26,788 research outputs found
High efficiency compound semiconductor concentrator photovoltaics
Special emphasis was given to the high yield pilot production of packaged AlGaAs/GaAs concentrator solar cells, using organometallic VPE for materials growth, the demonstration of a concentrator module using 12 of these cells which achieved 16.4 percent conversion efficiency at 50 C coolant inlet temperature, and the demonstration of a spectral splitting converter module that achieved in excess of 20 percent efficiency. This converter employed ten silicon and ten AlGaAs cells with a dichroic filter functioning as the beam splitter. A monolithic array of AlGaAs/GaAs solar cells is described
Category-specific enhancement of retrieval due to field perspective
Two memory perspectives have been distinguished: A field perspective where events are re-experienced in the first person, and an observer perspective where events are witnessed in the third person. Two experiments examined the influence of memory perspective on objective memory performance. In both experiments participants were presented with a series of verbal passages, each of which contained several different categories of information. For four of these categories (pertaining to affective reactions, physical sensations, psychological states, and associated ideas), recall was significantly higher when a field perspective was adopted than when an observer perspective was adopted, but for the five other categories (pertaining to physical actions, personal appearance, fine details, spatial relations, and peripheral details) there was no significant effect of perspective upon recall. The study is examined in the context of mental models and imagined episodic events
Continuous Crystallization in Hexagonally-Ordered Materials
We demonstrate that the phase transition from columnar-hexagonal liquid
crystal to hexagonal-crystalline solid falls into an unusual universality
class, which in three-dimensional allows for both discontinuous transitions as
well as continuous transitions, characterized by a single set of exponents. We
show by a renormalization group calculation (to first order in ) that the critical exponents of the continuous transition are precisely
those of the XY model, which gives rise to a continuous evolution of elastic
moduli. Although the fixed points of the present model are found to be
identical to the XY model, the elastic compliance to deformations in the plane
of hexagonal order, , is nonetheless shown to critically influence the
crystallization transition, with the continuous transition being driven to
first order by fluctuations as the in plane response grows weaker, .Comment: 4 pages, 2 figures (revised version
Instability of a membrane intersecting a black hole
The stability of a Nambu-Goto membrane at the equatorial plane of the
Reissner-Nordstr{\o}m-de Sitter spacetime is studied. The covariant
perturbation formalism is applied to study the behavior of the perturbation of
the membrane. The perturbation equation is solved numerically. It is shown that
a membrane intersecting a charged black hole, including extremely charged one,
is unstable and that the positive cosmological constant strengthens the
instability.Comment: 12 pages, 3 figures, to be published in Physical Review
OM-VPE grown materials for high efficiency solar cells
Organometallic sources are available for all the III-V elements and a variety of dopants; thus it is possible to use the technique to grow a wide variety of semiconductor compounds. AlGaAsSb and AlGaInAs alloys for multijunction monolithic solar cells were grown by OM-VPE. While the effort concentrated on terrestrial applications, the success of OM-VPE grown GaAs/AlGaAs concentrator solar cells (23% at 400 suns) demonstrates that OM-VPE is suitable for growing high efficiency solar cells in large quantities for space applications. In addition, OM-VPE offers the potential for substantial cost reduction of photovoltaic devices with scale up and automation and due to high process yield from reproducible, uniform epitaxial growths with excellent surface morphology
Curvature Corrections to Dynamics of Domain Walls
The most usual procedure for deriving curvature corrections to effective
actions for topological defects is subjected to a critical reappraisal. A
logically unjustified step (leading to overdetermination) is identified and
rectified, taking the standard domain wall case as an illustrative example.
Using the appropriately corrected procedure, we obtain a new exact (analytic)
expression for the corresponding effective action contribution of quadratic
order in the wall width, in terms of the intrinsic Ricci scalar and the
extrinsic curvature scalar . The result is proportional to with the
coefficient given by . The resulting form of the ensuing dynamical
equations is obtained in terms of the second fundamental form and the
Dalembertian of its trace, K. It is argued that this does not invalidate the
physical conclusions obtained from the "zero rigidity" ansatz used in
previous work.Comment: 19 pages plain TeX, 2 figures include
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