1. Department of Applied Physics, Graduate School of Engineering, Tohoku University, Aramaki Aoba, Aoba-ku, Sendai 980-8579
2. Advanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044
3. Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047
4. Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577
5. Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195