1. Experimentalphysik II, Universität Bayreuth, 95440 Bayreuth, Germany
2. Institut für Festkörperphysik, TU Darmstadt, Hochschulstrasse 6, 64289 Darmstadt,
Germany
3. Faculty of Mathematics & Computer Science, University of Warmia & Mazury in Olsztyn, 10710 Olsztyn, Poland