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

Paolo Sessi - Group of Prof. D^r Matthias Bode - University of Würzburg, Germany

Abstract

In a non-relativistic theory of solids, the spin degree of freedom appears just as an additional quantum number introduced to account for the Pauli exclusion principle. However, once the relativistic spin-orbit interaction is considered, spin moment and orbital motion become intimately coupled. In recent years, condensed matter systems characterized by strong spinorbit coupling have gain a lot of attention. This is due to the large variety of unconventional physical effects they host which, beyond their fundamental interest, represent a promising playground to develop new applications, being spintronics and magneto-electrics the most prominent examples. Since the key consequences of strong-spin orbit coupling usually manifest at boundaries, i.e. edges in 2D and surfaces in 3D, scanning probe techniques are ideal tools to visualize them with both high spatial and energy resolution. In my talk I will describe experiments that, by taking advantage of one of the basic concepts of quantum mechanics, i.e. the particle-wave duality, allow not only to visualize the presence of unconventional boundary modes, but also to demonstrate their unusual properties. In particular, I will discuss recent efforts focused on their controlled manipulation through coupling to welldefined perturbations. I will first examine topological insulators and show that, contrary to what generally believed, magnetic order and gapless states can coexist. I will illustrate how this is intimately related to the dual nature of magnetic dopants, which leads to the emergence of a two-fluid behavior where the competition in between opposite trends, i.e. gap- opening vs. gap-closing, is ultimately linked to the localized vs. delocalized nature of the perturbations-induced quasiparticles. I will then report more recent experiments on the new class of materials known as Weyl semimetals. The surfaces of these compounds host a new class of topologically protected electronic states that form Fermi arcs, which correspond to unclosed contours connecting Weyl points of opposite chirality. By quasi particle interference mapping, we detect the emergence of a rich scattering scenario. I will discuss how our experiments allow for a detailed verification of existing theoretical proposals dealing with intra- and inter-arc scattering events. More generally, I will discuss how electronic states propagate in these materials where the lack of inversion symmetry, combined with strong spin-orbit coupling, gives rise to a plethora of multi-band spinsplit states.