1. IBM Research-Zürich, Säumerstrasse 4, 8803 Rüschlikon, Switzerland
2. Electron Microscopy Center, EMPA, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
3. Department of Electrical and Information Technology, Lund University, Box 118, SE-221 00 Lund, Sweden