1. Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic
2. Otto-von-Guericke Universität Magdeburg, FNW/IEP/AHE, Universitätsplatz 2, 39106 Magdeburg, Germany
3. Department of Material Physics, Charles University, Ke Karlovu 5, 121 16 Prague, Czech Republic