Charge Correlations Imply Density Wave Causation

Abstract

This thesis presents X-ray scattering measurements of charge correlations in a range of superconducting transition metal oxides. Specifically, the charge correlations are charge density waves (CDW), and acoustic plasmons. CDW are static or low energy fluctuations of the charge density. Plasmons are characteristic long wavelength (high energy) oscillations of the charge density in a metal. An acoustic plasmon is the softening of the plasmon close to zero energy in a system formed of stacked 2D metallic layers. The superconducting transition metal oxides we look at are Tl2Ba2CuO6+δ (Tl2201), La2−xSrxCuO4
(LSCO), Sr2RuO4, and Nd1−xSrxNiO2 (NSNO). The broad aim of the research is to characterise these
charge correlations in detail, using state-of-the-art high resolution resonant inelastic X-ray scattering (RIXS) spectrometers I21, Diamond Light Source and ID32, ESRF. Thanks to the sensitivity of the RIXS technique to weak charge correlations, we are able to resolve subtle changes of the charge correlations with doping and temperature which we then relate to physical properties of the normal state of the superconductors.
We discover the presence of CDW in Tl2201, an electronically clean, overdoped cuprate superconduc-
tor. We relate the CDW to the doping evolution of the Hall number, which probes the carrier density,
by a Fermi surface reconstruction. Next, we measure the CDW and CDW fluctuations in LSCO. We
find they exist as two components: one is static within resolution, and is maximised at 1/8 hole doping. The other is inelastic (with an energy scale of around 6 meV), exists over a wide doping range, and has a distinct structure in (E,Q). Moving on from CDW, we measure the acoustic plasmon in Tl2201, LSCO, and Sr2RuO4. We compare the plasmon to two models: a model based on the random phase approximation (RPA) to describe the dispersion and a hydrodynamic model to describe the temperature scaling. We find the plasmons in the cuprates mostly obey both models, while the plasmon in Sr2RuO4 does not fit either quite well. We discuss possible reasons for this, including their different normal state properties. We also characterise a dispersive orbital excitation in Sr2RuO4. Finally, we perform RIXS measurements on a range of NSNO samples prepared under a wide range of growth conditions. The aim is to isolate intrinsic phenomena from byproducts of the growth procedure, which is currently an issue in this class of materials. We also perform extensive measurements on a scattering peak originating from a superstructure present in the IL nickelates, whose origin is not yet known. The results presented in this thesis add new information to the phase diagram of the superconducting systems studied, which will ultimately help to understand their superconducting state.

Date of Award	18 Jun 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Stephen M Hayden (Supervisor)