1. Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons Forschungszentrum Jülich GmbH Jülich 52428 Germany
2. Physikalisches Institut and Würzburg-Dresden Cluster of Excellence ct.qmat Universität Würzburg Würzburg 97074 Germany
3. The School of Electronic and Information Engineering Xi'an Jiaotong University Xi'an 710049 China