Affiliation: | Department of Physics, University of Turku, FIN-20014 Turku, Finland; Faculty of Mathematics and Information Science, Warsaw University of Technology, PL-00-661 Warszawa, Poland; Fachbereich Mathematik, Technische Universität Darmstadt, Darmstadt, Germany; Department of Physics, University of Turku, FIN-20014 Turku, Finland |