1. Condensed Matter Theory Laboratory, RIKEN CPR, Wako, Saitama 351-0198, Japan
2. Department of Physics, University of California, Berkeley, CA 94720, U.S.A.
3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A.
4. Department of Physics, University of Tokyo, Hongo, Tokyo 113-0033, Japan