1. Institut für Physik, Johannes Gutenberg Universität Mainz, 55128 Mainz, Germany.
2. Institute of Physics, Czech Academy of Sciences, Cukrovarnická 10, 162 00 Praha 6, Czech Republic.
3. Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121 16 Prague 2, Czech Republic.
4. Grupo de Investigación en Física Aplicada, Departamento de Física, Universidad del Norte, Barranquilla, Colombia.
5. School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK.