1. Institute of Applied Mathematics and Fundamental Science, Lviv Polytechnic National University, Lviv 79013, Ukraine
2. O.O. Galkin Institute of Physics and Engineering of NAS of Ukraine, Department of Physical Materials, Donetsk 03680, Ukraine
3. Fachbereich Physik, Institut für Experimentalphysik, Freie Universität Berlin, Berlin 14195, Deutschland