1. Department of Physics; Tohoku University; Sendai 980-8578 Japan
2. WPI Research Center; Advanced Institute for Materials Research; Tohoku University; Sendai 980-8577 Japan
3. Toyota Central R&D Labs.; Inc. Nagakute; Aichi 480-1192 Japan
4. JST, PRESTO; 4-1-8 Honcho Kawaguchi Saitama 332-0012 Japan