1. Institute of Electrical Engineering SAS, Dúbravská cesta 9, 84104 Bratislava, Slovakia
2. Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
3. Joint Institute for Nuclear Research, Dubna, Moscow Region, Russia
4. Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynská Dolina F1, 842 48 Bratislava, Slovakia