David A. Muller, Department of Applied and Engineering Physics, Cornell University, Ithaca, NY, will present seminar. Host TBA.

Seminar Title: A new Electron Microscope Probability Current Detector for Imaging Fields, Order Parameters, and Structure at Record Resolution

Abstract:

Orbital angular momentum and torque transfer play central roles in a wide range of magnetic textures and some analogous topological structures can also be found in ferroelectrics. While many techniques have been developed for imaging magnetic order, direct visualization of polar order is challenging and requires picometer sensitivity with Angstrom resolution, thus limiting the field of view. Here we show how previously hidden order parameters such the toroidal moment, the natural order parameter of ferrotoroidal phases, can be imaged using the transfer of orbital angular momentum from polarization vortices to a focused, high-energy electron beam. Its origin can be traced back to the torque transfer to the scattered beam, recovered from the measured probability current flow. Our ability to measure orbital angular momentum without having to compromise the shape or precision of the electron beam, and over arbitrarily large fields of view is made possible by a new design of a high-speed, momentum-resolved electron microscope pixel array detector (EMPAD) [1] whose high dynamic range allows us to record the complete angular distribution of transmitted electrons. 

Phasing of the full position-momentum space by Ptychography also numerically retrieves the probe and object functions with a spatial resolution limited not by the numerical aperture of the lens, but by the largest scattering angle collected. This has allowed us to reach world-record spatial resolutions, more than doubling the resolution of our  instrument [2]. The EMPAD is also proving valuable for imaging magnetic Skyrmion phases, strain profiles in fuel cell nanoparticle catalysts and 2D materials, where its high dynamic range allows us to detect small signals that would normally be swamped by large backgrounds.

[1] MW Tate, et al, Microscopy and Microanalysis 22 (2016), p. 237.

[2 ] Yi Jiang, et al., Nature 559, 343–349 (2018)

Poster available.

.