1. Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195
2. Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577
3. Institute for Material Research, Tohoku University, Katahira, Sendai 980-8577
4. Department of Physics, Brookhaven National Laboratory, Upton, NY 11973-5000, U.S.A.
5. Department of Physics, University of Toronto, Toronto, Ontario, Canada M5S 1A7
6. Department of Physics, University of California, Berkeley, Berkeley, CA 94720-7300, U.S.A.