1. State Key Laboratory of Electronic Thin Film and Integrated Devices; University of Electronic Science and Technology of China; Chengdu 610054 P. R. China
2. Key Laboratory of Ecophysics and Department of Physics; School of Science; Shihezi University; Xinjiang 832003 P. R. China
3. State Key Laboratory of New Ceramics and Fine Processing; School of Materials Science and Engineering; Tsinghua University; Beijing 100084 China
4. Department of Materials and Engineering; University of Washington; Seattle WA 98195-2120 USA