1. Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA
2. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
3. Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
4. Department of Engineering and Management of Advanced Technology, Chang Jung Christian University, Zhangda Rd, Guiren District, Tainan 711, Taiwan
5. Department of Electrical and Computer Engineering, Northwestern University, Evanston, Illinois 60208, USA