1. IBM Research, Zurich Research Laboratory, 8803 Rüschlikon, Switzerland
2. Debye Institute for Nanomaterials Science, University of Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands
3. Institute of Experimental and Applied Physics, University of Regensburg, 93053 Regensburg, Germany
4. Department of Physics, Tampere University of Technology, 33720 Tampere, Finland