The aimof this thesis is to investigate the impact of symmetry on light and how it alters its characteristics. Our research centers around the examination of complex photonic crystals rooted in the concept of photonic topological insulators, which are analogs of topological insulators initially introduced in condensed matter physics. Unlike typical insulating materials, these possess a unique ability to conduct along their surface or edges. Leveraging this fundamental property, photonic topological insulators have gained attention for designing transport circuits resistant to back-reflection and scattering mechanisms....@en

In this study, ultrathin films of the itinerant 4d ferromagnet SrRuO3 were epitaxially deposited on SrTiO3 and capped with a thin LaAlO3 layer. Top gates and a dielectric layer were patterned onto contacted films and magnetotransport properties were characterized at low temperatures as a function of top gate voltage. A particular focus was placed on the anomalous Hall resistivity. The magnitude of the Hall signal and the sheet resistance were shown to vary with top gate voltage. In particular, the anomalous Hall loops were compared to numerical tight-binding models. The model is proposed as an alternate explanation to the skyrmion picture and as a complement to the two-channel phenomenological model put forth to explain the unusual low-temperature anomalous signal of ultrathin SrRuO3 . Model predictions were found to be valid at low temperatures in semiconducting Ru-deficient SrRuO3 films.

This thesis is an experimental investigation of the physical properties of different transition metal oxide ultra-thin films. A common feature of these various materials and structures is that they exhibit a solid-state phase transition from a metallic to an insulating state, which is triggered upon changing sample composition, or by varying an external stimulus such as temperature, illumination or gas pressure. The experiments performed cover a broad spectrum of condensed matter, from material growth, structural characterisation and nanodevice fabrication to low-temperature magnetotransport, synchrotron microscopy and gas sensing.@en

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. @en

The realization of interfaces between different transition metal oxides has heralded a new era of materials and physics research. Notably, it enabled a uniquely diverse set of coexisting physical properties to be combined with an ever-increasing degree of experimental control. The primary focus of this thesis is the celebrated interface between the two wide band-gap insulators LaAlO3 and SrTiO3, which exhibits a variety of phenomena such as conductivity, superconductivity and spin–orbit coupling—all of which are gate-tunable, demonstrating the promise of this system for fundamental research and technological applications alike. We start by discussing the role of spin–orbit coupling in the magnetotransport properties of the system. Namely, we show how it can drive a giant in-plane magnetoresistance. On a more technically challenging perspective, we realize tunable Josephson junctions by means of lateral confinement and local side-gating. This technique, due to its simplicity, can be expanded to a broad group of interfacial systems. We then investigate LaAlO3/SrTiO3 interfaces along the (111) crystallographic direction, discovering that it condenses into a superconducting ground state and elucidating the important role played by electronic correlations. Finally, this thesis ends with a twist to the story. Moving away from epitaxial interfaces, we employ an innovative technique to obtain free-standing oxide films by etching a water-soluble sacrificial buffer layer. This exciting development paves the way to integrating complex oxides with van der Waals materials and engineering new phases in hybrid devices. @en

Modern materials synthesis techniques allowfor the layer-by-layer assimilation of structurally similar, yet compositionally different materials into artificial crystals, with atomic scale precision. At the resulting heterointerfaces, structural, electronic and magnetic reconstructions can lead to physical phenomena that are otherwise absent in the individual constituents. Composing so-called heterostructures is therefore one of the key approaches towards realizing the ultimate goal of designer materials with tailored properties. In this context, perovskite oxides represent a promising class of materials, owing to the combination of a delicate balance among competing electronic and magnetic interactions, as well as excellent structural compatibility among its members. This thesis describes a collection of investigations into interface-driven reconstructions in heterostructures composed of such perovskite oxides. Chapters 1 provides a brief introduction to the field of complex oxide interfaces, as well as the Berry curvature and its relationship to the so-called anomalous Hall effect. Chapter 2 provides an overview of the main experimental techniques used throughout this thesis; pulsed-laser deposition, X-ray diffraction, lithographic device fabrication and cryogenic magnetotransport characterization. Chapter 3 focuses on heterostructures composed of spin-orbit semimetal SrIrO3 and the bandgap insulator SrTiO3. Aided by transport measurements, synchrotron X-ray diffraction and DFT calculations, we demonstrate a coupling of orthorhombic structural domains in the film to tetragonal domains in the substrate. The results extend to a variety of orthorhombic materials, opening up possibilities to manipulate structural domain patterns to a wide variety of materials through interaction with a tetragonal substrate. Chapters 4 and 5 focus on the itinerant ferromagnet SrRuO3 and its intriguing intrinsic anomalous Hall effect. We show that through interfacing SrRuO3 with SrTiO3, SrIrO3 and LaAlO3, the sign of the momentum-space Berry curvature can be controlled. We propose a simple two-channel model to account for the unusual field dependence of the anomalous Hall effect in asymmetric heterostructures. The findings in these chapters underline oxide interfaces as a versatile platform for manipulating the geometric properties of wavefunctions in solid-state systems, as well as the potential of ultrathin SrRuO3 for spintronic applications. In Chapter 6, we synthesize SrRuO3 thin films on SrTiO3 (111) substates. Transport measurements indicate a distinct effect of electronic confinement on the electronic properties of (111) oriented SrRuO3 thin films as compared to their (001) counterparts, producing bands with a hole-like character in the ultrathin limit. This highlights crystal orientation and heteroepitaxial growth as an effective tuning parameter for controlling the electronic properties of oxide heterostructures. The final chapter summarizes the findings of this thesis and provides a number of research directions to be further explored.@en

