arXiv subjects

Gustau Catalan

Publications and source records attributed to Gustau Catalan.

At least 37 records · Page 2Linked to original sources

arXiv2023

Flexo-photovoltaic effect and above-bandgap photovoltage in halide perovskites

Halide perovskites have outstanding photovoltaic properties which have been optimized through interfacial engineering. However, as these materials approach the limits imposed by the physics of semiconductor junctions, it is urgent to explore alternatives, such as the bulk photovoltaic effect, whose physical origin is different and not bound by the same limits. In this context, we focus on the flexo-photovoltaic effect, a type of bulk photovoltaic effect that was recently observed in oxides under strain gradients. We have measured the flexo-photovoltaic effect of MAPbBr3 and MAPbI3 crystals under bending and found it to be orders of magnitude larger than for SrTiO3, the benchmark flexo-photovoltaic oxide. For sufficiently large strain gradients, photovoltages bigger than the bandgap can be produced. Bulk photovoltaic effects are additive and, for MAPbI3, the flexo-photovoltage exists on top of a native bulk photovoltage that is hysteretic, consistent with the electrically switchable macroscopic polarization of this material. The results suggest that harnessing the flexo-photovoltaic effect through strain gradient engineering can provide a functional leap forward for halide perovskites.

cond-mat.mtrl-sci↗

arXiv2022

Translational boundaries as incipient ferrielectric domains in antiferroelectric PbZrO3

In the archetypal antiferroelectric PbZrO3, antiparallel electric dipoles cancel each other, resulting in zero spontaneous polarisation at the macroscopic level. Yet in actual hysteresis loops, the cancellation is rarely perfect and some remnant polarization is often observed, suggesting the metastability of polar phases in this material. In this work, using aberration-corrected scanning transmission electron microscopy methods on a PbZrO3 single crystal, we uncover the coexistence of the common antiferroelectric phase and a ferrielectric phase featuring an electric dipole pattern of "up down up". This dipole arrangement, predicted by Aramberri et al. (2021) to be the ground state of PbZrO3 at 0K, appears at room temperature in the form of translational boundaries that aggregate to form wider stripe domains of the polar phase embedded within the antiferroelectric matrix.

cond-mat.mtrl-sci↗

arXiv2022

Switchable tribology of ferroelectrics

Artificially induced asymmetric tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe where down domains having lower friction coefficient than up domains can be used as smart masks as they show slower wear rate than up domains. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribological asymmetry is universal across ferroelectrics with different chemical composition and crystalline symmetry. Finally, using this switchable tribology and multi-pass patterning with a domain-based dynamic smart mask, we demonstrate three-dimensional nanostructuring exploiting the asymmetric wear rates of up and down domains, which can, furthermore, be scaled up to technologically relevant (mm-cm) size. These findings establish that ferroelectrics are electrically tunable tribological materials at the nanoscale for versatile applications.

physics.app-ph↗

arXiv2021

Piezoelectricity in Nominally Centrosymmetric Phases

Compound phases often display properties that are symmetry-forbidden relative to their nominal, average crystallographic symmetry, even if extrinsic reasons (defects, strain, imperfections) are not apparent. Here, we investigate macroscopic inversion symmetry breaking in nominally centrosymmetric materials and measure Resonant Piezoelectric Spectroscopy (RPS) and Resonant Ultrasound Spectroscopy (RUS) in 15 compounds, 18 samples, and 21 different phases, including unpoled ferroelectrics, paraelectrics, relaxors, ferroelastics, incipient ferroelectrics, and isotropic materials with low defect concentrations, i.e. NaCl,cfused silica, and CaF2. We exclude the flexoelectric effect as a source of the observed piezoelectricity yetcobserve piezoelectricity in all nominally cubic phases of these samples. By scaling the RPS intensities with those of RUS, we calibrate the effective piezoelectric coefficients using single crystal quartz as standard. Using this scaling we determine the effective piezoelectric modulus in nominally non-piezoelectric phases, finding that the "symmetry-forbidden" piezoelectric effect ranges from 1 pm/V to 10E-5 pm/V. The values for unpoled ferroelectric phases are only slightly higher than those in the paraelectric phase of the same material. The lowest coefficients are well below the detection limit of conventional piezoelectric measurements and demonstrate RPS as an ultra-highly sensitive method to measure piezoelectricity. We suggestt hat symmetry-breaking piezoelectricity in nominally centrosymmetric materials and disordered, unpoled ferroelectrics is ubiquitous.

