1. State
Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, P. R. China
2. Bruker (Beijing) Scientific Technology Co., Ltd., Beijing 100081, P. R. China
3. Institute
of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China
4. Department
of Materials Science and Engineering, The University of Arizona, Tucson, Arizona 85721, United States