1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Key Laboratory of Artificial Structures
and Quantum Control (Ministry of Education), Department of Physics
and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
4. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China
5. Department
of Physics, Harbin Institute of Technology, Harbin 150001, China
6. Department of Electrical Engineering and Computer Science, University of California at Berkeley, Berkeley, California 94720, United States
7. Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States