1. Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands, Faculty of Physics, University of Bucharest, P. O. Box MG-11, Bucharest 077125, Romania, and Laboratory Physics of Complex Systems, Faculty of Education, University of Maribor, Koroška 160, 2000 Maribor, Slovenia and Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia