1. Department of Electrical Engineering and Computer Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
2. School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
3. Department of Physics, Technische Universität München, Arcisstrasse 21, München 80333, Germany
4. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
5. Research Center for Applied Science, Academia Sinica, Taipei 11529, Taiwan
6. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139