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Title: Computational and experimental studies of exotic magnets and superconductors
Author: Kirschner, Franziska
ISNI:       0000 0004 7652 9795
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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The advent of metalworking during the bronze age marked a transition in humankind's relationship with materials: instead of materials existing only as they were found in nature, one could now manipulate their physical properties to make them fit for purpose. With the discovery of quantum mechanics, the new opportunities to manipulate materials were manifold. The collective behaviour of electrons inside a solid is much more than the sum of its parts; as such, in some bulk materials, new quantum mechanical behaviour can emerge. We can use our knowledge of quantum mechanics to manipulate these quantum materials, and thus we find ourselves not in the bronze or the iron age, but in the quantum age. This thesis is a series of four studies on a range of quantum materials. I begin with emergent magnetic monopoles in spin ice. While there have been many bulk studies of monopoles, there is yet to be a microscopic measurement of small ensembles of these emergent quasiparticles. I use Monte Carlo methods to simulate the expected magnetic field fluctuations arising from small numbers of monopoles passing close to a probe near the surface of a spin ice, and show that the presence of monopole excitations produces unique features in the magnetic noise spectrum. I also propose several experiments which may be able to detect these features. I then move onto CoTi2O5. Using muSR and neutron powder diffraction, as well as theoretical calculations, I find that this material undergoes a transition to an antiferromagnetic state, which is coupled to a spontaneous breaking of structural symmetry. This makes CoTi2O5 the lowest-known-symmetry crystal to undergo the spin Jahn-Teller effect. I then present a possible magnetostructural coupling regime. The remaining studies examine the importance of topology in the successful understanding of quantum material systems. I use μSR to investigate both the ACa2Fe4As4F2 (A = K, Rb, or Cs) and LuxZr1-xB12 families of superconductors. It is possible to alter the topological character of the superconducting gap in these families, either by manipulating the chemical spacer layers in the crystal structure of ACa2Fe4As4F2 or by changing x in LuxZr1-xB12. Along with the gap topologies, both the penetration depths and critical temperatures of these compounds appear to be sensitive to these chemical manipulations. I investigate how these topological crossovers are linked to the other superconducting properties of these materials, and discuss the consequences for our wider understanding of superconductivity.
Supervisor: Blundell, Stephen Sponsor: Lincoln College ; Oxford
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Condensed matter--Magnetic properties ; Consensed matter