1. Max Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
2. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
3. Department of Physics, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan
4. Institute of Physics, Goethe-University Frankfurt, 60438 Frankfurt am Main, Germany