Observation of a New Fluxon Resonant Mechanism in Annular Josephson Tunnel Structures

A novel dynamical state has been observed in the dynamics of a perdurbed sine-Gordon system. This resonant state, has been experimentally observed as a singularity in the dc current voltage characteristic of an annular Josephson tunnel junction, excited in the presence of a magnetic field. With this respect, it can be assimilated to self-resonances known as Fiske steps. Differently from these, however, we demonstrate, on the basis of numerical simulations, that its detailed dynamics involves rotating fluxon pairs, a mechanism associated, so far, to self-resonances known as zero-field steps.Comment: 4 pages, 2 figures, submitted to Physical Review Letter

Rutherford scattering with radiation damping

We study the effect of radiation damping on the classical scattering of charged particles. Using a perturbation method based on the Runge-Lenz vector, we calculate radiative corrections to the Rutherford cross section, and the corresponding energy and angular momentum losses.Comment: Latex, 11 pages, 4 eps figure

The Bernstein problem for intrinsic graphs in Heisenberg groups and calibrations

In this paper we deal with some problems concerning minimal hypersurfaces in Carnot-Caratheodory (CC) structures. More precisely we will introduce a general calibration method in this setting and we will study the Bernstein problem for entire regular intrinsic minimal graphs in a meaningful and simpler class of CC spaces, i.e. the Heisenberg group H^n. In particular we will positively answer to the Bernstein problem in the case n=1 and we will provide counterexamples when n>=5

Theoretical investigation of magnetoelectric effects in Ba2CoGe2O7

A joint theoretical approach, combining macroscopic symmetry analysis with microscopic methods (density functional theory and model cluster Hamiltonian), is employed to shed light on magnetoelectricity in Ba2CoGe2O7. We show that the recently reported experimental trend of polarization guided by magnetic field can be predicted on the basis of phenomenological Landau theory. From the microscopic side, Ba2CoGe2O7 emerges as a prototype of a class of magnetoelectrics, where the cross coupling between magnetic and dipolar degrees of freedom needs, as main ingredients, the on-site spin-orbit coupling and the spin-dependent O p - Co d hybridization, along with structural constraints related to the noncentrosymmetric structural symmetry and the peculiar configuration of CoO4 tetrahedrons.Comment: 5 pages, 4 figures, submitted for publicatio

Diagnostics for specific PAHs in the far-IR: searching neutral naphthalene and anthracene in the Red Rectangle

Context. In the framework of the interstellar polycyclic aromatic hydrocarbons (PAHs) hypothesis, far-IR skeletal bands are expected to be a fingerprint of single species in this class. Aims. We address the question of detectability of low energy PAH vibrational bands, with respect to spectral contrast and intensity ratio with ``classical'' Aromatic Infrared Bands (AIBs). Methods. We extend our extablished Monte-Carlo model of the photophysics of specific PAHs in astronomical environments, to include rotational and anharmonic band structure. The required molecular parameters were calculated in the framework of the Density Functional Theory. Results. We calculate the detailed spectral profiles of three low-energy vibrational bands of neutral naphthalene, and four low-energy vibrational bands of neutral anthracene. They are used to establish detectability constraints based on intensity ratios with ``classical'' AIBs. A general procedure is suggested to select promising diagnostics, and tested on available Infrared Space Observatory data for the Red Rectangle nebula. Conclusions. The search for single, specific PAHs in the far-IR is a challenging, but promising task, especially in view of the forthcoming launch of the Herschel Space Observatory.Comment: 13 pages, 13 figures, accepted for publication in A&

Gas phase formation of the prebiotic molecule formamide: insights from new quantum computations

New insights into the formation of interstellar formamide, a species of great relevance in prebiotic chemistry, are provided by electronic structure and kinetic calculations for the reaction NH2 + H2CO -> NH2CHO + H. Contrarily to what previously suggested, this reaction is essentially barrierless and can, therefore, occur under the low temperature conditions of interstellar objects thus providing a facile formation route of formamide. The rate coefficient parameters for the reaction channel leading to NH2CHO + H have been calculated to be A = 2.6x10^{-12} cm^3 s^{-1}, beta = -2.1 and gamma = 26.9 K in the range of temperatures 10-300 K. Including these new kinetic data in a refined astrochemical model, we show that the proposed mechanism can well reproduce the abundances of formamide observed in two very different interstellar objects: the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular shock L1157-B2. Therefore, the major conclusion of this Letter is that there is no need to invoke grain-surface chemistry to explain the presence of formamide provided that its precursors, NH2 and H2CO, are available in the gas-phase.Comment: MNRAS Letters, in pres

Localization of Gauge Fields and Monopole Tunnelling

We study the dynamical localization of a massless gauge field on a lower-dimensional surface (2-brane). In flat space, the necessary and sufficient condition for this phenomenon is the existence of confinement in the bulk. The resulting configuration is equivalent to a dual Josephson junction. This duality leads to an interesting puzzle, as it implies that a localized massless theory, even in the Abelian case, must become confining at exponentially large distances. Through the use of topological arguments we clarify the physics behind this large-distance confinement and identify the instantons of the brane world-volume theory that are responsible for its appearance. We show that they correspond to the (condensed) bulk magnetic charges (monopoles), that occasionally tunnel through the brane and induce weak confinement of the brane theory. We consider the possible generalization of this effect to higher dimensions and discuss phenomenological bounds on the confinement of electric charges at exponentially large distances within our Universe.Comment: 11 pages, 3 figures, improvements in the presentation, version to appear in Physical Review

The xx-Region of Shadowing Corrections in Nucleon Structure Functions

We discuss the experimental indications on the behaviour of F_2(x,Q^2) at small x both in proton and nuclear targets. By comparing the parametrizations of the data we conclude that shadowing correction effects in a proton target can appear at a noticeable level for x=(2 - 4)\times10^{-4} and Q^2\sim 10^1 GeV^2, namely inside the HERA regime.Comment: 5 pages, Latex, no figure

Spin analog of the controlled Josephson charge current

We propose a controlled Josephson spin current across the junction of two non-centrosymmetric superconductors like CePt_3Si. The Josephson spin current arises due to direction dependent tunneling matrix element and different momentum dependent phases of the triplet components of the gap function. Its modulation with the angle \xi between the noncentrosymmetric axes of two superconductors is proportional to \sin \xi. This particular dependence on \xi may find application of the proposed set-up in making a Josephson spin switch.Comment: 4 pages, 1 figure; title is changed; article is rewritte