4,333 research outputs found
Casimir effect for massless minimally coupled scalar field between parallel plates in de Sitter spacetime
Casimir effect for massless minimally coupled scalar field is studied. An
explicit answer for de Sitter spacetime is obtained and analized. Cosmological
implications of the result are discussed.Comment: 7 pages, 2 figure
It is not all about being sweet: differences in floral traits and insect visitation among hybrid carrot cultivars
Cytoplasmically male-sterile (CMS) carrot cultivars suffer from low pollination rates. In this study, insect visitation varied more than eightfold between 17 CMS carrot cultivars in a field-based cultivar evaluation trial. The visitation rates of honey bees, nectar scarabs, muscoid flies, and wasps each significantly differed among these cultivars. No significant difference in visitation rates was observed among cultivars of different CMS type (brown-anther or petaloid) or flower colour, but cultivars of Berlicumer root type had significantly higher insect visitation rates than Nantes. Six cultivars were further compared in regard to selected umbel traits: as umbel diameter increased, so did the visitation of soldier beetles, while that of honey bees decreased. Finally, nectar of these six cultivars was analysed for sugar content, which revealed monosaccharides to be the most common sugars in all. There was high variation in the levels of sugars from individual umbellets but no significant difference in nectar sugar composition among cultivars, suggesting that nectar sugar composition is of minor importance regarding pollinator attraction to hybrid CMS carrot umbels
About the initial mass function and HeII emission in young starbursts
We demonstrate that it is crucial to account for the evolution of the
starburst population in order to derive reliable numbers of O stars from
integrated spectra for burst ages t > 2 - 3 Myr. In these cases the method of
Vacca & Conti (1992) and Vacca (1994) systematically underestimates the number
of O stars. Therefore the current WR/O number ratios in Wolf-Rayet (WR)
galaxies are overestimated. This questions recent claims about flat IMF slopes
(alpha ~ 1-2) in these objects. If the evolution of the burst is properly
treated we find that the observations are indeed compatible with a Salpeter
IMF, in agreement with earlier studies.
Including recent predictions from non-LTE, line blanketed model atmospheres
which account for stellar winds, we synthesize the nebular and WR HeII 4686
emission in young starbursts. For metallicities 1/5 <= Z/Z_sun <= 1 we predict
a strong nebular HeII emission due to a significant fraction of WC stars in
early WR phases of the burst. For other metallicities broad WR emission will
always dominate the HeII emission. Our predictions of the nebular HeII
intensity agree well with the observations in WR galaxies and an important
fraction of the giant HII regions where nebular HeII is detected. We propose
further observational tests of our result.Comment: ApJ Letters, accepted. 8 pages LaTeX including 3 PostScript figures,
uses AASTeX and psfig macros. PostScript file also available at
ftp://ftp.stsci.edu/outside-access/out.going/schaerer/imf.p
Strong electron-photon coupling in one-dimensional quantum dot chain: Rabi waves and Rabi wavepackets
We predict and theoretically investigate the new coherent effect of nonlinear
quantum optics -- spatial propagation of Rabi oscillations (Rabi waves) in
one-dimensional quantum dot (QD) chain. QD-chain is modeled by the set of
two-level quantum systems with tunnel coupling between neighboring QDs. The
space propagation of Rabi waves in the form of traveling waves and wave packets
is considered. It is shown, that traveling Rabi waves are quantum states of
QD-chain dressed by radiation. The dispersion characteristics of traveling Rabi
waves are investigated and their dependence on average number of photons in
wave is demonstrated. The propagation of Rabi wave packets is accompanied by
the transfer of the inversion and quantum correlations along the QD-chain and
by the transformation of quantum light statistics. The conditions of
experimental observability are analyzed. The effect can find practical use in
quantum computing and quantum informatics.Comment: 16 pages, 15 figure
van der Waals coupling in atomically doped carbon nanotubes
We have investigated atom-nanotube van der Waals (vdW) coupling in atomically
doped carbon nanotubes (CNs). Our approach is based on the perturbation theory
for degenerated atomic levels, thus accounting for both weak and strong
atom-vacuum-field coupling. The vdW energy is described by an integral equation
represented in terms of the local photonic density of states (DOS). By solving
it numerically, we demonstrate the inapplicability of standard
weak-coupling-based vdW interaction models in a close vicinity of the CN
