1. Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113
2. The Institute of Physical and Chemical Research (RIKEN), Wako 351-01
3. Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
4. Department of Physics, Brookhaven National Laboratory, Upton, NY 11973, U.S.A.