Universität zu Köln

del Ser, Nina ORCID: 0000-0002-9080-8016 (2022). The Turn of the Screw and the Slide of the Skyrmion. PhD thesis, Universität zu Köln.

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Abstract

The experimental discovery of magnetic skyrmions in 2009 opened the door to the development of a new exciting field of physics: skyrmionics. Skyrmions are just one phase of the broader class of chiral magnets, which are characterised by broken inversion symmetry and twisting spin textures. Controlling chiral magnets is of fundamental interest if we want to use them in the technological devices of the future. One way of achieving this is to drive a chiral magnet out of equilibrium with time-oscillating magnetic fields. Even for weak fields, such driving universally activates the rotational and translational Goldstone modes of the magnet. For example, the helical phase starts to turn like an Archimedean screw, while a single skyrmion starts to slide at constant velocity. In this thesis we present a fully analytical theory of this effect for the first time. We also investigate what happens as we drive stronger. Above a critical driving strength, we observe the formation of a Floquet time quasicrystal, where a cascade of magnons carrying incommensurate frequencies and spatial momenta start populating the system. We show that this is triggered by magnon laser instabilities in the driven Floquet magnon spectrum. Finally, we show how the magnetic Archimedean screw can be used to pump electrons, generating large, in principle easily detectable DC currents. Because of their non-collinearity, chiral magnets can host topological charge. In the last part of the thesis, we consider what happens when this topological charge becomes fractional. We show firstly that this phenomenon is real, occurring for example at the meeting points of cubic magnets, or in the remains of exploding skyrmions. Finally, we discuss the physical properties of fractional charges, in particular their ability to act as strong scatterers for low energy magnons.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code del Ser, Nina ninadelser@gmail.com https://orcid.org/0000-0002-9080-8016 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-641111
Date:	10 November 2022
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute for Theoretical Physics
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language chiral magnet English skyrmion English helical magnet English Floquet English quasicrystal English electron pump English fractional charge English topological charge English non-equilbrium English oscillating magnetic field English scattering cross-section English Landau-Lifshitz-Gilbert English Archimedean Screw English Jellyfish English
Date of oral exam:	3 June 2022
Referee:	Name Academic Title Rosch, Achim Professor Diehl, Sebastian Professor Sinova, Jairo Professor
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/64111