Laboratoire de Physique Théorique

de la Matière Condensée

 

 

Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle  523 du LPTMC - Tour 12-13 


 

Baruch Meerson (Hebrew University, Jérusalem, Israël)

Geometrical optics of constrained Brownian motion: three short stories

Pascal Simon (LPS Orsay)

Majorana zero modes around skyrmionic  textures

Recent scanning tunneling spectroscopy measurements on a superconducting monolayer of lead(Pb) with nanoscale cobalt islands, have revealed puzzling quasiparticle in-gap states [1] which demand a better understanding of two-dimensional superconductivity in presence of spin-orbit coupling and magnetism.  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. In contrast to the case of a superconducting vortex [2], our spin-orbit defect does not create a tower of in-gap excitation states and our findings match the puzzling features observed in the experiment.  Additionally, these properties indicate that the system realizes a pair of well-protected Majorana zero mode (MZM) localized at the core and the rim of the defect [3]. We also discuss how the quasiparticle states of the defect relate to the states of superconductors on top of magnetic textures, such as skyrmions. Magnetic skyrmions are nanoscale particle-like spin configurations that are efficiently created and manipulated. They hold great promises for next-generation spintronics applications. I will focus on the theoretical analysis of magnetic skyrmions proximitized by conventional superconductors. I will show that a topological superconducting phase can emerge in these systems and uncover a whole almost flat band of these modes on the edge of the magnetic texture, in contrast to a previously reported MZM in the core of the skyrmion [4]. I will discuss in details the origin of these MZMs by relating this problem to the the extensively-studied Rashba nanowire model. We have found that these modes are remarkably stable to electronic and geometric perturbations which we investigate by a combination of analytical arguments and numerical tight-binding calculations. Additionally, this analysis reveals that the number of MZMs on the edge scales linearly with its perimeter [5].

[1] G.C. Ménard et al., Nature Comm. 8, 2040 (2017).

[2] C. Caroli, P.G. de Gennes, and J. Matricon, Physics Letters 9, 307(1964).

[3]  G. C. Ménard, et al., arXiv:1810.09541, Nature Comm. 10, 2587 (2019).

[4] G. Yang, P. Stano, J. Klinovaja & D. Loss, PRB 93, 224505 (2016).

[5] M. Garnier, A. Mesaros, P. Simon, arXiv:1909.1267, Comm. in Physics 2, 1 (2019).

Nicolas Levernier (IUSTI Marseille)

First-passage time of non-markovian random walks

Assa Auerbach (Technion, Haifa, Israël)
What determines the Hall and Thermal Hall signs of metals, magnets and superconductors?

Christof Wetterich (ITP Heidelberg)
Quantum mechanics and quantum computation from classical bits