1. Department of Physics, Niigata University, Niigata 950-2181, Japan
2. Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
3. Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
4. Core Facility Center, Osaka University, Toyonaka, Osaka 560-0043, Japan