1. Department of Physics, 2 Science Drive 3, National University of Singapore, Singapore 117542, Singapore, Institute of High Performance Computing, 1 Fusionopolis Way, 16-16 Connexis, Singapore 138632, Singapore, NanoCore, 5A Engineering Drive 4, National University of Singapore, Singapore 117576, Singapore, and Department of Physics, Soochow University, Suzhou 215006, China