1. University of California, Division of Physical Sciences, College of Letters and Science , Los Angeles, California , USA
2. Massachusetts Institute of Technology, Department of Materials Science and Engineering , Cambridge, Massachusetts , USA
3. Current affiliation: Forschungszentrum Jülich, Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons , Jülich , Germany
4. Brookhaven National Laboratory, Condensed Matter Physics and Materials Science Division , Upton, New York , USA