Affiliation: | RIKEN (The Institute of Physical and Chemical Research), 2-I Hirosawa, Wako, Saitama 351-01, Japan Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan Department of Applied Physics, Tokyo Institute of Technology, 2-12-1, Oh-okayama, Meguro, Tokyo 152, Japan |