1. Department of Physics, Sogang University, Seoul 04107, Republic of Korea
2. Department of Physics and Astronomy, Sejong University, Seoul 05006, Republic of Korea
3. National Nanofab Center, Daejeon 34141, Republic of Korea
4. Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
5. Department of Physics, Kyungpook National University, Daegu 41566, Republic of Korea
6. Institute of Physics, Johannes Gutenberg University Mainz, Mainz 55128, Germany