1. Department of Physics, University of California, Berkeley, California 94720-7300, USA
2. Institut für Physik, Johannes Gutenberg Universität-Mainz, 55128 Mainz, Germany
3. DWI-Leibniz Institute for Interactive Materials, Aachen 52074, Germany
4. Department of Chemistry, University of Houston, Houston, Texas 77204, USA
5. Helmholtz Institute Mainz, Johannes Gutenberg University, 55099 Mainz, Germany