1. Department of Physics and Optical Science, The University of North Carolina at Charlotte, Charlotte, North Carolina 28223, USA
2. Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan
3. Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan
4. Institute of Physics and Center for Emergent Functional Matter Science, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan
5. National Synchrotron Radiation Research Center (NSRRC), Hsinchu 30076, Taiwan