1. Department
of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, R.O.C.
2. Department
of Physics, National Tsing Hua University, Hsinchu, Taiwan 30043, R.O.C.
3. National
Nano Device Laboratories, National Applied Research Laboratories, 26, Prosperity Road I, Hsinchu, Taiwan 30078, R.O.C.
4. Department
of Physics, National Taiwan University, Taipei, Taiwan 10617, R.O.C.
5. Physics
Division, National Center for Theoretical Sciences, Hsinchu, Taiwan 30078, R.O.C.