1. Materials Sciences Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
2. Institute of Quantum Beam Science, Ibaraki University, Tokai, Ibaraki 319-1106, Japan
3. Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan
4. Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393, U.S.A.