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Search Results: 1 - 10 of 22938 matches for " Alfonso San Miguel "
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Ab initio study of charge doping effect on 1D polymerization of C60
Roberta Poloni,Alfonso San Miguel,Maria Victoria Fernandez-Serra
Physics , 2011, DOI: 10.1088/0953-8984/24/9/095501
Abstract: We study the interplay between charge doping and intermolecular distance in the polymerization of C60 fullerene chains by means of density functional theory (DFT)-based first principle calculations. The potential energy surface analysis shows that both the equilibrium intermolecular distance of the unpolymerized system and the polymerization energy barrier are inversely proportional to the electron doping of the system. We analyze the origin of this charge-induced polymerization effect by studying the behavior of the system's wavefunctions around the Fermi level and the structural modifications of the molecules as a function of two variables: the distance between the centers of the molecules and the number of electrons added to the system.
First-principles study of lithium-doped carbon clathrates under pressure
Nicolas Rey,Alfonso Munoz,Placida Rodriguez-Hernandez,Alfonso San Miguel
Physics , 2007, DOI: 10.1088/0953-8984/20/21/215218
Abstract: We present a theoretical study on the behavior under pressure of the two hypothetical C$_{46}$ and Li$_8$C$_{46}$ type-I carbon clathrates in order to bring new informations concerning their synthesis. Using \textit{ab initio} calculations, we have explored the energetic and structural properties under pressure of these two carbon based cage-like materials. These low-density meta-stable phases show large negative pressure transitions compared to diamond which represent a serious obstacle for their synthesis. However, we evidence that a minimum energy barrier can be reached close to 40 GPa, suggesting that the synthesis of the Li-clathrate under extreme conditions of pressure and temperature may be possible. Electronic band structure with related density of states behavior under pressure as well as the dependence of the active Raman modes with pressure are also examined.
Evidence for Scattering-Dependent Multigap Superconductivity in Ba8Si46
Yves Noat,Tristan Cren,Pierre Toulemonde,Alfonso San Miguel,Francois Debontridder,Vincent Dubost,Dimitri Roditchev
Physics , 2010,
Abstract: We have studied the quasiparticle excitation spectrum of the superconductor Ba8Si46 by local tunneling spectroscopy. Using high energy resolution achieved in Superconductor-Superconductor junctions we observed tunneling conductance spectra of a non-conventional shape revealing two distinct energy gaps, DeltaL = 1.3meV and DeltaS = 0.9meV. The analysis of tunneling data evidenced that DeltaL is the principal superconducting gap while DeltaS, smaller and more dispersive, is induced into an intrinsically non-superconducting band of the material by the inter-band quasiparticle scattering.
Structural and phonon properties of bundled single- and double-wall carbon nanotubes under pressure
Acrisio. L. Aguiar,Rodrigo B. Capaz,A. G. Souza Filho,Alfonso San-Miguel
Physics , 2012,
Abstract: In this work, we report a theoretical coupled study of the structural and phonons properties of bundled single- and double-walled carbon nanotubes (DWNTs), under hydrostatic compression. Our results confirm drastic changes in volume of SWNTs in high-pressure regime as assigned by a phase transition from circular to collapsed phase which are strictly dependent on the tube diameter. For the DWNTs, those results show first a transformation to a polygonized shape of the outer tube and subsequently the simultaneous collapse of the outter and inner tube, at the onset of the inner tube ovalization. Before the DWNT collapse, phonon calculations reproduce the experimentally observed screening effect on the inner tube pressure induced blue shift both for RBM and tangential G$_z$ modes . Furthermore, the collapse of CNTs bundles induces a sudden redshift of tangential component in agreement with experimental studies. The G$_z$ band analysis of the SWNT collapsed tubes shows that the flattened regions of the tubes are at the origin of their G-band signal. This explains the observed graphite type pressure evolution of the G band in the collapsed phase and provides in addition a mean for the identification of collapsed tubes
Enhancing the superconducting transition temperature of BaSi2 by structural tuning
José A. Flores-Livas,Régis Debord,Silvana Botti,Alfonso San Miguel,Miguel A. L. Marques,Stéphane Pailhès
Physics , 2010, DOI: 10.1103/PhysRevLett.106.087002
Abstract: We present a joint experimental and theoretical study of the superconducting phase of the layered binary silicide BaSi2. Compared with the layered AlB2 structure of graphite or diboride-like superconductors, in the hexagonal structure of binary silicides the sp3 arrangement of silicon atoms leads to corrugated sheets. Through a high-pressure synthesis procedure we are able to modify the buckling of these sheets, obtaining the enhancement of the superconducting transition temperature from 4 K to 8.7 K when the silicon planes flatten out. By performing ab-initio calculations based on density functional theory we explain how the electronic and phononic properties of the system are strongly affected by changes in the buckling. This mechanism is likely present in other intercalated layered superconductors, opening the way to the tuning of superconductivity through the control of internal structural parameters.
