Jacobo Santamaria

The ability to rationally design materials with specific properties is a major motivator in materials science and condensed matter physics. Driven by this motivation and by the potential to open new technological avenues, researchers are continually pushing the boundaries of established materials while pioneering new ones. This push has led to newly developed techniques for fabricating, handling, and transferring freestanding oxide membranes (FOMs), providing a new avenue to overcome traditional oxide epitaxial growth limitations, which relies on specific substrates and synthesis conditions. These advances enable the design and creation of novel materials with previously inaccessible interface symmetries, stacking configurations, and atomic reconstructions, opening exciting possibilities for exploring new physics and developing innovative device architectures using atomically engineered complex oxides. At the same time, these advancements present new challenges in the synthesis, membrane transfer, characterization, and final functionalization of FOMs. The field is still in its early stages and necessitates further scientific and technological progress. This roadmap offers an overview of the current state of the field and perspectives on its future, highlighting opportunities, challenges, and guiding principles for the research community.

The recent realization of freestanding membranes of perovskite oxides has enabled their deterministic mechanical assembly into twisted homo bilayers. Twisted oxide interfaces feature a non‐homogeneous pattern of shear strains determined by the atomic registry through the strong ionic‐covalent bonding of this family of compounds. Twisted ferroelectrics open an unprecedented opportunity to tailor topological polar landscapes in a way determined by the flexoelectric coupling of polarization to the large strain gradients developing with the modulation of the moiré coincidence pattern. Yet, flexoelectricity is a universal phenomenon which may render polar landscapes in non‐ferroelectric materials. Here we report a polar topology in twisted membranes of SrTiO ₃ (STO), a paraelectric centrosymmetric compound. The polar landscape is generated by the flexoelectrically induced polarization driven by the strain modulation triggered by twisting, as also supported by machine‐learned force fields based on first‐principles calculations. We further show that the strain and polarization patterns in top and bottom layers are correlated in a way which breaks inversion and mirror symmetries as dictated by the emergent chirality imposed by twisting. This finding opens exciting opportunities for the exploration of novel physical effects and functionalities.

Resistive switching phenomena have attracted increased attention over the years because of their technological relevance, particularly for applications like nonvolatile memories and memristors, which are of much interest for emerging neuromorphic computation schemes. Devices based on complex oxides are especially appealing, since these materials present a plethora of functionalities (magnetism, superconductivity, photoconductivity…) and resistive switching mechanisms. Determining their characteristic timescales is crucial, both for fundamental understanding and regarding the potential applications. Here, we analyze the resistive switching dynamics of tunnel junctions between a cuprate superconductor and a metallic counter-electrode, in which memristive behavior stems from an electrochemical redox reaction across their interface. We compare these junctions with similar ones in which a thin oxide ferroelectric layer is placed in between both electrodes, expected to promote faster ferroelectric switching effects. We find that all the junctions show similar dynamics, pointing to oxygen ion exchange as the dominant mechanism in all cases. Interestingly, very strong resistance switching effects are observed down to the 100 ns timescale, thus allowing for applications where fast switching is required, even at cryogenic temperatures at which superconducting effects can be exploited.

Two-dimensional electron gases (2DEGs) at oxide interfaces are promising for electronics because of desirable ingredients such as spin–orbit coupling and strong correlations that can be leveraged to bridge into spintronics or photonics. In this context, the ability to manipulate oxide 2DEGs via external knobs is particularly important. Here we show that a volatile high-mobility 2DEG can be photogenerated at the interface between SrTiO₃ (001) and infinite-layer NdNiO₂, where such an electronic state is otherwise absent. This allows us to optically switch the 2DEG between ON and OFF, leading to a giant, instantaneous conductivity variation. The key ingredients for this effect are the structural and electronic reconstructions at the NdNiO₂//SrTiO₃ interface, together with a built-in interfacial electric field that promotes the occupation of the Ti 3d_xy band by the photogenerated carriers. By contributing to understanding photoconductance at complex-oxide interfaces, our results pave the way to engineer the photoresponse of strongly correlated electrons.

