1. Department of Applied Physics, Graduate School of Engineering, Tohoku University, Aoba-yama 05, Aoba-ku, Sendai 980-8579
2. Department of Mechanical Engineering, Faculty of Engineering and Resource Science, Akita University, Tegata Gakuen-machi 1-1, Akita 010-0852
3. Institute for Materials Research, Tohoku University, Katahira 2-2-1, Aoba-ku, Sendai 980-8577