1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. National Institute for Materials Science, Tsukuba, Ibaraki 305-0003, Japan
3. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.
4. Division of Materials Research, National Science Foundation, Alexandria, Virginia 22314, U.S.A.