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SUMMARY:New Detectors for measuring probability current flow of an atomic-
 scale electron beam  - David A. Muller\, School of Applied and Engineering
  Physics\, Cornell University\, U.S.A.
DTSTART:20170127T150000Z
DTEND:20170127T160000Z
UID:TALK70360@talks.cam.ac.uk
CONTACT:Duncan Johnstone
DESCRIPTION:Complete information about the scattering potential of a sampl
 e is in principle encoded in the distribution of scattered electrons from 
 a localized beam propagating through it. A new generation of high-speed\, 
 momentum-resolved electron microscope detectors brings us closer to realiz
 ing this general goal and in doing so enables new imaging modes spanning s
 ub-Angstrom to multi-micron length scales.  Most of these new detectors we
 re adapted from x-ray sensors where fluxes are low.  Accordingly\, they pe
 rform well at the very low count rates appropriate for biological\, but ar
 e limited to count rates of less than 0.1 pA/pixel by the transit time in 
 the silicon sensor.  Our electron-microscope pixel array detector (EMPAD) 
 developed at Cornell is designed to overcome this fundamental weakness of 
 pulse counting with a hybrid architecture that preserves the single electr
 on sensitivity but also remains linear and unsaturated when exposed to the
  full electron beam. This enables not only measurements of probability cur
 rent flow that can be used to map electric and magnetic fields in thin sam
 ples\, but also the orbital angular momentum of an electron beam and resul
 tant torque transfer.  The detector has proved useful for a wide range of 
 quantitative applications including the imaging of strain fields in 2D mat
 erials\, polarization vortices in ferroelectrics\, and robust demonstratio
 ns of super-resolution imaging by ptychography.  
LOCATION:Goldsmiths 1\, Lecture room\, Dept. of Materials Science and Meta
 llurgy
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