QDev Seminar: Panagiotis Kotetes – University of Copenhagen

QDev Seminar: Panagiotis Kotetes

In this talk I will present recent work related to topological hybrid devices engineered by magnetic adatom chains deposited on a superconducting substrate. By treating the spins as classical, I present the numerically obtained magnetic phase diagram which consists of ferromagnetic (FM), antiferromagnetic (AFM) and spiral regimes [1]. In general, FM or AFM orderings are stabilized when the crystal field Ising anisotropy dominates over the Rashba spin-orbit coupling (SOC) of the substrate. Probably such a scenario has been recently experimentally realized in devices consisting of Fe FM chains on Pb [2],
where in addition, spatially resolved Majorana fermion (MF) fingerprints seem to have been observed.

Motivated by the above experiment, which indicates a strong Ising anisotropy, I provide an analysis of the accessible topological phases of FM and AFM adatom chains (see Fig.1a) by constructing a low energy model relying on Shiba states [1]. The latter constitute electronic bound states which are induced in the substrate by the magnetic adatoms and lie energetically within the bulk superconducting gap.
The resulting Shiba bands exhibit remarkable topological features supporting phases with one or two MFs per edge which can be accessed by varying the SOC strength, the adatom spacing, or the magnetic moment of the adatoms. Moreover, depending on the magnetic ordering, the resulting phase diagram and the MF wavefunctions demonstrate a diversity of distinctive features which are in principle detectable using spin-resolved scanning tunneling microscopy. Notably, AFM chains can also support MFs even without SOC, as long as a weak Zeeman field and a supercurrent are applied [3] (see Fig.1b).
In fact, in this system the electronic spin-polarization of the arising edge MF wavefunctions depends solely on the parity of the number of magnetic moments, which can serve as a unique signature of the MFs. The present work aims at motivating new experiments with atomic chains while it can open new perspectives for universal quantum computing using MFs.
[1] A. Heimes, D. Mendler, and P. Kotetes, New J. Phys. 17, 023051 (2015) .
[2] S. Nadj-Perge et al., Science 346, 602 (2014).
[3] A. Heimes, P. Kotetes, and G. Schön, PRB 90, 060507(R) (2014).