Learning from imperfections: constructing phase diagrams from atomic imaging of fluctuations

Materials characterization and property measurements are a cornerstone of material science, providing feedback from synthesis to applications. Traditionally, a single sample is used to derive information on a single point in composition space, and imperfections, impurities and stochastic details of material structure are deemed irrelevant or complicating factors in analysis. Here we demonstrate that atomic-scale studies of a single nominal composition can provide information on a finite area of chemical space. This information can be used to reconstruct the material properties in a finite composition and temperature range. We develop a statistical physics-based framework that incorporates chemical and structural data to infer effective atomic interactions driving segregation in a La5/8Ca3/8MnO3 thin-film. A variational autoencoder is used to determine anomalous behaviors in the composition phase diagram. This study provides a framework for creating generative models from diverse data and provides direct insight into the driving forces for cation segregation in manganites.Comment: 34 pages, 5 figures and supplementar

Knowledge Extraction from Atomically Resolved Images

Tremendous strides in experimental capabilities of scanning transmission electron microscopy and scanning tunneling microscopy (STM) over the past 30 years made atomically resolved imaging routine. However, consistent integration and use of atomically resolved data with generative models is unavailable, so information on local thermodynamics and other microscopic driving forces encoded in the observed atomic configurations remains hidden. Here, we present a framework based on statistical distance minimization to consistently utilize the information available from atomic configurations obtained from an atomically resolved image and extract meaningful physical interaction parameters. We illustrate the applicability of the framework on an STM image of a FeSe_<i>x</i>Te_1–<i>x</i> superconductor, with the segregation of the chalcogen atoms investigated using a nonideal interacting solid solution model. This universal method makes full use of the microscopic degrees of freedom sampled in an atomically resolved image and can be extended <i>via</i> Bayesian inference toward unbiased model selection with uncertainty quantification

Nanoscale Probing of Elastic–Electronic Response to Vacancy Motion in NiO Nanocrystals

Measuring the diffusion of ions and vacancies at nanometer length scales is crucial to understanding fundamental mechanisms driving technologies as diverse as batteries, fuel cells, and memristors; yet such measurements remain extremely challenging. Here, we employ a multimodal scanning probe microscopy (SPM) technique to explore the interplay between electronic, elastic, and ionic processes <i>via</i> first-order reversal curve <i>I</i>–<i>V</i> measurements in conjunction with electrochemical strain microscopy (ESM). The technique is employed to investigate the diffusion of oxygen vacancies in model epitaxial nickel oxide (NiO) nanocrystals with resistive switching characteristics. Results indicate that opening of the ESM hysteresis loop is strongly correlated with changes to the resonant frequency, hinting that elastic changes stem from the motion of oxygen (or cation) vacancies in the probed volume of the SPM tip. These changes are further correlated to the current measured on each nanostructure, which shows a hysteresis loop opening at larger (∼2.5 V) voltage windows, suggesting the threshold field for vacancy migration. This study highlights the utility of local multimodal SPM in determining functional and chemical changes in nanoscale volumes in nanostructured NiO, with potential use to explore a wide variety of materials including phase-change memories and memristive devices in combination with site-correlated chemical imaging tools

Surface Control of Epitaxial Manganite Films <i>via</i> Oxygen Pressure

The trend to reduce device dimensions demands increasing attention to atomic-scale details of structure of thin films as well as to pathways to control it. This is of special importance in the systems with multiple competing interactions. We have used <i>in situ</i> scanning tunneling microscopy to image surfaces of La_5/8Ca_3/8MnO₃ films grown by pulsed laser deposition. The atomically resolved imaging was combined with <i>in situ</i> angle-resolved X-ray photoelectron spectroscopy. We find a strong effect of the background oxygen pressure during deposition on structural and chemical features of the film surface. Deposition at 50 mTorr of O₂ leads to mixed-terminated film surfaces, with B-site (MnO₂) termination being structurally imperfect at the atomic scale. A relatively small reduction of the oxygen pressure to 20 mTorr results in a dramatic change of the surface structure leading to a nearly perfectly ordered B-site terminated surface with only a small fraction of A-site (La,Ca)O termination. This is accompanied, however, by surface roughening at a mesoscopic length scale. The results suggest that oxygen has a strong link to the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also pronounced: Ca surface segregation is decreased with oxygen pressure reduction

Dimensionality Controlled Octahedral Symmetry-Mismatch and Functionalities in Epitaxial LaCoO₃/SrTiO₃ Heterostructures

Epitaxial strain provides a powerful approach to manipulate physical properties of materials through rigid compression or extension of their chemical bonds via lattice-mismatch. Although symmetry-mismatch can lead to new physics by stabilizing novel interfacial structures, challenges in obtaining atomic-level structural information as well as lack of a suitable approach to separate it from the parasitical lattice-mismatch have limited the development of this field. Here, we present unambiguous experimental evidence that the symmetry-mismatch can be strongly controlled by dimensionality and significantly impact the collective electronic and magnetic functionalities in ultrathin perovskite LaCoO₃/SrTiO₃ heterojunctions. State-of-art diffraction and microscopy reveal that symmetry breaking dramatically modifies the interfacial structure of CoO₆ octahedral building-blocks, resulting in expanded octahedron volume, reduced covalent screening, and stronger electron correlations. Such phenomena fundamentally alter the electronic and magnetic behaviors of LaCoO₃ thin-films. We conclude that for epitaxial systems, correlation strength can be tuned by changing orbital hybridization, thus affecting the Coulomb repulsion, U, instead of by changing the band structure as the common paradigm in bulks. These results clarify the origin of magnetic ordering for epitaxial LaCoO₃ and provide a route to manipulate electron correlation and magnetic functionality by orbital engineering at oxide heterojunctions