Quantum materials can display physical phenomena rooted in the geometry of electronic wavefunctions. The corresponding geometric tensor is characterized by an emergent field known as the Berry curvature (BC). Large BCs typically arise when electronic states with different spin, orbital or sublattice quantum numbers hybridize at finite crystal momentum. In all the materials known to date, the BC is triggered by the hybridization of a single type of quantum number. Here we report the discovery of the first material system having both spin- and orbital-sourced BC: LaAlO₃/SrTiO₃ interfaces grown along the [111] direction. We independently detect these two sources and probe the BC associated to the spin quantum number through the measurements of an anomalous planar Hall effect. The observation of a nonlinear Hall effect with time-reversal symmetry signals large orbital-mediated BC dipoles. The coexistence of different forms of BC enables the combination of spintronic and optoelectronic functionalities in a single material.

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Suspended piezoelectric thin films are key elements enabling high-frequency filtering in telecommunication devices. To meet the requirements of next-generation electronics, it is essential to reduce device thickness for reaching higher resonance frequencies. Here, the high-quality mechanical and electrical properties of graphene electrodes are combined with the strong piezoelectric performance of the free-standing complex oxide, BaTiO₃ (BTO), to create ultrathin piezoelectric resonators. It is demonstrated that the device can be brought into mechanical resonance by piezoelectric actuation. By sweeping the DC bias voltage on the top graphene electrode, the BTO membrane is switched between the two poled ferroelectric states. Remarkably, ferroelectric hysteresis is also observed in the resonance frequency, magnitude and Q-factor of the first membrane mode. In the bulk acoustic mode, the device vibrates at 233 GHz. This work demonstrates the potential of combining van der Waals materials with complex oxides for next-generation electronics, which not only opens up opportunities for increasing filter frequencies, but also enables reconfiguration by poling, via ferroelectric memory effect.

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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

Although 2D materials hold great potential for next-generation pressure sensors, recent studies revealed that gases permeate along the membrane-surface interface, necessitating additional sealing procedures. In this work, we demonstrate the use of free-standing complex oxides as self-sealing membranes that allow the reference cavity beneath to be sealed by a simple anneal. To test the hermeticity, we study the gas permeation time constants in nanomechanical resonators made from SrRuO3 and SrTiO3 membranes suspended over SiO2/Si cavities which show an improvement up to 4 orders of magnitude in the permeation time constant after annealing the devices. Similar devices fabricated on Si3N4/Si do not show such improvements, suggesting that the adhesion increase over SiO2 is mediated by oxygen bonds that are formed at the SiO2/complex oxide interface during the self-sealing anneal. Picosecond ultrasonics measurements confirm the improvement in the adhesion by 70% after annealing.

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In oxide heterostructures, different materials are integrated into a single artificial crystal, resulting in a breaking of inversion symmetry across the heterointerfaces. A notable example is the interface between polar and nonpolar materials, where valence discontinuities lead to otherwise inaccessible charge and spin states. This approach paved the way for the discovery of numerous unconventional properties absent in the bulk constituents. However, control of the geometric structure of the electronic wave functions in correlated oxides remains an open challenge. Here, we create heterostructures consisting of ultrathin SrRuO3, an itinerant ferromagnet hosting momentum-space sources of Berry curvature, and LaAlO3, a polar wide-band-gap insulator. Transmission electron microscopy reveals an atomically sharp LaO/RuO2/SrO interface configuration, leading to excess charge being pinned near the LaAlO3/SrRuO3 interface. We demonstrate through magneto-optical characterization, theoretical calculations and transport measurements that the real-space charge reconstruction drives a reorganization of the topological charges in the band structure, thereby modifying the momentum-space Berry curvature in SrRuO3. Our results illustrate how the topological and magnetic features of oxides can be manipulated by engineering charge discontinuities at oxide interfaces.

