1. Physics Department and Electron Spin Science Center, Pohang University of Science and Technology, San 31, Hyoja-Dong, Namku, Kyungbuk 790-784, Republic of Korea, Nanodevice Research Center, Korea Institute of Science and Technology, P.O. Box 131, Chengryang, Seoul 130-650, Korea, and Department of Chemistry, Pohang University of Science and Technology, Pohang, 790-784, Republic of Korea