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COLLOQUIA
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19 June 2012, 12:00 h. Salón de Actos
MAGNETIC EXCITATIONS OF MOLECULES
ON THE SURFACE OF A SUPERCONDUCTOR
Jose Ignacio Pascual
Freie Universität Berlin, Alemania |
Magnetism and superconductivity are phenomena that cannot simultaneously exist in
the same region of a material: weak applied magnetic fields are expelled from the
superconductor, while strong magnetic fields destroy the superconducting properties.
An interesting playground to study the interplay between both phenomena are magnetic
impurities in a superconductor. They can either destroy locally the superconducting state
or be fully screened, depending on the specific details of how the local magnetic
moment interacts with the Cooper pairs.
Using scanning tunnelling spectroscopy, we investigate these interactions at the level of
single paramagnetic molecules on a superconducting lead surface. For the case of
sizeable coupling between magnetic moment and quasiparticles at the lead substrate,
we observe localized states in the superconducting energy gap [1,2,3]. These bound
states reflect the weakening of the superconducting paring [4] and allows us to track the
pair-breaking process. From our results we are able of resolving the delicate balance of
interactions of the magnetic impurity with either Cooper pairs or normal electrons. While
the former leads to the rupture of the superconducting state, the later screens the
magnetic moment of the impurity via the Kondo effect. The balance turns out to be
sensitive to the atomic scale environment, which thus determines the resulting ground
state of the system [4]. By simply modifying the molecular functionalisation and
environment interactions can be tuned, allowing us to explore different magnetic states
of a paramagnetic molecule in a superconducting-superconducting tunnelling junction.
[1] A. Yazdani, et al. Science 275, 1767 (1997)
[2] S.-H. Ji, et al. Phys. Rev. Lett. 100, 226801 (2008)
[3 K. J. Franke, G. Schulze, J. I. Pascual, Science 332, 940 (2011)
[4] H. Shiba, Prog. Theor. Phys. 40, 435 (1968)
332, 940–944 (2011) |
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29 May 2012, 12:00 h. Salón de Actos
Ultracold atoms in optical lattices as cond-mat simulators
Christiane Morais Smith
Institute for Theoretical Physics, Utrecht University, Netherlands |
| During the last years, cold atoms loaded into optical lattices emerged as an ideal playground to emulate condensed matter systems. Indeed, with optical lattices, not only the geometry and dimensionality of the system can be tuned, but also the amplitude of the hopping parameter. Besides, the nature of the particles (fermions, bosons, or mixtures) can be chosen at will. By using Feshbach resonances, it is possible to control the strength and sign (repulsive or attractive) of the interactions. In addition, the system is free of disorder, although the latter can also be introduced in a controlled way. Recently, it was shown that it is possible to generate gauge fields by using Raman excitations or by shaking the lattice.
In this talk, I will first introduce the main concepts of cold atom physics, before discussing how to use this system to simulate graphene, high-Tc superconductors, and topological insulators. In particular, the recent realization of a p-orbital Bose-Einstein condensate provides an interesting example of an emergent broken symmetry and the cold-atomic analog of topological states of matter.
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24 April 2012, 12:00 h. Sala de Seminarios, 182
Nanoestructuracion , minimalismo y autoorganizacion en un un sistema arquetipo altamente descorrelacionado:
Frentes de crecimiento de laminas delgadas de Au policristalino.
Jose Luis Sacedón
ICMM-CSIC |
J. L. Sacedon, E. Rodriguez, A. Gonzalez, E. Vasco
Polycrystalline thin films deposited at room temperature follows competitive columnar growth model. The surface growth fronts, composed by paraboloid-like surface protrusions, (SPs), have been recently decomposed into elemental paraboloidal meridian zones which allows a comprehensive statistical analysis of the film surface [1]. Using this analysis, local height distribution curves have been synthesized [2] and the interface width expressed as function of the statistical parameters [3]. The method evidences that the average value of the slopes obtained at the SP border remains constant during the growth [2]. The analysis of distribution curves of the terrace width (~ the inverse of the slope) at the nanoelements borders provides terrace-width limit values , constant during the growth [4] . We propose a self organization formalism, which applied to the interaction between surface nanoelements during the growth, allows to predict the limiting terrace width values observed, as well as their constancy .Additional effects of surface currents, driven by tensional [6] or chemical surface potential gradients, appear naturally included in the final model.
[1] E. Rodríguez-Cañas, J. A. Aznárez, A. I. Oliva, and J. L. Sacedón, Surf. Sci. 600, 3110 (2006)
[2] J. L. Sacedón, E. Rodríguez-Cañas, C. Munuera, A. I. Oliva, and J. A. Aznárez, Phys. Rev. B 72 195413 (2005)
[3] E. Rodríguez-Cañas, E. Vasco, and J. L. Sacedón, Appl. Phys. Lett. 90 013112 (2007)
[4] E. Rodríguez-Cañas, E. Vasco, J. A. Aznárez, A. Ruiz, C. Munuera, A. González-González , and J. L. Sacedon, Surf. Sci. 604, 974 (2010)
[5] J. Yu, and J. G. Amar, Phys. Rev. B 69, 045426 (2004)
[6] A. González-González, G. M. Alonzo-Medina, A. I. Oliva, C. Polop, J. L. Sacedón, and E. Vasco Phys. Rev. B 84, 155450 (2011)
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20 March 2012, 12:00 h. Salón de Actos
Building a viral particle: The morphogenetic pathway
José L. Carrascosa
Centro Nacional de Biotecnología, CSIC. |
| Viruses are excellent examples for optimization of the use of genetic information: Their limited genomic content forces them to use diverse strategies to perform complex functions with a minimum of different components. The building of a viral particle in complex double stranded DNA viruses is a good example on how a small number of structural components assemble to yield an infective viral particle making use of general principles and extensive symmetry constraints. Furthermore, the building of the viral particle follows an elaborate pathway, where a first assembled precursor is matured by sequential interaction with other structural components, following a unidirectional assembly line. We have studied the molecular bases of this fascinating process using as a model system the bacteriophage T7, but the results obtained so far might be extrapolated to many other viral systems. |
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07 February 2012, 12:00 h. Salón de Actos
"Nobel de Física de 2011:la aceleración de la expansión del universo"
Juan Garcia-Bellido
Instituto de Física Teórica -UAM/CSIC |
| Las observaciones de supernovas lejanas que sugieren la aceleracion actual del Universo han merecido el Premio Nobel de Fisica 2011. En esta charla-coloquio describiré dichas observaciones, asi como las dificultades que esto entraña para la Fisica Teórica. En un futuro próximo se llevaran a cabo cartografiados profundos del Universo, como el Dark Energy Survey, que permitirán explorar la naturaleza de la supuesta energía oscura responsable de la aceleracion del Universo." |
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17 January 2012, 12:00 h. Salón de Actos
Papel de los télomeros en el cáncer y envejecimiento
Maria Blasco
Centro Nacional de Investigaciones Oncológicas |
| Los telómeros protegen los extremos de los cromosomas frente a las actividades de reparación y degradación del DNA. La longitud de los telómeros y la integridad de las proteínas de unión a los telómeros son importantes para la protección telomérica. Además, los telómeros se transcriben y que el RNA telomérico permanece asociado a la cromatina telomérica. Nuestro grupo ha demostrado que el acortamiento de los telomeros a longitudes críticamente cortas es suficiente para causar envejecimiento del organismo y que si frenamos este proceso mediante la activacion de telomerasa, somos capaces de retrasar el envejecimiento y aumentar la logevidad. |
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