1. Department of Physics, Chungbuk National University, Cheongju 28644, South Korea
2. Research Institute for Nanoscale Science and Technology, Chungbuk National University, Cheongju 28644, South Korea
3. Department of Physics, UNIST, Ulsan 44919, South Korea
4. Functional Materials Department, Korea Institute of Materials Science, Changwon 51508, South Korea