1. Institute of Microelectronics of Barcelona (IMB-CNM, CSIC), Campus de la UAB, 08193, Bellaterra, Spain
2. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra 08193 Barcelona, Spain
3. Institute for Theoretical Physics, Georg-August University, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany