X-ray probes of dynamical correlations in cuprates and nickelates

Abstract

X-ray spectroscopy probes quantum materials on their intrinsic energy and time scales, providing direct access to their spin, charge, and electronic correlations. In strongly correlated systems, small changes to any of the microscopic parameters governing these correlations can have a large impact on the macroscopic phase that is realized. Thus, in many materials families, doping, pressure and strain, external magnetic or electric fields, and ultrafast optical excitation can tune the ground state across distinct phases. The layered square-planar and layered Ruddlesden-Popper nickelates are two new materials families whose ground state can be tuned across insulating, superconducting, and metallic phases by tuning the number of layers in the structural unit cell and modifying the lattice constants by pressure or epitaxial strain. Here I present resonant inelastic x-ray scattering (RIXS) studies of how the magnetic and electronic structure evolves with layer number across these two families of layered nickelates. The persistence of high energy magnetic excitations across the phase diagram presents a unifying theme between these nickelate materials and other unconventional superconductors, including cuprates and iron-pnictides. Next, I focus on time-resolved x-ray measurements of the spin- and charge-ordered trilayer square-planar nickelate La4Ni3O8, which reveal how structural and electronic dynamics shape this correlated phase which may compete with super- conductivity. By focusing separately on non-resonant x-ray diffraction (XRD) probes and resonant x-ray absorption spectroscopy (XAS) probes which are separately sensitive to only structural or electronic changes, we isolate how ultrafast optical excitation suppresses each of these components of the density wave order, and the subsequent slow recovery of both structural and electronic contributions to the ordered state. Finally, I demonstrate a new method to use RIXS to directly probe spin entanglement, revealing seven-partite entanglement in Sr2CuO3. By extending x-ray scattering to directly quantify quantum correlations, entanglement spectroscopy opens a new avenue for characterizing quantum phases in- and out-of-equilibrium by directly quantifying their entanglement correlations. Such entanglement measurements can reveal new insights into the microscopic interactions and quantum fluctuations that shape the phase diagrams of correlated materials.