1. Department of Physics, N. I. Lobachevsky University of Nizhny Novgorod, Nizhny Novgorod 603600, Russia
2. Centro de Física, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
3. International Iberian Nanotechnology Laboratory, Braga, Portugal
4. Institut für Halbleitertechnik, Universität Stuttgart, Pfaffenwaldring 47, D-70569 Stuttgart, Germany
5. QuantaLab, Universidade do Minho, Braga, Portugal