Aportes a la construcción de la identidad profesional del profesor de teatro
Folios , 2012,
Abstract: abstract this paper contributes to the debate on knowledge construction and professional identity of the drama teacher. from information gathered in practical classes of the teacher training program in performing arts at the universidad pedagógica nacional and supported on an anthropological view of knowledge and the methodology of the joint didactic action theory, it argues that knowledge and identity processes are set in social planes, in which body, interactions and mediations focused on the body are instances that produce the sense of the professional endeavor.
Aportes a la construcción de la identidad profesional del profesor de teatro Contributions to the Construction of the Drama Teacher Professional Identity
Miguel Alfonso
Folios , 2012,
Abstract: Resumen El siguiente escrito aporta al debate sobre la construcción del saber y la identidad profesional del profesor de teatro. A partir de información recogida en las clases de las asignaturas prácticas de la Licenciatura en Artes Escénicas de la Universidad Pedagógica Nacional, apoyado en una visión antropológica del saber y en la metofont sadología de la Teoría de la acción didáctica conjunta, plantea que los saberes y los procesos identitarios se configuran en planos sociales, en los que el cuerpo, las interacciones y las mediaciones centradas en el cuerpo son instancias que producen el sentido de la labor profesional. Abstract This paper contributes to the debate on knowledge construction and professional identity of the drama teacher. From information gathered in practical classes of the Teacher Training Program in Performing Arts at the Universidad Pedagógica Nacional and supported on an anthropological view of knowledge and the methodology of the joint didactic action theory, it argues that knowledge and identity processes are set in social planes, in which body, interactions and mediations focused on the body are instances that produce the sense of the professional endeavor.
Aguilera P,Alfonso; Ellena D,Miguel; Seguel B,Ivette; Montenegro B,Adolfo; San Martín A,José; Torralbo B,Luis; Rebolledo R,Ramón;
Idesia (Arica) , 2009, DOI: 10.4067/S0718-34292009000200004
Abstract: in 2005 there were published the first records of insects and other phytophagous invertebrates species associated with ugni molinae turcz. in chile. during 2005-2006 a survey of phytophagous insects was continued in natural habitat and artificial conditions as experimental cultivated species. in this work ten new records of phytopagous insects are added associated to u. molinae in farming conditions: two hemiptera (one pseudococcidae; one margarodidae), five lepidoptera (one arctiidae; one hepialidae; two saturnidae; one tortricidae) and three coleoptera (one scarabaeidae; two curculionidae) so, at present 32 species of phytophagous insects and other invertebrates are registered on u. molinae, a berry of southern chile. the phytophagous insects and considering two species of cicadellidae mentioned as vector s of phytoplasma known as witches broom, at present represent 88,24% of all phytophagous invertebrate organisms determined on u. molinae in chile.
Boletín de la Sociedad Chilena de Química , 2001, DOI: 10.4067/S0366-16442001000100007
Abstract: several new 2,3-functionally dialkyl-1,4-benzohydroquinone diacetates have been prepared by oxidative cleavage of epoxide compounds iii, obtained from the diels-alder condensation product between the monoterpene myrcene and 1,4-benzoquinone. the nature of the isolated products is depending of the functionality of the side chain
Phonon density of states, anharmonicity, electron-phonon coupling and possible multigap superconductivity in the clathrate superconductors Ba8Si46 and Ba24Si100: Why is Tc different in these two compounds?
Rolf Lortz,Romain Viennois,Alexander Petrovic,Yuxing Wang,Pierre Toulemonde,Christoph Meingast,Michael Marek Koza,Hannu Mutka,Alexei Bossak,Alfonso San Miguel
Physics , 2008, DOI: 10.1103/PhysRevB.77.224507
Abstract: We report a detailed study of specific heat, electrical resistivity and thermal expansion in combination with inelastic neutron and inelastic X-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 K and 1.5 K respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function and phonon anharmonicity from these measurements we discuss the important factors governing Tc and explain the difference between the two compounds.
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