1. School of Science and Engineering, Reykjavik University, Menntavegur 1, IS-101 Reykjavik, Iceland
2. Department of Theoretical Physics, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wroclaw, Poland
3. Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland
4. Istituto Nanoscienze-CNR, Via Campi 213a, I-41125 Modena, Italy