This thesis presents the results obtained from de Haas-van Alphen experiments in ironbased superconductors. Measurements of the quasi particle mass in the quantum critical system BaFe2(Asl- xPxh are presented, which show strong enhancement towards the critical composition Xc = 0.3. This is found to be in good agreement with the prediction of a diverging behaviour of the effective mass close to a quantum critical point. Further results obtained on the lower and upper superconducting critical field of this system will be presented, which are found to contradict the expectations from Ginzburg-Landau theory based on results of the quasi particle mass and London penetration depth. However we can reconcile the different experimental findings on superconducting and normal state properties, in this quantum critical system, by considering a significant contribution from Abrikosov vortex core states. The importance of understanding the normal state electronic structure and interactions is shown to be essential for an understanding of the superconducting ground state of a quantum critical system. Further we will show a detailed de Haas-van Alphen study of the il1-type iron-based superconductor LiFeP and its isovalent partner LiFeAs. To understand the formation of nodes on the superconducting gap structure in systems with little change in their Fermi surface topology, is essential for the formulation of a microscopic theory of the pairing mechanism in pnictide-superconductors. 'While we find both systems to be close to the geometric nesting condition, we are able to point to different possible scenarios of the origin of nodes based on quasi particle mass enhancement. Extending the study of quasi particle mass and its relation to the superconducting properties to the stoichiometric high-temperature cuprate superconductor YBa2 CU4 Os, we aim to study the Fermi surface evolution under hydrostatic pressure. As the system has a very stable oxygen stoichiometry which does not allow it to be doped by oxygen ordering, we use hydrostatic pressure to tune the system unexpectedly leading to an increase in the superconducting critical temperature with almost no change to the quasi particle mass .
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:680124 |
| Date | January 2014 |
| Creators | Putzke, Carsten Matthias |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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