1. World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1‐1 Namiki, Tsukuba, Ibaraki 305‐0044, Japan
2. Faculty of Science and Engineering, Waseda University, 3‐4‐1 Okubo, Shinjuku, Tokyo 169–8555, Japan, http://www.yamauchi‐labo.com/index.php