1. Institute of Applied Physics of the Academy of Sciences of Moldova, Academic Street 5, Chisinau 2028, Republic of Moldova
2. KTH and Stockholm University, Nordic Institute for Theoretical Physics, Roslagstullsbacken 23, 106 91 Stockholm, Sweden
3. St. Petersburg State University, Institute of Physics, Department of Solid State Physics, 1, Ulyanovskaya Street, Petrodvorets, 198504 St. Petersburg, Russia