1. Physics Department, Graduate School of Science, Tohoku University, Sendai 980-8545
2. Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Naka, Ibaraki 319-1195
3. Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012
4. Center for Low Temperature Science, Tohoku University, Sendai 980-8545