Degree Type

Dissertation

Date of Award

2011

Degree Name

Doctor of Philosophy

Department

Physics and Astronomy

First Advisor

Robert J. Mcqueeney

Second Advisor

Alan I. Goldman

Abstract

The study of iron based superconductors began in 2006 when the compound LaOFeP was found to have a superconducting transition at a modest temperature Tc of 3.2 K. Because elemental iron is a strong ferromagnet, this discovery was a great surprise due to the fact that magnetic moments are generally associated with magnetic pair breaking. Soon after this discovery, variations of these superconductors were made through chemical substitutions and the Tc rose to 55 K two years later. The explosion of research that has followed these discoveries has led to the synthesis of several families of iron based superconductors whose high values of Tc are second only to the cuprates and which bringing them into the field of high temperature superconductivity (SC).

This thesis focuses on how pressure and electron doping effect the crystallographic and magnetic properties of the iron based superconductors (Ca,Ba)Fe2 As2 which are part of a group of compounds known as 122s. Upon cooling, these compounds undergo a dual structural-magnetic transition from a tetragonal paramagnetic state to an orthorhombic and antiferromagnetic state. By tuning the pressure or dopant concentration knob, we have been able to show clearly that this has the effect of suppressing the magnetism and associated structural transition observed in un-doped or ambient pressure compounds. Neutron scattering measurements on CaFe2 As2 under pressure have demonstrated that the onset of superconductivity is observed after the stabilization of the room-temperature tetragonal phase at low temperatures under non-hydrostatic pressure. For the case of electron doping BaFe2 As2, it is clear that a sufficient suppression of structural and magnetic ordering transitions is necessary for the appearance and optimization of superconductivity. These studies have provided key information about the static magnetic ordering and the associated magnetic excitations, and have allowed for the attempt to address questions about the nature of magnetism in these materials and what role magnetism plays in the appearance of superconductivity.

Copyright Owner

Daniel Keith Pratt

Language

en

Date Available

2012-04-06

File Format

application/pdf

File Size

185 pages

Included in

Physics Commons

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