1. Department of Physics, Nagoya University, Nagoya 464-8602, Japan
2. Research and Education Institute for Semiconductors and Informatics, Kumamoto University, Kumamoto 860-8555, Japan
3. Department of Applied Chemistry, Keio University, Yokohama 223-8522, Japan
4. School of Materials and Chemical Technology, Tokyo Institute of Technology, Yokohama 226-8502, Japan