Anthony G. Gianfrancesco

Big data in reciprocal space: Sliding fast Fourier transforms for determining periodicity

Significant advances in atomically resolved imaging of crystals and surfaces have occurred in the last decade allowing unprecedented insight into local crystal structures and periodicity. Yet, the analysis of the long-range periodicity from the local imaging data, critical to correlation of functional properties and chemistry to the local crystallography, remains a challenge. Here, we introduce a Sliding Fast Fourier Transform (FFT) filter to analyze atomically resolved images of in-situ grown La5/8Ca3/8MnO3 (LCMO) films. We demonstrate the ability of sliding FFT algorithm to differentiate two sub-lattices, resulting from a mixed-terminated surface. Principal Component Analysis and Independent Component Analysis of the Sliding FFT dataset reveal the distinct changes in crystallography, step edges, and boundaries between the multiple sub-lattices. The implications for the LCMO system are discussed. The method is universal for images with any periodicity, and is especially amenable to atomically resolved pr...

Surface control of epitaxial manganite films via 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 in situ scanning tunneling microscopy to image surfaces of La5/8Ca3/8MnO3 films grown by pulsed laser deposition. The atomically resolved imaging was combined with in situ 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.

Atomic-scale electrochemistry on the surface of a manganite by scanning tunneling microscopy

The doped manganese oxides (manganites) have been widely studied for their colossal magnetoresistive effects, for potential applications in oxide spintronics, electroforming in resistive switching devices, and are materials of choice as cathodes in modern solid oxide fuel cells. However, little experimental knowledge of the dynamics of the surfaces of perovskite manganites at the atomic scale exists. Here, through in-situ scanning tunneling microscopy (STM), we demonstrate atomic resolution on samples of La0.625Ca0.375MnO3 grown on (001) SrTiO3 by pulsed laser deposition. Furthermore, by applying triangular DC waveforms of increasing amplitude to the STM tip, and measuring the tunneling current, we demonstrate the ability to both perform and monitor surface electrochemical processes at the atomic level, including formation of oxygen vacancies and removal and deposition of individual atomic units or clusters. Our work paves the way for better understanding of surface oxygen reactions in these systems.

Full information acquisition and analysis of reflection high energy electron diffraction data for epitaxial growth processes

Reflection high energy electron diffraction (RHEED) has become ubiquitous in growth chambers worldwide, and has been used since the 1980s for in-situ characterization of thin films grown by both molecular beam epitaxy as well as pulsed laser deposition (PLD). The technique is essentially a surface electron diffraction technique where a beam of electrons is directed towards the substrate at small (<~13°) grazing angles, resulting in a diffraction pattern that can provide information on the quality of the surface (roughness), the surface reconstructions, transitions between different growth modes, rate of growth of layers, etc. Given the non-ideal geometry of the diffraction as well as the dynamic nature of the scattering, most theoretical approaches to study the evolution of RHEED diffraction as the growth proceeds are highly involved, and as such application of RHEED is typically qualitative (or at most semi-quantitative). Much remains poorly understood about the time-evolution of RHEED diffraction patterns, and careful data analysis of the RHEED image sequences could open pathways towards both elucidation of the underlying physics, as well as control over the film growth process.

On the role of chalcogen vapor annealing in inducing bulk superconductivity in Fe

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