1. Department of Physics, Università degli Studi di Milano, and INSTM, Milano, Italy
2. Department of Physics, Università degli Studi di Pavia and INSTM, Pavia, Italy
3. Department of Physics and Astronomy, Università degli Studi di Bologna, Bologna, Italy
4. Physikalisches Institut, Universität Stuttgart, Stuttgart, Germany
5. INSTM and Department of Chemistry, University of Florence, Firenze, Italy
6. Departamento de Quimica Inorganica, Universidad del Pais Vasco, Bilbao, Spain
7. Centro S3, Istituto Nanoscienze - CNR, Modena, Italy