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A matter of parity - Silvano De Franceschi - Jeudi 6 mars 2014 à 16 h 30

Silvano De Franceschi - CEA-Grenoble, Institute for Nanoscience and Cryogenics

Jeudi 6 mars 2014 à 16 h 30 - INSP - 4 place Jussieu - 75252 PARIS Cedex 05 - Barre 22-23, 3e étage, salle 317

Abstract

« Low-band-gap semiconductor nanowires constitute a versatile platform for superconductor-semiconductor devices. Such hybrid devices, conjugating macroscopic superconducting properties and electrically tunable microscopic degrees of freedom, offer the opportunity to access a wide range of unexplored quantum phenomena and mesoscopic effects [1]. In particular, great interest has recently been drawn by the possibility to create Majorana-fermion quasiparticles that could allow for topologically protected quantum computation. In this talk, I will present recent experimental results [2,3] obtained with hybrid superconductor-semiconductor devices fabricated mainly from InAs/InP core/shell nanowires. Special attention will be devoted to the electronic properties of an elementary system consisting of a quantum dot strongly coupled to a superconductor. It will be shown that the ground state of such a system can be either a spin singlet (with odd fermion parity) or a spin doublet (with even parity). Which of the two states has the lowest energy depends on a complex competition between Coulomb interactions, Kondo correlations, and the superconducting proximity effect. Elementary excitations between singlet and doublet states can be measured as sub-gap resonances in tunnel spectroscopy and, as I will show, they exhibit nontrivial magnetic properties. Implications of the present work to current research on Majorana fermions will as well be discussed. »

References

[1] S. De Franceschi, L.P. Kouwenhoven, C. Schönenberger, W. Wernsdorfer, “Hybrid superconductor-quantum dot devices”, Nature Nanotechnology 5, 703 (2010). [2] E. J. H. Lee, X. Jiang, R. Aguado, G. Katsaros, C. M. Lieber, and S. De Franceschi, “Zero-bias anomaly in a nanowire quantum dot coupled to superconductors”, Phys. Rev. Lett. 109, 186802 (2012). [3] E.J.H. Lee, X. Jiang, M. Houzet, R. Aguado, C.M. Lieber, S. De Franceschi, “Spin-resolved Andreev levels and parity crossings in hybrid superconductor-semiconductor nanostructures”, Nature Nanotechnology 9, 79 (2014).