1. Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8503, Japan
2. Frontier Research Center, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
3. Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
4. Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan