1. Quantum Beam Science
Center, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Naka-gun, Tokai 319-1195, Japan
2. Department
of Applied Chemistry, Meiji University, 1-1-1 Higashi-Mita, Tama-ku, Kawasaki 214-8571, Japan
3. Institute for Solid
State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
4. Quantum Condensed Matter
Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, United States