1. Center for Soft Condensed Matter Physics and Interdisciplinary Research and School of Physical Science and TechnologySoochow University Suzhou 215006 P. R. China
2. Shaanxi Province Key Laboratory of Quantum Information and Quantum Optoelectronic DevicesSchool of ScienceXi'an Jiaotong University Xi'an 710049 P. R. China
3. Institute for Frontier MaterialsDeakin University Geelong 3216 Australia
4. National Laboratory of Solid State Microstructures and Department of PhysicsNanjing University Nanjing 210093 P. R. China