1. Department of Physics, Pohang University of Science and Technology, San 31 Hyoja-Dong, Namku, Pohang, Kyungbuk 790-784, Korea (D.E.K., K.J.Y.); and Research Institute of Industrial Science and Technology, San 32 Hyoja-Dong, Namku, Pohang, Kyungbuk 790-784, Korea (H.K.P., K.J.O., D.W.K.)