1. Department of Physics, Ewha Womans University, Seoul 03760, Korea
2. Department of Physics, Yokohama National University, Hodogaya-ku, Yokohama 240-8501, Japan
3. Danang University of Science and Technology, 54 Nguyen Luong Bang, Lien Chieu, TP Da Nang 550000, Vietnam
4. School of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea