1. Department of Applied Physics, Graduate School of Engineering, Tohoku University, 6-6-05 Aoba-yama, Aoba-ku, Sendai 980-8579
2. Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577
3. Research Center for Low Temperature Physics, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro-ku, Tokyo 152-8551