10,144 research outputs found
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
Linear response time-dependent density functional theory of the Hubbard dimer
The asymmetric Hubbard dimer is used to study the density-dependence of the
exact frequency-dependent kernel of linear-response time-dependent density
functional theory. The exact form of the kernel is given, and the limitations
of the adiabatic approximation utilizing the exact ground-state functional are
shown. The oscillator strength sum rule is proven for lattice Hamiltonians, and
relative oscillator strengths are defined appropriately. The method of Casida
for extracting oscillator strengths from a frequency-dependent kernel is
demonstrated to yield the exact result with this kernel. An unambiguous way of
labelling the nature of excitations is given. The fluctuation-dissipation
theorem is proven for the ground-state exchange-correlation energy. The
distinction between weak and strong correlation is shown to depend on the ratio
of interaction to asymmetry. A simple interpolation between carefully defined
weak-correlation and strong-correlation regimes yields a density-functional
approximation for the kernel that gives accurate transition frequencies for
both the single and double excitations, including charge-transfer excitations.
Many exact results, limits, and expansions about those limits are given in the
appendices.Comment: 22 pages, 14 figure
Transoesophageal detection of heart graft rejection by electrical impedance: using Finite Element Method simulations
Previous studies have shown that it is possible to evaluate heart graft rejection
level using a bioimpedance technique by means of an intracavitary catheter. However, this technique does not present relevant advantages compared to the gold standard for the detection of a heart rejection, which is the biopsy of the endomyocardial tissue. We propose to use a less invasive technique that consists in the use of a transoesophageal catheter and two standard
ECG electrodes on the thorax. The aim of this work is to evaluate different parameters affecting the impedance measurement, including: sensitivity to electrical conductivity and permittivity
of different organs in the thorax, lung edema and pleural water. From these results, we deduce the best estimator for cardiac rejection detection, and we obtain the tools to identify possible cases of false positive of heart rejection due to other factors. To achieve these objectives we have created a thoracic model and we have simulated, with a FEM program, different situations at the frequencies of 13, 30, 100, 300 and 1000 kHz. Our simulation demonstrates that the phase, at 100 and 300 kHz, has the higher sensitivity to changes in the electrical parameters of the heart muscle.Peer ReviewedPostprint (author’s final draft
High inclination orbits in the secular quadrupolar three-body problem
The Lidov-Kozai mechanism allows a body to periodically exchange its
eccentricity with inclination. It was first discussed in the framework of the
quadrupolar secular restricted three-body problem, where the massless particle
is the inner body, and later extended to the quadrupolar secular nonrestricted
three body problem. In this paper, we propose a different point of view on the
problem by looking first at the restricted problem where the massless particle
is the outer body. In this situation, equilibria at high mutual inclination
appear, which correspond to the population of stable particles that Verrier &
Evans (2008,2009) find in stable, high inclination circumbinary orbits around
one of the components of the quadruple star HD 98800. We provide a simple
analytical framework using a vectorial formalism for these situations. We also
look at the evolution of these high inclination equilibria in the non
restricted case.Comment: 11 pages, 6 figures. Accepted by MNRAS 2009 September 1
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
Lovelock inflation and the number of large dimensions
We discuss an inflationary scenario based on Lovelock terms. These higher
order curvature terms can lead to inflation when there are more than three
spatial dimensions. Inflation will end if the extra dimensions are stabilised,
so that at most three dimensions are free to expand. This relates graceful exit
to the number of large dimensions.Comment: 16 pages, 1 figure. v2: published version, added clarification
Theoretical study of Ga-based nanowires and the interaction of Ga with single-wall carbon nanotubes
Gallium displays physical properties which can make it a potential element to
produce metallic nanowires and high-conducting interconnects in
nanoelectronics. Using first-principles pseudopotential plane method we showed
that Ga can form stable metallic linear and zigzag monatomic chain structures.
The interaction between individual Ga atom and single-wall carbon nanotube
(SWNT) leads to a chemisorption bond involving charge transfer. Doping of SWNT
with Ga atom gives rise to donor states. Owing to a significant interaction
between individual Ga atom and SWNT, continuous Ga coverage of the tube can be
achieved. Ga nanowires produced by the coating of carbon nanotube templates are
found to be stable and high conducting.Comment: 8 pages, 8 figure
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
Magnetic Phases in Three-Flavor Color Superconductivity
The best natural candidates for the realization of color superconductivity
are quark stars -not yet confirmed by observation- and the extremely dense
cores of compact stars, many of which have very large magnetic fields. To
reliably predict astrophysical signatures of color superconductivity, a better
understanding of the role of the star's magnetic field in the color
superconducting phase that realizes in the core is required. This paper is an
initial step in that direction. The field scales at which the different
magnetic phases of a color superconductor with three quark flavors can be
realized are investigated. Coming from weak to strong fields, the system
undergoes first a symmetry transmutation from a Color-Flavor-Locked (CFL) phase
to a Magnetic-CFL (MCFL) phase, and then a phase transition from the MCFL phase
to the Paramagnetic-CFL (PCFL) phase. The low-energy effective theory for the
excitations of the diquark condensate in the presence of a magnetic field is
derived using a covariant representation that takes into account all the
Lorentz structures contributing at low energy. The field-induced masses of the
charged mesons and the threshold field at which the CFL MCFL symmetry
transmutation occurs are obtained in the framework of this low-energy effective
theory. The relevance of the different magnetic phases for the physics of
compact stars is discussed.Comment: Version to appear in PR
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