Advances in Scanning Transmission Electron Microscopy for Materials Discovery and Innovation

Abstract

Subtle interplay between structure, charge, and spin in the atomic lattice is the fundamental driver of the functional properties in crystalline quantum materials. These effects can be so subtle that in many cases just a few misplaced atoms are enough to change or suppress the desired material properties entirely. Probing these materials with quantitative detail at the atomic scale therefore offers key insights to the close link between material form and function. The scanning transmission electron microscope (STEM) is a powerful tool which grants access to detailed, quantitative measurements of such properties at the atomic scale. Here, a combination of high spatial-resolution imaging and high energy-resolution electron energy loss spectroscopy (EELS) is harnessed to probe local effects in a variety of quantum materials and these insights are further leveraged for the strategic design of new, tunable materials. Atomic-resolution imaging and spectroscopy rely on certain strict prerequisites ranging from sample suitability to environmental stability of the laboratory.Furthermore, the spatially localized measurements which are critical to understanding the fundamental physics at play in quantum systems can be limited by the practical realities of a material's robustness to measurement. Many quantum materials, however, are sensitive to radiation doses typically applied by the STEM probe, and the exotic phenomena they host exist only at cryogenic temperatures. These considerations have traditionally limited both the materials and the conditions which are studied, confined mostly to robust crystalline materials and ambient temperatures. Accessing exotic states in the STEM therefore necessitates significant advances in experimental capabilities, which are realized here through detector, sample stage, and electron source improvements. Together, these advances open the door to exploring both new materials and new phases in the STEM. Unconventional superconductivity is perhaps one of the most widely-studied phenomena in condensed matter physics, yet in many aspects remains one of the most mysterious. Here we study a subset of superconducting oxides exemplifies a few of the key questions in this field. Copper-based compounds are the prototypical "high-temperature'' superconductors, exhibiting remarkably robust superconductivity with critical temperatures reaching well over 100 K. Early theoretical predictions that nickel-based compounds with similar crystal and electronic structure could host similar properties were finally offered a platform for experimental validation with the discovery of superconducting infinite-layer nickelate thin films in 2019. Despite the nominal similarities between cuprates and nickelates, however, our STEM-EELS measurements reveals notable differences in the electronic landscape of these materials. These local measurements are key to disentangling which properties of the nickelate system reflect challenges of materials synthesis and which reflect the fundamental physics at work. But not all oxide superconductors are so closely related: in stark contrast to the cuprates, superconductivity in Sr2RuO4 is extremely sensitive to crystalline disorder. Local STEM-EELS measurements or Sr2RuO4thin films are used here to extract and characterize different types of disorder in the atomic lattice, thereby illustrating their distinct impacts on superconductivity. The STEM provides unique access to understanding the atomic-scale structures and interactions which impart functional or exotic properties to quantum materials. Detailed quantitative studies of oxide interfaces here inspire new approaches for symmetry templating oxide monolayers. Leveraging the insights of atomic-resolution STEM across a breadth of systems thus inspires and informs new methods for tunably controlling electronic properties in designer compounds.