Studies of Magnetism and Strain Tuning in Detwinned Iron Pnictides and Chalcogenides and Other Unconventional Systems
Tam, David W
Doctor of Philosophy
Uniaxial pressure can be used as a "clean" tuning parameter to smoothly change the atomic spacings and bond angles in a crystal lattice, induce phase transitions, or to break the rotational lattice symmetry and thereby provide access to underlying symmetries such as electron nematic phases. I present experiments (transport, neutron diffraction, inelastic neutron scattering, and muon spin rotation) that use uniaxial pressure as a platform for tuning iron-based superconductors and other unconventional magnetic systems. Simple clamps were used to break the in-plane symmetry of iron-based superconductors, enabling investigations of the magnetic ground state of a single domain. In the ground state of FeTe, anisotropy of spin fluctuations stemming from the magnetically ordered positions was observed to rapidly relax at an energy scale near E = 30 meV, which can be understood as the result of a competing quasi-degenerate magnetic ground state. Detwinning of NaFeAs, on the other hand, reveals orbital-selective spin fluctuations that are dominated by dyz-dyz spin fluctuations at low energy, while dxy-dxy scattering processes control the overall magnetic bandwidth. To study the uniaxial pressure dependence of materials with transport and scattering experiments, I developed a compressed air-based instrument which exerts controlled and repeatable uniaxial pressure on large samples, which we first used to show that in-plane pressure in Co- and Ni- doped BaFe2As2 increases the magnetic ordering temperature and ordered moment, while decreasing the superconducting Tc. Experiments with this instrument on many other systems are also reported here, including the uniaxial pressure dependence of Fe(Se,Te), BaFe2As2 and SrFe2As2 and their derivatives, Sr3Ru2O7, and other materials along various crystallographic directions. For example, comparing the transport properties of BaFe2As2 and SrFe2As2 under finely tuned in-plane uniaxial pressure reveals subtle differences that connect resistivity anisotropy to low-energy spin fluctuations, adding evidence for a spin-driven nematic phase in iron-based superconductors. In addition to uniaxial pressure experiments, I present theoretical calculations on the LiFeAs and BaFe2As2 systems and neutron scattering experiments on UPt3 that elucidate key aspects of the mechanism of magnetically-driven superconductivity in these systems. Finally, I discuss other experimental progress in magnetic systems such as the spin liquid candidate Ce2Zr2O7.