1. Department of Physics, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan
2. Researh Center for Low Temperature and Materials Sciences, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
3. NTT-AT, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan
4. NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan