1. Department
of Material Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China
2. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
3. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Department of Physics, Durham University, Durham DH1 3LE, United Kingdom
5. Institute of Physics, Academia Sinica, Taipei 155, Taiwan