1. Department of Physics, Faculty of Science, University of Ostrava, 30. dubna 22, 701 03 Ostrava, Czech Republic
2. ATRI, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, J. Bottu 25, 917 24 Trnava, Slovakia
3. Department of Physics and CHiPS, North Carolina State University, Raleigh, North Carolina 27695, USA