1. Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
2. Kyushu Hitachi Maxell, Ltd, Tagawa-gun, Fukuoka 822-1296, Japan
3. Takasaki Advanced Radiation Research Institute, Japan Atomic Energy Agency, Takasaki, Gunma 370-1292, Japan
4. Department of Electrical and Computer Engineering, University of California at Davis, Davis, California 95616 USA