1. Niigata Institute of Technology, 1719 Fujihashi, Kashiwazaki 945-11
2. Department of Physics, Faculty of Science, Niigata University, 2-8050 Ikarashi, Niigata 950-21
3. Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980