1. Department of Physics, National Cheng Kung University, Tainan 701, Taiwan
2. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan
3. Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan
4. Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba, 305-0044, Ibaraki, Japan
5. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Namiki 1-1, Tsukuba, 305-0044, Ibaraki, Japan
6. Physics Division, National Center for Theoretical Sciences, Taipei 106, Taiwan
7. Center for Quantum Frontiers of Research & Technology (QFort), National Cheng Kung University, Tainan 701, Taiwan