1. Department of Applied Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555
2. The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581
3. Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta, Midori, Yokohama 226-8503