[1] Songshan Lake Mat Lab, Dongguan 523808, Guangdong, Peoples R China
[2] Chinese Acad Sci, Inst Phys, Beijing 100190, Peoples R China
[3] Karlsruhe Inst Technol KIT, Inst Nanotechnol, Hermann von Helmholtz Pl 1, D-76344 Eggenstein Leopoldshafen, Germany