1. Department of Physics, University of Illinois at Urbana−Champaign, 1110 West Green Street, Urbana, Illinois 61801-3080, United States
2. Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana−Champaign, 104 South Goodwin Avenue, Urbana, Illinois 61801-2902, United States
3. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
5. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
6. Department of Physics, National Taiwan University, Taipei 10617, Taiwan