1. Graduate School of Engineering and Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
2. Department of Physics, Faculty of Science, Shinshu University, Matsumoto, Nagano 390-8621, Japan
3. Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
4. Center for Science and Technology under Extreme Conditions, Osaka University, Toyonaka, Osaka 560-8531, Japan
5. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
6. Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan