1. Department of Physics, University of Basel, Klingelbergstr. 82, 4056 Basel, Switzerland
2. Lehrstuhl für Angewandte Physik and Center for Nanoscience, Ludwig-Maximilians-University, Amalienstr. 54, 80799 Munich, Germany
3. Dipartimento di Scienze Fisiche, Informatiche e Matematiche, Università di Modena e Reggio Emilia, Via Campi 213/A, 41125 Modena, Italy