1. Institute of Mathematics, Information Technologies and Physics, Udmurt State University, 426034 Izhevsk, Russia
2. Theoretical Department, Vereshchagin Institute for High Pressure Physics, Russian Academy of Sciences, 108840 Troitsk, Moscow, Russia
3. Physikalisch-Astronomische Fakultät, Friedrich-Schiller-Universität, 07743 Jena, Germany