60 research outputs found
Beats of the Magnetocapacitance Oscillations in Lateral Semiconductor Superlattices
We present calculations on the magnetocapacitance of the two-dimensional
electron gas in a lateral semiconductor superlattice under two-dimensional weak
periodic potential modulation in the presence of a perpendicular magnetic
field. Adopting a Gaussian broadening of magnetic-field-dependent width in the
density of states, we present explicit and simple expressions for the
magnetocapacitance, valid for the relevant weak magnetic fields and modulation
strengths. As the modulation strength in both directions increase, beats of the
magnetocapacitance oscillations are observed, in the low magnetic field range
(Weiss-oscillations regime), which are absent in the one-dimensional weak
modulation case.Comment: 11 pages, 7 figures, accepted by Mod. Phys. Lett. B (March 2007
Isophots of Zodiacal Light and Its Terrestrial Component
Isophotes of zodiacal light and its terrestrial component due to interplanetary dust in vicinity of earth-moon syste
UBVRI twilight sky brightness at ESO-Paranal
Twilight studies have proved to be important tools to analyze the atmospheric
structure with interesting consequences on the characterization of astronomical
sites. Active discussions on this topic have been recently restarted in
connection with the evaluation of Dome C, Antarctica as a potential
astronomical site and several site-testing experiments, including twilight
brightness measurements, are being prepared. The present work provides for the
first time absolute photometric measurements of twilight sky brightness for
ESO-Paranal (Chile), which are meant both as a contribution to the site
monitoring and as reference values in the analysis of other sites, including
Dome C. The UBVRI twilight sky brightness was estimated on more than 2000 FORS1
archival images, which include both flats and standard stars observations taken
in twilight, covering a Sun zenith distance range 94-112 deg. The comparison
with a low altitude site shows that Paranal V twilight sky brightness is about
30% lower, implying that some fraction of multiple scattering has to take place
at an altitude of a few km above the sea level.Comment: 11 pages, 13 figures, accepted for publication in A&
16s rrna sequencing analysis of the gut microbiota in broiler chickens prophylactically administered with antimicrobial agents
In poultry production, gut microbiota (GM) plays a pivotal role and influences different host functions related to the efficiency of production performances. Antimicrobial (AM) use is one of the main factors affecting GM composition and functions. Although several studies have focused their attention on the role of AMs as growth promoters in the modulation of GM in broilers, the consequences of higher AM concentrations administered during prophylactic treatments need to be better elucidated. For this purpose, 16S rRNA gene sequencing was performed to evaluate the impact of different prophylactic AM protocols on the composition and diversity of the broiler GM. Diversity analysis has shown that AM treatment significantly affects alpha diversity in ileum and beta diversity in both ileum and caecum. In ileal samples, the Enterobacteriaceae family has been shown to be particularly affected by AM treatments. AMs have been demonstrated to affect GM composition in broiler. These findings indicate that withdrawal periods were not enough for the restoral of the original GM. Further studies are needed for a better elucidation of the negative effects caused by an altered GM in broilers
Nuclear muon-positron conversion mediated by Majorana neutrinos
We study lepton number violating (LNV) process of muon-positron conversion in
nuclei mediated by the exchange of light and heavy Majorana neutrinos. Nuclear
structure calculations have been carried out for the case of experimentally
interesting nucleus 48Ti in the framework of renormalized proton-neutron
Quasiparticle Random Phase Approximation. We demonstrate that the imaginary
part of the amplitude of light Majorana neutrino exchange mechanism gives an
appreciable contribution to the muon-positron conversion rate. This specific
feature is absent in the allied case of neutrinoless double beta decay. Using
the present neutrino oscillations, tritium beta decay, accelerator and
cosmological data we derived the limits on the effective masses of light
_{\mu e} and heavy _{\mu e} neutrinos. The expected rates of
muon-positron nuclear conversion, corresponding to these limits, were found to
be so small that even within a distant future the muon-positron conversion
experiments will hardly be able to detect the neutrino signal. Therefore,
searches for this LNV process can only rely on the presence of certain physics
beyond the trivial extension of the Standard Model by inclusion of massive
