1. Akhiezer Institute for Theoretical Physics, NSC KIPT, Akademichna 1, 61108 Kharkiv, Ukraine
2. Faculty of Physics, Adam Mickiewicz University, Uniwersytetu Poznańskiego 2, PL-61-614 Poznań, Poland
3. Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudingerweg 9, D-55099 Mainz, Germany