31,519 research outputs found
Solar energy conversion through the interaction of plasmons with tunnel junctions. Part A: Solar cell analysis. Part B: Photoconductor analysis
A solar cell utilizing guided optical waves and tunnel junctions was analyzed to determine its feasibility. From this analysis, it appears that the limits imposed upon conventional multiple cell systems also limit this solar cell. Due to this limitation, it appears that the relative simplicity of the conventional multiple cell systems over the solar cell make the conventional multiple cell systems the more promising candidate for improvement. It was discovered that some superlattice structures studied could be incorporated into an infrared photodetector. This photoconductor appears to be promising as a high speed, sensitive (high D sup star sub BLIP) detector in the wavelength range from 15 to over 100 micrometers
Microgravity Particle Research on the Space Station
Science questions that could be addressed by a Space Station Microgravity Particle Research Facility for studying small suspended particles were discussed. Characteristics of such a facility were determined. Disciplines covered include astrophysics and the solar nebula, planetary science, atmospheric science, exobiology and life science, and physics and chemistry
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A positive relationship between the abundance of ammonia oxidizing archaea and natural abundance δ15N of ecosystems
We present a significant relationship between the natural abundance isotopic composition of ecosystem pools and the abundance of a microbial gene. Natural abundance 15N of soils and soil DNA were analysed and compared with archaeal ammonia oxidizer abundance along an elevation gradient in northern Arizona and along a substrate age gradient in Hawai'i. There was a significant positive correlation between the abundance of archaeal amoA genes and natural abundance δ15N of total soil or DNA suggesting that ammonia oxidizing archaea play an important role in ecosystem N release. © 2013 Elsevier Ltd
Explaining the visible and the invisible: Public knowledge of genetics, ancestry, physical appearance and race in Colombia
This is the final version of the article. Available from SAGE Publications via the DOI in this record.Using data from focus groups conducted in Colombia, we explore how educated lay audiences faced with scenarios about ancestry and genetics draw on widespread and dominant notions of nation, race and belonging in Colombia to ascribe ancestry to collectivities and to themselves as individuals. People from a life sciences background tend to deploy idioms of race and genetics more readily than people from a humanities and race-critical background. When they considered individuals, people tempered or domesticated the more mechanistic explanations about racialized physical appearance, ancestry and genetics that were apparent at the collective level. Ideas of the latency and manifestation of invisible traits were an aspect of this domestication. People ceded ultimate authority to genetic science, but deployed it to work alongside what they already knew. Notions of genetic essentialism co-exist with the strategic use of genetic ancestry in ways that both fix and unfix race. Our data indicate the importance of attending to the different epistemological stances through which people define authoritative knowledge and to the importance of distinguishing the scale of resolution at which the question of diversity is being posed.This article arises out of two projects: ‘Race, genomics and mestizaje (mixture) in Latin America: a comparative approach’ funded by the Economic and Social Research Council (ESRC) (grant RES-062-23-1914) and ‘Public engagement with genomic research and race in Latin America’ funded by The Leverhulme Trust (grant RPG-044)
Roughness of tensile crack fronts in heterogenous materials
The dynamics of planar crack fronts in heterogeneous media is studied using a
recently proposed stochastic equation of motion that takes into account
nonlinear effects. The analysis is carried for a moving front in the
quasi-static regime using the Self Consistent Expansion. A continuous dynamical
phase transition between a flat phase and a dynamically rough phase, with a
roughness exponent , is found. The rough phase becomes possible due
to the destabilization of the linear modes by the nonlinear terms. Taking into
account the irreversibility of the crack propagation, we infer that the
roughness exponent found in experiments might become history-dependent, and so
our result gives a lower bound for .Comment: 7 page
Development of high critical current density in multifilamentary round-wire Bi2Sr2CaCu2O8+x by strong overdoping
Bi2Sr2CaCu2O8+x is the only cuprate superconductor that can be made into a
round-wire conductor form with a high enough critical current density Jc for
applications. Here we show that the Jc(5 T,4.2 K) of such Ag-sheathed
filamentary wires can be doubled to more than 1.4x10^5 A/cm^2 by low
temperature oxygenation. Careful analysis shows that the improved performance
is associated with a 12 K reduction in transition temperature Tc to 80 K and a
significant enhancement in intergranular connectivity. In spite of the