surface where the local photonic DOS effectively increases, giving rise to an
atom-field coupling enhancement. An inside encapsulation of atoms into the CN
has been shown to be energetically more favorable than their outside adsorption
by the CN surface. If the atom is fixed outside the CN, the modulus of the vdW
energy increases with the CN radius provided that the weak atom-field coupling
regime is realized (i.e., far enough from the CN). For inside atomic position,
the modulus of the vdW energy decreases with the CN radius, representing a
general effect of the effective interaction area reduction with lowering the CN
curvature.Comment: 15 pages, 5 figure
Spontaneous decay dynamics in atomically doped carbon nanotubes
We report a strictly non-exponential spontaneous decay dynamics of an excited
two-level atom placed inside or at different distances outside a carbon
nanotube (CN). This is the result of strong non-Markovian memory effects
arising from the rapid variation of the photonic density of states with
frequency near the CN. The system exhibits vacuum-field Rabi oscillations, a
principal signature of strong atom-vacuum-field coupling, when the atom is
close enough to the nanotube surface and the atomic transition frequency is in
the vicinity of the resonance of the photonic density of states. Caused by
decreasing the atom-field coupling strength, the non-exponential decay dynamics
gives place to the exponential one if the atom moves away from the CN surface.
Thus, atom-field coupling and the character of the spontaneous decay dynamics,
respectively, may be controlled by changing the distance between the atom and
CN surface by means of a proper preparation of atomically doped CNs. This opens
routes for new challenging nanophotonics applications of atomically doped CN
systems as various sources of coherent light emitted by dopant atoms.Comment: 10 pages, 4 figure
Effects of a powered ankle prosthesis on shock absorption and residual limb/socket interface pressure
Thesis (S.M.)--Massachusetts Institute of Technology, School of Architecture and Planning, Program in Media Arts and Sciences, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 83-86).Lower-extremity amputees face potentially serious post-operative complications, including increased risk of further amputations, excessive stress on both limbs, and discomfort at the stump/socket interface. State of the art; passive prostheses have improved many negative consequences associated with lower-limb loss, but we believe the limit of uninformed elastic prostheses has been reached. Further strides require a more biomimetic approach. Through integration of "smart" technology (sensors and actuators), a new phase of bionic lower-limb prostheses is upon us, which enables prosthetic devices to more closely mimic biological behavior by generating human-like responses and power outputs. The closer we come to natural biology, gait abnormalities in amputees will decline. This project compares the first bionic ankle prosthesis to commonly used passive prostheses to determine how more biomimetic adaptability and work generation in the prosthetic joint affects discomfort and joint stress. We have put forth several metrics to describe discomfort (elements of shock absorption, pressure distribution, etc.) and will conduct level-ground walking tests with three unilateral amputee subjects using both passive and power devices. We hope to make a case for the pursuit of more biomimetic designs for rehabilitative devices, by showing a positive effect on "comfort" and a restoration of normal gait dynamics when using a bionic ankle prosthesis.by David Allen Hill.S.M
Milling plant and soil material in plastic tubes over-estimates carbon and under-estimates nitrogen concentrations
Peer reviewedPostprin
Graphene Sheets Stabilized on Genetically Engineered M13 Viral Templates as Conducting Frameworks for Hybrid Energy-Storage Materials
Utilization of the material-specific peptideâsubstrate interactions of M13 virus broadens colloidal stability window of graphene. The homogeneous distribution of graphene is maintained in weak acids and increased ionic strengths by complexing with virus. This graphene/virus conducting template is utilized in the synthesis of energy-storage materials to increase the conductivity of the composite electrode. Successful formation of the hybrid biological template is demonstrated by the mineralization of bismuth oxyfluoride as a cathode material for lithium-ion batteries, with increased loading and improved electronic conductivity.National Institute for International Education (Korea) (Korean Government Scholarship Program)United States. Army Research Office (Institute for Collaborative Biotechnologies (ICB))National Institutes of Health (U.S.) (Materials Research Science and Engineering Centers program
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