1. Department of Solid-State Sciences, Ghent University, Krijgslaan 281/S1, 9000 Ghent, Belgium
2. Instituut voor Kern- en Stralingsfysica, KU Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium
3. Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium
4. IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, USA
5. Department of Physics, University of Cape Town, Rondebosch 7700, South Africa