Research Centre: Department of Mathematics
Title: : Engineering topological superconductivity and Majorana states with defects in 2D superconductors
Speaker: Pascal Simon (Paris Sud)
In recent years, a renewed interest in magnetic impurities in superconductors was driven by their potential as a new platform for topological superconductivity. Recent scanning tunneling spectroscopy measurements on a superconducting monolayer of lead (Pb) with nanoscale cobalt islands, have revealed puzzling quasiparticle in-gap states  which demand a better understanding of two-dimensional superconductivity in presence of spin-orbit coupling and magnetism. Tantalizingly, the quasiparticle states evoke general topologically protected states which haven't yet been explored in two-dimensional superconductors. Thus motivated, we theoretically study a model of two-dimensional s-wave superconductor with a fixed configuration of exchange field and spin-orbit coupling terms allowed by symmetry.
Using analytics and exact diagonalization of tight-binding models, we find that a vortex-like defect in the Rashba spin-orbit coupling binds a single Majorana zero-energy (mid-gap) state. Importantly, in contrast to the case of a superconducting vortex , our spin-orbit defect does not create a tower of in-gap excitation states. Our findings match the puzzling features observed in the experiment, particularly: (1) preservation of superconducting gap, and (2) short localization length of the zero-energy state compared to the superconductor coherence length . Additionally, these properties indicate that the system realizes the coveted well-protected Majorana zero mode (MZM), which is a key requirement for a potential realization of a topological qubit.
We also discuss how the MZMs can arise in a superconductor on top of a magnetic textures, such as magnetic skyrmions. We find a highly degenerate flat band of MZMs on the edge of the skyrmion that is robust to local perturbations be they electronic or geometric. In addition, the number of MZMs in the flat band surprisingly grows linearly with the perimeter of the edge of the texture, irrespective of its precise shape. In turn, this implies that the MZM are localized on the nanometer scale which potentially allows for their individual addressing .
1. G. C. Ménard et al., Nature Comm 11, 1013 (2017).
2. C. Caroli, P.G. de Gennes, and J. Matricon, Physics Letters 9, 307(1964).
3. G. C. Ménard, A. Mesaros et al., submitted.
4. M. Garnier, A. Mesaros, P. Simon, in preparation.
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When & where
3.00pm - 4.00pmTuesday 23rd October 2018