macroscopically untextured nature of the wire, overdoping is highly effective
in producing high Jc values.Comment: 4 figure
Report on a Periodic Extrinsic Infrared (PEIR) Photoconductor
An infrared photoconductor, designated as the Periodic Extrinsic InfraRed (PEIR) photoconductor, is proposed. A PEIR photoconductor will be useful for detecting wavelengths from 7 μm (1400 cm-1) to longer than 100 μm (100 cm-1). Through epitaxial growth, a PEIR photoconductor is made up of heavily doped layers separated by lightly doped layers. The heavily doped layers are doped such that an impurity band forms but are not doped high enough to cause the impurity band to merge with the conduction or valence band. The lightly doped layers are used to confine the carriers in the impurity bands and consequently, conduction can only occur due to carriers excited to the conduction (n-type device) or valence (p-type device) band. Radiation excites the carriers from the impurity band to the conduction or valence band. The impurity band layers are thin enough that even if the electric field in the impurity band layers is small, there is a high probability the excited carrier will scatter into the lightly doped layer and be swept away by the electric field in the lightly doped layer, A PEIR photoconductor will have two major advantages. First, the absorption coefficient will be high because of the high impurity concentration in the impurity band layers. The absorption coefficient will be from 103cm-1 to as high as 104cm-1. Additionally, a method has been found to approximately determine the highest absorption coefficient attainable in specific host semiconductor:impurity dopant materials systems. Consequently, one can determine the optimum host semiconductor:impurity dopant system to be used in a PEIR photoconductor designed to detect a certain wavelength. Second, some host semiconductors that are being considered are Si and GaAs, which are much easier to work with than HgCdTe (the material of choice for intrinsic photoconductors at wavelengths longer than 7 μm)
Complete control of a matter qubit using a single picosecond laser pulse
We demonstrate for the first time that a matter physical two level system, a
qubit, can be fully controlled using one ultrafast step. We show that the spin
state of an optically excited electron, an exciton, confined in a quantum dot,
can be rotated by any desired angle, about any desired axis, during such a
step. For this we use a single, resonantly tuned, picosecond long, polarized
optical pulse. The polarization of the pulse defines the rotation axis, while
the pulse detuning from a non-degenerate absorption resonance, defines the
magnitude of the rotation angle. We thereby achieve a high fidelity, universal
gate operation, applicable to other spin systems, using only one short optical
pulse. The operation duration equals the pulse temporal width, orders of
magnitude shorter than the qubit evolution life and coherence times.Comment: main text: 4 pages, 3 figures Supplemental material: 3 pages, 1
figur
About Superluminal motions and Special Relativity: A Discussion of some recent Experiments, and the solution of the Causal Paradoxes
Some experiments, performed at Berkeley, Cologne, Florence, Vienna, Orsay,
Rennes, etc., led to the claim that something seems to travel with a group
velocity larger than the speed c of light in vacuum. Various other experimental
results seem to point in the same direction: For instance, localized wavelet-
type solutions to Maxwell equations have been found, both theoretically and
experimentally, that travel with superluminal speed. [Even muonic and
electronic neutrinos [it has been proposed] might be "tachyons", since their
square mass appears to be negative]. With regard to the first-mentioned
experiments, it was recently claimed by Guenter Nimtz that those results with
evanescent waves (or tunneling photons) imply superluminal signal and impulse
transmission, and therefore violate Einstein causality. In this note we want to
stress that, on the contrary, all such results do not place relativistic
causality in jeopardy, even if they referred to actual tachyonic motions: In
fact, Special Relativity can cope even with superluminal objects and waves. For
instance, it is possible (at least in microphysics) to solve also the known
causal paradoxes, devised for faster than light motion, although this is not
widely recognized yet. Here we show, in detail and rigorously, how to solve the
oldest causal paradox, originally proposed by Tolman, which is the kernel of
many further tachyon paradoxes (like J.Bell's, F.A.E.Pirani's, J.D.Edmonds' and
others'). The key to the solution is a careful application of tachyon
mechanics, as it unambiguously follows from special relativity. At Last, in one
of the two Appendices, we propose how to evaluate the group-velocity in the
case of evanescent waves. [PACS nos.: 03.30.+p; 03.50.De; 41.20.Jb; 73.40.Gk;
84.40.Az; 42.82.Et ]Comment: LaTeX file: 26 pages, with 5 Figures (and two Appendices). The
original version of this paper appeared in the Journal below
Effects of impurities on radiation damage of silicon solar cells
Impurities effects on radiation damage of silicon solar cell
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