1. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
2. Materials and Life Science Division, J-PARC Center, Tokai, Ibaraki 319-1195, Japan
3. Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan
4. Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
5. Department of Physics and Astronomy, University of California, Riverside, CA 92521, U.S.A.