1. Department of Natural Sciences, Institute of Electrophysics, Kálmán Kandó Faculty of Electrical Engineering, Óbuda University, Tavaszmező u. 17, H-1084 Budapest, Hungary
2. Department of Natural Sciences, National University of Public Service, Ludovika tér 2, H-1083 Budapest, Hungary
3. Department of Physics, Institute of Physics, Budapest University of Technology and Economics, Muegyetem rkp. 3, H-1111 Budapest, Hungary