1. School of Liberal Arts, Korea University of Technology and Education, Cheonan 31253, South Korea
2. Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA
3. Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
4. Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA