1. Materials Sciences Research Center, Japan Atomic Energy Agency, Sayo, Hyogo 679-5148, Japan
2. Department of Physics, Faculty of Science, Kyoto Sangyo University, Kyoto 603-8555, Japan
3. Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313, Japan
4. Graduate School of Engineering and Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
5. Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan