1. Department of Applied Physics, College of Applied Science, Kyung Hee University, Suwon 446-701, Republic of Korea
2. Department of Materials Science and Engineering, and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
3. Department of Physics and EHSRC, University of Ulsan, Ulsan 680-749, Republic of Korea