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Séminaire Propriétés électroniques de l’INSP

« From paramagnetic atoms to ferromagnetic chains on superconductors » - B. Heinrich - 23 novembre 2016 à 14 h

INSP - UPMC - 4 place Jussieu - 75005 Paris - Barre 22-32, 2e étage, salle 201

Benjamin Heinrich - Université libre de Berlin

Abstract

From paramagnetic atoms to ferromagnetic chains on Pb – Insights in Yu-Shiba-Rusinov impurities for scanning tunneling microscopy In superconductors, magnetic impurities possess a pair-braking potential, which locally affects the quasiparticles and gives rise to Yu-Shiba-Rusinov bound states in the density of states. These states carry information on the magnetic coupling strength of the impurity with the superconductor, which determines the many body ground state properties of the system. Recently, the interest in Shiba physics was boosted by the prediction of topological superconductivity and Majorana modes in magnetically coupled chains and arrays of Shiba impurities. We use scanning tunneling microscopy and spectroscopy at 1.1 K to gain insights in the physics of such quantum systems from single adatoms to atomic chains on the type I superconductor lead (Pb) with high lateral and energy resolution. In the first part of the talk, we investigate tunneling processes into Yu-Shiba-Rusinov states, which are induced on Pb(111) by single manganese atoms. We show that, at large tip-sample distances, single-particle tunneling carries the current flowing from the tip to the sample, while Andreev reflections dominate the transport at smaller distances [1]. The analysis of distance-dependent measurements allows us to extract information on the relaxation mechanism of the Shiba states. We then investigate the distinct nature of these multiple Yu-Shiba-Rusinov (YSR) subgap states on different surface orientations of Pb [2]. An analysis of the number and spatial pattern suggest that the YSR states inherit their properties from the Mn d levels, which are split by the surrounding crystal field. The periodicity of the long-range YSR oscillations allows us to identify a dominant coupling of the d states to the outer Fermi sheet of the two-band superconductor Pb [3]. If there is time, I will also discuss other mechanisms, which bear multiple subgap states, e.g., magnetocrystalline anisotropy [4]. Our results highlight the importance of a careful interpretation of excitation spectra of 1 and 2D magnetic arrays in contact with a superconductor [5,], which are candidates to host Majorana zero modes as end or edge states. By means of measurements with a superconducting tip, we can, e.g., show that also trivial states can contribute to the spectral weight at zero energy and that the subgap excitation spectrum is altered by the finite length of the chain [7].

[1] M. Ruby et al., Phys. Rev. Lett. 115, 087001 (2015).
[2] M. Ruby et al., Phys. Rev. Lett. 117, 186801 (2016).
[3] M. Ruby et al., Phys. Rev. Lett. 114, 157001 (2015).
[4] N. Hatter et al., Nat. Commun. 6, 8988 (2015).
[5] S. Nadj-Perge et al., Science 346, 602 (2014).
[6] G. C. Ménard et al., arXiv:1607.06353.
[7] M. Ruby et al., Phys. Rev. Lett. 115, 197204 (2015).