ELFT HÍRADÓ

[Kormányos Andor <andor.kormanyos AT ttk.elte.hu>]

ELTE TTK Fizikai Intézet / ELTE Institute of Physics


ORTVAY KOLLOKVIUM / ORTVAY COLLOQUIUM
(https://physics.elte.hu/ortvayseminar)


Thursday, 9th November  2023, 15:00h.

Pócza Jenő Classroom,  Physics Building 1.71

Pázmány Péter sétány 1/A, 1117 Budapest.


Balázs  Újfalussy

Wigner Research Center for Physics

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

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.


Andor Kormányos
coordinator