Towards high pressure resonant X-ray diffraction experiments on I16
Item statusRestricted Access
Embargo end date31/07/2021
Povedano-Fuentes, María Isabel
The investigation of the properties of electron correlated materials under pressure remains a fertile arena for the discovery of novel electronic ground states, often beyond conventional wisdom. However, direct microscopic insight into pressure-driven quantum phenomena is certainly limited to a small number of techniques and usually hampered by the lack of suitable instrumentation. In this regard, resonant elastic x-ray scattering (REXS) is one of the best techniques that combines the elements of diffraction and spectroscopy, offering information about the atomic species, their positions in the crystal lattice and their electronic orbital configuration. Combining REXS with high-pressure (HP) presents an invaluable potential since pressure tuning of the interatomic electron-electron interactions in a crystal can be probed directly via the distortion of the lattice. Thus, HP-REXS experiments allow simultaneous observation of the crystallographic, magnetic and electronic degrees of freedom within the same experiment. This is extremely important in high-pressure studies where often inconsistencies stem from the use of different pressure devices or due to sample-dependent effects. Nevertheless, non-trivial technological challenges need to be overcome when developing the hardware and the methodology for HP-REXS experiments, mainly dictated by two factors. Firstly, the observation of electron-electron interatomic interactions frequently requires low-temperature, where the electronic ground state is free of thermal motion and, therefore, other energy scales such as the on-site Coulomb repulsion, the crystal field splitting or the spin-orbit coupling prevail over the electronic fluctuations induced by the temperature. Secondly, the absorption cross-section of materials is severe in the range of energies demanded to excite the elementary resonant processes of interest (typically below 15 keV), making the detection of weak magnetic reflections challenging. Moreover, the signal arising from these weak interactions gets further screened by the high-pressure device. The aim of this work is to provide a set of instrumentation for HP-REXS experiments on I16, the beamline for materials and magnetism at the Diamond Light Source (DLS). Likewise, to establish the working methodology and to collect REXS data at HP. The thesis begins with the introduction to synchrotron radiation and the fundamentals of REXS before describing the state-of-the-art of the HP instrumentation dedicated to x-ray studies under cryogenic conditions. Then, the new setup for HP-REXS experiments is described. It consists of a membrane-driven diamond anvil cell, a panoramic dome and an optical system for in situ pressure measurement using the ruby fluorescence method. The membrane cell presents an asymmetric layout for operating in back-scattering geometry, with a panoramic aperture of 100 degrees. This system allows the observation of resonant signals using excitation energies at least as low as ∼ 8 keV, within a temperature range of 30-300 K and up to 20 GPa for anvils of 500 µm in culet diameter. Finally, the thesis presents the results obtained from investigating the evolution of magnetic correlations in Sr3Ir2O7 and the lanthanum doped counterpart (Sr1-xLax)3Ir2O7 [x = 0.007(1)] upon application of hydrostatic pressure. The experimental evidence reveals the presence of long-range 3D magnetic order at up to at least 11 GPa of pressure. Combining the HP-REXS results with additional resonant inelastic x-ray scattering data and theoretical modelling a conclusion can be made about the presence of a spin-flop transition at the critical pressure of Pc ∼ 14 - 15 GPa, with putative short-range in-plane magnetic order above Pc. In summary, this thesis presents a set of instrumentation and detailed methodology for conducting REXS experiments under high-pressure. The experimental results demonstrate the viability of the proposed approach and provide a notion of the extraordinarily wealth information accessible, particularly beneficial for the investigation of the electronic properties of materials.