1. Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560
2. On leave from Institute of Spectroscopy, Russian Academy of Science, Troitsk, Moscow Region
3. CREST, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332
4. Research Group for Quantum Condensed Matter Systems, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11