Lorentzian compact manifolds: isometries and geodesics

In this work we investigate families of compact Lorentzian manifolds in dimension four. We show that every lightlike geodesic on such spaces is periodic, while there are closed and non-closed spacelike and timelike geodesics. Their isometry groups are computed. We also show that there is a non trivial action by isometries of \Heis_3(\RR) on the nilmanifold S^1\times (\Gamma_k \bsh \Heis_3(\RR)) for $\Gamma_k$ a lattice of \Heis_3(\RR).Comment: 17 page

Coherent radiation by molecular magnets

The possibility of coherent radiation by molecular magnets is investigated. It is shown that to realize the coherent radiation, it is necessary to couple the considered sample to a resonant electric circuit. A theory for describing this phenomenon is developed, based on a realistic microscopic Hamiltonian, including the Zeeman terms, single-site anisotropy, and dipole interactions. The role of hyperfine interactions between molecular and nuclear spins is studied. Numerical solutions of the spin evolution equations are presented.Comment: Latex file, 11 pages, 3 figure

Dipolar ordering in Fe8?

We show that the low-temperature physics of molecular nanomagnets, contrary to the prevailing one-molecule picture, must be determined by the long-range magnetic ordering due to many-body dipolar interactions. The calculations here performed, using Ewald's summation, suggest a ferromagnetic ground state with a Curie temperature of about 130 mK. The energy of this state is quite close to those of an antiferromagnetic state and to a glass of frozen spin chains. The latter may be realized at finite temperature due to its high entropy.Comment: 7 pages, no figures, submitted to EP

Magnetic qubits as hardware for quantum computers

We propose two potential realisations for quantum bits based on nanometre scale magnetic particles of large spin S and high anisotropy molecular clusters. In case (1) the bit-value basis states |0> and |1> are the ground and first excited spin states Sz = S and S-1, separated by an energy gap given by the ferromagnetic resonance (FMR) frequency. In case (2), when there is significant tunnelling through the anisotropy barrier, the qubit states correspond to the symmetric, |0>, and antisymmetric, |1>, combinations of the two-fold degenerate ground state Sz = +- S. In each case the temperature of operation must be low compared to the energy gap, \Delta, between the states |0> and |1>. The gap \Delta in case (2) can be controlled with an external magnetic field perpendicular to the easy axis of the molecular cluster. The states of different molecular clusters and magnetic particles may be entangled by connecting them by superconducting lines with Josephson switches, leading to the potential for quantum computing hardware.Comment: 17 pages, 3 figure

Roofing Slate Deposits in the Iberian Peninsula

España es el principal exportador mundial de pizarras, debido tanto a la calidad excepcional de sus yacimientos, como a los avances tecnológicos que se han ido incorporando en los últimos años en el proceso productivo. El presente artículo describe de manera general los principales litotectos de la Península Ibérica, incluidos dentro del área noroeste del Macizo Ibérico Hespérico, y entre los que destacan los pertenecientes a la Formación Pizarras de Luarca, que en Portugal se correlaciona con la Formación Pizarras de Valongo. En total, en la Península Ibérica se pueden distinguir 12 distritos mineros de pizarra para cubiertas, los cuales son valorados como yacimientos según criterios de tipo minero y petrológico.The exceptional quality of the slate of northwest Spain and the new technological advances incorporated in the last years into the production process have made the country the largest roofing slate producer in the World. This article describes the main slate lithotecs of the Iberian Peninsula. Most of them belong to the Luarca Slates Formation, which in Portugal correlates into the Valongo Slates Formation. Following mining and petrological criteria, a total of 12 mining districts of the Iberian Peninsula have been evaluated as roofing slates deposits.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEConselleria de Industria y Minas de la Xunta de Galiciapu

Pulsed field studies of the magnetization reversal in molecular nanomagnets

We report experimental studies of crystals of Mn12 molecular magnetic clusters in pulsed magnetic fields with sweep rates up to 4x10^3 T/s. The steps in the magnetization curve are observed at fields that are shifted with respect to the resonant field values. The shift systematically increases as the rate of the field sweep goes up. These data are consistent with the theory of the collective dipolar relaxation in molecular magnets.Comment: 4 pages, 4 figure

Sistema de inspección mediante imágenes de luminiscencia de módulos solares en funcionamiento

CIES2020 - XVII Congresso Ibérico e XIII Congresso Ibero-americano de Energia SolarRESUMEN: Los avances en la detección de defectos en módulos fotovoltaicos por luminiscencia se están viendo demorados por la necesidad que las técnicas de inspección actuales tienen de actuar sobre los contactos eléctricos y retirar los paneles de su estado de funcionamiento. En el presente trabajo se introduce un sistema que vence esta limitación, pudiendo tomar imágenes de luminiscencia sobre módulos solares en funcionamiento. Dicho sistema se pone a prueba mediante la implementación de un primer prototipo funcional, con el que se toman imágenes de un string de células completo para poder compararlas con las obtenidas utilizando las técnicas de inspección actuales, determinando así sus similitudes y diferencias, y corroborando la validez del método propuesto.ABSTRACT: Advances in the detection of defects in photovoltaic modules by luminescence are being delayed by the need that current inspection techniques have to act on the electrical contacts and remove the panels from their operating state. In the present work, a system is introduced that overcomes this limitation, being able to take images of luminescence on solar modules in operation. This system is put to the test by implementing a first functional prototype, with which images are taken of a complete string of cells to be able to compare them with those obtained using current inspection techniques, thus determining their similarities and differences, and corroborating the validity of the proposed method.info:eu-repo/semantics/publishedVersio