arXiv subjects

Christopher T. Nelson

Publications and source records attributed to Christopher T. Nelson.

12 recordsLinked to original sources

arXiv2026

Composition-Driven Phase Evolution in Sm-Doped BiFeO3 via Latent-Field Reconstruction of Atomically Resolved STEM Data

Functionalities of ferroelectric materials are governed by the spatial organization and coupling of polarization, strain, lattice rotation, and structural order accessible via atomically resolved scanning transmission electron microscopy (STEM) images. Quantitative interpretation of atomic-resolution STEM data has conventionally relied on locating atomic columns and converting their fitted coordinates into local structural descriptors. Here, we develop a field-based approach in which atomic-resolution images are represented by spatially varying latent Bragg fields, whose amplitudes and phases provide continuous maps of crystalline order, lattice displacement, strain, rotation, and mode-specific residual structure. The observed atomically resolved images are decoded from the latent fields. We apply this framework to image series of Sm-substituted BiFeO3 spanning 0-20% Sm and crossing the composition-driven boundary between the R3c ferroelectric phase and the orthorhombic, nonpolar Pnma phase. Conventional atom-resolved parameterization is used as an independent validation, showing that reconstructed Bragg amplitude tracks local atomic-column intensity and that field-derived shear reproduces unit-cell angular distortions obtained from atom fitting. The combined analysis reveals a systematic evolution from extended ferroelectric domains at low Sm concentration, through the appearance and growth of localized regions with period-doubled Pnma order at intermediate compositions, to a connected Pnma-dominated state at high Sm content. The period-doubled order is accompanied by enhanced shear and lattice rotation and by progressive reorganization of the ferroelectric domain structure. These results establish latent-field reconstruction as a physically interpretable complement to atom finding and provide a unified framework for resolving composition-driven phase evolution in ferroic materials.

cond-mat.mtrl-sci↗

arXiv2022

Unsupervised learning of ferroic variants from atomically resolved STEM images

An approach for the analysis of atomically resolved scanning transmission electron microscopy data with multiple ferroic variants in the presence of imaging non-idealities and chemical variabilities based on a rotationally invariant variational autoencoder (rVAE) is presented. We show that an optimal local descriptor for the analysis is a sub-image centered at specific atomic units, since materials and microscope distortions preclude the use of an ideal lattice as a reference point. The applicability of unsupervised clustering and dimensionality reduction methods is explored and are shown to produce clusters dominated by chemical and microscope effects, with a large number of classes required to establish the presence of rotational variants. Comparatively, the rVAE allows extraction of the angle corresponding to the orientation of ferroic variants explicitly, enabling straightforward identification of the ferroic variants as regions with constant or smoothly changing latent variables and sharp orientational changes. This approach allows further exploration of the chemical variability by separating the rotational degrees of freedom via rVAE and searching for remaining variability in the system. The code used in the manuscript is available at https://github.com/saimani5/ferroelectric_domains_rVAE.

cond-mat.mtrl-sci↗

arXiv2022

Surface-Driven Evolution of the Anomalous Hall Effect in Magnetic Topological Insulator MnBi2Te4 Thin Films

Understanding the effects of interfacial modification to the functional properties of magnetic topological insulator thin films is crucial for developing novel technological applications from spintronics to quantum computing. Here, we report that a large electronic and magnetic response is induced in the intrinsic magnetic topological insulator MnBi2Te4 by controlling the propagation of surface oxidation. We show that the formation of the surface oxide layer is confined to the top 1-2 unit cells but drives large changes in the overall magnetic response. Specifically, we observe a dramatic reversal of the sign of the anomalous Hall effect driven by finite thickness magnetism, which indicates that the film splits into distinct magnetic layers each with a unique electronic signature. These data reveal a delicate dependence of the overall magnetic and electronic response of MnBi2Te4 on the stoichiometry of the top layers. Our study suggests that perturbations resulting from surface oxidation may play a non-trivial role in the stabilization of the quantum anomalous Hall effect in this system and that understanding targeted modifications to the surface may open new routes for engineering novel topological and magnetic responses in this fascinating material.

cond-mat.mtrl-sci↗

arXiv2021

Self-regulated growth of candidate topological superconducting parkerite by molecular beam epitaxy

Ternary chalcogenides such as the parkerites and shandites are a broad class of materials exhibiting rich diversity of transport and magnetic behavior as well as an array of topological phases including Weyl and Dirac nodes. However, they remain largely unexplored as high-quality epitaxial thin films. Here, we report the self-regulated growth of thin films of the strong spin-orbit coupled superconductor Pd3Bi2Se2 on SrTiO3 by molecular beam epitaxy. Films are found to grow in a self-regulated fashion, where, in excess Se, the temperature and relative flux ratio of Pd to Bi controls the formation of Pd3Bi2Se2 due to the combined volatility of Bi, Se, and Bi-Se bonded phases. The resulting films are shown to be of high structural quality, the stoichiometry is independent of the Pd:Bi and Se flux ratio and exhibit a superconducting transition temperature of 800 mK and critical field of 17.7 +/- 0.5 mT, as probed by transport as well as magnetometry. Understanding and navigating the growth of the chemically and structurally diverse classes of ternary chalcogenides opens a vast space for discovering new phenomena as well as enabling new applications.

cond-mat.mtrl-sci↗

arXiv2021

Electric Field Control of Chirality

Polar textures have attracted significant attention in recent years as a promising analog to spin-based textures in ferromagnets. Here, using optical second harmonic generation based circular dichroism, we demonstrate deterministic and reversible control of chirality over mesoscale regions in ferroelectric vortices using an applied electric field. The microscopic origins of the chirality, the pathway during the switching, and the mechanism for electric-field control are described theoretically via phase-field modeling and second-principles simulations, and experimentally by examination of the microscopic response of the vortices under an applied field. The emergence of chirality from the combination of non-chiral materials and subsequent control of the handedness with an electric field has far-reaching implications for new electronics based on chirality as a field controllable order parameter.

cond-mat.mtrl-sci↗

arXiv2021

Deep learning polarization distributions in ferroelectrics from STEM data: with and without atom finding

Over the last decade, scanning transmission electron microscopy (STEM) has emerged as a powerful tool for probing atomic structures of complex materials with picometer precision, opening the pathway toward exploring ferroelectric, ferroelastic, and chemical phenomena on the atomic-scale. Analyses to date extracting a polarization signal from lattice coupled distortions in STEM imaging rely on discovery of atomic positions from intensity maxima/minima and subsequent calculation of polarization and other order parameter fields from the atomic displacements. Here, we explore the feasibility of polarization mapping directly from the analysis of STEM images using deep convolutional neural networks (DCNNs). In this approach, the DCNN is trained on the labeled part of the image (i.e., for human labelling), and the trained network is subsequently applied to other images. We explore the effects of the choice of the descriptors (centered on atomic columns and grid-based), the effects of observational bias, and whether the network trained on one composition can be applied to a different one. This analysis demonstrates the tremendous potential of the DCNN for the analysis of high-resolution STEM imaging and spectral data and highlights the associated limitations.

physics.data-an↗

arXiv2021

Exploring physics of ferroelectric domain walls via Bayesian analysis of atomically resolved STEM data

The physics of ferroelectric domain walls is explored using the Bayesian inference analysis of atomically resolved STEM data. We demonstrate that domain wall profile shapes are ultimately sensitive to the nature of the order parameter in the material, including the functional form of Ginzburg-Landau-Devonshire expansion, and numerical value of the corresponding parameters. The preexisting materials knowledge naturally folds in the Bayesian framework in the form of prior distributions, with the different order parameters forming competing (or hierarchical) models. Here, we explore the physics of the ferroelectric domain walls in BiFeO3 using this method, and derive the posterior estimates of relevant parameters. More generally, this inference approach both allows learning materials physics from experimental data with associated uncertainty quantification, and establishing guidelines for instrumental development answering questions on what resolution and information limits are necessary for reliable observation of specific physical mechanisms of interest.

cond-mat.mtrl-sci↗

arXiv2019

Melting of Spatially Modulated Phases in La-doped BiFeO3 at Surfaces and Surface-Domain Wall Junctions

The interplay between the surface and domain wall phenomena in multiferroic LaxBi1-xFeO3 in the vicinity of morphotropic phase transition is explored on the atomic level. Scanning Transmission Electron Microscopy (STEM) has enabled mapping of atomic structures of the material with picometer-level precision, providing direct insight into the spatial distribution of the order parameters in this material and their behavior at surfaces and interfaces. Here, we use the thermodynamic Landau-Ginzburg-Devonshire (LGD) approach to explain the emergence of spatially modulated phases (SMP) in La0.22Bi0.78FeO3 films, and establish that the change of polarization gradient coefficients caused by La-doping is the primary driving mechanisms. The suppression, or "melting", of the SMP in the vicinity of the domain wall surface junction is observed experimentally and simulated in the framework of LGD theory. The melting originated from the system tendency to minimize electrostatic energy caused by long-range stray electric fields outside the film and related depolarization effects inside it. The observed behavior provides insight to the origin of surface and interface behaviors in multiferroics.

cond-mat.mtrl-sci↗

arXiv2019

Building Free Energy Functional from Atomically-Resolved Imaging: Atomic Scale Phenomena in La-doped BiFeO3

