1. Department of Physics “A.Volta”, University of Pavia-CNISM, Via Bassi 6, I-27100 Pavia, Italy, Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany, Instituto de Ciencia Molecular (ICMOL), Universidad de Valencia, Polígono de la Coma, s/n, 46980 Paterna, Spain, and Department of Molecular Biosciences, University of Oslo, N-0316 Oslo, Norway