New Platforms for Topological Superconductivity with Magnetic Atoms

Workshop Report

Topological superconductors have received much attention in the past years, because they may host exotic low energy excitations such as Majorana bound states (MBS). The fact that these obey non-Abelian statistics renders them very attractive for topological quantum com- putation. Several different platforms to realize topological superconductivity are currently the subject of intensive research.

Among them, a rather simple recipe combining arrays of magnetic atoms or nanoparticles on top of a superconducting surface has been proposed to engineer topological superconductivity. Recent developments in scanning tunneling microscopy (STM) now permit probing and fabri- cating magnetic structures and many other artificial spin systems on top or embedded in super- conductors. The goal of this focused workshop was to gather the main specialists in this growing field in order to discuss the key experimental advances (material preparation, measurements), and to view how the exotic bound state excitations associated with these new superconductors may be unambiguously detected, manipulated and used within this scheme.

Ali Yazdani (Princeton) who was the first to find some possible signature of Majorana bound states in chains of iron on lead gave the colloquium. He described the recent progress of his group to find unambiguous fingerprints using spin-polarized STM as a way to distinguish stan- dard (Shiba) bound states from the Majorana bound states. His results were based on some general sum rules that the spin-resolved density of states has to satisfy. These sum rules and their derivation were presented by B. Bernevig (Princeton) earlier in the workshop. B. Bernevig also presented some evidence that Bismuth is a second-order 3D topological insulator with protected 1D hinge states. R. Wiesendanger (Hamburg) demonstrated a fully-controlled bottom-up fabri- cation of artificial 1D atomic chains from individual magnetic Fe adatoms on a superconducting Rhenium substrate, which are also suggested to host Majorana bound states at their ends. Ex- perimental results which probe the structure and Majorana wave function of mono-atomic Fe chains on superconducting lead with atomic force spectroscopy were shown by E. Meyer (Basel). Some more detailed theoretical framework to describe chains of magnetic atoms or more com- plicated clusters was explained by B. Braunecker (St Andrews) and T. Ojanen (Helsinki). F. von Oppen (Berlin) presented results of a successful experiment-theory collaborative work to scrutinize the physics of Shiba multiplets and associated wave functions taking into account the different orbitals of the adsorbed magnetic atoms. The theoretical picture of single impurities was further complemented by R. Zitko (Ljubliana).

Alternative platforms have also been discussed extensively along the workshop. A. MacDon- ald (Austin) exposed recent results on thin-film magnetically-doped topological insulators that are placed on a superconducting substrate as a promising route. C. Brun (Paris) showed exper- imental results on a system composed of magnetic Co clusters below a monolayer of lead which may offer an interesting route for 2D topological superconductivity. D. Loss (Basel) proposed a critical interpretation of the recent achievements of the physics of topological semiconducting wires. B. Sacepe (Grenoble) presented recent results on the ac Josephson effect in the 3D topo- logical insulator Bi2Se3. The physics of iron-based superconductors has also been covered by other invited speakers such P. Wahl (St Andrews) and C. Hess (Dresden). J. Hoffman (Harvard) presented SmB6 as an interesting topological material, whereas H. Suderow (Madrid) showed topological aspects of vortex states.

Finally, the program was considerably enriched by contributed talks of high quality on all of these topics together with a lively poster session.