1. Department of Physics, St. Petersburg State Maritime Technical University, Leninskii prospect 101, St. Petersburg 198262, Russia
2. Institut für Theoretische Physik der Johann Wolfgang Goethe-Universität, 60054 Frankfurt am Main, Germany
3. A.F.Ioffe Physical-Technical Institute of the Academy of Sciences of Russia, Polytechnicheskaya 26, St. Petersburg 194021, Russia