1. Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
2. Dresden High Magnetic Field Laboratory, Helmholtz-Zentrum Dresden-Rossendorf, D-01314 Dresden, Germany
3. Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany
4. Department of Electrical Engineering and Electronics, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan