1. Department of Physics; National Cheng Kung University; Tainan 70101 Taiwan
2. School of Materials Science and Engineering and Key Laboratory of Low Dimensional Materials & Application Technology of Ministry of Education; Xiangtan University; Xiangtan Hunan 411105 China
3. Department of Materials Science and Engineering; National Chiao Tung University; Hsinchu 30010 Taiwan
4. Department of Physics; Durham University; Durham DH1 3LE UK
5. Department of Mechanical Engineering; University of Washington; Seattle WA 98195-2600 USA
6. Institute of Physics; Academia Sinica; Taipei 11529 Taiwan