1. Center for Condensed Matter Sciences, National Taiwan University 1 , Taipei 10617, Taiwan
2. Center of Atomic Initiative for New Materials, National Taiwan University 2 , Taipei 10617, Taiwan
3. Department of Physics, National Taiwan University 3 , Taipei 10617, Taiwan
4. Department of Physics and Astronomy, Johns Hopkins University 4 , Baltimore, Maryland 21218, USA
5. Department of Physics, National Sun Yat-sen University 5 , Kaohsiung 80424, Taiwan