1. Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
2. Low-Temperature Physics Group, Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Central 2, Umezono, Tsukuba, Ibaraki 305-8568, Japan
3. Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan