Coupling of the A_{1g} As-phonon to magnetism in iron pnictides

Charge, spin and lattice degrees of freedom are strongly entangled in iron superconductors. A neat consequence of this entanglement is the behavior of the A_{1g} As-phonon resonance in the different polarization symmetries of Raman spectroscopy when undergoing the magneto-structural transition. In this work we show that the observed behavior could be a direct consequence of the coupling of the phonons with the electronic excitations in the anisotropic magnetic state. We discuss this scenario within a five orbital tight-binding model coupled to phonons via the dependence of the Slater-Koster parameters on the As position. We identify two qualitatively different channels of the electron-phonon interaction: a geometrical one related to the Fe-As-Fe angle and another one associated with the modification upon As displacement of the Fe-As energy integrals pdsigma and pdpi. While both mechanisms result in a finite B_{1g} response, the behavior of the phonon intensity in the A_{1g} and B_{1g} Raman polarization geometries is qualitatively different when the coupling is driven by the angle or by the energy integral dependence. We discuss our results in view of the experimental reports.Comment: 10 pages including 7 figure

Consideraciones florístico-ecológicas acerca de los roquedos calizos del Pirineo navarro. I. Foz de Mintxate.

Se ha estudiado la flora de las comunidades de Asplenietea rupestria Br.-Bl. y Meier 193^ y Thlaspietea rotundifolii Br.-Bl. 19^7 de una foz navarra situada en el extremo occidental de la provincia corológica pirenaica. Se hacen diversas consideraciones floristicas, corológicas, fitosociológicas y ecológicas que surgen del análisis de los dato

Flora fungica de las formaciones navarras de Pinus Sylvestris L.; I. Ascomycetes

Se estudian 24 especies de Ascomycetes recolectados durante las primaveras de 1978 y 1979 en pinares de Pinus sylveatris L. repartidos por el NE navarro. Cada especie, en las localidades reseñadas, se acompaña de un breve comentario ecológico. Un estudio descriptivo e iconográfico se ha llevado a cabo para todos los taxa

Doubly stochastic coherence via noise-induced symmetry in bistable neural models

The generation of coherent dynamics due to noise in an activator-inhibitor system describing bistable neural dynamics is investigated. We show that coherence can be induced in deterministically asymmetric regimes via symmetry restoration by multiplicative noise, together with the action of additive noise which induces jumps between the two stable steady states. The phenomenon is thus doubly stochastic, because both noise sources are necessary. This effect can be understood analytically in the frame of a small-noise expansion and is confirmed experimentally in a nonlinear electronic circuit. Finally, we show that spatial coupling enhances this coherent behavior in a form of system-size coherence resonance

Esfagnos y esfagnales de Navarra (España)

La Navarra silícea es muy favorable al desarrollo de turberas con esfagnos. Se han estudiado diversas localidades que permiten señalar la presencia de 14 taxa de esfagnos de los 33 que componen la flora española. 5 de ellos son novedades para la brioflora navarra. Resume: Sphaignes et tourbieres a sphaignes de Navarra (Espagne). La région de la Navarre siliceuse est tres favorable au développement des tourbieres a sphaignes. Diverses localités y ont été étudiées qui permettent de signaler la presence de 14 taxa de Sphaignes sur 33 de la flore espagnole. 5 entre elles sont nouvelles pour la brioflore de la Navarre

Theory of Ferromagnetism in Doped Excitonic Condensates

Nesting in a semimetal can lead to an excitonic insulator state with spontaneous coherence between conduction and valence bands and a gap for charged excitations. In this paper we present a theory of the ferromagnetic state that occurs when the density of electrons in the conduction band and holes in the valence band differ. We find an unexpectedly rich doping-field phase diagram and an unusual collective excitation spectrum that includes two gapless collective modes. We predict regions of doping and external field in which phase-separated condensates of electrons and holes with parallel spins and opposing spins coexist.Comment: 5 pages, 3 postscript file

Assisted hopping and interaction effects in impurity models

We study, using Numerical Renormalization Group methods, the generalization of the Anderson impurity model where the hopping depends on the filling of the impurity. We show that the model, for sufficiently large values of the assisted hopping term, shows a regime where local pairing correlations are enhanced. These correlations involve pairs fluctuating between on site and nearest neighbor positions

Briófitos del hayedo de peña Izaga (Navarra)

Se estudia la Flora Briofítica del hayedo de Peña de Izaga (Navarra). Se citan 64 especies (11 hepáticas y 53 musgos), comentando su autoecología. Se comparan áreas de hayedo típico (Helleboro/Fagetum 0. de Bolòs (1948) 1957) y comunidades limítrofes (comunidades importadoras), discutiéndose las posibles causas climáticas-ecológicas de las diferencias observadas. Résumé: On a étudié la Flore Muscinale des hêtraies de Peña Izaga (Navarra). La forêt climax (Helleboro-Fagetum 0. de Bolos (1948) 1957) et les communautés végétales limitrophes comprendent une bryoflore constituée par 64 espèces (11 hépatiques et 53 muscinées). On a fait une brève analyse de l'auto-écologie de chaque espèce et une discussion des facteurs du milieu qui causent leur distribution locale