Marin Alexe

There is rising interest in two-dimensional (2D) ferroelectrics, particularly as functional components in 2D heterostructures using their reconfigurable polarisation to create low-power memory devices. Understanding and optimising their application will require investigations that can unravel, at the nanoscale, the effect of ferroelectric switching on the local electronic structure. Using contact mode scanning tunnelling spectroscopy (CM-STS), we observe polarisation-dependent band-edge shifts in an all 2D heterostructure that combines the ferroelectric CuInP ₂ S ₆ (CIPS) with the semiconductor WSe ₂ . We employ scanning probe microscopy to characterise the polarisation and conductivity in the CIPS layer, and correlate these measurements with CM-STS spectra, revealing changes from p-type to n-type doping in the WSe ₂ layer as the out-of-plane polarisation of the underlying CIPS is switched by the external electric field. The data demonstrate the importance of considering the channel and contact resistances, along with tip-induced-band-bending, when using the tunnelling current to probe the local density of states (LDOS) in 2D semiconductor heterostructures. The significant band-edge shift of 0.8 eV demonstrates efficient coupling of the semiconductor to the polarisation field in the ferroelectric, consistent with clean interfaces in the heterostructure without dead layers. The results show that probing the LDOS by CM-STS gives valuable insight into 2D ferroelectric based electronics and enables direct correlation between electronic structure and functional properties.

Fast pixelated detectors in scanning transmission electron microscopy (STEM) enable acquisition of a two-dimensional diffraction pattern at every probe position, known as four-dimensional STEM (4D-STEM). In 4D-STEM, each measured intensity has dual character, forming a pixel in diffraction space, and equally a pixel in real space. Applying binary masks in diffraction space is often used to produce ‘virtual’ bright-field or annular-dark field images. Here we present a complementary method for atomic-resolution 4D-STEM, using correlation between real-space images (templates) and the data to create weighted masks in diffraction space. These weighted masks provide significant improvement over binary masks, and can produce images specific to different types of atom columns. We demonstrate this approach by obtaining separate high contrast images of Li and O columns in LiFePO ₄ and O columns in PbTiO ₃ . This method provides a computationally straightforward route to probe 4D-STEM data, and is particularly effective for specimens of moderate thickness where multiple scattering produces strong correlations in diffraction patterns.

Transient negative capacitance observed during ferroelectric polarization switching is often interpreted as an intrinsic material property, yet our study shows it can arise as a circuit artifact. We systematically varied the configuration of the measurement circuit used in fast voltage‐pulse experiments to characterise the ferroelectric polarization switching of epitaxial high‐quality thin‐film capacitors. We conclude that even modest resistances, such as the 50 Ω source impedance of a standard pulse generator, serially connected with the ferroelectric capacitor, significantly produced the characteristic voltage “dip” associated with negative capacitance, reducing the effective electric field across the ferroelectric and altering the polarization switching dynamics. Because the applied field is no longer constant, most of the theoretical models, such as conventional Kolmogorov–Avrami–Ishibashi or Landau–Khalatnikov, used to describe the ferroelectric switching, become invalid in the interpretation of the polarization switching in classical PUND (Positive‐Up‐Negative‐Down) measurements. To reveal intrinsic switching dynamics, we implemented a low‐impedance buffer amplifier. This buffered circuit configuration maintained a nearly constant field, eliminating the transient negative‐capacitance artifact, and revealed faster, higher‐current switching. We establish quantitative criteria for circuit design and device geometry to ensure accurate analysis of ferroelectric switching dynamics.

Probing novel properties, arising from twisted interfaces, has traditionally relied on the stacking of exfoliated two-dimensional materials and the spontaneous formation of van der Waals bonds. So far, investigations involving intimate covalent or ionic bonds have not been a focus. Yet, we show here that an established technique, involving thermocompressional wafer bonding, works well for creating twisted non-van der Waals interfaces. We have successfully bonded z-cut lithium niobate single crystals to create ferroelectric oxide interfaces with strong polar discontinuities and have mapped the associated emergent interfacial conductivity. In some instances, a dramatic change in microstructure occurs, involving local dipolar switching. A twist-induced collapse in the capability of the system to effec8tively screen interfacial bound charge is implied. Importantly, this only occurs around specific moiré twist angles with sparse coincident lattices and associated short-range aperiodicity. In quasicrystals, aperiodicity is known to induce pseudo-bandgaps and we suspect a similar phenomenon here.

Ferroelastic phase transitions lead to the formation of ferroelastic twins, resulting in spatial inhomogeneities in the crystal structures and functional properties of ferroelastic materials. Despite the importance of such nanoscale twins in oxide heterostructures, their direct, non‐invasive observation has relied on cumbersome and low‐throughput techniques such as transmission electron microscopy and synchrotron X‐ray diffraction, or been restricted to materials with coexistence of ferroelectric and ferroelastic states, by detecting the coupled ferroelectric order. In this study, the application of electron channeling contrast imaging (ECCI) in a scanning electron microscope for imaging ferroelastic domains in various oxide heterostructures is demonstrated. These include systems where ferroelasticity is coupled with critical functional properties such as ferroelectricity, magnetism and charge transport. The versatility of this imaging method is highlighted across different heterostructure geometries, from bare thin films to multilayers, achieving an impressive resolution of 6 nm. ECCI presents a powerful approach for exploring the rich spectrum of ferroelasticity‐coupled phenomena in oxide heterostructures.

We demonstrate that switching of ferroelectric thin films sandwiched between metallic electrodes can be controlled by laser-assisted heating, reminiscent of heat-assisted magnetic recording. We employ electrical switching cycles that quantify the electrically switchable remanent polarization Pr and show that 300 ns voltage pulses alone change the polarization by less than ΔPPr of the average polarization. The transient average temperature modeled by the heat equation can rationalize the polarization change observed for different relative timings Δt of the laser pulse if it arrives before the electrical pulse.

Domain structures play a crucial role in the electric, mechanical, and other properties of ferroelectric materials. In this study, we uncover the physical origins of the enigmatic zigzag domain structure in the prototypical multiferroic material BiFeO₃. Using phase-field simulations within the Landau-Ginzburg-Devonshire framework, we demonstrate that spatially-homogeneous defect charges result in domain structures that closely resemble those observed experimentally. The acquired understanding of the underlying physics of pyramidal-domain formation may enable the engineering of new materials with self-assembled domain structures exhibiting defined domain periodicity at the nanometer scale, opening avenues for advanced applications.

Crystals often have complex structural domains, but a general method to remove or deterministically control such local heterogeneity is lacking. The resulting heterogeneity in crystal orientations obscures our understanding of material properties and can reduce the reliability and performance of related applications. Here, using shear stress from an atomic force microscope tip, we ferroelastically write local crystal orientations in oxide thin films. Applying this deterministic and reversible control to SrRuO₃ and (La_0.7Sr_0.3)(Mn_0.9Ru_0.1)O₃ films, we realize twin-free single crystals and design specific crystal-orientation domain textures at the nanoscale. Furthermore, through magnetoelastic coupling, we can mechanically manipulate the local magnetic anisotropy, and thereby write and erase functional nanoscale magnetic textures unattainable by conventional methods. Thus, pure mechanical force emerges as a means to control structural heterogeneity on demand and may make it possible to program electronic and spintronic functionalities.