1. Department of Electrical Engineering and BioscienceWaseda UniversityShinjuku‐kuTokyo 169‐8555Japan
2. Research Institute for Materials Science and Technology, Waseda UniversityShinjuku‐kuTokyo 169‐8555Japan
3. Advanced Technology Research and Development Center, Mitsubishi Electric CorporationAmagasakiHyogo 661‐8661Japan