The effect of changing crystal structures on the magnetic and superconducting properties of the iron based superconductors and their parent compounds

Abstract

Superconductivity in high temperature superconducting materials arises due to the constituent electrons becoming strongly correlated. In particular, in the Iron based family of superconductors, antiferromagnetic fluctuations of the electron spins and charge fluctuations of the Iron d-orbitals are thought to be important. The strong correlations of the electrons in an Iron based superconductor lead to the electronic ground state adopting a lower symmetry than the crystal lattice. Transport studies of these systems under the application of uniaxial strain show a strongly anisotropic response of the materials to straining along different lattice directions, suggesting that the electronic correlations are in fact coupled to the lattice. In this thesis I present a study of the response of the magnetic and superconducting states of these Iron based materials to changes of their crystal structure induced by applying uniaxial strain or through doping. I first examine the response of the magnetism in Fe₁₊ₓTe, the parent compound of the Iron chalcogenides, to the change that occurs in the crystal structure as the system is doped with Iron. I present a spin polarized STM study of the magnetism throughout the phase diagram of Fe₁₊ₓTe using in-situ prepared magnetic tips to measure the spin structure. By using the STM tip to remove excess Iron atoms from the surface I show that it is possible to decouple the effects of the excess Iron atoms from the change of crystal structure. Therefore showing the effect of the structural change on the magnetism. Next I present an STM study of how the superconductivity in the tetragonal LiFeAs system responds to the application of uniaxial strain deforming the crystal structure. I describe the construction of a modifed STM sample holder that allows for the in-situ application of a variable Strain to the LiFeAs sample. I present the resulting impact of this straining process to the superconductivity of LiFeAs on an atomic scale and report the formation of a strain induced charge density wave state that coexists with the superconductivity. In the final chapter I report a quasiparticle interference study of the superconducting state of Sulphur doped FeSe and discuss the results in comparison of those from the literature for the undoped FeSe system.