Publicaciones
LECTURES AND NOTES OF J. FLOUQUET [2009-12-14]
Lectures and notes of Jacques Flouqet: On the heavy fermion road
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SUPERCONDUCTIVIDAD Y MAGNETISMO A TRAVÉS DE LA ESPECTROSCOPÍA TÚNEL DE BARRIDO CON PUNTA SUPERCONDUCTORA [2009-11-18]
Tesis doctoral de Vanesa Crespo Rozada.
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ORDEN Y DESORDEN EN LA SUPERCONDUCTIVIDAD [2009-11-18]
Tesis doctoral de Isabel Guillamón Gómez
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SEMANA DE LA CIENCIA 2009 [2009-10-27]
El CITECNOMIK organiza jornadas de puertas abiertas con experimentos de demostración los viernes 13 y 20 de noviembre.
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DIRECT OBSERVATION OF MELTING IN A TWO-DIMENSIONAL [2009-08-11]
Letter published in Nature Physics. doi:10.1038/nphys1368
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EFECTO FUENTE [2009-05-13]
Se muestra la salida de un recipiente de vidrio situado en un baño de helio superfluido, y conectado a este por ua superfuga, que no se ve en el vídeo. Dentro de dicho recipiente se situa un calentador, que se acciona al empezar el vídeo. Se observa como el líquido sale del recipiente a través de una chimenea situada en el centro del vídeo. Un gradiente de temperatura produce por lo tanto un gradiente de presión, que empuja el líquido de dentro del recipiente hacia fuera. El gradiente de presión está producido por flujo de helio superfluido al interior del recipiente, a través de la superfuga.
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NITRÓGENO SÓLIDO [2009-05-12]
This is a windows media file which shows the formation of solid nitrogen through reduction of the vapor pressure of the liquid.
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NANOSCALE SUPERCONDUCTIVITY IN AMORPHOUS W-BASED DEPOSITS [2009-03-05]
Reseña en Europhysics News
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GROWTH OF SINGLE CRYSTALS, BY P.C. CANFIELD [2009-02-26]
Lecture about "Growth of single crystals", by P.C. Canfield
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"SAMPLE CHARACTERIZATION" BY P.C. CANFIELD [2009-02-26]
Lecture about "Characterization of samples", by P.C. Canfield
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"17 POSITION KNOB" BY P.C. CANFIELD [2009-02-26]
Lecture about "17 position knob, rare earth materials", by P.C. Canfield
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SUPERCONDUCTIVITY BY P.C. CANFIELD [2009-02-26]
Superconductivity by P.C. Canfield
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FISHING THE FERMI SEA, P.C. CANFIELD [2009-02-26]
Fishing the Fermi sea, P.C. Canfield
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SEMINARIO EXTRAORDINARIO: DESIGN, DISCOVERY, GROWTH AND CHARACTERIZATION OF NOVEL INTERMETALLIC COMPOUNDS, POR EL DR. P.C. CANFIELD [2009-02-18]
Del 23 al 26 de febrero de 2009, de 15,30 a 17 h., en la Sala de Seminarios (C-III, 502) del Departamento de Física de la Materia Condensada de la Universidad Autónoma de Madrid
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THERMAL AND ACOUSTIC EXPERIMENTS ON POLYMORPHIC ETHANOL: B. KABTOUL, R.J. JIMÉNEZ RIOBÓO AND M.A. RAMOS [2009-01-28]
Calorimetric and acoustic experiments were conducted on pure ethanol in the temperature range around 80-180 K, in which different solid-solid and solid-liquid transitions are observed. Using a versatile calorimetric experimental system, the specific heats of the different phases were accurately measured and the phase transitions between them investigated. The latter investigation was supported by complementary Brillouin-scattering experiments. The influence of several factors in the kinetics of crystallization versus vitrification is demonstrated and briefly discussed.
