1. “Mekhanobr - Tekhnika” Corporation, 21-st liniya, 8a, 199026 St. Petersburg, Russia
2. Department of Physics, Lomonosov Moscow State University, 119899 Moscow, Russia
3. Arbeitsgruppe Nichtlineare Dynamik der Max-Plank-Gesellschaft an der Universita¨t Potsdam, Am Neuen Palais 19, PF 601553, D-14415, Potsdam, Germany