1. IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA.
2. Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany.
3. Max Planck Institute for Solid State Research, Heisenbergstasse 1, 70569 Stuttgart, Germany.
4. Department of Chemistry, Ludwig-Maximilians-Universität München, Butenandtstrasse 5-13, 81377 München, Germany.