1. Department of Physics, Chung-Ang University, Seoul 06974, Republic of Korea
2. School of Electrical Engineering, Korea University, Seoul 02841, Republic of Korea
3. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
4. WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
5. Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan