1,101 research outputs found
The role of center vortices in Gribov's confinement scenario
The connection of Gribov's confinement scenario in Coulomb gauge with the
center vortex picture of confinement is investigated. For this purpose we
assume a vacuum wave functional which models the infrared properties of the
theory and in particular shows strict confinement, i.e. an area law of the
Wilson loop. We isolate the center vortex content of this wave functional by
standard lattice methods and investigate their contributions to various static
propagators of the Hamilton approach to Yang-Mills theory in Coulomb gauge. We
find that the infrared properties of these quantities, in particular the
infrared divergence of the ghost form factor, are dominated by center vortices.Comment: 18 pages, 5 figure
Broad boron sheets and boron nanotubes: An ab initio study of structural, electronic, and mechanical properties
Based on a numerical ab initio study, we discuss a structure model for a
broad boron sheet, which is the analog of a single graphite sheet, and the
precursor of boron nanotubes. The sheet has linear chains of sp hybridized
sigma bonds lying only along its armchair direction, a high stiffness, and
anisotropic bonds properties. The puckering of the sheet is explained as a
mechanism to stabilize the sp sigma bonds. The anisotropic bond properties of
the boron sheet lead to a two-dimensional reference lattice structure, which is
rectangular rather than triangular. As a consequence the chiral angles of
related boron nanotubes range from 0 to 90 degrees. Given the electronic
properties of the boron sheets, we demonstrate that all of the related boron
nanotubes are metallic, irrespective of their radius and chiral angle, and we
also postulate the existence of helical currents in ideal chiral nanotubes.
Furthermore, we show that the strain energy of boron nanotubes will depend on
their radii, as well as on their chiral angles. This is a rather unique
property among nanotubular systems, and it could be the basis of a different
type of structure control within nanotechnology.Comment: 16 pages, 17 figures, 2 tables, Versions: v1=preview, v2=first final,
v3=minor corrections, v4=document slightly reworke
Dielectric function of the QCD vacuum
It is shown that the inverse of the ghost form factor in the Hamilton
approach to Yang-Mills theory in Coulomb gauge can be interpreted as the color
dielectric function of the QCD vacuum. Furthermore the horizon condition to the
ghost form factor implies that in the infrared the QCD vacuum is a perfect
color diaelectric medium and therefore a dual superconductor. The dielectric
function is explicitly calculated within a previously developed variational
approach, using a specific ansatz for the vacuum wave functional.Comment: 4 pages, 1 figure, revised version as published in Phys. Rev. Let
The Obama Factor: Change and Stability in Cultural and Political Anti-Americanism
Global public opinion toward the United States is an important factor in international politics. But to what degree are distinct dimensions of attitudes toward the United States associated with the person of the president and the consumption of U.S.-produced media content? Two surveys of German college students before and after the 2008 U.S. presidential election revealed that attitudes toward U.S. foreign policies improved from 2008 to 2009, and views on U.S. culture remained stable. Perceptions of Obama depended less on attitudes toward U.S. culture than perceptions of ordinary U.S. Americans, indicating a potential for the president to influence foreign political support, even in the face of cultural reservations. Consumption of some types of U.S. media was also associated with lower levels of anti-Americanism
Quantum Energies of Interfaces
We present a method for computing the one-loop, renormalized quantum energies
of symmetrical interfaces of arbitrary dimension and codimension using
elementary scattering data. Internal consistency requires finite-energy sum
rules relating phase shifts to bound state energies.Comment: 8 pages, 1 figure, minor changes, Phys. Rev. Lett., in prin
Functionalizing graphene by embedded boron clusters
We present a model system that might serve as a blueprint for the controlled
layout of graphene based nanodevices. The systems consists of chains of B7
clusters implanted in a graphene matrix, where the boron clusters are not
directly connected. We show that the graphene matrix easily accepts these
alternating boron chains, and that the implanted boron components may
dramatically modify the electronic properties of graphene based nanomaterials.
This suggests a functionalization of graphene nanomaterials, where the
semiconducting properties might be supplemented by parts of the graphene matrix
itself, but the basic wiring will be provided by alternating chains of
implanted boron clusters that connect these areas.Comment: 10 pages, 2 figures, 1 tabl
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