Újfalussy Balázs (Wigner Research Centre for Physics)

Shiba bands, topology and Majorana Zero Modes in artificial spin chains from first principles

Az előadás kivonata: Majorana-zero-modes (MZMs) were predicted to exist long ago as edge states of some particular physical systems. MZMs have become an important topic because of their potential applications in topological quantum computing, and simply because of the interest that they should host quasiparticles that are their own antiparticles. Despite the lot of buzz triggered by their possible applications as quantum bits with topological protection against decoherence, experimentally it is still very challenging to uniquely identify MZMs based solely on the spectral properties. At the same time, most theoretical models which claim to calculate Majoranas are forced to use entirely unrealistic parameter sets. Usually, in such situations, first-principles calculations are especially helpful, both in identifying and in designing such topological systems.

Here we apply a new formalism to study MZMs from first principles calculations in the superconducting state by solving the Dirac-Bogoliubov-de Gennes equations via band theoretical methods. Such an approach has the advantage that magnetism, superconductivity, and relativity is handled on the same footing in a material specific way, while the special geometry of the system can also be included.

In the talk, after a short study of single impurities as an introduction to the method, we will look at chains of magnetic atoms deposited on the surface of a superconductor. More specifically we will concentrate on a particular system considered prototypical: an Fe chain on the top of Au-covered Nb(110) surface, mostly because this system attracted significant experimental interest over the past two years. I will show that ferromagnetic chains do not support an MZM in this system, however, a broad range of spin-spirals can be identified with a robust zero energy state displaying all signatures of MZMs. We explore the formation and distinction of topologically trivial and non-trivial zero energy edge states, their stability against changes in the direction and magnitude of the local exchange field. Furthermore, we also discover from first-principles that the topological edge states form an exotic type of state: an internally antisymmetric triplet. Through various computational experiments I will demonstrate the emergence of the phenomena called topological fragmentation and show how Majorana Zero Modes can be shifted within the chain, as a first step towards the design and implementation of a braiding protocol.

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