14,628 research outputs found
Spectroscopy of drums and quantum billiards: perturbative and non-perturbative results
We develop powerful numerical and analytical techniques for the solution of
the Helmholtz equation on general domains. We prove two theorems: the first
theorem provides an exact formula for the ground state of an arbirtrary
membrane, while the second theorem generalizes this result to any excited state
of the membrane. We also develop a systematic perturbative scheme which can be
used to study the small deformations of a membrane of circular or square
shapes. We discuss several applications, obtaining numerical and analytical
results.Comment: 29 pages, 12 figures, 7 tabl
Development and fabrication of radiation resistant high efficiency solar cells Final periodic report, 24 Aug. 1966 - 24 Feb. 1967
Diffusion process for lithium doping of solar cells, and electrical characteristics and radiation resistance of solar cell
Radio Continuum Study of Supernova Remnants in the Large Magellanic Cloud - SNR J0519-6926
We present the results of new high resolution ATCA observations of SNR
J0519-6926. We found that this SNR exhibits a typical "horseshoe" appearance
with alpha = -0.55 +- 0.08 and D=28+-1 pc. No polarization (or magnetic fields)
are detected to a level of 1%. This is probably due to a relatively poor
sampling of the uv plane caused be observing in "snap-shot" mode.Comment: 6 pages 4 figures, to be published in Serbian Astronomical Journa
Bounds for solutions of a class of quasilinear elliptic boundary value problems in terms of the torsion function
In this paper maximum principles are employed to relate solutions of certain classes of nonlinear elliptic problems to solutions of the associated torsion problem. By this method a number of new isoperimetric inequalities are derived. In special cases solutions of the nonlinear problems are also related to solutions of the clamped membrane proble
Fabrication and evaluation of advanced titanium and composite structural panels
Advanced manufacturing methods for titanium and composite material structures are being developed and evaluated. The focus for the manufacturing effort is the fabrication of full-scale structural panels which replace an existing shear panel on the upper wing surface of the NASA YF-12 aircraft. The program involves design, fabrication, ground testing, and Mach 3 flight service of full-scale structural panels and laboratory testing of representative structural element specimens
A lower bound to the spectral threshold in curved tubes
We consider the Laplacian in curved tubes of arbitrary cross-section rotating
together with the Frenet frame along curves in Euclidean spaces of arbitrary
dimension, subject to Dirichlet boundary conditions on the cylindrical surface
and Neumann conditions at the ends of the tube. We prove that the spectral
threshold of the Laplacian is estimated from below by the lowest eigenvalue of
the Dirichlet Laplacian in a torus determined by the geometry of the tube.Comment: LaTeX, 13 pages; to appear in R. Soc. Lond. Proc. Ser. A Math. Phys.
Eng. Sc
The Strongest 100 Point Radio Sources in the LMC at 1.4 GHz
We present the 100 strongest 1.4 GHz point sources from a new mosaic image in
the direction of the Large Magellanic Cloud (LMC). The observations making up
the mosaic were made over a ten year period and were combined with Parkes
single dish data at 1.4 GHz to complete the image for short spacing. An initial
list of co-identifications within 10" at 0.843, 4.8 and 8.6 GHz consisted of
2682 sources. Elimination of extended objects and artifact noise allowed the
creation of a refined list containing 1988 point sources. Most of these are
presumed to be background objects seen through the LMC; a small portion may
represent compact H II regions, young SNRs and radio planetary nebulae. For the
1988 point sources we find a preliminary average spectral index of -0.53 and
present a 1.4 GHz image showing source location in the direction of the LMC.Comment: 8 pages, 2 figure
New Consequences of Induced Transparency in a Double-Lambda scheme: Destructive Interference In Four-wave Mixing
We investigate a four-state system interacting with long and short laser
pulses in a weak probe beam approximation. We show that when all lasers are
tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM)
field is strongly absorbed. The part which is not absorbed has the exact
intensity required to destructively interfere with the excitation pathway
involved in producing the FWM state. We show that with this three-photon
destructive interference, the conversion efficiency can still be as high as
25%. Contrary to common belief,our calculation shows that this process, where
an ideal one-photon electromagnetically induced transparency is established, is
not most suitable for high efficiency conversion. With appropriate
phase-matching and propagation distance, and when the three-photon destructive
interference does not occur, we show that the photon flux conversion efficiency
is independent of probe intensity and can be close to 100%. In addition, we
show clearly that the conversion efficiency is not determined by the maximum
atomic coherence between two lower excited states, as commonly believed. It is
the combination of phase-matching and constructive interference involving the
two terms arising in producing the mixing wave that is the key element for the
optimized FWM generation. Indeed, in this scheme no appreciable excited state
is produced, so that the atomic coherence between states |0> and |2> is always
very small.Comment: Submitted to Phys. Rev. A, 7 pages, 4 figure
The accelerating influence of humans on mammalian macroecological patterns over the late Quaternary
The transition of hominins to a largely meat-based diet ~1.8 million years ago led to the exploitation of other mammals for food and resources. As hominins, particularly archaic and modern humans, became increasingly abundant and dispersed across the globe, a temporally and spatially transgressive extinction of large-bodied mammals followed; the degree of selectivity was unprecedented in the Cenozoic fossil record. Today, most remaining large-bodied mammal species are confined to Africa, where they coevolved with hominins. Here, using a comprehensive global dataset of mammal distribution, life history and ecology, we examine the consequences of “body size downgrading” of mammals over the late Quaternary on fundamental macroecological patterns. Specifically, we examine changes in species diversity, global and continental body size distributions, allometric scaling of geographic range size with body mass, and the scaling of maximum body size with area. Moreover, we project these patterns toward a potential future scenario in which all mammals currently listed as vulnerable on the IUCN\u27s Red List are extirpated. Our analysis demonstrates that anthropogenic impact on earth systems predates the terminal Pleistocene and has grown as populations increased and humans have become more widespread. Moreover, owing to the disproportionate influence on ecosystem structure and function of megafauna, past and present body size downgrading has reshaped Earth\u27s biosphere. Thus, macroecological studies based only on modern species yield distorted results, which are not representative of the patterns present for most of mammal evolution. Our review supports the concept of benchmarking the “Anthropocene” with the earliest activities of Homo sapiens
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