28,840 research outputs found
Phonon Diodes and Transistors from Magneto-acoustics
By sculpting the magnetic field applied to magneto-acoustic materials,
phonons can be used for information processing. Using a combination of analytic
and numerical techniques, we demonstrate designs for diodes (isolators) and
transistors that are independent of their conventional, electronic formulation.
We analyze the experimental feasibility of these systems, including the
sensitivity of the circuits to likely systematic and random errors.Comment: 5 pages, 4 figure
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Learning salience amoung [sic] features through contingency in the CEL framework
Determining which features in an environment are salient given a task, salience assignment, is a central problem in Machine Learning. A related phenomenon, contingency (the conditions under which relative salience among environmental features is acquired), is central to learning and memory in animal psychology. This paper presents an analysis of a set of empirical data on contingency and an algorithm for the salience assignment problem. The algorithm presented is implemented in a working computer program which interacts with a simulated environment to produce contingent associative learning corresponding to relevant behavioral data. The model also makes specific empirical predictions that can be experimentally tested
Quantum Monte Carlo for minimum energy structures
We present an efficient method to find minimum energy structures using energy
estimates from accurate quantum Monte Carlo calculations. This method involves
a stochastic process formed from the stochastic energy estimates from Monte
Carlo that can be averaged to find precise structural minima while using
inexpensive calculations with moderate statistical uncertainty. We demonstrate
the applicability of the algorithm by minimizing the energy of the H2O-OH-
complex and showing that the structural minima from quantum Monte Carlo
calculations affect the qualitative behavior of the potential energy surface
substantially.Comment: 7 pages, 4 figure
New and Noteworthy Plants of Texas
Rhexia alifanus (Melastomataceae) and Rhynchospora inundatum (Cyperaceae), both previously reported in the state, are documented as part of the flora of the state. Ratibida pinnata (Asteraceae), Tradescantia fluminensis (Commelinaceae) and Saccharum ravennae (Poaceae) are reported as new to Texas. Phytologia 92(2): 249-255 (August 2, 2010).Waller Creek Working Grou
Umbral Moonshine and the Niemeier Lattices
In this paper we relate umbral moonshine to the Niemeier lattices: the 23
even unimodular positive-definite lattices of rank 24 with non-trivial root
systems. To each Niemeier lattice we attach a finite group by considering a
naturally defined quotient of the lattice automorphism group, and for each
conjugacy class of each of these groups we identify a vector-valued mock
modular form whose components coincide with mock theta functions of Ramanujan
in many cases. This leads to the umbral moonshine conjecture, stating that an
infinite-dimensional module is assigned to each of the Niemeier lattices in
such a way that the associated graded trace functions are mock modular forms of
a distinguished nature. These constructions and conjectures extend those of our
earlier paper, and in particular include the Mathieu moonshine observed by
Eguchi-Ooguri-Tachikawa as a special case. Our analysis also highlights a
correspondence between genus zero groups and Niemeier lattices. As a part of
this relation we recognise the Coxeter numbers of Niemeier root systems with a
type A component as exactly those levels for which the corresponding classical
modular curve has genus zero.Comment: 181 pages including 95 pages of Appendices; journal version, minor
typos corrected, Research in the Mathematical Sciences, 2014, vol.
Stress effects on the Raman spectrum of an amorphous material: theory and experiment on a-Si:H
Strain in a material induces shifts in vibrational frequencies, which is a
probe of the nature of the vibrations and interatomic potentials, and can be
used to map local stress/strain distributions via Raman microscopy. This method
is standard for crystalline silicon devices, but due to lack of calibration
relations, it has not been applied to amorphous materials such as hydrogenated
amorphous silicon (a-Si:H), a widely studied material for thin-film
photovoltaic and electronic devices. We calculated the Raman spectrum of a-Si:H
\ab initio under different strains and found peak shifts . This
proportionality to the trace of the strain is the general form for isotropic
amorphous vibrational modes, as we show by symmetry analysis and explicit
computation. We also performed Raman measurements under strain and found a
consistent coefficient of . These results
demonstrate that a reliable calibration for the Raman/strain relation can be
achieved even for the broad peaks of an amorphous material, with similar
accuracy and precision as for crystalline materials.Comment: 12 pages, 3 figures + supplementary 8 pages, 4 figure
Aviation security and terrorism: a review of the economic issues
Following the terrorist attacks on September 11, 2001, the passage of the Aviation and Transportation Act mandated a substantial increase in resources devoted to aviation security. This paper summarizes the specific changes stemming from this legislation. In addition, the paper examines the economic issues underlying the regulation and provision of aviation security. The fact that security at one airport can affect the well being of those at other airports and elsewhere, an example of a network externality (spillover), provides an economic justification for governmental involvement in aviation security. A fundamental question is whether the federal role should be restricted to setting and monitoring security standards or whether the role should also include the financing and implementation of security. A controversial change is that the federal government has assumed responsibility from the airlines and airports for the actual provision of aviation security. Proponents of this change argue that, relative to private provision, public provision reduces the incentives to reduce quality through cost reductions. On the other hand, a public agency might not provide security services efficiently because it can operate in a more-or-less monopolistic way. Furthermore, a public agency might provide an excessive amount of security and incur unnecessary expenses because it is likely to be judged on its security record and not on all the attributes encompassed by air transportation services for consumers. Thus, economic theory does not provide a clear answer to what is likely to be a continuing source of controversy - the appropriate scope of governmental involvement in aviation security.Economic conditions ; War - Economic aspects
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