Seminar: Visualización directa de la transición sólida orden-desorden de la materia de vórtices en Bi2212

SEMINAR

Visualización directa de la transición sólida orden-desorden de la materia de vórtices en Bi2212

Jazmín Aragón Sánchez

Laboratorio de Bajas Temperaturas, Centro Atómico Bariloche

When: Thursday 05 October, 15h30
Where: Sala de seminarios, modulo 03, planta 5
Abstract:

Las transiciones de fase sólido-sólido ocurren en muchos sistemas de materia condensada blanda tales como planos bidimensionales de electrones, plasmas y sistemas coloidales. Sin embargo, la observación directa de los cambios estructurales que se producen al atravesar esta transición no es sencilla. En este trabajo se estudian los cambios en las propiedades estructurales de la materia de vórtices en las cercanías de la transición orden-desorden del sólido de vórtices en muestras de Bi-2212 prístinas e irradiadas con electrones a diferentes dosis. Se empleó la técnica de decoración magnética para visualizar la estructura de vórtices formada en las distintas fases que presenta la materia de vórtices en su diagrama de fases B vs. T. Este último se determinó independientemente mediante la técnica de magnetometría Hall local con métodos AC y DC, midiéndose los campos de transición orden-desorden Bsp y de fusión Bf. Se caracterizaron las propiedades estructurales, densidad de defectos y factor de estructura, de la fase del vidrio de Bragg, B<Bsp y en el vidrio de vótices B>Bsp.

Seminar: Searching for the best thermoelectric materials

SEMINAR

Instituto Nicolás Cabrera

Searching for the best thermoelectric materials

Raquel Ribeiro

Universidad Federal do ABC & Ames Laboratory

When: Friday 21 April, 15h30
Where: Sala de seminarios, modulo 03, planta 5
Abstract:

Since Seebeck’s discovery in the 19th century, many materials have been found useful to generate
thermoelectricity. The first were based on electric conductors and semiconductors, such as
antimony and bismuth. Later, in the 20th century, many other thermoelectric materials were discovered
and developed, such as ceramics and composites. Nevertheless, even today semiconductors
remain among the basic materials for the production of thermoelectric devices. In this
lecture we will review recent efforts on improving thermoelectric efficiency in general. We will focus
on strategies to improved thermoelectric conversion efficiency, that is, the figure of merit for
thermoelectric performance as a basis for understanding the evolution of materials with good
thermoelectric properties. Particularly, several novel proof-of-principle approaches such as phonon
disordered in phonon-glass-electron crystal and charge-spin-orbital degeneracy in strongly
correlated systems on thermoelectric performance will be discussed.

 

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Upper critical field and pairing mechanism in ferromagnetic superconductor UCoGe

When: Friday 11 november 2016
Where: Seminar room, Modulo 03, pta 5, Departamento de Fisica de la Materia Condensada, Facultad de Ciencias UAM
Who: Beilun WU, Univ. Grenoble Alpes, CEA, INAC, SPSMS, Grenoble, France

Abstract: We present our transport study on the upper critical field of the orthorhombic ferromagnetic superconductor UCoGe. The bulk upper critical field curves at ambient pressure are obtained by thermal conductivity measurements for magnetic field H along all three crystal axes. For H//b in particular, the S-shape re-entrance phenomenon is well observed, and even more strongly than on resistivity measurements [1]. Surprisingly, for field above 8T along the b axis, bulk superconductivity appears to be “more robust” than observed on resistivity measurements: this is reminiscent of observations on cuprates High-Tc superconductors, where it was taken as a demonstration of the existence of a (resistive) vortex liquid phase. The Hc2 in UCoGe is a very strange case in terms of classical theories for superconductivity. We show that the numbers of features in Hc2 in UCoGe, including the huge anisotropy and the anomalous curvatures of the Hc2 along different field directions, can be consistently understood with a simple quantitative model for the field dependence of the pairing strength[2,3]. The consistency of the model is further checked by specific heat measurements. It points out the major role of ferromagnetic fluctuations as a mechanism for superconductivity in this system. [4]

