High-resolution 4D STEM dataset of SrTiO3 along the [1 0 0] axis at high magnification

This dataset can be used to test various analysis methods for high-resolution 4D STEM, including phase contrast methods such as ptychography. Scan and diffraction coordinates have been calibrated. The high scan magnification allows to identify individual atoms and easily distinguish them from reconstruction artifacts.

Data was acquired at a probe-corrected FEI Titan 80-300 STEM operated at 300 kV. The microscope was equipped with a Medipix Merlin for EM detector operated at an acquisition rate for individual diffraction patterns of 1 kHz. The scan size was 128 x 128 scan points and the recorded diffraction patterns had a dimension of 256 x 256 pixel.

The convergence angle of the incident probe was measured with a polycrystalline gold specimen. Employing parallel illumination first, the (111) gold diffraction ring was used to calibrate the diffraction space assuming a lattice constant of gold of 0.4083 nm. With the known wavelength the convergence semi-angle was determined to 22.1 mrad from a Ronchigram recorded in the same STEM setting as used in the actual experiment. The convergence semi-angle in pixel was determined from the size of the primary beam on the detector.

The rotation and handedness of the detector coordinate system with respect to the scan axes was determined by minimizing the curl of the first moment vector field and making sure that the divergence of the field is negative at atom positions. Note that, in theory, the curl of purely electrostatic fields should vanish. The pixel size in the scan dimension of 12.7 pm was taken from the STEM control software during live processing and verified by comparison with the known lattice constant of SrTiO₃. The residual scan distortion, that is, the translation of the diffraction pattern as a whole during scanning, was not compensated for since it turned out to be negligible at the atomic-resolution STEM magnifications used in this analysis.

The sample thickness was approximately 25 nm, determined by comparing the PACBED with simulation.

Parameters

Scan pixel size: 12.7 pm

Center y: 126 px

Center x: 123 px

Convergence semi-angle: 22.13 mrad, 15.5 px

Thickness: approx. 25 nm

Affine transformation of the direction of scan coordinates to detector coordinates using https://github.com/LiberTEM/LiberTEM/blob/master/src/libertem/corrections/coordinates.py:

transformation = rotate_deg(88) @ flip_y()
det_sy, det_sx = ((scan_sy, scan_sx) @ transformation)

See the included notebook for an exemplary analysis. See https://arxiv.org/abs/2106.13457 for more details.