1. Department
of Physics, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan
2. Department
of Physics, University of Illinois at Urbana−Champaign, 1110 West Green Street, Urbana, Illinois 61801-3080, United States
3. Frederick
Seitz Materials Research Laboratory, University of Illinois at Urbana−Champaign, 104 South Goodwin Avenue, Urbana, Illinois 61801-2902, United States