1. Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
2. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan
3. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
4. Center for Condensed Matter Sciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan