1. Center for Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.
2. Physics Department, Brookhaven National Laboratory, Upton, NY 11973, U.S.A.
3. Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
4. Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada