Ismail El Baggari1
Harvard University1
Scanning transmission electron microscopy (STEM) is highly sensitive to lattice degrees of freedom and enables tracking of lattice displacements with sub-Angstrom resolution and picometer precision. This has ushered novel visualizations and discoveries of functional atomic displacements in ferroelectric materials, magnetoelectric oxides, and 2D materials. Due to stringent stability requirements, the overwhelming majority of high-resolution STEM measurements are limited to room temperature or above. In many classes of correlated electronic materials, however, subtle and exotic ground states emerge exclusively below room temperature. Achieving cryogenic capabilities in STEM is therefore paramount to accessing and probing low temperature phases including high-temperature superconductivity, charge order, metal-insulator transitions, and more.<br/> <br/>I will illustrate recent, successful applications of high-resolution cryogenic STEM for discovering and understanding novel correlated phases at low temperature (~90 K) [1,2,3]. I will focus on charge order, a state in which the electrons and the lattice form superstructures that break the translational symmetry of the atomic lattice. In an overdoped manganite, we directly visualize intrinsic topological defects and show how these local fluctuations alter the periodicity and long-range order of incommensurate charge order stripes [1]. In a half-doped manganite, we address the long-standing question of whether charge order resides on the sites or bonds, discover an exotic intermediate state which breaks inversion symmetry, and find that non-linear couplings between distinct lattice distortion modes locally determine phase competition [3]. These cryogenic STEM results pave a clear path to imaging low temperature electronic phases with high resolution and precision.<br/> <br/>[1] El Baggari <i>et al.,</i> <i>Proceedings of the National Academy of Sciences</i> 115.7 (2018): 1445-1450.<br/>[2] El Baggari <i>et al.</i>, <i>Physical Review Letters</i> 125.16 (2020): 165302.<br/>[3] El Baggari, Baek <i>et al.,</i> <i>Nature Communications </i>12, 3747 (2021)