Excitation of optical transitions in solids using ultrashort pulses of light allows to selectively perturb microscopic degrees of freedom in order to change and control material properties on very short timescales. In this thesis we study how ultrafast resonant excitation of optical transitions can induce coherent structural dynamics in wide-bandgap insulators and control magnetic interactions, manipulate magnetic order and induce (propagating) spin dynamics in insulating antiferromagnets. In time-resolved all-optical pump-probe experiments,we use ultrashort pulses of light to target specific lattice vibrations, orbital resonances and electronic transitions in various insulating materials and optically probe the structural and magnetic dynamics on the picosecond timescale. Chapter 1 provides an introduction to the field that studies ultrafast optical control of solids, with a focus on resonant optical control of magnetic properties and the generation of propagating excitations. Chapter 2 discusses the basic concepts of magnetic interactions, magnetic order and spin waves and chapter 3 contains the main experimental methods and experimental setups used in this work. In chapter 4 we study the coherent structural dynamics initiated by ultrafast resonant excitation of an infrared-active lattice vibration in the wide-bandgap insulator LaAlO3. We observe the excitation of a coherent THz phonon mode, corresponding to rotations of the oxygen octahedra around a high-symmetry axis, and identify the underlying nonlinear phonon-phonon coupling through density functional theory calculations. The resonant lattice excitation is also shown to generate both longitudinal and transverse strain wavepackets, the result of optically induced anisotropic strain. In chapter 5 we demonstrate that light-driven infrared-active phonons can be used to control fundamental magnetic interactions and coherently manipulate magnetic states on picosecond timescales. Resonant optical excitation of lattice vibrations in the antiferromagnet DyFeO3 results in nonthermal, ultrafast and long-living changes in the exchange interaction between the Dy orbitals and the Fe spins. We identify phononinduced coherent lattice distortions as the underlying mechanism and show that we can use this change in magnetic interaction to induce picosecond coherent switching from a collinear antiferromagnetic ground state to a weakly ferromagnetic phase. Having explored the structural and magnetic dynamics following excitation of lattice vibrations, we explore the effect of optical excitation of orbital resonances in the van der Waals antiferromagnet NiPS3 in chapter 6. We demonstrate that ultrashort pulses of light, with the photon energy tuned in resonance with orbital transitions within the magnetic nickel d-orbital manifold, can excite a subterahertz magnon mode with twodimensional behaviour. We show that this selective excitation results from a photoinduced transient magnetic anisotropy axis, which emerges in response to excitation of the ground-state electrons to orbital states with a lower orbital symmetry.Finally, we show in chapter 7 that ultrashort pulses of light can generate a wavepacket of coherent propagating spin waves in insulating antiferromagnets. The nanometer confinement of ultrafast optical excitation in resonance with electronic charge-transfer transitions in the antiferromagnet DyFeO3 creates a strongly non-uniform spatial spin excitation profile close to the material surface. This results in the emission of a broadband wavepacket of coherent subterahertz spin waves into the material. We optically probe individual spectral components of this spin-wavepacket with wavelengths down to 125nm in a time-resolved fashion using the magneto-optical Kerr effect. Chapter 8 provides the main conclusions of the work presented in this thesis. Wereflect on unanswered questions and give possible directions for future research. @en