X - Ray scattering investigations of subtle ordering in correlated materials

The interaction of many particles can lead to spectacular new phases of matter whose properties and collective excitations bear little resemblance to the individual particles and interactions. Understanding how the macroscopic state transforms from one phase to another provides key insights into the underlying physics. In this thesis, we study two poorly understood states: the Hidden Order (HO) phase of URu 2 Si2 and the pseudogap of high T, cuprates. In the case of URu 2 Si2 , the HO phase causes a significant restructuring of the Fermi surface. Thermal conductivity and ultrasound measurements suggest that the lattice degrees of freedom couple strongly to this change. Additionally, torque magnetometry and x-ray diffraction suggest a breaking of C4 rotational symmetry. We directly study the lattice through x-ray scattering. We see no change of the acoustic phonon dispersions or of the phonon lifetimes from the HO transition. Calculations of phonon branch contributions to thermal transport suggest that magnetic excitations are responsible for the increase in thermal conductivity in the HO phase. For high T, cuprates, the pseudogap state is not well understood. It is not even clear if it is a true phase transition or if it is a crossover regime. Recent reports of circular dichroism at the copper K-edge in double-layer BSCCO suggest breaking of inversion symmetry in the pseudogap. We perform copper K-edge dichroism measurements on carefully aligned BSCCO. Azimuthal rotations reveal the circular dichroic signal the result of linear bleed through. Polar rotations suggest that the previous reports were likely caused by misalignment. Thesis Supervisor: Young S. Lee Title: Associate Professor