arXiv subjects

A. T. J. van Helvoort

Publications and source records attributed to A. T. J. van Helvoort.

5 recordsLinked to original sources

arXiv2024

Ferroelectric domain walls for environmental sensors

Domain walls in ferroelectric oxides provide fertile ground for the development of next-generation nanotechnology. Examples include domain-wall-based memory, memristors, and diodes, where the unusual electronic properties and the quasi-2D nature of the walls are leveraged to emulate the behavior of electronic components at ultra-small length scales. Here, we demonstrate atmosphere-related reversible changes in the electronic conduction at neutral ferroelectric domain walls in Er(Mn,Ti)O$_3$. By exposing the system to reducing and oxidizing conditions, we drive the domain walls from insulating to conducting, and vice versa, translating the environmental changes into current signals. Density functional theory calculations show that the effect is predominately caused by charge carrier density modulations, which arise as oxygen interstitials accumulate at the domain walls. The work introduces an innovative concept for domain-wall based environmental sensors, giving an additional dimension to the field of domain wall nanoelectronics and sensor technology in general.

cond-mat.mtrl-sci↗

arXiv2023

3D oxygen vacancy order and defect-property relations in multiferroic (LuFeO$_3$)$_9$/(LuFe$_2$O$_4$)$_1$ superlattices

Oxide heterostructures exhibit a vast variety of unique physical properties. Examples are unconventional superconductivity in layered nickelates and topological polar order in (PbTiO$_3$)$_n$/(SrTiO$_3$)$_n$ superlattices. Although it is clear that variations in oxygen content are crucial for the electronic correlation phenomena in oxides, it remains a major challenge to quantify their impact. Here, we measure the chemical composition in multiferroic (LuFeO$_3$)$_9$/(LuFe$_2$O$_4$)$_1$ superlattices, revealing a one-to-one correlation between the distribution of oxygen vacancies and the electric and magnetic properties. Using atom probe tomography, we observe oxygen vacancies arranging in a layered three-dimensional structure with a local density on the order of 10$^{14}$ cm$^{-2}$, congruent with the formula-unit-thick ferrimagnetic LuFe$_2$O$_4$ layers. The vacancy order is promoted by the locally reduced formation energy and plays a key role in stabilizing the ferroelectric domains and ferrimagnetism in the LuFeO$_3$ and LuFe$_2$O$_4$ layers, respectively. The results demonstrate the importance of oxygen vacancies for the room-temperature multiferroicity in this system and establish an approach for quantifying the oxygen defects with atomic-scale precision in 3D, giving new opportunities for deterministic defect-enabled property control in oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2023

Quantitative 3D mapping of chemical defects at charged grain boundaries in a ferroelectric oxide

Polar discontinuities and structural changes at oxide interfaces can give rise to a large variety of electronic and ionic phenomena. Related effects have been intensively studied in epitaxial systems, including ferroelectric domain walls and interfaces in superlattices. Here, we investigate the relation between polar discontinuities and the local chemistry at grain boundaries in polycrystalline ferroelectric ErMnO3. Using orientation mapping and different scanning probe microscopy techniques, we demonstrate that the polycrystalline material develops charged grain boundaries with enhanced electronic conductance. By performing atom probe tomography measurements, we find an enrichment of erbium and a depletion of oxygen at all grain boundaries. The observed compositional changes translate into a charge that exceeds possible polarization-driven effects, demonstrating that structural phenomena rather than electrostatics determine the local chemical composition and related changes in the electronic transport behavior. The study shows that the charged grain boundaries behave distinctly different from charged domain walls, giving additional opportunities for property engineering at polar oxide interfaces.

cond-mat.mtrl-sci↗

arXiv2023

Moiré Fringes in Conductive Atomic Force Microscopy

Moiré physics plays an important role for the characterization of functional materials and the engineering of physical properties in general, ranging from strain-driven transport phenomena to superconductivity. Here, we report the observation of moiré fringes in conductive atomic force microscopy (cAFM) scans gained on the model ferroelectric Er(Mn,Ti)O$_3$. By performing a systematic study of the impact of key experimental parameters on the emergent moiré fringes, such as scan angle and pixel density, we demonstrate that the observed fringes arise due to a superposition of the applied raster scanning and sample-intrinsic properties, classifying the measured modulation in conductance as a scanning moiré effect. Our findings are important for the investigation of local transport phenomena in moiré engineered materials by cAFM, providing a general guideline for distinguishing extrinsic from intrinsic moiré effects. Furthermore, the experiments provide a possible pathway for enhancing the sensitivity, pushing the resolution limit of local transport measurements by probing conductance variations at the spatial resolution limit via more long-ranged moiré patterns.

cond-mat.mtrl-sci↗

arXiv2021

Atomic-scale 3D imaging of individual dopant atoms in a complex oxide

A small percentage of dopant atoms can completely change the physical properties of the host material. For example, chemical doping controls the electronic transport behavior of semiconductors and gives rise to a wide range of emergent electric and magnetic phenomena in oxides. Imaging of individual dopant atoms in lightly doped systems, however, remains a major challenge, hindering characterization of the site-specific effects and local dopant concentrations that determine the atomic-scale physics. Here, we apply atom-probe tomography (APT) to resolve individual Ti atoms in the narrow band gap semiconductor ErMnO3 with a nominal proportion of 0.04 atomic percent. Our 3D imaging measures the Ti concentration at the unit cell level, providing quantitative information about the dopant distribution within the ErMnO3 crystal lattice. High-resolution APT maps reveal the 3D lattice position of individual Ti atoms, showing that they are located within the Mn layers with no signs of clustering or other chemical inhomogeneities. The 3D atomic-scale visualization of individual dopant atoms provides new opportunities for the study of local structure-property relations in complex oxides, representing an important step toward controlling dopant-driven quantum phenomena in next-generation oxide electronics.

cond-mat.mtrl-sci↗