arXiv subjects

Gustau Catalan

Publications and source records attributed to Gustau Catalan.

At least 19 recordsLinked to original sources

arXiv2026

Depth-Resolved Evolution of Buried Polar Topologies in a PbTiO3/SrTiO3 Superlattice

Polar topologies in complex oxides gives rise to a rich spectrum of emergent functionalities and are fundamentally governed by three-dimensional (3D) atomic structures. However, direct experimental determination of buried 3D polar configurations remains a longstanding challenge because conventional (scanning) transmission electron microscopy ((S)TEM) provides primarily projected structural information with limited depth sensitivity. Here, we combine depth-sectioning low-angle annular dark-field (LAADF) STEM, high-angle ADF (HAADF) STEM, and multislice electron ptychography (MEP) to directly visualize the depth-dependent atomic structure and polarization topology in a PTO/STO superlattice. Depth-sectioning STEM reveals pronounced focal-depth-dependent contrast variations and apparent splitting of Pb atomic columns, indicating significant structural heterogeneity along the beam direction. MEP reconstruction simultaneously resolves the Pb, Ti, and O sublattices with nanometer-scale depth resolution, enabling quantitative mapping of atomic displacements throughout the reconstructed volume. The resulting 3D atomic model reveals substantial depth-dependent displacements of Pb, Ti, and O atoms and a corresponding evolution of the polarization topology. Vortex-like polarization structures are observed near the specimen surfaces but become strongly suppressed within the interior, where distinct polarization configurations emerge. These findings show that polarization patterns observed in conventional projection images can arise from the superposition of multiple depth-dependent polar states and may obscure the underlying 3D polarization texture. Our findings establish a direct experimental link between local atomic displacements and depth-dependent polarization topology, opening new opportunities for investigating and engineering buried functional states in complex oxide nanostructures.

cond-mat.mtrl-sci↗

arXiv2026

Tilt-driven ferrielectricity in PbZrO$_3$

We reveal a tilt-driven mechanism for ferrielectricity in prototypical antiferroelectric PbZrO$_3$. Specifically, introducing an additional octahedral tilt into the antiferroelectric $Pbam$ phase breaks the symmetry constraint that enforces equal antiparallel dipoles, converting the compensated ``$\uparrow \uparrow \downarrow \downarrow$'' nonpolar configuration into an uncompensated ``$\uparrow \uparrow \downarrow \downarrow$'' polar $Pmc2_1$ phase. First-principles calculations show that the $Pmc2_1$ phase becomes stabilized under lattice contraction and gains increasing free-energy advantage over competing phases at finite temperatures. Atomic-scale imaging directly confirms the presence of $Pmc2_1$-like structures in thin films and single crystals. This work identifies a symmetry-governed, kinetically easily accessible pathway to ferrielectricity and establishes a form of ``competitive'' improper ferroelectricity, with broad implications for antiferroelectrics.

cond-mat.mtrl-sci↗

arXiv2026

Dynamic disentanglement of photoflexoelectricity and flexophotovoltage

The coupling between light and strain gradients shows two kinds of effects: light enhanced flexoelectricity (photoflexoelectricity) and gradient enhanced photovoltage (flexophotovoltage). Although these effects originate from fundamentally different physical mechanisms (one is light enhanced electromechanical coupling, the other is a bulk photovoltaic effect), in this article we show that dynamic flexoelectric measurements of semiconductors under illumination intrinsically contain contributions from both. To allow disentangling them, we have developed a general theoretical framework for their combined response in oscillating systems, demonstrating that the two contributions can be unambiguously separated through their distinct frequency and phase dependencies. We have validated these predictions using oscillating cantilever measurements on centrosymmetric perovskite semiconductors (SrTiO3 and methylammonium lead bromide, MAPbBr3), obtaining selfconsistent values for the coefficients both effects which are in excellent agreement with independent static measurements. Our results establish a general protocol for disentangling both light strain gradient couplings using only oscillatory measurements, and clarify the interpretation of flexoelectric measurements under illumination.

