1. Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397
2. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581
3. Department of Physics, Tohoku University, Sendai 980-8578
4. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195
5. Department of Physics, University of California, Riverside, CA 92521-0413, U.S.A.
6. Faculty of Integrated Arts and Sciences, The University of Tokushima, Tokushima 770-8502