57,418 research outputs found
Anomalous exchange interaction between intrinsic spins in conducting graphene systems
We address the nature and possible observable consequences of singular
one-electron states that appear when strong defects are introduced in the
metallic family of graphene, namely, metallic carbon nanotubes and nanotori. In
its simplest form, after creating two defects on the same sublattice, a state
may emerge at the Fermi energy presenting very unusual properties: It is
unique, normalizable, and features a wave function equally distributed around
both defects. As a result, the exchange coupling between the magnetic moments
generated by the two defects is anomalous. The intrinsic spins couple
ferromagnetically, as expected, but do not present an antiferromagnetic excited
state at any distance. We propose the use of metallic carbon nanotubes as a
novel electronic device based on this anomalous coupling between spins which
can be useful for the robust transmission of magnetic information at large
distances.Comment: 5 pages 5 fugure
Optimal Supervisory Policies and Depositor-Preferences Laws.
When supervisors have imperfect information about the soundness of banks, they may be unaware of insolvency problems that develop in the interval between on-site examinations. Supervising banks more often will alleviate this problem but will increase the costs of supervision. This paper analyzes the trade-offs that supervisors face between the cost of supervision and their need to monitor banks effectively. We first characterize the optimal supervisory policy, in terms of the time between examinations and the closure rule at examinations, and compare it with the policy of an independent supervisor. We then show that making this supervisor accountable for deposit insurance losses in general reduces the excessive forbearance of the independent supervisor and may also improve on the time between examinations. Finally, we extend our analysis to the impact of depositor-preference laws on supervisors' monitoring incentives and show that these laws may lead to conflicting effects on the time between examinations and closure policy vis-a-vis the social optimum.Deposit Insurance; Depositor Preference; Supervision.
G\"{o}del-type universes in f(R) gravity
The gravity theories provide an alternative way to explain the current
cosmic acceleration without a dark energy matter component. If gravity is
governed by a theory a number of issues should be reexamined in this
framework, including the violation of causality problem on nonlocal scale. We
examine the question as to whether the gravity theories permit
space-times in which the causality is violated. We show that the field
equations of these gravity theories do not exclude solutions with
breakdown of causality for a physically well-motivated perfect-fluid matter
content. We demonstrate that every perfect-fluid G\"{o}del-type solution of a
generic gravity satisfying the condition is necessarily
isometric to the G\"odel geometry, and therefore presents violation of
causality. This result extends a theorem on G\"{o}del-type models, which has
been established in the context of general relativity. We also derive an
expression for the critical radius (beyond which the causality is
violated) for an arbitrary theory, making apparent that the violation of
causality depends on both the gravity theory and the matter content. As
an illustration, we concretely take a recent gravity theory that is free
from singularities of the Ricci scalar and is cosmologically viable, and show
that this theory accommodates noncausal as well as causal G\"odel-type
solutions.Comment: 7 pages, V3: Version to appear in Phys. Rev. D (2009), typos
corrected, the generality of our main results is emphasized. The illustrative
character of a particular theory is also made explici
Ultracold atoms in optical lattices with random on-site interactions
We consider the physics of lattice bosons affected by disordered on-site
interparticle interactions. Characteristic qualitative changes in the zero
temperature phase diagram are observed when compared to the case of randomness
in the chemical potential. The Mott-insulating regions shrink and eventually
vanish for any finite disorder strength beyond a sufficiently large filling
factor. Furthermore, at low values of the chemical potential both the
superfluid and Mott insulator are stable towards formation of a Bose glass
leading to a possibly non-trivial tricritical point. We discuss feasible
experimental realizations of our scenario in the context of ultracold atoms on
optical lattices.Comment: 4 pages, 3 eps figure
The Self-Force of a Charged Particle in Classical Electrodynamics with a Cut-off
We discuss, in the context of classical electrodynamics with a Lorentz
invariant cut-off at short distances, the self-force acting on a point charged
particle. It follows that the electromagnetic mass of the point charge occurs
in the equation of motion in a form consistent with special relativity. We find
that the exact equation of motion does not exhibit runaway solutions or
non-causal behavior, when the cut-off is larger than half of the classical
radius of the electron.Comment: 17 pages, 1 figur
Kerr Geodesics, the Penrose Process and Jet Collimation by a Black Hole
We re-examine the possibility that astrophysical jet collimation may arise
from the geometry of rotating black holes and the presence of high-energy
particles resulting from a Penrose process, without the help of magnetic
fields. Our analysis uses the Weyl coordinates, which are revealed better
adapted to the desired shape of the jets. We numerically integrate the
2D-geodesics equations. We give a detailed study of these geodesics and give
several numerical examples. Among them are a set of perfectly collimated
geodesics with asymptotes parallel to the axis, with
only depending on the ratios and
, where and are the parameters of the Kerr black hole,
the particle energy and the Carter's constant.Comment: Accepted by Astronomy and Astrophysics. AA style with 3 EPS figures.
Content amended after AA's refereeing. Discussion of geodesics also corrected
and expanded earlier. Conclusions amended accordingl
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