2,523 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
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.
Dynamical coherent-potential approximation approach to excitation spectra in 3d transition metals
First-principles dynamical CPA (Coherent-Potential Approximation) for
electron correlations has been developed further by taking into account
higher-order dynamical corrections with use of the asymptotic approximation.
The theory is applied to the investigations of a systematic change of
excitation spectra in transition metals from Sc to Cu at finite
temperatures. It is shown that the dynamical effects damp main peaks in the
densities of states (DOS) obtained by the local density approximation to the
density functional theory, reduce the band broadening due to thermal spin
fluctuations, create the Mott-Hubbard type bands in the case of fcc Mn and fcc
Fe, and create a small hump corresponding to the `6 eV' satellite in the case
of Co, Ni, and Cu. Calculated DOS explain the X-ray photoelectron spectroscopy
data as well as the bremsstrahlung isochromat spectroscopy data. Moreover, it
is found that screening effects on the exchange energy parameters are
significant for understanding the spectra in magnetic transition metals.Comment: To be published in Phys. Rev.
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
Strong HI Lyman- variations from the 11 Gyr-old host star Kepler-444: a planetary origin ?
Kepler-444 provides a unique opportunity to probe the atmospheric composition
and evolution of a compact system of exoplanets smaller than the Earth. Five
planets transit this bright K star at close orbital distances, but they are too
small for their putative lower atmosphere to be probed at optical/infrared
wavelengths. We used the Space Telescope Imaging Spectrograph instrument
onboard the Hubble Space Telescope to search for the signature of the planet's
upper atmospheres at six independent epochs in the Ly- line. We detect
significant flux variations during the transits of both Kepler-444e and f
(~20%), and also at a time when none of the known planets was transiting
(~40%). Variability in the transition region and corona of the host star might
be the source of these variations. Yet, their amplitude over short time scales
(~2-3 hours) is surprisingly strong for this old (11.2+-1.0Gyr) and apparently
quiet main-sequence star. Alternatively, we show that the in-transits
variations could be explained by absorption from neutral hydrogen exospheres
trailing the two outer planets (Kepler-444e and f). They would have to contain
substantial amounts of water to replenish such hydrogen exospheres, which would
reveal them as the first confirmed ocean-planets. The out-of-transit
variations, however, would require the presence of a yet-undetected Kepler-444g
at larger orbital distance, casting doubt on the planetary origin scenario.
Using HARPS-N observations in the sodium doublet, we derived the properties of
two Interstellar Medium clouds along the line-of-sight toward Kepler-444. This
allowed us to reconstruct the stellar Ly- line profile and to estimate
the XUV irradiation from the star, which would still allow for a moderate mass
loss from the outer planets after 11.2Gyr. Follow-up of the system at XUV
wavelengths will be required to assess this tantalizing possibility.Comment: Accepted for publication in A&A Name of the system added to the title
in most recent versio
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
The Earth as an extrasolar transiting planet - II: HARPS and UVES detection of water vapor, biogenic O, and O
The atmospheric composition of transiting exoplanets can be characterized
during transit by spectroscopy. For the transit of an Earth twin, models
predict that biogenic and should be detectable, as well as water
vapour, a molecule linked to habitability as we know it on Earth. The aim is to
measure the Earth radius versus wavelength - or the atmosphere
thickness - at the highest spectral resolution available to fully
characterize the signature of Earth seen as a transiting exoplanet. We present
observations of the Moon eclipse of 21-12-2010. Seen from the Moon, the Earth
eclipses the Sun and opens access to the Earth atmosphere transmission
spectrum. We used HARPS and UVES spectrographs to take penumbra and umbra
high-resolution spectra from 3100 to 10400 Ang. A change of the quantity of
water vapour above the telescope compromised the quality of the UVES data. We
corrected for this effect in the data processing. We analyzed the data by 3
different methods. The 1st method is based on the analysis of pairs of penumbra
spectra. The 2nd makes use of a single penumbra spectrum, and the 3rd of all
penumbra and umbra spectra. Profiles are obtained with the three
methods for both instruments. The 1st method gives the best result, in
agreement with a model. The second method seems to be more sensitive to the
Doppler shift of solar spectral lines with respect to the telluric lines. The
3rd method makes use of umbra spectra which bias the result, but it can be
corrected for this a posteriori from results with the first method. The 3
methods clearly show the spectral signature of the Rayleigh scattering in the
Earth atmosphere and the bands of HO, O, and O. Sodium is detected.
Assuming no atmospheric perturbations, we show that the E-ELT is theoretically
able to detect the A-band in 8~h of integration for an Earth twin at
10pc.Comment: Final version accepted for publication in A&A - 21 pages, 27 figures.
Abstract above slightly shortened wrt the original. The ArXiv version has low
resolution figures, but a version with full resolution figures is available
here:
http://www.obs-hp.fr/~larnold/publi_to_download/eclipse2010_AA_v5_final.pd
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