1. Department of Physics, University of Konstanz, 78457 Konstanz, Germany
2. Max Planck Institute for Intelligent Systems, 70569 Stuttgart, Germany
3. Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
4. Institute of Condensed Matter Physics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
5. Institute of Physics, Johannes Gutenberg University of Mainz, 55099 Mainz, Germany
6. Laboratory for Mesoscopic Systems, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
7. Advanced Light Source, LBNL, Berkeley, California 94720, USA
8. Department of Solid State Sciences, Ghent University, 9000 Ghent, Belgium