[1] CNR, SPIN, Via Dodecaneso 33, I-16146 Genoa, Italy
[2] Univ Insubria, Ctr Nonlinear & Complex Syst, Dipartimento Sci & Alta Tecnol, Via Valleggio 11, I-22100 Como, Italy
[3] Ist Nazl Fis Nucleare, Sez Milano, Via Celoria 16, I-20133 Milan, Italy
[4] Free Univ Berlin, Dept Math & Comp Sci, Arnimallee 6, D-14195 Berlin, Germany
[5] Univ Genoa, Dipartimento Fis, Via Dodecaneso 33, I-16146 Genoa, Italy
[6] CNR, Ist Nanosci, NEST, Piazza San Silvestro 12, I-56127 Pisa, Italy