Keywords: glass transition; specific heat; Brillouin scattering; orientationallydisordered
crystals; ethanol; crystallization kinetics
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PHYSICAL REVIEW B 77, 035316 (2008)/ ELECTRON-ELECTRON INTERACTIONS AND CHARGING EFFECTS IN GRAPHENE QUANTUM DOTS. [2008-02-18]
We analyze charging effects in graphene quantum dots. Using a simple model, we show that when the Fermi level is far from the neutrality point, charging effects lead to a shift in the electrostatic potential and the dot shows standard Coulomb blockade features. Near the neutrality point, surface states are partially occupied and the Coulomb interaction leads to a strongly correlated ground state, which can be approximated by either a Wigner crystal or a Laughlin-like wave function. The existence of strong correlations modifies the transport
properties, which show nonequilibrium effects, similar to those predicted for tunneling into other strongly correlated systems.
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PHYSICAL REVIEW B 76, 125427 (2007) / DISSIPATION IN GRAPHENE AND NANOTUBE RESONATORS [2008-02-18]
Different damping mechanisms in graphene nanoresonators are studied: charges in the substrate, ohmic losses in the substrate and the graphene sheet, breaking and healing of surface bonds (Velcro effect), two level systems, attachment losses, and thermoelastic losses. We find that, for realistic structures and contrary to semiconductor resonators, dissipation is dominated by ohmic losses in the graphene layer and metallic gate. An extension of this study to carbon nanotube-based resonators is presented.
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PHYSICAL REVIEW B 76, 115407 (2007)/ORTHOGONALITY CATASTROPHE AND KONDO EFFECT IN GRAPHENE [2008-02-18]
Anderson?s orthogonality catastrophe (Phys. Rev. Lett. 18, 1049 1967) in graphene, at energies close to the Dirac point, is analyzed. It is shown that, in clean systems, the orthogonality catastrophe is suppressed due to the vanishing density of states at the Dirac point. In the presence of preexisting localized states at the Dirac energy, the orthogonality catastrophe shows features similar to those found in normal metals with a finite density of states at the Fermi level. The implications for the Kondo effect induced by magnetic impurities, and for the Fermi edge singularities in tunneling processes, are also discussed.
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PHYSICAL REVIEW B 75, 235433 (2007)/CHARGE DISTRIBUTION AND SCREENING IN LAYERED GRAPHENE SYSTEMS [2008-02-18]
The charge distribution induced by external fields in finite stacks of graphene planes, or in semi-infinite graphite is considered. The interlayer electronic hybridization is described by a nearest-neighbor hopping term, and the charge induced by the self-consistent electrostatic potential is calculated within the linear response theory (random phase approximation). The screening properties are determined by contributions from interband and intraband electronic transitions. In neutral systems, only interband transitions contribute to the charge
polarizability, leading to insulatinglike screening properties, and to oscillations in the induced charge, with a period equal to the interlayer spacing. In doped systems, we find a screening length that is equivalent to two-to-three graphene layers, superimposed to significant charge oscillations.
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PHYSICAL REVIEW B 75, 155424 (2007)/EXISTENCE AND TOPOLOGICAL STABILITY OF FERMI POINTS IN MULTILAYERED GRAPHENE [2008-02-18]
We study the existence and topological stability of Fermi points in a graphene layer and stacks with many layers. We show that the discrete symmetries (space-time inversion) stabilize the Fermi points in monolayer, bilayer, and multilayer graphenes with orthorhombic stacking. The bands near k=0 and e =0 in multilayers with the Bernal stacking depend on the parity of the number of layers, and Fermi points are unstable when the number of layers is odd. The low-energy changes in the electronic structure induced by commensurate perturbations which mix the two Dirac points are also investigated.
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PHYSICAL REVIEW B 76, 212507 (2007)/CURRENT REVERSAL IN COLLECTIVE RATCHETS INDUCED BY LATTICE INSTABILITY [2008-02-18]
A collective mechanism for current reversal in superconducting vortex ratchets is proposed. The mechanism is based on a two-dimensional instability of the ground state T=0 of the system. We illustrate our results with numerical simulations and experiments in Nb superconducting films fabricated on top of Si substrates with artificially induced asymmetric pinning centers.
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APPLIED PHYSICS LETTERS 91, 062505 (2007)/TRANSVERSE RECTIFICATION IN SUPERCONDUCTING THIN FILMS WITH ARRAYS [2008-02-18]
Superconducting Nb films have been grown on top of arrays of Cu nanotriangles. These asymmetric pinning centers strongly modify the vortex lattice dynamics. Two rectification effects have been observed: (i) longitudinal ratchet effect when the input currents are injected perpendicular to the triangle reflection symmetry axis and (ii) transverse rectification effect when the input currents are injected parallel to the triangle reflection symmetry axis and the output voltage drop occurs perpendicular to the triangle reflection symmetry axis. Increasing the applied magnetic field, the former shows a change of the output voltage polarity, the transverse output voltage does not show any polarity reversal. © 2007 American Institute of Physics. (DOI: 10.1063/1.2767199)
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EL HELIO LÍQUIDO CUMPLE 100 AÑOS [2008-02-13]
Anuncio de los actos conmemorativos y las sesiones de divulgación científica con motivo del centenario del helio líquido.
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JORNADA CIENTÍFICA PROGRAMA CIENCIA Y TECNOLOGÍA EN EL MILIKELVIN [2008-02-13]
El día 14 de febrero se celebrará en la Residencia "La cristalera" de la UAM una Jornada científica del programa I+D CITECNOMIK de la CM.
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PHYSICAL AND ENGINEERING SCIENCES (PESC) HIGHLIGHTS 2007 [2008-02-07]
El comité de física e ingeniería de la ESF (PESC-ESF) ha emitido un boletín
con resultados importantes de sus programas de cooperación a lo largo de
2007. En este documento se recoge, entre otras cosas,
uno de los resultados experimentales del programa Citecnomik (que participa
a su vez en el programa NES, Nanoscience and Engineering in
Superconductivity): la observación de modulaciones a escala atómica en la
densidad de estados superconductora del sistema 2H-NbSe2 (el manuscrito
correspondiente será publicado próximamente en este servidor). Científicos
del Citecnomik han demostrado que las propiedades de un superconductor
pueden presentar variaciones a escala atómica. Para entender la relevancia
de este resultado, recordemos que, hasta ahora, se partía de la suposición,
en el diseño de nuevos dispositivos basados en superconductores, que la
longitud de coherencia, en general del orden de algunas decenas de
nanómetros, es la escala más pequeña sobre la que se esperan variaciones de
las propiedades superconductoras aprovechables para las aplicaciones. Esta
suposición ofrecía limitaciones a la búsqueda de nuevas aplicaciones de la
superconductividad en las escalas de longitud más pequeñas, que, como queda
demostrado ahora, pueden superarse. Ver más en ................... HighlightsPESC01-Final_2.pdf
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LOW-TEMPERATURE CALORIMETRY ON MOLECULAR GLASSES AND CRYSTALS [2007-12-17]
Abstract
We have designed and developed a versatile calorimetric system especially intended for glass-forming liquids, with which we are able to
concurrently study and characterize the phase transitions in the range 77-300Kand also to measure their specific heat at lowtemperatures (0.3-30 K).
At the former, higher temperatures we employ either a continuous method or the quasi-adiabatic, Nernst method. For specific-heat measurements at
lower temperatures, we can use both quasi-adiabatic and thermal-relaxation methods. In particular, we have built up two complementary versions
of the thermal-relaxation method (the standard one and an alternative version useful for longer relaxation times). A user-friendly PC software has
been developed, allowing us to choose among the four calorimetric methods, as well as to tune all controlling parameters. We have applied this
experimental system to investigate the calorimetric and thermodynamic behavior of the different solid phases of ethanol, as well as other interesting
glass-forming systems such as alkyl-cyclohexanes or ionic liquids.
