The 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.
@article{arxiv.cond-mat/0312130,
title = {Entanglement in a first order quantum phase transition},
author = {J. Vidal and R. Mosseri and J. Dukelsky},
journal= {arXiv preprint arXiv:cond-mat/0312130},
year = {2007}
}