[1] Aoki, D.; Matsuda, T.D.; Taufour, V.; Hassinger, E.; Knebel, G. & Flouquet J., J. Phys. Soc. Jpn. 78, 113709 (2009).
[2] Y. Tada, S. Fujimoto, N. Kawakami, T. Hattori, Y. Ihara, K.Ishida, K. Deguchi, N. K. Sato, and I. Satoh, Journal of Physics: Conference Series 449 (2013) 012029
[3] V. P. Mineev, PRB 83, 064515 (2011)
[4] Hattori, T.; Ihara, Y.; Nakai, Y.; Ishida, K.; Tada, Y.; Fujimoto, S.; Kawakami, N.; Osaki, E.; Deguchi, K.; Sato, N. K. & Satoh, I., PRL 108, 066403 (2012)
 
Work made in collaboration with G.Bastien1, D.Braithewaite1, M. Taupin1,2, D. Aoki1,3, J.Flouquet1 and J.-P. Brison1
1 Univ. Grenoble Alpes, CEA, INAC, SPSMS, Grenoble, France
2 Low Temperature Laboratory, Aalto University, P.O. Box 13500, FI-00076 Aalto, Finland
3 Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan

INC seminar: Patterning Functional Nanostructures by Focused Beams Induced by Processing. Rosa Córdoba

INC seminar

Patterning Functional Nanostructures by Focused Beams Induced by Processing

Rosa Córdoba Castillo

Instituto de Ciencia de Materiales de Aragón (ICMA). CSIC-Universidad de Zaragoza)

Friday, June 17th, 2016, 12:30
Seminar room (5th floor), Módulo 3, Departamento de física de la Materia Condensada, Facultad de Ciencias UAM

Abstract:

Focused electron- and ion beams in combination with specific chemistry can be used for direct-writing of nanostructures at the three dimensions of space [1–5]. Doing so with magnetic materials provides an appealing route towards explorative research on future magnetic storage and spintronic technologies [6]. However, patterned materials often lack purity and/or proper functionality. The low metallic content in these deposits is caused by the incomplete precursor molecule dissociation and the lack of volatile products resulting from this process. That is why for the nanofabrication of binary systems and novel magnetic nanostructures new approaches are required. Here, we report two examples of functional nanostructures patterned by focused electron beam (FEB) induced processing. Firstly, we introduce a method to synthesize nickel-based deposits [7]. Our procedure consist of two sequential in situ steps at room temperature to further tune the deposited nickel-based structures. The 1st step: Ni grown by focused electron beam induced deposition (FEBID) and the 2nd step: FEB irradiation of the Ni FEBID under O2 flux at room temperature. By using this method, as-grown Ni deposits are transformed into homogeneous NiO deposits. Proof-of-concept studies

will be shown in which NiO deposits could display resistance switching, and so-called exchange-bias behaviour in NiO-Co bilayers, fully made by FEB processing. Second, we report a study on the magnetic switching behavior in a novel set of magnetic nanostructures. We manipulate the magnetization reversal in Fe FEBID nanostructures varying the ‘scanning strategy’ of the FEB. In particular, by changing the beam overlapping in one direction during the e-beam writing process, we introduce a subtle thickness modulation, enabling a new way to manipulate the local anisotropy and the nanomagnet’s switching field due to changes in magnetostatic interactions [8].

[1] Córdoba R, Sesé J, De Teresa J M and Ibarra M R 2010 Microelectron. Eng. 87 1550–3
[2] Córdoba R et al., 2012 J. Phys. D‐Applied Phys. 45 35001
[3] Lavrijsen R et al., 2011 Nanotechnology 22 25302
[4] Córdoba R 2014 Functional Nanostructures Fabricated by Focused Electron/Ion Beam Induced Deposition (Springer International Publishing)
[5] Rodríguez L A et al., 2015 Beilstein J. Nanotechnol. 6 1319–31
[6] Parkin S S P, Hayashi M and Thomas L 2008 Science (80‐. ). 320 190–4
[7] Córdoba R at al., 2016 Nanotechnology 27 065303
[8] Córdoba R, Han D‐S and Koopmans B 2016 Microelectron. Eng. 153 60–5

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