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SUMMARY:Where have all the speckles gone? Incoherence and decoherence in f
 luctuation electron microscopy - Mike Treacy\, Department of Physics\, Ari
 zona State University
DTSTART:20170119T160000Z
DTEND:20170119T170000Z
UID:TALK69612@talks.cam.ac.uk
CONTACT:Stephen Walley
DESCRIPTION: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 unde
 rstood. What was baffling\, however\, was the persistence of an anomalousl
 y high incoherence even when the illumination was highly coherent. The eff
 ect is not subtle: speckle intensities can be suppressed by up to two orde
 rs 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 dama
 ge\, and associated long-range strain fields contribute mainly to the effe
 ct. Atoms under a high energy electron beam move much more than is general
 ly acknowledged: popcorn under a blowtorch provides a dramatic analogy.\n\
 nThis talk will explain the basics of FEM\, and how it is used to probe am
 orphous 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.\n
LOCATION:Mott Seminar Room\, Cavendish Laboratory
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