1. Department of Physics and JILA, University of Colorado and NIST, Boulder, Colorado 80309, USA
2. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
3. Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201204, China
4. Institute of Experimental and Applied Physics, Kiel University, D-24098 Kiel, Germany
5. Ruprecht Haensel Laboratory, Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany