1. Solid State Physics Dept., Faculty of Physics and Nuclear Technique, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Kraków, Poland,
2. Institute of Physics and Informatics, Pedagogical University, Podchorążych 2, 30-084 Kraków, Poland,
3. Angewandte Physik, J. Kepler Universität, A-4040 Linz, Austria,
4. Van der Waals-Zeeman Laboratorium, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands