2,169 research outputs found
Coherent Exciton Lasing in ZnSe/ZnCdSe Quantum Wells?
A new mechanism for exciton lasing in ZnSe/ZnCdSe quantum wells is proposed.
Lasing, occurring below the lowest exciton line, may be associated with a
BCS-like condensed (coherent) exciton state. This state is most stable at low
temperatures for densities in the transition region separating the exciton Bose
gas and the coherent exciton state. Calculations show the gain region to lie
below the exciton line and to be separated from the absorption regime by a
transparency region of width, for example, about 80 meV for a 90 Angstrom
ZnSe/Zn_(0.75)Cd_(0.25)Se quantum well. Experimental observation of the
transparency region using differential spectroscopy would confirm this picture.Comment: 9 pages + 3 figs contained in 4 postscript files to appear Appl.
Phys. Lett. March 13, 199
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
The thermal conductivity reduction in HgTe/CdTe superlattices
The techniques used previously to calculate the three-fold thermal
conductivity reduction due to phonon dispersion in GaAs/AlAs superlattices
(SLs) are applied to HgTe/CdTe SLs. The reduction factor is approximately the
same, indicating that this SL may be applicable both as a photodetector and a
thermoelectric cooler.Comment: 5 pages, 2 figures; to be published in Journal of Applied Physic
Preface to the Special Issue on âThe Impact of Resilience in Developing Individual and Organizational Capacity to âBounce Backâ from Challengesâ
In this Preface to the Special Issue on âThe Impact of Resilience in Developing Individual and Organizational Capacity to âBounce Backâ from Challengesâ, I introduce the need to study resilience in HRD, and lay out the most fundamental concerns surrounding the use of the term resilience in contemporary workplaces and scientific discourse. I also introduce the papers published in the Special Issue, and link them to the overall narrative around resilience at work
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
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
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
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
Older Adolescentsâ Understanding of Participant Rights in the BlackBerry Project, a Longitudinal Ambulatory Assessment Study
For a longâterm, longitudinal study that used BlackBerry smartphones for passive ambulatory assessment among older adolescents, this study focused on three areas of ethical concern: (1) adolescentsâ competence to give assent; (2) understanding of confidentiality, the protection of information, and project goals; and (3) awareness of procedures and benefits, and comfort with the research design. One hundred and seventyâeight participants were 17 and 18 years old (84 girls). Results suggested that participants freely gave consent and understood most, but not all of the informed consent information. Participants reported a high level of satisfaction. Participants showed less understanding of when their confidentiality would be broken and how data would be protected
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