Quantum Monochromated Electron Microscopy: Combining High Spatial and Spectral Resolution to Observe Emergent Electronic and Vibrational Proprties in Nanosctructured Quantum Materials

Over the last 5-10 years, the scanning transmission electron microscope (STEM) has been turned to quantum materials more and more due to the ability to correlate the subtle structural signatures of quantum phase transitions with atomistic imaging and compositional analysis. Over this same time period, monochromation for electron energy-loss spectroscopy (EELS) in the STEM has also seen a resurgence. With typical energy resolutions improving by two orders of magnitude to enable a whole host of new experiments at the high spatial resolution of the STEM. The combination of the two is truly a new opportunity for quantum materials, as it allows the ability to probe the novel quasiparticles and shallow electronic structure that mediate emergent phenomena directly at the length scales over which they occur.<br/><br/>In this talk, I will discuss the application of monochromated STEM-EELS to quantum nanostructured superlattices. In SrTiO₃-CaTiO₃ (STO-CTO) superlattices, where the superlattice is in the growth direction with alternating layers of STO and CTO at different unit-cell thicknesses. In this system, the period of the superlattice dominates the macroscopic properties of the material through changing the number of interfaces between the STO and CTO. However, in short period superlattices, an emergent phonon response causes the interface-density-dependency to reverse and as the material takes a new property based off of the interface octahedral tilts. I will also discuss PbSe quantum dot superlattices, where the superlattice concerns lateral, self-assembled, epitaxially-connected quantum dots that form a network analog to a cubic crystal structure. Here the electronic states delocalize across the epitaxially connected QDs to create emergent electronic structure not present in the individual quantum dots. In both cases the combination of spatial and spectral information reveal emergent spectroscopic repsonses in the nanostructured superlattice.