1. Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
2. NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan
3. Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
4. National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
5. JST, PRESTO, Kawaguchi, Saitama 332-0012, Japan