1. Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of Education), School of Physics, Dalian University of Technology, Dalian 116024, People’s Republic of China
2. Department of Electrical Engineering and Information Science, Ruhr-University Bochum, D-44780 Bochum, Germany
3. Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Konkoly-Thege Miklós str. 29-33, H-1121 Budapest, Hungary