physics.app-ph↗

arXiv2026

Domain-induced control of latent heat in freestanding BaTiO$_3$ membranes

Thin ferroelectric BaTiO$_3$ films often exhibit continuous transitions instead of the first-order behavior of bulk crystals, a discrepancy usually attributed to epitaxial strain or dimensionality. Using quasi-adiabatic nanocalorimetry on freestanding BaTiO$_3$ membranes-free of clamping and substrate heat sinking-we show that domain morphology, not thickness or boundary conditions, controls the transition order. Thick membranes with large, monodomain-like regions display clear latent heat, whereas thinner membranes with dense 180$^{\circ}$ domain patterns show a continuous transition despite undergoing the same tetragonal-cubic structural change confirmed by x-ray diffraction. Piezoresponse force microscopy links this behavior to domain-size evolution, and a Ginzburg-Landau analysis demonstrates how reduced domain size lowers the free-energy barrier, rounding a nominally first-order instability. These results identify domain morphology as the key determinant of ferroelectric transition order in oxide membranes and establish design guidelines for enhancing caloric effects through domain engineering.

cond-mat.mtrl-sci↗

arXiv2025

Direct spatiotemporal imaging of carriers reveals a long-lived bulk photovoltaic mechanism

The bulk photovoltaic effect (BPVE), a manifestation of broken centrosymmetry, has attracted interest as a probe of the symmetry and quantum geometry of materials, and for use in photovoltaic and optoelectronic devices. However, so far the effect has not been captured directly in space and time. Here, we use contactless pump-probe microscopy to visualize the spatiotemporal evolution of photoexcited carriers in single-crystal, mono-domain $BiFeO_{3}$, a prototypical ferroelectric material. We observe asymmetric carrier transport along the polar axis, which confirms the intrinsic bulk- and symmetry-driven nature of the BPVE. Remarkably, this asymmetric transport persists for several nanoseconds after photoexcitation, which cannot be explained by conventional short-lived shift or phonon ballistic current BPVE mechanisms. Our Monte Carlo simulations show that asymmetric momentum scattering by defects, such as oxygen vacancies, leads to long-lived asymmetric carrier transport, as observed experimentally. Beyond fundamental insights, this paves the way towards controlling symmetry- and defect-driven photoresponses.

cond-mat.mtrl-sci↗

arXiv2025

Disorder-aided Early Warning Signals: Predicting Catastrophic Shifts in Athermal Systems

The early prediction of tipping points, distinguished by sudden and catastrophic shifts from stable states, poses a challenging task that would enable us to assess the impending threat across natural and engineered systems. This threat becomes particularly acute in low-fluctuation environments, where tipping occurs through saddle-node bifurcation without prior warning in noise dynamics. In this study, we investigate the tipping point dynamics of avalanche catastrophes in low-fluctuation domain, employing model system like the zero temperature random field Ising model and thermally deposited cobalt films. As the system approaches the tipping point, avalanche activity reveals pronounced critical behaviour, including critical slowing down, variance enhancement, and a growing spatial correlation length--hallmarks that may serve as early warning signals of impending collapse. Crucially, we demonstrate that increasing disorder in the system reduces its vulnerability to catastrophic failure. In highly disorder regimes, these early warning signals emerge well before the transition, thereby providing a large margin for anticipation and mitigation. This key finding suggests a protective role of disorder offering a novel perspective on resilience in complex systems. Our results not only deepen the understanding of tipping phenomena in disorder materials but also have broader implications for forecasting regime shift in diverse real-world systems.

cond-mat.stat-mech↗

arXiv2025

Polarization Vortices in a Ferromagnetic Metal via Twistronics

Recent advances in moire engineering provide new pathways for manipulating lattice distortions and electronic properties in low-dimensional materials. Here, we demonstrate that twisted stacking can induce dipolar vortices in metallic SrRuO3 membranes, despite the presence of free charges that would normally screen depolarizing fields and dipole-dipole interactions. These polarization vortices are correlated with moire-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 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 polarization topology design beyond dielectric materials and into metals.

cond-mat.mtrl-sci↗

arXiv2025

Flexoelectricity and surface ferroelectricity of water ice

The phase diagram of ice is complex and contains many phases, but the most common (frozen water at ambient pressure, also known as Ih ice) is a non-polar material despite individual water molecules being polar1,2. Consequently, ice is not piezoelectric and cannot generate electricity under pressure3. On the other hand, the coupling between polarization and strain gradient (flexoelectricity) is universal4, so ice may in theory generate electricity under bending. Here we report the experimental demonstration that ice is flexoelectric, finding a coefficient of 1.14+-0.13 nC/m, comparable to that of ceramics such as SrTiO3, TiO2, or PbZrO3. Additionally, and unexpectedly, the sensitivity of flexoelectric measurements to surface boundary conditions has also revealed a ferroelectric phase transition around ~160K confined in the near-surface region of the ice slabs. The electromechanical properties of ice may find applications for low-cost transducers made in-situ in cold and remote locations. Importantly, there are also consequences for natural phenomena. In particular, we have calculated the flexoelectric charge density generated in ice-graupel collisions, and found it to be comparable to the experimental charge transferred in such events, suggesting a possible participation of ice flexoelectricity in the charging up of thunderstorms.

cond-mat.mtrl-sci↗

arXiv2025

Ultrafast decoupling of polarization and strain in ferroelectric BaTiO$_3$

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 an above-bandgap laser excitation of the prototypical ferroelectric BaTiO$_3$, 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. This excited state could serve as a starting point to achieve stable and reversible polarization switching via THz light. Our results demonstrate a light-induced transient and reversible control of the ferroelectric polarization and offer a pathway to control by light both electric and magnetic degrees of freedom in multiferroics.

cond-mat.mtrl-sci↗

arXiv2025

Antiferroelectric switching inside ferroelastic domain walls

Ferroelastic materials (materials with switchable spontaneous strain) often are centrosymmetric, but their domain walls are always polar, as their internal strain gradients cause polarization via flexoelectricity. This polarization is generally not switchable by an external electric field, because reversing the domain wall polarity would require reversing the strain gradient, which in turn would require switching the spontaneous strain of the adjacent domains, destroying the domain wall in the process. However, domain wall polarization can also arise from biquadratic coupling between polar and non-polar order parameters (e.g. octahedral tilts in perovskites). Such coupling is independent of the sign of the polarization and thus allows switching between +P and -P. In this work, we seek to answer the question of whether the polarization of domain walls in ferroelastic perovskites is switchable, as per the symmetric biquadratic term, or non-switchable due to the unipolar flexoelectric bias. Using perovskite calcium titanate (CaTiO3) as a paradigm, molecular dynamics calculations indicate that high electric fields broaden the ferroelastic domain walls, thereby reducing flexoelectricity (as the domain wall strain gradient is inversely proportional to the wall width), eventually enabling switching. The polarization switching, however, is not ferroelectric-like with a simple hysteresis loop, but antiferroelectric-like with a double hysteresis loop. Ferroelastic domain walls thus behave as functional antiferroelectric elements, and also as nucleation points for a bulk phase transition to a polar state.

cond-mat.mtrl-sci↗

arXiv2025

Beware of the water: Hidden hydrogenation of perovskite membranes made by the water-soluble sacrificial layer method

The fabrication of perovskite oxide free-standing films (membranes) by lift-off methods using water-soluble sacrificial layers is appealing because of the new mechanical degrees of freedom that these membranes present over conventional epitaxial films. However, little is known about how their fabrication process, and in particular the exposure to water during the etching step, affects their properties. Here, we investigate how membranes of two perovskite archetypes, antiferroelectric PbZrO3 and paraelectric SrTiO3, are affected by water-based etching step. Using Raman spectroscopy and X-ray diffraction, we find evidence that hydrogen penetrates the perovskite structure. Concomitant with this protonation, the functional properties also change, and both materials display ferroelectric-like behavior that is absent in bulk ceramics or hydrogen-free films at room temperature. We also find that thermal annealing can be used to expel the hydrogen from the membranes, which henceforth recover bulk-like properties. The two main conclusions of this work are that (i) any perovskite membrane made by sacrificial layer hydrolysis is vulnerable to hydrogen penetration (protonation) that can induce important but extrinsic changes in functional properties, and (ii) the hydrogen can, and should, be expelled by annealing to recover intrinsic behaviour.