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In systems near phase transitions, macroscopic properties often follow algebraic scaling laws, determined by the dimensionality and the underlying symmetries of the system. The emergence of such universal scaling implies that microscopic details are irrelevant. Here, we locally investigate the scaling properties of the metal-insulator transition at the LaAlO₃/SrTiO₃ interface. We show that, by changing the dimensionality and the symmetries of the electronic system, coupling between structural and electronic properties prevents the universal behavior near the transition. By imaging the current flow in the system, we reveal that structural domain boundaries modify the filamentary flow close to the transition point, preventing a fractal with the expected universal dimension from forming.

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Oxide heterointerfaces constitute a rich platform for realizing novel functionalities in condensed matter. A key aspect is the strong link between structural and electronic properties, which can be modified by interfacing materials with distinct lattice symmetries. Here, we determine the effect of the cubic-tetragonal distortion of SrTiO₃ on the electronic properties of thin films of SrIrO₃, a topological crystalline metal hosting a delicate interplay between spin-orbit coupling and electronic correlations. We demonstrate that below the transition temperature at 105 K, SrIrO₃ orthorhombic domains couple directly to tetragonal domains in SrTiO₃. This forces the in-phase rotational axis to lie in-plane and creates a binary domain structure in the SrIrO₃ film. The close proximity to the metal-insulator transition in ultrathin SrIrO₃ causes the individual domains to have strongly anisotropic transport properties, driven by a reduction of bandwidth along the in-phase axis. The strong structure-property relationships in perovskites make these compounds particularly suitable for static and dynamic coupling at interfaces, providing a promising route towards realizing novel functionalities in oxide heterostructures.

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The correlated 4d transition metal oxide SrRuO₃ (SRO) features an anomalous Hall effect that originates from momentum-space sources of Berry curvature and depends sensitively on the magnetization. Here, we exploit this sensitivity and realize an epitaxial extraordinary Hall balance device, consisting of two ultrathin layers of SRO, separated by an insulating SrTiO₃ (STO) spacer. Our results highlight the potential of ultrathin SRO in the realization of oxide-based spintronic devices.

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Three-dimensional strontium ruthenate (SrRuO3) is an itinerant ferromagnet that features Weyl points acting as sources of emergent magnetic fields, anomalous Hall conductivity, and unconventional spin dynamics. Integrating SrRuO3 in oxide heterostructures is potentially a novel route to engineer emergent electrodynamics, but its electronic band topology in the two-dimensional limit remains unknown. Here we show that ultrathin SrRuO3 exhibits spin-polarized topologically nontrivial bands at the Fermi energy. Their band anticrossings show an enhanced Berry curvature and act as competing sources of emergent magnetic fields. We control their balance by designing heterostructures with symmetric (SrTiO3/SrRuO3/SrTiO3 and SrIrO3/SrRuO3/SrIrO3) and asymmetric interfaces (SrTiO3/SrRuO3/SrIrO3). Symmetric structures exhibit an interface-tunable single-channel anomalous Hall effect, while ultrathin SrRuO3 embedded in asymmetric structures shows humplike features consistent with multiple Hall contributions. The band topology of two-dimensional SrRuO3 proposed here naturally accounts for these observations and harmonizes a large body of experimental results.

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In bilayers of ferromagnets and heavy metals, which form the so-called spintronic emitters, the phenomena of ultrafast demagnetization and the inverse spin Hall effect (ISHE) conspire to yield remarkably efficient emission of electric pulses in the THz band. Light-induced demagnetization of the ferromagnet launches a pulse of spin current into the heavy metal, wherein it bifurcates into a radiative charge transient due to the ISHE. The influence of temperature on this combined effect should depend on both the magnetic phase diagram and the microscopic origin of spin Hall conductivity, but its exact dependence remains to be clarified. Here, we experimentally study the temperature dependence of an archetypal spintronic emitter, the Co/Pt bilayer, using electro-optic sampling of the emitted THz pulses in the time domain. The emission amplitude is attenuated with decreasing temperature, consistent with an inverse spin Hall effect in platinum of predominantly intrinsic origin.

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