756 research outputs found
Feather growth rate and mass in nearctic passerines with variablemigratory behavior and molt pattern
Bird species vary greatly in the duration of their annual complete feather molt. However, such variation is not well documented
in birds from many biogeographic areas, which restricts our understanding of the diversification of molt strategies. Recent research has revealed
that molt duration can be estimated in passerines from ptilochronology-based measurements of the growth rate of their tail feathers. We used
this approach to explore how molt duration varied in 98 Nearctic species that have different migratory strategies and molt patterns. As previously
documented for Palearctic species, migration was associated with a shortening of molt duration among species that molted during summer on
their breeding range. However, molts of winter-molting migratory species were as long as those of summer-molting sedentary species, which
suggests that winter molt also allows Nearctic migrants to avoid the temporal constraints experienced during summer. Our results also suggest
that migratory species that undergo a stopover molt within the Mexican monsoon region have the shortest molt duration among all Nearctic
passerines. Interestingly, and contrary to expectations from a potential tradeoff between molt duration and feather quality, observed variation
in feather growth rate was positively correlated with differences in tail feather mass, which may be caused by differences among groups in the
availability of resources for molting. We encourage the use of similar approaches to study the variation in molt duration in other geographic areas
where knowledge of the evolution of molt is limited.
Noncommutative quantum mechanics -- a perspective on structure and spatial extent
We explore the notion of spatial extent and structure, already alluded to in
earlier literature, within the formulation of quantum mechanics on the
noncommutative plane. Introducing the notion of average position and its
measurement, we find two equivalent pictures: a constrained local description
in position containing additional degrees of freedom, and an unconstrained
nonlocal description in terms of the position without any other degrees of
freedom. Both these descriptions have a corresponding classical theory which
shows that the concept of extended, structured objects emerges quite naturally
and unavoidably there. It is explicitly demonstrated that the conserved energy
and angular momentum contain corrections to those of a point particle. We argue
that these notions also extend naturally to the quantum level. The local
description is found to be the most convenient as it manifestly displays
additional information about structure of quantum states that is more subtly
encoded in the nonlocal, unconstrained description. Subsequently we use this
picture to discuss the free particle and harmonic oscillator as examples.Comment: 25 pages, no figure
Harmonic oscillator in a background magnetic field in noncommutative quantum phase-space
We solve explicitly the two-dimensional harmonic oscillator and the harmonic
oscillator in a background magnetic field in noncommutative phase-space without
making use of any type of representation. A key observation that we make is
that for a specific choice of the noncommutative parameters, the time reversal
symmetry of the systems get restored since the energy spectrum becomes
degenerate. This is in contrast to the noncommutative configuration space where
the time reversal symmetry of the harmonic oscillator is always broken.Comment: 7 pages Late
Formulation, Interpretation and Application of non-Commutative Quantum Mechanics
In analogy with conventional quantum mechanics, non-commutative quantum
mechanics is formulated as a quantum system on the Hilbert space of
Hilbert-Schmidt operators acting on non-commutative configuration space. It is
argued that the standard quantum mechanical interpretation based on Positive
Operator Valued Measures, provides a sufficient framework for the consistent
interpretation of this quantum system. The implications of this formalism for
rotational and time reversal symmetry are discussed. The formalism is applied
to the free particle and harmonic oscillator in two dimensions and the physical
signatures of non commutativity are identified.Comment: 11 page
Isolation of cDNAs of scrapie-modulated RNAs by subtractive hybridization of a cDNA library.
We have developed a subtractive cloning procedure based on the hybridization of single-stranded cDNA libraries constructed in pi H3M, a vector containing the phage M13 origin of replication. We have used this strategy to isolate three transcripts whose abundance is increased in scrapie-infected brain. DNA sequence analysis showed that they represent glial fibrillary acidic protein, metallothionein II, and the B chain of alpha-crystallin; the latter two may represent a response to stress
High-energy terahertz pulses from semiconductors pumped beyond the three-photon absorption edge
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Ultrafast modulation of the chemical potential in BaFe2As2 by coherent phonons
Time- and angle-resolved extreme ultraviolet photoemission spectroscopy is used to study the electronic structure dynamics in BaFe2As2 around the high-symmetry points Γ and M. A global oscillation of the Fermi level at the frequency of the A1g(As) phonon mode is observed. It is argued that this behavior reflects a modulation of the effective chemical potential in the photoexcited surface region that arises from the high sensitivity of the band structure near the Fermi level to the A1g(As) phonon mode combined with a low electron diffusivity perpendicular to the layers. The results establish a novel way to tune the electronic properties of iron pnictides: coherent control of the effective chemical potential. The results further suggest that the equilibration time for the effective chemical potential needs to be considered in the ultrafast electronic structure dynamics of materials with weak interlayer coupling. © 2014 American Physical Society
High-energy terahertz pulses from semiconductors pumped beyond the three-photon absorption edge
Coherent States on Hilbert Modules
We generalize the concept of coherent states, traditionally defined as
special families of vectors on Hilbert spaces, to Hilbert modules. We show that
Hilbert modules over -algebras are the natural settings for a
generalization of coherent states defined on Hilbert spaces. We consider those
Hilbert -modules which have a natural left action from another
-algebra say, . The coherent states are well defined in this
case and they behave well with respect to the left action by .
Certain classical objects like the Cuntz algebra are related to specific
examples of coherent states. Finally we show that coherent states on modules
give rise to a completely positive kernel between two -algebras, in
complete analogy to the Hilbert space situation. Related to this there is a
dilation result for positive operator valued measures, in the sense of Naimark.
A number of examples are worked out to illustrate the theory
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