Recent advances in ferroelectrics include the discovery of complex polarization textures such as non-Ising and chiral domain walls, bubble domains, labyrinth patterns, vortices, or polar skyrmions. New developments in this field focus on low-dimensional polar topological structures with unknown physical properties, and the first experimental methods with the potential to measure the associated local symmetry violations are emerging. A major challenge is that such topological structures develop on the sub-10 nm scale and only within the volume of the material. Therefore, their study, in terms of structure and functional properties, requires both high lateral resolution and access to the third dimension, allowing nanoscale studies of the sample volume. In this context, two-photon microscopy based on second-harmonic generation (SHG) microscopy with polarimetry analysis has emerged as a suitable method to probe the local symmetry and complex polarization textures.

In the presented work, we combine machine learning-based techniques with 3D SHG polarimetry to achieve a fast analysis of complex polarization textures and beat the resolution limit of the SHG optical microscopy method. The merits of K-means clustering and non-negative matrix factorization are demonstrated on 3D SHG polarimetry datasets measured in uniaxial ferroelectric crystals and multiaxial ferroelectric films. The results obtained not only show similarity, but offer significant improvements over the previously published manual analysis, allowing us to find additional behaviors revealed by machine learning that may have been overlooked or discarded as noise in the manual analysis. Finally, we extend the analysis to trained algorithms using high-quality experimental datasets to study sub-resolution polar nanostructures in ferroelectric films. The approach developed in this study foreshows new prospects for nonlinear optical microscopy studies by enabling enhanced sensitivity and high throughput analysis. It can be applied not only to the study of functional materials, such as ferroelectrics, but also to, e.g., the automatic detection of local spectroscopic signatures related to diseased cells in biomedical science.

Effectively managing heat flows remains a contemporary challenge crucial for enhancing the energy efficiency of household appliances. Precise control is vital to prevent electronic circuits from overheating and to optimize energy harvesting systems. Recent experimental results highlight ferroelectric and ferroelastic oxides as promising solutions for achieving dynamic control of heat flows across a wide temperature range, with significant variations in thermal conductivity. Ferroelectric and ferroelastic oxides spontaneously form domains, with boundaries known as domain walls, where slight variations in atomic positions occur. These walls play a direct role in influencing the propagation of heat-conducting phonons. Increased domain density, and subsequently domain wall density, leads to more collisions between phonons and domain walls, resulting in lower thermal conductivity. In addition, the orientation of the polarization with respect to the direction of the heat flow matters. In this study, we present a combined measurement of local thermal conductivity using scanning thermal microscopy and frequency domain thermoreflectance on a ferroelectric single crystal. Furthermore, we conducted thermal conductivity measurements at various temperatures for ferroelectric material-based ceramics with different grain sizes, revealing reversible changes in thermal conductivity under the in-situ application of an electric field. We discuss our results based on changes in polarization and domain structures induced by the electric field.

In semiconductor technology, conductivity is traditionally controlled in one of two ways: acceptor-donor doping, which generates additional charge carriers, or by employing an electric field to build up regions with excess mobile charge carriers to create conducting pathways, as exemplified in transistor architectures. Directly analogous principles are observable at the nanoscale in domain walls: pseudo-2D entities that form at the interface between two symmetrically equivalent regions of a crystal. The observation of enhanced currents at domain walls, typically in the tens of picoamps, is well-documented and generally attributed to the buildup of mobile charge carriers to screen polar discontinuities or the accumulation of ionic defects.

In this work, we highlight a fundamentally distinct approach to controlling conductivity in insulating materials. Utilizing the ferroelectric Mott insulators GaV4S8, we demonstrate a drastic increase in conductivity (reaching hundreds of nanoamps) at structural domain walls, coinciding with significant surface reconstructions. Synchrotron measurements show that such twin walls can harbor significant strain gradients, reducing the interatomic distances. This compaction is expected to increase orbital overlap in Mott insulators, significantly enhancing electron mobility and, consequently, conductivity. To underscore the universality of our findings, we induced mechanical deformations in our specimens, generating local strain gradients without the presence of domain walls. These regions exhibit increased conductivity, as expected, and, unlike domain walls, have no symmetry constraints on their locations. Our results illustrate a novel approach for the local tuning of conductivity, which is not solely limited to domain walls. Similar results are expected in any material whose electronic structure is sensitive to local strain gradients, such as Mott insulators, opening new avenues for the tailored engineering of material properties and offering broader applicability in the design of electronic devices.

Electric field-induced strain is one of the most well-known characteristics of ferroelectric materials which has widely been used for electromechanical applications. The intrinsic strain emerges from the electrostriction and piezoelectric effects of the materials while the extrinsic strain is mainly attributed to the domain wall movement in the polydomain materials. At a larger scale comprising several domain walls, the resultant of the extrinsic strains may result in macroscopic strain evolution. It has been observed that the electrostriction effect results in an elongation along the field direction, thus introducing microwave field assisted multifunctionality for utilisation in self-tuning composites (Li et al. Sci Reports, 2022: 7504. DOI: 10.1038/s41598-022-11380-9).
The current study has introduced functionalised ferroelectric barium titanate nanoparticles (BaTiO3) as a second phase dispersed into a high-performance grade epoxy matrix. A diglycidyl ether of bisphenol A epoxy with the hardener Aradur 3487 were modified with the BaTiO3 nanoparticles embedment. The silane coupling agent for the nanoparticles’ surface functionalisation was 3 glycidoxypropyl trimethoxysilane (3 GPS) that enables a uniform dispersion and improved interfacial filler/matrix bonding quality. Ultrasonication and solvent-aided mixing (ethanol, C2H6O, 99.9%) were employed to facilitate the dispersion of BaTiO3 nanoparticles having tetragonal crystal microstructure. The study has assessed the mechanical, thermoanalytical, morphological, rheological, electrical and dielectric properties of BaTiO3 modified epoxy at a lower weight loading up to 15 wt.% to achieve the scientific character of the microstructure of the functionalised BaTiO3-epoxy composite in the presence of BaTiO3 particles via X-ray diffraction, thermoanalytical analysis, electron microscopy, permeability, conductivity and permittivity analysis, Raman spectroscopy, atomic force microscopy, and rheological analysis.
The research was conducted for the quantification and the state of reversibility of the mechanical strains induced by microwave field in the composites. The microwave field excitation of functionalised ferroelectric BaTiO3 nanoparticles dispersed in epoxy is shown to introduce significant extrinsic strain into the epoxy as of result of microscopic domain wall movements due to the field induced polarisation. The non-linear evolution of field-induced strain and temperature with microwave power level and exposure time have been quantified using optical fibre Bragg grating sensors, and localised instantaneous reduction in strains has been observed.