arXiv subjects

Kasper A. Hunnestad

Publications and source records attributed to Kasper A. Hunnestad.

6 recordsLinked to original sources

arXiv2026

Correlation Between Dopant Atom Evaporation Field and Measured Site Preference in Atom Probe Tomography

Complex oxides possess a wide range of electric, magnetic, and optical properties that can be precisely tuned by chemical doping. The atomic-scale analysis of the property-controlling dopants, however, becomes increasingly difficult towards low doping levels. Atom probe tomography (APT) offers chemical sensitivity and spatial resolution to image individual dopant atoms down to a few parts per million. To reliably extract such information, detailed knowledge about the atom-specific field evaporation processes is required. Here we demonstrate a first insight into the APT-measured atomic position of dopant atoms and the field evaporation conditions, using Zr-doped ErMnO3 as a model system. Our analysis reveals a substantial preferential retention of both matrix and dopant atoms which strongly affects the dopant site determination and can lead to an incorrect interpretation. The retention effect is determined by intrinsic and extrinsic parameters, such as the dopant's evaporation field and concentration and the analysis temperature, respectively, as we explain based on field-evaporation simulations. Our results are important for the APT-based analysis of individual dopant atoms in solid systems and the understanding of field evaporation dynamics at the atomic level in general.

cond-mat.mtrl-sci↗

arXiv2026

Atom Probe Tomography as an Emerging Tool for Understanding Defect-driven Mechanisms in HfO$_{2}$-based Ferroelectrics

HfO$_{2}$-based ferroelectrics are essential for the next generation of CMOS-compatible memory and logic devices, yet their performance is governed by a complex interplay between oxygen vacancies, dopants, and structural defects that remains an active area of investigation. These defects shape the function-critical dynamic phenomena, such as polar phase stabilization, wake-up, fatigue, and imprint. In this Perspective, we review the limitations of established high-resolution structural characterization techniques and propose atom probe tomography (APT) as a powerful tool for the three-dimensional (3D), atomic-scale mapping of all constituent species in hafnia-based ferroelectric systems. By resolving individual dopants, vacancy clustering, and interfacial segregation, APT can facilitate a quantitative understanding of defect-property relations in hafnia-based ferroelectrics. We discuss current experimental challenges for APT application to ferroelectric oxides, demonstrate a proof-of-concept of atomic-scale reconstruction in a hafnia-based device stack, and highlight the potential of APT to guide the development of ferroelectric structures with enhanced reliability and performance.

cond-mat.mtrl-sci↗

arXiv2026

Chemical heterogeneity at conducting ferroelectric domain walls

Natural interfaces in ferroic oxides have developed into versatile playgrounds for studying electronic correlation effects in 2D systems. The microscopic origin of the emergent local electronic properties is often debated, however, as quantitative atomic-scale characterization remains challenging. A prime example is enhanced conductivity at ferroelectric domain walls, attributed to mechanisms ranging from local band gap reduction to point defect accumulations. Here, we resolve the microscopic mechanisms for domain wall conduction in the ferroelectric model system BiFeO3, by combining transport measurements with atom probe tomography to quantify the local chemical composition and correlate it with the electrical properties. Significant chemical variations along the walls are observed, demonstrating an outstanding chemical flexibility at domain walls, which manifest in spatially varying physical properties. The results give a unifying explanation for the diverse electronic behavior observed and establish the fundamental notion that multiple conduction mechanisms can coexist within individual domain walls.

cond-mat.mtrl-sci↗

arXiv2025

Hybrid antiferroelectric-ferroelectric domain walls in noncollinear antipolar oxides

Antiferroelectrics are emerging as advanced functional materials and are fertile ground for unusual electric effects. For example, they enhance the recoverable energy density in energy storage applications and give rise to large electromechanical responses. Here, we demonstrate noncollinearity in dipolar order as an additional degree of freedom, unlocking physical properties that are symmetry-forbidden in classical antiferroelectrics. We show that noncollinear order of electric dipole moments in K$_3$[Nb$_3$O$_6$|(BO$_3$)$_2$] leads to a coexistence of antiferroelectric and ferroelectric behaviors. Besides the double-hysteresis loop observed in antiferroelectrics, a pronounced piezoresponse and electrically switchable domains are observed, separated by atomically sharp and micrometer-long charged domain walls. Hybrid antiferroelectric-ferroelectric responses are expected in a wide range of noncollinear systems, giving a new dimension to the research on antiferroelectrics and multifunctional oxides in general.

cond-mat.mtrl-sci↗

arXiv2025

Local doping of an oxide semiconductor by voltage-driven splitting of anti-Frenkel defects

Layered oxides exhibit high ionic mobility and chemical flexibility, attracting interest as cathode materials for lithium-ion batteries and the pairing of hydrogen production and carbon capture. Recently, layered oxides emerged as highly tunable semiconductors. For example, by introducing anti-Frenkel defects, the electronic hopping conductance in hexagonal manganites was increased locally by orders of magnitude. Here, we demonstrate local acceptor and donor doping in Er(Mn,Ti)O$_3$, facilitated by the splitting of such anti-Frenkel defects under applied d.c. voltage. By combining density functional theory calculations, scanning probe microscopy, atom probe tomography, and scanning transmission electron microscopy, we show that the oxygen defects readily move through the layered crystal structure, leading to nano-sized interstitial-rich (p-type) and vacancy-rich (n-type) regions. The resulting pattern is comparable to dipolar npn-junctions and stable on the timescale of days. Our findings reveal the possibility of temporarily functionalizing oxide semiconductors at the nanoscale, giving additional opportunities for the field of oxide electronics and the development of transient electronics in general.

cond-mat.mtrl-sci↗

arXiv2023

Imaging and structure analysis of ferroelectric domains, domain walls, and vortices by scanning electron diffraction

Direct electron detectors in scanning transmission electron microscopy give unprecedented possibilities for structure analysis at the nanoscale. In electronic and quantum materials, this new capability gives access to, for example, emergent chiral structures and symmetry-breaking distortions that underpin functional properties. Quantifying nanoscale structural features with statistical significance, however, is complicated by the subtleties of dynamic diffraction and coexisting contrast mechanisms, which often results in low signal-to-noise and the superposition of multiple signals that are challenging to deconvolute. Here we apply scanning electron diffraction to explore local polar distortions in the uniaxial ferroelectric Er(Mn,Ti)O$_3$. Using a custom-designed convolutional autoencoder with bespoke regularization, we demonstrate that subtle variations in the scattering signatures of ferroelectric domains, domain walls, and vortex textures can readily be disentangled with statistical significance and separated from extrinsic contributions due to, e.g., variations in specimen thickness or bending. The work demonstrates a pathway to quantitatively measure symmetry-breaking distortions across large areas, mapping structural changes at interfaces and topological structures with nanoscale spatial resolution.

cond-mat.mtrl-sci↗