16,036 research outputs found
Gravitational signals due to tidal interactions between white dwarfs and black holes
In this paper we compute the gravitational signal emitted when a white dwarf
moves around a black hole on a closed or open orbit using the affine model
approach. We compare the orbital and the tidal contributions to the signal,
assuming that the star moves in a safe region where, although very close to the
black hole, the strength of the tidal interaction is insufficient to provoque
the stellar disruption. We show that for all considered orbits the tidal signal
presents sharp peaks corresponding to the excitation of the star non radial
oscillation modes, the amplitude of which depends on how deep the star
penetrates the black hole tidal radius and on the type of orbit. Further
structure is added to the emitted signal by the coupling between the orbital
and the tidal motion.Comment: 21 pages, 8 figres. Submitted to MNRA
Intermixture of extended edge and localized bulk energy levels in macroscopic Hall systems
We study the spectrum of a random Schroedinger operator for an electron
submitted to a magnetic field in a finite but macroscopic two dimensional
system of linear dimensions equal to L. The y direction is periodic and in the
x direction the electron is confined by two smooth increasing boundary
potentials. The eigenvalues of the Hamiltonian are classified according to
their associated quantum mechanical current in the y direction. Here we look at
an interval of energies inside the first Landau band of the random operator for
the infinite plane. In this energy interval, with large probability, there
exist O(L) eigenvalues with positive or negative currents of O(1). Between each
of these there exist O(L^2) eigenvalues with infinitesimal current
O(exp(-cB(log L)^2)). We explain what is the relevance of this analysis to the
integer quantum Hall effect.Comment: 29 pages, no figure
Bistability of persistent currents in mesoscopic rings
We study the persistent currents flowing in a mesoscopic ring threaded by a
magnetic flux and connected to a stub of finite length. Multistability
processes and Coulomb blockade are demonstrated to be present in this system.
These properties are functions of the magnetic flux crossing the ring which
plays the role that the external applied potential fulfills in the
multistability behaviour of the standard mesoscopic heterostructures.Comment: 13 pages (Revtex), 4 PostScript figures. Send e-mail to:
[email protected]
A Note on the Symmetries of the Gravitational Field of a Massless Particle
It is shown that the metric of a massless particle obtained from boosting the
Schwarzschild metric to the velocity of light, has four Killing vectors
corresponding to an E(2)\times \RR symmetry-group. This is in agreement with
the expectations based on flat-space kinematics but is in contrast to previous
statements in the literature \cite{Schueck}. Moreover, it also goes beyond the
general Jordan-Ehlers-Kundt-(JEK)-classification of gravitational pp-waves as
given in \cite{JEK}.Comment: 10pages, amslatex, TUW-94-12 and UWThPh-1994-2
Characterization of multilayer stack parameters from X-ray reflectivity data using the PPM program: measurements and comparison with TEM results
Future hard (10 -100 keV) X-ray telescopes (SIMBOL-X, Con-X, HEXIT-SAT, XEUS)
will implement focusing optics with multilayer coatings: in view of the
production of these optics we are exploring several deposition techniques for
the reflective coatings. In order to evaluate the achievable optical
performance X-Ray Reflectivity (XRR) measurements are performed, which are
powerful tools for the in-depth characterization of multilayer properties
(roughness, thickness and density distribution). An exact extraction of the
stack parameters is however difficult because the XRR scans depend on them in a
complex way. The PPM code, developed at ERSF in the past years, is able to
derive the layer-by-layer properties of multilayer structures from
semi-automatic XRR scan fittings by means of a global minimization procedure in
the parameters space. In this work we will present the PPM modeling of some
multilayer stacks (Pt/C and Ni/C) deposited by simple e-beam evaporation.
Moreover, in order to verify the predictions of PPM, the obtained results are
compared with TEM profiles taken on the same set of samples. As we will show,
PPM results are in good agreement with the TEM findings. In addition, we show
that the accurate fitting returns a physically correct evaluation of the
variation of layers thickness through the stack, whereas the thickness trend
derived from TEM profiles can be altered by the superposition of roughness
profiles in the sample image
GHASP: an H kinematic survey of spiral galaxies - X. Surface photometry, decompositions and the Tully-Fisher relation in the Rc-band
We present Rc-band surface photometry for 170 of the 203 galaxies in GHASP,
Gassendi H-Alpha survey of SPirals, a sample of late-type galaxies for which
high-resolution Fabry-Perot H{\alpha} maps have previously been obtained. Our
data set is constructed by new Rc-band observations taken at the Observatoire
de Haute-Provence (OHP), supplemented with Sloan Digital Sky Survey (SDSS)
archival data, obtained with the purpose of deriving homogeneous photometric
profiles and parameters. Our results include Rc-band surface brightness
profiles for 170 galaxies and profiles for 108 of these objects. We
catalogue several parameters of general interest for further reference, such as
total magnitude, effective radius and isophotal parameters -- magnitude,
position angle, ellipticity and inclination. We also perform a structural
decomposition of the surface brightness profiles using a multi-component method
in order to separate disks from bulges and bars, and to observe the main
scaling relations involving luminosities, sizes and maximum velocities.
We determine the Rc-band Tully Fisher relation using maximum velocities
derived solely from H rotation curves for a sample of 80 galaxies,
resulting in a slope of , zero point of and an
estimated intrinsic scatter of . We note that, different from
the TF-relation in the near-infrared derived for the same sample, no change in
the slope of the relation is seen at the low-mass end (for galaxies with
km/s). We suggest that this different behaviour of the Tully
Fisher relation (with the optical relation being described by a single
power-law while the near-infrared by two) may be caused by differences in the
stellar mass to light ratio for galaxies with km/s.Comment: 17 pages, 11 figure
The 1/N Expansion in Noncommutative Quantum Mechanics
We study the 1/N expansion in noncommutative quantum mechanics for the
anharmonic and Coulombian potentials. The expansion for the anharmonic
oscillator presented good convergence properties, but for the Coulombian
potential, we found a divergent large N expansion when using the usual
noncommutative generalization of the potential. We proposed a modified version
of the noncommutative Coulombian potential which provides a well-behaved 1/N
expansion.Comment: v2: resided version, to appear in PRD, 18 pages, 4 figure
Modelization of Thermal Fluctuations in G Protein-Coupled Receptors
We simulate the electrical properties of a device realized by a G protein
coupled receptor (GPCR), embedded in its membrane and in contact with two
metallic electrodes through which an external voltage is applied. To this
purpose, recently, we have proposed a model based on a coarse graining
description, which describes the protein as a network of elementary impedances.
The network is built from the knowledge of the positions of the C-alpha atoms
of the amino acids, which represent the nodes of the network. Since the
elementary impedances are taken depending of the inter-nodes distance, the
conformational change of the receptor induced by the capture of the ligand
results in a variation of the network impedance. On the other hand, the
fluctuations of the atomic positions due to thermal motion imply an impedance
noise, whose level is crucial to the purpose of an electrical detection of the
ligand capture by the GPCR. Here, in particular, we address this issue by
presenting a computational study of the impedance noise due to thermal
fluctuations of the atomic positions within a rhodopsin molecule. In our model,
the C-alpha atoms are treated as independent, isotropic, harmonic oscillators,
with amplitude depending on the temperature and on the position within the
protein (alpha-helix or loop). The relative fluctuation of the impedance is
then calculated for different temperatures.Comment: 5 pages, 2 figures, Proceeding of the 18-th International Conference
on Fluctuations and Noise, 19-23 September 2005, Salamanca, Spain -minor
proofreadings
Oxygen-stripes in La0.5Ca0.5MnO3 from ab initio calculations
We investigate the electronic, magnetic and orbital properties of
La0.5Ca0.5MnO3 perovskite by means of an ab initio electronic structure
calculation within the Hartree-Fock approximation. Using the experimental
crystal structure reported by Radaelli et al. [Phys. Rev B 55, 3015 (1997)], we
find a charge-ordering stripe-like ground state. The periodicity of the
stripes, and the insulating CE-type magnetic structure are in agreement with
neutron x-ray and electron diffraction experiments. However, the detailed
structure is more complex than that envisaged by simple models of charge and
orbital order on Mn d-levels alone, and is better described as a charge-density
wave of oxygen holes, coupled to the Mn spin/orbital order.Comment: 4 pages, 3 figures. Version accepted for publication in PR
Gravitational Waves from Rotating Proto-Neutron Stars
We study the effects of rotation on the quasi normal modes (QNMs) of a newly
born proto neutron star (PNS) at different evolutionary stages, until it
becomes a cold neutron star (NS). We use the
Cowling approximation, neglecting spacetime perturbations, and consider
different models of evolving PNS. The frequencies of the modes of a PNS are
considerably lower than those of a cold NS, and are further lowered by
rotation; consequently, if QNMs were excited in a sufficiently energetic
process, they would radiate waves that could be more easily detectable by
resonant-mass and interferometric detectors than those emitted by a cold NS. We
find that for high rotation rates, some of the g-modes become unstable via the
CFS instability; however, this instability is likely to be suppressed by
competing mechanisms before emitting a significant amount of gravitational
waves.Comment: 5 pages, proceedings of the 5th Edoardo Amaldi Conference On
Gravitational Wave
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