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VORTEX LATTICE VIEWED BY MILLIKELVIN STM ON NBSE2 [2007-09-28]
The movie is made of conductance maps on the vortex lattice of superconducting NbSe2 at 100 mK. The maps are made using a tip of Au in an area of about 500 nm x 500 nm, and the video is constructed from the normalized conductance for voltages well above the superconducting gap down to voltages at the Fermi level. About 1 s corresponds to a change in bias voltage of about 0.1 mV. At the beginning of the video, vortices are barely seen, and appear to be white, as the density of states outside vortices is higher than inside, due to the quasiparticle peaks of the superconducting regions. However, close to the Fermi level, the contrast changes and a myriad of rays give images with star-like features that exemplify the way the quasiparticles behave close to the Fermi level and inside vortex cores. These results reproduce closely the work of Hess et al. Phys. Rev. Lett. 62, 214 (1989) and Phys. Rev. Lett. 64 2711 (1990). See also http://mp.okayama-u.ac.jp/~hayashi/. Recently we have discovered an additional modulation of the superconducting density of states at atomic level. Corresponding videos and publication will soon be posted.
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SOME MODERN ASPECTS OF THE PHYSICS OF STRONGLY CORRELATED ELECTRON SYSTEMS [2007-05-15]
Libro Escuela N. Cabrera 1999
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EXPLORANDO SEALES FERROMAGNETICAS EN GRAFITO [2007-03-27]
El grafito, estado cristalino estable del carbono puro a presión y temperatura ambientes, es sabido que presenta un fuerte y anisotrópico diamagnetismo, achacable a sus electrones p deslocalizados. No obstante, en las últimas dos décadas diversos investigadores han reportado evidencias más o menos claras de señales de ferromagnetismo en carbono a temperatura ambiente, especialmente en el caso de muestras de grafito pirolítico altamente orientado (HOPG) irradiadas con protones, por parte del grupo del prof. P. Esquinazi. Distintos trabajos teóricos parecen apoyar la importancia del desorden y/o de las parejas protón-vacante de carbono a la hora de explicar el ferromagnetismo.
El interés de poder disponer de materiales orgánicos no metálicos con propiedades magnéticas es evidente. Por ello, distintos grupos de investigación localizados en el campus de Cantoblanco (Madrid) hemos emprendido el último año una línea de investigación al respecto, combinando distintas técnicas experimentales. Se están irradiando diferentes muestras de grafito de alta pureza, con protones y con otros iones, en el acelerador de iones tipo tandem de 5 MV sito en el CMAM de la Universidad Autónoma de Madrid. Al tiempo que se realiza la implantación, la técnica de PIXE permite medir la concentración local de otros elementos traza, comprobándose el grado de posibles impurezas ferromagnéticas. Se están realizando también mediciones de la magnetización total de la muestra a temperatura ambiente mediante SQUID, observándose en todas las muestras una (débil) señal ferromagnética que puede variar con el tiempo (meses), tanto antes como después de la irradiación. La superficie de las muestras irradiadas con protones se estudian mediante técnicas más locales como el microscopio de fuerzas magnéticas (MFM) o el efecto Kerr magneto-óptico (MOKE), con resultados todavía no concluyentes. En este trabajo presentamos algunos resultados experimentales preliminares obtenidos.
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TESIS DOCTORAL DE M. CRESPO: "ESPECTROSCOPÍA TÚNEL LOCAL EN SUPERCONDUCTORES MAGNÉTICOS: LOS BOROCARBUROS DE NÍQUEL TMNI2B2C Y ERNI2B2C" [2007-03-26]
En 1956 Ginzburg predijo que la superconductividad y el ferromagnetismo difícilmente podrían coexistir en un superconductor de tipo I, ya que la magnetización uniforme espontánea generaría un campo magnético interno que sobrepasaría fácilmente el campo crítico superconductor. Sin embargo, durante las últimas décadas se ha visto que la coexistencia de estos dos fenómenos resulta posible admitiendo modulaciones de alguno de los dos parámetros de orden (o de ambos), que producen fenómenos espectaculares a escala nanoscópica. El objetivo de este trabajo ha sido realizar un estudio a escala local de la densidad de estados de superconductores magnéticos, usando microscopía y espectroscopía túnel de barrido a muy bajas temperaturas.
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DON NICOLÁS CABRERA [2007-03-07]
Don Nicolás Cabrera Sánchez, un físico creador y organizador. Dos facetas de un gran Científico.
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