1. Department of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, Colorado 80309, United States
2. Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
3. Department of Electrical Engineering, Stanford University, Stanford, California 94305, United States
4. Ludwig-Maximilians-Universität and Center for Nanoscience, 80539 München, Germany