1. Department of Physics & Center for Quantum Coherence in Condensed Matter, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea
2. Korea Research Institute of Standards and Science, Daejeon 34113, Republic of Korea
3. Department of Physics, Jeonbuk National University, Jeonju 54896, Republic of Korea
4. Department of Nano Science, University of Science and Technology, Daejeon 34113, Republic of Korea