Bell inequality in an exactly soluble spin-1/2 XX chain displaying spin-Peierls transition

Quantum nonlocality is an experimentally measurable quantum correlation indicated by the violation of Bell inequality. Previous studies show that bipartite Bell inequality is not violated in various translationally invariant spin models. In this paper, we study bipartite Bell inequality in an exactly soluble spin-1/2 XX chain displaying spin-Peierls transition, where the system is in a uniform phase at high temperatures and in a dimerized phase in low temperatures. The dimerization is induced by spin-phonon interactions in the system. We find that the Bell inequality is violated in the dimerized phase, even if the dimerization parameter is very small. It suggests that two-qubit quantum nonlocality may be widely present in real materials in which spin-phonon interactions cannot be ignored. The relation between Bell inequality and the thermal phase transition in the chain is also studied. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2014