Scanning Transmission Electron Microscopy (STEM) has enabled mapping of atomic structures of solids with sub-pm precision, providing insight to the physics of ferroic phenomena and chemical expansion. However, only a subset of information is available, due to projective nature of imaging in the beam direction. Correspondingly, the analysis often relies on the postulated known form of macroscopic Landau-Ginzburg energy, and some predefined relationship between experimentally determined atomic coordinates and the order parameter field. Here, we propose an approach for exploring the structure of ferroics using reduced order parameter models constructed based on experimental data only. We develop a four sublattices model (FSM) for the analytical description of A-cation displacement in (anti)ferroelectric-antiferrodistortive perovskites of ABO3-type. The model describes the displacements of cation A in four neighboring unit cells and determines the conditions of different structural phases appearance and stability in ABO3. We show that FSM explains the coexistence of rhombohedral (R), orthorhombic (O) and spatially modulated (SM) phases, observed by atomic-resolution STEM in La-doped BiFeO3. Using this approach, we atomically resolve and theoretically model the sublattice asymmetry inherent to the case of the A-site La/Bi cation sublattice in LaxBi1-xFeO3 polymorphs. This approach allows exploring the ferroics behaviors from experimental data only, without additional assumptions on the nature of the order parameter.

cond-mat.mtrl-sci↗

arXiv2018

Intrinsic structural instabilities of domain walls driven by gradient couplings: meandering anferrodistortive-ferroelectric domain walls in BiFeO3

Using Landau-Ginzburg-Devonshire approach, we predict the intrinsic instability of the ferroelectric-ferroelastic domain walls in the multiferroic BiFeO3 emerging from the interplay between the gradient terms of the antiferrodistortive and ferroelectric order parameters at the walls. These instabilities are the interface analogue of the structural instabilities in the vicinity of phase coexistence in the bulk; and so they do not steam from incomplete polarization screening in thin films or its spatial confinement, electrostrictive or flexoelectric coupling. The effect of BiFeO3 material parameters on the 71 degree, 109 degree, and 180 degree walls is explored, and it is shown that the meandering instability appears at 109 degree, and 180 degree walls for small gradient energies, and the walls become straight and broaden for higher gradients. In contrast to the 180 degree and 109 degree domain walls, uncharged 71 degree walls are always straight, and their width increases with increasing the tilt gradient coefficient. The wall instability and associated intrinsic meandering provide a new insight into the behavior of morphotropic and relaxor materials, wall pinning, and mechanisms of interactions between order parameter fields and local microstructure.

cond-mat.mtrl-sci↗

arXiv2018

A high-energy density antiferroelectric made by interfacial electrostatic engineering

Dielectric capacitors hold a tremendous advantage for energy storage due to their fast charge/discharge times and stability in comparison to batteries and supercapacitors. A key limitation to today's dielectric capacitors, however, is the low storage capacity of conventional dielectric materials. To mitigate this issue, antiferroelectric materials have been proposed, but relatively few families of antiferroelectric materials have been identified to date. Here, we propose a new design strategy for the construction of lead-free antiferroelectric materials using interfacial electrostatic engineering. We begin with a ferroelectric material with one of the highest known bulk polarizations, BiFeO3. We show that by confining atomically-precise thin layers of BiFeO3 in a dielectric matrix that we can induce a metastable antiferroelectric structure. Application of an electric field reversibly switches between this new phase and a ferroelectric state, in addition, tuning of the dielectric layer causes coexistence of the ferroelectric and antiferroelectric states. Precise engineering of the structure generates an antiferroelectric phase with energy storage comparable to that of the best lead-based materials. The use of electrostatic confinement provides a new pathway for the design of engineered antiferroelectric materials with large and potentially coupled responses.

cond-mat.mtrl-sci↗

arXiv2016

The Nature of Electron Transport and visible light absorption in Strontium Niobate -- A Plasmonic Water Splitter

Semiconductor compounds are widely used for water splitting applications, where photo-generated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr$_{1-x}$NbO$_3$, 0.03 < x < 0.20) have shown competitive photocatalytic efficiency, opening up the material space available for finding optimizing performance in water-splitting applications. The origin of the visible light absorption in these powders was reported to be due to an interband transition and the charge carrier separation was proposed to be due to the high carrier mobility of this material. In the current work we have prepared epitaxial thin films of Sr$_{0.94}$NbO$_{3+δ}$ and found that the bandgap of this material is ~4.1 eV, which is very large. Surprisingly the carrier density of the conducting phase reaches 10$^{22}$ cm$^{-3}$, which is only one order smaller than that of elemental metals and the carrier mobility is only 2.47 cm$^2$/(V$\cdot$s). Contrary to earlier reports, the visible light absorption at 1.8 eV (~688 nm) is due to the bulk plasmon resonance, arising from the large carrier density, instead of an interband transition. Excitation of the plasmonic resonance results in a multifold enhancement of the lifetime of charge carriers. Thus we propose that the hot charge carriers generated from decay of plasmons produced by optical absorption is responsible for the water splitting efficiency of this material under visible light irradiation.

cond-mat.mtrl-sci↗