1. Department of Physics, University of California, Berkeley, California 94720, United States
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Kavli Energy NanoSciences Institute at the University of California, Berkeley, California 94720, United States
4. Physics Department E20, Technical University of Munich, James-Franck-Straße 1, D-85748 Garching, Germany
5. Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
6. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
7. Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BB, United Kingdom