1. Department of Quantum Matter, AdSM, Hiroshima University, Higashihiroshima, Hiroshima 739-8530, Japan
2. Institute for Advanced Materials Research, Hiroshima University, Higashihiroshima, Hiroshima 739-8530, Japan
3. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
4. Laboratoire Léon Brillouin, CEA-CNRS, CEA/Saclay, 91191 Gif sur Yvette, France
5. Faculty of Science, Ibaraki University, Mito 310-8512, Japan