16,586 research outputs found
The June 2012 transit of Venus. Framework for interpretation of observations
Ground based observers have on 5/6th June 2012 the last opportunity of the
century to watch the passage of Venus across the solar disk from Earth. Venus
transits have traditionally provided unique insight into the Venus atmosphere
through the refraction halo that appears at the planet outer terminator near
ingress/egress. Much more recently, Venus transits have attracted renewed
interest because the technique of transits is being successfully applied to the
characterization of extrasolar planet atmospheres. The current work
investigates theoretically the interaction of sunlight and the Venus atmosphere
through the full range of transit phases, as observed from Earth and from a
remote distance. Our model predictions quantify the relevant atmospheric
phenomena, thereby assisting the observers of the event in the interpretation
of measurements and the extrapolation to the exoplanet case. Our approach
relies on the numerical integration of the radiative transfer equation, and
includes refraction, multiple scattering, atmospheric extinction and solar limb
darkening, as well as an up to date description of the Venus atmosphere. We
produce synthetic images of the planet terminator during ingress/egress that
demonstrate the evolving shape, brightness and chromaticity of the halo.
Guidelines are offered for the investigation of the planet upper haze from
vertically-unresolved photometric measurements. In this respect, the comparison
with measurements from the 2004 transit appears encouraging. We also show
integrated lightcurves of the Venus/Sun system at various phases during transit
and calculate the respective Venus-Sun integrated transmission spectra. The
comparison of the model predictions to those for a Venus-like planet free of
haze and clouds (and therefore a closer terrestrial analogue) complements the
discussion and sets the conclusions into a broader perspective.Comment: 14 pages; 14 figures; Submitted on 02/06/2012; A&A, accepted for
publication on 30/08/201
Complete structure of Z_n Yukawa couplings
We give the complete twisted Yukawa couplings for all the Z_n orbifold
constructions in the most general case, i.e. when orbifold deformations are
considered. This includes a certain number of tasks. Namely, determination of
the allowed couplings, calculation of the explicit dependence of the Yukawa
couplings values on the moduli expectation values (i.e. the parameters
determining the size and shape of the compactified space), etc. The final
expressions are completely explicit, which allows a counting of the DIFFERENT
Yukawa couplings for each orbifold (with and without deformations). This
knowledge is crucial to determine the phenomenological viability of the
different schemes, since it is directly related to the fermion mass hierarchy.
Other facts concerning the phenomenological profile of Z_n orbifolds are also
discussed, e.g. the existence of non--diagonal entries in the fermion mass
matrices, which is related to a non--trivial structure of the
Kobayashi--Maskawa matrix. Finally some theoretical results are given, e.g. the
no--participation of (1,2) moduli in twisted Yukawa couplings. Likewise, (1,1)
moduli associated with fixed tori which are involved in the Yukawa coupling, do
not affect the value of the coupling.Comment: 60 page
Tight-binding study of bilayer graphene Josephson junctions
Using highly efficient simulations of the tight-binding Bogoliubov-de Gennes
model we solved self-consistently for the pair correlation and the Josephson
current in a Superconducting-Bilayer graphene-Superconducting Josephson
junction. Different doping levels for the non-superconducting link are
considered in the short and long junction regime. Self-consistent results for
the pair correlation and superconducting current resemble those reported
previously for single layer graphene except in the Dirac point where remarkable
differences in the proximity effect are found as well as a suppression of the
superconducting current in long junction regime. Inversion symmetry is broken
by considering a potential difference between the layers and we found that the
supercurrent can be switched if junction length is larger than the Fermi
length
Automorphism Group of : Applications to the Bosonic String
This paper is concerned with the formulation of a non-pertubative theory of
the bosonic string. We introduce a formal group which we propose as the
``universal moduli space'' for such a formulation. This is motivated because
establishes a natural link between representations of the Virasoro algebra
and the moduli space of curves. Among other properties of it is shown that
a ``local'' version of the Mumford formula holds on .Comment: 29 page
Giant magnetoresistance in ultra-small Graphene based devices
By computing spin-polarized electronic transport across a finite zigzag
graphene ribbon bridging two metallic graphene electrodes, we demonstrate, as a
proof of principle, that devices featuring 100% magnetoresistance can be built
entirely out of carbon. In the ground state a short zig-zag ribbon is an
antiferromagnetic insulator which, when connecting two metallic electrodes,
acts as a tunnel barrier that suppresses the conductance. Application of a
magnetic field turns the ribbon ferromagnetic and conducting, increasing
dramatically the current between electrodes. We predict large magnetoresistance
in this system at liquid nitrogen temperature and 10 Tesla or at liquid helium
temperature and 300 Gauss.Comment: 4 pages, 4 figure
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