8,805 research outputs found
Ultra-dense phosphorus in germanium delta-doped layers
Phosphorus (P) in germanium (Ge) delta-doped layers are fabricated in
ultra-high vacuum by adsorption of phosphine molecules onto an atomically flat
clean Ge(001) surface followed by thermal incorporation of P into the lattice
and epitaxial Ge overgrowth by molecular beam epitaxy. Structural and
electrical characterizations show that P atoms are confined, with minimal
diffusion, into an ultra-narrow 2-nm-wide layer with an electrically-active
sheet carrier concentration of 4x10^13 cm-2 at 4.2 K. These results open up the
possibility of ultra-narrow source/drain regions with unprecedented carrier
densities for Ge n-channel field effect transistors
Maximizing phonon thermal conductance for ballistic membranes
At low temperatures, phonon scattering can become so weak that phonon
transport becomes ballistic. We calculate the ballistic phonon conductance G
for membranes using elasticity theory, considering the transition from three to
two dimensions. We discuss the temperature and thickness dependence and
especially concentrate on the issue of material parameters. For all membrane
thicknesses, the best conductors have, counter-intuitively, the lowest speed of
sound.Comment: 4 pages, 4 figures, proceedings to phonons 2007 conferenc
Combinatorial Bounds and Characterizations of Splitting Authentication Codes
We present several generalizations of results for splitting authentication
codes by studying the aspect of multi-fold security. As the two primary
results, we prove a combinatorial lower bound on the number of encoding rules
and a combinatorial characterization of optimal splitting authentication codes
that are multi-fold secure against spoofing attacks. The characterization is
based on a new type of combinatorial designs, which we introduce and for which
basic necessary conditions are given regarding their existence.Comment: 13 pages; to appear in "Cryptography and Communications
The Location of the Nucleus of NGC 1068 and the Three-dimensional Structure of Its Nuclear Region
The HST archival UV imaging polarimetry data of NGC 1068 is re-examined.
Through an extensive estimation of the observational errors, we discuss whether
the distribution of the position angles (PAs) of polarization is simply
centrosymmetric or not. Taking into account the effect of a bad focus at the
time of the observation, we conclude that, within the accuracy of HST/FOC
polarimetry, the PA distribution is completely centrosymmetric. This means that
the UV polarization originates only from scattering of the radiation from a
central point-like source.
However, our analysis shows that the most probable location of the nucleus is
only ~0.''08 (~6pc) south from the brightest cloud called ``cloud B''. The
error circle of 99% confidence level extends to cloud B and to ``cloud A''
which is about 0.''2 south of cloud B. By this FOC observation, Cloud B is only
marginally rejected as the nucleus.
Assuming that the UV flux is dominated by electron-scattered light, we have
also derived a three-dimensional structure of the nuclear region. The inferred
distribution suggests a linear structure which could be related to the radio
jet.Comment: 19 pages, 14 figures, to be published in the Astrophysical Journa
Using frost sensitive cover crops for timely nitrogen mineralisation and soil moisture management
Cover crops can be utilised to lower soil nitrate leaching. However, depending on the species grown and cover crop termination management this may lead to nitrogen (N) immobilisation and/or depletion of soil moisture available to the following cash crop, potentially impacting on crop yields. Cover crop management is also dependent on using herbicides to terminate growth prior to planting the next crop. We used an alternative method for cover crop termination by capitalising on plant senescence by frost in a multi-species cover crop established over-winter between wheat and maize. The cover crops accumulated greater quantities of N than the control. However, upon cover crop senescence due to cold temperatures, the partially terminated cover crop significantly increased topsoil available N from December to late February. This available N in the topsoil could be susceptible to leaching although this was not observed in our study. Cover crops did not have a significant prolonged effect on soil moisture over winter and late spring. The following maize yields were not significantly different between the control and cover crop treatment. Frost sensitive cover crop species could not be reliably terminated under a temperate climate, but provided a continuous supply of soil available N as the plants senesced. Depending on the soil moisture and weather conditions in the spring there could be a N leaching risk although this could be mitigated by establishing early spring crops
Remote terminal system evaluation
An Earth Resources Data Processing System was developed to evaluate the system for training, technology transfer, and data processing. In addition to the five sites included in this project two other sites were connected to the system under separate agreements. The experience of these two sites is discussed. The results of the remote terminal project are documented in seven reports: one from each of the five project sites, Purdue University, and an overview report summarizing the other six reports
Life on ice, Antarctica and Mars
The study of the origin of life and the prospects for human exploration of Mars are two themes developed in a new 57-minute film, Life on Ice, Antarctica, and Mars, produced by the InnerSpace Foundation and WHRO Television for broadcast by the Public Broadcasting System (PBS). A brief explanation of the film and how it relates to the future human exploration of space is presented
Rapid Evolution of Testis Size Relative to Sperm Morphology Suggests that Post-Copulatory Selection Targets Sperm Number in \u3cem\u3eAnolis\u3c/em\u3e Lizards
Post-copulatory sexual selection is thought to be responsible for much of the extraordinary diversity in sperm morphology across metazoans. However, the extent to which post-copulatory selection targets sperm morphology versus sperm production is generally unknown. To address this issue, we simultaneously characterized the evolution of sperm morphology (length of the sperm head, midpiece and flagellum) and testis size (a proxy for sperm production) across 26 species of Anolis lizards, a group in which sperm competition is likely. We found that the length of the sperm midpiece has evolved 2–3 times faster than that of the sperm head or flagellum, suggesting that midpiece size may be the most important aspect of sperm morphology with respect to post-copulatory sexual selection. However, testis size has evolved faster than any aspect of sperm morphology or body size, supporting the hypothesis that post-copulatory sexual selection acts more strongly upon sperm production than upon sperm morphology. Likewise, evolutionary increases in testis size, which typically indicate increased sperm competition, are not associated with predictable changes in sperm morphology, suggesting that any effects of post-copulatory selection on sperm morphology are either weak or variable in direction across anoles. Collectively, our results suggest that sperm production is the primary target of post-copulatory sexual selection in this lineage
Free-space quantum key distribution
A working free-space quantum key distribution (QKD) system has been developed
and tested over a 205-m indoor optical path at Los Alamos National Laboratory
under fluorescent lighting conditions. Results show that free-space QKD can
provide secure real-time key distribution between parties who have a need to
communicate secretly.Comment: 5 pages, 2 figures, 2 tables. To be published in Physical review A on
or about 1 April 199
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