David Sanchez-Manzano

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.

Swirling spin textures, including topologically nontrivial states, such as skyrmions, chiral domain walls, and magnetic vortices, have garnered significant attention within the scientific community due to their appeal from both fundamental and applied points of view. However, their creation, controlled manipulation, and stability are typically constrained to certain systems with specific crystallographic symmetries, bulk or interface interactions, and/or a precise stacking sequence of materials. Recently, a new approach has shown potential for the imprint of magnetic radial vortices in soft ferromagnetic compounds making use of the stray field of YBa₂Cu₃O_7-δ superconducting microstructures in ferromagnet/superconductor (FM/SC) hybrids at temperatures below the superconducting transition temperature (T_C). Here, we explore the lower size limit for the imprint of magnetic radial vortices in square and disc shaped structures as well as the persistence of these spin textures above T_C, with magnetic domains retaining partial memory. Structures with circular geometry and with FM patterned to smaller radius than the superconductor island facilitate the imprinting of magnetic radial vortices and improve their stability above T_C, in contrast to square structures where the presence of magnetic domains increases the dipolar energy. Micromagnetic modeling coupled with a SC field model reveals that the stabilization mechanism above T_C is mediated by microstructural defects. Superconducting control of swirling spin textures, and their stabilization above the superconducting transition temperature by means of defect engineering holds promising prospects for shaping superconducting spintronics based on magnetic textures.

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.

SrIrO₃ is a correlated semimetal with narrow t_2g d-bands of strong mixed orbital character resulting from the interplay of the spin-orbit interaction due to heavy iridium atoms and the band folding induced by the lattice structure. In ultrathin layers, inversion symmetry breaking, occurring naturally due to the presence of the substrate, opens new orbital hopping channels, which in presence of spin-orbit interaction causes deep modifications in the electronic structure. Here, we show that in SrIrO₃ ultrathin films the effect of inversion symmetry breaking on the band structure can be externally manipulated in a field effect experiment. We further prove that the electric field toggles the system reversibly between a metallic and an insulating state with canted antiferromagnetism and an emergent anomalous Hall effect. This is achieved through the spin-orbit driven coupling of the electric field generated in an ionic liquid gate to the electronic structure, where the electric field controls the band structure rather than the usual band filling, thereby enabling electrical control of the effective role of electron correlations. The externally tunable antiferromagnetic insulator, rooted in the strong spin-orbit interaction of iridium, may inspire interesting applications in spintronics.

The Josephson effect results from the coupling of two superconductors across a spacer such as an insulator, a normal metal or a ferromagnet to yield a phase coherent quantum state. However, in junctions with ferromagnetic spacers, very long-range Josephson effects have remained elusive. Here we demonstrate extremely long-range (micrometric) high-temperature (tens of kelvins) Josephson coupling across the half-metallic manganite La_0.7Sr_0.3MnO₃ combined with the superconducting cuprate YBa₂Cu₃O₇. These planar junctions, in addition to large critical currents, display the hallmarks of Josephson physics, such as critical current oscillations driven by magnetic flux quantization and quantum phase locking effects under microwave excitation (Shapiro steps). The latter display an anomalous doubling of the Josephson frequency predicted by several theories. In addition to its fundamental interest, the marriage between high-temperature, dissipationless quantum coherent transport and full spin polarization brings opportunities for the practical realization of superconducting spintronics, and opens new perspectives for quantum computing.