The U(1) phase transition on toroidal and spherical lattices

We have studied the properties of the phase transition in the U(1) compact pure gauge model paying special atention to the influence of the topology of the boundary conditions. From the behavior of the energy cumulants and the observation of an effective \nu -> 1/d on toroidal and spherical lattices, we conclude that the transition is first order.Comment: LATTICE98(gauge

Study of the Coulomb-Higgs transition in the Abelian Higgs Model

The order of the Coulomb-Higgs transition in the U(1)-Higgs model with unfrozen modulus of the scalar field is studied. Large lattices (up to $24^4$ in one case) and high statistics are used. We fix $\beta =1.15$ and explore specially a region of $\lambda$-values where metastability is observed. We study the thermodynamical limit of several observables, in particular, the latent heat, the specific heat, the decrement of the free energy between the maxima and the central minimum of the two-peaked histogram, the Binder cumulant and the displacement of the critical coupling with the lattice size. The results point towards a second order transition for $\lambda \gtrsim 0.005$, while for smaller values of $\lambda$ the strong metastability growing with the lattice size seems to derive from a first order character.Comment: 10 pages, Latex, epsfig, uuencoded gzipped tar file, 4 figures include

A multisite microcanonical updating method

We have made a study of several update algorithms using the XY model. We find that sequential local overrelaxation is not ergodic at the scale of typical Monte Carlo simulation time. We have introduced a new multisite microcanonical update method, which yields results compatible with those of random overrelaxation and the microcanonical demon algorithm, which are very much slower, all being incompatible with the sequential overrelaxation results.Comment: 13 pages, 4 figure

El magmatismo (Ortoneises y volcanismo del Ordovícico Superior) del Paleozoico de los Catalánides.

En el presente trabajo se caracterizan petrológica y geoquímicamente las rocas metavolcánicas del Ordovícico Superior de los Catalánides (Gavarres y Guilleries) y los ortoneises de las Guilleries. Estos últimos se encuentran en disposición estratiforme en materiales metasedimentarios cuya edad supuesta es Proterozoico Superior o Cámbrico Inferior. Las rocas metavolcánicas están constituidas por metarriolitas masivas y tobas dacítico-riolíticas con abundante mezcla de material fragmentario. Corresponden a un volcanismo explosivo subaéreo y de carácter freatomagmático. Sus caracteres geoquímicos lo definen como los términos ácidos de una secuencia calcoalcalina de procedencia cortical emplazada postcolisionalmente dentro de un contexto extensivo. El modelo de REE indica que se trata de fundidos poco fraccionados procedentes de la fusión de una corteza reciclada. Los "Ortoneises de las Guilleries" corresponden a leuconeises biotíticos con granate. Sus caracteres geoquímicos los caracterizan como metagranilos alumínicos de procedencia cortical. Sus espectros geoquímicos (elementos incompatibles y REE) presentan pautas análogas a los de las rocas metavolcánicas e indican que se trata de fundidos de procedencia cortical de baja alcalinidad emplazados en un contexto anorogénico. La ausencia de correlación geoquímica y de pautas de fraccionación entre metavolcanitas y ortoneises descarta la idea de una fuente común para ambos

SU(1,1) Coherent States For Position-Dependent Mass Singular Oscillators

The Schroedinger equation for position-dependent mass singular oscillators is solved by means of the factorization method and point transformations. These systems share their spectrum with the conventional singular oscillator. Ladder operators are constructed to close the su(1,1) Lie algebra and the involved point transformations are shown to preserve the structure of the Barut-Girardello and Perelomov coherent states.Comment: 11 pages, 5 figures. This shortened version (includes new references) has been adapted for its publication in International Journal of Theoretical Physic

Interacting holographic tachyon model of dark energy

We propose a holographic tachyon model of dark energy with interaction between the components of the dark sector. The correspondence between the tachyon field and the holographic dark energy densities allows the reconstruction of the potential and the dynamics of the tachyon scalar field in a flat Friedmann-Robertson-Walker universe. We show that this model can describe the observed accelerated expansion of our universe with a parameter space given by the most recent observational results.Comment: 7 pages, 8 figures, accepted for publication in IJMP

A Proposal for a Modified Moller-Plesset Perturbation Theory

A modified version of the Moller-Plesset approach for obtaining the correlation energy associated to a Hartree-Fock ground state is proposed. The method is tested in a model of interacting fermions that allows for an exact solution. Using up to third order terms improved results are obtained, even more accurate in the limit of loosely bound particles. This result suggests the possible convenience of the scheme for the study of chemical bound problems.Comment: 10 pages, 1 figur

The Academic Resilience Approach in the promotion of young people’s mental health. Proposals for its use in schools

Els joves que gaudeixen de bona salut mental són més propensos a tenir un rendiment acadèmic més bo i un millor desenvolupament social. Davant l’increment de problemes de salut mental entre adolescents a Catalunya, es presenta l’Enfocament de Resiliència Acadèmica (ERA) com una pràctica estratègica per a centres d’educació secundària que involucra tota la comunitat educativa. La finalitat és ajudar que els joves, especialment els més vulnerables, aconsegueixin obtenir un rendiment acadèmic més òptim, malgrat les circumstàncies en les quals es troben. Les propostes per integrar l’ERA als centres de secundària es basen en la redefinició de les activitats que ja es desenvolupen i en el disseny de noves pràctiques educatives considerant els elements del Marc de Resiliència

La restauración de Sambucus palmensis en La Gomera: Conservación genética y modelización de nicho climático

La conservación de especies vegetales amenazadas a menudo conlleva la restauración de sus poblaciones naturales, ya sea mediante el refuerzo de las poblaciones existentes, la reintroducción de poblaciones extintas o la introducción de nuevos núcleos. Antes de comenzar acciones de restauración, es esencial conocer la biología de las especies con el fin de determinar los factores más importantes que limitan el crecimiento de la población fundadora (Heywood & Iriondo, 2003

Phase diagram of the (bosonic) Double-Exchange Model

The phase diagram of the simplest approximation to Double-Exchange systems, the bosonic Double-Exchange model with antiferromagnetic super-exchange coupling, is fully worked out by means of Monte Carlo simulations, large-N expansions and Variational Mean-Field calculations. We find a rich phase diagram, with no first-order phase transitions. The most surprising finding is the existence of a segment like ordered phase at low temperature for intermediate AFM coupling which cannot be detected in neutron-scattering experiments. This is signaled by a maximum (a cusp) in the specific heat. Below the phase-transition, only short-range ordering would be found in neutron-scattering. Researchers looking for a Quantum Critical Point in manganites should be wary of this possibility. Finite-Size Scaling estimates of critical exponents are presented, although large scaling corrections are present in the reachable lattice sizes.Comment: 17 pages, 14 figure