Forum DQMP

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Date(s) - 31/05/2016
13 h 00 min - 14 h 00 min

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Tuesday, May 31st 2016 – 13h00
Auditoire Stückelberg
Ecole de physique

Coffee and tea will be available from 12h50 at the entrance of the Auditoire

 

Structure and magnetocaloric properties of Gadolinium borohydrides
Emilie Didelot (group of Prof. Cerny)

Good candidates for magnetic coolers in sub-Kelvin applications are compounds in which Gd3+ ions are sufficiently isolated from each other to maintain paramagnetic behavior at cryogenic temperatures. In the last decade, Gd-based magnetic coolers have been sought after in coordination compounds with light organic molecules as ligands in order to obtain light-weight materials where decoupled magnetic centres lead to large magnetic/nonmagnetic atomic mass ratios which are essential characteristics to achieve good magnetocaloric properties per unit mass and volume.
We have synthesized novel Gd-based mixed-metal borohydrides of the type AnGd(BH4)n+3 (n=0,1,2,3; A = K, Cs) [1]. The differences in n lead to different dimensionalities of the Gd-BH4 substructure, which comprises 2D and 3D networks as well as isolated polyanions, and are reflected in magnetization measurements.The magnetocaloric effect was evaluated from isothermal magnetization measurements. In
K2Gd(BH4)5 the maximum entropy change –ΔSMmax reaches 54.6 J.kg-1.K-1 (around 2K) for the maximum field change (9 T).

[1] P. Schouwink, E. Didelot, Y.-S. Lee, T. Mazet and R. Černý J. Alloys Compd. 2016, 664, 368.

 

Superconducting shape resonance from quantum confinement at the LaAlO3/SrTiO3 interface 
Davide Valentinis (groups of Prof. Triscone and Prof. van der Marel)

The properties of materials are modulated by their geometry and dimensionality. In particular, quantum confinement effects arise when one or more spatial dimensions of a system are reduced to atomic scale. In confined geometry, superconductors exhibit many fascinating phenomena such as shape resonances: in quasi-2D configuration, superconducting properties like the critical temperature acquire an oscillatory evolution as a function of electron density and confinement length, due to the presence of quantized subbands. These confinement-induced resonances are relevant for superconducting thin films [1], as well as for heterostructures like the LaAlO3/SrTiO3 interface, where the low-density electron gas is confined to a layer only a few nanometers thick [2]. In a confined quasi-2D geometry, all subbands are coupled and superconductivity is of a multi-band nature, with pairing matrix elements depending on the shape of the confinement potential. We solve this problem for a square potential well, including self-consistency in the
chemical potential [3]. We study the various properties and their scaling laws both analytically and numerically, as a function of confinement length and density. We apply the model to the LaAlO3/SrTiO3 system, in order to understand the dome-shaped density dependence of Tc.

[1] C. J. Thompson and J. M. Blatt, Phys. Lett. 5, 6 (1963); J. M. Blatt and C. J. Thompson, Phys. Rev. Lett. 10, 332 (1963).
[2] A. D. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).
[3] D. Valentinis, D. van der Marel and C. Berthod, arXiv:1601.04521 (2016); arXiv:1601.04927 (2016).

 

Forum Committee : L. Foini, C. Lichtensteiger, N. Ubrig (24.05.2016)

Réalisation : Sur Mesure concept