1. Condensed Matter Science Division, Japan Atomic Energy Agency, Sayo, Hyogo 679-5148, Japan
2. Department of Physics, Kyoto Sangyo University, Kyoto 603-8555, Japan
3. Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
4. Graduate School of Science and Center for Low Temperature Science, Tohoku University, Sendai 980-8577, Japan