1. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
2. Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
3. Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan
4. Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan