The topological non-connectivity threshold in quantum long-range interacting spin systems F. Borgonovi, G. L. Celardo and R. Trasarti-Battistoni Eur. Phys. J. B, 50 1-2 (2006) 27-31 Published online: 08 February 2006 DOI: 10.1140/epjb/e2006-00035-y