1. Center for Vacuum Technology, Korea Research Institute of Standards and Science, Daejeon 34113, South Korea
2. Department of Physics, Chungnam National University, Daejeon 34134, South Korea
3. Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, South Korea
4. Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 34141, South Korea