Majorana neutrinos.Comment: 28 pages, 3 figures. Added comments, references and one figure. Typos
in eqs. (18), (19) correcte
Localized Wavefunctions and Magnetic Band Structure for Lateral Semiconductor Superlattices
In this paper we present calculations on the electronic band structure of a
two-dimensional lateral superlattice subject to a perpendicular magnetic field
by employing a projection operator technique based on the ray-group of
magnetotranslation operators. We construct a new basis of appropriately
symmetrized Bloch-like wavefunctions as linear combination of well-localized
magnetic-Wannier functions. The magnetic field was consistently included in the
Wannier functions defined in terms of free-electron eigenfunctions in the
presence of external magnetic field in the symmetric gauge. Using the above
basis, we calculate the magnetic energy spectrum of electrons in a lateral
superlattice with bi-directional weak electrostatic modulation. Both a square
lattice and a triangular one are considered as special cases. Our approach
based on group theory handles the cases of integer and rational magnetic fluxes
in a uniform way and the provided basis could be convenient for further both
analytic and numerical calculations.Comment: 19 pages, 5 figures. accepted to Int. J. Mod. Phys. B (April 2006
Direct detection of supersymmetric dark matter- Theoretical rates for transitions to excited states
The recent WMAP data have confirmed that exotic dark matter together with the
vacuum energy (cosmological constant) dominate in the flat Universe.
Supersymmetry provides a natural dark matter candidate, the lightest
supersymmetric particle (LSP). Thus the direct dark matter detection is central
to particle physics and cosmology. Most of the research on this issue has
hitherto focused on the detection of the recoiling nucleus. In this paper we
study transitions to the excited states, focusing on the first excited state at
50 keV of Iodine A=127. We find that the transition rate to this excited state
is about 10 percent of the transition to the ground state. So, in principle,
the extra signature of the gammai ray following its de-excitation can be
exploited experimentally.Comment: LaTex, 13 pages, 3 postscript figures, 1 table, to appear in IJMP
Dark Matter Spin-Dependent Limits for WIMP Interactions on 19-F by PICASSO
The PICASSO experiment at SNOLAB reports new results for spin-dependent WIMP
interactions on F using the superheated droplet technique. A new
generation of detectors and new features which enable background discrimination
via the rejection of non-particle induced events are described. First results
are presented for a subset of two detectors with target masses of F of
65 g and 69 g respectively and a total exposure of 13.75 0.48 kgd. No
dark matter signal was found and for WIMP masses around 24 GeV/c new limits
have been obtained on the spin-dependent cross section on F of
= 13.9 pb (90% C.L.) which can be converted into cross section
limits on protons and neutrons of = 0.16 pb and = 2.60 pb
respectively (90% C.L). The obtained limits on protons restrict recent
interpretations of the DAMA/LIBRA annual modulations in terms of spin-dependent
interactions.Comment: Revised version, accepted for publication in Phys. Lett. B, 20 pages,
7 figure
Searching for Supersymmetric Dark Matter - The Directional Rate and the Modulation Effect Due to Caustic Rings
The detection of the theoretically expected dark matter is central to
particle physics and cosmology. Current fashionable supersymmetric models
provide a natural dark matter candidate which is the lightest supersymmetric
particle (LSP). The allowed parameter space of such models combined with fairly
well understood physics (quark substructure of the nucleon and nuclear
structure) permit the evaluation of the event rate for LSP-nucleus elastic
scattering. The thus obtained event rates, which sensitively depend on the
allowed parameter space parameters, are usually very low or even undetectable.
So, for background reduction, one would like to exploit two nice features of
the reaction, the directional rate, which depends on the sun's direction of
motion and the modulation effect, i.e. the dependence of the event rate on the
earth's annual motion. In the present paper we study these phenomena in a
specific class of non isothermal models, which take into account the late
in-fall of dark matter into our galaxy, producing flows of caustic rings. We
find that the modulation effect arising from such models is smaller than that
found previously with isothermal symmetric velocity distributions and much
smaller compared to that obtained using a realistic asymmetric distribution
with enhanced dispersion in the galactocentric direction.Comment: 31 LATEX pages, 2 tables and 1 figure included. Accepted for
publication in Physical Review
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