1. Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan
2. Nano-Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taipei 11529, Taiwan
3. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
4. Department of Physics, National Taiwan University, Taipei 10617, Taiwan
5. Department of Physics, Lancaster University, LA1 4YB Lancaster, United Kingdom