1. Department of Physics and Earth Sciences, Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
2. Department of Physics, Kobe University, Kobe 657-8501, Japan
3. Department of Materials Science and Technology, Faculty of Engineering, Niigata University, Niigata 950-2181, Japan
4. Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan