Probing the Symmetry-Breaking Phases in Iron-based Superconductors with Combined Strain, Transport and X-ray Techniques

Thesis (Ph.D.)--University of Washington, 2021Spontaneous rotational symmetry breaking phases, such as the electronic nematic phase and charge and spin stripe phases, are commonly found in the phase diagram of strongly correlated materials including many of the high-temperature superconductors. This thesis is essentially a study of how tuning parameters, namely applied uniaxial stress and applied magnetic field, can interact with the rotational symmetry-breaking order parameters of these phases, either to probe the system and better understand its intrinsic properties or to create new states in the system that are not spontaneously generated. This thesis discusses the development of a methodology which combines synchrotron x-ray techniques and transport measurements of a single crystal sample mounted to a uniaxial stress device. This methodology is used in iron-based high temperature superconducting materials to probe the nematic and magnetic phases. In Co-doped BaFe2As2, the coupling between electronic nematicity, the crystal lattice and the resistivity anisotropy is probed by strain, and a temperature and phase independent transport-structural correspondence is demonstrated. In EuFe2As2, the effects of nematicity on the Eu localized magnetic order is demonstrated from a strongly anisotropic antiferromagnetic coupling between Eu moments. While the work in this thesis is primarily focused on iron-based superconductors, these ideas are very general to the field of condensed matter physics, and this methodology of applied fields combined with x-ray measurements developed here is broadly applicable to systems with structural domains, large magnetoelastic coupling, and other strongly correlated electron effects