Affiliation: | a Department of Physics, National Cheng Kung University, Tainan 701, Taiwan b Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan c Institute of Physics, St. Petersburg State University, Petrodvorets, St. Petersburg 198904, Russia d National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, People's Republic of China e A.F. Ioffe Physico-Technical Institute RAS, St. Petersburg 194021, Russia |