1. Institut de Physique, Université de Neuchâtel, rue Breguet 1, CH-2000 Neuchâtel, Switzerland
2. Institute for Theoretical Physics, University of Wrocław, pl. M. Borna 9, 50-204 Wrocław, Poland
3. Institute of Physics, Pedagogical University, pl. Słowiański 6, 65-029 Zielona Góra, Poland