1. Department of Physics, Taras Shevchenko National University of Kyiv, 6 Academician Glushkov Ave., Kyiv 03680, Ukraine
2. Bogolyubov Institute for Theoretical Physics, National Academy of Science of Ukraine, 14-b Metrolohichna Str., Kyiv 03680, Ukraine
3. CNR-SPIN, University “Tor Vergata,” Viale del Politecnico 1, I-00133 Rome, Italy