784 research outputs found
Christoffel functions and orthogonal polynomials for Erdšos weights on (ââ,â)
We establish bounds on orthonormal polynomials and Christoffel functions associated with weights on IR of the form W2 = eâ2Q, where Q : IR â IR is even,
and is of faster than polynomial growth at â (so-called Erdšos weights). Typical examples are Q(x) := expk |x| α1, α > 1, , where expk = expk (exp (... exp(·))) denotes
the kth iterated exponential. Further, we obtain uniform estimates on the spacing of
all the zeros and on the Christoffel functions. These results complement earlier ones
for the case where Q is of polynomial growth at â (so-called Freud weights) and for
exponential weights on (â1, 1)
The Landshoff-Nachtmann Pomeron on the Lattice
We investigate the Landshoff-Nachtmann two-gluon-exchange model of the
Pomeron using gluon propagators computed in the Landau gauge within quenched
lattice QCD calculations. We first determine an effective gluon-quark coupling
by constraining the Pomeron-quark coupling to its phenomenological value
\beta_0 = 2\, \gev^{-1}. We then provide predictions for a variety of
diffractive processes. As the propagators have been evaluated entirely from QCD
first principles (although in the quenched approximation), our results provide
a consistency check of the Landshoff-Nachtmann model. We address the issue of
the possible gauge-dependence of our results, which will be the object of a
future study.Comment: uuencoded, compressed tar file, 13 pages latex, 4 Postscript figures,
requires epsf.st
Nanoscale polar heterogeneities and branching Bi-displacement directions in K0.5Bi0.5TiO3
K0.5Bi0.5TiO3 (KBT)âone of the few perovskite-like ferroelectric compounds with room-temperature tetragonal symmetryâdiffers from other members of its family (BaTiO3 and PbTiO3) by the presence of a disordered mixture of K and Bi on cuboctahedral sites. This disorder is expected to affect local atomic displacements and their response to an applied electric field. We have derived nanoscale atomistic models of KBT by refining atomic coordinates to simultaneously fit neutron/X-ray total scattering and extended X-ray absorption fine-structure data. Both Bi and Ti ions were found to be offset relative to their respective oxygen cages in the high-temperature cubic phase; in contrast, the coordination environment of K remained relatively undistorted. In the cubic structure, Bi displacements prefer the âš100â© directions and the probability density distribution of Bi features six well-separated sites; a similar preference exists for the much smaller Ti displacements, although the split sites for Ti could not be resolved. The cation displacements are correlated, yielding polar nanoregions, whereas on average, the structure appears as cubic. The cubic â tetragonal phase transition involves both order/disorder and displacive mechanisms. A qualitative change in the form of the Bi probability density distribution occurs in the tetragonal phase on cooling to room temperature because Bi displacements âbranch offâ to âš111â© directions. This change, which preserves the average symmetry, is accompanied by the development of nanoscale polar heterogeneities that exhibit significant deviations of their polarization vectors from the average polar axis
Patchiness and Demographic Noise in Three Ecological Examples
Understanding the causes and effects of spatial aggregation is one of the
most fundamental problems in ecology. Aggregation is an emergent phenomenon
arising from the interactions between the individuals of the population, able
to sense only -at most- local densities of their cohorts. Thus, taking into
account the individual-level interactions and fluctuations is essential to
reach a correct description of the population. Classic deterministic equations
are suitable to describe some aspects of the population, but leave out features
related to the stochasticity inherent to the discreteness of the individuals.
Stochastic equations for the population do account for these
fluctuation-generated effects by means of demographic noise terms but, owing to
their complexity, they can be difficult (or, at times, impossible) to deal
with. Even when they can be written in a simple form, they are still difficult
to numerically integrate due to the presence of the "square-root" intrinsic
noise. In this paper, we discuss a simple way to add the effect of demographic
stochasticity to three classic, deterministic ecological examples where
aggregation plays an important role. We study the resulting equations using a
recently-introduced integration scheme especially devised to integrate
numerically stochastic equations with demographic noise. Aimed at scrutinizing
the ability of these stochastic examples to show aggregation, we find that the
three systems not only show patchy configurations, but also undergo a phase
transition belonging to the directed percolation universality class.Comment: 20 pages, 5 figures. To appear in J. Stat. Phy
Displacive orderâdisorder behavior and intrinsic clustering of lattice distortions in biâsubstituted NaNbO3
Perovskiteâlike NaNbO3âBi1/3NbO3 solid solutions are studied to understand the interactions between octahedral rotations, which dominate the structural behavior of NaNbO3 and displacive disorder of Bi present in Bi1/3NbO3. Models of instantaneous structures for representative compositions are obtained by refining atomic coordinates against Xâray total scattering and extended Xârayâabsorption fine structure data, with additional input obtained from transmission electron microscopy. A mixture of distinct cations and vacancies on the cuboctahedral Aâsites in Na1â3x Bix NbO3 (x †0.2) results in 3D nanoscale modulations of structural distortions. This phenomenon is determined by the inevitable correlations in the chemical composition of adjacent unit cells according to the structure typeâan intrinsic property of any nonmolecular crystals. Octahedral rotations become suppressed as x increases. Outâofâphase rotations vanish for x > 0.1, whereas inâphase tilts persist up to x = 0.2, although for this composition their correlation length becomes limited to the nanoscale. The loss of outâofâphase tilting is accompanied by qualitative changes in the probability density distributions for Bi and Nb, with both species becoming disordered over loci offset from the centers of their respective oxygen cages. Symmetry arguments are used to attribute this effect to different strengths of the coupling between the cation displacements and outâofâphase versus inâphase rotations. The displacive disorder of Bi and Nb combined with nanoscale clustering of lattice distortions are primarily responsible for the anomalous broadening of the temperature dependence of the dielectric constant
Streamlined Calibrations of the ATLAS Precision Muon Chambers for Initial LHC Running
The ATLAS Muon Spectrometer is designed to measure the momentum of muons with
a resolution of dp/p = 3% and 10% at 100 GeV and 1 TeV momentum respectively.
For this task, the spectrometer employs 355,000 Monitored Drift Tubes (MDTs)
arrayed in 1200 Chambers. Calibration (RT) functions convert drift time
measurements into tube-centered impact parameters for track segment
reconstruction. RT functions depend on MDT environmental parameters and so must
be appropriately calibrated for local chamber conditions. We report on the
creation and application of a gas monitor system based calibration program for
muon track reconstruction in the LHC startup phase.Comment: 25 pages, 21 figure
Pyrochlore Photons: The U(1) Spin Liquid in a S=1/2 Three-Dimensional Frustrated Magnet
We study the S=1/2 Heisenberg antiferromagnet on the pyrochlore lattice in
the limit of strong easy-axis exchange anisotropy. We find, using only standard
techniques of degenerate perturbation theory, that the model has a U(1) gauge
symmetry generated by certain local rotations about the z-axis in spin space.
Upon addition of an extra local interaction in this and a related model with
spins on a three-dimensional network of corner-sharing octahedra, we can write
down the exact ground state wavefunction with no further approximations. Using
the properties of the soluble point we show that these models enter the U(1)
spin liquid phase, a novel fractionalized spin liquid with an emergent U(1)
gauge structure. This phase supports gapped S^z = 1/2 spinons carrying the U(1)
``electric'' gauge charge, a gapped topological point defect or ``magnetic''
monopole, and a gapless ``photon,'' which in spin language is a gapless,
linearly dispersing S^z = 0 collective mode. There are power-law spin
correlations with a nontrivial angular dependence, as well as novel U(1)
topological order. This state is stable to ALL zero-temperature perturbations
and exists over a finite extent of the phase diagram. Using a convenient
lattice version of electric-magnetic duality, we develop the effective
description of the U(1) spin liquid and the adjacent soluble point in terms of
Gaussian quantum electrodynamics and calculate a few of the universal
properties. The resulting picture is confirmed by our numerical analysis of the
soluble point wavefunction. Finally, we briefly discuss the prospects for
understanding this physics in a wider range of models and for making contact
with experiments.Comment: 22 pages, 14 figures. Further minor changes. To appear in Phys. Rev.
Electrical transport studies of quench condensed Bi films at the initial stage of film growth: Structural transition and the possible formation of electron droplets
The electrical transport properties of amorphous Bi films prepared by
sequential quench deposition have been studied in situ. A
superconductor-insulator (S-I) transition was observed as the film was made
increasingly thicker, consistent with previous studies. Unexpected behavior was
found at the initial stage of film growth, a regime not explored in detail
prior to the present work. As the temperature was lowered, a positive
temperature coefficient of resistance (dR/dT > 0) emerged, with the resistance
reaching a minimum before the dR/dT became negative again. This behavior was
accompanied by a non-linear and asymmetric I-V characteristic. As the film
became thicker, conventional variable-range hopping (VRH) was recovered. We
attribute the observed crossover in the electrical transport properties to an
amorphous to granular structural transition. The positive dR/dT found in the
amorphous phase of Bi formed at the initial stage of film growth was
qualitatively explained by the formation of metallic droplets within the
electron glass.Comment: 7 pages, 6 figure
Interactions, Distribution of Pinning Energies, and Transport in the Bose Glass Phase of Vortices in Superconductors
We study the ground state and low energy excitations of vortices pinned to
columnar defects in superconductors, taking into account the long--range
interaction between the fluxons. We consider the ``underfilled'' situation in
the Bose glass phase, where each flux line is attached to one of the defects,
while some pins remain unoccupied. By exploiting an analogy with disordered
semiconductors, we calculate the spatial configurations in the ground state, as
well as the distribution of pinning energies, using a zero--temperature Monte
Carlo algorithm minimizing the total energy with respect to all possible
one--vortex transfers. Intervortex repulsion leads to strong correlations
whenever the London penetration depth exceeds the fluxon spacing. A pronounced
peak appears in the static structure factor for low filling fractions . Interactions lead to a broad Coulomb gap in the distribution of
pinning energies near the chemical potential , separating
the occupied and empty pins. The vanishing of at leads to a
considerable reduction of variable--range hopping vortex transport by
correlated flux line pinning.Comment: 16 pages (twocolumn), revtex, 16 figures not appended, please contact
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