origin of the superconductivity induced absorption bands
We report on results of our theoretical study of the c-axis infrared
conductivity of bilayer high-Tc cuprate superconductors using a microscopic
model involving the bilayer-split (bonding and antibonding) bands. An emphasis
is on the gauge-invariance of the theory, which turns out to be essential for
the physical understanding of the electrodynamics of these compounds. The
description of the optical response involves local (intra-bilayer and
inter-bilayer) current densities and local conductivities. The local
conductivities are obtained using a microscopic theory, where the
quasiparticles of the two bands are coupled to spin fluctuations. The coupling
leads to superconductivity and is described at the level of generalized
Eliashberg theory. Also addressed is the simpler case of quasiparticles coupled
by a separable and nonretarded interaction. The gauge invariance of the theory
is achieved by including a suitable class of vertex corrections. The resulting
response of the model is studied in detail and an interpretation of two
superconductivity-induced peaks in the experimental data of the real part of
the c-axis conductivity is proposed. The peak around 400/cm is attributed to a
collective mode of the intra-bilayer regions, that is an analogue of the
Bogolyubov-Anderson mode playing a crucial role in the theory of the
longitudinal response of superconductors. For small values of the bilayer
splitting, its nature is similar to that of the transverse plasmon of the
phenomenological Josephson superlattice model. The peak around 1000/cm is
interpreted as a pair breaking-feature that is related to the electronic
coupling through the spacing layers separating the bilayers.Comment: 18 pages, 15 figures, submitted to Phys. Rev.
Sparse aperture masking at the VLT II. Detection limits for the eight debris disks stars Pic, AU Mic, 49 Cet, Tel, Fomalhaut, g Lup, HD181327 and HR8799
Context. The formation of planetary systems is a common, yet complex
mechanism. Numerous stars have been identified to possess a debris disk, a
proto-planetary disk or a planetary system. The understanding of such formation
process requires the study of debris disks. These targets are substantial and
particularly suitable for optical and infrared observations. Sparse Aperture
masking (SAM) is a high angular resolution technique strongly contributing to
probe the region from 30 to 200 mas around the stars. This area is usually
unreachable with classical imaging, and the technique also remains highly
competitive compared to vortex coronagraphy. Aims. We aim to study debris disks
with aperture masking to probe the close environment of the stars. Our goal is
either to find low mass companions, or to set detection limits. Methods. We
observed eight stars presenting debris disks ( Pictoris, AU
Microscopii, 49 Ceti, Telescopii, Fomalhaut, g Lupi, HD181327 and
HR8799) with SAM technique on the NaCo instrument at the VLT. Results. No close
companions were detected using closure phase information under 0.5 of
separation from the parent stars. We obtained magnitude detection limits that
we converted to Jupiter masses detection limits using theoretical isochrones
from evolutionary models. Conclusions. We derived upper mass limits on the
presence of companions in the area of few times the diffraction limit of the
telescope around each target star.Comment: 7 pages, All magnitude detection limits maps are only available in
electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5
Temporal variations in the evaporating atmosphere of the exoplanet HD 189733b
Atmospheric escape has been detected from the exoplanet HD 209458b through
transit observations of the hydrogen Lyman-alpha line. Here we present
spectrally resolved Lyman-alpha transit observations of the exoplanet HD
189733b at two different epochs. These HST/STIS observations show for the first
time, that there are significant temporal variations in the physical conditions
of an evaporating planetary atmosphere. While atmospheric hydrogen is not
detected in the first epoch observations, it is observed at the second epoch,
producing a transit absorption depth of 14.4+/-3.6% between velocities of -230
to -140 km/s. Contrary to HD 209458b, these high velocities cannot arise from
radiation pressure alone and require an additional acceleration mechanism, such
as interactions with stellar wind protons. The observed absorption can be
explained by an atmospheric escape rate of neutral hydrogen atoms of about 10^9
g/s, a stellar wind with a velocity of 190 km/s and a temperature of ~10^5K.