cond-mat.mtrl-sci↗

arXiv2021

Deconvolution of phonon scattering by ferroelectric domain walls and point defects in a PbTiO3 thin film deposited in a composition-spread geometry

We present a detailed analysis of the temperature dependence of the thermal conductivity of a ferroelectric PbTiO3 thin film deposited in a composition-spread geometry enabling a continuous range of compositions from ~25% titanium-deficient to ~20% titanium-rich to be studied. By fitting the experimental results to the Debye model we deconvolve and quantify the two main phonon scattering sources in the system: ferroelectric domain walls (DWs) and point defects. Our results prove that ferroelectric DWs are the main agent limiting the thermal conductivity in this system, not only in the stoichiometric region of the thin film ([Pb]/[Ti]~1), but also when the concentration of cation point defects is significant (up to ~15%). Hence, DWs in ferroelectric materials are a source of phonon scattering at least as effective as point defects. Our results demonstrate the viability and effectiveness of using reconfigurable DWs to control the thermal conductivity in solid-state devices.

cond-mat.mtrl-sci↗

arXiv2021

Direct visualization of antiferroelectric switching dynamics via electrocaloric imaging

The large electrocaloric coupling in PbZrO3 allows using high-speed infrared imaging to visualize antiferroelectric switching dynamics via the associated temperature change. We find that in ceramic samples of homogeneous temperature and thickness, switching is nucleation-limited and fast, with devices responding in the milisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics from nucleation-limited to propagation-limited, whereby a single phase boundary sweeps across the sample like a cold front, at a speed of c.a. 20 cm/s. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response.

cond-mat.mtrl-sci↗

arXiv2021

Origin of the Large Negative Electrocaloric Effect in Antiferroelectric PbZrO3

We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electro-cooling of -3.5 K to an electro-heating of +5.5 K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics.

cond-mat.mtrl-sci↗

arXiv2021

Metallic Diluted Dimerization in VO2 Tweeds

The observation of electronic phase separation textures in vanadium dioxide (VO2), a prototypical electron-correlated oxide, has recently added new perspectives on the long standing debate about its metal-insulator transition and its applications. Yet, the lack of atomically-resolved information on phases accompanying such complex patterns still hinders a comprehensive understanding of the transition and its implementation in practical devices. In this work, atomic resolution imaging and spectroscopy unveils the existence of ferroelastic tweed structures on ~5 nm length scales, well below the resolution limit of currently employed spectroscopic imaging techniques. Moreover, density functional theory calculations show that such fine tweeds are formed by a metallic structure, formed by partially dimerized V-chains, that appears tetragonal (rutile) on average, but is locally monoclinic. These observations suggest that the metal/insulator coexistence among low-symmetry forms of VO2 is driven by lattice degrees of freedom, and provide a multiscale perspective for the interpretation of existing data, whereby phase coexistence and structural intermixing can occur all the way down to the atomic scale.

cond-mat.mtrl-sci↗

arXiv2020

Ferroelectric 180 degree walls are mechanically softer than the domains they separate

Domain walls are functionally different from the domains they separate, but little is known about their mechanical properties. Using scanning probe microscopy, we have measured the mechanical response of ferroelectric 180o domain walls and observed that, despite separating domains that are mechanically identical (non-ferroelastic), the walls are mechanically distinct -- softer -- compared to the domains. This effect has been observed in different ferroelectric materials (LiNbO3, BaTiO3, PbTiO3) and with different morphologies (from single crystals to thin films) so it appears to be universal. We propose a theoretical framework that explains the domain wall softening and justifies that the effect should be common to all ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2020

