1. Nanoelectronics Research Institute (NeRI), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568
2. Nanotechnology Research Institute, AIST, Tsukuba, Ibaraki 305-8568
3. CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012
4. Department of Applied Physics, Nagoya University, Nagoya 464-8603