1. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics; Chinese Academy of Sciences; 1295 Dingxi Road Shanghai 200050 China
2. University of Chinese Academy of Sciences; Beijing 100049 China
3. Materials Science and Engineering Department; University of Washington; Seattle Washington 98195-2120 USA
4. Department of Physics; University of Michigan; Ann Arbor Michigan 48109 USA