1. Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
2. Department of Physics, Niigata University, Niigata 950-2181, Japan
3. Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400-005, India
4. Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan