Spin–orbit coupling and geometric phases at oxide interfaces

In this work, we investigate electronic and magnetic phenomena in thin films and heterostructures of transition metal oxides with strong spin–orbit coupling. Ultrathin films are prepared by pulsed laser deposition, a technique which enables layer-by-layer growth of complex materials on atomically flat crystal surfaces. The properties of these heterostructures, which include materials such as strontium iridate (SrIrO3) and strontium ruthenate (SrRuO3), are probed by applying electric and magnetic fields. By varying parameters such as temperature, magnetic field strength and layer thickness, we obtain information about spin and charge transport in these atomically engineered crystals. Chapter 1 provides an introduction to the field of transition metal oxides, followed by a brief overview of the materials studied in this dissertation. Chapters 2 to 5 are dedicated to SrIrO3, a material that displays unexpected physical properties owing to the strong spin–orbit coupling of Ir. Chapter 2 starts with the growth and thermodynamic stability of SrIrO3, which is essential to obtain high-quality films and study their properties in the ultrathin limit. We develop a method to grow stoichiometric films by measuring their transport characteristics as a function of the target condition. We discover that the properties of SrIrO3 are sensitive to degradation in air and develop an encapsulation procedure to protect the filmsurface. SrIrO3 displays an exotic semimetallic state due to the interplay between electronic correlations, spin–orbit coupling, and octahedral rotations. In Chapter 3, we combine thermoelectric and magnetotransport measurements to quantitatively determine the transport coefficients of the different conduction channels. Despite their different dispersion relationships, electrons and holes are found to have strikingly similar transport coefficients. Chapters 4 and 5 focus on the electronic and magnetic properties of SrIrO3 in the two-dimensional limit. In Chapter 4, we discover a metal–insulator transition occurring at a critical thickness of 4 unit cells and an enhancement of spin fluctuations near the transition point. We investigate the magnetic state in Chapter 5, showing that a fourfold symmetric magnetoresistance component appears above a criticalmagnetic field. In Chapter 6, we interface ultrathin SrIrO3 with SrRuO3, an itinerant ferromagnet with an unconventional anomalous Hall conductivity. We discover that the presence of two dissimilar interfaces results in the emergence of two spin-polarized conduction channels. Having explored the influence of epitaxial interfaces, in Chapter 7 we develop a method to detach thin films fromtheir growth substrate using an epitaxial buffer layer. Using this approach, we prepare nanomechanical resonators of freestanding SrTiO3 and SrRuO3 films. By measuring the temperature dependence of theirmechanical response, we observe signatures of structural phase transitions in the SrTiO3, which affect the strain and mechanical dissipation of the resonators. Chapter 8 summarizes the findings of the previous chapters and provide perspectives for future work. We discuss ongoing experiments regarding Berry phase engineering and the manipulation of freestanding films.