1. BK21 Physics Research Division and Center for Nanotubes and Nanostructured Composites (CNNC), Sungkyunkwan University, Suwon 440-746, Republic of Korea,
2. Department of Physics, Tohoku University, Sendai, 980-8578, Japan,
3. Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea