361 research outputs found
Free energy of an SU(2) monopole-antimonopole pair
We induce an external monopole-antimonopole pair in an SU(2) lattice
gauge system and measure its free energy as a way to probe the vacuum
structure. We discuss the motivation and computational methodology of the
investigation and illustrate our preliminary results.Comment: LATTICE98(confine
Potential between external monopole and antimonopole in SU(2) lattice glu odynamics
We present the results of a study of the free energy of a monopole pair in
pure
SU(2) theory at finite temperature, both below and above the deconfinement
tran sition. We find a Yukawa potential between monopoles in both phases. At
low temp erature, the screening mass is compatible with the lightest glueball
mass. At hi gh temperature, we observe an increased screening mass with no
apparent disconti nuity at the phase transition.Comment: LATTICE 99 (Topology and Confinement
Free energy of an SU(2) monopole-antimonopole pair
We present a high-statistic numerical study of the free energy of a
monopole-antimonopole pair in pure SU(2) theory. We find that the
monopole-antimonopole interaction potential exhibits a screened behavior, as
one would expect in presence of a monopole condensate. Screening occurs both in
the low-temperature, confining phase of the theory, and in the high-temperature
deconfined phase, with no evidence of a discontinuity of the screening mass
across the transition. The mass of the object responsible for the screening at
low temperature is approximately twice the established value for the lightest
glueball, indicating a prevalent coupling to glueball excitations. At high
temperature, the screening mass increases. We contrast the behavior of the
quantum system with that of the corresponding classical system, where the
monopole-antimonopole potential is of the Coulomb type.Comment: Latex, 22 pages, 8 figures. A mistake in the computer program
implementing the multihistogram method has been corrected and all the
affected numerical data have been revised. The main conclusions of the paper
are unchanged, but the screening masses turn out somehow larger. (We thank
Philippe de Forcrand for correspondence which helped us find the error.
Polaronic excitations in CMR manganite films
In the colossal magnetoresistance manganites polarons have been proposed as
the charge carrier state which localizes across the metal-insulator transition.
The character of the polarons is still under debate. We present an assessment
of measurements which identify polarons in the metallic state of
La{2/3}Sr{1/3}MnO{3} (LSMO) and La{2/3}Ca{1/3}MnO{3} (LCMO) thin films. We
focus on optical spectroscopy in these films which displays a pronounced
resonance in the mid-infrared. The temperature dependent resonance has been
previously assigned to polaron excitations. These polaronic resonances are
qualitatively distinct in LSMO and LCMO and we discuss large and small polaron
scenarios which have been proposed so far. There is evidence for a large
polaron excitation in LSMO and small polarons in LCMO. These scenarios are
examined with respect to further experimental probes, specifically charge
carrier mobility (Hall-effect measurements) and high-temperature
dc-resistivity.Comment: 16 pages, 10 figure
Fresnel Representation of the Wigner Function: An Operational Approach
We present an operational definition of the Wigner function. Our method
relies on the Fresnel transform of measured Rabi oscillations and applies to
motional states of trapped atoms as well as to field states in cavities. We
illustrate this technique using data from recent experiments in ion traps [D.
M. Meekhof et al., Phys. Rev. Lett. 76, 1796 (1996)] and in cavity QED [B.
Varcoe et al., Nature 403, 743 (2000)]. The values of the Wigner functions of
the underlying states at the origin of phase space are W(0)=+1.75 for the
vibrational ground state and W(0)=-1.4 for the one-photon number state. We
generalize this method to wave packets in arbitrary potentials.Comment: 4 pages include 4 figures, submitted to PR
Vortex waistlines and long range fluctuations
We examine the manner in which a linear potential results from fluctuations
due to vortices linked with the Wilson loop. Our discussion is based on exact
relations and inequalities between the Wilson loop and the vortex and electric
flux order parameters. We show that, contrary to the customary naive picture,
only vortex fluctuations of thickness of the order of the spatial linear size
of the loop are capable of producing a strictly linear potential. An effective
theory of these long range fluctuations emerges naturally in the form of a
strongly coupled Z(N) lattice gauge theory. We also point out that dynamical
fermions introduced in this medium undergo chiral symmetry breaking.Comment: 17 pages, LaTex file with 7 eps figures, revised references, minor
comments adde
Fe-substituted mullite powders for the in situ synthesis of carbon nanotubes by catalytic chemical vapor deposition
Powders of iron-substituted mullite were prepared by combustion and further calcination in air at different temperatures. A detailed study involving notably Mo¨ssbauer spectroscopy showed that the Fe3+ ions are distributed between the mullite phase and a corundum phase that progressively dissolves into mullite upon the increase in calcination temperature. Carbon nanotube-Fe-mullite nanocomposites were prepared for the first time by a direct method involving a reduction of these powders in H2-CH4 and without any mechanical mixing step. The carbon nanotubes formed by the catalytic decomposition of CH4 on the smallest metal particles are mostly double-walled and multiwalled, although some carbon nanofibers are also observed
Dynamics of entanglement between two trapped atoms
We investigate the dynamics of entanglement between two continuous variable
quantum systems. The model system consists of two atoms in a harmonic trap
which are interacting by a simplified s-wave scattering. We show, that the
dynamically created entanglement changes in a steplike manner. Moreover, we
introduce local operators which allow us to violate a Bell-CHSH inequality
adapted to the continuous variable case. The correlations show nonclassical
behavior and almost reach the maximal quantum mechanical value. This is
interesting since the states prepared by this interaction are very different
from any EPR-like state.Comment: 9 page
Object Detection Through Exploration With A Foveated Visual Field
We present a foveated object detector (FOD) as a biologically-inspired
alternative to the sliding window (SW) approach which is the dominant method of
search in computer vision object detection. Similar to the human visual system,
the FOD has higher resolution at the fovea and lower resolution at the visual
periphery. Consequently, more computational resources are allocated at the
fovea and relatively fewer at the periphery. The FOD processes the entire
scene, uses retino-specific object detection classifiers to guide eye
movements, aligns its fovea with regions of interest in the input image and
integrates observations across multiple fixations. Our approach combines modern
object detectors from computer vision with a recent model of peripheral pooling
regions found at the V1 layer of the human visual system. We assessed various
eye movement strategies on the PASCAL VOC 2007 dataset and show that the FOD
performs on par with the SW detector while bringing significant computational
cost savings.Comment: An extended version of this manuscript was published in PLOS
Computational Biology (October 2017) at
https://doi.org/10.1371/journal.pcbi.100574
TRY plant trait database - enhanced coverage and open access
Plant traits-the morphological, anatomical, physiological, biochemical and phenological characteristics of plants-determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait-based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits-almost complete coverage for 'plant growth form'. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait-environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives
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