1. Institute of Physics, Czech Academy of Sciences, v.v.i., Cukrovarnická 10, CZ-162 00, Prague 6, Czech Republic
2. Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 12116 Prague 2, Czech Republic
3. Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravská cesta 9, 841 04 Bratislava, Slovak Republic