Investigation of the cellular response to bone fractures: evidence for flexoelectricity

The recent discovery of bone flexoelectricity (electrical polarization induced by strain gradient) suggests that flexoelectricity could have physiological effects in bones, and specifically near bone fractures, where flexoelectricity is theoretically highest. Here, we report a cytological study of the interaction between crack stress and bone cells. We have cultured MC3T3-E1 mouse osteoblastic cells in biomimetic microcracked hydroxyapatite substrates, differentiated into osteocytes and applied a strain gradient to the samples. The results show a strong apoptotic cellular response, whereby mechanical stimulation causes those cells near the crack to die, as indicated by live-dead and caspase staining. In addition, analysis two weeks after stimulation shows increased cell attachment and mineralization around microcracks and a higher expression of osteocalcin, an osteogenic protein known to be promoted by physical exercise. The results are consistent with flexoelectricity playing at least two different roles in bone remodelling: apoptotic trigger of the repair protocol, and electrostimulant of the bone-building activity of osteoblasts.

q-bio.TO↗

arXiv2019

Periodicity doubling cascades: direct observation in ferroelastic materials

Very sensitive responses to external forces are found near phase transitions. However, phase transition dynamics and pre-equilibrium phenomena are difficult to detect and control. We have directly observed that the equilibrium domain structure following a phase transition in BaTiO3, a ferroelectric and ferroelastic material, is attained by halving of the domain periodicity, sequentially and multiple times. The process is reversible, displaying periodicity doubling as temperature is increased. This observation is backed theoretically and can explain the fingerprints of domain period multiplicity observed in other systems, strongly suggesting this as a general model for pattern formation during phase transitions in ferroelastic materials.

cond-mat.mtrl-sci↗

arXiv2018

Flexoelectric fracture-filter effect in ferroelectrics

The propagation front of a crack generates large strain gradients and it is therefore a strong source of gradient-induced polarization (flexoelectricity). Herein, we demonstrate that, in piezoelectric materials, a consequence of flexoelectricity is that crack propagation will be helped or hindered depending on whether it is parallel or antiparallel to the piezoelectric polar axis. This means that the theory of fracture physics can no longer assume mechanical symmetry in polar materials. The discovery of fracture asymmetry also has practical repercussions for the electromechanical fatigue of ferroelectrics and piezoelectric transducers, as well enabling a new degree of freedom for crack-based nanopatterning.

cond-mat.mtrl-sci↗

arXiv2018

Piezoelectric mimicry of flexoelectricity

The origin of "giant" flexoelectricity, orders of magnitude larger than theoretically predicted, yet frequently observed, is under intense scrutiny. There is mounting evidence correlating giant flexoelectric-like effects with parasitic piezoelectricity, but it is not clear how piezoelectricity (polarization generated by strain) manages to imitate flexoelectricity (polarization generated by strain gradient) in typical beam-bending experiments, since in a bent beam the net strain is zero. In addition, and contrary to flexoelectricity, piezoelectricity changes sign under space inversion, and this criterion should be able to distinguish the two effects and yet "giant" flexoelectricity is insensitive to space inversion, seemingly contradicting a piezoelectric origin. Here we show that, if a piezoelectric material has its piezoelectric coefficient be asymmetrically distributed across the sample, it will generate a bending-induced polarization impossible to distinguish from true flexoelectricity even by inverting the sample. The effective flexoelectric coefficient caused by piezoelectricity is functionally identical to, and often larger than, intrinsic flexoelectricity: the calculations show that, for standard perovskite ferroelectrics, even a tiny gradient of piezoelectricity (1% variation of piezoelectric coefficient across 1 mm) is sufficient to yield a giant effective flexoelectric coefficient of 1 $μ$C/m, three orders of magnitude larger than the intrinsic expectation value.

