4Department of Physics and Astronomy, University of Florence, 50019 Sesto Fiorentino, Italy
5ICFO—Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, Avinguda Carl Friedrich Gauss 3, 08860 Castelldefels (Barcelona), Spain
6ICREA, Passeig de Lluís Companys 23, 08010 Barcelona, Spain
7Institute for Condensed Matter Physics and Complex Systems, DISAT, Politecnico di Torino, I-10129 Torino, Italy