1. Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581
2. Graduate School of Science and Technology, Chiba University, Chiba 263-8522
3. Department of Engineering, University of Tokyo, Tokyo 113-8656
4. Institute für Festkörperphysik Forschungszentrum, 76021 Karlsruhe 3640, Germany