Author: Haiyu Lu

Publisher:

Published: 2021

Total Pages:

ISBN-13:

One of the key ideas in the condensed matter physics at a low energy scale is to treat an interacting many-body electron system in terms of weakly interacting emergent particles, namely elementary excitations. Resonant inelastic X-ray scattering (RIXS) has emerged as a powerful tool to study elementary excitations, thanks to the exciting instrumentation improvement and the advent of X-ray synchrotron radiation facilities. By tuning the incident X-ray photon energy to the resonance condition of a specific element, RIXS can resonantly enhance the scattering cross section between the incident photon and the elementary excitation in the material, enabling element-specific measurements. In this dissertation, we investigated magnetic excitations in the recently discovered superconducting infinite-layer nickelate thin films. Besides a similar crystal structure, the infinite-layer nickelate is nominally isoelectronic to the high temperature cuprate superconductors. At this early stage, it is important to experimentally study the similarities and differences between cuprates and nickelates, particularly the respective electronic and magnetic structures. Here, by utilizing the high-resolution RIXS, we found that the parent compound of the infinite-layer nickelate is not a charge-transfer material, like cuprates. Instead, it is probably closer to a Mott compound with an additional coupling with rare-earth metallic states. Furthermore, we also found a branch of dispersive magnetic excitations with a bandwidth of approximately 200 meV in the parent compound nickelates. Although these modes are significantly damped, they are reminiscent of the spin-wave of strongly coupled, anti-ferromagnetically aligned spins on a square lattice. Upon doping, the spectral weight and energy decrease slightly, while the modes become overdamped. The dispersion, doping dependence, damping effect of the magnetic excitation, and the added complexity due to the presence of the rare-earth metallic pockets, together with its comparison with the magnetic excitation in cuprates, will be discussed.