1. Max-Planck-Institute for Chemical Physics of Solids, D-01187 Dresden, Germany
2. Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581
3. Graduate School of Science, Hokkaido University, Sapporo 060-0810
4. Graduate School of Science, Osaka University, Toyonaka 560-0043
5. Institute of Low Temperature and Structure Research, Polish Academy of Science, 50-950 Wroclaw 2 B. D. Box 937, Poland