1. Department of Physics, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
2. Advanced Research Institute of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
3. Laboratoire Structures, Propriétés et Modélisation des Solides, Ecole Centrale Paris, 92295 Châtenay-Malabry Cedex, France
4. Laboratoire Léon Brillouin, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
5. Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226-8503