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Title: Magnetotransport experiments in the ferropnictides
Author: Moseley, Dominic
ISNI:       0000 0004 5349 637X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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This thesis concerns the magnetotransport properties of the iron-based superconductors, and in particular, the ferropnictides. In the low doped ferropnictides, linked structural and magnetic transitions occur which significantly alter the electronic behaviour. Simultaneous to the establishment of the magnetic ordering is the creation of small Fermi surface pockets. It has been shown that some of these Fermi surface pockets have Dirac Cone characteristics. The primary work in this thesis focuses on the existence of non-saturating quasi-linear magnetoresistance in the underdoped ferropnictides. This feature has been seen as the hallmark of Dirac cone physics due to the commonly applied quantum linear magnetoresistance model. We have explored this hypothesis by performing a series of magnetotransport experiments using the van der Pauw method on undoped BaFe$_{2}$As$_{2}$, low cobalt doped BaFe$_{1.985}$Co$_{0.015}$As$_{2}$ and superconducting BaFe$_{1.96}$Co$_{0.04}$As$_{2}$. Scattering centres have been systematically introduced using 3-MeV proton irradiation. The quantum linear magnetoresistance model predicts the quasi-linear magnetoresistance should vary with carrier scattering. We describe these experiments, and draw the conclusion that the quantum linear magnetoresistance model is incorrectly applied. Other models to explain the quasi-linear magnetoresistance are reviewed. Speculation as to the cause of magnetic hysteresis in the magnetoresistance found in some of the parent crystals studied is presented. The Hall resistivity in the parent and underdoped ferropnictides shows a clear non-linear response suggesting that the single carrier model is invalid. We find that the Hall resistivity is insensitive to the introduction of disorder. Various models are reviewed including the anomalous Hall Effect and the antiferromagnetism related anisotropic quasiparticle lifetime model. Furthermore, magnetotransport scaling techniques are considered. Only the modified Kohler's rule is satisfied and this is shown to have an intriguing Co doping dependence.
Supervisor: Branford, William; Cohen, Lesley Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral