1. Department of Physics, Kindai University, Higashi-Osaka, Osaka 577-8502, Japan
2. Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan
3. Department of Physics, Fukuoka University of Education, Munakata, Fukuoka 811-4192, Japan