1. Institute of Experimental and Applied Physics; University of Regensburg; 93053 Regensburg Germany
2. Institute of Nanotechnology (INT); Karlsruhe Institute of Technology (KIT); 76344 Eggenstein-Leopoldshafen Germany
3. Institut de Physique et Chimie des Matériaux de Strasbourg (IMCMS); CNRS-Université de Strasbourg; 67034 Strasbourg France