1. Joint Research Center for Atom Technology—Angstrom Technology Partnership (JRCAT-ATP), Higashi 1-1-4, Tsukuba, Ibaraki 305, Japan;
2. Present address: Sony Corporation, Yokohama Technology Center, 134 Goudo-cho, Hodogaya-ku, Yokohama 240, Japan;
3. Materials Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439-4838;
4. JRCAT, National Institute for Advanced Interdisciplinary Research, Higashi 1-1-4, Tsukuba, Ibaraki 305, Japan;