1. Department of PhysicsFaculty of Science and TechnologyTokyo University of Science 2641 Yamazaki Noda Chiba 278‐8510 Japan
2. Institute for Solid State PhysicsThe University of Tokyo 5‐1‐5 Kashiwanoha Kashiwa Chiba 277‐8581 Japan
3. Department of Materials Science and EngineeringTokyo Institute of Technology 2‐12‐1 Ookayama Meguro‐ku Tokyo 152‐8552 Japan