1. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China
2. Department of Physics, Tam Kang University, New Taipei 251, Taiwan, Republic of China
3. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China
4. Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China