1. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
2. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
3. Nano Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taipei 11529, Taiwan
4. Physics Division, National Center for Theoretical Sciences, Hsinchu 30013, Taiwan
5. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
6. Department of Physics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan
7. Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan