1. Department of Quantum Engineering, Nagoya University, Furocho, Chikusa-ku, Nagoya 464–8603, Japan
2. National Institute for Materials Science, 1–2–1 Sengen, Tsukuba, Ibaraki 305–0047, Japan
3. Inorganic Materials Research Laboratory, Tosoh Corporation, 4560 Kaisei-cho, Shunan, Yamaguchi 746-8501, Japan