70,008 research outputs found
Power Spectra in Global Defect Theories of Cosmic Structure Formation
An efficient technique for computing perturbation power spectra in field
ordering theories of cosmic structure formation is introduced, enabling
computations to be carried out with unprecedented precision. Large scale
simulations are used to measure unequal time correlators of the source stress
energy, taking advantage of scaling during matter and radiation domination, and
causality, to make optimal use of the available dynamic range. The correlators
are then re-expressed in terms of a sum of eigenvector products, a
representation which we argue is optimal, enabling the computation of the final
power spectra to be performed at high accuracy. Microwave anisotropy and matter
perturbation power spectra for global strings, monopoles, textures and
non-topological textures are presented and compared with recent observations.Comment: 4 pages, compressed and uuencoded RevTex file and postscript figure
Uhlenbeck-Donaldson compactification for framed sheaves on projective surfaces
We construct a compactification of the Uhlenbeck-Donaldson type
for the moduli space of slope stable framed bundles. This is a kind of a moduli
space of slope semistable framed sheaves. We show that there exists a
projective morphism , where is
the moduli space of S-equivalence classes of Gieseker-semistable framed
sheaves. The space has a natural set-theoretic stratification
which allows one, via a Hitchin-Kobayashi correspondence, to compare it with
the moduli spaces of framed ideal instantons.Comment: 18 pages. v2: a few very minor changes. v3: 27 pages. Several proofs
have been considerably expanded, and more explanations have been added. v4:
28 pages. A few minor changes. Final version accepted for publication in
Math.
Reduced dynamics with renormalization in solid-state charge qubit measurement
Quantum measurement will inevitably cause backaction on the measured system,
resulting in the well known dephasing and relaxation. In this report, in the
context of solid--state qubit measurement by a mesoscopic detector, we show
that an alternative backaction known as renormalization is important under some
circumstances. This effect is largely overlooked in the theory of quantum
measurement.Comment: 12 pages, 4 figure
Infrared study of the charge-ordered multiferroic LuFe(2)O(4)
The reflectivity of a large LuFe(2)O(4) single crystal has been measured with
the radiation field either perpendicular or parallel to the c axis of its
rhombohedral structure, from 10 to 500K, and from 7 to 16000 cm-1. The
transition between the two-dimensional and the three-dimensional charge order
at T_(CO) = 320 K is found to change dramatically the phonon spectrum in both
polarizations. The number of the observed modes above and below T_(CO),
according to a factor-group analysis, is in good agreement with a transition
from the rhombohedral space group R{bar 3}m to the monoclinic C2/m. In the
sub-THz region a peak becomes evident at low temperature, whose origin is
discussed in relation with previous experiments.Comment: Physical Review B in pres
Generalized Dynamic Scaling for Critical Relaxations
The dynamic relaxation process for the two dimensional Potts model at
criticality starting from an initial state with very high temperature and
arbitrary magnetization is investigated with Monte Carlo methods. The results
show that there exists universal scaling behaviour even in the short-time
regime of the dynamic evolution. In order to describe the dependence of the
scaling behaviour on the initial magnetization, a critical characteristic
function is introduced.Comment: Latex, 8 pages, 3 figures, to appear in Phys. Rev. Let
Polarization of the Microwave Background in Defect Models
We compute the polarization power spectra for global strings, monopoles,
textures and nontopological textures, and compare them to inflationary models.
We find that topological defect models predict a significant (1 microK)
contribution to magnetic type polarization on degree angular scales, which is
produced by the large vector component of the defect source. We also
investigate the effect of decoherence on polarization. It leads to a smoothing
of acoustic oscillations both in temperature and polarization power spectra and
strongly suppresses the cross-correlation between temperature and polarization
relative to inflationary models. Presence or absence of magnetic polarization
or cross-correlation would be a strong discriminator between the two theories
of structure formation and will be testable with the next generation of CMB
satellites.Comment: 4 pages, 3 figures, RevTeX fil
A Component Based Heuristic Search Method with Evolutionary Eliminations
Nurse rostering is a complex scheduling problem that affects hospital
personnel on a daily basis all over the world. This paper presents a new
component-based approach with evolutionary eliminations, for a nurse scheduling
problem arising at a major UK hospital. The main idea behind this technique is
to decompose a schedule into its components (i.e. the allocated shift pattern
of each nurse), and then to implement two evolutionary elimination strategies
mimicking natural selection and natural mutation process on these components
respectively to iteratively deliver better schedules. The worthiness of all
components in the schedule has to be continuously demonstrated in order for
them to remain there. This demonstration employs an evaluation function which
evaluates how well each component contributes towards the final objective. Two
elimination steps are then applied: the first elimination eliminates a number
of components that are deemed not worthy to stay in the current schedule; the
second elimination may also throw out, with a low level of probability, some
worthy components. The eliminated components are replenished with new ones
using a set of constructive heuristics using local optimality criteria.
Computational results using 52 data instances demonstrate the applicability of
the proposed approach in solving real-world problems.Comment: 27 pages, 4 figure
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