1. Department of Mathematics and Physics, Lappeenranta University of Technology, PO Box 20, FIN-53851 Lappeenranta, Finland
2. Institute of Applied Physics, Academy of Sciences of Moldova, Academiei Str. 5, MD-2028 Chisinau, Republic of Moldova
3. Ioffe Institute, Politehnicheskaya Str. 26, St. Petersburg 194021, Russian Federation