1. Department of Materials Science and Engineering, and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea
2. Department of Physics, University of Ulsan, Ulsan 680-749, Korea
3. School of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea
4. Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea