1. Graduate School of Science and Technology, Niigata University, Igarashi, Niigata-shi, Niigata 950-21
2. Department of Materials Science and Processing, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita-si, Osaka 565
3. Department of Physics, Faculty of Science, Niigata University, Igarashi, Niigata-shi, Niigata 950-21