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 $f(R)$ gravity theories provide an alternative way to explain the current cosmic acceleration without a dark energy matter component. If gravity is governed by a $f(R)$ 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 $f(R)$ gravity theories permit space-times in which the causality is violated. We show that the field equations of these $f(R)$ 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 $f(R)$ gravity satisfying the condition $df/dR > 0$ 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 $r_c$ (beyond which the causality is violated) for an arbitrary $f(R)$ theory, making apparent that the violation of causality depends on both the $f(R)$ gravity theory and the matter content. As an illustration, we concretely take a recent $f(R)$ 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 $\rho =\rho_{1}$ parallel to the $z-$ axis, with $\rho_{1}$ only depending on the ratios $\frac{\mathcal{Q}}{E^{2}-1}$ and $\frac{a}{M}$, where $a$ and $M$ are the parameters of the Kerr black hole, $E$ the particle energy and $\mathcal{Q}$ 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