Cryogenic Electron Microscopy for Quantum Materials

Quantum materials refers to a class of materials with exotic properties that arise from the quantum mechanical nature of their constituent electrons, exhibiting, for example, high-temperature superconductivity, colossal magnetoresistivity, multiferroicity, and topological characteristics. Quantum materials often have incompletely filled d- or f-electron shells with narrow energy bands and the behavior of their electrons is strongly correlated. One distinct characteristic of the materials is that their electronic states are often spatially inhomogeneous, thus is well suited for study using spatially resolved electron beams with its great scattering power and sensitivity to atomic ionicity. Furthermore, most of these exotic properties only manifest at very low temperatures, posing a challenge to modern electron microscopy. It requires extraordinary instrument stabilities at cryogenic temperatures with critical spatial, temporal, and energy resolutions in both static and dynamic manner to probe these materials. On the other hand, the ability to directly visualize the atomic, electronic and spin structures and inhomogeneities of quantum materials and correlate them to their functionalities creates enormous opportunities. At the most elementary levels of condensed matter physics, understanding the competing orders of electron, spin, orbital, and lattice and their degrees of freedom, the impacts of defects and interfaces, and the site-specific quantum phenomena and phase transitions that give rise to the emergent behaviors allows us to discover and control novel materials for quantum information science and technologies [1].<br/> <br/>In this presentation, several of our research examples are selected to highlight the use of Cryo-EM to study strongly correlated quantum materials. We focus on the critical roles of heterogeneity, interfaces, and disorder in crystal structure, electronic structure, and spin structure to understand the physical properties of the materials. We show how electron diffraction and diffuse scattering analysis at low temperatures empowers us to reveal the nature of structural disorder and phonons under thermal equilibrium and far-from equilibrium [1]; how atomically resolved imaging and electron energy-loss spectroscopy at 10K can be used to understand the interface-enhanced superconductivity (Fig.1); and how to use Lorentz phase microscopy to explore the intriguing transformations among various topological chiral spin states under applied magnetic field at various cryogenic temperatures (Fig.2). Finally, we review our recent unprecedented development of a closed-cycle cryocooler for Cryo-EM without the need of refilling liquid He. The system is designed to reach a temperature as low as 4k with temperature stability and control better than a few mK. <br/> <br/><b>References</b><br/>[1] Zhu, Y., “Cryogenic Electron Microscopy on Strongly Correlated Quantum Materials”, Invited review article, special issue on Cryogenic Electron Microscopy. Guest editors: Y. Cui and L. Kourkoutis, Accounts of Chemical Research, 54, 3518−3528 (2021).<br/> <br/><b>Acknowledgement</b><br/>The research was supported by the US DOE-BES, Materials Sciences and Engineering Division, under Contract No. DE-SC0012704. Collaborations with and assistance from the group members at BNL in the past 30 years are acknowledged.