1. Institute of Fundamental and Frontier SciencesUniversity of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. Department of Materials Science and NanoengineeringRice University Houston TX 77005 USA
3. Department of Materials Science and EngineeringNational Tsing Hua University Hsinchu 30013 Taiwan ROC
4. Department of PhysicsNational Sun Yat‐Sen University Kaohsiung 80424 Taiwan ROC
5. Frontier Research Center on Fundamental and Applied Sciences of MattersNational Tsing Hua University Hsinchu 30013 Taiwan ROC