1. Department of Applied Physics, Osaka University, 2-1, Yamadaoka,
Suita, Osaka 565-0871, Japan
2. PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
3. Department of Electrical and Electronic Engineering, Kobe University, 1-1, Rokkodai-cho, Nada, Kobe 657-8501, Japan