1. Department of Physics; CICECO − Aveiro Institute of Materials and I3N; University of Aveiro; 3810-193 Aveiro Portugal
2. Department of Physics and CICECO − Aveiro Institute of Materials; University of Aveiro; 3810-193 Aveiro Portugal
3. Department of Mechanical Engineering; University of Minnesota Minneapolis; MN 55455 USA
4. Department of Physics and I3N; University of Aveiro; 3810-193 Aveiro Portugal
5. Walter Schottky Institut and Physik-Department; Technische Universität München; Am Coulombwall 4 Garching 85748 Germany