1. Department of Physics, University of California, Santa Cruz, California 95064, United States
2. Department of Physics, University of California, Berkeley, California 94720, United States
3. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
4. Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
5. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
6. Kavli Energy NanoSciences Institute at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States