1. Faculty of Cross-Cultural Studies, Kobe University, Tsurukabuto, Nada, Kobe 657, Japan
2. Graduate School of Science and Technology, Kobe University, Rokkodai, Kobe 657, Japan
3. Department of Physics, Kobe University, Rokkodai, Kobe 657, Japan
4. Physics Department, Graduate School of Science, Tohoku University, Sendai 980-77, Japan