1. Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153, Japan; Niihama National College of Technology, 7-1 Yagumocho, Niihama, Ehime 792, Japan; and Advanced Materials Research Laboratory, Matsushita Research Institute Tokyo, Inc., 3-10-1 Higashimita, Tama-ku, Kawasaki 214, Japan