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Magnetotransport experiments in the ferropnictides

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Title: Magnetotransport experiments in the ferropnictides
Authors: Moseley, Dominic
Item Type: Thesis or dissertation
Abstract: 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.
Content Version: Open Access
Issue Date: Oct-2014
Date Awarded: Apr-2015
URI: http://hdl.handle.net/10044/1/24798
DOI: https://doi.org/10.25560/24798
Supervisor: Branford, William
Cohen, Lesley
Sponsor/Funder: Engineering and Physical Sciences Research Council
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses

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