Light-matter interaction in polar and strongly correlated oxides

Abstract

Light-matter interactions are ubiquitous in nature and play a pivotal role in understanding the functioning of the universe. In the context of solid state physics, light-matter interactions are indispensable not only as a probe of material properties but also in using light to modify structure or properties, giving rise to new phenomenon. When absorbed, photons can excite electrons into higher energy states. The behaviour of these photoexcited electrons can be dictated by the periodic atomic potentials and, conversely, the photo-excited electrons can themselves modify the behaviour of the material system. In this thesis, I focus on three different materials: multiferroic BiFeO3 , ferroelectric BaTiO3 , and strongly correlated material VO2 , and investigate different interplays between the crystal lattice and the photoexcited electrons in these materials. In Chapter 1, I lay down the basic concepts of ferroelectricity and metal-insulator transitions, introduce the materials and their crystal structures and phase diagrams. Besides, I also explain the specific types of light-matter interaction, relevant for discussion. I dedicate Chapter 2 in explaining the experimental methods that have been used in this thesis with great detail, specifically the techniques such as interferometry, ultrafast spectroscopy, and femtosecond X-ray diffraction. Due to asymmetric crystal structure of ferroelectrics, the dynamics and the movement of photoexcited electrons in the band depends on the crystal potentials along specific wavevectors. The Bulk photovoltaic effect (BPVE) in ferroelectrics exemplifies one instance of how crystal symmetry affects carrier generation and dynamics. While the theoretical understanding about BPVE has been sound, the microscopic charge transport mechanism and the intrinsic efficiency of the BPVE still remains elusive. In particular, direct visualization of carrier dynamics in BPVE was still missing. In Chapter 3, using an ultrafast spatiotemporal microscopy technique on multiferroic BiFeO3 , I directly visualize the spatial transport of photoexcited carriers on ultrafast time scales that take part in the BPVE. From the experimental observations, I conclude that the direction of ferroelectric polarization dictates an asymmetric drift of electrons along the polar axis and that BPVE intrinsically is an efficient process compared to macroscopic efficiencies reported for devices. In Chapter 4, instead of looking at how crystal structure and polarization affects photocarriers and their movement, I study how the presence of photocarriers affects the electrical polarization and spontaneous strain, causing photostriction. On freestanding circular membranes of BaTiO3 , I show that photoinduced strain can be used to induce mechanical resonances and extract large deflections from the membranes, in contrast with the case of paraelectric SrTiO3 . Subsequently, I prove that the presence of ferroelectricity is indispensable for extracting large photostrictive responses, and that the photostrictive mechanism is the screening of ferroelectric polarization by photocarriers. Strongly correlated electron systems, like VO2 , can switch between insulating and metallic states. The same photoexcited carriers that generate strain in a ferroelectric can drive a phase transition in such materials. In Chapter 5, I explore the dynamics of the photoinduced phase transition in an epitaxial thin film that has four co-existing phases. Using femtosecond X-ray diffraction, I track the dynamics and evolution of the coexisting phases and demonstrate how to access different parts of the phase diagram under non-equilibrium conditions. Additionally, in specific fluence regimes, we observe transition pathways that are usually inaccessible in thermal transitions. We also observe relaxation pathways that can be different from the excitation pathways giving rise to metastable states which only exist under delicate geometric/crystallographic constraints in the steady state. Finally, in Chapter 6, I summarise the findings of the thesis. Additionally, a follow up research question to Chapter 4 has been explored and the preliminary findings are presented in Appendix A.

Date of Award	27 May 2024
Original language	English
Supervisor	Gustau Catalan Bernabeu (Director), Gabriele De Luca (Director) & David Pesquera Herrero (Director)