Dynamics of a Rydberg hydrogen atom near a topologically insulating surface

We investigate the classical dynamics of a Rydberg hydrogen atom near the surface of a planar topological insulator. The system is described by a Hamiltonian consisting of the free-hydrogen part and the hydrogen-surface potential. The latter includes the interactions between the electron and both image electric charges and image magnetic monopoles. Owing to the axial symmetry, the $z$ component of angular momentum $l_{z}$ is conserved. Here we consider the $l_{z} = 0$ case. The structure of the phase space is explored extensively by means of numerical techniques and Poincar\'{e} surfaces of section for the recently discovered topological insulator TlBiSe$_{2}$. The phase space of the system is separated into regions of vibrational and rotational motion. We show that vibrational-rotational-vibrational type transitions can be tuned with the topological magnetoelectric polarizability.Comment: Accepted for publication in Europhysics Letter

Three-dimensional decomposition of galaxies with bulge and long bar

Some observations indicate that the Milky Way has two inner components, a bulge and a long bar, which present a misalignment of about 20 degrees that is against the predictions of some theoretical models that are based on numerical simulations. In this paper, we wish to determine whether this misalignment between the bar and the bulge can be observed in barred galaxies other than the Milky Way. For that, each galaxy of our sample was decomposed based on its Ks-band 2MASS image by fitting and modelling in a three-dimensional (3D) space the following components: a disc, a bar, and a bulge. The chi-square goodness-of-fit estimation allowed retrieving the best-fit angle values for the bar and the bulge to detect any misalignment. From the 3D decomposition of six barred galaxies, we have detected at least three galaxies (NGC 2217, NGC 3992, and NGC 4593) that present a significant misalignment between the bar and the bulge of more than 20 degrees.Comment: 8 pages, 1 figure. Accepted for publication in A&A. Corrected typo

Rethinking Ephemeral Architecture. Advanced Geometry for Citizen-Managed Spaces

In recent years there have been a high amount of citizen initiatives that address the complex problems of the contemporary city. There are empty or disused spaces that have been reused for urban gardens, for social use, to encourage integration and civic activities activation, etc. Traditional architectural processes do not provide realistic solutions to these initiatives that, along with limited financial resources, have led to the emergence of architectures and self-constructed facilities, almost as an emergency mode, without necessary planning, media and constructive knowledge. The democratization of technology, thanks to laboratories of digital production, combined with knowledge of the properties of different surfaces through the CAD-CAM tools, offers new opportunities for the development of a lightweight, flexible and low impact architecture, very according to the needs of citizens' initiatives that naturally arise in our cities. The new existing scenario contemplates the figure of the architect, or engineer, not only as an agent of the market, but as a professional able to propose efficient solutions to problems from within, bringing their specific knowledge and serving as bridges between the new technological solutions and the challenges of society

The Diffuse Nature of Stromgren Spheres

In this Letter, we argue that the standard analytical derivations of properties of HII regions, such as the speed, shape and asymptotic position of ionisation fronts require a more precise treatment. These derivations use the on the spot approximation, which in effect ignores the diffuse component of the radiation field. We show that, in fact, HII regions are diffusion dominated. This has as a result that the morphology of inhomogeneous HII regions will be drastically different, because shadowing effects have a less profound impact on the apparent shape. Moreover, it will have influence on the propagation speed of ionisation fronts. We quantify our claims by analytically deriving the internal radiation structure of HII regions, taking diffusion fully into account for several different cosmologically relevant density distributions.Comment: 4 pages, 2 figures, accepted for publication in A&A Letter