This article was originally published online on 30 May 2024 with an error in affiliation 2 and the acknowledgments. Affiliation 2 is correct as it appears above This article was originally published online on 30 May 2024 with an error in affiliation 2 and the acknowledgments. Affiliation 2 is correct as it appears above

The interplay between ferromagnetism and superconductivity has attracted substantial interest due to its potential for exotic quantum phenomena and advanced electronic devices. Although ferromagnetism and superconductivity are antagonistic phenomena, ferromagnets (F) can host spin-triplet superconductivity induced via proximity with superconductors (S). To date, most of the experimental effort has been focused on single S/F/S junctions. Here, we have found the fingerprints of long-range superconducting proximity effect in micrometric weak-link arrays, formed by embedding YBa2Cu3O7 superconducting islands in a half-metallic ferromagnet La0.7Ca0.3MnO3 film. These arrays show magnetoresistance oscillations that appear at temperatures below the critical temperature of YBa2Cu3O7 for currents below a threshold, indicating their superconducting origin. This realization paves the way for device architectures displaying macroscopic quantum interference effects, which are of interest for field sensing applications, among others.

The wealth of complex polar topologies^1–10 recently found in nanoscale ferroelectrics results from a delicate balance between the intrinsic tendency of the materials to develop a homogeneous polarization and the electric and mechanical boundary conditions imposed on them. Ferroelectric–dielectric interfaces are model systems in which polarization curling originates from open circuit-like electric boundary conditions, to avoid the build-up of polarization charges through the formation of flux-closure^11–14 domains that evolve into vortex-like structures at the nanoscale^15–17 level. Although ferroelectricity is known to couple strongly with strain (both homogeneous¹⁸ and inhomogeneous^19,20), the effect of mechanical constraints²¹ on thin-film nanoscale ferroelectrics has been comparatively less explored because of the relative paucity of strain patterns that can be implemented experimentally. Here we show that the stacking of freestanding ferroelectric perovskite layers with controlled twist angles provides an opportunity to tailor these topological nanostructures in a way determined by the lateral strain modulation associated with the twisting. Furthermore, we find that a peculiar pattern of polarization vortices and antivortices emerges from the flexoelectric coupling of polarization to strain gradients. This finding provides opportunities to create two-dimensional high-density vortex crystals that would enable us to explore previously unknown physical effects and functionalities.

Generation, manipulation, and sensing of magnetic domain walls are cornerstones in the design of efficient spintronic devices. Half‐metals are amenable for this purpose as large low field magnetoresistance signals can be expected from spin accumulation at spin textures. Among half metals, La_1−xSr_xMnO₃ (LSMO) manganites are considered as promising candidates for their robust half‐metallic ground state, Curie temperature above room temperature (T_c = 360 K, for x = 1/3), and chemical stability. Yet domain wall magnetoresistance is poorly understood, with large discrepancies in the reported values and conflicting interpretation of experimental data due to the entanglement of various source of magnetoresistance, namely, spin accumulation, anisotropic magnetoresistance, and colossal magnetoresistance. In this work, the domain wall magnetoresistance is measured in LSMO cross‐shape nanowires with single‐domain walls nucleated across the current path. Magnetoresistance values above 10% are found to be originating at the spin accumulation caused by the mistracking effect of the spin texture of the domain wall by the conduction electrons. Fundamentally, this result shows the importance on non‐adiabatic processes at spin textures despite the strong Hund coupling to the localized t_2g electrons of the manganite. These large magnetoresistance values are high enough for encoding and reading magnetic bits in future oxide spintronic sensors.

The proximity effect between superconductors and ferromagnets may become long range due to the generation of triplet pairs. The recent finding of a long, one micron-range unconventional Josephson effect between YBa₂Cu₃O₇ high Tc cuprates separated by a half metallic La_0.7Sr_0.3MnO₃ manganite ferromagnet has uncovered a novel unconventional triplet proximity effect. In this paper, we examine the temperature dependence of the critical current in planar Josephson junctions. We find that the critical current—normal resistance product follows the predictions of traditional superconductor-normal metal-superconductor junctions, which implies that triplet pairs in a ferromagnet are transported in the diffusive limit similarly to singlet pairs in a normal metal. This result calls for theoretical studies of the new triplet Josephson effect and underlines its potential in future superconducting spintronics.