2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/99358The phase diagram of spins 1/2 embedded in a magnetic field mutually interacting antiferromagnetically is determined. Contrary to the ferromagnetic case where a second order quantum phase transition occurs, a first order transition is obtained at zero field. The spectrum is computed for a large number of spins and allows one to study the ground state entanglement properties which displays a jump of its concurrence at the critical point.4 pages, 3 EPS figuresStrongly Correlated ElectronsQuantum Physicstext