Tuning the Spin-Orbit Coupling and the Spin Dynamics of Atomic Chains on Superconductors

Abstract

The ever-growing demand for high-performance computational power can no longer be met by the miniaturization of existing Si-based integrated circuit technology. New approaches that replace the electrical charge as the information carrier such as nanospintronics or overall new computing schemes such as quantum computation are promising technological routes to address these issues and are at the heart of modern physics research. While research towards quantum computation has made large leaps forward in recent years, it also became apparent that the decoherence time of the so-far realized qubits is too short. A heavily pursued approach to solve this issue is the use of topologically protected qubits that rely on exotic non-Abelian Majorana bound states (MBS), which can be realized as excitations in condensed-matter systems e.g. at the boundaries of topological superconductors.

This Ph.D. thesis addresses the experimental realization and fundamental properties of both technological concepts (nanospintronics and topological qubits) in the platform of artificial atomic spin chains assembled on the surfaces of s-wave superconductors. The methods of scanning tunneling microscopy (STM) and - spectroscopy (STS) are employed to construct such chains and different operational modes are used to investigate their magnetic and electronic properties as well as their spin dynamics. The experiments are carried out in a setup with a base temperature of 320 mK and an external magnet capable of applying a 12 T magnetic field.

Three material systems are investigated in the course of this thesis in order to determine suitable properties for the realization of MBS in spin chains on superconductors. These properties are closely linked to the local bound states that magnetic adatoms induce in superconductors - Yu-Shiba-Rusinov (YSR) states - as well as their magnetic and electronic interactions in ensembles of multiple adatoms, which give rise to Shiba bands. The first material platform investigated in this thesis are single adatoms, artificial dimers, and chains of Mn adatoms on Nb(110). The platform is characterized by an unprecedented energy resolution in STS experiments and a highly reproducible control and fabrication of nanostructures tailored from individual adatoms. This enables the indirect observation of Shiba band formation in momentum space and reveals the effect of SOC on these bands - a topological gap in a Shiba band. The experiments reveal that a low SOC is the limiting factor, preventing the realization of a large topological gap hosting isolated MBS in this material platform.

A comparative study on similar Mn structures on Ta(110) is performed to further investigate the role of SOC, which is enabled by the fact that the main significant differences between niobium and tantalum are their atomic masses and their SOC strengths. A comparison of similar Mn chains and Shiba bands on both substrates reveals that the size of the topological gap can indeed be increased by heavy substrates.

This highlights that proximitized films of a high-Z layer on Nb(110) can be ideal substrates for Shiba chains to realize isolated MBS, which is the third material platform investigated in this thesis. It is demonstrated that Fe chains on one atomic layer of Au on Nb(110) can be tailored into a single Shiba band regime.

Last, the spin dynamics of antiferromagnetically coupled spin chains on Ta(110) are investigated using time-resolved spin-polarized STM. By tuning external parameters, such as the magnetic field, we drive the system from the quasiclassical case into the decoupled quantum limit, where the chain is largely decoupled from itinerant quasiparticles of the metallic substrate by the superconducting gap. We conclude that spin chains on superconductors are promising systems for future studies of quantum coherence effects.