1. RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, 351-0198 Saitama, Japan
2. Okinawa Institute of Science and Technology, 1919-1 Tancha, Onna-son, 904-0495 Okinawa, Japan
3. Department of Physics, University of Konstanz, Universitätstrasse 10, 78464 Konstanz, Germany
4. International College of Semiconductor Technology, National Chiao Tung University, Hsinchu 300, Taiwan