cond-mat.mtrl-sci↗

arXiv2025

Vortices and antivortices in antiferroelectric PbZrO3

Although ferroelectric materials are characterised by their parallel arrangement of electric dipoles, in the right boundary conditions these dipoles can reorganize themselves into vortices, antivortices and other non-trivial topological structures. By contrast, little is known about how (or whether) antiferroelectrics, which are materials showing an antiparallel arrangement of electric dipoles, can exhibit vortices or antivortices. In this study, using advanced aberration-corrected scanning transmission electron microscopy, we uncover the existence of atomic-scale (anti)vorticity in ferroelastic domain walls of the archetypal antiferroelectric phase of PbZrO3. The finding is supported, and its underlying physics is explained, using both second-principles simulations based on a deep-learning interatomic potential, and continuum field modelling. This discovery expands the field of chiral topologies into antiferroelectrics.

cond-mat.mtrl-sci↗

arXiv2024

Hierarchical domain structures in buckled ferroelectric free sheets

Flat elastic sheets tend to display wrinkles and folds. From pieces of clothing down to two-dimensional crystals, these corrugations appear in response to strain generated by sheet compression or stretching, thermal or mechanical mismatch with other elastic layers, or surface tension. Extensively studied in metals, polymers and, more recently, in van der Waals exfoliated layers, with the advent of thin single crystal freestanding films of complex oxides, researchers are now paying attention to novel microstructural effects induced by bending ferroelectric-ferroelastics, where polarization is strongly coupled to lattice deformation. Here we show that wrinkle undulations in BaTiO3 sheets bonded to a viscoelastic substrate transform into a buckle delamination geometry when transferred onto a rigid substrate. Using spatially resolved techniques at different scales (Raman, scanning probe and electron microscopy), we show how these delaminations in the free BaTiO3 sheets display a self-organization of ferroelastic domains along the buckle profile that strongly differs from the more studied sinusoidal wrinkle geometry. Moreover, we disclose the hierarchical distribution of a secondary set of domains induced by the misalignment of these folding structures from the preferred in-plane crystallographic orientations. Our results disclose the relevance of the morphology and orientation of buckling instabilities in ferroelectric free sheets, for the stabilization of different domain structures, pointing to new routes for domain engineering of ferroelectrics in flexible oxide sheets.

cond-mat.mtrl-sci↗

arXiv2024

Coexistence of ferroelectric and ferrielectric phases in ultrathin antiferroelectric PbZrO3 thin films

Whereas ferroelectricity may vanish in ultra-thin ferroelectric films, it is expected to emerge in ultra-thin anti-ferroelectric films, sparking people's interest in using antiferroelectric materials as an alternative to ferroelectric ones for high-density data storage applications. Lead Zirconate (PbZrO3) is considered the prototype material for antiferroelectricity, and indeed previous studies indicated that nanoscale PbZrO3 films exhibit ferroelectricity. The understanding of such phenomena from the microstructure aspect is crucial but still lacking. In this study, we fabricated a PbZrO3 film with thicknesses varying from 5 nm to 80 nm. Using Piezoresponse Force Microscopy, we discovered the film displayed a transition from antiferroelectric behaviour in the thicker areas to ferroelectric behaviour in the thinner ones, with a critical thickness between 10 and 15 nm. In this critical thickness range, a 12 nm PZO thin film was chosen for further study using aberration-corrected scanning transmission electron microscopy. The investigation showed that the film comprises both ferroelectric and ferrielectric phases. The ferroelectric phase is characterized by polarisation along the pseudocubic [011] projection direction. The positions of Pb, Zr, and O were determined using the integrated differential phase contrast method. This allowed us to ascertain that the ferroelectric PbZrO3 unit cell is half the size of that in the antiferroelectric phase on the ab plane. The observed unit cell is different from the electric field-induced ferroelectric rhombohedral phases. Additionally, we identified a ferrielectric phase with a unique up-up-zero-zero dipole configuration. The finding is crucial for understanding the performance of ultrathin antiferroelectric thin films and the subsequent design and development of antiferroelectric devices.

