Extended Data Fig. 4: Procedure of q-selective EELS data collection.

a, Low magnification STEM image before EELS acquisition. The central region with a size of 2 nm × 2 nm (the yellow box) was used to acquire an atomic resolution q-selective EELS hyperspectral image dataset. b, High magnification STEM image acquired at the yellow box in (a) with monochromation before EELS acquisition. c, Hyperspectral image dataset with 60×60 pixel area. The displayed intensity is the integrated raw signal in the 10–20 meV range. The total acquisition time for this dataset is about 60 min. d, High magnification STEM image acquired at the yellow box in (e) with monochromation after EELS acquisition. e, Low magnification STEM image after EELS acquisition. The atomic structure of STO overlaps on the atomic resolution STEM images in (b) and (d). The region of interest is chosen with perfect STO crystalline structure in absence of any defects. The crystalline structure is still retained after the long EELS acquisition. No detectable contamination occurs during this experiment as shown in (e). The red dashed circle indicates a defective region to be used to measure the sample drift before and after the EELS acquisition. The sample drift was 0.20 nm in this case with a sample drift rate of about 0.2 nm/h. f, Sequential STEM images captured during multi-frame EELS acquisition on STO of 3 nm × 3 nm. The frame number and time are labeled on top of each image with an acquisition time of 200 s per frame. A single-unit-cell atomic structure of STO is overlapped on atomic resolution STEM images, and the green, cyan, black, purple, and orange dots denote Sr, Ti, O1, O2 (apical oxygen), and O3 (equatorial oxygen), respectively.