An X-ray flare from the active star seen with Swift/XRT 8 hours before the
second-epoch observation supports the idea that the observed changes within the
upper atmosphere of the planet can be caused by variations in the stellar wind
properties, or by variations in the stellar energy input to the planetary
escaping gas (or a mix of the two effects). These observations provide the
first indication of interaction between the exoplanet's atmosphere and stellar
variations.Comment: To be published in A&A Letters, June 28, 201
Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman- line
The TRAPPIST-1 system offers the opportunity to characterize terrestrial,
potentially habitable planets orbiting a nearby ultracool dwarf star. We
performed a four-orbit reconnaissance with the Space Telescope Imaging
Spectrograph onboard the Hubble Space Telescope to study the stellar emission
at Lyman-, to assess the presence of hydrogen exospheres around the two
inner planets, and to determine their UV irradiation. We detect the
Lyman- line of TRAPPIST-1, making it the coldest exoplanet host star
for which this line has been measured. We reconstruct the intrinsic line
profile, showing that it lacks broad wings and is much fainter than expected
from the stellar X-ray emission. TRAPPIST-1 has a similar X-ray emission as
Proxima Cen but a much lower Ly- emission. This suggests that
TRAPPIST-1 chromosphere is only moderately active compared to its transition
region and corona. We estimated the atmospheric mass loss rates for all
planets, and found that despite a moderate extreme UV emission the total XUV
irradiation could be strong enough to strip the atmospheres of the inner
planets in a few billions years. We detect marginal flux decreases at the times
of TRAPPIST-1b and c transits, which might originate from stellar activity, but
could also hint at the presence of extended hydrogen exospheres. Understanding
the origin of these Lyman- variations will be crucial in assessing the
atmospheric stability and potential habitability of the TRAPPIST-1 planets.Comment: Published in A&A as a Letter to the Edito
Accumulated environmental risk in young refugees – A prospective evaluation
Background: Recently, we reported a strong, disease-independent relationship between accumulated pre-adult environmental risks and violent aggression later in life. Risk factors were interchangeable, and migration was among the explored risks. Alarmed by these data, we assessed collected risk loadin young ‘healthy’ refugees as a specifics group of current migration streams and evaluated first signals of behavioral abnormalities. Methods: In 9 German refugee centers, n=133 young refugees, not previously in contact with the health system, were recruited, many of them unaccompanied minors. Risk factors experienced apart from migration/refuge were carefully assessed: Traumatic experiences before/during/after flight (including war,genocide, human trafficking, torture, murder, slavery, terrorist attacks), urbanicity, physical and sexual abuse, problematic alcohol and cannabis use (lifetime). Evaluation comprised physical exam and psychopathology screening. Findings: Refugees arrived in Germany via Eastern Mediterranean/Balkanroute (34.6%), from Africa via Central Mediterranean route (39.1%), by plane (17.3%) or other routes, such as Western Mediterranean or Atlantic (9.0%). Flight reasons were war/expulsion (25.6%), persecution/threats to life (51.9%), economical/others (22.5%). Interpretation: refugees from hosting countries with alarming 'risk burden', should be considered as highly vulnerable towards development of global functional deficits, behavioral abnormalities, and neuropsychiatric disorders. Rapid proactive integration or sustainable support of those who will return to rebuild their countries are mandatory
Tuning Fermi-surface properties through quantum confinement in metallic meta-lattices: New metals from old atoms
We describe a new class of nanoscale structured metals wherein the effects of
quantum confinement are combined with dispersive metallic electronic states to
induce modifications to the fundamental low-energy microscopic properties of a
three-dimensional metal: the density of states, the distribution of Fermi
velocities, and the collective electronic response.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Detection of oxygen and carbon in the hydrodynamically escaping atmosphere of the extrasolar planet HD209458b
Four transits of the planet orbiting the star HD209458 were observed with the
STIS spectrograph on board HST. The wavelength domain (1180-1710A) includes HI
as well as CI, CII, CIV, NV, OI, SI, SiII, SiIII and SiIV lines. During the
transits, absorptions are detected in HI, OI and CII (5+/-2%, 13+/-4.5% and
7.5+/-3.5%, respectively). No absorptions are detected for other lines. The 5%
mean absorption over the whole HI Lyman alpha line is consistent with the
previous detection at higher resolution (Vidal-Madjar et al. 2003). The
absorption depths in OI and CII show that oxygen and carbon are present in the
extended upper atmosphere of HD209458b. These species must be carried out up to
the Roche lobe and beyond, most likely in a state of hydrodynamic escape.Comment: 6 pages, 4 figures, 1 table, submitted to ApJ Letters, revised
version with slightly revisited absorption depth estimate
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