1. School
of Physics, CRANN, AMBER Research Centre, Trinity College Dublin, College Green, Dublin 2, Ireland
2. Dipartimento
di Fisica “A. Volta”, Università degli Studi di Pavia, via Bassi 6, 27100 Pavia, Italy
3. NEST, Instituto
Nanoscienze-CNR and Scuola Normale Superiore, Piazza S. Silvestro 12, 56127 Pisa, Italy
4. Department
of Physics, Chair for Applied
Physics, Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Staudtstr. 7, 91058 Erlangen, Germany