11,535 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
Constraints on the three-fluid model of curvaton decay
A three fluid system describing the decay of the curvaton is studied by
numerical and analytical means. We place constraints on the allowed interaction
strengths between the fluids and initial curvaton density by requiring that the
curvaton decays before nucleosynthesis while nucleosynthesis, radiation-matter
equality and decoupling occur at correct temperatures. We find that with a
continuous, time-independent interaction, a small initial curvaton density is
naturally preferred along with a low reheating temperature. Allowing for a
time-dependent interaction, this constraint can be relaxed. In both cases, a
purely adiabatic final state can be generated, but not without fine-tuning.
Unlike in the two fluid system, the time-dependent interactions are found to
have a small effect on the curvature perturbation itself due to the different
nature of the system. The presence of non-gaussianity in the model is
discussed.Comment: 9 pages, 10 figure
Direct Calculation of Spin-Stiffness for Spin-1/2 Heisenberg Models
The spin-stiffness of frustrated spin-1/2 Heisenberg models in one and two
dimensions is computed for the first time by exact diagonalizations on small
clusters that implement spin-dependent twisted boundary conditions. Finite-size
extrapolation to the thermodynamic limit yields a value of for
the spin-stiffness of the unfrustrated planar antiferromagnet. We also present
a general discussion of the linear-response theory for spin-twists, which
ultimately leads to the moment sum-rule.Comment: 11 pgs, TeX, LA-UR-94-94 (to be published in Phys. Rev. B
Estimation of the mechanical properties of the eye through the study of its vibrational modes
Measuring the eye's mechanical properties in vivo and with minimally invasive
techniques can be the key for individualized solutions to a number of eye
pathologies. The development of such techniques largely relies on a
computational modelling of the eyeball and, it optimally requires the synergic
interplay between experimentation and numerical simulation. In Astrophysics and
Geophysics the remote measurement of structural properties of the systems of
their realm is performed on the basis of (helio-)seismic techniques. As a
biomechanical system, the eyeball possesses normal vibrational modes
encompassing rich information about its structure and mechanical properties.
However, the integral analysis of the eyeball vibrational modes has not been
performed yet. Here we develop a new finite difference method to compute both
the spheroidal and, specially, the toroidal eigenfrequencies of the human eye.
Using this numerical model, we show that the vibrational eigenfrequencies of
the human eye fall in the interval 100 Hz - 10 MHz. We find that compressible
vibrational modes may release a trace on high frequency changes of the
intraocular pressure, while incompressible normal modes could be registered
analyzing the scattering pattern that the motions of the vitreous humour leave
on the retina. Existing contact lenses with embebed devices operating at high
sampling frequency could be used to register the microfluctuations of the
eyeball shape we obtain. We advance that an inverse problem to obtain the
mechanical properties of a given eye (e.g., Young's modulus, Poisson ratio)
measuring its normal frequencies is doable. These measurements can be done
using non-invasive techniques, opening very interesting perspectives to
estimate the mechanical properties of eyes in vivo. Future research might
relate various ocular pathologies with anomalies in measured vibrational
frequencies of the eye.Comment: Published in PLoS ONE as Open Access Research Article. 17 pages, 5
color figure
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
The discrimination capabilities of Micromegas detectors at low energy
The latest generation of Micromegas detectors show a good energy resolution,
spatial resolution and low threshold, which make them idoneous in low energy
applications. Two micromegas detectors have been built for dark matter
experiments: CAST, which uses a dipole magnet to convert axion into detectable
x-ray photons, and MIMAC, which aims to reconstruct the tracks of low energy
nuclear recoils in a mixture of CF4 and CHF3. These readouts have been
respectively built with the microbulk and bulk techniques, which show different
gain, electron transmission and energy resolutions. The detectors and the
operation conditions will be described in detail as well as their
discrimination capabilities for low energy photons will be discussed.Comment: To be published in the proceedings of the TIPP2011 conference
(Physics Procedia
Astrophysical constraints on superlight gravitinos
I review the constraints on the mass of gravitinos that follow from
considerations on energy loss in stars and from Big Bang Nucleosynthesis
arguments.Comment: Invited talk at the 5th Workshop on High Energy Physics
Phenomenology(WHEPP-5), Pune, India, 12-26 January 199
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.
Gravitinos from Gravitational Collapse
We reanalyse the limits on the gravitino mass in superlight
gravitino scenarios derived from arguments on energy-loss during gravitational
collapse. We conclude that the mass range is excluded by SN1987A data. In terms of the
scale of supersymmetry breaking , the range is not allowed.Comment: 6 pages, latex, no figures. Numerical typo correcte
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