1. Department of Materials Science & Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States
2. Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States
3. Laboratory for Solid State Physics, ETH Zürich, Zürich CH-8093, Switzerland
4. QuTech and Kavli Institute of NanoScience, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands