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Tristan Cren - UPMC

Long range coherent magnetic bound states in superconductors

Individual local magnetic moments act destructively on Cooper pairs, leading to discrete spin-polarized states inside the superconducting energy gap as predicted by Yu, Shiba and Rusinov (YSR) [1,2,3]. Rusinov suggested that around magnetic atoms the decaying YSR wavefunction should have a spatially oscillating structure. We will show that in superconductors with a two-dimensional electronic band structure the YSR bound states indeed give rise to long range coherent magnetic quantum state [4]. We experimentally evidence coherent bound states with spatially oscillating particle-hole asymmetry extending tens of nanometers from individual iron atoms embedded in a 2H-NbSe2 crystal and in Pb/Si(111) monolayers. We theoretically elucidate how reduced dimensionality enhances the spatial extent of these bound states and describe their energy and spatial structure. These spatially extended magnetic states could be used as building blocks for coupling coherently distant magnetic atoms in new topological superconducting phases.

Recently a new type of electronic excitations being their own antiparticles were predicted to appear at the edges of a hybrid system constituted of a chain of magnetic atoms coupled to a superconductor [5]. These so-called Majorana end-states were claimed to have been observed in the case of chains of iron atoms on Pb (110) [6], however their spatial extent is restricted to a few atomic distances, making difficult to handle them for braiding. Enhancing the spatial extent of YSR bound states would facilitate the remote coupling of magnetic systems through a superconducting state, opening the route towards an easier manipulation of Majorana quasiparticles and the creation of new topological quantum devices.

[1] L. Yu, Bound state in superconductors with paramagnetic impurities, Acta Phys. Sin. 21, 75 (1965).

[2] H. Shiba, Classical Spins in Superconductors, Prog. Theor. Phys. 40, 435 (1968).

[3] A.I. Rusinov, On the theory of gapless superconductivity in alloys containing paramagnetic impurities, JETP Lett. 9, 85 (1969).

[4] G. Ménard et al., Coherent long-range magnetic bound states in a superconductor, Nature Physics (2015) DOI : 10.1038/NPHYS3508

[5] B. Braunecker and P. Simon, Interplay between Classical Magnetic Moments and Superconductivity in Quantum One-Dimensional Conductors : Toward a Self-Sustained Topological Majorana Phase, Phys. Rev. Lett. 111, 147202 (2013).

[6] S. Nadj-Perge, I.K. Drozdov, J. Li, H. Chen, S. Jeon, J. Seo, A.H. MacDonald, B.A. Bernevig, A. Yazdani, Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor, Science 346, 602 (2014).


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