1. NEST, Istituto Nanoscienze—CNR and Scuola Normale Superiore, Piazza San Silvestro 12, I-56127 Pisa, Italy
2. IMEM—CNR, Parco Area delle Scienze 37/A, I-43124 Parma, Italy
3. Physics Department, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
4. Physics Department, Sapienza University of Rome, P.le Aldo Moro 2, 00185 Rome, Italy