1. Wide-Gap Semiconductor GroupNational Institute for Materials Science Tsukuba 305-0044 Japan
2. High Magnetic Field Measurement GroupNational Institute for Materials Science Tsukuba 305-0003 Japan
3. Department of Condensed Matter PhysicsGraduate School of ScienceHokkaido University Sapporo 060-0810 Japan
4. Department of Electric Engineering and ElectronicsGraduate School of EngineeringKogakuin University Hachioji Tokyo 192-0015 Japan