1. Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397
2. Advanced Science Research Center, Japan Atomic Energy Research Institute, Ibaraki 332-0012
3. DRFMC, SPSMS, CEA-Grenoble, 38054 Grenoble, France
4. Laboratoire Léon Brillouin, CEA-CNRS, CEA/Saclay, 91191 Gif-sur-Yvette, France