1. Department of Experimental Physics, Faculty of Mathematics Physics and Informatics, Comenius University, Mlynská dolina, 842 48 Bratislava, Slovakia and Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Račianska 75, 831 02 Bratislava, Slovakia
2. Department of Experimental Physics, Faculty of Mathematics Physics and Informatics, Comenius University, Mlynská dolina, 842 48 Bratislava, Slovakia
3. ICAMS, Ruhr-Universität Bochum, 44780 Bochum, Germany and Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden