1. Department of Physics, Kyushu University, Ropponmatsu, Chuo-ku, Fukuoka 810-8560
2. Institute of Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8582
3. Institute for Materials Research, Tohoku University, Sendai 980-8577
4. CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012