1. Department of Applied Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
2. Department of Physics, Hokkaido University, Sapporo 060-0810, Japan
3. Theoretical Division and CNLS, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
4. Department of Physics, University of Virginia, Charlottesville, VA 22904, U.S.A.
5. Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, U.S.A.
6. Quantum Condensed Matter Division and Shull-Wollan Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.