cond-mat.mtrl-sci↗

arXiv2023

Switching dynamics and improved efficiency of free-standing antiferroelectric capacitors

We report the switching dynamics of antiferroelectric Lead Zirconate (PbZrO3) free standing capacitors compared to their epitaxial counterparts. Frequency dependence of hysteresis indicates that freestanding capacitors exhibit a lower dispersion of switching fields, lower residual polarization, and faster switching response as compared to epitaxially-clamped capacitors. As a consequence, freestanding capacitor membranes exhibit better energy storage density and efficiency.

cond-mat.mtrl-sci↗

arXiv2023

Non-destructive tomographic nanoscale imaging of ferroelectric domain walls

Extraordinary physical properties arise at polar interfaces in oxide materials, including the emergence of two-dimensional electron gases, sheet-superconductivity, and multiferroicity. A special type of polar interface are ferroelectric domain walls, where electronic reconstruction phenomena can be driven by bound charges. Great progress has been achieved in the characterization of such domain walls and, over the last decade, their potential for next-generation nanotechnology has become clear. Established tomography techniques, however, are either destructive or offer insufficient spatial resolution, creating a pressing demand for 3D imaging compatible with future fabrication processes. Here, we demonstrate non-destructive tomographic imaging of ferroelectric domain walls using secondary electrons. Utilizing conventional scanning electron microscopy (SEM), we reconstruct the position, orientation, and charge state of hidden domain walls at distances up to several hundreds of nanometers away from the surface. A mathematical model is derived that links the SEM intensity variations at the surface to the local domain wall properties, enabling non-destructive tomography with good noise tolerance on the timescale of seconds. Our SEM-based approach facilitates high-throughput screening of materials with functional domain walls and domain-wall-based devices, which is essential for monitoring during the production of device architectures and quality control in real-time.

cond-mat.mtrl-sci↗

arXiv2023

Probing the Surface Polarization of Ferroelectric Thin Films by X-ray Standing Waves

Understanding the mechanisms underlying a stable polarization at the surface of ferroelectric thin films is of particular importance both from a fundamental point of view and to achieve control of the surface polarization itself. In this study, it is demonstrated that the X-ray standing wave technique allows the polarization near the surface of a ferroelectric thin film to be probed directly. The X-ray standing wave technique is employed to determine, with picometer accuracy, Ti and Ba atomic positions near the surface of three differently strained $\mathrm{BaTiO_3}$ thin films grown on scandate substrates, with a $\mathrm{SrRuO_3}$ film as bottom electrode. This technique gives direct access to atomic positions, and thus to the local ferroelectric polarization, within the first 3 unit cells below the surface. By employing X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing species adsorbed on the surface, upon exposure to ambient conditions, is obtained. The combination of structural and spectroscopic information allows us to conclude on the most plausible mechanisms that stabilize the surface polarization in the three samples under study. The different amplitude and orientation of the local ferroelectric polarizations are associated with surface charges attributed to the type, amount and spatial distribution of the oxygen-containing adsorbates.

cond-mat.mtrl-sci↗

arXiv2023

Photostrictive actuators based on freestanding ferroelectric membranes

Complex oxides offer a wide range of functional properties, and recent advances in fabrication of freestanding membranes of these oxides are adding new mechanical degrees of freedom to this already rich functional ecosystem. Here, we demonstrate photoactuation in freestanding thin film resonators of ferroelectric Barium Titanate (BaTiO3) and paraelectric Strontium Titanate (SrTiO3). The free-standing films, transferred onto perforated supports, act as nano-drums, oscillating at their natural resonance frequency when illuminated by a frequency-modulated laser. The light-induced deflections in the ferroelectric BaTiO3 membranes are two orders of magnitude larger than in the paraelectric SrTiO3 ones. Time-resolved X-ray micro-diffraction under illumination and temperature-dependent and holographic interferometry provide combined evidence for the photostrictive strain in BaTiO3 originating from partial screening of ferroelectric polarization by photo-excited carriers, which decreases the tetragonality of the unit cell. These findings showcase the potential of photostrictive freestanding ferroelectric films as wireless actuators operated by light.

cond-mat.mtrl-sci↗