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Séminaire « Propriétés opto-électroniques des solides » de l’INSP

Coherent long-range magnetic bound states in a superconductor - Gerbold Ménard - Mardi 29 mars 2016 à 11 h

INSP - 4 place Jussieu - 75252 PARIS Cedex 05 - Barre 22-32 - 2e étage, salle 201

Gerbold Ménard, doctorant dans l’équipe Spectroscopie de nouveaux états quantiques


The combination of superconductivity and magnetism has been known to give rise to localized states called Shiba states inside the superconducting gap [1]. These states are due to the breaking of a Cooper pair by the magnetic interaction and can be probe by STM. The measured amplitude and position of the peak inside the gap associated to these Shiba states depend on the specific parameters describing the interaction between the impurity and the conduction electrons. The precise control and tuning of magnetic impurity chains could lead to the observation of Majorana fermions which could be of great interest in quantum computation due to their topological properties [2].

Using low temperature scanning tunneling spectroscopy we accessed through the local density of states to the spectra associated to magnetic impurities as well as their spatial dependence. We studied samples of superconducting 𝑁𝑏𝑆𝑒2 containing a really small concentration of native magnetic impurities. We observed the appearance of star-shaped structures around individual impurities with a size of the order of the coherence length of the superconductor (≃10 𝑛𝑚). Though this type of structures where expected in this material under specific conditions [3], the fine study of our data revealed an oscillation of the density of states along the star branches. More specifically these oscillations were found to be phase shifted between electron and hole-like states. To further analyze our results, we performed tight-binding calculations which reproduced the spatial symmetry we witness around our magnetic impurities. A semi-analytical calculation also enabled us to establish a relation between the different spatial scales observed and the physical quantities associated to superconductivity, and to understand the role of dimensionality in such systems.

[1] Hiroyuki Shiba. Classical spins in superconductors. Progress of Theoretical Physics, 40 : 435–451, September 1968.
[2] A. Yazdani and al. Probing the local effects of magnetic impurities on superconductivity. Science, 275:1767–1770, March 1997.
[3] Michael E. Flatté and David E. Reynolds. Local spectrum of a superconductor as a probe of interactions between magnetic impurities. Physical Review B, 61 :14810–14814, June 2000.

Coherent long-range magnetic bound states in a superconductor

Gerbold C. Ménard, Sébastien Guissart, Christophe Brun, Stéphane Pons, Vasily S. Stolyarov, François Debontridder, Matthieu V. Leclerc, Etienne Janod, Laurent Cario, Dimitri Roditchev, Pascal Simon & Tristan Cren
Nature Physics 11, 1013–1016 (2015)