PGI-1 Talk: Raffaele Aliberti

Quantum Theory of Materials Seminar

Real-space mapping of the Yu-Shiba-Rusinov states around magnetic defects on superconducting surfaces

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Over the past ten years, there has been a significant surge in the study of low-dimensional hybrid systems combining superconductors and magnets. In these systems, magnetic impurities disrupt the superconducting order, creating pair-breaking potentials for Cooper pairs. This locally influences the Bogoliubov quasi-particles, leading to the formation of Yu-Shiba-Rusinov (YSR) states. These bound states are detectable within the gap of the density of states, which is shaped by the superconducting order parameter. It has been recognized that these states may play a key role in topological superconducting phases and potentially host Majorana zero modes (MZMs). These exotic quasi-particles are considered promising candidates for the development of topologically protected qubits, which could achieve an higher degree of fault-tolerance in the field of quantum computing. A crucial area of research in this field involves understanding how the electronic structure of the host material, the magnetic properties of impurities, and the spatial decay and orientation of YSR states interact. This is essential for advancing the understanding of these systems. The objective of this thesis is to examine the behavior of YSR states using Density Functional Theory (DFT), which can account for the material-specific properties critical to their emergence. Specifically, we aim to map out the spatial distribution, decay, and directionality of YSR states on superconducting surfaces, while addressing material-specific effects such as spin-orbit coupling (SOC) and crystal field splitting. To achieve this, we simulate Scanning Tunneling Microscopy (STM) experiments, which are the workhorse for accessing the spatial data on an atomic scale. The computational framework for this analysis is based on DFT, in particular we use the Korringa-Kohn-Rostoker (KKR) method. KKR is a multiple-scattering approach, which is particularly suited for systems with defects due to its use of Green’s function formalism. By integrating the Kohn-Sham-Bogoliubov-de Gennes (KS-BdG) method, we can accurately simulate all the required elements for the study of the YSR states: (i) the realistic electronic structure of materials, (ii) first-principles impurity embedding, and (iii) multi-band superconductivity, enabling predictions of YSR state properties in real materials. To automate and streamline the process of obtaining spatial information about YSR states, we have developed a computational workflow using the AiiDA environment, in particular this workflow represents the latest implementation within the AiiDA-kkr plugin. It automates all steps needed for STM-type simulations, making it easier for researchers to conduct such studies in the future. As a case study, we apply this tool to analyze the interaction of a single Mn adatoms on superconducting Ta, a material of increasing interest in the context of superconducting quantum circuits. Through this work, we aim to connect real-space charge density oscillations, as observed in Quasi-Particle Interference (QPI) experiments, to the underlying electronic structure of the host material.

Contact

Prof. Dr. Stefan Blügel
Phone: +49 2461 61-4249
Email: s.bluegel@fz-juelich.de

Raffaele Aliberti
Phone: +49 2461 61-6263
Email: r.aliberti@fz-juelich.de