1. Grup de Fisíca Estadística, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
2. Institut d’Estudis Catalans, Carme, 47, 08001 Barcelona, Spain
3. Department of Mathematical and Computer Sciences, Physical Sciences and Earth Sciences, University of Messina, Viale F. Stagno d’Alcontres, 31, 98166 Messina, Italy