cond-mat.mtrl-sci↗

arXiv2018

Flexoelectricity in antiferroelectric ceramics

Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but it is only relatively recently that it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. This is the case, for example, of antiferroelectrics: materials with an antipolar but switchable arrangement of dipoles. And yet, these materials could be flexoelectrically relevant because it has been hypothesised that the origin of their antiferroelectricity might be flexoelectric. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO3 and AgNbO3) as a function of temperature, up to and beyond their Curie temperature. Neither flexoelectricity nor the flexocoupling coefficients are anomalously high, but the flexocoupling shows a sharp peak at the antiferroelectric phase transition.

cond-mat.mtrl-sci↗

arXiv2017

On the persistence of polar domains in ultrathin ferroelectric capacitors

The instability of ferroelectric ordering in ultra-thin films is one of the most important fundamental issues pertaining realization of a number of electronic devices with enhanced functionality, such as ferroelectric and multiferroic tunnel junctions or ferroelectric field effect transistors. In this paper, we investigate the polarization state of archetypal ultrathin (several nanometres) ferroelectric heterostructures: epitaxial single-crystalline BaTiO$_3$ films sandwiched between the most habitual perovskite electrodes, SrRuO$_3$, on top of the most used perovskite substrate, SrTiO$_3$. We use a combination of piezoresponse force microscopy, dielectric measurements and structural characterization to provide conclusive evidence for the ferroelectric nature of the relaxed polarization state in ultrathin BaTiO$_3$ capacitors. We show that even the high screening efficiency of SrRuO$_3$ electrodes is still insufficient to stabilize polarization in SrRuO$_3$/BaTiO$_3$/SrRuO$_3$ heterostructures at room temperature. We identify the key role of domain wall motion in determining the macroscopic electrical properties of ultrathin capacitors and discuss their dielectric response in the light of the recent interest in negative capacitance behaviour.

cond-mat.mtrl-sci↗

arXiv2015

Giant reversible nanoscale piezoresistance at room temperature in Sr2IrO4 thin films

Layered iridates have been the subject of intense scrutiny on account of their unusually strong spin-orbit coupling, which opens up a narrow gap in a material that would otherwise be a metal. This insulating state is very sensitive to external perturbations. Here, we show that vertical compression at the nanoscale, delivered using the tip of a standard scanning probe microscope, is capable of inducing a five orders of magnitude change in the room temperature resistivity of Sr2IrO4. The extreme sensitivity of the electronic structure to anisotropic deformations opens up a new angle of interest on this material, and the giant and fully reversible perpendicular piezoresistance makes iridates a promising material for room temperature piezotronic devices.

cond-mat.mtrl-sci↗

arXiv2013

Origin Of the enhanced flexoelectricity of relaxor ferroelectrics

We have measured the bending-induced polarization of Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals with compositions at the relaxor-ferroelectric phase boundary. The crystals display very large flexoelectricity, with flexocoupling coefficients an order of magnitude bigger than the theoretical upper limit set by the theories of Kogan and Tagantsev. This enhancement persists in the paraphrase up to a temperature T* that coincides with the start of elastic softening in the crystals. Analysis of the temperature dependence and cross-correlation between flexoelectric, dielectric and elastic properties indicates that the large bendinginduced polarization of relaxor ferroelectrics is not caused by intrinsically giant flexoelectricity, but by the reorientation of polar nanotwins that become ferroelastically active below T*.

cond-mat.mtrl-sci↗

arXiv2013

On the link between Octahedral Rotations and Conductivity in the Domain Walls of BiFeO3

We analyze the hypothetical link between octahedral straightening and increased conductivity inside the domain walls of BiFeO3. Our calculations for 109 degree walls predict a lattice parameter expansion of c.a. 1 percent in the direction perpendicular to the wall, and an associated straightening of the octahedral rotation angle of 4 degrees, which is comparable to that observed in the high temperature metallic phase of BiFeO3. On the other hand, in the closely related family of rare-earth orthoferrites, straighter octahedra do not correlate with increased bandgap, which suggests that the correlation between octahedral straightening and bandgap reduction in BiFeO3 is perhaps fortuitous and not necessarily the cause of increased conductivity at the walls.

cond-mat.mtrl-sci↗