11,166 research outputs found
Effects of Bose-Einstein Condensation on forces among bodies sitting in a boson heat bath
We explore the consequences of Bose-Einstein condensation on
two-scalar-exchange mediated forces among bodies that sit in a boson gas. We
find that below the condensation temperature the range of the forces becomes
infinite while it is finite at temperatures above condensation.Comment: 10 pages, 2 figure
Color Magnetic Flux Tubes in Dense QCD
QCD is expected to be in the color-flavor locking phase in high baryon
density, which exhibits color superconductivity. The most fundamental
topological objects in the color superconductor are non-Abelian vortices which
are topologically stable color magnetic flux tubes. We present numerical
solutions of the color magnetic flux tube for diverse choices of the coupling
constants. We also analytically study its asymptotic profiles and find that
they are different from the case of usual superconductors. We propose the width
of color magnetic fluxes and find that it is larger than naive expectation of
the Compton wave length of the massive gluon when the gluon mass is larger than
the scalar mass.Comment: 24 pages, 5 figures; v2: typos corrected, references added, minor
changes; v3: published versio
Long range neutrino forces in the cosmic relic neutrino background
Neutrinos mediate long range forces among macroscopic bodies in vacuum. When
the bodies are placed in the neutrino cosmic background, these forces are
modified. Indeed, at distances long compared to the scale , the relic
neutrinos completely screen off the 2-neutrino exchange force, whereas for
small distances the interaction remains unaffected.Comment: 8 pages, 2 figure
Long Range Forces from Pseudoscalar Exchange
Using dispersion theoretic techniques, we consider coherent long range forces
arising from double pseudoscalar exchange among fermions. We find that Yukawa
type coupling leads to spin independent attractive potentials whereas
derivative coupling renders spin independent repulsive potentials.Comment: 27 pages, REVTeX, 3 figures included using epsfi
Scale-free Networks from Optimal Design
A large number of complex networks, both natural and artificial, share the
presence of highly heterogeneous, scale-free degree distributions. A few
mechanisms for the emergence of such patterns have been suggested, optimization
not being one of them. In this letter we present the first evidence for the
emergence of scaling (and smallworldness) in software architecture graphs from
a well-defined local optimization process. Although the rules that define the
strategies involved in software engineering should lead to a tree-like
structure, the final net is scale-free, perhaps reflecting the presence of
conflicting constraints unavoidable in a multidimensional optimization process.
The consequences for other complex networks are outlined.Comment: 6 pages, 2 figures. Submitted to Europhysics Letters. Additional
material is available at http://complex.upc.es/~sergi/software.ht
Domain-wall profile in the presence of anisotropic exchange interactions: Effective on-site anisotropy
Starting from a D-dimensional XXZ ferromagnetic Heisenberg model in an
hypercubic lattice, it is demonstrated that the anisotropy in the exchange
coupling constant leads to a D-dependent effective on-site anisotropy
interaction often ignored for D>1. As a result the effective width of the wall
depends on the dimensionality of the system. It is shown that the effective
one-dimensional Hamiltonian is not the one-dimensional XXZ version as assumed
in previous theoretical work. We derive a new expression for the wall profile
that generalizes the standard Landau-Lifshitz form. Our results are found to be
in very good agreement with earlier numerical work using the Monte Carlo
method. Preceding theories concerning the domain wall contribution to
magnetoresistance have considered the role of D only through the modification
of the density of states in the electronic band structure. This Brief Report
reveals that the wall profile itself contains an additional D dependence for
the case of anisotropic exchange interactions.Comment: 4 pages; new title and abstract; 1 figure comparing our results with
earlier numerical work; a more general model containing the usual on-site
anisotropy; new remarks and references on the following two topics: (a)
experimental evidence for the existence of spin exchange anisotropy, and (b)
preceding theories concerning the domain wall contribution to
magnetoresistance; to appear in Phys. Rev.
Higgs- and Goldstone bosons-mediated long range forces
In certain mild extensions of the Standard Model, spin-independent long range
forces can arise by exchange of two very light pseudoscalar spin--0 bosons. In
particular, we have in mind models in which these bosons do not have direct
tree level couplings to ordinary fermions. Using the dispersion theoretical
method, we find a behaviour of the potential for the exchange of very
light pseudoscalars and a dependence if the pseudoscalars are true
massless Goldstone bosons.Comment: 13 pages (REVTeX), 2 figure
Radiopurity of Micromegas readout planes
Micromesh Gas Amplification Structures (Micromegas) are being used in an
increasing number of Particle Physics applications since their conception
fourteen years ago. More recently, they are being used or considered as readout
of Time Projection Chambers (TPCs) in the field of Rare Event searches (dealing
with dark matter, axions or double beta decay). In these experiments, the
radiopurity of the detector components and surrounding materials is measured
and finely controlled in order to keep the experimental background as low as
possible. In the present paper, the first measurement of the radiopurity of
Micromegas planes obtained by high purity germanium spectrometry in the low
background facilities of the Canfranc Underground Laboratory (LSC) is
presented. The obtained results prove that Micromegas readouts of the microbulk
type are currently manufactured with radiopurity levels below 30 microBq/cm2
for Th and U chains and ~60 microBq/cm2 for 40K, already comparable to the
cleanest detector components of the most stringent low background experiments
at present. Taking into account that the studied readouts were manufactured
without any specific control of the radiopurity, it should be possible to
improve these levels after dedicated development.Comment: 15 pages, 2 figure
Full analytical solution of finite-length armchair/zigzag nanoribbons
Finite-length armchair graphene nanoribbons can behave as one dimensional
topological materials, that may show edge states in their zigzag-terminated
edges, depending on their width and termination. We show here a full solution
of Tight-Binding graphene rectangles of any length and width that can be seen
as either finite-length armchair or zigzag ribbons. We find exact analytical
expressions for both bulk and edge eigen-states and eigen-energies. We write
down exact expressions for the Coulomb interactions among edge states and
introduce a Hubbard-dimer model to analyse the emergence and features of
different magnetic states at the edges, whose existence depends on the ribbon
length. We find ample room for experimental testing of our predictions in N = 5
armchair ribbons. We compare the analytical results with ab initio simulations
to benchmark the quality of the dimer model and to set its parameters. A
further detailed analysis of the ab initio Hamiltonian allows us to identify
those variations of the Tight-Binding parameters that affect the topological
properties of the ribbons
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