1. Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea
2. KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Korea
3. National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
4. Department of Physics and Applied Physics, Harvard University, Cambridge, Massachusetts 02139, United States