64,361 research outputs found
Persistent currents in mesoscopic cavities and the effect of level crossings as random variables
In the present article we perform analytical and numerical calculations
related to persistent currents in 2D isolated mesoscopic annular cavities
threaded by a magnetic flux. The system considered has a high number of open
channels and therefore the single particle spectrum exhibits many level
crossings as the flux varies. We determine the effect of the distribution of
level crossings in the typical persistent current.Comment: 23 pages, including 6 figure
Gromov, Cauchy and causal boundaries for Riemannian, Finslerian and Lorentzian manifolds
Recently, the old notion of causal boundary for a spacetime V has been
redefined in a consistent way. The computation of this boundary
for a standard conformally stationary spacetime V = R x M, suggests a natural
compactification associated to any Riemannian metric on M or, more
generally, to any Finslerian one. The corresponding boundary is
constructed in terms of Busemann-type functions. Roughly,
represents the set of all the directions in M including both, asymptotic and
"finite" (or "incomplete") directions. This Busemann boundary is
related to two classical boundaries: the Cauchy boundary and the Gromov
boundary. Our aims are: (1) to study the subtleties of both, the Cauchy
boundary for any generalized (possibly non-symmetric) distance and the Gromov
compactification for any (possibly incomplete) Finsler manifold, (2) to
introduce the new Busemann compactification , relating it with the
previous two completions, and (3) to give a full description of the causal
boundary of any standard conformally stationary spacetime.Comment: Final version with minor modifications, to appear in Memoirs of the
American Mathematical Society. 80 pages, 10 figures, late
Drinfel'd Twists and Algebraic Bethe Ansatz
We study representation theory of Drinfel'd twists, in terms of what we call
F matrices, associated to finite dimensional irreducible modules of quantum
affine algebras, and which factorize the corresponding (unitary) R matrices. We
construct explicitly such factorizing F matrices for irreducible tensor
products of the fundamental representations of the quantum affine algebra sl2
and its associated Yangian. We then apply these constructions to the XXX and
XXZ quantum spins chains of finite length in the framework of the Algebraic
Bethe Ansatz.Comment: 45 pages, late
Hausdorff separability of the boundaries for spacetimes and sequential spaces
There are several ideal boundaries and completions in General Relativity
sharing the topological property of being sequential, i.e., determined by the
convergence of its sequences and, so, by some limit operator . As emphasized
in a classical article by Geroch, Liang and Wald, some of them have the
property, commonly regarded as a drawback, that there are points of the
spacetime non -separated from points of the boundary .
Here we show that this problem can be solved from a general topological
viewpoint. In particular, there is a canonical minimum refinement of the
topology in the completion which -separates the spacetime
and its boundary ---no matter the type of completion one
chooses. Moreover, we analyze the case of sequential spaces and show how the
refined -separating topology can be constructed from a modification
of the original limit operator . Finally, we particularize this procedure to
the case of the causal boundary and show how the separability of and
can be introduced as an abstract axiom in its definition.Comment: 29 pages, 8 figures, late
On the final definition of the causal boundary and its relation with the conformal boundary
The notion of causal boundary for a strongly causal spacetime
has been a controversial topic along last decades: on one hand, some
attempted definitions were not fully consistent, on the other, there were
simple examples where an open conformal embedding i:M\hookarrow M_{0} could
be defined, but the corresponding conformal boundary disagreed
drastically with the causal one. Nevertheless, the recent progress in this
topic suggests a definitive option for , which is developed here in
detail. Our study has two parts: (I) To give general arguments on a boundary in
order to ensure that it is admissible as a causal boundary at the three natural
levels, i.e., as a point set, as a chronological space and as a topological
space. Then, the essential uniqueness of our choice is stressed, and the
relatively few admissible alternatives are discussed. (II) To analyze the role
of the conformal boundary . We show that, in general,
may present a very undesirable structure. Nevertheless, it is
well-behaved under certain general assumptions, and its accessible part
agrees with the causal boundary. This study justifies both
boundaries. On one hand, the conformal boundary , which
cannot be defined for a general spacetime but is easily computed in particular
examples, appears now as a special case of the causal boundary. On the other,
the new redefinition of the causal boundary not only is free of inconsistencies
