Cryogenic electron microscopy for quantum science
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Introduction Quantum phenomena such as superconductivity, charge density waves, skyrmions, magnetic spin ice, and device physics for quantum information science applications are more easily and commonly studied at low temperatures (closer to mK than 77 K).1 In most cases, these phenomena only exist at low temperatures. However, electron microscopes are designed to work optimally at room temperature. Currently, most ultralow-temperature experimental measurements for quantum materials are performed spectroscopically or with only near-surface imaging. Scanning tunneling microscopy (STM) can provide atomic imaging and electronic structure information with spectroscopic resolution of 11 µeV for exploring quantum phenomena at temperatures down to 10 mK.2 Scanning tunneling microscopes can provide a wide array of information,3–6 providing direct observations of inhomogeneous structures such as charge puddles in graphene at 4.8 K7 and atomic-scale surface features in Weyl semimetals.8 Beyond the impressive advancements in scanning probe measurement technology of the recent three decades, one of the primary reasons low temperatures provide inherent stability is that as temperatures approach zero K, so do coefficients of thermal expansion. Therefore, even if there is drift and movement, it is small, allowing modern atomic force microscopes
and scanning tunneling microscopes to image the same atoms for weeks at a time.9 However, scanning probe techniques only provide information from near the sample surface, which by definition cannot provide structural information about internal defects or the three-dimensional microstructure. Liquid He-based cryogenic sample stages for electron microscopy have been available since the 1960s.10–14 While it has been technically possible to cool samples down to as low as 1.5 K in a transmission electron microscope,15 mechanical vibrations and sample drift have limited the resolution, stability, and practical possibilities of ultralow-temperature transmission electron microscopy (TEM). Nonetheless, TEM has been used to image phenomena in quantum materials at more limited resolutions than is available at room temperature. Harada et al. produced the first-ever images of “real-time” observations of vortices in high-Tc superconductors at 9 K.16 Mori and colleagues17 used TEM imaging at 95 K to observe patterns of charge localization in the charge-ordered phase of a manganese oxide that displays colossal magnetoresistance. More recently, cryo-transmission electron microscopy (cryo-TEM) with modern electron microscopes has enabled direct imaging of quantum phase transformations in relation to defects and external stimuli. For instance, Carbone and colleagues used Lorentz TEM (a method of phase contrast
Andrew M. Minor, University of California, Berkeley, and Lawrence Berkeley National Laboratory, USA; [email protected] Peter Denes, Lawrence Berkeley National Laboratory, USA; [email protected] David A. Muller, School of Applied and Engineering Physics, Cornell University, USA; [email protected] doi:10
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