8,343 research outputs found
Thermal conduction in molecular chains: Non-Markovian effects
We study the effect of non-Markovian reservoirs on the heat conduction
properties of short to intermediate size molecular chains. Using classical
molecular dynamics simulations, we show that the distance dependence of the
heat current is determined not only by the molecular properties, rather it is
also critically influenced by the spectral properties of the heat baths for
both harmonic and anharmonic molecular chains. For highly correlated reservoirs
the current of an anharmonic chain may exceed the flux of the corresponding
harmonic system. Our numerical results are accompanied by a simple single-mode
heat conduction model that can capture the intricate distance dependence
obtained numerically
Quantum heat transfer in harmonic chains with self consistent reservoirs: Exact numerical simulations
We describe a numerical scheme for exactly simulating the heat current
behavior in a quantum harmonic chain with self-consistent reservoirs.
Numerically-exact results are compared to classical simulations and to the
quantum behavior under the linear response approximation. In the classical
limit or for small temperature biases our results coincide with previous
calculations. At large bias and for low temperatures the quantum dynamics of
the system fundamentally differs from the close-to-equilibrium behavior,
revealing in particular the effect of thermal rectification for asymmetric
chains. Since this effect is absent in the classical analog of our model, we
conclude that in the quantum model studied here thermal rectification is a
purely quantum phenomenon, rooted in the quantum statistics
Twisted equivariant K-theory, groupoids and proper actions
In this paper we define twisted equivariant K-theory for actions of Lie
groupoids. For a Bredon-compatible Lie groupoid, this defines a periodic
cohomology theory on the category of finite CW-complexes with equivariant
stable projective bundles. A classification of these bundles is shown. We also
obtain a completion theorem and apply these results to proper actions of
groups.Comment: 26 page
Models of Civil-Military Relationships at the Elite Level
http://deepblue.lib.umich.edu/bitstream/2027.42/50840/1/58.pd
Testing the Hubble Law with the IRAS 1.2 Jy Redshift Survey
We test and reject the claim of Segal et al. (1993) that the correlation of
redshifts and flux densities in a complete sample of IRAS galaxies favors a
quadratic redshift-distance relation over the linear Hubble law. This is done,
in effect, by treating the entire galaxy luminosity function as derived from
the 60 micron 1.2 Jy IRAS redshift survey of Fisher et al. (1995) as a distance
indicator; equivalently, we compare the flux density distribution of galaxies
as a function of redshift with predictions under different redshift-distance
cosmologies, under the assumption of a universal luminosity function. This
method does not assume a uniform distribution of galaxies in space. We find
that this test has rather weak discriminatory power, as argued by Petrosian
(1993), and the differences between models are not as stark as one might expect
a priori. Even so, we find that the Hubble law is indeed more strongly
supported by the analysis than is the quadratic redshift-distance relation. We
identify a bias in the the Segal et al. determination of the luminosity
function, which could lead one to mistakenly favor the quadratic
redshift-distance law. We also present several complementary analyses of the
density field of the sample; the galaxy density field is found to be close to
homogeneous on large scales if the Hubble law is assumed, while this is not the
case with the quadratic redshift-distance relation.Comment: 27 pages Latex (w/figures), ApJ, in press. Uses AAS macros,
postscript also available at
http://www.astro.princeton.edu/~library/preprints/pop682.ps.g
A Linearization Beam-Hardening Correction Method for X-Ray Computed Tomographic Imaging of Structural Ceramics
Computed tomographic (CT) imaging with both monochromatic and polychromatic x-ray sources can be a powerful NDE method for characterization (e. g., measurement of density gradients) as well as flaw detection (e. g., detection of cracks, voids, inclusions) in ceramics. However, the use of polychromatic x-ray sources can cause image artifacts and overall image degradation through beam hardening (BH) effects [1]. Beam hardening occurs because (i) x-ray attenuation in a given material is energy dependent and (ii) data collection in CT systems is not energy selective. Without an appropriate correction, the BH effect prevents the establishment of an absolute scale for density measurement. Thus, quantitative density comparisons between samples of the same material but of different geometrical shape becomes unreliable [2]
The Uncertainty of Fluxes
In the ordinary quantum Maxwell theory of a free electromagnetic field,
formulated on a curved 3-manifold, we observe that magnetic and electric fluxes
cannot be simultaneously measured. This uncertainty principle reflects torsion:
fluxes modulo torsion can be simultaneously measured. We also develop the
Hamilton theory of self-dual fields, noting that they are quantized by
Pontrjagin self-dual cohomology theories and that the quantum Hilbert space is
Z/2-graded, so typically contains both bosonic and fermionic states.
Significantly, these ideas apply to the Ramond-Ramond field in string theory,
showing that its K-theory class cannot be measured.Comment: 33 pages; minor modifications for publication in Commun. Math. Phy
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