1. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
2. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
3. Department of Physics, Niigata University, Niigata 950-2181, Japan
4. Center for Advanced High Magnetic Field Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
5. Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8531, Japan
6. Research Center for Development of Far-Infrared Region, University of Fukui, Fukui 910-8507, Japan