Josep Santiso

Ferroelectric memristive devices based on hafnia are promising systems for neuromorphic electronics, yet the interplay between polarization-modulated resistive changes and defect-mediated transport often leads to complex and debated switching mechanisms. Here, we investigate this competition in epitaxial Hf0.5Zr0.5O2/La0.67Sr0.33MnO3 heterostructures with Pt top electrodes by combining structural, ferroelectric, and memristive characterization with a statistical analysis across a broad range of device areas spanning three orders of magnitude. We identify two distinct memristive regimes with opposite resistance–voltage chiralities. Small devices exhibit a low-resistance state that scales inversely with area, consistent with area-distributed electronic transport, while larger devices display an area-independent resistance indicative of localized conductive channels. A statistical nucleation model quantitatively captures this behavior and yields a crossover characteristic area of A*≈103μm2. This crossover also correlates with the onset of ferroelectric wake-up for the larger devices, linking conductive-channel nucleation and oxygen-vacancy redistribution within a unified physical picture. These results establish lateral device size as a key parameter controlling the dominant transport mechanism in epitaxial hafnia-based devices.

Epitaxial strain engineering in oxide thin films offers a powerful strategy for tuning the electronic and thermoelectric properties by modulating the defect density, electronic band structure, and phonon scattering. In this study, we show that the gradual relaxation of in-plane epitaxial strain in layered double perovskite GdBaCo₂O_5.5+δ thin films grown on SrTiO₃ (001) substrates leads to a pronounced enhancement in thermoelectric power near the critical film thickness, where the epitaxial strain begins to relax, and a structural phase transition begins to occur. Our analysis suggests that this enhancement arises from an increase in carrier effective mass, likely caused by the reconstruction of electronic bands.

An amorphous carbon-rich conductive phase formed at 85 °C and 1 sun operation, causing carrier shunt losses in perovskite solar cells, is mitigated by additives at grain boundaries. MXene-interlayer prevents delamination during thermal expansion.

A fundamental understanding of the interplay between lattice structure, polarization and electrons is pivotal to the optical control of ferroelectrics. The interaction between light and matter enables the remote and wireless control of the ferroelectric polarization on the picosecond timescale, while inducing strain, i.e., lattice deformation. At equilibrium, the ferroelectric polarization is proportional to the strain, and is typically assumed to be so also out of equilibrium. Decoupling the polarization from the strain would remove the constraint of sample design and provide an effective knob to manipulate the polarization by light. Here, upon above-bandgap laser excitation of the prototypical ferroelectric BaTiO₃, we induce and measure an ultrafast decoupling between polarization and strain that begins within 350 fs, by softening Ti-O bonds via charge transfer, and lasts for several tens of picoseconds. We show that the ferroelectric polarization out of equilibrium is mainly determined by photoexcited electrons, instead of the strain.

Recent advances in moiré engineering have revealed new pathways for manipulating lattice distortions and electronic properties in low-dimensional materials. Here, we demonstrate that twisted-stacking can induce polarization vortices in metallic SrRuO ₃ membranes, despite the presence of free charge screening. These polarization vortices are correlated with moiré-periodic flexoelectricity induced by shear strain gradients, and exhibit a pronounced dependence on the twist angle. In addition, multiferroic behavior emerges below the ferromagnetic Curie temperature of the films, whereby polarization and ferromagnetism coexist and appear to compete, showing opposite twist-angle dependencies of their respective magnitudes. Density functional theory calculations provide insights into the microscopic origin of these observations. Our findings extend the scope of topological polarization design beyond dielectric materials and into metals.