1. Department of Physics and Frederick Seitz Materials Research Laboratory, University of Illinois Urbana−Champaign; Urbana, Illinois 61801, United States
2. Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan
3. Physics Division, National Center for Theoretical Sciences, Taipei 10617, Taiwan
4. Department of Physics, National Taiwan University, Taipei 10617, Taiwan