1. State Key Laboratory of Precision Measurement Technology and Instruments, School of Precision Instrument and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China
2. Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, Department of Physics, Center for Joint Quantum Studies, Tianjin University, Tianjin 300350, China
3. Institute of Experimental Physics, Johannes Kepler University Linz, A-4040 Linz, Austria