1. State Key Laboratory for Mechanical Behavior of Materials and International Center of Dielectric Research (ICDR); Xi'an Jiaotong University; Xi'an 710049 China
2. International Center of Dielectric Research (ICDR); The School of Electronic and Information Engineering; Xi'an Jiaotong University; Xi'an 710049 China
3. Peter Grünberg Institute and Ernst Ruska Center for Microscopy and Spectroscopy with Electrons; Forschungszentrum Jülich; D-52425 Jülich Germany
4. Max Planck Institute of Microstructure Physics; Weinberg 2 D-06120 Halle Germany
5. Max Planck Institute for Solid State Research; Heisenbergstr. 1 D-70569 Stuttgart Germany
6. Department of Physics; University of Warwick; Coventry CV47AL UK