1. Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195
2. Department of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro-ku, Tokyo 152-8551
3. Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581
4. Department of Physics, Sophia University, Kioi-cho, Chiyoda-ku, Tokyo 102-8554