and applicable to any strongly causal spacetime, but also recovers the expected
structure in the cases where a natural conformal boundary is available. The
cases of globally hyperbolic spacetimes and asymptotically conformally flat
ends are especially studied.Comment: Revisded version, including the correction of Appendix 3.6 (51 pages,
13 figures). To appear in Adv. Th. Math. Phy
The Magnetic Fields of the Quiet Sun
This work reviews our understanding of the magnetic fields observed in the
quiet Sun. The subject has undergone a major change during the last decade
(quiet revolution), and it will remain changing since the techniques of
diagnostic employed so far are known to be severely biased. Keeping these
caveats in mind, our work covers the main observational properties of the quiet
Sun magnetic fields: magnetic field strengths, unsigned magnetic flux
densities, magnetic field inclinations, as well as the temporal evolution on
short time-scales (loop emergence), and long time-scales (solar cycle). We also
summarize the main theoretical ideas put forward to explain the origin of the
quiet Sun magnetism. A final prospective section points out various areas of
solar physics where the quiet Sun magnetism may have an important physical role
to play (chromospheric and coronal structure, solar wind acceleration, and
solar elemental abundances).Comment: Review talk presented to the 6th Solar Polarization Workshop held in
Maui HI, USA, May-June 201
Brain Damage and Motor Cortex Impairment in Chronic Obstructive Pulmonary Disease: Implication of Nonrapid Eye Movement Sleep Desaturation
Nonrapid eye movement (NREM) sleep desaturation may cause neuronal damage due
to the withdrawal of cerebrovascular reactivity. The current study (1) assessed
the prevalence of NREM sleep desaturation in nonhypoxemic patients with chronic
obstructive pulmonary disease (COPD) and (2) compared a biological marker of
cerebral lesion and neuromuscular function in patients with and without NREM
sleep desaturation
Quantitative Stellar Spectral Classification. II. Early Type Stars
The method developed by Stock and Stock (1999) for stars of spectral types A
to K to derive absolute magnitudes and intrinsic colors from the equivalent
widths of absorption lines in stellar spectra is extended to B-type stars.
Spectra of this type of stars for which the Hipparcos Catalogue gives
parallaxes with an error of less than 20% were observed with the CIDA 1-meter
reflector equipped with a Richardson spectrograph with a Thompson 576x384 CCD
detector. The dispersion is 1.753 A/pixel using a 600 lines/mm grating in the
first order. In order to cover the spectral range 3850 A to 5750 A the grating
had to be used in two different positions, with an overlap in the region from
4800 A to 4900 A. A total of 116 stars was oberved, but not all with both
grating positions. A total of 12 measureable absorption lines was identified in
the spectra and their equivalent widths were measured. These were related to
the absolute magnitudes derived from the Hipparcos Catalogue and to the
intrinsic colors (deduced from the MK spectral types) using linear and second
order polynomials and two or three lines as independent variables. The best
solutions were obtained with polynomials of three lines, reproducing the
absolute magnitudes with an average residual of about 0.40 magnitudes and the
intrinsic colors with an average residual of 0.016 magnitudes.Comment: 17 pages including 4 figures, accepted by Rev.Mex.A.
Quantitative Stellar Spectral Classification. III. Spectral Resolution
The method developed by Stock and Stock (1999) to derive absolute magnitudes
and intrinsic colors is applied to simulated low-resolution spectra. The
simulation is made by convolving real spectra with a Gaussian function,
(the full width at half maximum) related to the final spectral
resolution. The accuracy with which the stellar parameters are determined
indicates that the method may be applied to typical objective-prism spectra. We
show that changes in the spectral resolution do not significantly affect the
stellar parameters obtained with this method for early-type stars, whereas for
later-type stars an improved approach is necessary.Comment: 12 pages, 7 figures embedded, Accepted by RevMexA
Algorithmic Complexity in Noise Induced Transport Systems
Time correlated fluctuations interacting with a spatial asymmetry potential
are sufficient conditions to give rise to transport of Brownian particles. The
transfer of information coming from the nonequilibrium bath, viewed as a source
of negentropy, give rise to the correlated noise. The algorithmic complexity of
an object provides a means of quantitating its information contents. The
Kolmogorov information entropy or algorithmic complexity is investigated in
order to quantitate the transfer of information that occurs in computational
models showing noise induced transport. The complexity is measured in terms of
the average number of bits per time unit necessary to specify the sequence
generated by the system.Comment: 7 pages, 2 figure
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