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. Institute
of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
5. School
of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
6. Department
of Physics, National Taiwan University, Taipei 10617, Taiwan