1. Graduate Group in Applied Science and Technology, University of California, Berkeley, CA 94720, USA.
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3. Department of Physics, University of California, Berkeley, CA 94720, USA.
4. Institute of Science and Technology Austria, 3400 Klosterneuburg, Austria.
5. School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China.
6. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
7. Electronics and Photonics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568, Japan.
8. Department of Physics, Harvard University, Cambridge, MA 02138, USA.