1. Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
2. Nanoelectronics Research Institute (NeRI), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
3. Core Research for Evolutional Science and Technology (CREST), Saitama 332-0012, Japan