1. Department of Physics, University of California, Berkeley, CA 94720, USA.
2. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
3. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
4. Center for Spintronics, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea.
5. Department of Physics, University of California, Davis, CA 95616, USA.
6. Department of Physics and State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China.
7. Korea Research Institute of Standards and Science, Yuseong, Daejeon 305-340, Republic of Korea.