1. Center for Microscopy-Microanalysis and Information Processing, National University for Science and Technology Politehnica of Bucharest, 060042 Bucharest, Romania
2. Faculty of Electrical Engineering, National University for Science and Technology Politehnica of Bucharest, 060042 Bucharest, Romania
3. Department of Science and Engineering of Oxide Materials and Nanomaterials, Faculty of Chemical Engineering and Biotechnologies, National University for Science and Technology Politehnica of Bucharest, 060042 Bucharest, Romania
4. State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China