f(R)f(R) quantum cosmology: avoiding the Big Rip

Extended theories of gravity have gathered a lot of attention over the last years, for they not only provide an excellent framework to describe the inflationary era but also yields an alternative to the elusive and mysterious dark energy. Among the different extended theories of gravity, on this work we focus on metric $f(R)$ theories. In addition, it is well known that if the late-time acceleration of the universe is stronger than the one induced by a cosmological constant then some future cosmic singularities might arise, being the Big Rip the most virulent one. Following this reasoning, on this work, we analyse the Big Rip singularity in the framework of $f(R)$ quantum geometrodynamics. Invoking the DeWitt criterium, i. e. that the wave function vanishes at the classical singularity, we proof that a class of solutions to the Wheeler-DeWitt equation fulfilling this condition can be found. Therefore, this result hints towards the avoidance of the Big Rip in metric $f(R)$ theories of gravity.Comment: V1:13 pages. Dedicated to the memory of Prof. Pedro F. Gonzalez-Diaz (our former PhD supervisor). V2: 9 pages (new style), minor clarifications included, no physics changes, 6 references added. Version accepted for publication in Physical Review

Ab initio calculations of structures and stabilities of (NaI)_nNa+ and (CsI)_nCs+ cluster ions

Ab initio calculations using the Perturbed Ion model, with correlation contributions included, are presented for nonstoichiometric (NaI)_nNa+ and (CsI)_nCs+ (n=1-14) cluster ions. The ground state and several low-lying isomers are identified and described. Rocksalt ground states are common and appear at cluster sizes lower than in the corresponding neutral systems. The most salient features of the measured mobilities seem to be explained by arguments related to the changes of the compactness of the clusters as a function of size. The stability of the cluster ions against evaporation of a single alkali halide molecule shows variations that explain the enhanced stabilities found experimentally for cluster sizes n=4, 6, 9, and 13. Finally, the ionization energies and the orbital eigenvalue spectrum of two (NaI)_13Na+ isomers are calculated and shown to be a fingerprint of the structure.Comment: 8 pages plus 13 postscript figures, LaTeX. Accepted for publication in Phys, Rev. B; minor changes including a more complete comparison to pair potential result

Orbital-Free Molecular Dynamics Simulations of Melting in Na8 and Na20: Melting in Steps

The melting-like transitions of Na8 and Na20 are investigated by ab initio constant energy molecular dynamics simulations, using a variant of the Car-Parrinello method which employs an explicit electronic kinetic energy functional of the density, thus avoiding the use of one-particle orbitals. Several melting indicators are evaluated in order to determine the nature of the various transitions, and compared with other simulations. Both Na8 and Na20 melt over a wide temperature range. For Na8, a transition is observed to begin at approx. 110 K, between a rigid phase and a phase involving isomerizations between the different permutational isomers of the ground state structure. The ``liquid'' phase is completely established at approx. 220 K. For Na20, two transitions are observed: the first, at approx. 110 K, is associated with isomerization transitions between those permutational isomers of the ground state structure which are obtained by interchanging the positions of the surface-like atoms; the second, at approx. 160 K, involves a structural transition from the ground state isomer to a new set of isomers with the surface molten. The cluster is completely ``liquid'' at approx. 220 K.Comment: Revised version, accepted for publication in J. Chem. Phys. The changes include longer simulations for the Na20 microcluster, a more complete comparison to previous theoretical results, and the discussion of some technical details of the method applie

Location of franchises and large retail chains following the Great Recession. The case of the city of Zaragoza (Spain)

The current retail activity model is rapidly evolving, in order to adapt to consumer changes. For a long time, shopping centres have stood out as the most attractive model. However, the implementation of franchises and large commercial chains in cities is transforming the uses of urban space and configuring new spatial relationships. The city of Saragossa serves as a case study

Evaluacion de la union adhesiva en la fabricacion de tableros contrachapados de grado exterior ignifugados

100 p.En este estudio se confeccionaron tableros contrachapados con chapas tratadas con un producto retardarte de llama (Stop-fire), unidas con un adhesivo fenolico, (Oxilite 2200), con el objetivo de obtener una unión satisfactoria en la conformación de un tablero contra chapado; debido a que existen probadas sustancias ignifugas de origen chileno en el mercado, que interfieren en el fraguado de los adhesivos fenolicos. Para la obtención de una unión adhesiva de calidad, se modifico el pH del retardante de llama. Una vez incorporado este a la madera, se aplico un tratamiento superficial a la chapa para mejorar la calidad de la unión. Fabricados los tableros, se extrajeron las probetas para realizar los ensayos de resistencia de la unión adhesiva, resistencia mecánica en flexión y de reacción al fuego. Estas propiedades fueron medidas de acuerdo a las normas PS 1-95, ASTM D 3043 - 87 y ISO 5660-1. El análisis de resultados indica que el tratamiento superficial aplicado a las chapas no mejoró la calidad de la unión adhesiva. El tratamiento ignifugo que se preparo con un pH de 9,6 logro aprobar el ensayo de unión adhesiva en la conformación de un tablero contrachapado, sin alterar las propiedades ignifugas del producto retardante de llama, solo mostrando una leve disminución en la resistencia mecánica en flexión de los tableros