1. Department of Physics, Hankuk University of Foreign Studies, Yongin, Gyeonggi 449-791, Korea
2. Department of Electrophysics, Kwangwoon University, Seoul 139-701, Korea
3. Division of Quantum Phases and Devices, Department of Physics, Konkuk University, Seoul 143-701, Korea
4. Korea Research Institute of Standards and Science, Daejeon 305-340, Korea
5. Department of Physics and Division of Energy Systems Research, Ajou University, Suwon, Gyeonggi 443-749 Korea