*Bowen Sheng1,2, Xiantong Zheng1, Ping Wang1, Gordon Schmidt2, Frank Bertram2, Peter Veit2, Juergen Blaesing2, Zhaoying Chen1, Andre Strittmatter2, Juergen Christen2, Bo Shen1, Xinqiang Wang1
(1. State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, 100871 Beijing, China, 2. Institute of Physics, Otto-von-Guericke-University Magdeburg, 39106 Magdeburg, Germany)