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