1. University Education Center, Ibaraki University, Mito 310-8512, Japan
2. Department of Electronics and Computer Science, Tokyo University of Science, Yamaguchi, Sanyo Onoda, Yamaguchi 756-0884, Japan
3. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
4. Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashihiroshima, Hiroshima 739-8530, Japan