1. Center
for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan
2. National
Center for Theoretical Sciences and Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
3. Department
of Automatic Control Engineering, Feng Chia University, Taichung 40724, Taiwan
4. Department
of Electrical Engineering, University of California, Los Angeles, California 90095, United States