1. Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 30123-970, Brazil
2. Department of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden
3. Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstraße 5-13, 81377 Munich, Germany
4. Institute of Solid State Physics, Technische Universität Berlin, 10623, Berlin, Germany
5. Department of Physics, Philipps-Universit ät Marburg, 35037 Marburg, Germany