1. Department of Physics, Dong-A University, 840 Hadan-Dong, Saha-Gu, Pusan 604-714, Korea
2. Korea Research Institute of Standards and Science, PO Box 102, Yusong, Taejon 305-600, Korea
3. Department of Physics, Dongguk University, Pil-Dong 3-26, Chung-Gu, Seoul 100-715, Korea
4. Electronics and Telecommunications Research Institute, PO Box 106, Yusong, Taejon 305-600, Korea