1. Institute of Technical Physics, Riga Technical University, Laboratory of Semiconductor Physics, Paula Valdena Str. 3/7, LV-1048, Riga, Latvia
2. Institute of Mathematical Sciences and Information Technologies, University of Liepaja, 14 Liela Street, Liepaja LV–3401, Latvia
3. The MS2 Discovery Interdisciplinary Research Institute, Wilfrid Laurier University, Waterloo, Ontario, Canada, N2L 3C5, Canada
4. BCAM - Basque Center for Applied Mathematics, E48009 Bilbao, Spain