Abstract

Fluctuation electron microscopy (FEM) examines the statistics of speckle arising in dark-field scattering from disordered materials. The role of illumination spatial incoherence in coherent speckle is well understood. What was baffling, however, was the persistence of an anomalously high incoherence even when the illumination was highly coherent. The effect is not subtle: speckle intensities can be suppressed by up to two orders of magnitude. The likely culprit is displacement decoherence, which is a type of enhanced Debye-Waller effect originating from quasi-elastic and inelastic momentum exchanges between the beam and the material. Beam damage, and associated long-range strain fields contribute mainly to the effect. Atoms under a high energy electron beam move much more than is generally acknowledged: popcorn under a blowtorch provides a dramatic analogy.

This talk will explain the basics of FEM , and how it is used to probe amorphous materials. The role of incoherence, and its role as an adjustable parameter, will be explained. The physics of speckle, and our inference of displacement decoherence, will be outlined.