1. State Key Laboratory of Tribology; Department of Mechanical Engineering; Tsinghua University; Beijing 100084 China
2. Key Laboratory of Space Applied Physics and Chemistry; Ministry of Education and Department of Applied Physics; School of Science; Northwestern Polytechnical University; Xi'an 710072 China
3. State Key Laboratory of New Ceramics and Fine Processing; School of Materials Science and Engineering; Tsinghua University; Beijing 100084 China
4. Ames Laboratory and Department of Physics and Astronomy; Iowa State University; Ames IA 50011 USA
5. Institute of Electronic Structure and Lasers (IESL), FORTH; 71110 Heraklion Crete Greece