1. Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA
2. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
3. Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea
4. Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43212, USA
5. Department of Physics, Ewha Womans University, Seoul 120-750, South Korea