1. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
2. Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China
3. Department of Physics, Uppsala University, Box 530, S-75121 Uppsala, Sweden
4. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
5. Department of Chemistry and Biochemistry, University of California at Santa Cruz, Santa Cruz, California 95064, USA